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BEL-2015 Interview

CONTENTS

1. General guidelines PAGE (1-2)

2. Important terms used in Telecom technology PAGE (3-25)

3. Technical questions asked in interview PAGE (26-44)

(i) Communication system

(ii) Microwave

(iii) Electromagnetic theory

(iv) Electronics device

(v) Analog and Digital

(vi) Instrumentation

4. Important study material for technical part PAGE (45-160)

(i) EDC &Analog

(ii) Digital

(iii) EMT& Microwave

(iv) RADAR, Satellite& OFC

5. Important terms used in Electronics PAGE (161-176)

6. Latest technology material PAGE (161-188)

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Chapter-1: General Guidelines

BEL interview contains 15/85 weight age of total marks. These marks are very crucial for final selection and

there is cut-off marks for interview. Dress must be simple and formal. Try to wear neat and clean formal

clothes. Color of dress must be light. Don‘t use perfumes in interview. One should look decent and formal.

Generally time allotted is 10-15 minutes which can vary between 5-20 minutes. BEL interview is totally

technical interview and most of the questions will be technical in nature.

Marks: BEL interview contains 15 marks. These marks are very crucial for final selection out of 100.

There are separate cut-off marks for interview.

Interview Board: There are 4 members in interview panel BEL Interview. Chairman will be BEL higher

authority person and other three members will be technical persons who are senior persons from BEL or

from academic backgrounds.

Dress code: Dress must be simple and formal. Try to wear neat and clean formal clothes. Color of dress

must be light. Don‘t use perfumes in interview. One should look decent and formal. Shoes should be

polishes and preferably black.

Time for interview: Generally time allotted is 20 minutes which can vary between 15-25 minutes.

Generally there are 5-6 members in one session.

Nature of interview: BEL interview is technical interview and most of the questions will be technical in

nature. Most of the questions will be related to your subject, job experience, job profile, your hobbies and

latest technology in field of Electronics&Communication field. Most of the questions will be related to your

core subject like Microwave, RADAR satellite, Optical fiber Analog electronics and latest technology in

field of Electronics&communication. If you are comfortable with English then you can use this language. If

you are not comfortable then you can use mixture of Hindi and English.

Language used in interview: If you are comfortable with English then you can use this language. If you are

not comfortable then you can use mixture of Hindi and English. Many students have got good marks by

performing in mixed language.

Golden rules (Do’s and Do not’s) for BEL Interview:

1. Just after entering in interview board room greet everyone .If lady member is there then greet her

first and then rest of members. No need of greeting one by one you can say good-morning/evening

all of you sirs (by making eye contact to everybody).

2. If lady member is there then say good morning/evening mam first and then say good

morning/evening to all of you sirs. (Never use madam).

3. When chairman offer chair then say thank you mam/sir to chairman of your board.

4. Distance between chairman of board and you will be much so your voice must be audible to

chairman. Many times voice is not audible and that create a very negative impact in interview.

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5. Never interrupt in between first listen their question properly and with patience. Listening is also

very important part of interview.

6. If you don‘t know answer of any question then say sorry sir I don‘t know or you can say I studied

earlier but this time I m not able to recall.

7. Try to avoid giving wrong answers .if you are not sure about answer of any question you simply say

sir I m not sure, but I think answer may be this.

8. If any members tells you correct answer say him/her thank you very much sir/mam.

9. If they offer Tea/water to you then avoid it if possible but don‘t forget to say thank-you sir/mam.

10. Try to answer technical question in a simple language so that chairman can also understand it.

Chairman of board is decision maker of your marks and he is generally a non technical person.

11. Don‘t contradict yourself in questions related to your bio-data.

12. If you have written anything related to your hobby, sports, extra activities etc. then your answer must

be balanced and must not be contradictory.

13. In interview never get tensed if you are being then hide it. Always give answer with light smiling

face.

14. Never give negative answer to anything. Never say that job is bad, low profile etc.

15 Never blame system for anything. Never blame anybody for anything during your interview process.

You can‘t say our whole system is corrupt .our system is responsible for that etcs.

16. Never blame government policies and never make system responsible for everything.

17. There are 4 members during interview and they ask questions one by one. So maintain eye contact to

person who is asking questions and in mean time you can also see other members also. But main

focus must be on that person only who is asking questions.

18. Answers must be given in very simple language and up to the points. Try to avoid formulas during

answering always try to give logical explanation.

19. Be more cautious in questions related to your bio-data and non technical questions. In these

questions you can‘t say I don‘t know eg. If they ask why do you want to join IES you can‘t say sorry

sir I don‘t know. If you have written something in your bio-data then you are supposed to know it.

20. Wear proper dress and if possible don‘t use perfume etc. Your cloths must be neat and clean and

shoes must be polished.

21. Never use words like social status, social-value, job security& cool job etcs.

22. Try to use words with which you are familiar. For Example if you are answering about routers and

not good in packet switching then try to avoid packet switching word, while explaining about router.

But at the same time if you are good in packet switching then use word packet switching

intentionally.

23. Never show your ego during interview.

24. Never try to show that you are superior. Your behavior must be down to earth. Your attitude must be

in such a manner that you can work in a team effectively.

25. Before leaving room don‘t forget wishing every one and saying thank you to all of you sirs.

26. Don‘t disclose your medical history. e.g. if you are color blind then don‘t tell them in interview

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Chapter-2: Important terms used in Communication

Access Channels: Dedicated channels giving nondiscriminatory access to a local cable system by the

public, government agencies or educational institutions.

ACCUNET Switched Digital Services: High-speed dial-up digital data services offered by AT&T for full

duplex digital transmission at speeds of 56, 64, 384 and 1536 kbps.

Acoustic Modem: A modulator-demodulator unit that converts data signals to telephone tones and back

again.

Active Satellite: A satellite that transmits a signal, in contrast to a passive satellite that only reflects a signal.

The signal received by the active satellite is usually amplified and translated to a different frequency before

it is retransmitted.

ADPCM - Adaptive differential pulse code modulation: A method of compressing audio data by

recording the differences between successive digital samples rather than full value of the samples. There are

many different types of ADPCM standards; this refers to the standard as defined in the CD-ROM XA and

CD-I standards.

ADSL - Asymmetric Digital Subscriber Loop: ADSL uses a regular phone line (twisted pair) without a

dial tone (a dry pair) to allow transfer speeds of up to 7Mb downstream with slower speeds going back up

(faster than a T1). The Telcos are using ADSL to maintain market share rather than allowing the cable

industry to deploy cable modems.

Analog: Information represented by a continuous electromagnetic wave encoded so that its power varies

continuously with the power of a signal received from a sound or light source.

Analog-to-Digital - A/D Conversion: The conversion of an analog signal into a digital equivalent. An A/D

converter samples or measures an input voltage and outputs a digitally encoded number corresponding to

that voltage.

Analog Transmission: Transmission of a continuously variable signal as opposed to a discrete signal.

Physical quantities such as temperature are described as analog while data characters are coded in discrete

pulses and are referred to as digital.

Arpanet: A predecessor of the Internet. Started in 1969 with funds from the Defense Department's

Advanced Research Projects Agency.

ASCII: American Standard Code for Information Interchange; pronounced "Askee." An eight-level code for

data transfer adopted by the American Standards Association to achieve compatibility between data services.

Aspect Ratio: The ratio of picture width to height (4 to 3 for North American NTSC broadcast video).

Asynchronous Communication: Takes place in different time frames and accessed at the user's

convenience. Synchronous communication takes place in the same time frame such as a live teleconference.

Asynchronous Time-Division Multiplexing: An asynchronous signal transmission mode that makes use of

time-division multiplexing.

Asynchronous Transmission: A technique in which the time interval between characters may be of

unequal length. Transmission is controlled by start and stop elements at the end of each character. Used for

low-speed terminal links.

ATM - Asynchronous Transfer Mode: ATM switching protocol can handle all types of traffic - voice,

data, image, and video.

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Attenuation: The difference between transmitted and received power due to loss through equipment, lines,

or other transmission devices; usually expressed in decibels. The loss in power of electromagnetic signals

between transmission and reception points.

ATV - Advanced Television: An agglomeration of techniques, based largely on digital signal processing

and transmission, that permits far more program material to be carried through channels than existing analog

systems can manage. In this sense, HDTV (high definition television) is a subset of ATV. ATV does not

automatically signify improved picture or sound performance. Those are things that can be accomplished

with ATV in systems designed for such purposes, but it can also carry ten somewhat lower- quality signals

where only one could exist previously, or permit ghost cancellation for ordinary NTSC signals. In each case,

the new features derive from the use of digital techniques of one form or another.

Audio Bridge: An audio bridge connects the telephones at remote sites, equalizes the noise distortion and

background noise for a live audio teleconference.

Audio Frequency: A frequency lying within the audible spectrum (the band of frequencies extending from

about 20 Hz to 20 kHz).

Audiographic: Teleconference system which uses narrow band telecommunications channels (telephone

lines or subcarriers); transmits audio and graphics. Graphics can be transmitted by facsimile transceivers

(transmitter-receiver), computers (text or graphic display), or electronic drawing systems (such as electronic

blackboard) which allow a participant to draw or write on an electronic screen which is transmitted to a

remote site where participants can see it.

Aural Cable: Services providing FM-only original programming to cable systems on alease basis.

Azimuth: Angle between an antenna beam and the meridian plane, measured along a horizontal plane. How

far east or west in the southerly sky the satellite is located in relation to the local meridian, or north-south

plane. It is measured in degrees, clockwise from true north.

B-Mac: A method of transmitting and scrambling television signals where MAC (multiplexed analog

component) signals are time-multiplexed with a digital burst containing digitized sound, video

synchronizing, authorization, and information.

Backbone: A high-speed network that connects several powerful computers. In the U.S., the backbone of

the Internet is often the NSFNet, a government funded link between a handful of supercomputer sites across

the country.

Backbone Microwave System: A series of directional microwave paths carrying common information to be

relayed between remote points. The backbone microwave system is engineered to allow the insertion of

signals, the dropping off of signals and the switching of signals along its length at designated relay points. In

order to maintain the signals in the highest possible quality, the equipment used in the backbone microwave

system is normally of a higher technical performance level than other microwave electronics in the network.

Antennas are always directional.

Backhaul: A term used for the transmission of a signal (normally video) from the ends of transmission

systems such as microwave to a central point. For a satellite videoconference, a backhaul refers to a signal

brought in from a secondary site to the origination site, mixed with the primary signal, and sent out over the

program out satellites.

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Bandwidth: Determines the rate at which information can be transmitted across that a medium. The rates

are measured in bits (bps), kilobits (kbps), megabits (Mbps), or gigabits per second (Gbps). Typical

transmission services are 64 kbps, 1.544 mbps (T1), and 45 Mbps (T3). The space between the top and

bottom limit of airwave frequencies that are transmitted over a communications channel. The maximum

frequency (range), measured in Hertz, between the two limiting frequencies of a transmission channel; the

range of frequencies that can be carried by a transmission medium without undue distortion. Narrowband

uses lower frequency signals such as telephone frequencies of about 3,000 Hertz and radio subcarrier signals

of about 15,000Hertz. Broadband uses a wide range of frequencies (broadcast and cable TV, microwave and

satellite; carries a great deal of information in a short time; more expensive to use. C band is in the 4 to 6

Giga-Hertz (GHz) Ku Band is 12 and 14 GHz .14.0 and 14.5 GHz are used to uplink; 11.7 and 12.2 GHz

are used to downlink. A receiver with dual band capability can receive C and Ku band signals.

Base Band: The unmodulated signal that is delivered from a satellite receiver.

Basic Rate Interface - BRI: The basic subscriber loop for one or two users, which delivers two 64 Kbps B

channels and one 16 kbps D channel over a standard twisted pair loop. Each circuit-switched B channel can

transmit voice or data simultaneously. The D channel transmits call control messages and user packet data.

Baud: A unit of digital transmission signaling speed derived from the duration of the shortest code element.

Speed in bauds is the number of code elements per second. 300 Baud is low, 2400 Baud and 9600 Baud are

much faster and common for transmitting data by computer.

BBS - Bulletin Board System/Service: The BBS is an area within a network where users can "post"

information for public display, in much the same way one posts information on a regular bulletin board.

Most networks dedicate a bulletin board to special interest areas, such as education or computer care.

Beyond the Horizon Region: That physical region beyond the optical horizon with which line-of- sight

radio communications is not normally possible, but can occur if atmospheric conditions are such to cause

beam bending or forward scattering of the radio signal.

Bit Error Rate: Fraction of a sequence of message bits that are in error. A bit error rate of 10-6

means that

there is an average of one error per million bits.

Bit Rate: Speed at which bit positions are transmitted, normally expressed in bits per second

Blanking Pulse: 1. A signal used to cut off the electron beam and thus remove the spot of light on the face

of a television picture tube or image tube. 2. A signal used to suppress the picture signal at a given time for a

required period.

Bridge: Device which interconnects three or more telecommunication channels, such as telephone lines. A

telephone conference audio bridge links three or more telephones (usually operated assisted). Usually a

meet-me audio bridge or provides a teleconference direct dial access number. Both connect remote sites and

equalize noise distortion.

Bridges, Gateway, Routers: Devices that convert LANs to other LANs, computers and WANs by allowing

systems running on different media (copper wire, fiber optics, etc.) and protocols (rules to communicate).

Bridging Amplifier: An amplifier connected directly into the main trunk of the CATV system. It serves as a

sophisticated tap, providing isolation from the main trunk, and has multiple high level outputs that provide

signal to the feeder portion of the distribution network. Synonymous with bridger and distribution amplifier.

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Broadband: Communications channels that are capable of carrying a wide range of frequencies. Broadcast

television, cable television, microwave and satellite are examples of broadband technologies. These

technologies are capable of carrying a great deal of information in a short amount of time, but are more

expensive to use than technologies like telephone which require less band width.

Broadband (Wideband)distribution systems: A telecommunications medium that carries high frequency

signals includes television frequencies of 3 to 6 megahertz. Broadband distribution systems work like cable

TV, in that up to twenty channels are available from a single coaxial cable. A main trunk cable will originate

at the control room, and run down the hallways of the viewing area. Smaller cables can tie into the main

cable at any point along its length. Any room that is near the main cable run can have access to all of the

channels Telecommunications on the system. Normal television sets are used, and a variety of channels can

be received by simply changing channels on the television set.

Broadband Network: A local area network (LAN) residing on coaxial cable capable of transporting

multiple data, voice and video channels.

Broadcasting: The dissemination of any form of radio electric communications by means of Hertzian waves

intended to be received by the public. Transmission through space, utilizing pre assigned radio frequencies,

which are capable of being received aurally or visually by an audience. The one-way transmission of

information.

Buffer: Temporary storage facility used as an interface between system elements whose data rates are

different; Memory area in computer or peripheral device used for temporary storage of information that has

just been received. The information is held in the buffer until the computer or device is ready to process it.

Hence, a computer or device with memory designated as a buffer area can process one set of data while

more sets are arriving.

Business Television - BTV: Corporate use of video transmission for meetings/training via satellite.

Burst: 1. In data communication, a sequence of signals counted as one unit in accordance with some

specific criterion or measure. 2. A color burst.

Burst Modem: In satellite communications, an electronic device used at each station that sends high-speed

bursts of data which are interleaved with one another. These bursts must be precisely timed to avoid data

collisions with multiple stations.

Bus: A circuit or group of circuits which provide an electronic pathway between two or more central

processing units (CPUs) or input/output devices.

Byte: A group of bits treated as a unit used to represent a character in some coding systems. The values of

the bits can be varied to form as many as 256 permutations. Hence, one byte of memory can represent an

integer from 0 to 255 or from -127 to+128. The unit of computer memory typically consists of eight bits;

64K, 64,000 bytes or64 kilobytes.

C-Band: A category of satellite transmissions which transmit from earth at 4.0 to 6.0 GHz and receive from

the satellite at between 3.7 and 4.2 GHz which are also shared with terrestrial line-of-sight microwave users.

This band of transmissions has less path loss than the other standard used for satellites (Ku-Band) but must

have a large antenna for the same receiver input power level due to its use of longer wavelength frequencies.

Other problems relating to the use of C-Band include the shared use of these frequencies with terrestrial

microwave transmission which cause interference with the weaker satellite signals in certain areas.

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Cable/Cable Television: A broadband communications technology in which multiple television channels as

well as audio and data signals are transmitted either one way or bi directionally through a distribution

system to single or multiple specified locations. Uses coaxial cable to transmit programs. Direct-by-wire

transmission to homes from a common antenna to which these homes are linked. Cable companies provide

the service in most cases. Distinguished from television reception through a roof-top antenna that picks up

the broadcast signal. The only acronym was CATV, denoting community antenna television.

Cache: In a processing unit, a high-speed buffer storage that is continually updated to contain recently

accessed contents of main storage. Its purpose is to reduce access time. A holding area for data within the

CD-ROM drive itself or on its interface board, that allows the system a method for matching data transfer

rates and presentation speed requirements.

CATV- Community Antenna Television: A broadband communications system capable of delivering

multiple channels of entertainment programming and non entertainment information from a set of

centralized antennas, generally by coaxial cable, to a community. Many cable television designs integrate

microwave and satellite links into their overall design, and some now include fiber optics.

Carrier: Vendor of transmission services operating under terms defined by the FCC as a common carrier.

Owns a transmission medium and rents, leases or sells portions for a set tariff to the public via shared

circuits.

CAV - Constant Angular Velocity: A disk that rotates at a constant rate of speed. Examples are hard

drives, floppy disks, magneto-optical discs and some videodiscs. ACAV videodisc permits access to video

within seconds, allows for up to 54,000 still frames, or may contain up to 30 minutes of full motion video

(or any combination of stills and video).

CCITT: Consultative Committee on International Telephony and Telegraphy

CCL - Connection Control Language: A scripting language that allows the user to control a modem.

CCTV: Closed-Circuit Television. The system for sending cable signals to subscribers or designated

locations.

CD-ROM Compact Disc - Read Only Memory: CD-ROM discs can store a variety of data types including

text, color graphics, sound, animation and digitized video that can be accessed and read through a computer.

A disc can store up to 600 megabytes of data, much more information that can be stored on a 3.5 inch

compute disk, which hold up to 1.4 megabytes. This makes CD-ROM an inexpensive medium for storing

large amounts of data. Because CD-ROM was not designed to store digitized, full-motion video,

compression technology is important in compressing data to fit on a disc as well as decompressing data for

playback.

Center Clipper: Variable attenuator which is used to eliminate any residual echo left by the echo canceller.

A key difference between one canceller and another is the manner in which this center clipper operates. In a

high quality canceller, the center clipper will operate very rapidly and smoothly, resulting in no residual

echo during double-talk and no clipping of syllables. The center clipper is in essence a level- activated

switch. Signals above the threshold level are passed unaltered and signals below the threshold are blocked.

When speech is present in both directions, the center clipper tends to mutilate the speech signal, adding

audible amounts of harmonic and intermodulation distortion. This distortion is often referred to as "glitch"

and sounds remarkably like its name when it occurs. The transmit signal can be totally chopped out if the

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level of the transmit signal drops below the estimated level of returning echo.

Channel: A signal path of specified bandwidth for conveying information.

1. A half circuit;

2. A radio frequency assignment (which is dependent upon the frequency band and the geographic location).

Channel capacity in a cable television system is the number of channels that can be simultaneously carried

on the system. Generally defined in terms of the number of 6 MHz (television bandwidth) channels.

CFDA - Catalog of Federal Domestic Assistance: The CFDA is a government-wide compilation of federal

programs, projects, services and activities that provide assistance or benefits to the American public. The

primary purpose of the CFDA is to assist users in identifying programs that meet specific objectives of a

potential applicant, and to obtain general information on federal assistance programs.

Charge-Coupled Device - CCD: A solid-state device used in many television cameras to convert optical

images into electronic signals. These imagers are organized into rows and columns called pixels. The charge

pattern formed in the CCD pixels when light strikes them forms the electronic representation of the image.

Chroma Key: In color television, an electronic matting process of inserting one image over a background.

Used very commonly with weathercasters who are standing in front of a blank wall painted process blue.

The electronics remove the blue and insert the weather map so that on the television screen the two images

merge and the weathercaster appears to be standing in front of a large map painted on the wall.

Chrominance Signal: The color signal component in color television that represents the hue and saturation

levels of the colors in the picture.

Circuit: Means of two-way communication between two or more points. 1. In communication systems, an

electronic, electrical, or electromagnetic path between two or more points capable of providing a number of

channels. 2. Electric or electronic part. 3. Optical or electrical component that serves a specific function or

functions. Circular Polarization: A mode of transmission in which signals are down linked in a rotating

corkscrew pattern. A satellite's transmission-capacity can be doubled by using both right-hand and left-hand

circular polarization.

Closed Circuit Television - CCTV: A private television system in which signals are sent usually via cable,

to selected viewing points throughout the distribution system but are not broadcast to the public. The signal

does not have to meet FCC commercial specifications.

CLV - Constant Linear Velocity: A disc that rotates at a varying rate of speed. Examples are CD-Audio,

CD-ROM, CD-I, CD-ROM XA, and some videodiscs. CLV videodiscs may contain up to one hour of full

motion video, but still frames and quick access time are forfeited.

C/N - C/NR - Carrier to Noise Ratio: Refers to the ratio of the satellite carrier (or signal) to noise level in a

given channel. Usually measured in dB at the LNA output. Coaxial Cable - Coax: A type of metal cable

used for broadband data and cable systems. It has excellent broadband frequency characteristics, noise

immunity and Telecommunications

Co-Channel Interference: Interference on a channel caused by another signal operating on the same

channel.

Codec: A Coder-Decoder converts analog signals, (voice or video) into digital form (1or 0) for transmission

over a digital medium and, upon reception at a second codec, reconverts the signals to the original analog

form. Two codecs are needed - one at each end of the channel.

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Color Burst: In NTSC terminology, refers to a burst of approximately nine cycles of 3.58 MHz subcarrier

on the back porch of the composite video signal. This serves as a color synchronizing signal to establish a

frequency and phase reference for the chrominance signal.

Color Signal: Any signal at any point in a color television system for wholly or partially controlling the

chromaticity values of a color television picture.

Color Subcarrier: In NTSC color, the 3.58 MHz subcarrier whose modulation sidebands are interleaved

with the video luminance signal to convey color information.

Color Transmission: A method of transmitting color television signals which can reproduce the different

values of hue, saturation, and luminance which together makeup a color picture.

Communications Satellite: Relay system in orbit above earth for telecommunications signals (voice, video,

data); require earth stations to transmit and receive signals at the ground locations. Commonly called a

"bird."

Compression: The application of any of several techniques that reduce the amount of information required

to represent that information in data transmission. This method reduces the required bandwidth and/or

memory.

Compressed Video: Processes video images; transmits changes from one frame to the next which reduces

the bandwidth to send them over a telecommunications channel reduces cost. Also called bandwidth

compression, data compression or bit rate reduction. The most publicized compression techniques are

proposed by two expert groups, that of JPEG (Joint Photographic Expert Group) and MPEG (Moving

Picture Expert Group), who are defining methods for image compression in still frame and real time video.

The algorithm used by these two groups is called discrete cosine transform (DCT). DCT transforms a block

of pixels into a matrix of coefficients and estimates redundancy in the matrix. The advantage of JPEG and

MPEG is that the algorithms are symmetrical; that is, the same amount of processing is required for the

encode and decode functions. These are ideal for two-way applications such as videoconferencing.

Crosstalk: 1. Undesired transfer of signals from one circuit to another circuit. 2. The phenomenon whereby

a signal transmitted on one circuit or channel of a communications system is detectable or creates an

undesirable effect in another circuit or channel.

D1 and D2: Digital tape component and composite formats (respectively) used for professional video

recording. D1 is costlier than D2. Both can go through many generations of dubbing without visible loss of

picture quality.

Daisy Chain: A way to connect computers - one after another along a single line.

DARPA: Defense Advanced Research Projects Agency of the Pentagon. Replaced ARPA.

Data Communications: 1. The movement of encoded information by means of electrical or electronic

transmission systems. 2. The transmission of data from one point to another over communications channels.

Data Compression: A technique that saves storage space by eliminating gaps, empty fields, redundancies,

or unnecessary data to shorten the length of records or blocks.

DBS - Direct Broadcast Satellite: Service uses high powered satellites to broadcast multiple channels of

TV programming to inexpensive, small-dish antennas at homes for direct on-site reception of signals.

DCT - Discrete Cosine Transform: Compression algorithm

Dedicated Lines: Leased telecommunications circuits that are devoted to a specific application; a circuit

designated for exclusive use by two users; i.e., for interactive portion of teleconference.

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Dial up - Dialup: To call another computer via modem. A connection or line reached by modem, as in "a

dialup line."

Dial-Up Teleconferencing: Using public phone line to connect with a tele conference, either with or

without operator assistance.

Digital: Discrete bits of information in numerical steps. A form of information that is represented by signals

encoded as a series of discrete numbers, intervals or steps, as contrasted to continuous or analog circuits.

Digital signals can be sent through wire or over the air. The method allows simultaneous transmission of

voice and data. All digital technology is emerging as the primary transmission mode for voice, video, data

and facsimile; Information represented by signals encoded as a series of discrete numbers, intervals or steps.

Can be sent through wire or over the air. Allows simultaneous transmission of voice, video and data.

Digital Transmission: The transmission of information in the form of "1s" and "0s." Information

customarily sent in this form is related to computer data traffic which is already in digital form. Other

communications include audio and video.

Digitized Audio: Allows the incorporation of audio materials with other media to present information over

a computer network or through the Internet. With this method,Telecommunications the audio is

contained.various software programs "stream" the audio signal so that it can be heard as it is being said.

Digitizer: A device that converts an analog signal (either images or sound) into a digital signal that can be

manipulated on the computer. Video capture boards convert video images from video sources such as the

VCR or video camera, while sound digitizers take any sounds, the spoken word as well as music off of a

cassette or CD player, and turn them into digital data. That data can be edited using sound editing and

multimedia software.

Disk Drive: A computer data storage device in which data is stored on the magnetic coating (similar to that

on magnetic tape) of a rotating disk.

Dish: Parabolic antenna. Primary element of a satellite earth station; sends and/or receives satellite signals.

Usually bowl-shaped; concentrates signals to a single focal point. The antenna cross section exposed to the

signal is the aperture.

Distortion: An undesired change in wave form of a signal in the course of its passage through a

transmission system.

Downlink: Transmission of radio frequency signals from a satellite to an earth station (verb). A satellite

receiving station (noun).

Download: 1. Transfer data from a main computer or memory to a remote computer or terminal. 2. There

are several different methods, or protocols, for downloading files, most of which periodically check the file

as it is being copied to ensure no information is inadvertently destroyed or damaged during the process.

Some, such as XMODEM, only let you download one file at a time. Others, such as batch-YMODEM and

ZMODEM, let you type in the names of several files at once, which are then automatically downloaded.

Drive bay: The opening in a computer chassis designed to hold a floppy drive, hard drive, CD-ROM drive,

tape drive or other device. May be half-height or full-height, exposed or internal. Drop-Outs: Black or white

lines or spots appearing in a television picture originating from the playback of a video tape recording.

DS1: Digital signal level 1; a digital transmission format in which 24 voice channels are multiplexed into

one T1 channel.

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DS3: Digital signal level 3; a telephony term describing the 45 mbps signal carried on a T3 facility. It is

most often associated with broadcast video transmission. Although the broadcast purest will rightfully point

out that as a digital signal it is not a true broadcast quality RS-250B standard signal, it is the nearest

approximation to a broadcast signal in a digital environment.

DSP: Digital signal processing. Telecommunications

DTMF - Dual Tone Multiple Frequencies: Standard telephone signaling technique which can be used

through any transmission medium of voice grade or better. The technique is often used for remote switching

control functions. Dual Band Capability: Many receivers are capable of both C and Ku band operation.

Duplex: In a communications channel the ability to transmit in both directions.

DVI - Digital Video Interactive: DVI is a programmable (variable bit and frame rate) compression and

decompression technology developed by Intel offering two distinct levels and qualities of compression and

decompression for motion video.

E-Layer: A heavily ionized signal-reflecting region location 50-70 miles above the surface of the earth,

within the ionosphere.

E-mail - Electronic mail: The term for private messages sent as files from one computer to another, either

over a local area network (LAN), or via modem over the phone lines. E-mail is like having your own private

mail box on a network. Used as both a noun and verb. Mail can be sent between Internet and commercial

services such as American On Line.

Earth Station: The location antenna used to send or receive signals to satellites normally located in the

geostationary orbit. A parabolic antenna and associated electronics for receiving or transmitting satellite

signals.

Echo: The reflections of signal energy that cause it to return to the transmitter or to the receiver.

Echo Canceller: Eliminates audio transmission echo. A telephone line echo canceller produces a synthetic

replica of the echo it expects to see returning and subtracts it from the transmitted speech. The replica it

creates is based on the transmission characteristics of the telephone cable between the echo canceller and the

telephone set.

Electromagnetic Spectrum: The frequency range of electromagnetic radiation that includes radio waves,

light and X-rays. At the low frequency end are sub-audible frequencies (e.g., 10 Hz) and at the other end,

extremely high frequencies (e.g., X-rays, cosmic rays).

Electronic Blackboard or Whiteboard: A device that looks like an ordinary blackboard or whiteboard, but

has a special conductive surface for producing free-hand information that can be sent over a

telecommunications channel, usually a telephone line.

Electro-Mechanical Pen: A device that has an electronic pen with a mechanical arm for producing free-

hand information that can be sent over a telecommunication channel, usually a telephone line.

Electronic Mail - E-Mail: A system of electronic communication whereby an individual sends a message to

another individual or group of people; includes computer mail and facsimile (FAX).

Encoder: A device that electronically alters a signal (encrypts) so that it can be clearly seen only by

recipients that have a decoder which reverses the encryption process.

Encryption: An encoder electronically alters a signal so that it can be clearly seen only by recipients who

have a decoder to reverse encryption. Selective address ability/scrambling designates receivers to

descramble a signal. Each decoder has a unique "address."

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End User: The ultimate last user of a telecommunications system whether or not it is a student within a

school, business or a subscriber on a cable television system.

EROM - Erasable Read-Only Memory: In a computer, the read-only memory (ROM) that can be erased

and reprogrammed. Synonymous with erasable-programmable read only

Ethernet: Baseband protocol and technology developed by Xerox and widely supported by manufacturers;

a packet technology that operates at 10 mbps over coaxial cable and allows terminals, concentrators, work

stations and hosts to communicate with each other.

Facsimile - FAX: A device which uses a form of electronic transmission allowing movement of hard-copy

documents from widely separated geographic areas via a telecommunications channel, usually a telephone

line. Usually called a FAX machine now but previously was called a tele copier.

FDDI Fiber Distributed Data Interface: 1. Transports data up to speeds of 100 Mbps.

2. FDDI is a high-speed (100Mb) token ring LAN.

FDMA - Frequency Division Multiple Access: Refers to the use of multiple carriers within the same

satellite transponder where each uplink has an assigned frequency slot and bandwidth.

Federal Communications Commission (FCC): An independent government agency established by the

Communications Act of 1934 to regulate the broadcasting industry. The Commission later assumed

authority over cable. The FCC is administered by seven commissioners and reports to Congress. The FCC

assigns broadcasting frequencies, licenses stations, and oversees interstate communications.

FEC - Forward Error Correction: Adds unique codes to the digital signal at the source so errors can be

detected and corrected at the receiver.

Feeder Cables: The coaxial cables that take signals from the trunk line to the subscriber area and to which

subscriber taps are attached. Synonymous with feeder line.

Fiber Optics: Communications medium based on a laser transmission that uses a glass or plastic fiber

which carries light to transmit video, audio, or data signals. Each fiber can carry from 90 to 150 megabits of

digital information per second or 1,000 voice Telecommunications channels. Transmission can be simplex

(one-way) or duplex (two-way) voice, data, andvideo service.

File Server: A component of a local area network, or LAN, which stores information for use by clients, or

workstations.

FM Microwave Radio: Ultra-high frequency often used to provide the return link in fully interactive

systems (simplex). It can also be used in duplex to provide two-way full motion video and audio

interactivity.

FM Broadcast Band: The band of frequencies extending from 88 to 108 MHz.

Four-Wire Circuit: A circuit that has two pairs of conductors (four wires), one pair for the send channel

and one pair for the receive channel; allows two parties to talk and be heard simultaneously.

Frame: Full screen or frame of video is made up of two fields. Thirty frames is one second of video.

Frame Relay: A high speed interface between switches and T1 or T3 multiplexers. Frame relay is a

connection-oriented interface that initially will be incorporated into private T1 and T3 multiplexers. While

some carriers have committed to offer public frame relay service, others consider frame relay to be an

"interim technology" and are focusing on cell relay .T1 and T3 multiplexers equipped with frame relay will

provide a packet-oriented, HDLC-framed interface to routers and X.25 packet switches. The packets will be

routed to the proper destination by the multiplexers. Minimal protocol processing enables frame relay

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multiplexers to achieve high throughput. Initially, permanent virtual circuits will be supported; later, it is

likely that switched virtual circuits services may also be provided by frame relay. The major advantage of

frame-relay-equipped multiplexers is that only a single connection is required from the customer premises

equipment (routers or X.25 packet switches) to the multiplexer. Also, with frame relay support in

multiplexers, users contend for bandwidth provided via the multiplexer, and thus line cost efficiencies can

be improved.

Freeware: Software that doesn't cost anything. It can be distributed freely. However, the author still holds

the copyright which means that the software can't be modified. Freeze Frame: Repeating or holding one

frame so that it appears that the action has stopped.

Frequency: The number of times a complete electromagnetic wave cycle occurs in a fixed unit of time,

usually one second. The rate at which a current alternates, measured in Hertz on a telecommunications

medium.

Frequency Modulation: The range of frequencies within which an audio device will function.

FTP - File-transfer Protocol: A system for transferring files across the Internet. Anonymous FTP is a

conventional way of allowing you to sign on to a computer on the Internet and copy specified public files

from it. Some sites offer anonymous FTP to distribute software and various kinds of information. You use it

like any FTP, but the username is "anonymous". Many systems will allow any password and request that the

password you choose is your user id. If this fails, the generic password is usually "guest".

Full Duplex Audio Channel: An audio channel which allows conversation to take place interactively and

simultaneously between the various parties, without electronically cutting off one or more participants if

someone else is speaking. With a Half Duplex Audio Channel, only one party can speak at a time without

cutting off the other end.

Full-motion Video: Not compressed. A standard video signal of 30 frames per second, 525 horizontal lines

per frame, capable of complete action.

Gateway: A machine that exists on two networks, such as the Internet and BITNET, and that can transfer

mail between them.

Gateway: A network element (node) that performs conversions between different coding and transmission

formats. The gateway does this by having many types of commonly used transmission equipment to provide

a means for interconnection.

Geostationary Orbit - Geosynchronous - Clarke Belt: An orbital path approximately 22,300 miles above

the earth. This unique satellite orbit has the characteristic that objects located in it rotate at the same relative

speed as the surface of the earth. Objects placed in this orbit such as communications satellites can be

considered fixed with respect to antennas located on the surface of the earth which are oriented towards

them. Satellites in this orbit are always positioned above the same spot on the earth and from the earth, they

appear fixed in space. Microwave transmission from these earth located antennas can be sent to the

relatively fixed satellites in this orbit which serve as microwave repeaters back to the surface of the earth.

British physicist and science fiction writer, Sir Arthur C. Clarke, invented satellite communication in his

1954 paper Wireless World, which explained this east-west orbit, 22,300 miles above the equator; three

satellites based in this orbit could provide world-wide communications.

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GIF - Graphic Interchange Format: A format developed in the mid-1980s by CompuServe for use in

photo-quality graphics images. Now commonly used everywhere on-line. The filename extension generally

given to GIF files is .gif.

Glitch: 1. A narrow horizontal bar moving vertically through a television picture. 2. A short duration pulse

moving through the video signal at approximately reference black level on a wave-form monitor. 3. A

random error in a computer program. 4. Any random, usually short, unexplained malfunction.

Gopher and Gopher Server: An Internet information retrieval system. Software following a simple

protocol for tunneling through a TCP/IP Internet, and running errands, especially the retrieval of

"documents." This information system is technically known as a Gopher Server and is part of an

international network of Gopher Servers. The Gopher concept was created and initially implemented at the

University of Minnesota. The software they created has migrated around the Internet and is now serving the

public at large. Since its initial conception, many other organizations have contributed software to this effort.

Half-Duplex: A communications channel over which both transmission and reception are possible but only

in one direction at one time; e.g., a two-wire circuit.

Handshake: Two modems trying to connect first do this to agree on how to transfer data.

Handshaking: Exchange of predetermined signals when a connection is established between two data-set

devices.

HDSL - High-Bit-Rate Digital Subscriber Line: A method of providing high-speed data services over

unconditioned copper wires at a top speed of 1.544 mbit/s. VHDSL (for very high-bit-rate digital subscriber

line) is double that at 3 mbit/s. The key advantage of HDSL and VHDSL is that they allow telcos to provide

services like frame relay, SMDS, and high-quality compressed video over existing telephone lines which is

much less expensive than pulling fiber or installing additional repeaters. VHDSL is based on carrier less

amplitude/phase modulation (CAP) and has applications beyond HDSL and VHDSL such as a video-on-

demand service (being tested by Bell Atlantic) using CAP in which subscribers can interactively request

videos, which are then transmitted over high-speed lines.

HDTV: Higher (than normal) definition TV. HDTV is generally defined as a system that offers, as a

minimum, certain specific features and characteristics. These are Wide aspect ratio (now agreed as 19:9 or

1.778:1.; effectively doubled horizontal and vertical resolution (compared to existing systems); absence of

encoding/decoding artifacts (requires component operation); and compact disc quality stereo sound. The

technology applied to make HDTV transmittable in existing 6 MHz channels is essentially the same as the

technology necessary for multichannel operation in those same channels.

HTML - HyperText Markup Language: The language used to mark up text files with links for use with

World Wide Web browsers. This is a file format, based on SGML, for hypertext documents on the Internet.

It is very simple and allows for the embedding of images, sounds, video streams, form fields and simple text

formatting. References to other objects are embedded using uniform resource locators (URLs).

Telecommunications Glossary/41

HTTP - Hypertext Transfer Protocol: The Internet protocol, based on TCP/IP, used to fetch hypertext

objects from remote hosts. See also TCP/IP. Hub: 1. A signal distribution point for part of an overall system.

2. The master station through which all communications to, from and between micro terminals must flow.

Hue: The attribute of color perception that determines whether the color is red, yellow, green, blue, purple,

etc.

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Hyperband: The band of cable television channels above 300 MHz.

IM: Intermodulation distortion occurs when two or more signals are passed through a nonlinear device such

as an amplifier.

IMAP: A new protocol for the storage and retrieval of e-mail. Much like POP - the Post Office Protocol.

International Telecommunication Union - ITU: Organization composed of the telecommunications

administrations of the participating nations. Focus is the maintenance and extension of international

cooperation for improving telecommunications development and applications.

Internet: A worldwide system for linking smaller computer networks together-governmental institutions,

military branches, educational institutions, and commercial companies. Networks connected through the

Internet use a particular set of communications standards to communicate, known as TCP/IP. Internet is the

name given to the overall connectivity of all its various sub-networks, including USENET, APRAnet,

CSnet, BITNET, etc. There is no surcharge to send or receive messages through Internet. Only ASCII

messages up to 50,000 characters can be sent through this system. With a lowercase "i", an internet is a

group of connected networks.

Intranet: Intranets differ from the Internet because they are private networks, set off from the rest of the

world by firewalls. They can often connect disparate corporate networks. They often access corporate

resources and databases that were built with non-Internet technology in mind.

IP - Internet Protocol: The main protocol used on the Internet.

IRC - Internet Relay Chat: A service where users can "talk" via typing to people around the world

ISDN - Integrated Services Digital Network: A set of standards provide a common architecture for the

development and deployment of digitally integrated communications services. A set of standardized

customer interfaces and signaling protocols for delivering digital circuit-switched voice/data and packet-

switched data services. ISDN is designed to provide standard interfaces to custom premises equipment such

as computers, telephones, and facsimile machines through basic rate interface (BRI) to PBXs, host

computers, and LANs through primary rate interface (PRI); to the pubic Telecommunications switched

network through SS7; and to local packet data terminals and the public packet-switched network through

X.25 and X.75/X.75' packet services. The key to ISDN is out-of-band signaling which permits the users'

equipment and the network to exchange control and signaling information over a separate channel from that

which carries user information. A digital telecommunications channel that allows integrated transmission of

voice, video and data. ISDN lines used to access network services are divided into bearer, or "B" channels,

and a supervisory, or "D" channel, for out-of-band signaling. B Channels carry digitally encoded customer

information such as voice and data traffic, while the D channel provides the information required to set up,

route and disconnect calls on B Channels. D channels can also carry other information such as caller

identification. Twenty-three B channels and one D channel form a Primary Rate Interface or "23B+D". PRI

B channels can be used for any combination of voice, data, and image transmission at 64 kpbs. In addition,

B channels can be grouped together to create wider bandwidths for applications like video transmission.

IVDS - Interactive Video and Data Services: Name for license which will be granted by the FCC to

devices called Interactive TV Appliances (ITAs). ITAs include TV Answer, a two-way television service for

consumers for game shows, sporting events and respond instantly to news polls and interactive advertising

as well as participate in distance learning. The system will also let viewers shop, bank, pay bills, organize

TV programming and order a pizza.

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IVR: The IVR unit answers the call, greets the caller, and guides the caller through possible responses with

a series of voice prompts. The desired information is provided via prerecorded voice fragments (words) or

computer-generated speech.

Ka-Band: A satellite transmission in the 20 and 30 gigahertz frequency spectrum. Kbps, kb/s - Kilobits per

Second: A unit of measure of data of 1,000 bits per second or 1,000 Baud.

Ku Band: A category of satellite transmissions higher in frequency than those used as "c band" which are

being transmitted from satellites placed in the geostationary orbit. The group of microwave frequencies from

12 to 18 GHz and the band of satellite downlink frequencies from 11.7 to 12.2 GHz. The higher frequencies

(12 GHz versus 4 GHz) have created the possibility of smaller receive antennas and the realization of direct

broadcast satellite (DBS) signals to the end user without the necessity of going through a cable television

system or other shared use receive site due to the factors of size and cost.

LAN - Local Area Network: Private transmission network interconnecting offices within a building or

group of buildings and usually designed to convey traffic; e.g., voice, data, facsimile, video. Usually

associated now with a computer network made up of computers, printers, and mass storage units. MAN

Metropolitan area network. WAN - Wide Area Network.

Laser - Light Amplification by Stimulated Emission of Radiation:

1. A device for generating coherent electromagnetic signals (e.g., light). Low powered lasers are frequently

used to transmit light signals into optical fibers.

2. Laser light contains waves that have the same phase, as opposed to conventional light, whose individual

wave phases are unrelated to the phases of the others.

LCD - Liquid Crystal Display: A method of creating alphanumeric displays by reflecting light on a special

crystalline substance. Frequently used in electronic games and watches, and in portable electronic

instruments.

Leased Lines: A term used to describe the leased or rented use of dedicated lines from point to point. Lines

could include fiber optic cables, telephone cables, microwave or other transmission systems.

LEC: Local exchange carrier of a telephone company.

LED - Light Emitting Diode: A semiconductor which emits light when a proper voltage is applied to its

terminal

LNA - Low Noise Amplifier: Located at the antenna. Refers to electronic equipment, used in conjunction

with satellite reception, intended to amplify extremely weak satellite signals without introduction of noise.

They are rated in different noise temperatures, expressed in degrees Kelvin. The lower the noise temperature

figure, the higher the carrier-to-noise ratio, and the better the picture.

Local Exchange Carrier (LEC): Carriers that can carry only intra-LATA traffic. Local telephone

companies such as US West, Contel, Centel etc.

Local Loop: The local loop gets the signal from the receive site to the viewing room. Microwave, fiber

optics, cable and sometimes broadcast are used to distribute the signal. Also referred to as the "Last Mile

Low Earth Orbiting Satellite - LEO: Low earth orbit satellites which require 77 small, smart satellites to

provide linkage around the world. The satellites move overhead in Telecommunications their low orbit.

Motorola's Iridium (from the element Iridium which has 77 electrons) uses the concept to provide, digital,

satellite-based personal communications via small, handheld transportable receivers. With the system, voice,

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fax, or data calls can be made or received anywhere. The user will have one universal telephone number for

the phone. Local gateways will store customer billing information, keep track of user locations, and

interconnect with terrestrial carriers worldwide. The dual-mode phone will access customers' regular cellular

service first, switching to Iridium only when there is no terrestrial signal, to assure least-cost routing.

Mean Time Between Failure - MTBF: A statistical quantitative value for the time between episodes of

equipment or component failure.

Minicomputer: An intermediate range computer, between full-size mainframes and 16-bit microcomputers.

Historically, minicomputers have served dedicated uses, such as in scientific and laboratory work.

Modem - Modulator/Demdulator: Device that connects computer terminals and hosts through analog links

by converting data signals to analog signals and back again. Transmission rate of 300 Baud is slow; 2400 is

faster.

Modular: Constructed with standardized units or dimensions for flexibility and variety in use; allows for

easy replacement, substitution, expansion or reconfiguration of modules or sub-assemblies.

Modulator: A device which converts the video signal and audio signal onto a viewable TV channel. It takes

the video and audio signals that are separated by the receiver and combines them into a signal that can be

received by an ordinary TV set. This signal is called an "RF" signal, meaning radio frequency, and is usually

set for either channel 3 or 4. The advantage of using a modulator is that it permits the use of standard TV

receivers for displays, but the signal quality is not as good as using a direct video and audio feed to a

monitor TV display.

Mosaic: A free graphical front end to the Internet that supports browsing of multimedia data that includes

plain and formatted text, picture, video and sound. The data is based on the hypertext document format

where text or pictures can act as links to other places in the same or different documents. Mosaic provides a

simple graphical user interface (GUI) that enables easy access to the data stored on the Internet. These data

may be simple files or hypertext documents. The document a user points to on the receiving desktop could

be on the same machine or on another computer elsewhere on the Internet. Mosaic clients are currently

available for Windows, Macintosh, the X Window System and many flavors of UNIX.

Multimedia: The combination of multiple digitized data types; text, sound, computer generated graphics

and animations, photographs and video. The merger of digital technologies based on the use of computers.

The technologies that are converging are computing, television, printing and telecommunications.

Multiple Access: The ability of more than one user to use a transponder. Transponders have three basic

resources frequency, time and space. The frequency domain is used in FDMA. Time domain multiple access

is used in TDMA by time-sharing transponder. Space domain multiple access makes use of either the

polarization discrimination, orthogonal digital codes or through spread-spectrum techniques. Multiple Audio

Subcarrier Tuning: Essential to take advantage of radio and data services riding piggy-back on video signals.

Multiplexing: Transmission of two or more information streams over a single physical medium at the same

time such that each data source has its own channel. Allows a number of simultaneous transmissions over a

single circuit. Common methods are frequency division multiplexing (FDM) where the frequency bands are

split to constitute a distinct channel and time division multiplexing (TDM) where the common channel is

allotted to several different information channels, one at a time.

Multitasking: Pertaining to the concurrent execution of two or more tasks by a computer.

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N + 1: Created by the FCC, this formula forms the basis by which the FCC regulates expansion of channel

capacity for non-broadcast use. The FCC requires that if the government, education, public access, and

leased channels are in use at least 80 percent of the Monday-through-Friday period for at least 80 percent of

the time during any three-hour period for six consecutive weeks, then within six months the system's

channel capacity must be expanded by the operator.

Network: 1. Two or more information sources or destinations (points or nodes) linked via communications

media to exchange information. 2. It can be as simple as a cable strung between two computers a few feet

apart or as complex as hundreds of thousands of computers around the world linked through fiber optic

cables, phone lines and satellites.

Network Interface Card: Also known as NIC. Add-in circuit board that allows a PC to be connected to a

local area network (LAN).

NIC - Network Information Center: An organization which provides network users with information

about services provided by the network. As close as an Internet- style network gets to a hub; it's usually

where you'll find information about that particular network.

Node: An addressable unit in a network, which can be a computer, work station or some type of

communications control unit. Noise - Audio: Unwanted sounds (static) that interfere with the intended

sounds; or unwanted sound signals. Noise Temperature: The amount of thermal noise present in a system,

expressed in degrees Kelvin. The lower the noise temperature, the better. Noise - Video: Unwanted

electronic interference that shows up as snow.

NTSC: National Television System Committee; defined the 52 5-line color video signal frequency spectrum

which extends from 30 Hz to 4.2 MHz. NTSC video consists of 525 interlaced lines, with a horizontal

scanning rate of 15,734 Hz, and a vertical (field) rate of 59.94 Hz. A color subcarrier at 3.579545 MHz

contains color hue (phase) and saturation (amplitude) information. 30-frame-per-second color TV standard

in use in U.S., Canada, Mexico, Japan and a few other countries.

OEM: Original equipment manufacturer.

Off-Line: Mode of operation in which terminals, or other equipment, can operate while disconnected from a

central processor. Contrast with "on-line" where there is a direct connection to a host computer.

On-line: When a computer is connected to an on-line service, bulletin-board system or public-access site, it

is on-line.

Open Systems Interconnect - OSI: Generally open systems and networks are based on standards and the

OSI model, providing applications and data portability, providing interoperability between systems, having

common user interfaces, providing transparency below the application level, and provided by multiple

sources, with multiple sources having input on development.

Optical Character Recognition - OCR: The machine identification of printed characters through use of

light-sensitive devices; often used as a method of entering data.

Optical Fiber: An extremely thin, flexible thread of pure glass able to carry one thousand times the

information possible with traditional copper wire. See Fiber Optics. Orange Book: Colloquial name of the

standard that describes CD-Recordable equipment, media and formats. An extension of the "Yellow Book"

standard which includes specifications for incremental writes or multiple sessions. It specifies standards for

CD-R and magneto-optical cartridge systems as well as Kodak's Photo CD. Most CD-ROM drives today can

only read "single session" discs or the first session of a multisession disc.

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OSI: Open Systems Interconnection.

Outbound: Direction of a signal relative to the hub of a local area network (LAN) or other

telecommunications system. Outbound signals would be traveling away from the primary hub in the forward

direction to the extremities of the system.

PABX - Private Automatic Branch Exchange: A private automatic telephone exchange, usually located at

the user's site, that routes and interfaces the local business telephones and data circuits to and from the

public telephone network.

Packet: The unit of data sent across a packet switching network. The term is used loosely. While some

Internet literature uses it to refer specifically to data sent across a physical network, other literature views the

Internet as a packet switching network and describes IP data grams as packets.

Packet Switching: A communications data transmission method that breaks down messages into smaller

units of standard sized pieces called packets, which are individually addressed and routed through a

network; the network link is occupied only during packet transmission. Packet switching increases

efficiency in transport.

PAL: Phase Alternation by Line, the 625-line, 25-frame-per-second TV standard used in Western Europe,

India, China, Australia, New Zealand, Argentina, and parts of Africa.

Parabolic Dish: A satellite antenna, usually bowl-shaped, that concentrates signals to a single focal point.

See reflector.

Parity Check: A check of the accuracy of data being transmitted. To accomplish this, an extra parity bit is

added to a group of bits so that the number of ones in the group is, according to the specification, even or

odd. Then, at the receiving end, the bits in the word are added, the parity bit needed for that total is

determined, and the total is then compared with the parity bit transmitted.

PBX - Private Branch Exchange: A private telephone exchange that serves a particular organization and

has connections to the public telephone network; refers to a multi-line telephone exchange terminal with

various features for voice and data communications.

PCM - Pulse Code Modulation: A method of converting analog sound into digital representation by use of

successive samples.

PDA - Personal Digital Assistant: Small, hand-held devices that combine computer power with graphics,

sound, video and communication capabilities. They will take several forms including electronic note takers

and portable display telephone. They hold various programs, address files and databases depending on the

user's needs. Many feature a modem, fax, radio mail and computer.

Point-to-Multipoint: A teleconference broadcast from one location to several receiving locations (also

known as downlink sites.)

Point-to-Point: Teleconference between two locations. Point-to-Multipoint – one location to many sites.

Polarization: A characteristic of the electric field on an electromagnetic wave in space. The directional

aspects of a signal. Signals can have circular or planar polarization. Four types of polarization are used with

satellites; horizontal, vertical, right-hand circular and left-hand circular. Electromagnetic waves have the

ability to vibrate in different radial directions. Typically, satellite signal polarization is either horizontal or

vertical. The signal coming from the satellite to the dish will either be vibrating along a horizontal or vertical

plane. The receiving equipment must be adjusted to receive the correct polarization.

POP - Post Office Protocol: A protocol for the storage and retrieval of e-mail. Eudora uses POP.

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Port: In software, the act of converting code so that a program runs on more than one type of computer. In

networking, a number that identifies a specific "channel" used by network services. For instance, Gopher

generally uses port 70 but is occasionally set to use other ports on various machines.

PPP - Point-to-Point Protocol: PPP provides a method for transmitting datagrams over serial point-to-point

links.

PRI - Primary rate interface: PRI is a CCITT-defined ISDN trunking technology that delivers 64 kbps

clear channels and standardized out-of-band signaling. PRI can serve customer premise equipment (CPE)

such as a PBX, LAN gateway, or host computer or can serve as a trunk interface between central offices.

PROM - Programmable read-only memory: A type of read-only memory that can be programmed by the

computer user. This programming usually requires special equipment.

Protocol: A set of rules and procedures for establishing and controlling conversations on a line. The set of

messages has specific formats for exchanging communications and assuring end-to-end data integrity of

links, circuits, messages, sessions and application processes. Usually associated with communications over

computer. It is the language that computers use when talking to each other. The method used to transfer a

file between a host system and your computer. A formal description of message formats and the rules two

computers must follow to exchange those messages. There are several types, such as Kermit, YMODEM

and ZMODEM. Protocols can describe lowlevel details of machine-to-machine interfaces (e.g., the order in

which bits and bytes are sent across a wire) or high-level exchanges between allocation programs (e.g., the

way in which two programs transfer a file across the Internet). Public Switched Network: Any switching

system that provides a circuit switched to many customers. Pulse Code Modulation: A time division

modulation technique in which analog signals are sampled and quantized at periodic intervals. The values

observed are typically represented by a coded arrangement of 8 bits of which one may be for parity.

Rain Attenuation - Rain Losses: The attenuation (loss) of a signal due to rainfall. If you are receiving a

teleconference on a Ku band dish, local rainstorms can drastically weaken the signal strength of the

program. The result will be sparkles which interfere with the ability to see the program. During a heavy

downpour or thunderstorm, signal reception may be lost temporarily. The noise temperature perceived by

the receiving antenna may increase due to rain being present in the link.

RAM: Random-access memory. A volatile memory used by a computer's central processing unit as a

chalkboard for writing and reading information. RAM is measured in multiples of 4096 bytes (4K bytes),

and serves as a rough measurement of a computer's capacity.

Raster: The scanned (illuminated) area of a television picture tube.

Reflector: Antenna's main curved "dish," which collects and focus signals onto the secondary reflector or

the feed.

Repeater: A term used to describe the process of reprocessing and send a weak signal on to a more distant

service area. The weak signal condition develops as the initially strong signal passes through the miles of

air, moisture, rain and snow which gradually attenuates or reduces its power level.

Retrace: The return of a scanning beam to a desired position.

RF - Radio Frequency: Radio frequencies are generally considered as any electromagnetic signal from

normal radio to microwave transmission. RGB - Red, Green Blue: Method of transmitting video signals that

feeds red, green, and blue channels over separate wires; provides highest-quality video signal and is the

format for most computer equipment.

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Ring Network: A local area network in which devices are connected in a closed loop or ring as opposed to a

bus network.

ROM: Read only memory. A type of permanent, non-erasable memory that plugs directly into the wiring

of a computer, and contains computer programs. Some computers are supplied with some built-in ROM,

whereas others have external slots for inserting ROM cartridges.

Routing: Selecting the minimum delay path (and/or minimum cost path) in a network for a message or

packet to reach its destination.

RS-232-C: Standard interface between a piece of equipment and a telephone circuit.

S/N - S/NR - Signal to Noise Ratio: Final relationship between the video or audio signal level to the noise

level. Ratio of the signal power to the noise power in a specified band width, expressed in dBW.

Satellite: An electronics retransmission device serving as repeater normally placed in orbit around the earth

in the geostationary orbit for the purpose of receiving and retransmitting electromagnetic signals. It normally

receives signals from a single source and retransmits them over a wide geographic area. Satellite C/Ku band

Domestic communications satellites operate on two frequency ranges designated C and Ku band. Each

require specific electronic equipment. C band is less expensive; operates at 4 kHz. Ku-band operates at 12

kHz. Some teleconferences are broadcast on both bands. Many satellites are now built with both C and Ku

band capacity.

Satellite Relay: An active or passive satellite repeater that relays signals between two earth stations.

SCPC - Single Channel Per Carrier: Signal transmission technique often used in satellite transmission

which concentrates one channel of information on a single transmitted carrier for relay through the satellite.

The channel may be digital, analog or multiplexed analog in nature provided that its information may be sent

on a single narrow band carrier. The Single Channel Per Carrier transmission technique allows multi-

channel operation in the satellite with access from any location on the earth.

Scrambler: A device that transposes or inverts signals or otherwise encodes a message at the transmitter to

make it unintelligible to a receiver not equipped with an appropriate descrambling device. Synonymous with

encoder.

Server: A computer that can distribute information or files automatically in response to specifically worded

e-mail requests.

Serial Input/Output: Data transmission in which the bits are sent one by one over a single wire.

Shareware: Software that is freely available on the Internet. If you like and use the software, you should

send in the fee requested by the author, whose name and address will be found in a file distributed with the

software.

SIMM - Single In-line Memory Module: Devices used to add memory to computers.

Simplex: A circuit capable of transmission in one direction only. Contrast with half duplex and full duplex.

SLIP - Serial Line Internet Protocol: SLIP is currently a de facto standard, commonly used for point-to-

point serial connections running TCP/IP. It is not an Internet standard but is defined in RFC 1055.

SMDS - Switched Multimegabit Data Service: A public network service that will enable customers to

send packets between LANs at either T-1 or T-3 rates. Switched

SMTP - Simple Mail Transfer Protocol: The Internet standard protocol for transferring electronic mail

messages from one computer to another. SMTP specifies how two mail systems interact and the format of

control messages they exchange to transfer mail. SONET - Synchronous Optical Network: Will offer

23

dedicated point-to-point lines via fiber, with bandwidths ranging from 51.84 mbps to over 2gbps. SONET

defines optical interfaces for high speed digital transmission - ranging from 51.84 mbps to more than 2 gbps

in multiples of 51.84 mbps. The purpose of the SONET standard is to guarantee that fiber, and fiber

terminating equipment (e.g. digital loop carrier systems) from different central office vendors, can all

interface with each other. While many trials are currently under way to test the SONET central office

standards, all new fiber deployment is expected to be compliant with this standard.

SS7 - Signaling System 7: Increases both the efficiency of the telcos' interoffice trunking facilities and

their opportunities for revenue generation by enabling networkwide services. With SS7 trunk signaling,

premium services such as ISDN and Custom Local Area Signaling Service can be easily and efficiently

extended across the network.

Star Network: A network configuration in which there is only one path between a central or controlling

node and each end-point node. Supercomputers: The fastest and most powerful computing systems that are

available at any given time.

Switch: Mechanical or solid-state device that opens or closes circuits, changes operating parameters or

selects paths for circuits on a space or time division basis.

Switched Circuit: A circuit that may be temporarily established at the request of one or more stations.

Switched Network: Any network in which switching is present and is used to direct messages from the

sender to the ultimate recipient. Usually switching is accomplished by disconnecting and reconnecting lines

in different configurations in order to set up a continuous pathway between the sender and the recipient.

Switched System: A communications system (such as a telephone system) in which arbitrary pairs or sets of

terminals can be connected together by means of switched communications lines.

Synchronous Communication: Communication which takes place in the same time frame. Examples are

live teleconferences which must be viewed when they are broadcast. If the teleconference is taped and

viewed later, it becomes asynchronous communication-communication which takes place at the convenience

of the end user through the technology of video tape recording.

Synchronous Transmission: Data characters and bits are transmitted at a fixed rate with the transmitter and

receiver synchronized. This eliminates the need for start-stop elements, thus providing greater efficiency.

T1 (DS-1) Channel: High-speed digital data channel/carrier with a bit rate of 1.544 mbps which requires a

bandwidth of approximately 2.1616 MHz to transmit in a television type cable environment. (1.4 x 1.544 =

2.1616); a general term for a digital carrier (DS-1. available for high-volume voice or data traffic; often used

for compressed video teleconferencing networks. Each T1 circuit can accommodate 24 voice channels. A

video codec operating at the T1 rate uses the equivalent of 24 voice channels. A codec operating at 56 or 64

Kbps is operating in the range of one voice channel. A standard video signal digitized at 90 Mbps has

approximately 1400 voice channels. The compressed video signal quality and the cost decreases as the

transmission speed decreases.

TCP/IP - Transmission Control Protocol/Internet Protocol: The combination of TCP and IP. The

particular system for transferring information over a computer network that is at the heart of the Internet.: IP

is the network layer protocol for the Internet. It is a packet switching, datagram protocol .

TDMA - Time Division Multiple Access: Form of multiple access where a single carrier is time shared by

many users. Signals from earth stations reaching the satellite consecutively are processed in time segments

without overlapping.

24

Telecommunications: Communicating over a distance. Use of wire, radio, optical or other electromagnetic

channels to transmit or receive signals for voice, video and data communications.

Telecommuter - telecommuting: Ability to work from home, local office, or from the road because of

equipment. Equipment allows the telecommuting employee to work from anywhere. The equipment includes

a telephone, fax, modem and as the NII is deployed, video.

Teleconference: Electronic communications between two or more groups, or three or more individuals, who

are in separate locations via audio, audiographics, video or computer. Audio teleconference - two-way

communication between two or more groups, or three or more individuals, in separate locations. Video

teleconference - one (or more) uplink and downlink sites. May be fully interactive voice and video, two-way

voice and one-way video; full-motion, compressed, or freeze-frame video.

Telemetry: The science of sensing and measuring information at some remote location and transmitting the

data to a convenient location to be read and recorded.

Telnet: The Internet standard protocol for remote terminal connection service. Allows a user at one site to

interact with a remote time sharing system at another site as if the user's terminal was connected directly to

the remote computer (see "rlogin"). On the Macintosh, NCSA Telnet is the standard.

TI - Terrestrial Interference: TI is normally generated as a result of relatively strong terrestrial microwave

signals overpowering the weak satellite transmissions which are the primary signals of interest at a satellite

earth station.

Time Sharing: Pertaining to the interleaved use of time on a computer system that enables two or more

users to execute computer programs concurrently.

Token Ring: A type of LAN. Examples are IEEE 802.5, ProNET-10/80 and FDDI. The term "token ring" is

often used to denote 802.5Telecommunications Glossary/86

Transponder - Channel - Downlink Frequency: A satellite microwave repeater (receiver and transmitter)

receives the signal from an uplink, amplifies it, down converts the frequency of a received band of signals,

and re-transmits the signal back to earth. Satellites have 12, 24, or more transponders each with the capacity

for one color TV signal and two audio channels. Typically transponder with 24 transponders have twelve

polarized for vertical and twelve for horizontal transmissions in order to optimize the bandwidth of the

satellite and the respective transponders.

Twisted Pair: A pair of wires used in transmission circuits and twisted about one another to minimize

coupling with other circuits. UTP - unshielded twisted pair.

Two-Wire Circuit: A typical telephone circuit on the public switched network; a circuit formed by two

conductors insulated from each other to provide a send and receive channel in the same frequency.

Uplink: An earth station that transmits a radio frequency signal to a communications satellite. The

transmitting facility, or uplink, consists of a large dish-shaped antenna and high-power amplifiers. The

uplink is like the transmitter of a radio or television station, except that it concentrates it signals in one

direction by means of a parabolic dish antenna that delivers a strong pinpoint signal to a specific satellite in

space.

URL - Uniform Resource Locator: A standard for writing a text reference to an arbitrary piece of data in

the WWW. A URL looks like "protocol://host/localinfo" where protocol specifies a protocol to use to fetch

the object (like HTTP or FTP), host specifies the Internet name of the host on which to find it, and local info

is a string (often a file name) passed to the protocol handler on the remote host.

25

Virtual Private Network: Use of the public switched telephone system to provide a capability similar to

that of a private network.

Virtual Reality - VR: Loosely defined as putting users into a computer-generated environment, rather than

merely reacting to images on a display screen. Full immersion VR can include a helmet that senses head

movement and changes the view seen through small TV screens mounted in front of each eye along with

gloves that allow users to touch objects in the virtual world

VSAT - Very Small Aperture Terminal: Small earth stations with a satellite dish usually 4 to 6 feet (1.2 to

1.8 meters) in diameter used to receive high speed data transmission; can also transmit slow- speed data. A

VSAT uplink for compressed video a C-Band frequencies is approximately 4.5 meters in diameter for most

satellite applications.

World Wide Web - WWW: The web of systems and the data in them that is the Internet. Presents

information in a user friendly hypertext format. WWW displays pages of information, with links to other

pages. Mosaic is the program that really makes Web materials come alive. Different systems display the

links differently, by highlighting the link items or by putting a code (such as a number in brackets) after the

item. Others put the link in boldface or in color.

WORM - Write Once Read Many: A type of permanent optical storage which allows the user to record

information on a blank optical storage disc. Information may be added until the disc is full, but not erased or

changed.

Workstation: Computers that are generally targeted at technical users, interface over a network easily, often

run UNIX, come standard with more compute power than PCs and are capable of fast graphics. Distinctions

between high-end personal computers and workstations are blurring. For high-end animation work such as

3-D logs, morphing or animated characters, workstations provide the compute power and graphics

performance the animator needs. Workstations are used for computer-generated imagery (often called CGI)

because they take less time than PCs to render images.

X.25: Set of packet-switching standards published by the CCITT. An international standard for control of

data communications between two or more computers or terminals using packet-switching technology.

26

Chapter-3: Technical questions asked in interview:

We can divide our whole syllabus into following parts from interview point of view.

Part-1: Material science, Electronics device circuit, Power electronics

Part-2: Analog electronics, digital electronics and microprocessor

Part-3: Communication system, Electromagnetic theory and Microwave.

Part-4: Signal&system, Network, Control &Instrumentation

Questions asked from Communication System:

Q. What is meaning of communication?

Q. What is the meaning of Tele communication?

Q. What is the meaning of effective communication?

Q. What is difference between simplex, half duplex and full duplex system?

Q. What codes are used for communication system?

Q. What is meaning of radio frequency used in communication? and what are FCC rule used in

communication?

Q. What are medium wave, short wave and FM wave ? what propagations are used for them?

Q. Why FM propagation can‘t be used for long distance while AM&SW wave propagation can be used for

long distance?

Q. What is the meaning of modulation and what are its advantages and disadvantages?

Q. What is meaning of order of modulation?

Q. Can you define under modulation and over modulation in Amplitude modulation?

Q. What is the meaning of bandwidth of a signal in terms of time &frequency domain?

Q. How bandwidth is related to speed?

Q. Can you explain envelop detector method for detection of AM?

Q. When Envelop detector is used?

Q. How value of RC is designed in case of envelop detector?

Q. What is diagonal clipping in case of envelop detector?

Q. How diagonal clipping can be removed?

Q. What is the consequence of diagonal clipping on detection?

Q. What is difference between synchronous detection and envelop detection?

Q. How envelop detector is an asynchronous detector?

Q. How DSB-SC has power saving in compare to AM while both has same bandwidth?

Q. How much power is saved in DSB-SC?

Q. What are the advantages and disadvantages of DSB-SC?

Q. Whether DSB-SC is better method for one to many transmission or not?

Q. What is main role of ring modulator?

Q. How carriers are suppressed by ring modulator?

Q. Whether ring modulator is the best method for DSB-SC?

Q. Whether envelop detector can be used for DSB-SC or not ? if yes how and if not why?

27

Q. What is the best method for detection of DSB-SC and how that method is the best?

Q. What is Quadrature null effect in DSB-SC?

Q. How will you define attenuation and distortion and what is the difference between both?

Q. Which one is more dangerous attenuation or distortion?

Q. What is Hilbert Transform?

Q. What is physical meaning of Hilbert transform?

Q. How Hilbert and Fourier transform are different?

Q. What are applications of Hilbert transform?

Q. How SSB-SC is different from DSB-SC?

Q. How power is saved in SSB-SC? How B.W is saved in SSB-SC?

Q. Whether SSB-FC is possible? Where it is used?

Q. What are various methods for generation for SSB-SC?

Q. SSB-SC requires minm

power and minm

bandwidth but still it is not used always why?

Q. Can you explain practical use of SSB-SC?

Q. What are the limitations of SSB-SC?

Q. How SSB-SC is used in FDM? And why SSB-SC is preferred for FDM?

Q. How SSB-SC is used in Basic telephony system?

Q. What are the voice frequency? And why SSB-SC is preferred for Voice communication?

Q. Whether voice frequency or audio frequency are same? If not how they are different?

Q. What is use of VSB-SC and how it is used?

Q. Why SSB-SC is not used in TV?

Q. Compare performances of DSB-FC, DSB-SC, and SSB-FC&SSB-SC in terms of all parameters?

Q. What are the advantages of FM over AM?

Q. What is difference between NBFM and WBFM?

Q. Where NBFM is used and whether envelop detector can be used for detection in NBFM?

Q. Where WBFM is used?

Q. Can u explain VCO method for generation of FM and what are its disadvantages?

Q. How FET reactance modulator is the best direct method for generation of FM?

Q. How Armstrong method is good for FM generation?

Q. Can you explain why frequency multiplier is required in Armstrong method?

Q. What is frequency multiplier circuit? And what are the devices inside it?

Q. What is non linear device inside freqn multiplier and what is band pass filter?

Q. What is the maximum freqn deviation in FM?

Q. What is the maximum freqn in case of FM?

Q. How PLL can be used for detection of FM?

Q. PLL is which type of filter?

Q. Why PLL is the best method for detection of FM?

Q. What is difference between PM&FM and how will you distinguish between FM and PM?

Q. Can you compare FM and PM noise performance?

Q. Which is better PM or FM and why?

Q. What is pulse modulation and what are its disadvantages?

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Q. Whether pulse modulation or digital modulation is same?

Q. What is need of sampling? And what is instantaneous sampling?

Q. What are the limitations of instantaneous sampling?

Q. What is sampling theorem?

Q. What is the condn

for sampling?

Q. What is aliasing? What is Nyquist rate?

Q. What is oversampling, under sampling and sampling?

Q. What is natural sampling & flat top sampling?

Q. What is difference between natural&flat sampling?

Q. Which modulation technique is used in FDM? What is the basic concept of FDM? What is the

application of FDM?

Q. What is the use of guard band in case of FDM?

Q. What is the value of bandwidth in case of FDM?

Q. What is the basic concept of PCM?

Q. What is the use of PCM?

Q. Where PCM technology is used?

Q. How PCM technology play a role in digital communication?

Q. Whether PCM technology is used for transmission?

Q. How PCM is digital modulation technique?

Q. How quantizer&encoder works in case of PCM?

Q. What is the use of quantizer and what type of mapping it is?

Q. Quantizer is many to one or many to many mapping and if yes how?

Q. Which type of encoder is used in PCM technology?

Q. What is the value of bandwidth and SNR in case of PCM?

Q. Can you give practical application of PCM?

Q. Can PCM may be used for compression techniques?

Q. How DPCM is different from PCM and what is the advantage of DPCM?

Q. What is the bandwidth in case of DPCM and what about SNR in case of DPCM?

Q. How DM is one bit DPCM?

Q. What type of Noise are present in case of DM?

Q. What is slope over load problem in case of DM?

Q. How this problem can be overcome in case of DM?

Q. What is positive slope overload and negative slope over load problem?

Q. Can you justify how slope overload occurs only at low frequency?

Q. How slope over load is more dangerous?

Q. What is granular noise effect? And what is this problem? Granular noise effect is less dangerous?

Q. What is the shape of output in case of granular noise effect?

Q. Compare slope overload problem and granular noise problem?

Q. Which one is oversampled PCM or DM?

Q. Compare Noise performance of PCM or DM?

Q. Compare bandwidth requirement in DM&PCM?

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Q. What is Matched filter? And where it is used?

Q. Why Matched filter is called matched filter?

Q. How will you calculate matched filter for a rectangular pulse?

Q. What are the advantages of digital communication over analog communication?

Q. What is disadvantage of digital communication?

Q. How MODEM is used in case of digital communication?

Q. What is MODEM and what is the advantage of MODEM?

Q. Why transmission in case of digital communication is analog only?

Q. Why digital transmission is not possible in case of digital communication?

Q. What are the problems with digital transmission in case of long distance?

Q. Can you differentiate role of PCM and Modem in case of Digital communication?

Q. Whether MODEM is used at transmitter and receiver end both? If yes why?

Q. Can you explain PSK Modulation? What is BPSK?

Q. How Bandwidth utilization can be increased in case of QPSK?

Q. If QPSK is faster than BPSK then why we always not prefer QPSK?

Q. What are the advantages and disadvantages of QPSK?

Q. Can you compare constellation diagram of BPSK and QPSK?

Q. What is the relation between bandwidth of BPSK and QPSK?

Q. What is the synchronization problem in detection of BPSK&QPSK?

Q. What is the practical applications of BPSK&QPSK?

Q. Can you compare QPSK, 8ary PSK and 16 ary PSK in terms of bandwidth and speed?

Q. What is FSK? and can you explain its basic concept?

Q. What is MSK and what is basic meaning of MSK?

Q. What is ASK and how it works like OOK?

Q. Can you compare constellation diagram of ASK, PSK, QPSK and FSK?

Q. Can you compare bandwidth of ASK, PSK, QPSK and FSK?

Q. Whether FSK is 3 dB inferior or superior to PSK?

Q. Whether ASK is inferior to PSK and FSK and if by how much amount?

Q. Which is the best method for MODEM and Why?

Q. What is QAM modem and what are its advantages?

Q. Can you explain constellation diagram of QAM and its applications?

Q. Morse code is used in ASK technique? How?

Q. What are different codes used in digital communication technique?

Q. What is the difference between synchronous communication & asynchronous communication?

Q. What are the start and stop bits in case of synchronous communication?

Q. What is the difference between channel capacity and bandwidth?

Q. Whether both channel capacity and bandwidth are same?

Q. What is the meaning of Baud rate in communication system?

Q. What is the difference between bit rate and baud rate in communication system?

Q. What is the baud rate for BPSK and QPSK?

Q. What is ISI and how ISI can be reduced?

30

Q. What is Roll-off factor in case of ISI and what is its effect on its bit rate?

Q. What is eye diagram and eye pattern?

Q. What is the use of satellite communication and how it is used?

Q. What is the frequency range of satellite communication? what is the general range of satellite freqn

Q. What are the advantages of satellite communication?

Q. What are the disadvantages of satellite communication?

Q. What is the value of delay in satellite communication? and why there is this delay in satellite?

Q. What is the value of round trip delay in satellite communication?

Q. What bands are used in satellite communication?

Q. Why C-band and Ku-band are preferred in satellite communication?

Q. What is the value of uplink and down link freqn in C-band and Ku-band?

Q. Why uplink freqn is higher than down link in case of satellite ?

Q. Why uplink freqn is lower than down link in case of mobile communication?

Q. Which type of antenna is used in case of satellite communication?

Q. Why 6/4 GHZ system is preferred over 14/12 GHz

Q. In rain attenuation C-Band is preferred why?

Q. In DTH Ku-band transponder is used in satellite communication why?

Q. Why DTH system does not work properly in rain?

Q. What is the bandwidth of satellite communication and how many numbers of channels can be used?

Q. Why bandwidth of C-band and Ku-band satellite remains same?

Q. How frequency reuse concept can be used in satellite communication?

Q. How Solar cell is used in satellite communication?

Q. How solar cell works? and how power may be achieved with solar cell without any biasing?

Q. What is the power source in case of emergency in case of solar cell?

Q. How 3 satellites can cover whole earth except polar region? Why polar region is not covered by 3

satellites?

Q. What is difference between passive and active satellite?

Q. What do you understand by geostationary satellite?

Q. Why this is called geostationary?

Q. What is the height of geostationary satellite?

Q. What is time period and angular velocity of geostationary satellite?

Q. What is the meaning of Geo synchronous satellite?

Q. How it is Geo synchronous with orbit?

Q. What is the height of geosynchronous orbit?

Q. What is time period and angular velocity of geosynchronous satellite?

Q. What is the difference between geostationary and geosynchronous satellite?

Q. Whether both are called communication satellite?

Q. Can you give me name of some communication satellites launched recently?

Q. From where communication satellites are launched?

Q. What are Kepler‘s law in satellite communication?

Q. What is apogee and perigee in satellite communication?

31

Q. What is the path of satellite ? elliptical/circular or linear?

Q. What is optical fiber ? and what are its advantages in communication system?

Q. What type of light flow in OFC? Infrared/visible/Ultraviolet?

Q. Generally OFC is secure communication why?

Q. Generally losses are less in case of OFC why?

Q. What is the main advantage of OFC in communication?

Q. Why there is less interference in case of OFC?

Q. What is the basic principle of Optical fiber communication?

Q. How Total-Internal-Reflection play a role in transmission of OFC?

Q. What is skew ray in case of optical fiber?

Q. What are the factors which affect propagation of light in optical fiber?

Q. How composition of fiber and size of fiber affects propagation in fiber?

Q. Can you explain how voice/data or any information is transmitted in optical fiber?

Q. What is light source in OFC and what is light detector used in OFC?

Q. LED/Laser diode which is preferred as light source in OFC?

Q. Photo diode or Avalanche photo diode which one is preferred for photo detection?

Q. What are the advantages of APD over normal diode?

Q. Can you use PIN diode in optical fiber?

Q. What is responsivity in case of photo detector?

Q. What materials are used for Optical fiber?

Q. What is the glass material used in OFC?

Q. Why Plastic core and Glass clad is not used in OFC?

Q. For high speed what is preferred plastic or glass?

Q. What is Numerical aperture in case of OFC?

Q. What is the physical significance of Numerical aperture?

Q. Numerical aperture depends upon what factors?

Q. Whether Numerical aperture depends upon dimension of fiber?

Questions asked from Microwave Engineering:

Q. What is use of transmission line?

Q. How propagation occurs in case of transmission line?

Q. Propagation in transmission line is TE/TM or TEM?

Q. How information is transmitted in transmission line?

Q. How will you define microwave?

Q. What is the range of frequency in case of microwave?

Q. How will you link microwave with Feet?

Q. Why microwave is called microwave?

Q. Whether VHF&UHF lies in range of microwave?

Q. What is the range of voice frequencies?

Q. What is the range of frequency for Medium wave and short wave?

32

Q. What is the range of frequency for FM whether VHF&UHF both lies in FM wave?

Q. Can you arrange Infrared, Ultraviolet, Visible, X-rays and Gama Rays in order of frequency and wave

length?

Q. What are the frequencies of different microwave bands?

Q. What are the frequencies of C-band&Ku-band?

Q. Advantages of Microwave frequency?

Q. How bandwidth is increased in case of microwave frequency?

Q. What about fading effect in case of microwave?

Q. How fading is decreased in case of microwave?

Q. Can you explain lumped elements and distributed elements?

Q. Why lumped elements are used at low frequency?

Q. How power is calculated in case of lumped elements?

Q. Why distributed elements are used at high frequency?

Q. How power is calculated in case of distributed elements?

Q. Can you explain Bolometer method for low microwave power calculation?

Q. How Bolometer works on concept of variable resistance?

Q. How Calorimetric is used for high power measurements?

Q. What is double minima method used for high value of VSWR?

Q. Why double minima method is called double minima?

Q. What is reflectometer and what is the use of it?

Q. How S-Matrix can measure value of power directly?

Q. What is H-Plane Tee Junction and how it works?

Q. What is E-Plane Tee junction and how it works?

Q. How E-plane Tee junction and H-plane Tee junction works like 3 dB splitter?

Q. Whether both E-plane Tee and H-plane Tee are doubling the value of power?

Q. What is the basic difference between E-plane Tee and H-plane Tee?

Q. Why H-plane Tee is called as Current Junction?

Q. Why E-plane Tee is also called as Voltage junction?

Q. Where E-plane Tee or H-Plane Tee are used in Microwave?

Q. What is Magic Tee and why it is called magic Tee?

Q. What are the applications of Magic Tee?

Q. What is the directional coupler? And what is the basic structure of directional coupler?

Q. What is the parameter which decides a directional coupler?

Q. What is gyrator?

Q. What is isolator?

Q. What is circulator? and how it is unsymmetrical device?

Q. What is the effect of transit time at microwave frequency?

Q. What are the problems arise due to transit time effect?

Q. What are the solutions of transit time problem?

Q. How Microwave utilize concept of transit time?

Q. What are the O type tube and how these tubes are linear beam tubes?

33

Q. Whether TWT& Klystron are examples of linear beam tube? If yes how?

Q. What are M type tubes and how these are cross field devices?

Q. How Magnetron is an example of M type tube?

Q. Can you compare O type and M type tubes?

Q. Can you explain working of Two Cavity Klystron?

Q. What are the name of 2 cavities in 2cavity klystron?

Q. What are the function of both cavity?

Q. How velocity modulation occurs in buncher cavity?

Q. What is the result of velocity modulation?

Q. How Current modulation occurs in catcher cavity?

Q. What is density modulation in drift space?

Q. What is the meaning of bunching?

Q. How gain is increased due to bunching?

Q. How gain is measured in terms of 2 cavity klystron?

Q. Can you compare Velocity modulation, density modulation and current modulation?

Q. How Oscillations can occur in case of 2 cavity klystron?

Q. Whether 2 cavity klystron is an amplifier or oscillator?

Q. What is the maximum efficiency of 2 cavity klystron amplifier?

Q. What is the general efficiency in case of 2 cavity?

Q. How Length of drift space varies with applied RF voltage?

Q. What is the maximum value of bunching parameter in case of 2 cavity?

Q. What is the need of multi-cavity klystron and what are the distances between 2 cavities in case of multi

cavity?

Q. What is reflex klystron?

Q. How reflex klystron is used? How it is used as low power microwave source?

Q. Reflex klystron is oscillator or amplifier explain?

Q. How many cavities are there in reflex klystron? One or two?

Q. How one cavity can help in oscillations?

Q. Velocity modulation/Current modulation/Drift occurs in which portion of reflex klystron?

Q. Why transit in repeller space of reflex klystron must be n+3/4 ?

Q. Why reflex klystron is preferred over 2 cavity oscillator?

Q. Reflex klystron is known a slow power oscillator how?

Q. What is maximum efficiency in case of 2 reflex klystron?

Q. What is TWT? amplifier or oscillator?

Q. How gain of TWT is higher than 2 cavity klystron?

Q. How interaction between e beam and RF field is continuous in case of TWT?

Q. What is the need of slow wave structure in case of TWT?

Q. How Interaction between e beam and RF field prolongs in case of TWT?

Q. TWT is broad band amplifier how?

Q. How focusing is done in case of TWT?

Q. What is the role of attenuator in case of TWT?

34

Q. How oscillations are prevented by use of Attenuator?

Q. TWT is preferred over Klystron due to higher value of gain or due to high value of bandwidth?

Q. Magnetron is a high power oscillator? how?

Q. Where Magnetron is used?

Q. What is the dominant mode in Magnetron?

Q. What is Tunnel diode? How it is highly doped diode?

Q. How tunnel diode is negative resistance device?

Q. What is the order of doping in case of Tunnel diode?

Q. What is the biasing in case of Tunnel diode?

Q. What are the materials used in Tunnel diode?

Q. What is the range of frequency in case of Tunnel diode?

Q. What is the order of output in case of Tunnel diode?

Q. What is the equivalent circuit in case of Tunnel diode?

Q. What are the applications of Tunnel diode?

Q. What is backward diode and how it is linked with Tunnel diode?

Q. Can you call backward diode as highly doped Zener diode?

Q. What is the basic concept of varactor diode?

Q. How Capacitance varies with Rev.bias in case of Varactor diode?

Q. What is the symbol of varactor diode and where it is used?

Q. Whether varactor diode is negative resistance device or not?

Q. How Varactor diode is used for tuning?

Q. How varactor diode is used in parametric amplifier?

Q. Whether varactor can be used as amplifier or oscillator?

Q. What is the equivalent circuit of varactor diode?

Q. What is step recovery diode? Why it is called as Snap-off varactor?

Q. Step recovery works in F.bias or R.bias?

Q. What is difference between varactor and step recovery in terms of applications?

Q. Can you explain Metal Semiconductor diode? is it same as Schottkey diode?

Q. What are the materials used in case of Schottky barrier diode?

Q. What type of biasing is used in Schottky barrier diode?

Q. Is there any depletion layer in Schottky diode?

Q. Why Schottky diode is called ―Hot carrier diode‖

Q. What is the difference between Schottky and Normal diode?

Q. Why cut-in voltage is less in Schottky and why reverse current is high in compare to normal diode?

Q. Why Schottky diodes are used for fast switching?

Q. Whether schottky diode is active or passive device?

Q. Can you explain PIN diode?

Q. Why PIN diode is PIN diode?

Q. Can you explain doping of P,I &N regions?

Q. How PIN diode is used as Microwave switch?

Q. Why SI material is used for fabrication of PIN diode?

35

Q. What is Gunn diode?

Q. Gunn diode is negative resistance device? If yes how?

Q. Why Gunn diode is used for amplification&oscillation?

Q. For Gunn diode only direct band gap is used?

Q. Why Gunn diode are called as TED?

Q. What is TED?

Q. Can you explain 2 valley model in Gunn diode?

Q. Why 2 valleys are used in Gunn diode?

Q. How domains are formed in Gunn diode?

Questions asked from EDC and Power Electronics :

Q. Why silicon is preferred material for semiconductor? why not others?

Q. Why Carbon can‘t be used as Semiconductor it is also from 4th

group?

Q. What is concept of hole and did they exist?

Q. Does hole physically exist?

Q. How will u define mobility?

Q. Why electron mobility is always higher than holes?

Q. What is concept of effective mass?

Q. At what factor effective mass depends?

Q. What are the disadvantages of Ge as a semiconductor material?

Q. How silicon helps in fabrication?

Q. What is use of SiO2 in case of fabrication?

Q. What is use of Al in case of fabrication?

Q. What are basic steps of fabrication?

Q. If we want a high speed diode will be prefer Si only?

Q. What is value of intrinsic concentration (ni) in case of silicon and Ge at room temperature? And how

they changes with temperature?

Q. How conductivity varies with temp for semiconductor and metal and why?

Q. Why in a SC conductivity increases with temp and not in metal?

Q. How extrinsic semiconductor are more conductive than intrinsic S.C explain?

Q. What is effect of temperature on extrinsic SC

Q. What are super conductors?

Q. What is ideal behavior of a super conductor?

Q. Why their conductivity is very high?

Q. What are applications of super conductor?

Q. Give some example of super conductors?

Q. What is BCS theory for super conductor?

Q. How N-type SC and P type SC forms?

Q. What is effect on doping if temp is increased in case of N type and P-type?

36

Q. What are direct and indirect band gap SC and their application?

Q. Explain direct band gap and indirect band gap SC

Q. What are applications of direct and indirect band gap SC?

Q. What are degenerate semiconductor? And where they are used?

Q. What is advantage of degenerate SC in tunnel diode

Q. What is the position of Fermi level in degenerate SC

Q. What happens on position of Fermi level if doping is increased)

Q. How will you define Fermi level?

Q. Fermi level is probability of what?

Q. Where is Fermi level in case of N-type &P –type?

Q. What is Fermi level at absolute temperature?

Q. Where is Fermi level for metals?

Q. What are laws other than Fermi-Dirac distribution?

Q. What is Hall effect? and its application?

Q. Discuss Hall effect for metal, SC&Insulator?

Q. For Intrinsic SC Hall coeff is –ve why?

Q. For extrinsic hall coeff will be?

Q. Can u tell difference between drift and diffusion current?

Q. Which current plays a major role in current of diode and how?

Q. What is ohms law for semiconductor?

Q. What is concept of drift current? And how it can be increased?

Q. Can u explain reverse current in diode and how it varies with reverse voltage?

Q. Explain concept of photo diode and how it is different form solar cell?

Q. Explain working of solar cell?

Q. What are photo detector and give some example of it?

Q. What is LED and how it works? How it different form LASER?

Q. Where LED is used? How it is used in Optical fiber?

Q. Can u explain concept Of Optical fiber?

Q. What is opto-isolator in case of OFC?

Q. Which light is used in case of OFC?

Q. What is Total Internal reflection in OFC?

Q. What is difference between Pin diode and APD? Which one is used where?

Q. What is working of APD?

Q. What is working of PIN diode?

Q. What is break down in zener diode?

Q. What is avalanche multiplication in zener diode? And what is tunneling?

Q. What is basic difference between 2 mechanisms?

Q. Which one is used for high doping and which one is used for low doping?

Q. What is effect of temp in case of avalanche breakdown in zener diode?

Q. Why voltage increases if temp is increased in avalanche breakdown in zener diode?

Q. Can u explain Zener as a constant voltage regulator?

37

Q. How zener diode can be used in DC power supply?

Q. What are the basic blocks of Power supply?

Q. What is the ripple used in a filter?

Q. Can u compare HWR and FWR on basis of ripple frequency? And which one is better?

Q. Which type of transformer is used in power supply and why?

Q. What is use of bleeder resistor in case of power supply?

Q. What is Form Factor and what is its significance?

Q. What is voltage regulation for a power supply?

Q. Can u differentiate between clipper and clamper?

Q. Is clipper same as Rectifier followed by clamper?

Q. What is voltage doubler /Tripler etc?

Q. How forward voltage in a diode changes with temp?

Q. Is it same for both Ge and Si?

Q. Why this value is negative?

Q. What is ideality factor in case of diode why this 1 for Ge and 2 for Si?

Q. Why reverse current change with voltage in case of Si but not in case of Ge.

Q. How will u define rectifier and what is its use?

Q. What is reverse recovery diode in case of diode used as a switch?

Q. How transistor work as an amplifier?

Q. Why there is 180 degree phase shift in case of CE mode and not in case of CB and CC?

Q. What is physical meaning of Beta in case of transistor?

Q. Why base width is narrow and doping light in case of transistor?

Q. For high speed transistor what will u do with base doping?

Q. What is transit time in case of transistor?

Q. What are use of CE CC and CB mode?

Q. Why CE as an amplifier?

Q. Why CC for impedance matching? And how it is used there?

Q. How CB is used as constant current source?

Q. What is early effect in case of transistor?

Q. What is punch through?

Q. How breakdown occurs before punch through in a transistor?

Q. What are advantage of FET over BJT?

Q. What are disadvantage of FET over BJT?

Q. Why FET is more temp stable than BJT?

Q. Why FET is less noisy than BJT?

Q. Why FET has high impedance than BJT?

Q. Is fabrication easy in case of FET?

Q. Compare gain bandwidth product b/w FET&BJT?

Q. Why BJT as a switch has higher speed than FET?

Q. How and when FET is used as Voltage variable Resistor?

Q. How FET is used as an amplifier in saturation region?

38

Q. Can u explain saturation and active region in FET?

Q. What is difference between FET&MOSFET?

Q. Why MOSFET has high impedance than FET? And what is its order?

Q. Can u draw ac model of a transistor?

Q. How can u draw h-parameter model with the help of ac model?

Q. Why h-parameter is called hybrid parameter?

Q. Explain variation of different h parameters with respect to temperature?

Q. Compare Input impedance, output impedance, voltage gain and current gain for CE CC and CB mode.

Q. Why CC is used as impedance matching?

Q. What is cascade and Cascode combination?

Q. Why Gain Band width product of Cascode is higher?

Q. How Band width is increased in Cascode?

Q. What are advantages of Cascode configuration?

Q. Can you give some examples of constant current sources?

Q. How Zener diode can be used in constant current source?

Q. How SCR works?

Q. Compare power diode and power BJT?

Q. Explain latching and holding current in SCR?

Q. What is the application of SCR?

Q. What is dv/dt and di/dt effect in case of SCR?

Q. How dv/dt and di/dt can be avoided?

Questions asked from Analog &Digital electronics:

Q. Why analog is called analog?

Q. What is difference between analog and digital?

Q. How will u calculate voltage amplification of BJT?

Q. Can u draw frequency response of BJT?

Q. Can u explain every part of frequency response?

Q. What are low frequency region mid band frequency region and high frequency region?

Q. Why gain decreases in low frequency and high frequency?

Q. Why gain remains constant in mid band frequency region?

Q. What are the role of coupling capacitor by pass capacitor and blocking capacitor in BJT amplifier?

Q. Which one of the following capacitor determines lower cut-off?

Q. Why gain decreases in high frequency region?

Q. What is parasitic capacitance in BJT amplifier?

Q. What are the capacitances available in parasitic capacitance?

Q. What happens to lower cut-off and higher-cut off in case of multistage amplifier?

Q. Please draw RC high pass filter?

Q. What happens when a step is given as input to it?

39

Q. When it works like a differentiator?

Q. Do values of R&C decide is it differentiator or not?

Q. What is the condition for RC high pass as a differentiator circuit?

Q. What is the output of RC differentiator if square wave is given as input?

Q. Same questions can be asked for RC low pass which is an integrator?

Q. What is tilt in case of High pass filter?

Q. How will u define rise time in case of RC LPF?

Q. Can u draw π model of a transistor?

Q. What new changes occurs when a transistor is operated at high frequency?

Q. What is base spreading resistance?

Q. Why base spreading resistance occurs?

Q. Why junction capacitance occurs at high frequency in case of BJT?

Q. What are various junction/parasitic capacitance in case of BJT?

Q. Which capacitance is dominated for calculation of higher cut-off?

Q. Why h-parameter can‘t be used for power amplifier?

Q. Can u explain concept of oscillator?

Q. Is it same for sinusoidal &triangular wave generator?

Q. What concept is used for sinusoidal generator?

Q. What concept is used for triangular wave generator?

Q. How RC phase shift oscillators used for sinusoidal wave generator?

Q. What are the ranges of RC phase shift oscillator?

Q. What is concept of offset nonlinearity in case of RC phase shift?

Q. Can it be made by FET only or BJT also is used?

Q. What is difference between RC phase shift by use of BJT&FET.

Q. Why RC BJT oscillator uses voltage shunt feed back while RC phase shift FET uses voltage series

explain?

Q. What factors change frequency of oscillation in both cases?

Q. What is concept of wein bridge oscillator and where it is used?

Q. What are audio frequency and radio frequency oscillator?

Q. Do both have same concept?

Q. What is basic concept of radio frequency oscillator?

Q. What elements are used for radio frequency oscillator?

Q. Is Radio frequency oscillator and Crystal oscillator same?

Q. How relaxation oscillators are used for triangular wave generator?

Q. What is basic concept of relaxation oscillator?

Q. What are Bistable, Monostable &Astable multi-vibrator?

Q. How will u decide which one is bistable, monostable&astable.

Q. What is stable stable&which is quasi stable state?

Q. What is difference between stable state& quasi stable state?

Q. How will u use Bistable as a Flip-flop?

Q. How will u use mono stable as Pulse width modulator?

40

Q. How astable is used as voltage to frequency convertor?

Q. How astable works as it has 2 quasi stable states?

Q. What is Schmitt trigger?

Q. How it is different from astable muti-vibrator?

Q. Can u draw input and output for Schmitt Trigger?

Q. Show UTP&LTP in terms of input and output?

Q. Which mutivibrator is used for clock generation?

Q. What are the advantages of negative feedback?

Q. Compare negative feedback and positive feedback in terms of stability?

Q. Why negative feed-back are stabilized while positive feedback are unstable?

Q. How negative feedback improves stability?

Q. When one uses negative feedback and when positive feedback?

Q. What are DC amplifiers? And what are their applications?

Q. What is the problem with DC amplifiers?

Q. What is the drift problem in case of DC amplifier?

Q. How Differential amplifier can remove this problem?

Q. Explain working of differential amplifiers?

Q. What are different modes in differential amplifier?

Q. What are differential mode and common mode in case of differential amplifier?

Q. What is CMRR and what is physical significance of it?

Q. Is CMRR link with noise rejection? What is meaning of infinite value of CMRR?

Q. Where differential amplifiers are used and what is its use in case of OPAMP?

Q. How will u define digital?

Q. Is digital& binary are same?

Q. What is difference between digital&discrete or both are same?

Q. Are analog &continuous same?

Q. How will u define continuous&analog?

Q. What is difference between both continuous and analog?

Q. What is meaning of universal gate?

Q. Can MUX be used as a universal gate?

Q. For (n) variable function implementation what should be size of MUX

Q. What is difference between MUX and encoder?

Q. What are encoder?

Q. Can u tell one application of encoder?

Q. What is decoder and how it is different from encoder?

Q. What is priority encoder?

Q. Where this priority encoder can be used?

Q. How will u implement higher order decoder/MUX by lower order?

Q. How will u implement 8x1Mux by use of 4X1 MUX?

Q. How will u define select lines in case of MUX?

Q. Can u design a Full adder with half adder only?

41

Q. What is difference between combination circuit and sequential circuit?

Q. What are the advantages of sequential circuits in digital?

Q. Is feedback digital circuits same as sequential?

Q. What are the basic applications of sequential circuits?

Q. How sequential circuits used as a memory elements?

Q. Are sequential same as Latch?

Q. What is difference between latch and flip-flop?

Q. What is basic use of latch?

Q. What are transparent latch?

Q. Can we differentiate latch and flip flop based upon triggering of clock?

Q. In microprocessor how a latch is used?

Q. What is difference between level triggering and edge triggering?

Q. What is basic problem with level triggering?

Q. How above problem can be removed by use of edge triggering?

Q. What are set up time and hold up time? And what are their values and their significance?

Q. What is their significance in case of level triggering and edge triggering?

Q. What is Meta stability problem in case of flip-flop?

Q. Is it linked with setup time and hold up time?

Q. How Meta stability can be removed?

Q. What is race around condition?

Q. Why does it occur?

Q. Does it occur in case of both level and edge triggering?

Q. How one can remove race around condition?

Q. What is master slave flip-flop?

Q. How does it remove race around condition?

Q. Can u explain working of Master Slave FF?

Q. Can u design MOD-10 counter by use of 4 FFs and NOR gate?

Q. In place of NOR gate can we use AND gate?

Q. Why we will use only NAND and OR gate in above case?

Q. For ripple counter can we use D FF or S FFS?

Q. Why only T&JK FFs are used?

Q. What are special feature of JK and T FF.

Q. Does triggering affects MOD of ripple counter?

Q. What is basic problem with ripple counter?

Q. Can u explain glitch problem in ripple counter?

Q. How this glitch problem can be removed in ripple counter?

Q. What are the advantages and disadvantages of synchronous over ripple?

Q. How will u design a synchronous binary counter?

Q. What components are required for design of MOD-16 synchronous counter?

Questions asked from Electronics measurement

42

1. What is measurement ? and what is instrument ?

2. Essential characteristics of electrical measurement ?

3. What is absolute instrument ?

4. Is there any relative instrument ?

5. Give example of absolute instrument ?

6. What is secondary instrument ?

7. Use of secondary instrument ?

8. Difference between analog and digital instrument ?

9. Difference between deflecting instrument and null deflecting instrument?

10. How defecting instrument are less accurate?

11. Why Null deflecting instrument are more accurate?

12. Example of null deflecting instrument ?

13. Ammeter and Voltmeter are which type of instrument ?

14. AC&DC bridge are under category of which type of instrument ?

15. What is the basis for classification of errors?

16. What is gross error?

17. If pointer of an indicating instrument moves away from the origin then what are absolute limiting error

and relative limiting error?

18. What are essentials of indicating instrument ?

19. What are various torque required for indicating instrument ?

20. What is main function of deflecting torque

21. What is the main function of controlling torque ?

22. Discuss gravity control and spring control mechanism?

23. Which control mechanism has uniform scale?

24. Spring control mechanism is used for which type of instrument ?

25. Gravity control mechanism is used for which type of instrument ?

26. What is the use of damping torque ?

27. What should be approximate value of damping factor for indicating instrument ?

28. What are various types of damping mechanism used for an instrument ?

29. Difference between eddy current damping and air current damping ?

30. PMMC uses which type of damping and what type of damping it uses?

31. Why PMMC uses eddy current damping?

32. PMMC works on which effect?

33. PMMC has uniform or non uniform scale ?

34. What is use of spring control mechanism in PMMC?

35. Why Al former is used in PMMC?

36. Why no need of magnetic shielding in PMMC?

37. Why PMMC can‘t be used for AC and only for DC?

38. What is swamping resistor in PMMC?

43

39. Electrostatic type instruments used for ?

40. Hot wire instrument works on which effect? and used for ?

41. Energy meter is which type of instrument ?

42. What is sensitivity of voltmeter?

43. What is difference between attraction and repulsion type moving iron instrument ?

44. What damping and control mechanism is used for both? Whether these same or different ?

45. Whether both have uniform or non uniform scale?

46. What is basic principle of electrodynamometer ?

47. How electrodynamometer is used as transfer instrument ?

48. What are various applications of electro dynamometer

49. Power factor meter works on which principle ?

50. Why there is no need of controlling torque in case of power factor meter?

51. How Ac power is calculated by electrodynamometer ?

52. What are various methods for measuring low, medium and high resistance?

53. Advantages of digital instrument over analog instrument ?

54. What is transducer ?

55. Difference between primary and secondary transducer?

56. Difference between active and passive transducer?

57. Example of active transducer ?

58. Example of Variable R transducer ?

59. Example of variable L transducer ?

60. Example of variable C transducer ?

61. What is basic concept of LVDT?

62. Use of LVDT?

63. In LVDT is core made up of ?

64. Temperature effect can be reduced in LVDT how ?

65. What is use of capacitive transducer?

66. Basic concept of piezo electric transducer ?

67. Compare thermocouple and thermistor?

68. Thermistor is made up of ?

69. Use of semiconductor strain gauge?

70. What is use of Lissajous pattern?

71. What is Time base circuit?

72. Time base circuit used in CRO?

73. Use of sampling oscilloscope ?

74. Use of storage oscilloscope?

75. Which focusing method is used in CRO?

76. What is use of aquadag in CRO

77. What is graticules in CRO

78. What is use of tacho meter ?

79. Difference between LED and LCD?

44

80. Why no power is consumed in potentiometer ?

81. Wein bridge is used at which frequency?

82. What is use of Wagner earthing device?

83. Q meter works on which principle ?

84. Spectrum analyzer is used for ?

85. What is use of vector voltmeter ?

86. What is phantom loading ?

87. What is creeping in energy meter?

88. How creeping is solved in energy meter?

89. Why high power consumption in hot wire instrument ?

90. What is frequency range of electrodynamometer?

91. Attraction type Moving iron is more accurate than repulsion type why?

92. Moving iron instrument is not used in DC ?

93. What is basic principle of Megger?

94. Why power consumption is low in PMMC?

95. What is CRDX

96. How damping can be achieved in D‘arsonval instrument ?

97. Bifilar winding ?

98. What are various type of errors in wheat stone bridge ?

99. What is significance of torque/weight ratio

100. What is IEEE4888?

45

Donorions

Electrons

N-TypeP-Type

Acceptorions

Hole

+

–

P Type N Type

–x1VBx2

Concentration of holesConcentration ofacceptors

Chapter-4: Important study material for technical part:

Study material for Electronics device and Analog Semiconductor diodes:

P–N Junction:

If donor impurities are introduced into one side and acceptors into other side of a single crystal of a

semiconductor then P-N Junction is formed. Here donor ion is represented by a plus sign because after this

process impurity atom donates an electron& it becomes a positive ion. Here acceptor ion is indicated by a

minus sign because after this process atom accepts an electron& it becomes a negative ion.

The uncircled charges on each side of junction represents the free carriers ie. plus sign on the p-side

represents free holes and minus sign on the n-side represents free electrons. The charges shown in circles

represent ionized impurities which cannot move

Formation of depletion layer:

Due to density gradient across the junction holes will diffuse to right across the junction and electrons to

left. Positive holes which diffuse across the junction will disappear as a result of recombination with

electrons. Similarly electrons will also recombine with holes after crossing the boundary.

The un-neutralized ions in the neighborhood of the junctions are referred as uncovered charges.

Since the region of junction is depleted of mobile charges it is called depletion region, the space charge

region or transition region.

Thickness of this region is of the order of visible light (0.5 m). within this very narrow space charge layers

there are no mobile carriers. when a sufficient number of donar & acceptor are uncovered then further

diffusion is stopped. It is because a barrier is setup against further movement of charge carriers. This is

called potential barrier or junction barrier. The potential barrier is of the order of 0.1 – 0.3 volt.

Depletion region:

46

qv0

N-Type

P-Type

P N

V0

The region across P-N junction in which potential changes from positive to negative is called depletion

region. The width of region is of the order of 10–8

meter. Since this region has immobile ions which are

electrically charged and it is called as space charge region.

This potential barrier discourages the diffusion of majority carrier across junction, but this potential barrier

helps minority carriers to drift across the junction.

Open circuited P-N junction:

No electric current can flow because no electric field is applied, and circuit is open – circuited

Both P type and N type samples are at different positions with respect to band edges. But for thermal

equilibrium Fermi-level must be at same level.

When a P-N junction is formed Fermi level in N-Type is at higher level than in P-Type. Due to this

difference electrons will flow from material which has a higher Fermi-level to a material which has lower

Fermi-level until they attain a common level.

Conduction band of P-Type is shifted upward by qV0 over conduction band of N-Type. Where V0 is the

potential barrier developed across the junction.

Q. Although a potential barrier V0 exists at the P-N junction a voltmeter connected across P-N

junction will not read this why?

The reason is as follows:

If this is possible then let us assume that a current flows due to barrier voltage in a short circuit P-N Junction

diode. This current will heat the connecting metal wire. Since there is no external source of energy the

heating of wire must take place with a simultaneous cooling of P-N Junction diode. But under thermal

equilibrium this situation can‘t exist. Hence we conclude that current through the circuit is zero. This means

that barrier voltage must be balanced by metal to Semiconductor contact potentials at the ends of diode. A

voltmeter connected to terminals of diode shows zero voltage readings.

Application of voltage across a P-N junction:

There are no free or mobile charge carriers in depletion layer. But it contains only immobile positive and

negative ions. A barrier potential is developed across depletion layer represented by symbol V0. The value of

V0 for Ge is 0.3 volt and value for Si is 0.7 volt. It has been observed that there is no diffusion of electrons

or holes across the junction due to barrier potential, unless an external field is established across the P-N

junction.

47

P N

V -V0

Free electonflow

I

Forward biased P-N junction diode:

When an external voltage is applied to P-N junction in such a direction that it cancels the potential barrier

and permits the current flow is called as forward bias.

To apply a forward bias positive terminal of a battery is connected to P-type Semiconductor while negative

terminal is connected to N-Type Semiconductor. The applied forward potential establishes an electric field

opposite to potential barrier. Potential barrier is very small (0.3V for Ge and 0.7 V for Si) therefore a small

forward voltage is sufficient to eliminate the barrier. At same forward voltage the potential barrier at P-N

junction can be eliminated altogether then the junction resistance will become almost zero and a low

resistance path is established for the entire circuit. Thus a large current is generated in circuit even for small

potential applied. Such a circuit is called Forward bias circuit and current is called Forward bias current.

Current flow in forward biasing:

In case of Forward bias the holes from p type S.C. are repelled by positive battery terminal towards the

junction and simultaneously the es in N-Type are repelled by negative battery terminal towards the junction.

when the electron hole combination takes place near the junction a covalent bond near the positive terminal

of battery breaks down. This causes liberation of an electron which enters the positive terminal. This action

creates a new hole which move towards the junction. For each electron in N-region that combines with a

hole from P-region an electron enters the crystal from negative terminal. The constant movement of

electrons towards the positive terminal and holes toward the negative terminal produces a high forward

current. As the applied voltage is further increased the electrons and holes having lower energy will be able

to cross the potential barrier and current will increase further.

Reverse biased P-N junction diode:

When an external voltage is applied to P-N junction in such a direction that it increases the potential barrier

then it is called Reverse bias. For Reverse bias positive terminals of battery is connected to N-Type and

negative terminal is connected to P-type.

This applied Reverse voltage establishes an electric field which acts in same direction of potential barrier.

Therefore the resultant field is strengthened and barrier height is increased. Thus a high resistance path is

established for the entire circuit and current does not flow through the junction i.e. effect of reverse bias is to

() potential barrier and allows a very little current to flow.

48

P N

V -V0

I

V

Rh

R

P N

mA

VBR

Fo

rward

curr

en

t

Forward voltage

When junction is reverse biased then es in N-Type S.C. and holes in P-type S.C. are attracted away from

junction under the action of applied voltage. Since there is no recombination of electron-hole pairs thus

current is negligibly small ie. junction has a high resistance.

Conclusion: we conclude that when a P-N, junction is Forward biased it has a low resistance path and hence

current flows in circuit. On the other hand when it is Reverse biased it has high resistance path and no

current flows in circuit.

Thus P-N junction diode is a one way device which offers a low resistance when forward biased and

behaves like an insulator when reverse biased. So it can be used as a rectifier.

V–I characteristic of P-N junction:

Voltmeter and milli-ammeters are connected to record the values of voltage and current respectively.

V–I Characteristic in P–N junction:

when applied voltage is zero no current flows through the circuit but as applied voltage is increased by

adjusting the potential divider Rh a small current flows in circuit. Once voltage is increased then resistance

of P–N junction reduces and voltage increases sharply.

0.3 for

0.7 for

V V Ge

V Si

49

In case of Rev. bias current is due to flow of minority carriers. If voltage is further increased in reverse

direction then reverse current does not increase. At the breakdown of junction, value of current increases

abruptly.

Diode equation for VI characteristic:

So 0I I

I0 is independent of Reverse bias voltage. In case of forward bias total current is sum of current caused by

holes entering N region and current caused by electrons entering free region.

Variation of Reverse current with Temp :

0 0

0

p n n p

p n

D p D nI Aq

L L

For N-Type 2 2

0 0&i in p

D A

n np n

N N

So 2

0

p ni

P D n A

D DI Aq n

L N L N

But

2 3GE

KTi on A T e

For Ge and Si Dp & Dn vary inversely to temp.

So I0 T2

/GE KTe

In above discussion effect of carrier generation and recombination has been ignored. This is valid for Ge but

not for Si.

From above discussion it is clear that I0 becomes double for every 10ºC rise in temperature.

2 10 2 0 1( ) ( ) 2

10

T TI T I T

Effect of Temp on P-N junction diode:

0

0

G

T

V

VmI K T e

For Ge: = 1 m = 2, VG0 = 0.785 V

For Si: = 2 m 1.5, VG0 1.21V

2.1 /º

2.3 º

dVmv C for Ge

dT

mv C for Si

But in general it can be assumed that

2.5 /ºdV

mV CdT

for both Ge and Si.

Diode Resistance:

1. Static resistance / DC resistance:

Static resistance of a diode is defined as ratio V/I of the voltage to current. Static resistance R is equal to

reciprocal of slope of a line joining Q pt. to origin.

Typical values are VF = 0.8 volt at IF = 10 mA corresponds to RF = 80. IR = 10 A at VR 50 V & Rv = 500

M.

50

xp

–xn

ND

0

–NA

x

x = 0

Charge density

x

ID

VD

VD

I (mA)D

The application of DC voltage to a circuit containing a S.C. diode will result in an operating point on

characteristic curve which will not change with time. DC resistance of diode is independent of shape of

characteristics in region surrounding point of interest.

DD

D

VR

I

2. Dynamic resistance or AC resistance:

For small signal operation dynamic or incremental resistance is an important parameter and is defined as

reciprocal of slope of voltage-ampere characteristics. Dynamic resistance is not constant but depends upon

operating voltage.

Different junction in diode:

1. Step graded junction:

Consider a junction in which there is an abrupt charge from acceptor ions on one side to donor side on other

side. Such a junction is formed experimentally for example by placing a Indium which is trivalent against

N-Type Ge and heating combination to a high temperature for a short time.

For step graded junction

NA >> ND

Vj = 2

1/2

2

Dj

q N ww V

2. Linearly graded junction:

It is obtained by drawing a single crystal from a melt of Ge whose type is changed during drawing process

by adding first P-Type and then N-type impurities.

Here total width of space charge layer w is 1

3jw V

Different type of capacitances in P-N J’n diode:

1. Space charge or Transition capacitance:

51

CT

R

RS

When a P-N junction is reverse biased then depletion region acts like an insulator or dielectric material

while P and N regions on either side have low resistance and acts as the plate. So this P-N junction in Rev.

bias may be regarded as a parallel plate capacitor if A is area of parallel plate capacitor and w is width of

depletion layer then

Varactor Diode:

Voltage Variable capacitances are called Varactors. Varicaps or Voltcaps. A circuit model for a Varactor

diode under reverse bias is shown

Rs Body / ohmic resistance of diode Typical values of CT and RS are 20 pF and 8.5 respectively at a

reverse bias of 4v. The value of R is large (> 1M).

2. Diffusion or storage capacitance (CD):

When forward biased P-N junction is applied then potential barrier is reduced and then majority carriers

diffuse away from the junction and progressively recombine. The density of carriers is high near the junction

and decays exponentially with distance so that a charge is stored on both side of junction when a forward

bias voltage is applied. It is clear that amount of charge stored varies with applied potential as for a true

capacitor so origin of diffusion capacitance lies in injected charge stored near the junction outside the

transition region. So rate of change of injected charge with voltage called diffusion capacitance or storage

capacitance.

Junction diode switching time:

In a given P-N junction diode when external voltage is suddenly reversed in a diode circuit which has been

carrying current in forward direction the diode current will not immediately fall to its steady state reverse

voltage value until injected or excess minority carrier density pn – pn0 has dropped normally to zero the

diode will continue to conduct easily and current will be determined by the external resistance in diode

circuit. Value of trr is generally of the order of 400 n sec.

52

PN

Clear Plastic

Radiation

(Photo - diode)

R

Opto electronics:

1. Bulk type photo conductors:

If energy of photon is greater than energy gap of semiconductor then covalent bonds of SC will be broken

and new EHPs are created which will increase conductivity of semiconductor. This effect is known as

Photoconductive effect.

Due to increase in conductivity resistivity will decrease i.e. resistance will decrease hence device is called

photo resistor or photo conductor. Photo conductive cell is a device in form of either a slab of a

Semiconductor in bulk form or a thin film of semiconductor deposited on an insulating substrate with ohmic

contacts at opposite ends.

Examples of photo conductive cells are Cds, Cdse ,PbS&Tls.The most popularly used photo conductive cell

in visible spectrum is Cds cell. In case of Cds in absolute darkness resistance is as high as 2 MΩ but in

strong incident light it has resistance as small as 10 Ω.

Cds photo conductive cells has merits:

1. High dissipation capability

2. Excellent sensitivity in visible spectrum

3. Low resistance when irradiated by light.

Application of photo conductive cells:

1. To measure the intensity of light

2. As a voltage regulator

3. To record modulated light intensity

4. Used in counting applications

2. Junction type photo conductors:

(a) P-N Photo diodes

(b) Solar cells

(c) PIN Photo diodes

(d) Avalanche Photo diodes

(e) NPN transistors

(a) P-N Photodiodes:

A P-N photo diode is essentially a reverse biased P-N Junction diode in which light is permitted on one

surface of junction. This device consists of a P-N junction embedded in clear plastic as indicated in figure.

Radiation is allowed to fall upon one surface across the junction. The remaining sides of plastics are either

painted black or enclosed in a metallic case.

53

100

300

400

–40 –30 –20 –10 0V

V-I characteristics of Photo diode:

If reverse voltage in excess of a few tenth of a volt are applied then a reverse current independent of

magnitude of reverse voltage is obtained. This dark current corresponds to reverse current due to thermally

generated minority carriers. If light falls upon surface additional EHPs are created proportional to number of

Incident photons .So here Total current Itotal is given by Itotal=Io+Is where Is is the short circuit current

proportional to light intensity.

Typical V-I characteristics of photo diode are shown in figure:

The curves don‘t pass through origin (with the exception of dark current curve).

Note: Radiation must be focused near the junction. If radiation is focused into a small spot away from the

junction the injected minority carrier can recombine before diffusing the junction and hence a much

smaller current will result.

Application of photo diode:

1. High speed reading of computer punch cards and tapes

2. Light detection system

3. Reading of film sound track

4. Light operated switches

5. Switching

6. Optical communication

Photo voltaic potential:

In case of Photo diode if forward bias is applied then potential barrier is lowered and majority current

increase. When majority current equals to minority current then this total current is reduced to zero.

The voltage at which zero current is obtained is called the photo voltaic potential.

When light falls on the P-N junction diode then reverse current increases and forward current also increases

to make total current equal to zero. This photo voltaic potential is of the order of magnitude of 0.5 volt for

silicon and 0.1 volt for Ge.

In open circuited condition for I=0 then photo voltaic voltage is generated and voltage is Vmax

max ln 1 sT

o

IV V

I

Since s

o

I

I>>1 so Vmax increases logarithmically with Is. Here Is changes with

intensity of light.

54

P NI

V

Solar energy converters:

It is clear that a definite non zero current is obtained for zero applied voltage. Hence a junction photo cell

may be used under short circuited conditions. As here Is is proportional to light intensity. The current drain

from a photo voltaic cell may be used to power electronic equipment or more commonly to charge auxiliary

storage batteries. Such energy converters using sun light as primary energy are called solar batteries and are

used in solar cells.

(b)Solar cell:

The solar cell is basically a P-N junction diode that converts sunlight directly to electrically with large

conversion efficiency. The action of solar cell is explained as follows:

When a P-N junction diode is exposed to light photons are absorbed and EHPs are generated in both P&N

side of junction. The electrons move to N side and holes move to P side.

When the P-N junction diode is open circuited the accumulation of electrons and holes on two side of

junction give rise to an open circuited voltage Vo .If a load resistance is connected across the diode a current

will flow in circuit. The maximum current called short circuited current is obtained when an electric short is

connected across the diode terminals. Note that current flows as long as the diode is exposed to sun light and

magnitude of current is proportional to light intensity.

Solar cells are used extensively in satellite and space vehicle as most important long duration power supply.

Solar cells are constructed with Silicon, Gallium arsenide, Cadmium sulphide and with many other

semiconductors.

(c) PIN Photo diode:

An intrinsic silicon layer is inserted between heavily doped P and N type silicon materials. The intrinsic

silicon reduces the transit time of photo induced electron hole pairs. The reason is that carriers generated by

light photons incident on middle of this layer have less distance to travel than if generated at one side or the

other of the layer. The response time of PIN photo diode is ultra fast with a switching speed of nano-second.

So PIN phto diodes are used in ultra fast switching and logic circuits.

(d) Avalanche Photo diode:

When photo diode is operated in break down region then this diode is Avalanche Photo Diode (APD).

Current sensitivity is increased by 30-100 times in case of APD. These diodes are operated at high reverse

bias voltage so that break down of diode takes place. Main advantage of APD is its high sensitivity.

55

P N

V

Metal Contact

Cathode

MetalContact

Anode

(e) Photo transistors:

The photo transistor is a junction transistor with collector base junction exposed to light. It is similar to

photo diode but has a sensitivity gain of 50 to 100 times more. Generally NPN transistors are used to its

increased gain and greater sensitivity.

Light Emitting Diode (LED):

It is a forward biased P-N junction diode .In this case holes move from P side to N side and electrons from

N side to P side due to diffusion process. When electrons from N side cross the junction then they recombine

with holes on the P side. When holes from P side cross the junction then they recombine with electrons on

the N side. In case of Si and Ge this recombination takes place through traps and liberated energy goes into

crystal as a heat. But in semiconductors like GaAs there is considerable amount of direct recombination

without the aid of traps.

Under such circumstances the energy released when electron falls from CB into VB appears in form of

radiation. Such a PN diode is called Light Emitting Diode. The efficiency of the process of light generation

increases with the injected current and with a decrease in temperature. GaAs emits light in Infra-red region.

GaP& GaAsP emits light in visible region.

Advantages of LED:

1. Low working voltage &current

2. Less power consumption

3. Very fast action

4. Small size and weight

5. Long life

Liquid Crystal Display (LCD):

LCD has the distinct advantage of having a low power requirement than LED. It is typically of the order of

Microwatt for display as compared to same order of mill watts for LEDs. It however require an external or

internal light source and is limited to a temperature range of about 00C to 60

0C.Life time is an area of

concern because LCD can chemically degrade. LCD are much slower than LED.LCD typically have

response times in range of 100-300 m-sec. while LED are available with response time slower below 100

nsec. There is greater range of color choice in case of LCD.

Rectifier converts AC in pulsating DC. A diode rectifier forms an essential building block of DC power

supplies required to power electronic equipment. As indicated the power supply is fed from 120V (rms)

60HZ ac line and it delivers a DC voltage V0(usually in range of 5-20 Volt) to an electronic circuit

represented by the load block.

56

Transformer Rectifier FilterIC

RegulatorLoad

Rectifier &Filter:

Block diagram of a power supply:

1. Power transformer: It consists of two separate coils wound around an iron core that magnetically

couples the two windings. The primary winding having N1 turns is connected to 120 V ac supply and

the secondary winding having N2 turns is connected to circuit of DC power supply.

In addition to providing the appropriate sinusoidal amplitude for the DC power supply the power

transformer provides the electrical isolation between electronic equipment and the power line circuit.

This isolation minimizes the risk of electric shock to the equipment user.

2. Rectifier: The diode rectifier converts the input sinusoid to a unipolar output which can have

pulsating waveform. But this waveform has a non zero average or DC Component. Its pulsating

nature makes it unsuitable as a DC source for electronic circuit hence there is no need of filter.

3. Filter: The output of the rectifier filter is much more constant than without the filter but it still

contains a time dependent component known as ripple.

4. Voltage regulator: To reduce the ripple and to stabilize the magnitude of DC a voltage regulator is

employed against variations caused by changes in load current.

Summary of HWR and FWR

HWR FWR

1. Aveg. Value of O/p Vm/ 2Vm/

2. RMS value of O/P Vm/2 Vm/ 2

3. Maxm

efficiency 40.6% 81.2%

4. Ripple factor 1.21 0.48

5. Ripple frequency same as i/p Twice of i/p

6. PIV Vm Vm Bridge

2Vm Centre tapped

7. % Regulation 100%f

L

R

R 100%

f

L

R

R

8. Form factor 1.57 1.11

9. TUF 0.287 0.693

THYRISTOR

Thyristor (also called silicon controlled rectifier or SCR) consists of alternate p and n layers (i.e., p-n-p-n)

forming three p-n junctions. The anode terminal is outside the p layer. A contact welded to inner p layer

(i.e., p2) forms the gate. Its symbol is as shown in Figure.

57

V-I Characteristics of Thyristor:

The v-i characteristic of a thyristor is shown in Figure. The behavior of a Thyristor can be explained by two

transistor model as shown in figure.

Thyristor can be turned on by applying a positive gate signal. Other triggering methods are dv/dt triggering,

high temperature triggering and light triggering. When a thyristor is triggered the gate loses control. It can

be turned off by decreasing the current to less than holding current. The turn on time of Thyristors is less

than about 3 µ-s and turn off time is between 10-300 µ-s.

Thyristors are available upto about 10 KV and 1200 A rating. For high voltage and high current circuits

series and parallel connection of thyristors are used. When connected in series, the voltages across the

thyristors are not equal. When connected in parallel, the currents through them are non-equal. Therefore

thyristors in series need static and dynamic equalizing circuits.

It is necessary to turn on and turn off a thyristor at proper instant. The turned on is done by applying a

positive gate signal. The turn off methods are called commutation methods. Commutation can be natural or

58

forced. When an SCR is connected to an ac source, the current goes through its natural current zero at the

end of every half cycle and a reverse voltage automatically appears across it. This is called natural

commutation. In dc circuits there is no natural current zero and forced commutation (class B commutation)

Auxiliary commutation (class C commutation), Complementary commutation (Class D commutation) and

external pulse commutation (Class E commutation). Natural commutation is also called line commutation or

class F commutation.

Comparison between Power MOSFET and BJT:

1. Power MOSFET has slower switching losses but its on resistance and conduction losses are more. A

BJT has higher switching losses but lower conduction loss. So at high frequency power MOSFET is the

obvious choice but at lower frequency BJT is superior.

2. BJT is a current controlled device and Power MOSFET is a voltage controlled device.

3. MOSFET has positive temperature coefficient for resistance this makes parallel operation of MOSFET

easy. A BJT has negative temperature coefficient.

4. MOSFET is a unipolar device while BJT is bipolar device

5. Base current in BJT is much larger than the control signal (or gate current)required in MOSFET.

6. Gate circuit impedance in MOSFET is extremely high.

7. BJT suffers from secondary break down voltage where as MOSFET is free from this problem because

BJT has negative temperature coefficient while MOSFET has positive temperature coefficient.

Two-transistor model of a Thyristor:

Two-transistor model of a Thyristor can be used for explaining turn-on and turn-off mechanisms of

Thyristor. For turning on mechanisms are

1. Gate triggering

2. Forward voltage triggering

3. dv/dt triggering

4. Temperature triggering

5. Light triggering

Comparison of Thyristor and Transistor operation:

1. Once a Thyristor is on by a gate signal it remains latched in on state due to internal regeneration action.

However in case of transistor a continuous base signal must be given to remain in on state.

2. In order to turn-off a thyristor a reverse voltage must be applied across its anode cathode terminals.

However a transistor turns off when its base signal is removed.

Thyristor protection:

In general a Thyristor may be subjected to over voltages or over currents. During SCR turn on di/dt may be

prohibitively large and there may be false triggering of SCR by high value of dv/dt.So thyristors must be

protected against all such abnormal conditions for satisfactory and reliable operations.

di/dt problem:

When a thyristor is forward biased and is turned on by a gate pulse conduction of anode current begins in

immediate neighborhood of gate cathode junction therefore current spreads across the whole area of

junction. The thyristor design permits the spread of conduction to whole junction area as rapidly as possible.

But if rate of rise of anode current i.e. di/dt is large as compared to spread velocity of carriers local hot spot

59

N P N

EBJ CBJB MetalContact

EC

B

C

E

P N P

B

EC B

C

E

will be formed near the gate connection on account of high current density. This localized heating may

destroy the transistor.

So the rate of rise of anode current at the time of turn on must be kept below the specified limiting value.

Solution of di/dt problem:

The value of di/dt can be maintained below acceptable limits by using a small inductor called di/dt inductor

in series with the anode circuit. Typical limit values of di/dt are 20-500 A/micro-sec.

dv/dt problem:

If the rate of rise of forward voltage dv/dt is high the charging current I will be more .This charging current

plays the role of gate current and turns on SCR even when gate signal is zero. Such phenomena of turning

on a thyristor called a dv/dt turn must be avoided as it leads to false operation of thyristor circuit. For

controllable operation of thyristor, the rate of rise of forward anode to cathode voltage dv/dt must be kept

below the specified rated limit. Typical values of dv/dt are 20-500 V/micro sec.

Solution of dv/dt problem:

False turn-on of a thyristor by large dv/dt can be prevented by using a snubber circuit in parallel with the

device. A snubber circuit consists of a series combination of resistance RS and a capacitance CS in parallel

with the thyristor.

Transistor:

Transistor is a 3 layer semiconductor device consisting of either two N and one P type layer of material or

Two P and one N type layers of material. The former is called NPN transistor while later is called PNP

transistor. The emitter is heavily doped base is lightly doped and collector only lightly doped. The outer

layers have width much greater than sandwiched P or N type material. Ratio of total width to central width is

around 150:1.

Transistors are made from Ge or Si are shown in figure:

Direction at emitter denotes convention direction of current. In case of PNP current flows from Emitter to

base while in case of NPN it is from Base to emitter.

3 important parts of BJT are:

1. Emitter region

2. Base region

3. Collector region

60

E

P

B

N

C

P

VCC

VEE

|V B|C

|V B|E

V0

np0

np

np0

np0

np

Minority CarrierConcentration

pn

Ic C

IB

VEB VCB

InE

IpE Ipc1

Ipco

Inco

Ico

ICE

Main function of emitter is supply majority charge carriers to the base region and hence it must be heavily

doped in compare to other parts. Base region is lightly doped and thin so that it may pass most of the

injected charge to the collector region. Collector collects majority charge carriers after passing through the

base region. Generally size of collector is larger than that of emitter because collector has to dissipate much

more power and i.e. why emitter and collectors are not interchangeable.

Transistor biased in active region:

For active region Emitter base junction is forward biased and collector base junction is reverse biased.

Forward biasing of emitter base junction lowers the emitter base potential by EBV whereas reverse

Biasing of collector junction increases the collector base potential by CBV .

EBJ forward biased:

Due to emitter base junction as forward biased width of depletion region between emitter and base will be

reduced and there will be heavy flow of majority carriers from P type (Emitter) to N type (Base). When

these holes pass through base region then they are recombined with electrons and contribute to base current.

CBJ reverse biased:

Due to Collector base junction reverse biased width of depletion layer will be increased so there will be no

direct diffusion process of carriers. But these holes which are majority carriers will move easily to collector.

The reason for relative ease with which these majority carriers can cross the reverse biased junction is easily

understood if we consider that for the reverse biased junction diode the injected majority carrier will appear

as minority carriers in N type material.

Current components in case of a transistor in active mode: (PNP)

61

VCC

VEB

RL

Emitter region:

The forward biased on emitter junction will cause current to flow across this junction. This current will

consists of two components

1. Holes injected from emitter to base region (IpE)

2. Electrons injected from base to emitter region ( InE)

It is highly desirable to have the first component at a much higher level than second component .Due to this

region only emitter is highly doped and base is lightly doped it means device will have a high density of

holes in emitter region and low density of electrons in base region.

Different modes of operations in BJT:

Transistor consists of two P-N junctions Emitter Base Junction (EBJ) and Collector Base Junction (CBJ)

Depending on the bias condition (forward or reverse) of each of these junctions different mode of operations

of BJT are obtained.

The active mode which is also known as forward active mode is the one used if transistor is to operate as an

amplifier.

Switching applications (eg. logic circuits) use both cut-off and saturation mode.

The reverse active mode or inverse active mode has very limited applications but is conceptually important.

BJT Mode of operations:

MODE Emitter-Base J’n Collector-Base J’n NPN PNP

Cut-off Mode Reverse Reverse VBE<0 &VBC<0 VEB<0 &VCB<0

Active Forward Reverse VBE>0 &VBC<0 VEB>0 &VCB<0

Saturation Forward Forward VBE>0 &VBC>0 VEB>0 &VCB>0

Reverse Active Reverse Forward VBE<0 &VBC>0 VEB<0 &VCB>0

Transistor as an amplifier:

A load resistor RL is in series with the collector supply voltage VCC. A small voltage change iV between

emitter and base causes a relatively large emitter current change EI we define a symbol ' that fraction of

this current change which is collected and passed through RL so here '

C EI I

The change in output voltage across load resistor will be '

L L C L EV R I R I

But i e EV r I

So Voltage gain

' '

26

L L E LV

i e E e

e

E

V R I RA

V r I r

rI

Where EI is quiescent emitter current in mA.

62

: ( )

100%

E pE nE

pE

pE nE

I I I

Emitter injection Effeciency

Ifor ideal case

I I

Base - region:

Holes injected from emitter will enter into base region .These holes will be minority carriers in base region

and will recombine with electrons. Base in usually very thin and proportions of holes lost through this

recombination will be very small

Base current IB will be composed of two components:

1. First component is due to electrons injected form base into emitter

2. second component is due to electrons that have to be supplied by external circuit in order to replace the

electrons lost from the base through recombination process.

Value of common emitter current gain (β):

is constant for a particular transistor and for modern NPN transistor it lies in range of

50-200 but it can be as high as 1000 for special devices.

depends upon following factors:

1. width of the base region

2. relative doping of emitter to base region

From above result it is clear that for high value of

1. Base width should be thin

2. Base should be lightly doped

3. Emitter must be highly doped

Relation between α and β:

(1)

(2)

(3)

1

C E

C B

E B C

I I

I I

I I I

By above equations

is a constant for a given transistor and has value less than one but very close to unity. If =100 then

value of will be 0.99. Typical values lie in range of 0.90 to 0.995.

For very small change in there is large change in value of . So transistors of same type may have

widely different values of .

Early effect or base width modulation:

Width w of depletion region of a diode increases with magnitude of reverse bias voltage. Since emitter

Junction is forward biased and collector Junction is reverse biased in active region then barrier width at JE is

negligible as compared with space charge width at JC.

As applied voltage across Junction increases then transition region penetrates deeper into collector and base.

Doping in base is smaller than collector so more depletion layer portion will be in base region. If width of

base region is wB then by application of reverse bias voltage effective width of base is decreased.

63

So Modulation of effective base width by collector voltage is known as early effect. 1BEqV

CEKTc s

A

vi I e

v

Effect of base width modulation:

Value of both and are increases because there is less chance of recombination within the base region.

Punch through or Reach through. If collector voltage is increased then there is chance that base width will

become zero and this phenomenon is known as punch through

Break down voltages:

In case of a transistor before punch through breakdown of transistor occurs.

BVCEO is break down voltage for CE configuration & B VCBO is break down voltage for CB configuration.

1n

CEO CBO

FE

BV BVh

If n = 6 and hFE = 50

Transistor Biasing:

Transistor is a magical device that that can raise the level of applied ac input without assistance of an

external energy source. In actual the improved output ac power level is the result of transfer of energy from

applied DC power supply. The analysis or design of any electronic amplifier has two components the dc

portion and ac portion. So amplification in a transistor depends upon DC characteristics of BJT.So Gain in

BJT will be affected by DC level parameters.

For Faithful amplification following conditions must be full filled

1. Emitter-Base Junction must be forward biased

2. Collector-Base Junction must be revere biased

3. Value of IC and VCE must be proper.( Both values are taken as DC values)

Co-ordinates of (VCE, IC) are known as operating points. For proper amplification position of Q point

Must be in middle of DC load line. Maintenance of Q point for proper amplification is known as biasing.

Biasing: when Q point of a transistor is not in active region i.e. it is not properly biased then BJT will work

inefficiently and produces distortion in output signal.So a transistor is properly biased with the help of

batteries and external resistor to maintain Q point in proper position. This method is known as biasing and

associated circuits are known as biasing circuits.

Q. Why operating point shifts?

There are following reasons for shift in Q point of BJT?

1. Transistor parameters are temperature sensitive

2. Replacement of transistor with another transistor

3. Thermal run away

Variation of transistor parameters with temperature:

DC collector current(IC) in BJT varies with temperature which also effects values of VCE and finally

Operating point shifts due to change in temperature.

1. ICO becomes double for every 100C rise in temperature

(1 )C B COI I I so value of IC will change with change in temperature.

2. Value of increase with increase in temperature which will effect value of collector current(IC)

3. VBE decrease by -2.5 mV/0C which will also effect value of collector current.

64

BEdV

dT -2.5 mV/

0C

Replacement of transistor:

When a transistor is replaced by another transistor of same type then it is possible that value of and VBE

don‘t remain same and hence Q point is changed. So Q point must be stabilized in such a manner that it

remains fixed irrespective of replacement of transistor.

Thermal run away:

Typically junction temperature range for Ge and Si transistor are

Ge: 600C-110

0C and for Si: 150

0C-225

0C

If due to increase in temperature collector current increase then large collector current may heat up the

transistor and it may burn out. So variation in collector current with respect to temperature must be

maintained. Self destruction of a transistor without any biasing method is known as thermal run away.

Various methods for stabilization:

1. Stabilization methods:

In stabilization method resistor and power supplies are designed in such manner that value of collector

current IC becomes constant with respect to change in temperature. It means there is very less change in

value of IC with respect to change in ICO,VBE and .

Different methods used are:

1. Fixed bias circuit

2. Emitter stabilized bias circuit

3. Voltage divider bias

4. Voltage Feedback biasing

2. Compensation method:

Different compensation methods used are

1. Diode compensation for VBE

2. Diode compensation for ICO

3. Thermistor compensation

Temperature range of a transistor:

Si: Temp. Range is from 150 – 225ºC

Ge: Temp. Range is from 60 – 100ºC

Thermal runaway:

Due to power dissipation at junction, junction temperature increases and this will () collector current with a

subsequent increase in power dissipation. This phenomenon is referred as Thermal runaway and it may

damage transistor permanently.

Thermal resistance:

Steady state temperature rise at collector junction is proportional to power dissipation at junction.

Tj – TA = PD

Tj junction Temp.

TA Ambient Temp.

Thermal resistance

65

R = 5KC R1

R2

V = 24VCC

PD Power dissipated in watt at collector

Condition for thermal stability:

The rate at which heat is released at collector Junction must not exceed the rate at which heat can dissipate

So To avoid thermal run away:

1C

j

P

T

or VCE <

2

CCV

Heat sink:

Power transistors are mounted in a large metal case to provide a large area from which heat generated by

device may be radiated. The metal sheet that helps to dissipate the additional heat from transistor is known

as heat sink.

Field Effect Transistor:

The Field Effect Transistor is a semiconductor device which depends upon its operation on the control of

current by an electric field. There are two types of FETs

1. Junction Field Effect Transistor (JFET or simply FET)

2. Insulated Gate Field Effect Transistor (IGFET) More commonly known as MOS or MOSFET

Difference between BJT &FET:

1. Its operation depends upon flow of majority carriers only so it is unipolar

2. It is simpler to fabricate and occupies less space in IC form

3. It exhibits a high input impedance typically Mega ohm

4. It is less noisy than BJT because partition noise is absent in case of FET.

5. BJT is Current controlled current device while FET is Voltage Controlled current device.

6. It exhibits no offset voltage at zero drain current and hence makes an excellent signal chopper

7. Gain Bandwidth product (GBWP) of FET is small in compare to BJT.

8. FET has better thermal stability in compare to BJT.

9. FET is less affected by radiation

10. Due to low value of trans-conductance FET has low voltage gain in compare to BJT.

Basic structure of JFET:

JFET is a 3 terminal device with one terminal capable of controlling the current between other two. Basic

structure of N channel JFET is shown in figure in which N type material forms channel between embedded

layers of P type material. The top of N type channel is connected through an ohmic contact to terminal drain

(D) while the lower end of same material is connected through an ohmic contact to a terminal known as

Source(S). Both source(S) and Drain (D) can be interchanged.

66

P

D

N

G

S

P

D

NG

S

N

G

D

S

G

D

S

N-channel JFET

Drain (D)

G

Source (S)

P N P

N-channel Depletion region

Parts of FET:

1. Source: It is the terminal through which majority carrier enter the semiconductor bar.

2. Drain: It is the terminal through which majority carrier enter the semiconductor bar

3. Gate: These are two heavily connected heavily doped impurity regions which forms two P-N Junctions.

4. Channel: It is the space between two gates through which majority carriers pass.

For N-channel FET ID and VDS are positive while value of VGS is negative

For P-Channel FET ID and VDS are negative while value of VGS is positive.

The direction of arrow at the gate indicates the direction in which the gate current flows when gate junction

is forward biased.

Zero biased condition:

In absence of any applied potential JFET has two P-N junctions under no bias conditions. The result is a

depletion region at each junction as shown in figure. This is same as region of a diode under no bias

conditions.

67

P

D

P

A

B

S

S

VDD

D

Depletionlayer

Q. Why depletion region is wider at drain side than source side?

Due to current flow there will be a uniform voltage drop while going from drain to source. Let VA and VB be

potential drop at these points and value of VA>VB due to progressive voltage drop along length of channel.

So reverse biasing effect on P-N junction is stronger near the drain than near source.

Operation of N-channel JFET:

Case-1 when VGS=0 and VDS>0

When no potential is applied between gate and source and a potential is applied between drain and source.

Drain current will flow and it will have maximum value because channel is widest.

Case-2 when VGS<0 and VDS>0

If gate is reverse biased by applying a negative voltage between gate and source then width of depletion

layer will increased and thereby decreases cross section of N channel. Due to decrease in cross sectional area

of N channel the Drain current ID will decrease. When gate biased in increased further and is equal to Pinch-

off voltage (VGS= -VP) then value of drain current becomes equal to zero. So drain current is a function of

voltage and i.e. why FET is a voltage controlled current device.

Enhancement type of MOSFET (MOS):

Enhancement type MOSFET differs from the depletion type that no continuous channel exists between

source and drain. In N-channel enhancement MOSFET P type substrate extends all the way to metal Oxide

layer. Since it does not conduct when VGS=0 hence it is called OFF MOSFET also.

Working of N-MOS:

P type substrate insulating dielectric SiO2 layer and metal layer of gate forms a parallel plate capacitor.

When a positive voltage is applied to gate with respect to substrate then minority carriers in P substrate

electrons are drawn towards the dielectric. Due to electrons negative charge is induced on P type substrate

and forms an inversion layer. If positive voltage on gate is increased then magnitude of induced negative

charges in semiconductor increases and thus conductivity of induced N-channel increases leading to higher

drain current because negative charge carriers are increased. Value of threshold voltage is in range of 2-4

68

N

N

Sio2

D

G

S

MetallicContact

No

chan

nel

P TypeSubstrate SS

Volts. Here drain current has been increased by application of positive voltage so this MOSFET is termed as

Enhancement type MOSFET.

Enhancement NMOS characteristics:

Case-1: VGS=0 volt and VDS>0

Since channel is absent and will result in a current of effectively zero ampere. In this case it is not sufficient

to have a large accumulation of charge carriers (electrons) at the drain and source. With VDS some positive

voltage VGS at 0 volt and terminal SS directly connected to the source there are in fact two reverse biased

P-N junctions between N doped region and P substrate to oppose any significant flow between drain and

source.

Case-2: VGS>0 and VDS>0

In this case drain and gate are at positive potential with respect to source. The positive potential at gate will

pressure the holes in P substarate along the edge of SiO2 layer to leave the area and enter deeper regions of

P substrate and result is a depletion region near the SiO2 insulating layer void of holes. But the electrons in P

substarate will be attracted to the positive gate and accumulate in region near the surface of SiO2 layer. The

SiO2 layer and its insulating qualities will prevent the negative carriers from being absorbed at the gate

terminal.

Case-3: VGS=VT and VDS>0

If value of VGS increases in magnitude then concentration of electrons near the SiO2 surface increase until

eventually the induced N type region can support a measurable flow between drain and source. The level of

VGS that that results in significant increase in drain current is called Threshold voltage and is given symbol

VT .since the channel is nonexistent with VGS=0 and enhanced by application of a positive gate to source

Voltage this type of MOSFET is called enhancement type MOSFET.

Case-4: VGS>VT and VDS>0

As value of VGS is increased beyond VT the density of free carriers in the induced channel will increase

resulting in an increased level of drain current. If we hold VGS constant and increase the level of VDS the

drain current will eventually reach a saturation level as occurred for JFET and Depletion type MOSFET. If

VGS is held fixed and value of VDS is increased then gate will become less and less positive with respect to

drain. The reduction in gate to drain voltage will in turn reduce the attractive forces for free carriers

(electrons) and causes a reduction in effective channel width. In this case the channel will be reduced to

point of pinch-off and a saturation conditions will be established. If value of VDS is further increased at a

fixed value of VGS then saturation current ID will not be affected.

69

Ie Ie Ie Ie

E

B

C

B

e

b

c

b

0 CBZ

Ie Ic

e

b

c

b

re

Ie Ic

e

b

c

b

re

Transistor as an amplifier:

Transistor is a magical device which converts an low level ac input into high level ac output. It amplifies an

ac signal by use of DC power supply. There are various models for expressing amplification in transistor.

If bev is the ac input and ci is the ac output then

c m bei g v Where Cm

T

Ig

V

There are various models for explaining transistor models.

re transistor model:

The re model employs a diode and controlled current source to duplicate the behavior of a transistor in the

region of interest. Recall that a current-controlled current source is one where the parameters of the current

source are controlled by a current else-where in the network. In fact in general BJT transistor amplifiers are

referred to as current-controlled devices.

Common Base transistor:

Ic = Ie

26e

E

mVr

I

The subscript e of re was chosen to emphasize that it is the dc level of emitter current that determines the ac

level of the resistance of the diode .

Input impedance and output impedance for CB:

i e CBZ r

For the common-base configuration typical values of Zi range from a few ohms to a maximum

of about 50 .

In general for the common-base configuration the input impedance is relatively small and the output

impedance quite high.

Voltage gain and Current gain for CB:

If resistance RL is connected between CB terminals and Vi is connected between EB terminals.

70

Ii I0

+

–

+

–

Vi V0

2

2'

1

1'

If I0 is the load current between RL.

Vcb = –I0RL = –(–Ic)RL = IeRL

eb i e i e eV V I Z I r

So that

and o e Lv

i e e

V I RA

V I r

For the current gain:

1o c ei i CB

i e e

I I IA A

I I I

h-parameter model for transistor:

The quantities hie, hre, hfe, and hoe are called the hybrid parameters and are the components of a small-signal

equivalent circuit to be described shortly. For years the hybrid model with all its parameters was the chosen

model for the educational and industrial communities. Presently however the re model is applied more

frequently but often with the hoe parameter of the hybrid equivalent model to provide our description of the

hybrid equivalent model will begin with the general two-port system.

The following set of equations is only one of a number of ways in which the four variables can be related. It

is the most frequently employed in transistor circuit analysis, however, and therefore is discussed in detail in

this chapter.

11 12 0

0 21 22 0

(1)

(2)

i i

i

V h I h V

I h I h V

If we arbitrarily set V0 = 0 (short circuit the output terminals) and solve for h11 in Eq.(1)&(2) the following

will result:

0

11

0

i

i V

Vh

I

Ohms

If Ii is set equal to zero by opening the input leads the following will result for h12:

12

0 0i

i

I

Vh

V

unit less

0

021

0i V

Ih

I

unit less

71

S

D

Vgs rdg Vm gs

G

S

+

–

RC

RR

VCC

CE

Q1

E

Q

R

RB

V0

L

RB1

V1

B

CB

022

0 0iI

Ih

V

Siemens

FET AC Equivalent Circuit:

A model for the FET transistor in the ac domain can be constructed. The control of Id by Vgs is included as a

current source gmVgs connected form drain to source as shown in Figure.

The current source has its arrow pointing from drain to source to establish a 180º phase shift between output

and input voltages as will occur in actual operation.

The input impedance is represented by the open circuit at the input terminals and the output impedance by

the resistor rd from drain to source. Note that the gate to source voltage is now represented by Vgs (lower-

case subscripts) to distinguish it from dc levels. In addition take note of the fact that the source is common to

both input and output circuits while the gate and drain terminals are only in ―touch‖ through the controlled

current source gmVgs.

Cascode connection:

A cascade connection has one transistor on top of (in series with) another. Figure shows a cascade

configuration with a common-emitter (CE) stage feeding a common-base (CB) stage. This arrangement is

designed to provide high input impedance with low voltage gain to ensure that the input Miller capacitance

is at a minimum with the CB stage providing good high-frequency operation. A practical BJT version of a

cascade amplifier is provided in Figure.

Darlington connection:

A very popular connection of two bipolar junction transistors for operation as one ―super beta‖ transistor is

the Darlington connection shown in Figure. The main feature of the Darlington connection is that the

composite transistor acts as a single unit with a current gain that is the product of the current gains of the

72

Q2

E

C

Q1B

QD

E

C

B

E

C

B

I = 10 mA

V = –4VDSS

P

I =10 mA

ID

individual transistors. If the connection is made using two separate transistors having current gains of 1 and

2 , the Darlington connection provides a current gain of

1 2D

If the two transistors are matched so that 1 = 2 = the Darlington connection provides a current gain of

2

D

A Darlington transistor connection provides a transistor having a very large current gain, typically a few

thousand.

Feedback pair:

The feedback pair connection is a two-transistor circuit that operates like the Darlington circuit. Notice that

the feedback pair uses a pnp transistor driving an npn transistor, the two device acting effectively much like

one pnp transistor. As with a Darlington connection, the feedback pair provide very high current gain (the

product of the transistor current gains). A typical application uses a Darlington connection and a feedback

pair connection to provide complementary transistor operation. A practical circuit using a feedback pair is

provided in Figure .Some consideration of the dc bias and ac operation will provide better understanding of

how the connection works.

Constant current source:

1. JFET Current Source

A simple JFET current source is that of Figure with VGS set to 0 V the drain current I fixed at

10D DSSI I mA

The device therefore like a current source of value 10 mA. While the actual JFET does have an output

resistance, the ideal current source would be a 10-mA supply as shown in figure.

73

VE

I =IERE

+VBE(on)–+

–VZ

VB

R1

–VEE

I

Q2Q1

IX RX

+VCC

2. Transistor/Zener Constant-Current Source

Replacing resistor R2 with a Zener diode, as shown in Figure provides an improved constant-current source

The Zener diode results in a constant current calculated using the base-emitter KVL (Kirchhoff Voltage

Loop) equation. The value of I can be calculated using

Z BEE

E

V VI I

R

A major point to consider is that the constant current depends on the Zener diode voltage, which remains

quite constant and the emitter resistor RE. The voltage supply VEE has no effect on the value of I.

Current mirror circuits

A current mirror circuit provides a constant and is used primarily in integrated circuits. The constant current

is obtained from an output current, which is the reflection or mirror of a constant current developed on one

side of the

The current IX and I can be obtained using the circuit currents listed in Figure.We assume that the emitter

current (IE) for both transistors is the same (Q1 and Q2 being fabricated near each other on the same chip).

The two transistor base currents are then approximately

1

E EB

I II

The collector current of each transistor is then

IC IE

Finally, the current through resistor RX, IX, is

2 2 2E E E

X E E E

I I II I I I

In summary, the constant current provided at the collector of Q2 mirrors that of Q1. Since

CC BEX

X

V VI

R

74

Vi

C

V0

L

+VCC

RFC

–VBB

+

_Sawtoothgenerator

AmplifierLow-pass

filter

Feedback

Vi

V0

Converts digitalback to sinusoidal

+

–Vi

+

–V =AV0 i

+

–A

Class C Amplifier

A class C amplifier as that shown in Figure is biased to operate for less than 180º of the input signal cycle.

The tuned circuit in the output however will provide a full cycle of output signal for the fundamental or

resonant frequency of the tuned circuit (L and c tank circuit) of the output. This type of operation is

therefore limited to use at one fixed frequency, as occurs in a communications circuit for example.

Operation of a class C circuit is not intended primarily for large-signal or power amplifiers.

Class D Amplifier:

A class D amplifier is designed to operate with digital or pulse-type signals. An efficiency of over 90% is

achieved using this type of circuit making it quite desirable in power amplifiers. It is necessary however to

convert any input signal into a pulse-type waveform before using it to drive a large power load and to

convert the signal back to a sinusoidal-type signal to recover the original signal. Figure shows how a

sinusoidal signal may be converted into a pulse-type signal using some form of saw tooth or chopping

waveform to be applied with the input into a comparator-type op-amp circuit so that a representative pulse-

type signal is produced. While the letter D is used to describe the next type of bias operation after class C the

D could also be considered to stand for ―Digital,‖ since that is the nature of the signals provided to the class

D amplifier.

Basic concept of oscillation:

75

Steady-state envelopelimited by circuit saturation

Nonsinusoidal waveformdue to saturation

Initial noisevoltage

C C C

RRR

Phase-shift oscillator:

An example of an oscillator circuit that follows the basic development of a feedback circuit is the phase-shift

oscillator. An idealized version of this circuit is shown in Figure. Recall that the requirements for oscillation

are that the loop gain A is greater than unity and that the phase shift around the feedback network is 180º

(providing positive feedback). In the present idealization we are considering the feedback network to be

driven by a perfect source (zero source impedance) and the output of the feedback network to be connected

into a perfect load (infinite load impedance). The idealized case will allow development of the theory behind

the operation of the phase-shift oscillator. Practical circuit versions will then be considered.

FET Phase-Shift Oscillator:

A practical version of a phase-shift oscillator circuit is shown in Figure. The circuit is drawn to show clearly

the amplifier and feedback network. The amplifier stage is self-biased with a capacitor bypassed source

resistor Rs and a drain bias resistor RD. The FET device parameters of interest are gm and rd. From FET

amplifier theory, the amplifier gain magnitude is calculated from

|A| = gmRL

76

RE

R1RC

VCC

CE

R2

C C CR’

R R

f =1

6+4(R /R)c

VDD

C

RS CS

g , rm d

RD

C C

RRR

f=

Transistor Phase-Shift Oscillator:

If a transistor is used as the active element of the amplifier stage, the output of the feedback network is

loaded appreciably by the relatively low input resistance (hie) of the transistor. Of course, an emitter-

follower input stage followed by a common-emitter amplifier stage could be used. If a single transistor stage

is desired, however the use of voltage-shunt feedback (as shown in Fig.) is more suitable. In this connection

the feedback signal is coupled through the feedback resistor R in series with the amplifier stage input

resistance (Ri).

1 1

2 6 4( / )C

fRC R R

23 29 4 Cfe

C

RRh

R R

77

+

_

a

b

c

d

R1

C1

C2

R2

R3

R4

+VCC

–VEE

Outputsinusoidalsignal

X1 X2

X3

–

+

R4

Output

–VEE

R3

R2

R1+VCC

Wein bridge oscillator:

A practical oscillator circuit uses an op-amp and RC bridge circuit with the oscillator frequency set by the R

and C components. Figure shows a basic version of a Wien bridge oscillator circuit. Note the basic bridge

connection. Resistors R1 and R2 and capacitors C1 and C2 form the frequency-adjustment elements while

resistors R3 and R4 form part of the feedback path. The op-amp output is connected as the bridge input at

points a and c. The bridge circuit output at points b and d is the input to the op-amp.

0

1 1 2 2

1

2f

R C R C

Q. Calculate frequency of oscillation:

Solution

Using Equation:

0 3 6

1 13120.7 H

2 2 (51 10 )(0.001 10 )f

RC

Radio frequency oscillator:

78

VDD

L

RG

RFC

CC

V0

C2C1

L

CERE

V0

C1

C2

RFC

VCC

R1

R2

CC

Reactance Element

Oscillator Type X1 X2 X3

Colpitts oscillator C C L

Hartley oscillator L L C

Tuned input, tuned output LC LC —

Colpitts Oscillator

FET colpitts oscillator:

A practical version of an FET Colpitts oscillator is shown in Figure. The circuit is basically the same form as

shown above with the addition of the components needed for dc bias of the FET amplifier. The oscillator

frequency can be found to be

0

1

2 eq

fLC

Where 1 2

1 2

eq

C CC

C C

BJT Colpitts oscillator:

79

VBB

R1

R2

B2

B1

CT

EVE

RT

VB1

VB2

R

L

C

CM

|Z|

0 f1 f2

(series-resonance) (antiresonances)

Crystal oscillator:

Quartz crystal exhibits the property that when mechanical stress is applied across the faces of a crystal a

difference of potential is developed across the opposite faces of crystal. This property is called piezo electric

effect. Similarly a voltage applied across one set of faces of crystal causes mechanical distortion in crystal

shape.

Equivalent circuit of crystal:

Crystal impedance versus frequency:

Unijunction oscillator

A particular device the Unijunction transistor can be used in a single-stage oscillator circuit to provide a

pulse signal suitable for digital-circuit applications. The unijunction transistor can be used in what is called a

relaxation oscillator as shown by the basic circuit of Figure shown.

Resistor RT and capacitor CT are the timing components that set the circuit oscillating rate. The oscillating

frequency may be calculated which includes the unijunction transistor intrinsic stand-off ratio as a factor

(in addition to RT and CT) in the oscillator operating frequency.

80

VBB

R2

CT

RT

Vout

RC

R1

RB

VBBVE

V =VE P

VEmin

0VTime

Time

Time

0V

VB1

VB2

0V

VBB

R1

CT

RT

VBB

R2

CT

RT

0

1

n 1/(1 )T T

fR C l

Wave forms at B1 B2 and E in UJT:

Frequency of oscillation in UJT:

Typically a unijunction transistor has a stand-off ratio from 0.4 to 0.6. Using a value of = 0.5 we get

0

1 1.44 1.44

In 1/(1 0.5) In 2

1.5

T T T T T T

T T

fR C R C R C

R C

81

Qn

Qn

J

K

VCC

CLK

1

0

Q = 0n

Q0

Q0

T0

FF0

Q1 Q2

Q1 Q2

T1 T2

Ff1 Ff2CLK

1 1 1

Study material for Digital

Digital Counter:

Counter is a sequential circuit consisting of a set of FFS connected in suitable manner to count the sequence

of input pulses presented to it in digital form. Counters are classified in 2 categories

1. Asynchronous counter

2. Synchronous counter

Asynchronous counter can be designed by use of Minm

hardware and each Flip-flop is triggered by output

from previous Flip-flop. Settling time in an asynchronous counter is cumulative sum of individual settling

times of Flip-flops. It is also called as serial counter.

In Synchronous counter clock pulse is applied simultaneously to all Flip-flops which leads to propagation

delay of a single Flip-flop. These are also known as parallel counter.

Counters are important sub system in digital equipments. They are used for pulse counting, frequency

division, time measurement and control & timer operation.

Modulus N counter:

Counter goes through N states before resetting to 0 or original state.

So a MOD-8 counter goes through 8 states from 000 to 111 and it recycles to 0 at the 9th

state.

MOD-2 counter:

A single JK or T FF works as MOD-2 counter.

From Above waveform it is clear that for one clock pulse we obtain a half pulse at the output. So this is

divide by 2 counter. If we have a series combination of N FFS then it will be divide by 2N counter or simply

by N counter.

Binary and Non binary counter:

If maximum binary number is equal to 2N then it is a binary counter if not it is a non binary counter.

MOD-16 is binary counter whereas MOD-10 is not a binary counter.

Asynchronous or ripple counter:

All Flip-flops are not under control of a single clock but clock is applied to 1st Flip-flop i.e. LSB of counter

and successive Flip-flop is triggered by output of previous Flip-flop. Here trigger move through the Flip-flop

like a ripple it is called a ripple counter.

MOD-8 ripple up-counter:

82

1

0 1 0 1 0 1 0

0 1 1 0 0 1 1 0

0 0 0 1 1 1 1 0Q = 00

Q = 00

Q = 00

000 001 010 011

111 110 101 100

Q2

Q1

Q0

Q2

Q2

Q2

Q1

Q1

Q1

Q0

Q0

Q0

0

1

2

3

Q2

Q2

Q2

Q2

Q1

Q1

Q1

Q1

Q0

Q0

Q0

Q0

4

5

6

7

Figure shows a 3-bit binary ripple counter, this binary counter is constructed by T FFS or J K FFS. The

system clock a square wave drives FFO, output of FFO drives FF 1 and output of FF 1 drives FF2. Overall

propagation delay is sum of individual delay of all FFS.

State diagram:

Here No. of states read are 8, so it will be MOD-8 counter. If freq. of input clock is f then freq

. at output will

be (f/8). Such a circuit is known as divide by 8, counter.

Ripple counter with decoded outputs:

A decoding gate is one which can be connected to output of a counter and its output will be high only when

counter content is equal to the given state.

5 will be decoded for Q2 Q1 Q0 = 101

So logic expression for decoding gate is Q5 = Q2 1Q Q0

Decoder circuit for a 3-bit binary counter:

83

2ts

Ts

QA

QB

QC

Q4

CLK

Q0

Q0

T0

FF0

Q1 Q2

Q1 Q2

J1 J2

FF1 FF2

LSBVCC

K0

VCC

K1 K2

VCC

MSB

Problem of Glitch:

Each decoding output will be high only when the counter content is equal to a given state and this state

occurs only once during a cycle of 2n states of counter, where n is number of Flip-flops in a counter.

In an asynchronous counter decoding gate produces a high output more than once during a cycle of 2n states.

Such undesired high or low pulses that appear at decoding gate output at undesired time instants are called

glitches or false spikes which are normally of short duration. The reason for these glitches is cumulative

propagation delay in asynchronous counter.

Solution of Glitch:

Problem of glitch can be solved by use of any one of the following method:

(i) Clock input to strobe decoding gates

(ii) By use of synchronous counter

Thus by strobing decoding gates with clock inputs glitches can be avoided completely.

Clock frequency for ripple counter:

If tpd is propagation delay of each Flip-flop then total propagation delay of N-bit ripple counter must be N

tpd.

If TS is additional time interval for strobing then clock period T N tpd + TS

Maxm

clock frequency 1 1

( )( )pd S

fT N t T

Q. Design MOD-8 ripple down counter.

Input pulse is applied as clock input. Here 0Q output serves as clock input for FF1 and 1Q as clock input for

FF2.

FF0 toggle at negative transition of clock, FF1 toggle when Q0 goes from low to high and FF2 toggle when

Q1 goes from low to high.

84

Q2

Q1

Q0

000 111 110 101

001 010 011100

000 001 010 011 100

101

000

Q0T Q1 Q2T T

CLK

1 1

1

Negative transition at 0 1Q & Q will be same as positive transition at Q0 and Q1.

If initial counter content is 000 at its first negative transition of clock the counter content becomes 111 at the

second negative transition content becomes 110 and so on.

State diagram of MOD-8 down counter :

Design of Ripple counter with Modulus < 2n :

To construct MOD-N counter method is:

1. Find the number of Flip-flops required for desired MOD-N. 2n–1

N 2n

2. Connect all n Flip-flops as a ripple counter.

3. Find the binary number for N.

4. Construct all FF outputs for which Q = 1 when count is N, as inputs to NAND gates.

5. Connect the NAND gate output to clear input of each Flip-flop.

Q. Design MOD-6 UP counter by use of NAND gate.

Sol. State diagram of MOD-6 UP counter is

For MOD-6 UP counter just make 110 to 000 by use of NAND gate.

Above MOD-6 UP counter can also be designed by use of OR gate also.

85

Q0T Q1 Q2T T

CLK

1 11

Q0 Q1 Q2

Q2J2

Q2K2

Q1J1

Q1K1

Q0J0

Q0K0

Count down

CLK

VCC

Countup

Synchronous 3 bit UP/Down counter:

Synchronous counter:

Ripple counter has low speed of operation but it requires less hardware. Another problem with ripple

counter is glitch at decoding gate outputs. Glitch problem can be solved by applying clock pulses to all FFS

simultaneously which is done in synchronous counter.

In a parallel counter all FFS change their state simultaneously i.e. they are all synchronized with negative

transition of input clock signal. Because of common clocking of all FFS glitches can be avoided completely

in synchronous counter.

Q. Design a 3-bit synchronous up counter.

The No. of FFS required is 3 and let FFS are FFO, FF1 and FF2.

For FF0 inputs are J0 and K0 & output is Q0.

For FF1 inputs are J1 and K1 & output is Q1.

For FF2 inputs are J2 and K2 & output is Q2.

Counter state FF0 FF1 FF2

Q2 Q1 Q0 J0 K0 J1 K1 J2 K2

0 0 0 1 × 0 × 0 ×

0 0 1 × 1 1 × 0 ×

0 1 0 1 × × 0 0 ×

0 1 1 × 1 × 1 1 ×

1 0 0 1 × 0 × × 0

1 0 1 × 1 1 × × 0

1 1 0 1 × × 0 × 0

1 1 1 × 1 × 1 × 1

0 0 0

86

Q2J2

Q2K2

Q1J1

Q1K1

Q0J0

Q0K0

Synchronous counter with ripple carryCLK

VCC

Q2J2

Q2K2

Q1J1

Q1K1

Q0J0

K0

CLK

VCC

Q3J3

Q3K3

Calculate value of J0 in terms of Q2 Q1 Q0 by use of K-Map :

By solving K-Map values will be:

J0 = 1&K0 = 1

J1 = Q0 K1 = Q0

J2 = Q0 Q1 K2 = Q0 Q1

Synchronous counter with parallel carry:

Total delay = Propogation delay of 1 FF+ Propogation delay of NAD gate

= tp + tg

Maxm

frequency of operation fmax = p g

1

t + t

Synchronous counter with ripple carry:

max

1

( 2)p g

ft n t

n No. of Flip-flops stages.

Design of synchronous Down counter:

J0 = K0 = 1

J1 = K1 = 0Q

J2 = K2 = 0 1Q Q

J3 = K3 = 0 1 2Q Q Q

Shift Registers and memory:

Register are binary storage elements which are used as temporary memory device they belong to family of

sequential logic circuit.

In a register if stored bit is shifted in left or right then register is known as shift register. For shift registers

FF S used are D FFS or SR & J K FFS.

87

QDQDQD

CLK

QD

n-bitSerial data

inputSerial data

output

n-bitSerial data

input

MSB LSB

Parallel data outputs.

n-bitSerial data

input

Parallel data inputs

Shift register permits the stored data to move from a particular location to some other location within

register.

Types of shifting:

1. Serial shifting

2. Parallel shifting

Serial shifting method shifts one bit at a time for each clock pulse in serial beginning with either MSB or

LSB.

A 4-bit shift register requires 4 clock pulses to shift a bit from input to output. While in parallel shifting all

the data or inputs get shifted simultaneously during a single clock pulse.

Basic 4-bit shift register:

Classification of shift Register:

1. Serial – In – Serial – out:

Data can be moved in & out of a register one bit at a time.

2. Serial – In – Parallel – out:

Data is loaded Serially one bit at a time but data stored can be read simultaneously.

3. Parallel – In – Serial – out :

Data can be loaded simultaneously but data can be removed from register one bit at a time by clock

pulse.

4. Parallel – In – Parallel – out :

Data can be loaded into stages simultaneously and can also be taken out to read simultaneously.

88

n-bit

Parallel datainputs

Parallel data outputs

Q0D

CLK

Q1D Q2

D Q3D

FF0 FF1 FF2 FF3

Serial data input Serial data output

DatainputQ3J

Q3K

Q2J

Q2K

Q1J

Q1K

Q0J

Q0K

o/p

QAD

CLK

QBD QC

D QDD

0100

1. S I S O

Right Shift Register:

Let 11001 is entered in Serial In / Serial out shift register. 1101

FF0 FF1 FF2 FF3

0 0 0 0

1 0 0 0

0 1 0 0

1 0 1 0

1 1 0 1

2. Serial – In – Parallel out shift Register:

89

Q2DQ1DQ0D

CLK

A

CBShift/Load

3 – bit P I SO S H R :

Q0D

CLK

Q1D Q2

D Q3D

1000 0100 0010 0001

Q0

CLK

Q1D Q2

D Q3D

CLR CLR CLR

Pulse

PR

3. Parallel In Serial out Shift Register:

4. Parallel Input parallel output Shift Register

Ring counter:

The output Q of last FF is connected back to Serial input of 1st FF and only one FF is set at any particular

time while other FFS are cleared. The Flip-flops are connected in such a way that information shifts either

from left to right and back around from Q3 to Q0 or from right to left and back around from Q0 to Q3.

When low pulse is given then Q0 = 1, Q1 = 0, Q2 = 0 and Q3 = 0.

SoQ0 Q1 Q2 Q3 = 1000 is initial state of given ring counter.

So 4 valid states there will be 4 valid states.

This circuit can be used for counting no. of pulses. The number of pulses counted is read by nothing which

Flip-flop is in 1 state. No decoding circuitry is required. Since there is one pulse at the output for each of N

clock pulses this circuit is referred to as divide by N counter or N : 1 scalar.

90

Q3J3

Q3K3

Q2J2

Q2K2

Q1J1

Q1K1

Q0J0

Q0K0

CLK

Q0

Q4

Q4

Q3

0

1

Twisted Ring Counter:

In Ring counter if output Q is connected to input of 1st FF then it is twisted ring counter.

Connection have been switched from normal to complementary and taken generally from last or tail of stage

so it is called switch tail ring counter.

In case of N-bit ring counter no. of states are N, while for N-bit switch tail ring counter no. of states are 2N.

So In case of switch tail ring counter there is 100% saving in number of FFS.

Switch tail ring counter is also known as Johnson or Mobeious counter.

Moebius counter is a divide by 2N counter.

For decoding the count two input AND gates are required. The decoder circuit for a 5-stage counter is

shown in figure.

Q0 Q1 Q2 Q3

0 0 0 0

1 0 0 0

1 1 0 0

1 1 1 0

1 1 1 1

0 1 1 1

0 0 1 1

0 0 0 1

0 0 0 0

So it is a MO D–8 counter.

Decoding logic for a 5 stage twisted ring counter.

No. of Unused in a Johnson counter are given by 2N – 2N. For a MOD–8 counter N = 4

So No. of unused states are 24 – 2×4 = 8

Integrated circuits-Logic families:

In the implementation of logic functions discrete transistors and circuit elements are not used. Fabrication of

enormous quantum of circuits on a single chip has made possible the integrated logic families. These

integrated circuits are classified according to the number of gates.

91

Category Numbers of equivalent

basic gates on a single chip

Total number of

components on single chip

SSI

MSI

LSI

VLSI

ULSI

Less than 12

12 – 99

100 – 999

1000 – 9999

10,000 and more

Less than 100

100 – 999

1000 – 9999

10,000 to 99,999

100,000 and more

Integrated circuits are classified into 2 general categories Linear and Digital.

Linear IC‘s are operating with continuous signals and used to construct electronic circuit such as amplifiers

and voltage comparators etc. while Digital integrated circuits are operating with binary signals and so

constructed with integrated circuits.

Digital IC:

Various logic families can be placed into 2 broad categories according to IC fabrication process. These are

designed as bipolar and MOS.

Bipolar IC:

Bipolar IC are classified as saturated logic and non saturated logic family. Bipolar technology is employed

for fabricating transistors, diodes and resistors on a chip. SSI and MSI utilize bipolar technology because it

is faster.

1. Saturated logic family:

1. RTL (Resistor Transistor Logic)

2. DCTL (Direct Coupled Transistor Logic)

3. DTL (Diode-Transistor Logic)

4. HTL (High Threshold Logic)

5. TTL (Transistor –Transistor Logic)

6. I2L(Integrated Injection Logic)

2. Non saturated logic family:

1. Schottky TTL

2. Emitter Coupled Logic (ECL)

MOS IC: MOS (Metal Oxide Semiconductor) technology is preferred in LSI and VLSI as more MOSFETs

can be fabricated on a small chip area. Currently an improved version called CMOS (Complementary Metal

Oxide Semiconductor) technology is widely used. CMOS ICs contain parts comparable to insulated gate

field effect transistors (IGFETs).

1. PMOS-P channel MOSFET

2. NMOS-Channel MOSFET

3. CMOS-Complementary MOSFET

The circuits using TTL and CMOS families are logic gates, flip-flops, encoders and decoders, multiplexers,

latches and registers. While NMOS is used in microprocessors and memories, CMOS finds use in

calculators, wrist watches and battery operated computers because of low power consumption. PMOS is

almost obsolete.

Characteristics of Digital IC:

92

3

2

1

0.2

0.5 0.65 0.8 1 VOH

Vin

VIL VIH

VOL

VOH

Vout

1. Speed of operation: It is defined as time taken for output of a gate to change after inputs have changed.

It is defined in terms of propagation delay .Here propagation delay is a function of switching time of

individual transistor or MOSFET.Typically, in a switching transistor circuit, while the input change from

Low (0) to High (1), the output has to change from High (1) to Low (0). But the two changes are not

simultaneous and there is a time delay which is known as propagation delay tpd. This delay is due to the

fact that the output voltage has to change from cut off region to saturation region via the forward active

region. Also the junction and stray capacitances along with the base and collector resistances RB and RC

constitute a time constant. This time constant depends on the number of gates connected at the input and

output, fan-in and fan-out. So, tpd increases for a high fan-out. Propagation delay time tpd is a measure of

the speed of operation of the logic gate.

2. Power dissipation: It is a measure of power consumed by logic gate when fully driven by all its inputs

and is expressed in milli-watts or nano-watts.Avergae value of power dissipation is the product of DC

supply voltage and mean current taken from that power supply.

3. Fan-in: It is the number of inputs connected to a gate without any degradation in voltage levels. It

determines functional capabilities of a logic circuit.

4. Fan-out: It is the maximum number of similar logic gates that can drive without any degradation in

voltage levels.Fanout is a function of output impedance of driving gate and input impedance of driven

gate.

5. Speed power product: It is the product of propagation delay power dissipation of the gate. This is

intended decide on the choice of gate for a particular application, high speed or low power.

6. Noise-immunity: It is the maximum noise voltage that may appear at the input of a logic gates without

changing logical state of its output. A quantative measure of noise immunity is called noise margin.

7. Noise margin: Difference between operating input logic voltage level and threshold voltage level is

called as noise margin of circuit. Noise margin refers to amplitude of noise voltage that may cause logic

level to change.

The input-output characteristics of a switching transistor circuit will be as follows:

Hence If Vi VIL max, then V0 VOH min If Vi VIH min, then V0 VOL max

Undesirable signals called noise such as picked up from power supply lines should not affect the functioning

of the gate. VOH min is greater than VIH min. This is the region of noise immunity for high level logic (1).

Similarly VIL max is greater than VOH max and the difference is the noise immunity for low level logic (0).

Low level noise margin NML =VIL max – VOL max

High level noise margin NMH = VOH min – VOL min

93

+VCC

RC

Y

T2T1

RBBRB

(a) circuit

A

B

Y

For reliability the noise margins should be high. If the noise amplitude is higher than the noise margin

voltage it will get added to VOL max and exceed VIL max of next stage and cause false triggering. For high

level, VOH min will get reduced and be below VIH min of next stage

Other specifications are:

1. Supply voltage VCC

2. Operating free air temperature

3. Input voltages for logic 0 and 1 at output

4. Output voltages for logic 0 and 1 at output

5. Input currents for logic 0 and 1 at output

Resistor Transistor Logic (RTL)

It uses resistors and transistors. It was one of the earliest technologies and is not used now-a-days

Both transistors T1&T2 are in cut off then output Y is high (+VCC). Any one transistor with high input makes

the output low because that transistor will enter in saturation region. When both transistors are in saturation

the output is low. The collector resistor RC is called the pull up resistor. This is so because the output voltage

rises from 0 to VCC by the time constant = RC C where C is the capacitance due to base emitter junction

and stray effects.

Truth table for RTL gate:

A B Vout A B Vout

L L H 0 0 1

L H L 0 1 0

H L L 1 0 1

H H L 1 1 0

The truth table is that of a NOR gate. So RTL gate behaves like a NOR gate. Symbol for RTL gate is as

follows:

94

V (+)CC

A . B

A

B

YY1

Y2

A

B

A

B

Wired OR connection in RTL

V (+)CC

RC

Specifications of RTL gate:

Power Supply Voltage VCC 3.8 V

Noise Margin VNML 0.2 V

Fan-out FO 4

Propagation delay tpd 12 ns

Power Dissipation Pd 30 – 100 mW

RTL circuit for a NAND gate is

Wired-OR connection in RTL gate:

Y A B C D A B C D

Direct Coupled Transistor Logic (DCTL):

It is same as in RTL except that base resistances RB are not used in this case. Noise margin of this circuit is

very poor leading to false operation.

This type of circuit also performs NOR logic.

Current hogging problem in DCTL:

The chief deficiency of a DCTL gate is what is known as current hogging. When one gate is driving a

number of gates in parallel, it is likely that the gate which requires the least voltage for saturation may hog

or consume large current leaving inoperative the other gates. We know that the voltage required for

saturation of a BE junction, V is 0.75 V. Due to differences in manufacture or operational temperature

differences, VBE(sat) may be marginally different, say, 0.74 or 0.76. 0.74 junction may hog the current leaving

less current for other transistors to attain saturation.

95

T1

0.76

T2 T3

0.74 0.75

RC

VCC

T1P

RDA

DB

A

B

T2

RC

VCC

Y

V (A)1

V (B)2

D2

D1

V (Y)0

RL

VCC

G1

G2

Y

A

B

C

D

CCB2 B1 B3

C

V 0.74I I and I

R

For 0.76 and 0.75.

Diode Transistor Logic (DTL)

Figure shows 2 input NAND gates using DTL. This logic uses transistor and diodes.

Working of diode as AND gate:

Voltages Logic

V1 V2 V0 A B Y

0 V 0V +0.7 V 0 0 0

0 V +5V +0.7 V 0 1 0

+5 V 0V +0.7 V 1 0 0

+5 V +5V +5 V 1 1 1

This is the truth table of an AND gate. AND gate output is followed by NOT gate so DTL behaves like a

NAND gate.

Wired-AND connection in DTL gate:

Y = (AB)(CD) ( )( )A B C D

High Threshold Logic (HTL):

In high noise surrounding a HTL, DTL gate is used. But noise margins depend on supply voltage which

determines the High logic. By increasing the supply voltage better noise margin will be achieved.

96

A

B

C

D

VP

12K

Q1

Z

VZ 5K

Q2

3K

15KRC

V = 15 VCC

Y = ABCD

Q2

Q4

Q3

R3

Q1

D1

Input OutputD2

R4R2R1

+V = +5 VCC

Diode in DTL is replaced by a Zener diode whose breakdown voltage is 7 V.

It has 3 times propagation delay that of DTL delay. It has the highest noise immunity among all available

logic families. Fan out is restricted to about 10.

Transistor-Transistor Logic (TTL)

This logic gate developed as an improvement over the large propagation delay of DTL gate. With TTL we

can obtain inverter, AND, OR, NAND, NOR gates.

TTL inverter: The circuit is as follows:

Transistor Q1 plays a key role in the functioning of the circuit. D1 is called clamping diode, Q1 input

coupling transistor, Q2 a phase splitter, Q3, Q4 the output circuit referred to as totem pole arrangement as Q4

sits over Q3, and D2 is intended for Q4 to turn off when Q2 is HIGH.

Working of TTL as Inverter:

Input High: When the input is High at +5 V, the emitter-base diode of transistor Q1 is reverse biased and

cut-off. The base-collector junction of Q1 is forward biased and a current flows through R1 to the base of Q2.

This current is of the order of 0.7 mA and it saturates the base of Q2. This heavy current flow in the base

emitter of Q2 causes a voltage drop which forward biases Q3 and turns it on. The flow of IC in Q3 causes a

drop in VCC and the output is Low(0). Thus, we get a Low output for a HIGH input, an inverter action.

During this action as Q2 is on, the collector voltage of Q2 becomes low and keeps Q4 off.

Input Low: When the input is low the emitter-base diode of transistor Q1 is forward biased and is saturated.

Output of Q1 will be low and which will make Q2 in cut-off and finally Q3 also in cut-off so output will be

high.

97

A

B

C

D

Q2

Q4

Q3

Q1IN OUT

D2

V + 5 VCC

D3

INHIBITINHIBIT

A

B

C

Q1

R1

VBO

R2

RC

Q2

Q4

D2

Q3

CL

R3

+VCC

V =A.B.C0

TTL as NAND gate

A logical extension of the TTL inverter circuit is the TTL (T2L) NAND gate. In the TTL NAND gate, the

first transistor Q1 is manufactured as a multiple emitter transistor i.e. it has a single collector and base but a

number of emitters.

The truth table for NAND gate is

A B C Y

0 0 0 1

0 0 1 1

0 1 0 1

0 1 1 1

1 0 0 1

1 0 1 1

1 1 0 1

1 1 1 0

Wired-logic: The TTL gate outputs cannot be connected or paralleled as the two collector resistances come

in parallel and the output impedance is very low drawing heavy current. But an open collector output can be

ANDed. In open collector the collectors of transistors are tied and connected to the power supply through a

suitable resistor. This is called passive pull-up as against active pull-up. The purpose of wire ANDing is to

create an additional logic function as for instance.

Tri-state TTL

We saw that wired-OR circuits are not possible with TTL NAND circuit. In order to make it possible 3 state

(tri-state) TTL circuits have been developed.

Its truth table is

98

+ –0.4

VCC

ID

IB

V =0.7VBE

+–

+

V =0.3CE

INHIBIT IN OUT

0 V 0 V +2.5 V to +5 V

0 V +5 V 0 V

+5 V 0 V open circuit

+5 V +5 V open circuit

The INHIBIT input is called also as ENABLE . when the signal is LOW at IN and INHIBIT at the emitter of

Q1, both inputs are LOW and we get a HIGH output as an inverter. The same function gats inverted when

input is HIGH, INHIBIT is LOW. INHIBIT LOW is HIGH at emitter of Q1 so that both inputs being HIGH,

we get LOW output. If the INHIBIT input is HIGH at +5V, at the INHIBIT point it is low and diode D3 gets

forward biased shorting Q2 and cutting off Q3, Q4. So whatever the input at IN, the output is an open circuit

and high impedance.

TTL sub families

Many improvements have been carried out to the standard TTL studied previously. The improvements are in

speed and/or power consumption. This has given rise to TTL sub families. They are, apart from standard

TTL with 10 ns delay and 10 mW power consumption.

1. Low Power TTL

2. Low Power Schottky TTL

3. Schottky TTL

4. Advanced Low Power Schottky TTL

5. Advanced Schottky TTL

Low power TTL

The power consumption of a standard TTL is reduced to 1 mW per gate by increasing the value of all

resistances ten fold. . As we increasing the resistance reduces the speed (increased propagation delay) of

35 ns.

Low power Schottky TTL (TTLLS)

Schottky Transistor

A transistor can be fabricated with a Schottky diode connected between collector and base.

Circuit and symbol are shown in Figure. We know that the cut-in voltage of the Schottky diode is 0.4 V. If

the base to collector voltage exceeds 0.4, the diode conducts. This property can be used to prevent the

transistor from going into saturation as delay is mainly caused by minority carriers returning to equilibrium

conduction when emerging from saturated condition which will increase the speed of transistor.

Emitter Coupled Logic (ECL)

The logic circuits discussed so far was based on transistors driven into saturation for on. When switched off,

there was delay due to charge carriers having to be swept back into equilibrium level. Emitter coupled logic

overcomes this basic defect by operating the transistor in the forward active and cut-off modes. Hence ECL

99

Q1Input

Vi

RC RCIC1 IC2

+VCC

Q2

V01 V02

IE

VEE

Reference

VR

ABCD

has the fastest switching speed of all logic families, 1 ns. However, its power dissipation is high 20 mW per

gate and sensitive to temperature variations.

The basic circuit of ECL is the differential amplifier circuit with one input used as a reference. Hence it is a

voltage comparator circuit. Let us briefly review the differential amplifier. The circuit is

If instead of the reference voltage VR, the input voltage is V02 then the output responds to the difference of

input voltages Vd = V02 – V02. The sum of the currents IC1 + IC2 = F IE IE. If the difference voltage Vd

4VT = 100 mV = 0.1 V, then IC1 IE and V02 is maximum as IC2 0. The situation reverses if VD is negative.

If |Vd| 4VT, then IC2 is maximum and V01 is maximum as IC1 0.

If instead of V2 we have a reference voltage VR and if the input voltage Vi is above VR i.e., VR + 0.1

corresponding to HIGH input then Q1 is on, V01 is LOW. At the same time Q2 is off and V02 is HIGH. The

HIGH voltage input to Q1 switches from VR + 0.1 to VR – 0.1, HIGH to LOW.

When the input voltage swings from VR – 0.1 to VR + 0.1, current IE flows through Q2 switching it on. V02

becomes LOW and as Q1 is off V01 is HIGH.Thus we obtain the logic 1 and 0 simultaneously at the two

outputs V01 and V02 without the transistors going into saturation.

ECL gate as OR and NOR gate:

100

Current source

transistor

T1

T2

C1

C2

Multiplecollectortransistor

T1

T2

VS VS

T1

T2

VS

A

B

C

D

Wired-OR connection

By connecting the outputs of ECL gates wired-OR configurations are possible.

Characteristics of ECL gate:

1. Operation in the cut-off and active region without going into saturation makes ECL the fastest logic gate.

2. Due to differential amplifier circuit, the input impedance is high > 100 K and emitter follower output

gives it low output impedance = 15 . On account of this it has largest fan-out among bipolar devices.

3. Complementary outputs are available simultaneously.

4. Grounded operation of collector practically eliminates current spikes.

5. A number of logic functions can be derived by wired-OR connection.

6. Gate parameters do not degrade over long periods due to temperature.

7. By connecting a – 2 V supply to the output emitter and keeping them floating transmission of logic

symbols through short distance transmission lines of 50 is possible without deterioration.

Disadvantages of ECL gate:

1. Voltage swing between logic levels is small (0.8 V)

2. Noise margin is low (2 to 3 mV)

3. Power dissipation per gate is high, 60 mW

4. Level shifters are necessary to interface with other logic families.

5. Propagation delay gets increased by capacitive loading.

Integrated Injection Logic (I2L)

It is a bipolar technology capable of very high component densities. It has low power requirement and good

switching speed. I2L was developed to improve the performance of DCTL which suffered from current

hogging. It uses very little ‗real estate‘ with the result the density of components on the chip is the highest

among bipolar technologies (at least ten times that of TTL). It also consumes very little power and hence is

very much suited for medium and large scale integration.

101

VGG

T2

G

T1

GS

B

D

S

B

D

VDD

Output V0

InputVi

Vin

Vin

A

B

T1

T2

T3

V =–15VDD V =–15VDD

A B

T3

T2T1

V =Y=ABO Y=A+B

NAND gate NOR gate

FET/MOSFET and CMOS:

Comparison of p and n channel FETs

PMOS enhancement type has an advantage from the point of mobile positively charged ions within the

substrate tending to migrate towards the gate. NMOS has the advantage of higher mobility of electrons

compared to holes and consequently higher conductivity and low rd. This also makes for higher packing

density. A higher packing density means less junction capacitances making for faster switching speeds.

NMOS requires extensive process control in manufacturing which makes it costlier. Otherwise, NMOS is

used more widely than PMOS.

MOSFET as a switch: The basic circuit of a FET switch is as follows:

This is an enhancement NMOS switch circuit. The lower transistor T1 is called the driver and top transistor

T2 in totem pole arrangement is called the load. By earthing the substrate B of both FETs the substrate may

be common. In transistor T2, G and D are tied so that VGG = VDD.

To function as a switch the driver transistor T1 should be capable of high drain current so that the output

voltage becomes nearly 0 for LOW logic. On the other hand, the load transistor need to deliver only a low

current to function as a large resistor. This opposing requirement is met by proper ratio of channel length to

width.

MOS Logic Gates

From the fundamental function as an inverter by a PMOS or NMOS gate many other logic functions can be

derived.

PMOS Gates

102

G2

T2

D2

D1

V0

S2

+VSS

p channel

n channelG1

T1

Vi

The logic as we have noted in the case of inverter is negative, Transistor T3 is always conducting with its

gate tied to drain and forms a low resistance of 2 K.

Both A and B LOW (0): If the input voltage is less than –4 V, say, VIL = 2.0 V, both transistors are off. The

output voltage will be HIGH (1) – 15 – (–4) = – 11 V.

Either A or B LOW and the other HIGH : The LOW input transistor will be off and prevent the other

HIGH input transistor becoming on as the circuit will not be through, the transistors being in series.

Both A and B HIGH: With the inputs being HIGH at – 11 V, the transistors go into saturation. The output

voltage becomes LOW at about – 2V vide Figure 83. The truth table is

A B Y

0 0 1

0 1 1

1 0 1

1 1 0

This is the truth table of a NAND gate.

NOR gate

The operation is opposite of the NAND gate; the two transistors T1 and T2 are parallel connected. When

both T1 and T2 are off, the output is HIGH. Either one or both on with a HIGH input, the output is LOW.

The truth table is

A B Y

0 0 1

0 1 0

1 0 0

1 1 0

This is the truth table of NOR gate.

CMOS Gates: In the discussion on PMOS and NMOS circuits we made out that PMOS goes to the on state

if VGS is greater than – 4 the threshold voltage and NMOS switches on if VGS is greater than +4 V. So while

PMOS requires a negative going input, NMOS requires a positive going input. So by a complementary set of

PMOS and NMOS we can alternately switch on and off the two transistors with a single pulse. Such a

device is CMOS- Complimentary Metal-Oxide Semiconductor.

CMOS circuit is used with the gates and drains tied respectively to each other of p and n channel so that

even during manufacture they are interconnected and common lead brought out. To S of p channel is

connected the supply voltage and S of n earthed so that supply voltage is designated as VSS.

CMOS inverter

103

When Vi VT(p), T1 is off. If Vi VT(n) is 2V (logic 0) and VSS = 15 V, VGS(2) will be – 13 V and so T2 will

be on. The output voltage will be VSS, logic 1. if input voltage is Vi VSS – VT(p), T2 will be off and T1 will

be on. That is, HIGH voltage for logic 1 input should be 15 – 4 = 11 so that VGS(2) < 15 – 11 4.

For HIGH input T1 will be on and driven to saturation so that V0 is LOW. Thus we obtain inversion

Input/ logic Output/ logic

< 4V 0 VSS 1

> 11 1 + 0.5 0

Comparison of Logic families:

TTL ECL COMS

Series

54AS/74

AS

Series

54ALS/74ALS

Series

10000H

Series

III

Series

74HC

Typical values of

output voltage

High, V

Low, V

3.5

0.5

3.4

0.4

–0.9

–1.75

–0.9

–1.75

VCC = –0.1

0.1

Noise margin

Low, V

High, V

1.15

1.95

1.15

1.95

0.27

0.23

0.2

0.2

2.3

2.7

Fan out 33 20 90 70 More than 50

Supply voltage, V 5 5 –5.2 5.2 6

Power dissipation

per gate 8 mW 1.2 mW 25 mW 60 mW 2.5 W

Propagation delay

time, ns 1.7 4 1.1 0.7 8

104

Study material for Microwave

Transmission line:

Transmission line may be thought of as a device for transmitting or guiding electrical energy from one point

to another. It guides electrical energy from one place to another place. It is a transmission medium between

transmitting station and antenna.

Application of transmission line:

1. One of most common and widely used applications of transmission lines is to carry RF power from one

point to another.

2. Other application of RF transmission lines is a pure reactive element.

3. The third application of transmission line is in impedance transformation.

Types of transmission line:

Open wire line.

Co-axial cable

Two-Wire Transmission Line

For a two wire transmission line with conductors of radius a and conductivity c with distance between the

two conductors as d in a medium of parameters , and , the capacitance per unit length at high or low

frequencies is expressed as:

1

& ( )ln ( / )

cosh2

C C a dd d a

a

1cosh2

dL

a

ln ( )

dL a d

a

(At high frequencies)

2

2

c

Ra

(At low frequencies)

Characteristics impedance of open wire line:

1. The characteristic impedance (ZO) of two conductor parallel wire line with air dielectric is given by:

0 10

276 2log

SZ

dK

Ohm

S Center to Center spacing of conductors

d Diameter of each conductor

K Dielectric constant

2. It has characteristic impedance of the order of 150 – 600 ohm.

3. It is balanced transmission line.

4. It is used in frequency range of less than 1 GHZ.

5. Problem in this transmission line is of radiation loss, and which occurs at high frequency.

Characteristics impedance of Co-axial cable:

1. The characteristic impedance (Z0) of a Co-axial cable is given by

105

ac

b

0 10

138log

DZ

dK

D Inner diameter of outer conductor

d Outer diameter of inner conductor

2. It has characteristic impedance of the order of 40 to 150 ohm.

3. It is an unbalanced transmission line.

4. It is used in frequency range less than 18GHZ, but more than 1 GHZ.

5. In co-axial cable radiation losses are smaller but dielectric losses are higher.

6. Possible modes in co-axial cable are TE, TM and TEM.

All two conductor lines including co-axial lines belong to TEM mode. Power flows along and between

conductors.

Coaxial transmission line

For a coaxial line as shown in Figure the capacitance C per unit length

2

ln

Cb

a

The inductance of the coaxial transmission line is

0 ln2

bL

a

Balun: There is a balancing transformer called, BALUN, Connected between the feeder and receiver input.

Typical example of such a case is 300 ohm line feeder feeding a 75 ohm Television receiver input.

Transmission line losses:

There are 3 major sources of losses in transmission lines.

1. Copper losses (known as I2R loss)

2. Dielectric loss.

3. Radiation loss.

Copper loss is due to resistance associated with the conductors constituting the transmission line. This loss

appears in form of heat, and is frequency dependent and increases with increase in frequency.

Dielectric loss also appears in the form of heat and increases with increases in frequency. This loss is due to

leakage through dielectric.

Radiation loss is due to radiation of RF power to free space or heat by circuits. It is negligible in shielded

transmission line such as co-axial cables.

Equivalent circuit of a transmission line:

A transmission line can be assumed to be made of R, L, G & C.

106

R L R L

G C G C

L L L

CCC

Value of R, L, G, & C are taken as per unit length.

But at high frequency, effect of R & G is neglected and, transmission can be assumed to made of L & C

only.

Generally high frequency transmission line can be assumed as an ideal transmission line.

Propagation-constant: Propagation constant (P) of a propagating wave along a transmission line can be

mathematically expressed as

.P Z Y

Where Z is the distributed series impedance per unit length and Y is the distributed shunt admittance per unit

length

( )( )P R jwL G jwC j

= Attenuation constant Neper / km

β= Phase shift constant Rad / km

Here, β and P are known as secondary constant, while R, L, G & C are known as primary constant.

Characteristic Impedance (Zo) of transmission line:

Zo of a transmission line is its input impedance if it was infinity long. Thus the Zo of line is its characteristic

parameter and is independent of line length. Also a finite length of a line terminated in its Zo will have its

input impedance equal to characteristic impedance.

Conceptually the Zo of a line depends upon geometry, spacing and size of conductors and also on dielectric

separating the conductors.

Zo of a transmission line is secondary parameter.

Value of & β in a transmission line:

1. 0

02 2

GZR

Z

2.

21

18

R GLC

L C

Lossless transmission line:

107

0

1. 0

2. 0 Zero attenuation

3. is a linear function of frequency

14.

5. is a purely resistive

p

R G

LC

vLC

LZ It

C

Distortion in transmission line:

Distortion is due to and β. If is a function of frequency and β is non linear function of frequency, then

distortion will be present.

For distortion less transmission β must be a linear function of frequency.

21

18

R GLC

L C

So here R G

L C is essential condition for distortion less transmission line.

Loading in transmission line:

To maintain distortion less condition value of Inductors are increased, and which is called as loading in

transmission line.

Various parameters in distortion less transmission line:

0

1. function of frequency

2. is a linear function of frequency

13.

4. is a purely resistive

p

RG is

LC or phase shift

vLC

L RZ It

C G

So if only transmission line is purely resistive then it may be either distortion less or lossless transmission

line.

Input impedance of a transmission line:

Input impedance of a loss less transmission line of length l which is terminated with load ZR and

characteristic impedance is Z0

00

0

tan

tan

Ri

R

Z jZ lZ Z

Z jZ l

Short circuited transmission line: If Transmission line is of characteristic impedance Z0

Zi l

108

Input impedance of a short circuit transmission line is 0 tanSCZ jZ l

If l < /4 then Short circuited transmission line behaves like an Inductor.

If l = /4 then Short circuited transmission line behaves like a tank circuit or parallel tuned circuit.

If l > /4 then Short circuited transmission line behaves like a capacitor.

Open-circuited transmission line:

Input Impedance of __________________________

An open circuited

Transmission line is

0 cotSCZ jZ l

Zi l

If l < /4, then open circuited transmission line behaves like a capacitor.

If l = /4, then open circuited transmission line behaves like a series tuned circuit.

If l > /4, then open circuited transmission line behaves like an inductor.

/4 length transmission line:

ZO ZR 2

0i

R

ZZ

Z

Zi l = /4

Quarter wave length transmission line behaves like an impedance inventor if ZR is purely reactive element

Quarter wave length transmission line is used for impedance transformer if load is purely resistive only.

So /4 length transmission line works like a step up and step down transformer if load is purely resistive

network.

/2 Length transmission line:

i RZ Z

i.e. Load is reflected at the input side. ZR

Zi l = /2

/8 Length transmission line: This transmission line is used for Maximum impedance transformation here

input impedance is equal to characteristic impedance of transmission line. Bu load must be purely resistive

only.

Reflection in case of transmission line:

When a transmission line is terminated in load impedance which is not equal to its characteristic impedance

part of signal energy sent down the live is reflected back. The ratio of reflected signal amplitude to incident

one is defined as Reflection coefficient (K) at load -end.

0

0

L

L

Z ZK

Z Z

109

0

1. 0 1

2. 0

3. 1

L

L

L

If Z then K

If Z Z then K

If Z then K

So Value of reflection coefficient varies between – 1 and + 1.

Standing wave formation:

whenever a signal traveling along a transmission line comes across a discontinuity or whenever line is

terminated in a load other than Zo of line a part of whole of incident energy is reflected back. Under such

circumstances we have two counter propagating waves in transmission line.

At all those points where they add produce a signal maximum and at all those points where they are out of

phase they produce a signal minimum.

Thus we have points of signal maxima and signal minima along the live except for the case where there is no

discontinuity and where the line is terminated in its characteristic impedance. Since these points of signal

maxima & minima are motionless, standing waves are said to exist.

Standing wave Ratio:

VSWR on abbreviation for voltage standing wave Ratio is the ratio of max minE to E .

It is a measure of the mismatch at the discontinuity. Higher is the value of VSWR indicator greater

mismatching between load and line.

1

1

KVSWR

K

Value of VSWR lies between 1 and ∞. VSWR has minimum value when line is terminated with

characteristic impedance. While VSWR has maximum value when line is terminated with either open circuit

or short circuit.

VSWR for purely resistive load:

0

0

00

0 Characteristic impedance of line

LL

L

L

L

RVSWR If R R

R

RVSWR If R R

R

R

R Load

Transmission coefficient for transmission line:

K + 1 = T

K Reflection Coefficient

T Transmission coefficient

Note:

1. Reflection Coefficient varies from -1 to + 1.

2. Voltage standing wave ratio various from 1 to ∞

3. Transmission coefficient various from 0 to 2.

4. Input impedance various from 0 to ∞

ZO ZL

110

Stub Matching:

A stub is a shorted or open transmission lines section used in conjunction with transmission line to provide

impedance matching and cancel out reflections.

Introduction of stub does not absorb any power because both short circuit and then circuit stub are purely

reactance.

Impedance transformation from stub:

If ZR is complex load and one want to get impedance match between transmission line and load then Point P

on line is designed in such a manner that normalized admittance seen at point P is 1+Jx. This is decided by

value of LS. Now length of stub (lt), which is connecting in shunt is designed in such a manner that it cancel

out the effect of Jx, and presence an admittance of – Jx. Thus there is a perfect match.

A stub a can also be added in series by cutting the main transmission line.

Generally short circuit stubs are preferred because open circuit stubs have radiation problems.

Limitations of single stub matching:

Length of stub and location of stub are frequency dependent, so it can be used at fixed frequency only.

So single stub matching is a narrow band system.

Some time it is impractical to put the stub at the located point. It is quite often so in co-axial lines and

waveguides.

Double stub tuner: For Transmission load matching over r a range of frequency it is best to use it. In double

stub matching two stubs are put across the line at fixed point spaced 3 / 8 apart or even closer.

These stubs have adjustable shorting plungers and these plungers can be adjusted to cancel out most

reflections.

Smith Chart:

It is a plot of normalized impedance or admittance with angle and Magnitude of a generalized reflection

coefficient in a unity circle. It is valid for lossy as well as loss less transmission line. It is also valid for wave

guide. It contains 2 set of lines. The first set of line referred to as constant resistance lines and other set of

line is referred as constant reactance lines.

Constant Resistance circle: (R-circle)

2

2 2 1( )

1 1x y

RK K

R R

Where xK and yK are on x & y axis respectively. Above equation represent family of circles.

Here each circle represents a fixed resistance i.e. All the points on a particular resistance circle represent the

same resistance. The resistance which there circles represent are marked on horizontal diameter at the points

of intersection of these circles and horizontal line. The outermost resistance circle cutting the horizontal line

on left extreme represents zero resistance .The centre of horizontal diameter is labeled as 1. There 1 means

ZO, because all impedances on this chart have been normalized with respect to Z0.

111

Centre of R circle is: , 01

R

R

Constant Reactance circles: (X-circle):

These lines are not complete circle like the lines of constant resistance but are arcs of complete circles.

The lines in upper half represents positive reactance, while those in lower half represents negative reactance.

So if we superimpose R- circle & X-circle then we will get a smith chart.

Important points about smith chart:

1. A complete revolution about smith chart represents a distance of /2 odd lines.

2. Clock wise movement of chart is regarded as moving towards generator or away from load.

3. Since /2 distance or line corresponds to a movement of 3600 on chart, while a distance corresponds to

7200 movement on chart.

4. Smith chart can be used as both impedance and admittance chart.

5. The circle on smith chart represents normalized constant or conductance while the arcs of circles on

smith chart represent normalized reactances or susceptances.

Application of Smith chart:

(1) SWR for any given transmission line load Mis-match.

(2) Magnitude and angle of reflection coefficient.

(3) The length of short circuited transmission line section of known Z0 to provide the desired input

impedance.

Wave guide:

A waveguide does the same job at microwaves which transmission lines usually do at relatively lower

frequencies. It is a conducting tube through which energy is transmitted in form of electromagnetic waves.

Waveguide is an alternative of transmission line at high frequency especially at microwave frequency.

Waveguide is nothing but conducting medium in which energy is transmitted in form of electromagnetic

waves. The waveguide can assure any shape theoretically but analysis of irregular shape becomes difficult.

Two popular types of waveguide are rectangular and circular.

Generally a rectangular waveguide is characterized by its wide dimension and narrower dimension. A

circular waveguide is characterized by its internal diameter (d).

Generally waveguides are used above 1 GHZ.

Comparison of wave guide and transmission line:

Waveguides is a high pass filter while transmission line is an all pass filter.

In waveguide propagation is in terms of E&H, while in transmission line it is in terms of V&I.

Waveguide does not support TEM mode while transmission line support TEM mode.

D.C. does not pass through waveguide, while in transmission D.C. can pass.

Propagation in rectangular waveguide:

112

In case of rectangular waveguide wave transmits by successive reflections from inner walls of tube. Here

conduction of energy does not take place through conductive walls but through dielectric medium which is

generally air and that‘s why waveguides are less lossy. Here wave is sent in such a manner that field is

minimum at walls but maximum at centre.

Phase and Group velocity:

Group velocity is the velocity of propagation of modulated signal envelop in a waveguide. While phase

velocity is the velocity of propagation of phase of carrier. Group velocity is less than velocity of light while

phase velocity is greater than velocity of light.

2

1

p c

c

cv f Cut off frequency

f

f

2

1 cg

f Frequency of operation

fv c

f

Waveguide modes:

There will be infinite no of possible electric and magnetic field configuration inside the waveguide, if there

was no upper limit for the frequency of signal to be transmitted. Each of these filed configurations is called a

mode.

TE and TM modes:

In TE-Mode the electric field of lines are entirely transverse to direction of propagation of E.M. wave.

Whereas magnetic field has a component along direction of propagation.

In TM mode the magnetic field of lines are entirely transverse to direction of propagation of E.M. wave

where as electric field has a component along direction of propagation.

Significance of TE mn Mode:

m indicates no of half wave various of electric field in wide dimension of waveguide where as the second

script n, indicates the no. of half wave various of electric field along narrow dimension of waveguide.

Possible modes in TE mn and TM mn:

TE mn : m = 0, 1, 2, 3, 4 …………….

n = 0, 1, 2, 3, 4 ……………..

Values of m and n can not be zero together. So TEoo mode is not possible.

TM mn : m = 1, 2, 3, 4, ………….

n = 1, 2, 3, 4 …………..

So here TM mo and TMon modes are not possible.

Rectangular parallel plane waveguide / Guided wave:

The cut- off wavelength (c) of a rectangular parallel plane waveguide with broader dimension (a) is given

by 2

c

a

m where m is no of half wavelength of electric intensity to be established between the walls.

The largest value of cut-off wavelength = 2a

Propagation constant in rectangular waveguide:

113

Propagation constant (P) is given by 2 2 where P j cP

2 2

2

c

m n

a b

Attenuation constant Neper / km or dB / km

β Phase shift Radian / km

Observation in wave guide:

1. If < c then P will be real and wave will have only attenuation. This type of mode is called as

Evanescent mode. In this mode waveguide behaves like an attenuator.

2. If > c then P will be imaginary and wave will have only phase shift. Here waveguide behaves like

a high pass filter.

The modes that can exist and sustain in a waveguide are decided by frequency of propagating signal in

addition to waveguide dimension.

Each waveguide mode is characterized by a cut – off frequency and if frequency of propagating signal is

greater than cut off frequency then wave will propogate and modes will sustain in waveguide.

TEM- mode does not exist in hollow waveguide but it is the dominant mode of co-axial cable.

Dominant mode in rectangular waveguide:

In case of rectangular waveguide dominant mode is TE10. Dominant mode has the highest cut – off

wavelength and the lowest cut – off frequency. This mode has the lowest possible distortion.

10( ) 2c r rTE a where a is broader dimension of wave guide

10( ) 2c TE a For free space

10( )2

c

r r

cf TE

a

10( )2

c

cf TE For free space

a

Relation between 0 , &c g :

The wavelength of propagating signal inside the waveguide is different from what it is in free space.

For a given waveguide the two wavelengths are inter-related by:

2 2 2

0

0

1 1 1

Free space wave length

Cut off wave length

Guided wave length

c g

c

g

Circular waveguide:

Except formulas of cut- off wave length and cut - off frequency all formulas remain same.

For TEnm 2

'c

nm

a

P

a Radius of Circular waveguide.

114

For TM nm 2

c

nm

a

P

'nmP & nmP are solutions of Bessel function equation.

Possible modes in circular waveguide:

TEnm n = 0, 1, 2, 3………….

m = 1, 2, 3 ………….

So in case of circular waveguide TE10, TE20, TE30………. Tno are not possible. Lowest possible mode is

TE11. In case of circular waveguide dominant mode is TE11.

De generates modes: For different value of m&n if modes have same cut-off frequency then these modes

are called degenerate modes. In case of rectangular waveguide TEmn & TMmn are degenerate modes.

Important points about circular waveguide:

1. If a rectangular waveguide operating in dominant mode is gradually deformed into circular cross section

in direction of propogation then mode at output is TE11.

2. Ratio of cross section of circular waveguide to rectangular waveguide for same cut off frequency in

dominant mode is around 2.19.

3. Advantage of circular waveguide is its symmetrical nature.

Cavity resonator:

At high frequency RLC, circuits are insufficient for resonance due to large size of circuit are replaced by

cavity resonator. Cavity is Band pass Filter.

Formation of Cavity:

Cavity is a rectangular waveguide shorted at both ends. Here energy oscillates from entirely electric to

entirely magnetic twice per cycle. So after every quarter time period energy is transferred from electric to

magnetic and magnetic to electric.

Possible modes in rectangular cavity:

TE mnp : m = 1, 2, 3 ………

n = 1, 2, 3, …………

p = 0, 1, 2, 3,……….

TM mnp : m = 0, 1, 2, 3,……….

n = 0, 1, 2, 3,……….

p = 1, 2, 3,……….

Dominant modes in rectangular cavity:

In general case, dominant mode is TE101.

So lowest possible mode or dominant mode in case of a cubic resonator of side (a) is

fc (101) = (c/a√2)

Cylindrical cavity:

d

If d ≥ 2a then dominant mode is TE111 mode.

115

Cylindrical cavity is used in wave meter and mode used for it is TE010, which is not dominant mode.

Equivalent circuit for cavity:

So a cavity can be represented by LCR circuit

Cavity resonators are used as turned circuits at Microwave frequencies. The cavity suffers from

disadvantage that their resonant frequencies are harmonically related and it is a serious draw back in

situations where pulses of energy are fed to cavity.

Waveguide coupling:

Probe coupling and loop coupling are the two commonly used waveguide coupling techniques. Both probe

and loop coupling can be used to launch a particular mode in the waveguide. Probes couple primarily to an

electric field, while loops couple to a magnetic field.

Waveguide diaphragm / Apertures / Irises:

Waveguide diaphragms or apertures or irises are used to provide impedance matching in waveguides as

stubs in case of transmission lines. The diaphragm may be capacitive, inductive or resonant.

Waveguide posts:

Waveguide posts are also used for impedance matching. A cylindrical post extending into the waveguide

from one of the two broadsides has the effect of providing a lumped reactance at that point. The reactance

provided by post may be inductive or capacitive depending upon how for it extends into waveguide. Posts

are easier to adjust as compared to diaphragms.

A coupling probe is places at a point where electric filed has maximum intensity. A coupling loop is placed

at a point of maximum magnetic field intensity.

Antenna &wave propagation:

Antenna is a region of transition between guided wave and free space. An antenna is a passive device and

behavour like an inductor.

116

Hertzian dipole / Short antenna / short dipole:

1. Near filed or Fresnel zone 1/r2

2. Far field or Fraunhoper zone 1 /r

For field represents radiation of antenna and contributes flow of energy away from source. Near field or

induction field represents energy started in magnetic field. Both near and for field are equal at r = ( /2)

Value of E and H in case of hertzian dipole:

The value of fields at point pare calculated.

sin

2

sin120 .120

2

sin.60

m

m

m

I dlH

r

E I dlE

H r

I dlE

r

Effective height of antenna:

In case of antenna effective height is different from its actual height. But in case of sort antenna actual

height & effective height are same.

.

avg

e

m

avg

m

m

Il l

I

I Aveg valueof current

I Max valueof current

l Actual height of antenna

1. In case of linear current distriubution

2

maveg

II

2

avg

e

m

I ll l

I

2. In case of sinusoidal current distribution

2 maveg

II

2avg

e

m

I ll l

I

Vertical grounded antenna:

Radiation resistance of vertical grounded antenna is

117

2

2

2

2

160

1& 1

160

e rr

r

r r

er

lR

For free space

lR

This vertical grounded antenna is used for transmission at medium frequency. Power radiated by an antenna

is 2

rms rW I R

Value of electric field radiated by vertical grounded antenna:

Value of electric field at a distance r by an antenna which radiates power rW

90 r

rms

WE

r so

1rmsE

r

Value of electric field at a distance r by an isotropic antenna which radiates power tW

30 t

rms

WE

r so

1rmsE

r

Note:

1. Radiation resistance of / 2 dipole is 73.0 ohm

2. Radiation resistance of / 4 monopole is 36.5 ohm.

Important formulas of antennas:

1. Radiation intensity ( , )U : It is nothing but power per unit solid angle.

2

2

( , )4

44

avg

avg

Total powerU

Pr

P r

2. Average value of Radiation Intensity is given by

4

( , )sin

radavg

rad

PU

P U d d

3. Directive gain (Gd): It is ratio of radiation intensity to average value of radiation intensity.

( , )( , )d

avg

UG

U

4. Directivity(D): It is the value of antenna is given as maximum value of directive gain

maxmax

( , )( , )d

avg

UD G

U

5. Power gain(GP):

4 ( , )P

in

UG

P

118

6. Directive gain (Gd):

4 ( , )

d

rad

UG

P

7. Efficiency of antenna:

Radiation Resistance

Ohmic loss in antenna

p rad r

d in r l

r

l

G P R

G P R R

R

R

Radiation equation:

Tx Rx

d

If Pt is power transmitted by antenna and Pr is power received by antenna. If Gt and Gr are gain of Tx and Rx

antenna.

2

x x

4

operating wavelength

distance between T and R antenna

(P ) -(P ) 22 20log (G ) (G )

r t t r

t dB r dB t dB r dB

P PG Gd

d

d

Isotropic antenna:

1. Gain value is 1.

2. Gain value is 0-dB

3. Ae = 0.082

Hertzian- dipole:

1. Gain value is 1.5

2. Gain value is 1.76 dB

3. Rr = 802 (l/)

2

4. Ae = 0.12 2

/ 2 dipole antenna:

1. Gain value is 1.64

2. Gain value is 2.15 dB

3. Radiation resistance is 73.0 ohm

4. Ae = 0.13 2

/ 4 monopole antenna:

1. Gain value is 3.28

119

2. Gain value is 5.15 dB

3. Radiation resistance is 36.5 ohm

4. Ae = 0.26 2

Radiation resistance of Loop antenna:

Rr = 320 4 (A /

2)2

Here A is area of loop antenna

A = r2: r is radius of Loop.

If Loop antenna has N- turns then

Rr = 320 4 N

2 (A /

2)2

Rr N

2

Directivity of antenna:

4

HPBW

d

HPBW

G

where &

HPBWHPBW are in Radians.

41000

HPBW

d

HPBW

G

where &HPBWHPBW are in degree.

Gain of antenna:

Directivity of antenna is

2

4eD A

Here is operating wavelength and Ae is the effective area of antenna.

Q. Calculate the gain of an antenna with a circular aperture of diameter 3 meter at a frequency of 5

GHz.

Sol: 2

2 2

4 4 3: 24649

(3 / 50) 4eSol D A

Parabolic Reflector:

In case of parabolic reflector effective area is around 60% of total area.

G = 6 (D /)2 G Gain of parabolic antenna

D Diameter of antenna

Wavelength.

140BWFN

D

In degree

70BWFN

D

In degree

Note: From above formula it is clear that by increasing the frequency value of decreases which means

BWFN decreases and value of directivity increases.

Aperture number:

Ratio of focal length to diameter is f/D = 0.25 Cot / 2

Generally f/D is between (0.2 – 0.5)

Antenna array:

120

By putting antenna in a proper arrangement one can get directivity of antenna in desired direction this

arrangement is called antenna array.

Advantages of antenna array:

1. Broad side array:

It is a linear array of n, isotropic point sources, which have some magnitude and phase. In broad side array

maximum radiation will be perpendicular to axis AA‘, while mill radiation will be along AA‘. To avoid

grating to be /2

1 2 3 4

A A‘

2. End fire array:

It is a linear array of n, isotropic point sources, which have same magnitude but out of phase in this case end

fire array maximum radiation will be along axis AA‘, while null radiation will be perpendicular to AA‘.

Broad side array:

1. Length of array = (n – 1) d n.d

2. HPBW=057.3

/L Here L = n.d

3. BWFN =0114.6

/L

4. Directivity = (2L/)

End fire array:

1. HPBW= 0 2

57.3/L

2. BWFN=0 2

114.6/L

3. Directivity=(4L/)

Note: The BWFN for End fire array is always greater than that of Broad side array. So For End fire array

under identical conditions the range of transmission is less and the area of coverage is more than that of

broad side array.

Antenna&wave propagation:

Different modes of propagation for EM wave are:

1. Ground wave propagation (< 2MHZ)

2. Ionospheric wave propagation (2 – 30 MHZ)

3. Line of sight wave propogation (>30MHZ)

4. Tropospheric / Beyond the horizon (>300MHZ)

Ground wave propagation:

It is also known as medium wave propogation. It is generally used for broadcasting medium wave & long

wave. Ground wave propagation is a wave which is guided along earth as in case of a waveguide.

Ground wave propogation is produced by vertical antenna only (which is vertically polarized) because

horizontal component of EM wave will be short circuited and i.e. why horizontal antenna is not used.

121

Ground wave propagation cannot be used at a frequency which is greater than 2MHZ due to high

attenuation.

Sky wave propagation:

Ionosphere is made up of 4 layers D, E, F1 & F2. In night both D&E layers will disappear & F1 & F2 will

merge together. So at night only 1 layer F layer is present.

Concept of sky wave propagation:

Ionosphere has free electrons and ions. Due to EM wave dielectric constant will be reduced and it will cause

reflection of EM wave from ionosphere. If any wave which has a frequency greater than 30 MHZ will

penetrate ionosphere and transmit the ionosphere.

R.I=2

811

N

f

Q. Calculate the value of frequency at which an EM wave must be propagated through the D region

with an index of refraction of 0.5 and an electron density of 4 33.24 10 /electrons m

Sol: 4

2 2

81 81 3.24 101 0.5 1

N

f f

1870.61f Hz

Maximum Usable Frequency and Critical frequency:

Critical frequency is the highest frequency reflected by ionosphere at vertical incidence but it is not the

highest frequency for general angle of incidence.

Critical frequency max9cf N

Here Nmax is the maximum ionic density.

2

sec 12

Skip distance

Virtual height

c c

DMUF f i f

h

D

h

Skip distance is the minimum value of distance for a particular value of incident angle.

Fading: Fluctuation of signal strength at receiver end is called as fading. Fading occurs at high frequency

and can be overcome by:

1. Automatic volume control

2. Frequency density

3. Space density

122

Space wave propagation:

Radio- horizon = 4.12 th km where th is in meter.

Radio horizon=4

3 Optical horizon

Distance between X XT and R antenna is

4.12 4.12 &t r t rh h Km where h h arein meter

Electric field at receiver antenna is

2

2

88

1

t rR

R t r

R

P h hE

d

E h h

Ed

Transpospheric scattering wave propagation:

It is used at VHF & UHF frequency range.

Super refraction or ducting:

It is used in frequency range from (300 MHZ 30 GHZ)

Microwave:

Microwave is the range of frequencies which lies between 1-300GHz. Microwaves are the wavelengths

which are measured in centimeter. Microwave is a signal which has wavelength less than 1Foot. Microwave

signal Contains UHF, SHF&EHF.

Important frequency range:

1. Medium wave: 300 KHz-3MHz.

2. Short wave:3MHz-30MHz

3. Very High Frequency:30MHz-300MHz

4. Ultra High Frequency:300MHz-3GHz

5. Super High Frequency:3GHz-30GHz

6. Extra High Frequency:30GHz-300GHz

Microwave frequency band:

1. L-band: (1-2 GHz)

2. S-band:(2-4 GHz)

3. C-band:(4-8GHz)

4. X-band:(8-12GHz)

5. Ku-band:(12-18GHz)

6. K-band:(18-27GHz)

123

Microwavesource

Wavemeter Calibratedattenuator

Transmittinghorn antenna

Receivinghorn antenna

Output tooscilloscope orpower meter

Waveguidetermination

Crystalmount

Waveguides

StandStandStand

7. Ka-band:(27-40GHz)

Advantages of microwave:

1. Ability to use high gain antenna

2. Ability to use high directive antenna

3. Greater privacy or secure communication

4. Increased value of bandwidth

Applications of microwave:

1. Used in telecommunication

2. Used in Radar

3. Used in Electronic Count Measure(ECM)

4. They are also used in industrial, scientific and medical field.

Microwave systems:

A microwave system normally consists of a transmitter subsystem, including a microwave oscillator,

waveguides, and a transmitting antenna, and a receiver subsystem that includes a receiving antenna,

transmission line or waveguide, a microwave amplifier, and a receiver.

Figure shows a typical microwave system.

In order to design a microwave system and conduct a proper test of it, an adequate knowledge of the

components involved is essential. Besides microwave devices, the text therefore describes microwave

components, such as resonators, cavities, microstrip lines, hybrids, and microwave integrated circuits.

Microwave measurement:

The quantities required to be measured in microwave circuits are frequency, power and impedance.

Microwave frequency measurement is done by slotted line, resonant cavities

And transfer oscillator. Microwave measurement uses bolometer and microwave power

meters. Microwave impedance measurement is done by measurement of reflection coefficient and

VSWR.slotted line and probe is a basic tool for these measurements.

Measurement of power:

1. Low power (0.01 mw-10mw): For low power measurement Bolometer or thermocouple method is used.

Concept of bolometer is based upon change of resistance with respect to power. If positive temperature

coefficient of resistance then Barretrs and for negative temperature coefficient of resistance thermistor is

used.

124

2. Medium power(10 mw-1w)&high power(>10w): For this power measurement

Calorimetric wattmeter method is used.

Measurement of VSWR:

1. Low value of VSWR(S<10): Instrument used for low value of VSWR is VSWR meter.

2. High value of VSWR(S>10): For high value of VSWR method used is double minima method.

Measurement of Impedance:

1. High value of Magic-T

2. By slotted line method

3. By reflectometer

Calculation of VSWR by reflectometer:

If Pi is the incident power and Pr is the reflected power in case of reflectometer then value of reflection

coefficient will be

K= (Pr/Pi)1/2

Once value of reflection coefficient is known then one can calculate VSWR.

Wave meter:

It is a single cylindrical cavity .It uses a short circuit plunger which can change the resonance frequency of

cavity by changing its length. Most suitable mode for cylindrical

Cavity is TE011due to its higher value of quality factor.

Scattering parameter(S-Matrix):

At microwave frequency elements behave like distributed elements so measurement of voltage and current is

not possible and i.e. why impedance parameters are not possible

but at microwave frequency direct measurement of power is possible and i.e. why scattering matrix is

possible. These parameters are called scattering parameters because of multiple reflections at microwave

junction.

Properties of S-matrix:

1. It is a square matrix

2. It is a symmetric matrix

3. It is unitary matrix

Waveguide Corners, Bends, and Twists:

The waveguide corner, bend, and twist are shown in Figure. These waveguide components are normally

used to change the direction of the guide through an arbitrary angle.

In order to minimize reflections from the discontinuities, it is desirable to have the mean length L between

continuities equal to an odd number of quarter-wavelengths. That is,

2 14

gL n

where n = 0, 1, 2, 3, …., and g is the wavelength in the waveguide. If the mean length L is an odd number

of quarter wavelengths, the reflected waves from both ends of the waveguide section are completely

canceled. For the waveguide bend, the minimum radius of curvature for a small reflection is given by

Southworth [2] as where a and b are the dimensions of the waveguide bend as illustrated in

125

1 2

3H-PLAN TEE

...................

.

.

...................

.

.

L

a

b

R L

(a) (b) (c) (d)

Tee junctions:

In microwave circuits a waveguide or coaxial-line junction with three independent ports is commonly

referred to as a tee junction. From the S-parameter theory of a microwave junction it is evident that a tee

junction should be characterized by a matrix of third order containing nine elements, six of which should be

independent. The characteristics of a three-port junction can be explained by three theorems of the tee

junction. These theorems are derived from the equivalent-circuit representation of the tee junction. Their

statements follow:

1. A short circuit may always be placed in one of the arms of a three-pot junction in such a way that no

power can be transferred through the other two arms.

2. If the junction is symmetric about one of its arms, a short circuit can always be placed in that arm so that

no reflections occur in power transmission between the other two arms.

(That is the arms present matched impedances.)

It is impossible for a general three-port junction of arbitrary symmetry to present matched impedances at all

three arms

Waveguide junctions:

Waveguide junctions are used whenever it is desired to combine two or more signals into one or split a

signal into two or more components in a waveguide system. The commonly used waveguide junctions

include T junction and hybrid junctions .E-plane Tee junction and H-plane Tee junctions are the two popular

Tee junction. In E-plane Tee all the arms lie in plane of electric field while all the three arms of H-plane Tee

lie in plane of magnetic field.

H plane Tee:

It is formed by cutting a rectangular slot along the width of main waveguide. Here port 1 &2 are called

collinear ports while port-3 is called as H-arm or side arm. All three arms of H-plane T lie in plane of

magnetic field .It is also called as shunt-T or current junction.

A signal in H arm splits equally into coplanar arms. If lengths of coplanar arm are equal then output electric

fields are in phase. It is clear that power coming out of port 1 and port 2 is 3dB down or reduced to half with

respect to port-3.It is also called as 3dB splitter.

126

21

3

E-PLANE TEE

Arm

-3

Arm-2Arm-1

MAGIC-TEE

E plane Tee:

It is formed by cutting a rectangular slot along the broader dimension of main waveguide. Here port 1 &2

are called collinear ports while port-3 is called as E-arm .All three arms of E-plane T lie in plane of electric

field .It is also called as series -T or voltage junction.A signal in E arm splits equally into coplanar arms. If

the Tee is fed at port 3 and lengths of port 1 and port 2 are equal then electric fields at two outputs are 1800

out of phase. The power delivered at port 1 and ports 2 are equal. It is clear that power coming out of port 1

and port 2 is 3dB down or reduced to half with respect to port-3.It is also called as 3dB splitter.

The S-matrix for E-plane Tee is as follows.

Magic Tee: It is a four port device and is a combination of E-plane Tee and H-plane Tee. Ports 1 and port 2

are called coplanar arms .Port 3 is called as E-arm and port 4 is called as H-arm. The features of magic Tee

are

1. All ports are perfectly matched

2. If a signal is fed at coplanar arms 1 or (2) it splits equally between E&H arms. At each output port

Pout=0.5Pin and Vout=0.707Vin.There will be no output at other coplanar arms 2or(1)

3. If a signal is fed in H-arm it splits equally between coplanar arms the outputs in coplanar arms being in

phase equidistant from junction. There will be no output at E port.

4. If a signal is fed in E-arm it splits equally between coplanar arms the output being 1800 out of phase

equidistant from junction.

The Magic Tee has a property that the arms 3 &4 are both individually connected to arms 1 and 2 i.e. arms 3

is connected to arm 1 and arm 2 and arm 4 is also connected to arms 1 and arm2 but they are not connected

to each other. One of the common applications of Magic tee is in the front end of a microwave receiver. In

this application the antenna and local oscillator may respectively feed arms 3 and arms 4 with mixer

connected to arm 2 and a matched termination to arm 1.

Applications of magic Tee:

1. Used as a mixer

2. Used as a duplexer

3. Used for impedance measurement

127

2 Z0

Z0 Z0

2 Z02 Z0

2 Z0Z0

Port 4 Port 2

Port 3Port 1

Z0

PORT-1

PORT-3

PORT-2

PORT-4

Hybrid rings:

It is also called as Rat-race but it has a annular ring to which are connected four ports. The ports 1 and ports

2 are decouple while ports 3 and ports 4 are decoupled. Their features are

1. A signal entering port 1 or port 2 is split equally between port 3 and port 4.

2. A signal entering port 3 or port 4 is split equally between ports 1 and ports 2.

Directional coupler:

Wave guides directional couplers like transmission lines directional couplers are mainly used for

unidirectional power flow measurement and SWR measurement.

It is a 4 port device in which portions of forward and reverse traveling waves are separately coupled to two

of the ports. In simple terms it consists of two transmission lines and a mechanism for coupling signals

between them. Their features are

1. A portion of wave traveling from port 1 to port 2 is coupled to port 4 but not to port 3.

2. A portion of wave travelling from port 2 to port 1 is coupled to port 3but not to port 4 .

3. A portion of wave travelling from port 3 to port 4 is coupled to port 2 but not port 1.

4. A portion of wave travelling form port 4 to port 3 is coupled to port 1and not port 2.

Coupling factor:

It is the ratio of incident power to forwarded power

C=10log(P1/P4) Coupling factor measures of how much incident power is being sampled.

Directivity:

It is the ratio of forwarded power to backward power

D=10log (P4/P3) Directivity measures distinction between forward and reverse travelling wave power.

Isolation factor: It is the sum of both coupling factor and directivity.

128

L = (2n + 1)

Primary waveguide

Port 2

Added Port 4

Port 1

Port 3 Canceled

Secondary waveguide

S-matrix of 2-hole directional coupler is

12 14

12 23

23 34

14 34

0 0

0 0

0 0

0 0

S S

S S

S S

S S

It can also be written in other form as follows:

0 0

0 0

0 0

0 0

p jq

p jq

jq p

jq p

Two-Hole Directional Couplers:

A two-hole directional coupler with traveling waves propagating in it is illustrated in Fig. 4-5-3. The spacing

between the centers of two holes must be

(2 1)4

gL n

where n is any positive integer.

A fraction of wave energy entered into port 1 passes through the holes and is radiated into the secondary

guide as the holes act as slot antennas. The forward waves in the secondary guide are in the same phase,

regardless of the hole space, and are added at port 4. The backward waves in the secondary guide (waves are

progressing from right to left) are out of phase by (2L/g)2 rad and are canceled at port 3.

S- Matrix of a Directional Coupler:

In a directional coupler all four ports are completely matched. Thus the diagonal elements of the S matrix

are zeros and S11 = S22 = S33 = S44 = 0

As noted, there is no coupling between port 1 and port 3 and between port 2 and port 4. Thus

S13 = S31 = S24 = S42 = 0

Consequently the S matrix of a directional coupler becomes

12 14

21 23

32 34

41 43

0 0

0 0

0 0

0 0

S S

S SS

S S

S S

129

Port 4

Port 2

Port 3Port 1

Phase shifter

Phase shifter

Primary guide

90º90º 180º

180º

270º

90º

Coupler 1 Coupler 2

Port 1

Port 3

Port 2

Port 4

Secondary guide

Microwave Circulators:

A microwave circulator is a multiport waveguide junction in which the wave can flow only from the nth port

to the (n + 1)th port in one direction .Although there is no restriction on the number of ports, the four-port

microwave circulator is the most common. One type of four-port microwave circulator is a combination of

two 3-dB side-hole directional couplers and a rectangular waveguide with two nonreciprocal phase shifters

as shown in Figure.

The operating principle of a typical microwave circulator can be analyzed with the aid of given Figure.

Each of the two 3-dB couplers in the circulator introduces a phase shift of 90º, and each of the two phase

shifters produces a certain amount of phase change in a certain direction as indicated. When a wave is

incident to port 1, the wave is split into two components by coupler 1. The wave in the primary guide arrives

at port 2 with a relative phase change of 180º. The second wave propagates through the two couplers and the

secondary guide and arrives at port 2 with a relative phase shift of 180º. Since the two waves reaching port 2

are in phase, the power transmission is obtained from port 1 to port 2. However, the wave propagates

through the primary guide, phase shifter, and coupler 2 and arrives at port 4 with a phase change of 270º.

The wave travels through coupler 1 and the secondary guide, and it arrives at port 4 with a phase shift of 90º.

Since the two waves reaching port 4 are out of phase by 180º, the power transmission from port 1 to pot 4 is

zero. In general, the differential propagation constants in the two directions of propagation in a waveguide

containing ferrite phase shifters should be

1 3

2 4

(2 1) /

2 /

m rad s

n rad s

Where m and n are any integers, including zeros. A similar analysis shows that a wave incident to port 2

emerges at port 3 and so on. As a result, the sequence of power flow is designated as 1 2 3 4 1.

A perfectly matched, lossless, and nonreciprocal four-port circulator has an S matrix of the form

12 13 14

21 23 24

31 32 34

41 42 43

0

0

0

0

S S S

S S SS

S S S

S S S

Using the properties of S parameters as described previously, the above S matrix can be simplified to

130

PORT-3

PORT-1

PORT-2

PORT-1 PORT-2GYRATOR

SOURCE ISOLATOR LOAD

0 0 0 1

1 0 0 0

0 1 0 0

0 0 1 0

S

Circulators:

It is a 3port device and in this case signal is transmitted from port 1 to port 2(but not to port 3) from port 2

to port 3(but not port 1 ) and then from port 3 to port 1(but not port 2). S-matrix of a circulator is as follows

13

21

32

0 0

0 0

0 0

S

S

S

It is impossible to construct a perfectly matched lossless reciprocal 3 port junction. Circulators are used in

duplexer, parametric amplifier and tunnel diode.

Ferrite devices: A ferrite is a ceramic like material with relative permeability of several thousands and

specific resistance of about 1014

times that of metals. The chemical composition is manganese, oxygen and

ferrous etc. Ferrites are used in fabrication of microwave isolators, circulators and resonators etc.

Yitrium Iron Garnet (YIG) is a rare earth material having ferromagnetic properties but very low losses. It is

used in fabrication of microwave resonators and its resonance frequency can be changed by changing

magnetic field strength.

Gyrators: It is a device in which a relative phase difference f 1800 from port 1 to port 2 but no phase

difference from port 2 to port 1.

Isolators: It is a 2 port non reciprocal device which provides a minimum attenuation to EM wave in one

direction and very high attenuation in opposite direction. Ferrite based isolators are of two types namely

Faraday rotation isolators used for powers up to a few hundred watts and resonant absorption isolators used

for higher powers.

Microwave Isolators:

An isolator is a nonreciprocal transmission device that is used to isolate one component from reflections of

other components in the transmission line. An ideal isolator completely absorbs the power for propagation in

one direction and provides lossless transmission in the opposite direction. Thus the isolator is usually called

uniline. Isolators are generally used to improve the frequency stability of microwave generators, such as

klystrons and magnetrons, in which the reflection from the load affects the generating frequency. In such

131

45º

45º

H

Magneticfield

Outputwaveguide

Directionof rotation

Resistivevane

Inputwaveguide

Reflectedwavevector

y

x

z

Ferrite rod

E

cases, the isolator placed between the generator and load prevents the reflected power from the unmatched

load from returning to the generator. As a result, the isolator maintains the frequency stability of the

generator.

Isolators can be constructed in many ways. They can be made by terminating ports 3 and 4 of a four-port

circulator with matched loads. On the other hand, isolators can be made by inserting a ferrite rod along the

axis of a rectangular waveguide as shown in given Figure. The isolator here is a Faraday-rotation isolator. Its

operating principle can be explained as follows .The input resistive card is in the y-z plane, and the output

resistive card is displaced 45º with respect to the input card. The dc magnetic field, which is applied

longitudinally to the ferrite rod, rotates the wave plane of polarization by 45º. The degrees of rotation

depend on the length and diameter of the rod and on the applied dc magnetic field. An input TE10 dominant

mode is incident to the left end of the isolator. Since the TE10 mode wave is perpendicular to the input

resistive card, the wave passes through the ferrite rod without attenuation. The wave in the ferrite rod section

is rotated clockwise by 45º and is normal to the output resistive card. As a result of rotation, the wave arrives

at the output

Microwave amplifiers and oscillators:

Microwave devices:

These devices are used for high power and high frequency. These devices can be categorized as follows.

1. Cross field devices: Example is magnetron which is a high power oscillator.

2. Velocity modulated devices: examples are 2-cavity klystron, multi cavity klystron and reflex klystron. In

which 2 cavity and multi cavity are amplifiers while reflex klystron is Oscillator.

3. Traveling wave tubes: It is a broad band amplifier. It can be categorized into helix and coupled cavity

type of TWT‘s

O-type tubes (linear beam tubes): These are linear beam tubes. Examples are TWT and klystron type of

tubes.O-type tubes can be categorized into 3 types

1. Cavity type: Klystron is an example of it.

2. Slow-wave structure: TWT is an example of it. It is of 2 types forward wave and backward wave. In

forward it behaves like an amplifier while in backward it behaves like an oscillator.

3. Hybrid tubes: It is combination of TWT and klystron

M-type tubes (cross field device): Here magnetic field is perpendicular. It is again of 2 types

132

1. Resonant structure: Magnetron is the best example of resonant structure

2. Non resonant structure:

1. FW-CFA:

2. FW-CFO: Dematron

3 .BW-CFA: Amplitron

4 .BW-CFO: Carcinotron

Comparision of O-type and M-tube:

1. In O-type energy conversion is from kinetic energy to radio frequency energy but in case of M-type it is

from potential energy to radiofrequency energy.

2. In O-type E&H fields are parallel to direction of electron flow and H is used for focusing while in case

of M-type E&H are perpendicular to each other and also perpendicular to direction of propagation.

3. In O type analyses is easy while in M type it is difficult

4. O type has low value of gain but M-type has moderate gain

5. O type has moderate efficiency and moderate output while M type has high efficiency and high output.

6. In O type noise is low but in M type it is high.

Microwave triodes:

Oscillator circuits using vacuum tubes have the following limitations at very high frequencies or microwave

frequencies.

1. The stray capacitances and inductances become important and affect the operation of the circuit.

2. At low frequencies the transit time between cathode and anode is a small fraction of period of

oscillation. However at microwave frequencies this transit time becomes comparable to time period of

oscillations.

If a vacuum tube a triode for instance is to be used at microwave frequencies the internal capacitances and

inductances must be reduced and transit time must be minimized.

Problems due to transit time effect:

1. There is problem in design

2. More power is absorbed or grid loss is increased

3. Effect of noise will be increased

Solution to overcome transit time effect:

1. Both electrodes are brought close together

2. Voltage is increased at anode

3. Higher anode current is used.

Two-cavity klystron:

Two cavity klystron tubes make use of transit time effect for this operation. A high velocity electron beam is

produced by oxide coated indirectly heated cathode and is focused and accelerated by focusing electrode.

This beam is transmitted through a glass tube. The beam passes a gap in the buncher cavity. The RF signal

to be amplified is applied to buncher cavity. As electrons move ahead they see an accelerating field for half

cycles and retarding field for the other half cycle. Therefore some electrons are accelerated and some are

retarded. This process is called velocity modulation. The velocity modulation causes bunching of electrons.

This bunching effect converts velocity modulation into density modulation of beam.

133

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Drift space

Catchercavity

Output

Collector

Bunchercavity

Input

Cathode

Note:

The beam is allowed to drift freely till it reaches another gap in the catcher cavity. Oscillations are excited in

the catcher cavity the power of these oscillations is much higher than that in buncher cavity. The drift space

is pretty long and transit time in this space is made use of to form electron bunches as shown. The gaps must

be so small that the voltage across them does not change significantly during the passage of a bunch of

electrons.

Different modulations in 2-cavity klystron:

1. Velocity modulation in buncher cavity

2. Density modulation in drift space

3. Current modulation in catcher cavity

Bunching is not complete in 2 cavity klystron so gain value will be very low.

Efficiency of 2 cavity klystron:

Maximum efficiency of 2 cavity klystron is 58% but in general case this value is 15-30%.

Oscillations in 2 cavity klystron:

If a portion of signal in a catcher cavity is coupled back to buncher cavity then oscillations will take place

but condition is that feedback of output to input cavity must be in proper phase and it must have correct

polarity and sufficient amplitude.

Multi-cavity klystron:

To improve efficiency and power gain of 2 cavity klystron extra cavities are used and up to 7 cavities can be

used. In case of multi cavity klystron each intermediate cavity is placed at a distance of 1.841 away from

previous cavity and this will increase velocity modulation. Here each intermediate cavity is buncher cavity.

If intermediate cavities are tuned to same frequency (Synchronous tunning) then gain and efficiency will

increase but not the bandwidth. If cavities are tuned to different frequency (Stagger tunning) then other than

gain and efficiency bandwidth is also increased.

Important features of multi cavity:

1. Frequency range: 0.25GHz to 100GHz

2. Power output: 10kw to several hundred KW

3. Power gain: 60dB

4. Efficiency: about 40%

Reflex klystron:

It is a klystron like microwave tube with only one cavity (which can be used as both buncher and catcher

cavity) and a repeller electrode. Its parts are electron gun, resonator, repeller and output coupling. This

operates on the principle of positive feedback. The repeller electrode with a very high negative potential

134

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.. . .

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......

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..

.. ..

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.

.

.

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..

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. ..

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.

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..... .. ..... .....

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. .. ... . .

..

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.

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....

. . . .. ....

..

... .

Cavity (Anode)

Electron beamRepeller

Focussingelectrode Output

.

. ......

..

provides the feedback and converts tube into oscillator. Reflex klystron is a low efficiency low power

microwave oscillator.

It has small electron gun compared to multi cavity because device is short and does not require focusing.

Operation of reflex klystron:

Here electrons never reach the repeller because it has a high negative voltage and after covering some

distance electrons will be turned back. If voltages are properly chosen then returning electrons will give

more energy to gap than it took from gap in onward journey and continuous oscillation will take place. Here

the positive voltage on the anode and negative voltage on the repeller electrode are critical to operation and

need to be carefully adjusted and also need to be highly regulated.

Transit time in reflex klystron:

If energy delivered by bunching electrons to cavity in returning energy is greater than energy loss in cavity

then cavity can sustain oscillations at resonance frequency of cavity

So optimum transit time for bunches to arrive at cavity is n+3/4 cycles after beam initially left the cavity.

Different values of n will give different modes. These modes in a reflex klystron give same frequency but

different transit times.

Specification of reflex klystron:

1. Operating frequency is 4-200 GHz

2. Output power is 100mw.

3. Maximum efficiency is 22.7% but for low power oscillations it is in range of 10%

4. Reflex klystron is preferred to 2 cavity klystron because reflex klystron is easier to tune than 2-cavity

klystron.

Applications of reflex klystron:

1. Signal source in microwave generator

2. Local oscillator in microwave receiver

3. Pump oscillator for parametric amplifier.

4. Included in radar receiver

Travelling wave tube:

A travelling wave is basically a microwave amplifier like a klystron. This is a linear beam amplifier, has

high value of bandwidth &gain and also has low noise figure. However it is different from klystron in sense

that interaction between electron beam and RF field is continuous. In klystron the resonant structure limits

the bandwidth but TWT provides large value of bandwidth and can be used as a medium or high power

pulsed or CW microwave amplifier.

135

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Attenuator

Magneticfield

Inputguide

Outputguide

CollectorElectronbeamGlass

tube

Helix

Types of slow wave structure:

1. Helix: It has non resonant structure and can provide large value of bandwidth. But at high frequency and

high power performance of helix is reduced.

2. Coupled cavity: It is used for high power and high frequency but it is limited up to 100GHz Only.

3. Ring bar: This is used for frequency more than 100GHz.

Focusing in TWT: Focusing of electron beam is done with the help of

1. Permanent magnet

2. Electro magnet

3. Solenoid

4. Periodic permanent magnet

In general case periodic permanent magnet is used to reduce the bulk of permanent magnet.

Attenuators in TWT:

In case of TWT gain is increased continuously and due to continuous bunching there may be chance of

oscillations and thats why there is need of attenuator to reduce gain and prevent oscillations. The attenuator

may be lossy metallic coating and kanthal&aquadag.

Operation of Travelling Wave Tube amplifier:

A narrow electron beam is sent through the centre of a long axial helix which is made positive with respect

to cathode. The collector is made more positive with respect to helix. The beam acquires a high velocity.

The RF signal is applied to input end of helix this field propagates around the helix with a speed equal to

velocity of light in free space. The axial velocity of this electric field is however equal to velocity of light

multiplied by ratio of helix pitch to circumference. The axial velocity can be made equal to electron beam

velocity. The axial RF field and the electron beam interact continuously

The electron beam bunching gives energy to the field. The interaction between the beam and RF field is

similar to that of magnetron.TWT is preferred to magnetron for use in Radar transmitter because it is

capable of longer duty cycle.

Why high value of gain in TWT:

Here RF field and electron beam interact continuously and gives almost complete bunching and which will

give high value of gain.

Backward wave amplifier: It is similar to TWT but only difference is that position of output and inputs are

interchanged. But bandwidth of backward amplifier is less than TWT.

136

Anodecavities

Cathode

Output

Interactionspace

Gaps

Backward oscillator: This oscillator is obtained by BWA by some modification.

BWO is based upon TWT.

Twystron: It is combination of both TWT and Klystron and generally at input side

multi-cavity klystron is used and output side TWT is used.

Magnetron:

1. It is a cross field amplifier.

2. It is a high power amplifier

3. It has high efficiency

Concept of magnetron:

If a magnetic field is zero then all electrons will move from cathode to anode but if certain value of magnetic

field is applied then electrons will not reach at the anode.

But if a magnetic field greater than cut-off magnetic field is applied then all electrons will come back at

cathode forming a cloud known as Brillioun cloud. In case of magnetron permanent magnets are used to

ensure that electrons will orbit around the cathode.

Mode jumping: Magnetron acts as a transmission line filter in which various modes differ very little in

frequency from each other so due to nearness of frequency one mode will jump to another mode. Strapping

is used to avoid mode jumping and by use of strapping separation between frequency of two modes become

25-30%.It means wires are connected in magnetron.

Rising sun magnetron: No of modes in this magnetron are same as strapped magnetron but here half no of

modes are resonant at a frequency above π mode and other half are resonant at a frequency above π mode.

But in case of strapped magnetron all modes are resonant below π mode. In case of rising sun magnetron

frequency separation is increased.

Tunnel Diode:

Tunnel diode is a voltage controllable negative resistance device having V-I characteristics expressed by

single valued function of voltage negative resistance differential resistance. It can behaves like an oscillator

or amplifier in microwave band. In this output power is very low and is of the order of mill watt .In this case

doping value is very high and generally 1000 times higher than normal diode. Tunnel diode has high speed,

low power operation, low weight and low value of noise. Materials used for tunnel diode is Ge,Si,GaAs and

GaSb. But generally used materials are Ge not Si because Si has higher value of noise, high forbidden gap

and low mobility.

137

J

EO

J

EO

Applications of tunnel diode:

1. Used for microwave amplifications

2. Used for high speed switching

3. Used as low noise amplifier and mixer

4. Due to high speed device and high speed switching logic it is used for binary memory.

5. They are used as moderate to low noise preamplifiers in all kinds of microwave receivers up to a

frequency range of 50GHz.

Backward diode: If tunnel diode works in Reverse bias with large value of reverse current then these types

of diode are called as backward diodes. But for higher values of reverse bias there will be zener break down.

Varactor diode: This diodes work in reverse bias and value of capacitance decreases with increase in

reverse bias voltage. This is also known as voltage variable capacitor.

Varactor diode is used for frequency multiplication due to variation of its capacitor.

Application of varactor diodes are in electronic tuning and parametric amplifier.

Most important application of varactor diode is in frequency multiplication and can never be used for

frequency oscillation within the useful operating region the varactor diode is equivalent to capacitance in

series with a resistance at high frequency. The high frequency cut-off is used as the figure of merit for

varactors.

Step recovery diode: It is similar in construction to varactor diode. It stores charge however when it is

conducting with a forward bias and it generates a current pulse rich in harmonics when it is made to

discharge by applying a reverse bias. It works like varactor diode and is used for frequency multiplication

but with higher value of factor than varactor diode. Varactor diodes are used at very high frequency while

step recovery at lower than varactor diode. It is also known as snap-off varactor.

Gunn-diode:

Transferred Electron Effect (TED) (Gunn Effect):

If sufficient voltage is applied across material like GaAs, InP,CdTl&InAs then electrons will acquire higher

value of energy and will transfer to higher energy band form lower energy band .This will result decrease in

mobility i.e. value of current is decreased by increasing voltage and shows negative resistance phenomenon.

This effect occurs in N-type material only.

RIDLEY–WATKINS–HILSUM (RWH) THEORY

Differential Negative Resistance

The fundamental concept of the Ridley–Watkins–Hilsum (RWH) theory is the differential negative

resistance developed in a bulk solid-state III-V compound when either a voltage (or electric field) or a

138

I

High field

Low field

High field

Low field

J

EO

J

EO

J1

J0

J2

E1 E0 E2

J1

J0

J2

E1 E0 E2

current is applied to the terminals of the sample. There are two modes of negative-resistance devices:

voltage-controlled and current-controlled modes as shown in given figures.

In the voltage-controlled mode the current density can be multivalued, whereas in the current-controlled

mode the voltage can be multivalued. The major effect of the appearance of a differential negative-resistance

region in the current-density-field curve is to render the sample electrically unstable. As a result, the initially

homogeneous sample becomes electrically heterogeneous in an attempt to reach stability. In the voltage-

controlled negative-resistance mode high-field domains are formed, separating two low-field regions. The

interfaces separating low and high-field domains lie along equipotentials thus they are in planes

perpendicular to the current direction as shown in Fig-1 below In the current-controlled negative-resistance

mode splitting the sample results in high-current filaments running along the field direction as shown in

Fig-2 below.

Expressed mathematically, the negative resistance of the sample at a particular region is

dI dJ

dV dE = negative resistance

If an electric field E0 (or voltage V0) is applied to the sample, for example, the current density J0 is

generated. As the applied field (or voltage) is increased to E2 (or V2), the current density is decreased to J2.

When the field (or voltage) is decreased to E1 (or V1), the current density is increased to J1. These

phenomena of the voltage-controlled negative resistance are shown in Fig-1 Similarly for the current-

controlled mode the negative-resistance profile is as shown in Fig-2.

Two-Valley Model Theory

According to the energy band theory of the n-type GaAs, a high-mobility lower valley is separated by an

energy of 0.36 eV from a low-mobility upper valley as shown in Figure below.

139

Lower valley

Upper valley

E = 1.43 eVg

K

Conductionband

Forbiddenband

Valenceband

< 100 >•

•

•

•

•

E

00 K

E

00 K

E

00 K

Data for two valleys in GaAs:

Valley Effective Mass

Me

Mobility

Separation

E

Lower

Upper eM = 0.068

1.2euM

2

2

8000cm /V-sec

180cm /V-secu

E = 0.36 eV

E = 0.36 eV

The data for the two valleys in the n-type GaAs and Table shows the data for two-valley semiconductors.

Electron densities in the lower and upper valleys remain the same under an equilibrium condition. When the

applied electric field is lower than the electric field of the lower valley (E < E ), no electrons will transfer to

the upper valley as shown in Fig-a when the applied electric field is higher than that of the lower valley and

lower than that of the upper valley ( uE E E ), electrons will begin to transfer to the upper valley as

shown in Fig-(b). And when the applied electric field is higher than that of the upper valley (Eu < E), all

electrons will transfer to the upper valley as shown in Fig-(c)

(a) E < EQ (b) EQ < E < Eu (c) Eu < E

Data for two-valley semiconductors:

Semiconductor

Gap energy

(at 300ºK)

Eg(eV)

Separation energy

between two valleys

E(eV)

Threshold

field

Eth(KV/cm)

Peak velocity

p(107 cm/s)

Ge

GaAs

InP

CdTe

InAs

InSb

0.80

1.43

1.33

1.44

0.33

0.16

0.18

0.36

0.60

0.80

0.51

1.28

0.41

2.3

3.2

10.5

13.0

1.60

0.6

1.4

2.2

2/5

1.5

3.6

5.0

140

If electron densities in the lower and upper valleys are n and nu, the conductivity of the n-type GaAs is

( )u ue n

where e = the electron charge

= the electron mobility

n = n + nu is the electron density

Gunn diode: A Gunn diode uses GaAs which has a negative differential mobility i.e. a decrease in carrier

mobility with increase in electric field. The impedance of Gunn diode is tens of ohms. The Gunn effect is

instrumental in generation of microwave oscillations.

This diode differs from the other semiconductor diodes in the sense that they depend for their operation on

bulk properties of the semiconductor rather than their junction properties. It is also a negative resistance

device. Here negative resistance is due to electron transfer to a less mobile energy band. The Two-valley

method is used for analysis of Gunn diode.

Modes of operation in Gunn diode:

Since Gunn first announced his observation of microwave oscillation in the n-type GaAs and n-type InP

diodes, various modes of operation have been developed, depending on the material parameters and

operating conditions. As noted, the formation of strong space-charge instability depends on the conditions

that enough charge is available in the crystal and that the specimen is long enough so that the necessary

amount of space charge can be built up within the transit time of the electrons.

This requirement sets up a criterion for the various modes of operation of bulk negative-differential-

resistance devices. Copeland proposed four basic modes of operation of uniformly doped bulk diodes with

low-resistance contacts.

Gunn oscillation mode: This mode is defined in the region where the product of frequency multiplied by

length is about 107 cm/s and the product of doping multiplied by length is greater than 10

12/cm

2. In this

region the device is unstable because of the cyclic formation of either the accumulation layer or the high-

field domain. In a circuit with relatively low impedance the device operates in the high-field domain mode

and the frequency of oscillation is near the intrinsic frequency. When the device is operated in a relatively

high-Q cavity and coupled properly to the load, the domain is quenched or delayed (or both) before

nucleating. In this case, the oscillation frequency is almost entirely determined by the resonant frequency of

the cavity and has a value of several times the intrinsic frequency.

1. Stable amplification mode: This mode is defined in the region where the product of frequency times

length is about 107 cm/s and the product of doping times length is between 10

11 and 10

12/cm

2.

2. LSA oscillation mode: This mode is defined in the region where the product of frequency times length is

above 107 cm/s and the quotient of doping divided by frequency is between 2 × 10

4 and 2 × 10

5.It has

advantage that operating frequency is not limited by transit time effect .So sample length can be made

longer and device can sustain higher voltage and higher power dissipation.LSA mode is preferred when

high output power is necessary at high frequency but this mode is generally not used for frequency

greater than 20GHz.

3. Bias-circuit oscillation mode: This mode occurs only when there is either Gunn or LSA oscillation, and

it is usually at the region where the product of frequency times length is too small to appear in the figure.

141

When a bulk diode is biased to threshold, the average current suddenly drops as Gunn oscillation begins.

The drop in current at the threshold can lead to oscillations in the bias circuit that are typically 1 kHz to

100 MHz.

Applications of Gunn diode:

1. It is used in continuous wave radar

2. It is used in pulsed radar

3. It is used in microwave receivers

4. It is used as pump oscillator and can be used as broad band amplifier

5. It can be used as medium power oscillator

Frequency of oscillation in Gunn diode:

Frequency of oscillation of Gunn diode is

f=vd/L

Here Vd is drift velocity and L is effective length of diode.

IMPATT diode :( Impact Ionization Avalanche Transit Time)

IMPATT diodes don‘t depend for their operation on the junction properties. It exhibits negative resistance

which may be defined as the voltage across diode being 1800out of phase with the current flowing through

it. First delay is involved in generating avalanche multiplication which gives 900 phase shift and second

delay is due to transit time through drift space which gives another 900 phase shift.

Materials for IMPATT:

IMPATT diodes are made from Silicon or GaAs or even Indium Phosphide .Silicon IMPATT diodes can be

used in excess of 200GHz.IMPATT diodes are used as microwave amplifiers and oscillators.

Important features of IMPATT:

1. The impedance of IMPATT diode is a few ohms

2. Frequency of IMPATT is from 1GHz-300GHz

3. Power output is in the range of 0.5W to 5W for single diode circuit.

4. Efficiency is around 20%.

5. Frequency of oscillation for IMPATT is f=vd/2L

Where f is the frequency of oscillation and vd is drift velocity and L is the length of active region.

Limitation of IMPATT:

The noise is the biggest problem with these diodes. The noisy performance arises from the avalanche

process where the high operating current generates shot noise. When used as amplifiers they produce noise

figures of the order of 30dB.IMPATT oscillators are not as good as klystrons or Gunn diodes. IMPATT

diodes are more efficient and more powerful than Gunn diode but due to noise problem they are not able to

replace Gunn diodes.

So IMPATT diodes are known as narrow band device while Gunn diodes are broadband devices.

Applications of IMPATT diodes:

1. Police radar system

2. Low power microwave transmitter

142

TRAPATT Diode :( Trapped Plasma Avalanche Triggered Transit)

TRAPATT diode is very similar to an Impatt diode and is only a different method of operating the Impatt

diode. It is also a negative resistance device capable of producing high pulse microwave powers at relatively

low frequencies.

Specifications of TRAPATT:

1. Frequency 3-50GHz

2. Power output 1-3 watt

3. Efficiency of about 25%

Applications of TRAPATT:

1. Low power Doppler radar

2. Microwave beacon landing system

3. TRAPATT behaves like class-C and i.e. why used for pulsed operation.

BARITT diode:

It is Barrier Injected Transit time diode and is less noisy than IMPATT.

PIN Diode:

A PIN diode has an intrinsic layer (i) between P and N layers. The effective width of depletion layer

increases by the width of intrinsic layer .It can be used as a voltage controlled attenuator. This PIN diode is

used for high frequency switching circuits, limiters and modulators etc.

MESFET:

It is a field effect transistor for microwave frequencies It is fabricated in GaAs and uses a metal

semiconductor schottky junction for the gate contact .It is a low noise amplifier

and is used in high frequency logic circuits.

Parametric amplifier:

Parametric amplifiers are used for low noise amplification at microwave frequency.

It is a low noise amplifier as it does not involve any resistance in amplifying process.

It makes use of a device whose reactance is varied in such a manner so to yield amplification. Varactors are

commonly used as the active devices.

Difference b/w parametric&normal amplifier:

A conventional amplifier depends for its amplification process on a variable resistance (which is the

collector resistance in transistor amplifier) and DC power supply.

The parametric amplifier uses a variable reactance and an AC power supply. The reactance is varied at a

frequency called the pump frequency.

Concepts of parametric amplifier:

Amplification is obtained by electronically variation of reactance in some predetermined fashion at some

frequency higher than frequency of signal being amplified. Here energy

is taken from pump source and is added to signal at signal frequency and will result in amplification.

Power gain in parametric amplifier:

If fp and fs are pump frequency and signal frequency then

Power gain of up-convertor= ( fp+ fs)/ (fs): If the pump frequency is much higher than

Signal frequency then configuration is called an Up-convertor.

143

w

h

Stripconductor

t

Ground plane

x z

y0

E

EE

H

Power gain of down-convertor= ( fs)/ (fs+ fp): If the pump frequency is only slightly higher so that the idler

frequency is less than signal frequency a down convertor results.

Degenerate modes:

If fp= 2fs then modes will be degenerate modes. But if pump frequency is other than twice the signal

frequency the two signals beat and a difference signal called the idler frequency appears. It may be noted

that there is no requirement of pump frequency to be a multiple of signal frequency in a non degenerate

parametric amplifier.

Applications of parametric amplifier:

1. Radar tracking

2. Communication

3. Earth satellite station

MASER:

Microwave Amplification by Stimulated Emission of Radiation

This provides extremely low noise microwave amplification. The basic principle of operation of a MASER

is as following:

Certain materials have atomic systems that can be made to resonate magnetically at frequencies depending

upon the atomic structure of material and the applied strength magnetic field. Resonant absorption takes

place when such a resonance is stimulated by an external signal at that frequency. If the material is suitably

excited or pumped from another source emission will occur.

The materials used could be gaseous such as Ammonia or solid state such as Ruby.

Ruby Maser is the more practical maser amplifier .Noise figures of a fraction of a dB are common in masers.

Strip-lines and micro-strip lines

Strip lines and Micro strips are also conducting media developed as an alternate to waveguide for

applications that require miniaturization.Striplines evolved from a coaxial line and propagation in a strip line

is by means of a TEM mode. Microstrip is analogous to parallel wire line .Semiconductor microwave

devices are often packaged so that they can be directly connected to a strip line or a microstrip. In recent

years with the introduction of Monolithic Microwave Integrated circuits(MMICS) microstrip lines and

coplanar strip lines have been used extensively because they provide one free and accessible surface over

which solid state device can be placed.

Schematic diagram of strip lines:

144

wt

hh

d

Important points about Microstrip lines:

1. It is also called as open strip line

2. Modes on microstrip lines are only Quasi Transverse Electric and Magnetic(TEM)

3. Radiation loss is a problem in case of micro-strip line particularly at such discontinuities as short

circuited posts ,corners and so on

4. By use of thin high dielectric material radiation loss of open strip line is reduced

Characteristic impedance of microstrip line:

0

60 4ln

r

hZ for h d

d

h is height from the centre of the wire to the ground plane .

d is diameter of the wire

r is the dielectric constant of ambient medium.

Empirical equation of effective dielectric constant ( re ):

0.475 0.67re r

re is the effective relative dielectric constant for a microstrip line

r is the relative dielectric constant for micro strip line

Comparison of Microstrip and strip line:

1. Microstrip has advantage over strip line that it is simpler in construction and easier integration with

semiconductor devices

2. There is a greater tendency with microstrip to radiate from irregularities and sharp corners.

3. There is a lower isolation between adjoining circuits in microstrip than strip line

4. Both Q and power handling are lower with microstrip line.

Comparison of strip line and wave guide:

1. Advantages of strip line over wave guide are reduced bulk and greater bandwidth

2. Another advantage of strip line over wave guide is greater compatibility for integration with microwave

devices.

3. But strip line has greater losses, lower Q and lower power handling capability than wave guides.

4. Another disadvantage of strip-line and consequently of microstrip is that components made of it are not

readily adjustable.

Modes in strip line and microstrip line:

In case of strip line mode is purely TEM while for microstrip line mode is quasi TEM.

145

Study material for RADAR

Radar i.e. Radio Detection and Ranging is used for gathering information about distant objects or targets by

E.M waves. It is used for detecting static or mobile objects. It is also an effective method for guiding pilots

with regard to his location in space. Frequency generally used by radar lies in upper UHF and Microwave

range. Radar consists of a transmitter and receiver each connected to a directional antenna .Here both

transmitter and receiver antennas are of same type generally. Transmitter used for radar is klystron, TWT &

transistor amplifier while antenna used are parabolic reflector, planar arrays&phased arrays.

Note:

Radar can see through fog, rain&snow and can determine location of target very accurately.

Radar can‘t resolve like eye and also can‘t recognize color of object.

Block diagram of an elementary Pulsed Radar:

Duplexer:

Radar which is using same antenna for transmitter and receiver is called as duplexer.

Such type of radars is called as mono-static radar. Duplexer uses quarter wavelength transmission lines.

The functions of duplexer are:

1 .To isolate transmitter and receiver

2. To protect receiver from transmitter

3. To help use a single transmitter and receiver antenna

Devices used for duplexer are solid state ferrite circulator &solid state devices.

Range of target:

Range =C.T/2 Here T is the time taken by radar signal to travel to target and back

Range (Km) =0.15 T (microsecond)

Maximum Unambiguous Range (MUR):

After the radar pulse has been transmitted a sufficient rest time must be allowed for the echo to return so as

not to interface with next transmit pulse. This PRT or Pulse Repetition Time determines the maximum

distance to the target to be measured.

Any signal arriving after transmission of the second pulse is called a second return echoes and is called

Maximum Un ambiguous Range (MUR). So the range beyond which object appears as second around

echoes is called the MUR.

2 12.2

sec

C PRTMUR

PRF

C Velocity of light

PRT is in

Transmitter

Receiver

Duplexer

146

If PRF is too high then ambiguity in majority range might be present. If a large reflector object is very close

the echoes may return before the complete pulse may be transmitted.

To eliminate this ambiguity the receiver is blocked or turned-off.

Factors affecting range of a radar:

Maximum range of RADAR is given by:

1/42

max 2

min

min

4

Peak Transmitted pulse power

S capture area of target

Effective area of transmitting antenna

Operating wavelength

Minimum detectable power

t e

t

e

P A Sr

S

P

A

S

Radar displays:

1. A-scope display: This is the most popular and similar to ordinary CRO .It displays system which

indicates the range of target. A-scope displays range v‘s amplitude of the received echo signals .It is

suited for tracking of radar rather than surveillance of radar.

2. B-scope display: It displays signal range v‘s angle. Here amplitude is a function of azimuth.

Normally this is used in air-borne radar.

3. Plane Position Indicator (PPI): It is a presentation that maps in polar coordinates and the location

of target in azimuth&range.This is used for surveillance radar.

Scanning of radars: Scanning refers to the way in which the antenna keeps moving in azimuth and

elevation for covering an area which has the desired target. So scanning is nothing but movement of an

antenna to cover an area in which target is present.

1. Horizontal scanning: ship to ship radar

2. Elevation/vertical/nodding scanning:

3. Helical scanning:

4. spiral scanning:

Tracking of radars: After scanning method it is necessary to locate its path very accurately to predict its

future position. This is called antenna tracking. For tracking target we need range, velocity, azimuth angle&

elevation angle of target.

Methods of tacking:

An antenna having narrow pencil shaped beam helps in antenna tracking. But just a pencil beam is not

sufficient for accurate tracking of target because accuracy of this type of antenna is insufficient in itself.

Three techniques are used for tracking.

1. Lobe switching technique: It is also called sequential lobing. Here direction of antenna beam is rapidly

switched between two positions in this system so that strength of echo from target will fluctuate at the

switching rate unless target is exactly mid way between two directions.

147

Target direction

Alternate

lobe positions

Path of lobe tip

Lobe

A single beam is switched between two squinted angular positions to obtain angular measurement.

Thus sequential lobing is used for tracking a target accurately in one plane.

2. Conical scanning/or switching. After a target has been acquired the best scanning system for tracking is

conical. It is a logical extension of lobe switching and is achieved by mounting the parabolic antenna

slightly-off centre and then rotating it about the axis of parabola the rotation is slow compared to PRF.

The name conical scan is derived from the surface described in space by pencil radiation pattern of

antenna as the tip of pattern moves in a circle. The conical system is just as accurate in elevation as in

azimuth.

Disadvantages of lobe scanning &conical scanning:

1. The motion of antenna is more complex in both scanning

2. More than one returned pulse is required to locate a target(Minimum 4 pulses are required)

3. Amplitude comparisons are not particularly accurate.

Mono-pulse tracking:

An ideal system in which all information can be tracked only in 1 pulse is known as monopulse.

Generally 4 pulses are required in case of conical scanning. Most important advantage is that it is not

affected by amplitude fluctuations of target echo. It is preferred when acute angle measurement is required.

If target cross section is changing then best system for accurate tracking is mono-pulse.

The disadvantage of mono-pulse is that it requires two extra receiving channels and a more complex feeding

arrangement which makes the system bulky and expensive.

Limitation in tracking:

1. Glint: Angle noise which affects all tracking radars.

2. Amplitude fluctuations of target echo

3. Receiver noise

Beam shapes for radar:

1. Pencil-beam: Beam width of pencil beam in horizontal beam is equal to horizontal beam width in

vertical plane. Beam width of pencil beam is generally less than a few degrees. It is used in tracking

radar, 3-D radar &phased array radar.

2. Fan-beam: It has one angle small compared with others. Here azimuthal beam width might be one or

few degree while elevation beam width might be 4-10 times the azimuthal beam width.

This beam is generally modified to obtain more complete coverage example is cosecant squared shaped

beam. Surveillance radar and airborne ground surveillance radar use it.

148

Doppler Effect: when target is moving relative to radar then there will be a shift in carrier frequency of the

received signal and this effect is called as Doppler effect. The shift in frequency is Doppler shift and it is a

measure of velocity of target. CW Doppler Radar is the based upon concept of Doppler Effect.

If fd is Doppler frequency and vR is the relative frequency then2 r

d

vf

Hz. Pulse to pulse change in

frequency is known as frequency agility. The same magnitude of Doppler shift is observed regardless of

whether a target is moving towards the radar or away from it with a given velocity. Radars which use

concept of Doppler effect are CW, FM-CW and MTI radar.

Note:

Doppler Effect is observed only for radial motion and not for tangential motion. Thus no Doppler effect is

noticed if a target moves across the field of view of a radar.

CW Doppler radar: It is used for calculating speed of moving objects. This radar is limited in maximum

power it transmits and this place a limit on its maximum range. This radar is not capable of indicating range

of target and shows only its velocity.

Advantages of CW Doppler Radar:

1. It is capable of giving accurate measurement of relative velocities using low transmitter powers,

simple circuitry, low power consumption and small size equipments.

2. It is unaffected by presence of stationary targets

3. It is also capable of measuring a large range of target speeds quickly and accurately.

4. CW radar can even the measure the direction of the target in addition to its target

Disadvantages of CW Doppler Radar:

1. It is limited in maximum power it transmits and this naturally places a limit on maximum range.

2. It is rather easily confused by presence of a large number of targets

3. Radar is incapable of indicating the range of the target. It can only show its velocity because the

transmitted signal is un-modulated. The receiver can‘t sense which particular cycle of oscillation is

being received at the moment and therefore can‘t tell how long ago this particular cycle was

transmitted so the range can‘t be measured.

Applications of CW Doppler radar:

This radar is generally is used for

1. Mobile applications,

2. Police radars and also in

3. Aircraft navigation.

4. Radar speed meter

Clutter: Echoes from natural environment such as land, sea &weather are called as clutter. Clutter echoes

can be many order of magnitude larger than aircraft echoes and aircraft might n‘t be detectable.

149

MTI-Radar or Pulse Doppler radar:

Pulse radar that employs Doppler shift for removing clutter problem in case of moving targets is called as

MTI radar or a pulse radar. The basic principle of an MTI radar is to compare a set of received echoes with

those received during the previous sweep and canceling out those whose phase has remained unchanged.

Moving targets will give change of phase and are not cancelled. Thus clutter due to stationary targets both

man-made and natural are removed from display and this allows easier detection of moving targets.

Delay line canceller is used in MTI radar and is an example of time domain filter that rejects stationary

clutter at zero frequency. It has a frequency function that can be derived from time domain representation of

signals. The function of quartz delay in MTI radar is to help in subtracting a complete scan from previous

scan.

Blind speed: If target has uniform velocity the successive sweeps will have Doppler phase shifts of exactly

360 degree and target appears stationary and gives wrong radar Indication .The speed corresponding to this

condition is called as blind speed. If any object is moving with blind speed then it will look stationary.

Methods for reducing blind speed:

1. Operate the radar at long wave length i.e. low frequency.

2. Operate with high Pulse Repetition Frequency

3. Operate with more than one PRF frequency is used.

Frequency Modulated CW radar:

This radar is used for measuring height of aero plane This FM-CW radar overcome limitation of CW radar

and can calculate height as well as speed of objects.

FM/CW radar is mostly used as altimeter in aircraft due to shorter ranges and lower power requirements as

compared to pulsed radar.

Radio navigational system:

1. LORAN(Long Range Navigational Aid):This is based on principle on time difference between receipt

of signals from a pair of radio transmitters .A given constant time difference between signals from two

stations is represented by hyperbolic line of position .It is based upon pulsed system &hyperbolic system.

LORAN-A: 2 MHZ and LORAN-C: 90-110 KHZ

2. VOR :( VHF Omini-directional Range): It is a radio navigational system used for aircraft .It broadcasts

a VHF signal. VOR is using LOS propagation and limits in range of 240 km only. All VOR beacons use a

phased antenna array such that signal is rotated electronically. Frequency range is 108-136 MHZ.

3. OMEGA: It is the 1st truly global navigational system for aircraft .OMEGA employed hyperbolic radio

navigational technique and chain is operated in VLF range of 10-14 KHZ.

4. DECCA: It is also hyperbolic system.

LORAN is pulsed system while DECCA &OMEGA are continuous system. While LORAN&DECCA are

hyperbolic system.

Radar beacons: Radar is small radar consisting of a receiver, a separate transmitter and an antenna which is

often ominidirectional. One of the functions of a beacon may be to identify itself This beacon may be

installed on target such as an aircraft and will transmits a specific code. This system is used in airport traffic

control and also for military purposes where it is called as Identification Friend or Foe. It is used for

1. Target identification 2. Navigation 3.Very significant extension of maximum range

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Phased array: It is a group of antenna in which relative phases of respective signals feeding antenna are

varied in such a way that effective radiation pattern of array is reinforced in a desired direction and

suppressed in undesired direction. It is used in SONAR, optical communication & warship of several navies.

It is used for

1. Very fast scanning 2.scan&track together 3.can track many target together 4.complex circuit.

Distance measuring equipment: It is secondary radar used in aircraft.DME system consists of a pulse

transmitter and receiver in aircraft and pulse transmitter and receiver on earth station. The equipment in

aircraft is called the interrogator and equipment on ground is called as transponder. An important feature of

the transponder is that any aircraft fitted with interrogator equipment can measure its distance from

transponder.

Instrument Landing System: This instrument landing system provides the pilot information about his

position in relation to prescribed approach path continuously with the help of instrument on ground and in

the aircraft. This instrument must have 3 units

Localizer, glide-slope& marker beacons.

Ground Control Approach system: It is a high precision radar system located near the runaway of

airports. The radar system is used to bring the aircraft into approach zone and guide it along the correct path

of descent to a point very near to run away. GCA

Comprises of 2 radars one called surveillance Radar element and other called Precision Approach Radar.

Radomes: It is a sheltering structure for antennas which must be operated in severe weather conditions

Radomes must be mechanically strong and no interference with normal operation of antenna.

151

Study material for satellite communication

A communication satellite is basically a microwave link repeater between many transmitting stations and

many receiving stations. It receives by an earth station and returns to earth at a frequency of about 2 GHz

away .This prevents interference between the uplink and down link frequency. Communication satellites

have same angular velocity as the earth and so they appear to be stationed over one spot on globe. Frequency

of satellite is in the range of 1-10 GHz while for long distance frequency is in the range of 3-6 GHz. A

satellite in circular orbit 35800 km away from earth will complete a revolution in 24 hours as does the earth

below it and that is why it appears stationary.

Merits of satellite:

1. Satellite remains fixed in its position relative to earth

2. It requires a limited earth station antenna tracking

3. It is capable of providing continuous and uninterrupted communication over the desired area.

4. Satellite communications are unaffected by Doppler frequency shift.

Demerits of satellite:

1. A costly vehicle is required for launching

2. Regions near the north and south poles are not covered in communication range of satellite.

3. There is a time delay of about 300 m sec between a transmitted and received signal.

4. It requires increased EIRP as compared with low altitude system.

Working of satellite: Thus in geosynchronous satellite a message signal is transmitted from an earth station

via an uplink to a satellite amplified in transponder on board the satellite and then retransmitted from down

link to another earth station. For C-band

Frequency used are 6/4 GH z. For Ku band it is in the range of 14/12 GHz and for Ka band it is 31/21 GHz.

Typical signal received from geosynchronous satellite is of the order of pico-watt. Satellite use technique

concept of frequency reuse.

Round trip-delay: It is approximately 480 msec but if we calculate exact time then it is around 540 msec.

Different bands used in satellite:

Satellite uses C band and Ku-bands for satellite communication In C-band uplink frequency is from 5.925

GHz to 6.425 GHz and down link frequency is from 3.7GHz to 4.2GHz. So satellite bandwidth in C-band is

0.5GHz or 500 MHz.

In Ku-band transponder uplink frequency is form 14 to 14.5 GHz and down link frequency is from 11.7 to

12.2 GHz so band width is again 0.5 GHz or 500 MHz.

Why uplink has higher frequency than down link frequency:

High power uplink is transmitted to satellite and weak signal is received at ground station from satellite.

Noise picked up by down link frequency is lower than uplink frequency.

Down link weak signals can be easily amplified by earth station.

Advantages of 6/4 GHz:

1. Relative inexpensive microwave equipment.

2. Low attenuation due to rain fall

3. In significant sky back ground noise

152

6/4 GHz prefers over 14/12 GHz due to rain attenuation while by use of 14/12 GHz one can use small

component

Note: Satellite angle of elevation must be greater than 5 degree to prevent excessive signal attenuation

&noise in atmosphere.

Bandwidth of satellite: Satellite communication is a wide band RF repeater .Uplink transmission

frequencies employed range from 5.9 GHz to 6.4 GHz .In the satellite the signals are down converted to a

frequency range of 3.7 GHz to 4.2 GHz and retransmitted towards earth .So a bandwidth of 500 MHz is

required for satellite communication. The bandwidth of 500 MHz is often divide into 12 channels each of 40

MHz bandwidth .Each channel can be used for TV signal.

Frequency reuse in satellite:

The use of polarization to increase the available frequency bandwidth is to referred as frequency reuse.

It will also be observed that uplink signals in each group are polarized in the opposite sense to down

link signals. Right Hand Circular Polarization (RHCP) and LHCP may also be used in addition to vertical

and horizontal polarization which permits a further increase in frequency reuse.

Power sources for satellite: Power requirement of satellite are met by solar cells&storage batteries. .Solar

cell converts solar energy into electrical energy &these are capable of providing a power of 5 Kw while the

usual power requirement of power satellite is of the order of 1 Kw. The power from storage batteries is used

when satellite comes in the shadow zone of earth. No of days when earth‘s shadow falls on geosynchronous

satellite is 88 days.

Telemetery&telecommand:

Telemetry is a system which gives information about satellite. When a partial parameter of a satellite

deviates from its signal value then a particular is sent and this is called telecommand.

Types of satellite:

1. Passive satellite: They don‘t have any type of equipments and don‘t use any kind of electrical

energy. They act only as reflectors in space for reflecting the signals transmitted from ground.

2. Active satellite: These satellites need power for operating these equipments which is derived from

sun in form of solar energy. The solar cells convert solar energy into electrical energy. Availability

of solar fuels decides life of a particular satellite.

Coverage of earth: Three satellites can cover the entire‘s earth surface except small areas near south and

north poles but here condition is that minimum elevation angle required for earth station does not exceed 10

degree .For coverage of polar region Molina orbit is required which is highly eccentric.

So one satellite can cover 40% of earth‘s satellite .Earth coverage satellite antennas from synchronous

altitude have a beam width of 17.3 degree.

Geostationary orbit (Clarke-orbit): An orbit in which a satellite appears stationary relative to any point on

the earth is called geostationary orbit. Here satellite is placed in an equatorial orbit of about 3600 km above

earth .This orbit is achieved by synchronizing the revolution of satellite around earth with the speed of

earth‘s rotation about its own axis .

It is the orbit which remains stationary relative to earth. There is only one geostationary orbit but this is

occupied by a large number of satellite.

153

Requirement for geostationary orbit:

1. It must have zero inclination. Any other inclination would carry the satellite over some range of

latitudes and hence would not be geo stationary.

2. Second condition is that geostationary satellite should travel eastwards at the same rotational velocity as

the earth.

3. Orbit must be circular because velocity is constant.

4. The earth makes one complete rotation relative to fixed stars in approximately 23 h 56 min since it is

slightly less than the time required for one complete rotation about its own axis which is 24 hr.

5. Radius of geostationary orbit is 42164Km. The earth‘s equatorial radius is approximately 6378Km and

hence the height of geostationary orbit above earth is h=42164-6378=35786 Km

Drift in position of satellite:

The gravitational field of moon and to a lesser extent that of sun causes a drift in angle of inclination which

amounts to be about 0.85 degree per year.

For satellite operating in C band (6/4 GHz) the drift must be kept must within 00.1 and for Ku band (14/12)

GHz satellites it must be within 00.05

Look angles in satellite:

To maximize transmission and reception the direction of maximum gain of the earth station antenna referred

to as antenna bore sight must point directly at the satellite.

To align the antenna in this way two angles must be known these are azimuth or angle measured from the

true north and the elevation or angle measured up from the local horizontal plane as shown in figure:

The azimuth and elevation angles are usually referred as the look angles. It is often necessary to know the

range of distance from the earth station to satellite.

Mean radius of earth R= 6378Km

Radius of geostationary orbit: 42164geoa Km

Geosynchronous orbit: A satellite whose period of revolution equals to period of rotation of earth about its

axis. In this orbit a satellite has 24 hours non equatorial orbit.

Synchronous orbits are circular in nature. Life span of geosynchronous satellite is around 10 years. Radius

of geosynchronous orbit is 42250 km.

Note:

So geostationary orbit lies in equatorial orbit plane while orbital plane of geosynchronous is inclined to

equatorial plane. All geostationary orbits are geosynchronous but reverse is not true. Generally

communication satellites are not used in low orbit because they don‘t provide 24 hours time period.

Path loss in satellite:

Path loss (dB)=32.5+20logf+20logd where f is in MHz and d is distance and is in Km.

So if frequency is increased 2 times then path loss will be increased by 6 dB but if f is reduced 2 times then

path loss will be decreased by 6 dB.

Basic transmission loss for a satellite signal for uplink frequency of 6 GHz will be around 200 dB.

EIRP (Effective Isotropic Radiated Power):

EIRP=PT.GT PT is transmitted power and GT is gain of transmitted antenna.

Figure of merit:

154

F.O.M=G/T where G is the gain of antenna and T is the total noise temperature.

F.O.M (dB) = G (dB)-10 logT unit of FOM is in db/degree Kevin

Carrier to noise ratio:

C/N=EIRP+M-La+228.6 here C/N is in dB. M is figure of merit and La is path loss.

Apogee&perigee:

Maximum height of an elliptical orbit is called the apogee while minimum height of an elliptical orbit is

called perigee. Apogee is the point farthest from earth and perigee is the point of closest approach to earth.

Ascending node is the point where the orbit crosses the equatorial planes going from south to north.

155

Study material for optical fiber communication Optical fiber:

Optical fibres are used for light and infrared transmission in a manner identical to waveguide at microwave

frequencies. Optical fibres are increasingly replacing wire. Optical fiber concept is extension of microwave

link. In fibre optic system a light carrier is modulated by information and the modulated light carrier is

carried through fiber. The main advantage of fibre optic is low loss and high data carrying capacity.

Basic Fibre optic system:

The optical fibre use light as the carrier of information. An optical fibre use light as the carrier of

information.It uses light as the carrier of information. In optical fibre a light carrier is modulated by the

information and the modulated light carrier is carried through a fibre.The main advantage of fibre optic

technologies are the low loss and high data carrying capacity.

A laser or light emitting diode generates a light beam .This light carrier is modulated by the digital

information to be carried then modulated signal is injected into a fiber optic cable.

The light rays bounce back and forth between the walls of fibre cable until they reach the receiving end of

fibre. At the receiving end a photo detector is used to translate the modulated beam back into an electrical

signal. So optical fibre is a medium in which voice, data or video is transmitted in form of light.

Propagation of optical fibre:

Total Internal Reflection forms the basis of light propagation through a optical fibre .This analysis consider

only meridional rays –those that pass through the fibre axis each time they are reflected .Other rays called

skew rays travel down the fibre without passing through the axis .The path of a skew ray is typically helical

wrapping around the central axis. Fortunately skew rays are ignored in most fibre optics analysis.

Propagation of light through fibre depends upon:

1. The size of fibre

2. The composition of fibre

3. The light injected into the fibre

Sequence of transmission in optical fibre:

1. Information is encoded into electrical signal

2. Electrical signals are converted into light signals

3. Light travel down the fibre.

4. A detector changes the light signal into electrical signal

5. Electrical signals are decoded into information

Light source: It may be Light Emitting Diode (LED) or LASER (Light Amplification by Stimulated

Emission).Laser is nearly perfect single optical frequency source .Radiation from Laser is due to stimulated

emission.

156

Photo detector: Photo detectors may be divided into photo emissive, photovoltaic or photo conductive type

they convert optical signals into electrical current. These may be Photodiode or Avalanche Photo Diode

(APD).

Responsivity:

Light wave receivers or detectors are the final device in our basic optical communication. These detectors

are usually low power, low noise PIN diodes coupled to a FET amplifier.

The main consideration in choice of detector is repsonsivity.

Repsonsivity=A Diodecurrent

W Incident light

Advantages of optical fibre communication:

1. Light weight

2. Larger information

3. Less space and easy installation

4. Immunity to interference

5. No capacitance &inductance information

6. Secure communication

7. Resistivity to temperature &environment changes

8. Cheaper cable

9. A single fiber can handle as many voice channels as 1500 pair cable.

Optical fibre construction: An optical fire consists of a central core and an outer cladding. The material of

core has higher index of refraction as compared to cladding. Light can propagate through an optical fibre by

total internal reflection and continuous refraction. Optical fibres that propagate light by TIR are most widely

used. Materials used for optical fibres are glass (fused silica) and plastic. Glass material has superior optical

quantities but is fragile and more expensive as compared to plastic. There are three varieties of optical fibre

available today.

1. Glass core with glass cladding

2. Glass core with plastic cladding

3. Plastic core with plastic cladding

Glass fibres are more costly as compared to plastic fibres. Glass fibres propagate light more efficiently than

plastic. Glass fibres are used for high speed applications and also used for long transmission paths. Plastic

fibres are more flexible and rugged than flux.

Total internal reflection in optical fibre:

157

01 2

0

1

2

2

2

1

2 2

1 1 2

sin sin& 90

sin 1 sin 90

sin cos 1

sin sin

a cc

c

a

n nbut

n

n

n

n n n

Numerical aperture: The Numerical Aperture is measure of light gathering ability of an optical fibre. The

fiber with a large numerical aperture accepts more light as compared to fibre with small numerical aperture.

The numerical aperture of a fibre is important because it gives an indication of how the fibre accepts and

propagates light. A fibre with a large numerical aperture accepts light well while fibre with a low numerical

aperture requires high directional light.

2 2

0 1 2. sin aN A n n n

Losses in fibers:

1. Rayleigh losses: These losses are induced because of this scattering effect vary inversely with the

fourth power of wavelength so that their effects are reduced to less than about 0.3dB per km at wave

length of 1.3 km.

2. Absorption losses: Ultraviolet absorption, infrared absorption and ion resonance absorption

contribute to absorption losses.

3. Ultraviolet absorption: Takes place because of pure fused silica, valance electrons can be ionized

into conduction electrons by light with a centre wavelength of about 0.14 micro meter.

4. Infrared absorption: It takes place because photons of light energy are absorbed by the atoms

within glass molecules and converted to the random mechanical vibrations typical of heating.

5. Ion resonance absorption: It takes place because photons of light energy are absorbed by the atoms

within the glass molecules and converted to the random mechanical vibrations typical of heating.

6. Bending losses: It occurs because small bends acts as scattering which cause mode coupling to

occur. Since micro-bends are randomly distributed over the length of fibre, losses resulting from

them will be randomly distributed and a total figure for a fibre can be obtained.

Types of fiber index:

The refractive index profile describes the relation between the indices of core& cladding.

It is of 2 types:

1. Step index 2.Greaded index

The step index fibre has a core with uniform index throughout the profile shows a sharp step at the junction

of core and cladding .In contrast the graded index has a non uniform core .The index is highest at centre and

gradually decreases until it matches with that of cladding. There is no sharp break in indices between core

and cladding.

Modes of propagation:

Modes are the various paths the light can take in travelling down the fiber. The light can travel in single

mode and multi mode. If there is only one path for light to take down the cable it is called a single mode. If

there are more than one path it is called multi mode. In single mode propagation the central core of fibre is

so small that there is only one path the light can take as it propagates down the fibre.

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Single mode

In multi mode propagation the central core is much larger and in this case the

By this classification there are 3 types of fibres:

1. Multi mode step index fibre

2. Multi mode graded index fibre

3. Single mode step index fibre

Step index multi mode fibre:

This fibre is called step index because the refractive index changes abruptly from clad to core .The cladding

has a refractive index somewhat lower than the refractive index of core glass. The paths along which the

modes of this step index fibre travel differ depending on their angles relative to the axis. As a result the

different modes in a pulse will arrive at the far end of the fibre at different times resulting in pulse spreading

which limits the bit rate of a digital signal which can be transmitted.

Modal dispersion in step index fibre:

The multi mode step index has a fiber core of diameter from 100 to 970 micro meter. with this large core

diameter there are many paths through which light can travel(multi mode).Therefore the light ray travelling

the straight path through the centre reaches the end before the other rays. The difference in the length of time

it takes the various light rays to exist the fiber is called modal dispersion. This is a form of signal distortion

which limits the bandwidth of the fiber.

Graded index multi mode fibre:

This fibre is called graded index because there are many changes in the refractive index with larger values

towards the centre. As light travels faster in a lower index of refraction so the farther the light is from center

axis the greater is its speed. Graded index fibers have core diameter of 50, 62.5 or 85 micro meter and a

cladding diameter of 125 micro meter.

Modal dispersion in graded index fibre:

Since light travel faster through the lower index of refraction the light at the fiber core travels more slowly

than the light nearer the surface. Therefore both light rays arrive at the exits point at almost the same time.

Thus reducing modal dispersion. The graded index reduces model dispersion to 1ns/km or less. The graded

index fibre is used in application requiring a wide bandwidth and a low model dispersion. The no of modes

in fibre is about half that of step index fibre having the same diameter &Numerical aperture.

Step index single mode fibre:

Another way to reduce model dispersion is to reduce the core‘s diameter until the fibre only propagates one

mode efficiently. The single mode fibre has a small core diameter of only 5-10 micro meter. Standard

cladding diameter is 125 micro meter. Since the fibre carries only 1 mode so model dispersion does not

exist. So single mode fibre easily have a potential bandwidth of 50-100 GHz km .The core diameter is so

small that the splicing technique and measuring techniques are very difficult .One advantage of single mode

159

is that once they are installed the system‘s capacity can be increased. Single step index has a high critical

angle of 077 .

Single mode step index fibers are the most widely used in today‘s wide band communication. In this fiber a

light ray can travel on only one path therefore modal dispersion is zero.

Specification of single mode fiber:

1. The bandwidth is from 50 to 10 GHz/km

2. The digital communication rate is in excess of 2000Mbytes/sec

3. More than 100,000 voice channels are available

4. Light wavelength approach core diameter therefore high frequency capabilities are achieved

5. The Mode Field Diameter (MFD, spot size) is larger than core diameter.

Calculation of number of mode in a multi mode fiber:

The normalized frequency of cut-off (Also called cut-off parameter or V number) is useful for determining

how many modes a given fiber will support. It can be proved that

2 2

1 2

0 0

1 2

.

. . & .

d dV n n N A d diameter of core

N A Numerical aperture n R I of core n R I of cladding

If the waveguide parameter V found is considerably larger than unity then the approximate number of modes

which the fiber will support is given by: 2

Number of modes2

V

Numerical aperture& number of modes:

In general fibers with a high bandwidth have a lower numerical aperture. They thus allow fewer modes

means less dispersion and hence greater bandwidth. A larger numerical aperture promotes more modal

dispersion since more paths for the rays are provided numerical aperture although it can be defined for a

single mode fibre is essentially meaningless as a practical. NA ranges from 0.50 for plastic to 0.21 for

graded index fibers.

Window frequency in fibre:

Attenuation of fibre for optical power varies with wavelengths of light. Windows are low loss regions where

fibres carry light with little attenuation.

A narrow window is defined as the range of wavelengths at which a fibre best operates.

1. First window: 800-900 nm –Wavelength=850nm

2. Second window: 1250-1350 nm –Wavelength=1300 nm

3. Third window: 1500-1600 nm –Wavelength=1550nm

Attenuation:

It is defined as the loss of optical power over a set distance a fibre with lower attenuation will allow more

power to reach a receiver than fibre with high attenuation.

Attenuation is of 2 types:

1. Extrinsic attenuation

2. Intrinsic attenuation

160

Intrinsic attenuation: It is loss due to inherent or within the fibre .It may occur as

1. Absorption: natural impurities in the glass absorb light energy

2. Scattering: light rays traveling in the core reflect from small imperfections into a new pathway that may

be lost through the cladding.

Extrinsic attenuation: It is lost due to external sources. It may occur as

1. Micro bending: The fibre is sharply bent so that the light traveling down the fibre can‘t make the turn

&is lost in cladding. It is due to small surface irregularities in the cladding causes light to be reflected at

angles where there is no further reflection.

2. Macro bending: Macro bending or small bends in fibre caused by crushing contraction etc. These bends

may not be visible with the naked eyes. It causes certain modes not to be reflected and therefore causes

losses to cladding.

Attenuation losses in optical fiber:

Attenuation is the loss of optical energy as it travels through the fiber. This loss is measured in decibels per

Km. These losses may vary from 300dB/Km for expensive fiber to as low 0.21 dB/Km for high quality

single mode fibers.

Dispersion:

It is the spreading of light pulse as it travels down the length of an optical fiber.

Dispersion limits the bandwidth or information capacity of a fibre. Dispersion is of 3 types.

1. Modal dispersion: It occurs only in modal dispersion

2. Material dispersion:

3. waveguide dispersion: It is most significant in single mode fibre.

OFC splicing:

Splices are permanent connection between two fibers .The splicing involves cutting of the edges of two

fibers to be spliced .The following 3 methods are used for splicing.

1. Adhesive bonding or glue splicing

2. Mechanical splicing

3. Fusion splicing

This technique is the most popular technique used for achieving very low splice losses.

Splice loss measurement is to be taken by an OTDR (Optical Time Domain Reflectometer)

161

Chapter-5: Important terms used in Electronics&communication

Absorption: Loss or dissipation of energy as it travels through a medium Example: radio waves lose some

of their energy as they travel through the atmosphere.

AC coupling: Circuit that passes an AC signal while blocking a DC voltage.

AC load line: A graph representing all possible combinations of AC output voltage and current for an

amplifier.

Active component: A component that changes the amplitude of a signal between input and output.

Active filter: A filter that uses an amplifier in addition to reactive components to pass or reject selected

frequencies.

Active region: The region of BJT operation between saturation and cutoff used for linear amplification.

Admittance :( symbol "Y") Measure of how easily AC will flow through a circuit. Admittance is the

reciprocal of impedance and is measured in Siemens.

Alternating current: An electric current that rises to a maximum in one direction, falls back to zero and

then rises to a maximum in the opposite direction and then repeats.

Amplifier: A circuit that increases the voltage, current, or power of a signal.

Amplitude: Magnitude or size of a signal voltage or current.

Analog: Information represented as continuously varying voltage or current rather than in discrete levels as

opposed to digital data varying between two discrete levels.

Antenna, transmitting: A device that converts an electrical wave into an electromagnetic wave that

radiates away from the antenna.

Antenna, receiving: A device that converts a radiated electromagnetic wave into an electrical wave.

Armstrong oscillator: An oscillator that uses an isolation transformer to achieve positive feedback from

output to input.

Astable multivibrator: An oscillator that produces a square wave output from a DC voltage.

Attenuate: To reduce the amplitude of an action or signal. The opposite of amplification.

Band-pass filter: A tuned circuit designed to pass a band of frequencies between a lower cut-off frequency

(f1) and a higher cut-off frequency (f2). Frequencies above and below the pass band are heavily attenuated.

Band-stop filter: A tuned circuit designed to stop frequencies between a lower cut-off frequency (f1) and a

higher cut-off frequency (f2) of the amplifier while passing all other frequencies.

Bandwidth: width of the band of frequencies between the half power points.

Barrier potential: The natural difference of potential that exists across a forward biased pn junction.

Baud: A unit of signaling speed equal to the number of signal events per second. Not necessarily the same

as bits per second.

Bias: A DC voltage applied to a device to control its operation.

Bistable multivibrator: A multivibrator with two stable states. An external signal is required to change the

output from one state to the other. Also called a latch.

Bleeder current: A current drawn continuously from a source. Bleeder current is used to stabilize the

output voltage of a source.

Bridge rectifier: A circuit using four diodes to provide full wave rectification. Converts an AC voltage to a

pulsating DC voltage.

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Buffer: An amplifier used to isolate a load from a source.

Bulk resistance: The natural resistance of a "P" type or "N" type semiconductor material.

Butterworth filter: A type of active filter characterized by a constant gain (flat response) across the

midband of the circuit and a 20 dB per decade roll-off rate for each pole contained in the circuit.

Bypass capacitor: A capacitor used to provide an AC ground at some point in a circuit.

Capacitance: The ability of a capacitor to store an electrical charge. The basic unit of capacitance is the

Farad.

Capacitive reactance: The opposition to current flow provided by a capacitor. Capacitive reactance is

measured in ohms and varies inversely with frequency.

cascaded amplifier: An amplifier with two or more stages arranged in a series configuration.

Cascode amplifier: A high frequency amplifier made up of a common-source amplifier with a common-

gate amplifier in its drain network.

Center frequency: Frequency to which an amplifier is tuned. The frequency half way between the cut-off

frequencies of a tuned circuit.

Center tap: Midway connection between the two ends of a winding.

Center tapped transformer: A transformer with a connection at the electrical center of a winding.

Center tapped rectifier: Circuit that make use of a center tapped transformer and two diodes to provide full

wave rectification.

Ceramic capacitor: Capacitor in which the dielectric is ceramic.

Charge: Quantity of electrical energy.

Chebyshev filter: A type of active filter characterized by high roll-off rates (40 dB per decade per pole) and

midband gain that is not constant.

Choke: Inductor used to oppose the flow of alternating current.

Circuit: Interconnection of components to provide an electrical path between two or more components.

Circuit breaker: A protective device used to open a circuit when current exceeds a maximum value. In

effect a reusable fuse.

Clamper: A diode circuit used to change the DC level of a waveform without distorting the waveform.

Clapp oscillator: A variation of the Colpitts oscillator. An added capacitor is used to eliminate the effects of

stray capacitance on the operation of the basic Colpitts oscillator.

Clipper: A diode circuit used to eliminate part of a waveform

Coaxial cable: Transmission line in which the signal carrying conductor is covered by a dielectric and

another conductor.

Colpitts oscillator: An oscillator with a pair of tapped capacitors in the feedback network.

Common-mode rejection ratio :( CMRR) The ratio of op-amp differential gain to common-mode gain. A

measure of an op-amp's ability to reject common-mode signals such as noise.

Comparator: An op-amp circuit that compares two inputs and provides a DC output indicating the polarity

relationship between the inputs.

Complementary symmetry amplifier: A class B amplifier using matched complementary transistors. Does

not require a phase inverter for push-pull output.

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Complementary transistors:

Two transistors, one NPN and one PNP having near identical charastictic N-channel and P-channel FETs

can also be complementary.

Constant current circuit: Circuit used to maintain constant current to a load having resistance that changes.

Crossover distortion: Distortion caused by both devices in a class B amplifier being cut-off at the same

time.

Crystal: Natural or synthetic piezoelectric or semiconductor material with atoms arranged with some degree

of geometric regularity.

Crystal-controlled oscillator: Oscillator that uses a quartz crystal in its feedback path to maintain a stable

output frequency.

Current: Measured in amperes, it is the flow of electrons through a conductor. Also know as electron flow.

Current amplifier: Amplifier to increase signal current.

Current-limiting resistor: Resistor in the path of current flow to control the amount of current drawn by a

device.

Current mirror: Term used to describe the fact that DC current through the base circuit of a class B

amplifier is approximately equal to the DC collector current.

Cutoff: Condition when an active device is biased such that output current is near zero or beyond zero.

Cutoff frequency: Frequency at which the power gain of an amplifier falls below 50% of maximum.

Darlington pair: An amplifier consisting of two bipolar junction transistors with their collectors connected

together and the emitter of one connected to the base of the other. Circuit has an extremely high current gain

and input impedance.

DC load line: A graph representing all possible combinations of voltage and current for a given load resistor

in an amplifier.

DC offset: The change in input voltage required to produce a zero output voltage when no signal is applied

to an amplifier.

Degenerative feedback: Also called negative feedback. A portion of the output of an amplifier is inverted

and connected back to the input. This controls the gain of the amplifier and reduces distortion and noise.

Delay time: The time for collector current to reach 10% of its maximum value in a BJT switching circuit.

Depletion region: The area surrounding a P-N junction that is depleted of carriers.

Depletion mode: In a FET, an operating mode where reverse gate-source voltage is used to deplete the

channel of free carriers. This reduces the size of the channel and increases its resistance.

Dielectric: Insulating material between two plates where an electrostatic field exists.

Dielectric constant: Property of a material that determines how much electrostatic energy can be stored per

unit volume when unit voltage is applied.

Dielectric strength: The maximum voltage an insulating material can withstand without breaking down.

Differential amplifier: An amplifier in which the output is in proportion to the differences between voltages

applied to its two inputs.

Diffusion: Tendency of conduction band electrons to wander across a pn junction to combine with valence

band holes.

Digital: Relating to devices or circuits that have outputs of only two discrete levels. Examples: 0 or 1, high

or low, on or off, true or false etc.

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Diode: A two terminal device that conducts in only one direction.

Direct coupling: Where the output of an amplifier is connected directly to the input of another amplifier or

to a load. Also known as DC coupling because DC signals are not blocked.

Discrete component: Package containing only a single component as opposed to an integrated circuit

containing many components in a single package.

Dissipation: Release of electrical energy in the form of heat.

Distortion: An undesired change in a waveform or signal.

Distributed capacitance: Any capacitance other than that within a capacitor. For example, the capacitance

between adjacent turns of wire in a coil.

Distributed inductance: Any inductance other than that within an inductor. Example inductance in any

conductor.

Domain: A moveable magnetized area in a magnetized material. Also known as magnetic domain.

Doping: The process of adding impurity atoms to intrinsic (pure) silicon or germanium to improve the

conductivity of the semiconductor material.

Drift: A problem that can develop in tuned amplifiers when the frequency of the tuned circuit changes due

to temperature or component aging.

Dual in-line package: Integrated circuit package having two rows of connecting pins.

Dual trace oscilloscope: Oscilloscope that can simultaneously display two signals.

Efficiency: The amount of power delivered to the load of an amplifier as a percentage of the power required

from the power supply.

Electric charge: Electric energy stored on the surface of a material. Also known as a static charge.

Electric field: A field or force that exists in the space between two different potentials or voltages. Also

known as an electrostatic field.

Electricity: Science states that certain particles possess a force field or charge. The charge possessed by an

electron is negative while the charge possessed by a proton is positive. Electricity can be divided into two

groups, static and dynamic. Static electricity deals with charges at rest and dynamic electricity deals with

charges in motion.

Electromagnetic communication: Use of an electromagnetic wave to pass information between two points.

Also called wireless communication.

Electromagnetic wave: wave that consists of both electric and magnetic variation.

Emitter feedback: Coupling from the emitter output to the base input of a bipolar junction transistor.

Emitter follower: A common collector amplifier. Has a high current gain, high input impedance and low

output impedance.

Enhancement-mode MOSFET:A field effect transistor in which there are no charge carriers in the channel

when the gate source voltage is zero.

Feedback: A portion of the output signal of an amplifier which is connected back to the input of the same

amplifier.

Feedback amplifier: An amplifier with an external signal path from its output back to its input.

Fiber optics: Laser‘s light output carries information that is conveyed between two points by thin glass

optical fibers.

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Field effect transistor :(FET) A voltage controlled transistor in which the source to drain conduction is

controlled by gate to source voltage.

Flip flop: A bistable multivibrator. A circuit which has two output states and is switched from one to the

other by means of an external signal (trigger).

Floating ground: Common connection in a circuit that provides a return path for current but is not

connected to an earth ground.

Flywheel effect: Sustaining effect of oscillation in an LC circuit.

Free running multivibrator:A multivibrator that produces a continuous output waveform without any

signal input. A square wave generator used to produce a clock signal.

Frequency-division multiplex :( FDM) Transmission of two or more signals over a common path by using

a different frequency band for each signal.

Frequency-domain analysis: A method of representing a waveform by plotting its amplitude against

frequency.

Function generator: Signal generator that can produce sine, square, triangle and saw tooth output

waveforms.

Gain: Increase in voltage, current and/or power. Gain is expressed as a ratio of amplifier output value to the

corresponding amplifier input value.

Gain bandwidth product: A device parameter that indicates the maximum possible product of gain and

bandwidth. The gain bandwidth product of a device is equal to the unity gain frequency (funity) of the device.

Ground: An intentional or accidental conducting path between an electrical system or circuit and the earth

or some conducting body acting in place of the earth. A ground is often used as the common wiring point or

reference in a circuit.

Gunn diode: A semiconductor diode that utilizes the Gunn effect to produce microwave frequency

oscillation or to amplify a microwave frequency signal.

half power point: A frequency at which the power is 50% of maximum. This corresponds to 70.7% of

maximum current or voltage.

h-parameters:(hybrid parameters) Transistor specifications that describe the component operating limits

under specific circumstances.

Hartley oscillator: An oscillator that uses a tapped inductor in the feedback network.

High-pass filter: A tuned circuit designed to pass all frequencies above a designated cut-off frequency.

Frequencies below the cut-off frequency are rejected or attenuated

Hole: A gap left in the covalent bond when a valence electron gains sufficient energy to jump to the

conduction band

Hybrid circuit: Circuit that combines two technologies (passive and active or discrete and integrated

components) onto one microelectronic circuit. Passive components are usual made by thin film techniques,

while active components are made with semiconductor techniques.

Hysteresis: Amount that the magnetization of a material lags the magnetizing force due to molecular

friction. In Schmitt Trigger circuits, the difference between the upper and lower trigger points.

IC: Abbreviation for "integrated circuit"

IC voltage regulator: Three terminal device used to hold the output voltage of a power supply constant

over a wide range of load variations.

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Impedance :(Z) Measured in ohms it is the total opposition to the flow of current offered by a circuit.

Impedance consists of the vector sum of resistance and reactance.

Impedance matching: Matching the output impedance of a source to the input impedance of a load to attain

maximum power transfer.

Inductance: Property of a circuit to oppose a change in current. The moving magnetic field produced by a

change in current causes an induced voltage to oppose the original change.

Input impedance: Opposition to the flow of signal current at the input of a circuit or load.

Integrated: When two or more components are combined into a circuit and then incorporated into a single

package.

Integrator: A device that approximates and whose output is proportional to an integral of the input signal. A

low pass filter.

Intermediate frequency amplifier :In a superheterodyne radio it amplifies a fixed frequency lower than the

received radio frequency and higher than the audio frequency.

Internal resistance: Every source has some resistance in series with the output current. When current is

drawn from the source some power is lost due to the voltage drop across the internal resistance. Usually

called output impedance or output resistance.

Intrinsic material: A semiconductor material with electrical properties essentially characteristic of ideal

pure crystal. Essentially silicon or germanium crystal with no measurable impurities.

Intrinsic stand-off ratio: A unijunction transistor (UJT) rating used to determine the firing potential of the

device.

Junction: Contact or connection between two or more wires or cables. The area where the p-type material

and n-type material meet in a semiconductor.

Junction diode: A semiconductor diode in which the rectifying characteristics occur at a junction between

the n-type and p-type semiconductor materials.

Kirchhoff"s current law: The sum of the currents flowing into a point in a circuit is equal to the sum of the

currents flowing out of that same point.

Kirchhoff"s voltage law: The algebraic sum of the voltage drops in a closed path circuit is equal to the

algebraic sum of the source voltages applied.

Knee voltage: The voltage at which a curve joins two relatively straight portions of a characteristic curve.

For a PN junction diode, the point in the forward operating region of the characteristic curve where

conduction starts to increase rapidly. For a zener diode, the term is often used in reference to the zener

voltage rating.

Laser: Device that produces a very narrow intense beam of light. The name is an acronym for "light

amplification by stimulated emission of radiation.

LED: Abbreviation for "light emitting diode."

Lenz's law: The current induced in a circuit due to a change in the magnetic field is so directed as to oppose

the flux, or to exert a mechanical force to oppose the motion.

Level detector: An op-amp circuit that compares two inputs and provides a DC output indicating the

polarity relationship between the inputs.

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Lie-detector: Piece of electronic equipment also called a polygraph used to determine whether a person is

telling the truth by looking for dramatic changes in blood pressure, body temperature, breathing rate, heart

rate and skin moisture in response to questions.

Light-emitting diode: A semiconductor diode that converts electric energy into electromagnetic radiation at

a visible and near infrared frequencies when its pn junction is forward biased.

Limiter: Circuit or device that prevents some portion of its input from reaching the output. A clipper.

Linear: Relationship between input and output in which the output varies in direct proportion to the input.

Loading effect: Large load impedance will draw a small load current and so loading of the source is small.

(light load). Small load impedance will draw a large load current from the source. (heavy load).

Load regulation: The ability of a voltage regulator to maintain a constant output voltage under varying load

currents.

Load resistance: Resistance of a load.

Logic: Science of dealing with the principle and applications of gates, relays and switches.

Magnetic poles: Points of a magnet from which magnetic lines of force leave (north pole) and arrive (south

pole).

Matched impedance: Condition that occurs when the output impedance of a source is equal to the input

impedance of a load.

Matching: Connection of two components or circuits so that maximum power is transferred between the

two.

Maximum power transfer: A theorem that states that maximum power will be transferred from source to

load when input impedance of the load equals the output impedance of the source.

Metal oxide field effect transistor :( MOSFET) A field effect transistor in which the insulating layer

between the gate electrode and the channel is a metal oxide layer.

Microphone: Electro acoustic transducer that converts sound energy into electric energy.

Microwave: Band of very short wavelength radio waves within the UHF, SHF and EHF bands.

Midband gain: Gain of an amplifier operating within its bandwidth.

Miller's theorem: A theorem that allows you to represent a feedback capacitor as equivalent input and

output shunt capacitors.

Minority carriers: The conduction band holes in n-type material and valence band electrons in p-type

material. Most minority carriers are produced by temperature rather than by doping with impurities.

Mismatch: Term used to describe a difference between the output impedance of a source and the input

impedance of a load. A mismatch prevents the maximum transfer of power from source to load.

Modulation: Process by which information signal (audio for example) is used to modify some characteristic

of a higher frequency wave known as a carrier (radio for example).

Monostable multivibrator: A multivibrator with one stable output state. When triggered, the circuit output

will switch to the unstable state for a predetermined period of time and then return to the stable state. A

timer.

MOSFET: Abbreviation for "metal oxide field effect transistor" also known as an "insulated gate field

effect transistor). A field effect transistor in which the insulating layer between the gate electrode and the

channel is a metal oxide layer.

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Multivibrator:A class of circuits designed to produce square waves or pulses. Astable multivibrators

produce continuous pulses without an external stimulus or trigger. Monostable multivibrators produce a

single pulse for some predetermined period of time only when triggered. Bistable multivibrators produce a

DC output which is stable in either one of two states. Either high or low. An external stimulus or trigger is

required for the bistable circuit to change states, either high to low or low to high.

Negative feedback: A feedback signal 180° out of phase with an amplifier input signal. Used to increase

amplifier stability, bandwidth and input impedance. Also reduces distortion.

Negative resistance: A resistance such that when the current through it increases the voltage drop across the

resistance decreases.

Negative temperature coefficient: A term used to describe a component whose resistance or capacitance

decreases when temperature increases.

Network: Combination of interconnected components, circuits or systems.

Noise: Unwanted electromagnetic radiation within an electrical or mechanical system. An operational

amplifier circuit having no phase inversion between the input and output.

Non-inverting input: The terminal on an operational amplifier that is identified by a plus sign.

Norton's theorem: Any network of voltage sources and resistors can be replace by a single current source

in parallel with a single resistor.

Notch filter: A filter which blocks a narrow band of frequencies and passes all frequencies above and below

the band.

NPN transistor: A bipolar junction transistor in which a p-type base element is sandwiched between an n-

type emitter and an n-type collector.

Offset null: An op amp control pin used to eliminate the effects of internal component voltages on the

output of the device.

Ohm's law: Relationship between voltage, current and resistance. Ohm's law states that current in a

resistance varies in direct proportion to voltage applied and inversely proportional to resistance.

Ohms per volt: Refers to a value of ohms per volt of full scale defection for a moving coil meter

movement. The number of ohms per volt is the reciprocal of the amount of current required to produce full

scale deflection of the needle. A meter requiring 50 microamps for full scale deflection has an internal

resistance of 20 kW per volt. The higher the ohms per volt rating, the more sensitive the meter.

One-shot: Monostable multivibrator.

Op-amp: Abbreviation for operational amplifier.

Open loop gain: Gain of an amplifier when no feedback is present.

Open loop mode: An amplifier circuit having no means of comparing the output with the input. (No

feedback.)

Operational amplifier: A high gain DC amplifier that has high input impedance and a low output

impedance. Op-amps are the most basic type of linear integrated circuits.

Oscillate: To produce a continuous output waveform without an input signal present.

Oscillator: An electronic circuit that produces a continuous output waveform with only DC applied.

Oscilloscope: An instrument used to display a signal graphically. Shows signal amplitude, period and

waveshape in addition to any DC voltage present. A multiple trace oscilloscope can show two or more

waveforms at the same time for phase comparison and timing measurements.

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Output: Terminal at which a component, circuit or piece of equipment delivers current, voltage or power.

Output impedance: Impedance measured across the output terminals of a device without a load connected.

Overload: condition that occurs when the load is greater than the system was designed to handle. (Load

resistance too small, load current too high.) Overload results in waveform distortion and/or overheating.

Overload protection: Protective device such as a fuse or circuit breaker that automatically disconnects a

load when current exceeds a predetermined value.

Paper capacitor: Fixed capacitor using oiled or waxed paper as a dielectric.

Parallel resonant circuit: Circuit having an inductor and a capacitor in parallel with one another. Circuit

offers a high impedance at resonant frequency. Sometimes called a "tank circuit."

Pass band: The range of frequencies that will be passed and amplified by a tuned amplifier. Also the range

of frequencies passed by a band pass filter.

Passive component: Component that does not amplify a signal. Resistors and capacitors are examples.

Passive filter: A filter that contains only passive or non amplifying components.

Passive system: System that emits no energy. It only receives. It does not transmit or reveal its position.

Peak inverse voltage:(PIV) The maximum rated value of a AC voltage acting in the direction opposite to

that in which a device is designed to pass current.

Peak to Peak: Difference between the maximum positive and maximum negative values of an AC

waveform.

Pentavalent element: Element whose atoms have five valence electrons. Used in doping intrinsic silicon or

germanium to produce n-type semiconductor material. Most commonly used pentavalent materials are

arsenic and phosphorus.

Percent of regulation: The change in output voltage that occurs between no-load and full-load in a DC

voltage source. Dividing this change by the full-load value and multiplying the result by 100 gives percent

regulation.

Percent of ripple: The ratio of the effective rms value of ripple voltage to the average value of the total

voltage. Expressed as a percentage.

Phase shift oscillator: An oscillator that uses three RC networks in its feedback path to produce the 180°

phase shift required for oscillation.

Phase splitter: Circuit that takes a single input signal and produces two output signals that are 180° apart in

phase.

Photoconductive cell: Material whose resistance decreases or conductance increases when exposed to light.

Photoconduction: A process by which the conductance of a material is change by incident electromagnetic

radiation in the visible light spectrum.

Photo-detector: Component used to detect or sense light.

Photodiode: A semiconductor diode that changes its electrical characteristics in response to illumination.

Photon: Discrete portion of electromagnetic energy. A small packet of light.

Photo resistor: Also known as a photoconductive cell or light dependent resistor. (LDR) A device whose

resistance decreases with exposure to light.

Photovoltaic cell: Component commonly called a solar cell used to convert light energy into electrical

energy.

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Piezoelectric crystal: Crystal material that will generate a voltage when mechanical pressure is applied and

conversely will undergo mechanical stress when subjected to a voltage.

Piezoelectric effect: The production of a voltage between opposite sides of a piezoelectric crystal as a result

of pressure or twisting. Also the reverse effect which the application of a voltage to opposite sides causes a

deformation to occur at the frequency of the applied voltage. (Converts mechanical energy into electrical

energy and electrical energy into mechanical energy.)

Pinch-off region: A region on the characteristic curve of a FET in which the gate bias causes the depletion

region to extend completely across the channel.

Pole: In an active filter, a single RC circuit. A one pole filter has one capacitor and one resistor. A two pole

filter has two RC circuits and so on.

Positive feedback: A feedback signal that is in phase with an amplifier input signal. Positive feedback is

necessary for oscillation to occur.

Potentiometer: A variable resistor with three terminals. Mechanical turning of a shaft can be used to

produce variable resistance and potential. Example: A volume control is usually a potentiometer.

Power: Amount of energy converted by a circuit or component in a unit of time, normally seconds.

Measured in units of watts. (Joules/second).

Power amplifier: An amplifier designed to deliver maximum power output to a load. Example: In an audio

system, it is the power amplifier that drives the loudspeaker.

Power dissipation: Amount of heat energy generated by a device in one second when current flows through

it.

Programmable UJT: Unijunction transistor with a variable intrinsic stand-off ratio.

Quality factor of an inductor or capacitor:

It is the ratio of a component's reactance (energy stored) to its effective series resistance (energy dissipated).

For a tuned circuit, a figure of merit used in bandwidth calculations. Q is the ratio of reactive power to

resistive power in a tuned circuit. Also the symbol for charge in coulombs (Q for quantity).

Quiescent: At rest. For an amplifier the term is used to describe a condition with no active input signal.

Quiescent point :( Q point) A point on the DC load line of a given amplifier that represents the quiescent

(no signal) value of output voltage and current for the circuit.

Radio communication: Term used to describe the transfer of information between two or more points by

use of radio or electromagnetic waves.

Radio-frequency amplifier: Amplifier having one or more active devices to amplify radio signals.

Radio-frequency generator: Generator capable of supplying RF energy at any desired frequency in the

radio-frequency spectrum.

Radio-frequency probe: Probe used in conjunction with an AC meter to measure radio-frequency signals.

RC time constant: Product of resistance and capacitance in seconds.

Reactance: Symbol "X". Opposition to current flow without the dissipation of energy. Example: The

opposition provided by inductance or capacitance to AC current.

Recombination: Process by which a conduction band electron gives up energy (in the form of heat or light)

and falls into a valence band hole.

Rectangular coordinates: A Cartesian coordinate of a Cartesian coordinate system whose straight-line axes

or coordinate planes are perpendicular.

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Regenerative feedback: Positive feedback. Feedback from the output of an amplifier to the input such that

the feedback signal is in phase with the input signal. Used to produce oscillation.

Regulated power supply: Power supply that maintains a constant output voltage under changing load

conditions.

Regulator: Device or circuit that maintains a desired output under changing conditions.

Relay: Electromechanical device that opens or closes contacts when a current is passed through a coil.

Relaxation oscillator: Free running circuit that outputs pulses with a period dependent or one or more RC

time constants.

Reluctance: Resistance to the flow of magnetic lines of force.

Residual magnetism: Magnetism remaining in the core of an electromagnet after the coil current is

removed.

Resistance: Symbolized "R" and measured in ohms. Opposition to current flow and dissipation of energy in

the form of heat.

Resistive power: Amount of power dissipated as heat in a circuit containing resistive and reactive

components. True power as opposed to reactive power.

Resistor: Made of material that opposes flow of current and therefore has some value of resistance.

Resonance: Circuit condition that occurs at the frequency where inductive reactance (XL) equals capacitive

reactance (XC).

Reverse bias: Bias on a PN junction that allows only leakage current (minority carriers) to flow. Positive

polarity on the n-type material and negative polarity to the p-type material.

Reverse breakdown voltage: Amount of reverse bias that will cause a PN junction to break down and

conduct in the reverse direction.

Reverse current: Current through a diode when reverse biased. An extremely small current also referred to

as leakage.

Reverse saturation current: Reverse current through a diode caused by thermal activity. This current is not

affected by the amount of reverse bias on the component, but does vary with temperature.

Ripple frequency: Frequency of the ripple present in the output of a DC source.

Ripple voltage: The small variations in Dc voltage that remain after filtering in a power supply.

Rise time: Time for the leading edge of a pulse to rise from 10% of its peak value to 90% of its peak value.

RL differentiator: An RL circuit whose output voltage is proportional to the rate of change of the input

voltage.

RL filter: Selective circuit of resistors and inductors that offers little or no opposition to certain frequencies

while blocking or attenuating other frequencies.

RL integrator: RL circuit with an output proportionate to the integral of the input signal.

Rms value: Rms value of an AC sine wave is 0.707 times the peak value. This is the effective value of an

AC sine wave. The rms value of a sine wave is the value of a DC voltage that would produce the same

amount of heat in a heating element.

Roll-off rate: Rate of change in gain when an amplifier is operated outside of its bandwidth.

Rotary switch: Electromechanical device that has a rotating shaft connected to one terminal capable of

making or breaking a connection to one or more other terminals.

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R-2R ladder: Network or circuit composed of a sequence of L networks connected in tandem. Circuit used

in digital to analog converters.

Saturation: Condition in which a further increase in one variable produces no further increase in the

resultant effect. In a bipolar junction transistor, the condition when the emitter to collector voltage is less

than the emitter to base voltage. This condition puts forward bias on the base to collector junction.

Saw tooth wave: Repeating waveform that rises from zero to maximum value linearly drops back to zero

and repeats. A ramp waveform.

Schmitt trigger: Circuit to convert a given waveform to a square wave output.

Schottky diode: High speed diode that has very little junction capacitance. Also known as a "hot-carrier

diode" or a "surface-barrier diode."

Selectivity: Chrematistic of a circuit to discriminate between wanted and unwanted signals.

Self biasing: Gate bias for a field effect transistor in which source current through a resistor produces the

voltage for gate to source bias.

Self inductance: Property that causes a counter electromotive force to be produced in a conductor when the

magnetic field expands or collapses with a change of current.

Semiconductor: An element which is either a good conductor or a good insulator, but rather lies somewhere

between the two. Characterized by a valence shell containing four electrons. Silicon, germanium and carbon

are the semiconductors most frequently used in electronics.

Series circuit: Circuit in which the components are connected end to end so that current has only one path

to follow through the circuit.

Series resonance: Condition that occurs in a series LC circuit at the frequency where inductive reactance

equals capacitive reactance. Impedance is minimum, current is maximum limited only by resistance in the

circuit.

Seven segment displays: Device made of several light emitting diodes arranged in a numeric or

alphanumeric pattern. By lighting selected segments numeric or alphabet characters can be displayed.

Shield: Metal grounded cover used to protect a wire, component or piece of equipment from stray magnetic

and/or electric fields.

Short circuit: Also called a short. Low resistance connection between two points in a circuit typically

causing excessive current.

Signal: Electrical quantity that conveys information.

Signal to noise ratio: Ratio of the magnitude of the signal to the magnitude of noise usually expressed in

decibels.

Silicon :( Si) Non metallic element (atomic number 14) used in pure form as a semiconductor.

Silicon-controlled rectifier :(SCR) Three terminal active device that acts as a gated diode. The gate

terminal is used to turn the device on allowing current to pass from cathode to anode.

Silicon controlled switch: An SCR with an added terminal called an anode gate. A positive pulse either at

the anode gate or the cathode gate will turn the device on.

Silicon dioxide: Glass like material used as the gate insulating material in a MOSFET.

Simplex: Communication in only one direction at a time. Example: FAX.

Simulcast: Broadcasting a program simultaneously in two different forms, for example a program broadcast

in both AM and FM.

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Single pole double throw :(SPDT) Three terminal switch in which one terminal can be connected to either

one of the other terminals.

Single pole single throw :(SPST) Two terminal switch or relay that can open or close one circuit.

Single sideband :(SSB) AM radio communication technique in which the transmitter suppresses one

sideband and therefore transmits only a single sideband.

Single throw switch: Switch containing only one set of contacts which can be either opened or closed.

Sink: Device such as a load that consumes power or conducts away heat.

Skin effect: Tendency of high-frequency (rf) currents to flow near the surface layer of a conductor.

Slew rate: The maximum rate at which the output voltage of an op-amp can change.

Software: Program of instructions that directs the operation of a computer.

Solar cell: Photovoltaic cell that converts light into electric energy. Especially useful as a power source for

space vehicles.

solid state: Pertaining to circuits where signals pass through solid semiconductor material such as transistors

and diodes as opposed to vacuum tubes where signals pass through a vacuum.

Sonar: Acronym for "sound navigation and ranging." A system using reflected sound waves to determine

the position of some target.

Sound wave: Pressure waves propagated through air or other plastic media. Sound waves are generally

audible to the human ear if the frequency is between approximately 20 and 20,000 vibrations per second.

(Hertz)

Source follower: FET amplifier in which signal is applied between gate and drain with output taken

between source and drain. Also called "common drain."

Source impedance: Impedance through which output current is taken from a source.

Spectrum: Arrangement or display of light or other forms of electromagnetic radiation separated according

to wavelength, energy or some other property.

Spectrum analyzer: Instrument used to display the frequency domain of a waveform plotting amplitude

against frequency.

Speed-up capacitor: Capacitor added to the base circuit of a BJT switching circuit to improve the switching

time of the device.

Stop band: Range of frequencies outside the pass band of a tuned amplifier.

Storage time: In a BJT switching circuit, it is the time required for collector current to drop from 100% to

90% of its maximum value.

Stranded conductor: Conductor composed of a group of strands of wire twisted together.

Stray capacitance: Undesirable capacitance that exists between two conductors such as two leads or one

lead and a metal chassis.

Superconductor: Metal such as lead or niobium that, when cooled to within a few degrees of absolute zero,

can conduct current with no resistance.

Super heterodyne receiver: Radio receiver that converts all radio frequencies to a fixed intermediate

frequency to maximize gain and bandwidth before demodulation.

Super high frequency :( SHF) Frequency band between 3 GHz and 30 GHz. So designated by Federal

Communications Commission (FCC).

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Superposition theorem: Theorem designed to simplify networks containing two or more sources. It states

that in a network containing more than one source, the current at any one point is equal to the algebraic sum

of the currents produced by each source acting separately.

Sweep generator: Test instrument designed to produce a voltage that continuously varies in frequency over

a band of frequencies. Used as a source to display frequency response of a circuit on an oscilloscope.

Switch: Electrical device having two states, on (closed) or off (open). Ideally having zero impedance when

closed and infinite impedance when open.

Switching transistor: transistor designed to change rapidly between saturation and cut-off.

Synchronization: Also called sync. Precise matching of two waves or functions.

Synchronous: Two or more signals in step or in phase.

System: Combination of several pieces of equipment to perform in a particular manner.

Tank circuit: Parallel resonant circuit containing only a coil and a capacitor. Both the coil and capacitor

store electrical energy for part of each cycle.

Telegraphy: Communication between two points by sending and receiving a series of current pulses either

through wire or by radio.

Telemetry: Transmission of instrument readings to a remote location either by wire or by radio.

Telephone: Apparatus designed to convert sound waves into electrical waves which are sent to and

reproduced data distant point.

Telephone line: Wires existing between subscribers and central stations in a telephone system.

Television :System that converts both audio and visual information into corresponding electrical signals

which are then transmitted through wires or by radio waves to a receiver which reproduces the original

information.

Thermal runaway: Problem that can develop in an amplifier when an increase in temperature causes an

increase in collector current. The increase in collector current causes a further increase in temperature and so

on. Unless the circuit is designed to prevent this condition, the device can be driven into saturation.

Thermal stability: The ability of a circuit to maintain stable characteristics in spite of increased

temperature.

Thermistor: Temperature sensitive semiconductor that has a negative temperature coefficient of resistance.

As temperature increases, resistance decreases.

Thermocouple: Temperature transducer consisting of two dissimilar metals welded together at one end to

form a junction that when heated will generate a voltage.

Thermostat: Device that opens or closes a circuit in response to changes in temperature.

The venin’s theorem: Theorem that replaces any complex network with a single voltage source in series

with a single resistance.

Threshold voltage: For an enhancement MOSFET, the minimum gate source voltage required for

conduction of source drain current.

Thyristor: A term used to classify all four layer semiconductor devices. SCRs and triacs are examples of

thyristors.

Time constant :(t) Time required for a capacitor in an RC circuit to charge to 63% of the remaining

potential across the circuit. Also time required for current to reach 63% of maximum value in an RL circuit.

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Time constant of an RC circuit is the product of R and C. Time constant of an RL circuit is equal to

inductance divided by resistance.

Time division multiplexing :( TDM) Transmission of two or more signals on the same path, but at different

times.

Transconductance: Also called mutual conductance. Ratio of a change in output current to the change in

input voltage that caused it.

Transmission line: Conducting line used to transmit signal energy between two points.

Triggering: Initiation of an action in a circuit which then functions for a predetermined time. Example: The

duration of one sweep in a cathode ray tube.

Trimmer: Small value variable capacitor, resistor or inductor used to fine tune a larger value.

Tuned circuit: Circuit that can have its component values adjusted so that it responds to one selected

frequency and rejects all others.

Tunnel diode: Heavily doped junction diode that has negative resistance in the forward direction of its

operating range.

Turn-off time: Sum of storage time and fall time.

Turn-on time: Sum of delay time and rise time.

Ultrasonic: Signals that are just above the frequency range of human hearing of approximately 20 kHz.

Unijunction transistor: Three terminal device that acts as a diode with its own internal voltage divider

biasing circuit.

Varactor diode: PN junction diode with a high junction capacitance when reverse biased. Most often used

as a voltage controlled capacitor. The varactor is also called: varicap, tuning diode

Very high frequency :(VHF) Electromagnetic frequency band from 30 MHz to 300 MHz

Very low frequency :(VLF) Frequency band from 3 kHz to 30 kHz.

Video amplifier: Amplifier having one or mare stages designed to amplify video signals.

Virtual ground: Point in a circuit that is always at approximately ground potential. Often a ground for

voltage, but not for current.

voice synthesizer: Synthesizer that can simulate speech by stringing together phonemes.

Voltage amplifier: Amplifier designed to build up signal voltage. By design amplifiers can have a large

voltage gain or a large current gain or a large power gain. Voltage amplifiers are designed to maximize

voltage gain often at the expense of current gain or power gain.

Voltage controlled oscillator: Oscillator whose output frequency depends on an input control voltage.

Voltage follower: Operational amplifier circuit characterized by a high input impedance, low output

impedance and unity voltage gain. Used as a buffer between a source and a low impedance load.

Voltage gain: Also called voltage amplification. Ratio of amplifier output voltage to input voltage usually

expressed in decibels.

Voltage multiplier: Rectifier circuit using diodes and capacitors to produce a DC output voltage that is

some multiple of the peak value of AC input voltage. Cost effective way of producing higher DC voltages.

Voltage doublers and voltage triplers are examples.

Waveguide: Rectangular or circular pipe used to guide electromagnetic waves at micro frequencies.

Wien-bridge oscillator: Oscillator that uses an RC low-pass filter and an RC high-pass filter to set the

frequency of oscillations.

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Wheatstone bridge: Four arm bridge circuit used to measure resistance, inductance or capacitance.

Wideband amplifier: Also called "broadband amplifier." Amplifier with a flat response over a wide range

of frequencies.

Woofer: Large loudspeaker designed primarily to reproduce low frequency audio signals.

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Chapter-6: Latest technology material for interview:

Anti-collision device:

The Anti-Collision Device (ACD) is a self-acting microprocessor-based data communication device

designed and developed by Konkan Railway.

When installed on locomotives (along with an auto-braking unit - ABU), guard vans, stations and level-

crossing gates (both manned and unmanned), the network of ACD systems prevents high-speed collisions in

mid-sections, station areas and at level-crossing gates.

The ACD uses both radio frequency and Global Positioning System (GPS) through satellites, whereby a

train is automatically brought to a halt if the track ahead is not clear. The train starts braking 3 kms ahead of

a blockade.

Block section in railway:

Trains cannot collide with each other if they are not permitted to occupy the same section of track at the

same time, so railway lines are divided into sections known as blocks. In normal circumstances, only one

train is permitted in each block at a time. This principle forms the basis of most railway safety systems.

Train detection circuit:

1. Track circuit

One of the most common ways to determine whether a section of line is occupied is by use of track circuit.

The rails at either end of each section are electrically isolated from the next section, and an electrical current

is fed to both running rails at one end. A relay at the other end is connected to both rails. When the section is

unoccupied, the relay coil completes an electrical circuit, and is energized. However, when a train enters the

section, it short-circuits the current in the rails, and the relay is de-energized.

2. Axle counter:

An alternative method of determining the occupied status of a block is using devices located at its beginning

and end that count the number of axles entering and leaving. If the same number leave the block as enter it,

the block is assumed to be clear. Although axle counters can provide similar functionality to track circuits,

they also exhibit a few other characteristics. In a damp environment an axle counted section can be far

longer than a track circuited one.

Global Positioning system:

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24

satellites placed into orbit by the U.S. Department of Defence. GPS was originally intended for military

applications, but in the 1980s, the government made the system available for civilian use. GPS works in any

weather conditions, anywhere in the world 24 hours a day. There are no subscription fees or setup charges to

use GPS.

Working of GPS:

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth.

GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially,

the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The

time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a

few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude

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and longitude) and track movement. With four or more satellites in view, the receiver can determine the

user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS

unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination,

sunrise and sunset time and more.

GPS Satellite system:

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us.

They are constantly moving, making two complete orbits in less than 24 hours. These satellites are travelling

at speeds of roughly 7000 miles an hour.GPS satellite are powered by solar energy. They have backup

batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small

rocket boosters on each satellite keep them flying in the correct path.

Generation of Mobile:

1G: Purely analog

2G: It is mainly for voice and slow transmission

2.5G: So the cellular services combined with GPRS became 2.5G.

GPRS could provide data rates from 56 kbit/s up to 114 kbit/s. It can be used for services such as Wireless

Application Protocol (WAP) access, Multimedia Messaging Service (MMS), and for Internet

communication services such as email and World Wide Web access.

2.75G: GPRS networks evolved to EDGE networks with the introduction of 8PSK encoding. Enhanced Data

rates for GSM Evolution (EDGE), Enhanced GPRS (EGPRS), or IMT Single Carrier (IMT-SC) is a

backward-compatible digital mobile phone technology that allows improved data transmission rates, as an

extension on top of standard GSM.

EDGE can be considered a 3G radio technology and is part of ITU's 3G definition, but is most frequently

referred to as 2.75G. EDGE was deployed on GSM networks .

3G:

International Mobile Telecommunications-2000 (IMT-2000):

Better known as 3G or 3rd

Generation, is a family of standards for wireless communications defined by the

International Telecommunication Union, which includes EDGE, CDMA2000, the UMTS family as well as

DECT and WiMAX. Services include wide-area wireless voice telephone, video calls, and wireless data, all

in a mobile environment. Compared to 2G and 2.5G services, 3G allows simultaneous use of speech and

data services and higher data rates (up to 14.4 Mbit/s on the downlink and 5.8 Mbit/s on the uplink with

HSPA+). Thus, 3G networks enable network operators to offer users a wider range of more advanced

services while achieving greater network capacity through improved spectral efficiency.

GPRS (General Packet Radio Service):

It offers high speed data services in GSM network. It uses Packet Mode Technique to transfer data and

provides connectivity to Internet. Users will be able to browse Internet using handsets supporting Internet

browsing. They will also be able use their e-mail accounts as is being done through landline Internet access.

Also browsing of Internet from Laptops and Desktop computers is possible by connecting the computer with

the GPRS enabled mobile handset through a data cable or Infrared connectivity.

Using GPRS you can download in your mobile the following:

• Polyphonic ring tones

• MP3 tones

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• Colour logos

• Wallpapers

• Videos

GSM&CDMA:

GSM: Global system for Mobile Communication

CDMA: Code Division Multiple Access.

GSM networks operate in a number of different frequency ranges (separated into GSM frequency ranges for

2G and UMTS frequency bands for 3G). Most 2G GSM networks operate in the 900 MHz or 1800 MHz

bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900

MHz bands because the 900 and 1800 MHz frequency bands were already allocated.

Most 3G GSM networks in Europe operate in the 2100 MHz frequency band.

GSM-900 uses 890–915 MHz to send information from the mobile station to the base station (uplink) and

935–960 MHz for the other direction (downlink), providing 125 RF channels (channel numbers 0 to 124)

spaced at 200 kHz. Duplex spacing of 45 MHz is used.

In some countries the GSM-900 band has been extended to cover a larger frequency range. This extended

GSM, E-GSM uses 880–915 MHz (uplink) and 925–960 MHz (downlink), adding 50 channels (channel

numbers 975 to 1023 and 0) to the original GSM-900 band. Time division multiplexing is used to allow

eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio

timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use

alternate frames in the same timeslot. The channel data rate for all 8 channels is 270.833 kbit/s, and the

frame duration is 4.615 ms.The transmission power in the handset is limited to a maximum of 2 watts in

GSM850/900 and 1 watt in GSM1800/1900.

SIM:

One of the key features of GSM is the Subscriber Identity Module, commonly known as a SIM card. The

SIM is a detachable smart card containing the users subscription information and phone book. This allows

the user to retain his or her information after switching handsets. Alternatively the user can also change

operators while retaining the handset simply by changing the SIM. Some operators will block this by

allowing the phone to use only a single SIM, or only a SIM issued by them, this practice is known as SIM

locking, and is illegal in some countries.

WAP (Wireless Application Protocol):

It provides a standardized way of linking the Internet to mobile phones. WAP is an application

communication protocol. WAP is used to access services and information. It is inherited from Internet

standards. It is used for handheld devices such as mobile phones and PDAs. It is a protocol designed for

micro browsers. It enables the creating of web-applications for mobile devices WAP uses the mark-up

language called WML instead of regular HTML. The WAP facility is available over CSD & GPRS for the

Cell One customers.

MMS (Multimedia Messaging Service):

Mobile Messaging is evolving beyond SMS text messaging with the introduction of MMS (Multimedia

Messaging Service). MMS delivers a total communication experience allowing personalized multimedia

content such as images, audio, text, video and combinations of these.

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Multimedia Messaging Service (MMS) is a store and forward messaging service that allows mobile

subscribers to exchange multimedia messages with other mobile subscribers. As such it can be seen as an

evolution of SMS, with MMS supporting the transmission of additional media types.

MMS is an important emerging service, which allows the sending of multiple media in a single message,

and the ability to send a message to multiple recipients. The originator can easily create a Multimedia

Message, either using a built-in or accessory camera, or can use images and sounds stored previously in the

phone (and possibly downloaded from a web site).Even if the recipient phone is not switched on, the

Multimedia Message will be stored and sent to the recipient as soon as they switch on their phone. If the

recipient has not subscribed to the MMS service, still he/she can view the MMS through internet based on

the SMS notification he/she gets.

A number of Multimedia Messages can be stored in the users handset and reviewed or forwarded at a later

date.

ATM&PVC:

(Asynchronous Transfer Mode & Permanent Virtual circuit):

ISDN:(integrated circuit Digital Network):

ISDN Has emerged as a powerful tool worldwide for provisioning of different services like voice, data and

image transmission over the telephone line through the telephone network. ISDN is being viewed as the

logical extension of the digitalization of telecommunication network and most developed countries are in

different stages of implementing ISDN.

An ISDN subscriber can establish two simultaneous independent calls (except when the terminal equipment

is such that it occupies two 'B' channels for one call itself like in video conferencing etc.) on existing pair of

wires of the telephone line (Basic rate ISDN) where as only one call is possible at present on the analog line

/telephone connection. The two simultaneous calls in ISDN can be of any type like speech, data, image etc.

Services Offered By ISDN

Normal Telephone & Fax (G3)

Digital Telephone -with a facility to identify the calling subscriber number and other facilities.

G4 Fax

Data Transmission at 64 Kbps with ISDN controller card

Video Conferencing at 128 Kbps

Video Conferencing at 384 Kbps (Possible with 3 ISDN lines)

Broadband service:

BSNL is in the process of commissioning of a world class, multi-gigabit, multi-protocol, convergent IP

infrastructure through National Internet Backbone-II (NIB-II), that will provide convergent services through

the same backbone and broadband access network. The Broadband service will be available on DSL

technology (on the same copper cable that is used for connecting telephone), on a countrywide basis

spanning 198 cities. In terms of infrastructure for broadband services NIB-II would put India at par with

more advanced nations. The services that would be supported includes always-on broadband access to the

Internet for residential and business customers, Content based services, Video multicasting, Video-on-

demand and Interactive gaming, Audio and Video conferencing, IP Telephony, Distance learning

messaging: plain and feature rich, Multi-site MPLS VPNs with Quality of Service (QoS) guarantees.

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The subscribe will be able to access the above services through Subscriber Service Selection System (SSSS)

portal.

Important Terms in modern communication:

Wi-Fi: Wireless fidelity

1. Wi-Fi allows local area networks (LANs) to be deployed without wires for client devices, typically

reducing the costs of network deployment and expansion. Spaces where cables cannot be run, such as

outdoor areas and historical buildings, can host wireless LANs.

2. Wireless network adapters are now built into most laptops. The price of chipsets for Wi-Fi continues to

drop, making it an economical networking option included in even more devices. Wi-Fi has become

widespread in corporate infrastructures.

3. Different competitive brands of access points and client network interfaces are inter-operable at a basic

level of service. Products designated as "Wi-Fi Certified" by the Wi-Fi Alliance are backwards

compatible. Wi-Fi is a global set of standards. Unlike mobile telephones, any standard Wi-Fi device will

work anywhere in the world.

4. Wi-Fi is widely available in more than 220,000 public hotspots and tens of millions of homes and

corporate and university campuses worldwide. The current version of Wi-Fi Protected Access encryption

(WPA2) is not easily defeated, provided strong passwords are used. New protocols for Quality of

Service (WMM) make Wi-Fi more suitable for latency-sensitive applications.

Wi-MAX: worldwide Interoperability for Microwave Access

WiMAX, meaning Worldwide Interoperability for Microwave Access, is a telecommunications technology

that provides wireless transmission of data using a variety of transmission modes, from point-to-multipoint

links to portable and fully mobile internet access. The technology provides up to 3 Mbit/sec broadband

speed without the need for cables. The technology is based on the IEEE 802.16 standard (also called

Broadband Wireless Access). The name "WiMAX" was created by the WiMAX Forum, which was formed

in June 2001 to promote conformity and interoperability of the standard. The forum describes WiMAX as "a

standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to

cable and DSL

What is electronics?

Electronics is flow of electrons or other electrically charged particles in semiconductor in a controlled

manner but electrical is study of presence or movement of electric charge.

Microwave oven:

It uses Magnetron microwave and generates microwave at frequency of 2.45 GHz for purpose of cooking of

food. This is based on molecule of water and other components and other compounds which rotate or vibrate

and due to this vibration heat is generated. Every organic matter is generally made up of matter.

Different microwave frequency band:

L, S, C, X, Ku, K, Ka (1-40GHz) and other higher frequency bands are as follows:

1. Q-band(30-50)

2. U-Band(40-60)

3. V-band(50-75)

4. E-band(60-90)

5. W-band(75-110)

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6. F-Band(90-140)

7. D-band(110-170)

Cable TV: CATV (Community Antenna TV)

CATV is a system of providing TV FM radio programming and other services to consumer via radio

frequency signals transmitted directly to people‘s TV through optical fiber and Co axial cable. In case of

radio broad casting use of different frequency allow many channels to be distributed through same cable

without separates wires for each. Here tuner of TV-VCR (Video-Cassette-Recorder). Since here a point is

operating like an antenna from where many households are taking cables so it is known as community

antenna TV.

Plasma TV:

It is an emissive flat panel display. It is lighter and much thinner than traditional TV and video display .In

Plasma TV light is emitted by phosphorous which is excited by plasma discharge between two flat panels of

glass. Thickness of plasma is less than 10 cm. There is Neon and Xenon gas in plasma TV which is

contained between hundred of tiny cells. Phosphorus in a plasma display give-off colored light when they

are excited. Contrast ratio of plasma TV is 5000:1. Main advantage of plasma TV is that a very wide screen

can be produced using extremely thin materials. In plasma each pixel is lit individually so image is very

bright and looks good from almost every angle.

Plasma state: It is an ionized gas and considered to be a distinct phase of matter, here ionized means at

least one electron has been disassociated from a significant fraction of molecule. Plasma are most common

phase of matter and entire visible universe outside solar system is plasma.

LCD:

It is a thin flat display device made of any number of color or monochrome pixels arrayed in front of a light

source or reflector. It uses a very small amount of power and i.e. Why suitable for use in battery powered

electronic devices. LCD works on concept of optical polarizer. For color LCD 3sub pixels are used which

are colored as red, green and blue. Main advantage of LCD is less power consumption and its disadvantages

are lower contrast ratio and larger response time.

Analog Digital TV:

Analog TV encodes television picture information as an analog signal i.e. by varying the voltage or

frequency of signal. Common Analog TV system are NTSC (National Television system Committee) and

PAL (Phase Alternating Line).

Digital TV uses digital modulation and compression to broad cast video audio and data signals. It can be

used to carry more channels in some amount of bandwidth than analog TV and it receive high definition

programming. This digital signal eliminates common analog broad casting defects such as ghosting, static

noise etcs.

Aspect ratio in case of digital TV is 16:9 while in analog it is 4:3.

High-Definition-TV (HDTV):

It has higher resolution than traditional formats like NTSC, PAL etcs. HDTV is broadcasted generally and

therefore coincides with introduction of digital TV. High definition means TV or display is able to accept

video over a HDMI connection using a new connector known as HDMI. HDTV has aspect ratio of 16:9 and

thus effective resolution is increased.

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High Definition Multimedia Interface (HDMI):

It provides an interface between any compatible digital audio video/audio source such as set-top-box, DVD

player and digital TV. HDMI supports standard, enhanced or high definition video plus multichannel audio

on a single cable.

Direct to Home (DTH):

It is a term that describes satellite television service which is delivered via communication satellite.

Radio frequency:

It is a portion of electromagnetic spectrum in which electromagnetic waves can be generated by AC fed to

an antenna. Generally RF range is between 3Hz-300GHz.

Sub-marine cable:

This is cable laid beneath the sea to carry telecommunication between countries. Normal radio

communication can‘t travel through thick conductors such as salt water. VLF(3-30 KHz) can penetrate sea

water down to a depth of roughly 20 meter hence a submarine staying at shallow depth can use these

frequencies.

Repeater:

It is an analog device that amplifies an input signal which may be either digital or analog. Repeaters are used

in both copper wire cables and optical fiber carrying light. Repeaters are used in broadcasting where they are

known as booster.

FCC- Rules :( Federal Communication Commission)

These are certain rules and which governed all radio spectrums in world wide.

Short wave: (3MHz-30MHz)

Short wave frequencies are capable of reaching other side of planet because they can be reflected by

ionosphere. SW are used for domestic broad casting in countries with a widely dispersed population and also

for international broad casting.

Medium wave (300 KHz-3MHz)

It is standard AM broad cast band. These waves have property of following curvature of earth i.e ground

wave at all times and also reflect ionosphere. Medium wave is ideal for both local and continent worldwide.

Long wave (<500KHz)

Frequency below 500 KHz in these wave follow curvature of earth unlike SW they don‘t reflect or refract

from ionosphere.

Different broad cast frequency:

1. AM radio :535-1605 KHZ(LF)

2. TV band-I :54MHz-88MHZ(VHF)

3. FM radio band-II: 88MHz-108MHz(VHF)

4. TV band-III: 174MHz-216MHz

5. TV band-IV&V:512MHz-806MHz(UHF)

Audio frequency:

It has range between 20-20KHz. These range are audible to human ear. It contains bands ELF, SLF,

ULF&VLF.

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Nano technology:

This technology is based upon fact that properties of material becomes totally different when their size

approaches that of a few hundred or tens of atoms. By control of matter at dimension of roughly 1-100nm

property will change. 2 phenomenonas occur by going at that level.

1. Quantum confinement 2. Gibbs-Thomson effect.

So nano technology means there is plenty of room at bottom

Wi-Fi :( Wireless-fidelity)

By this technique a computer/laptop/PDA will connect to internet. There is a hot spot which is radiating

internet signals in air. Wi-Fi uses IEEE802.11 standard. For access to internet person‘s laptop must be Wi-Fi

enabled. In this case hot-spot will broadcast its SSID (Service Set Identifier Network) via packets.

Wi-Max (Worldwide Interoperability of Microwave Access):

It is similar to Wi-Fi concept but it is used for long distance access. Wi-MAX is a wireless MAN. It is a

network that connects IEEE 802.11(hot spot) with each other and to other parts of internet and provides a

wireless alternative to cable and DSL. IEEE 802.16 provides upto 50 km of linear service area range and

allows connectivity between users without a direct LOS.

ISDN: (Integrated service Digital Network)

It is a type of circuit switched telephone network which is designed to allow digital transmission of voice

and data over ordinary telephone copper wires which result in better quality and higher speed than available

with analog system. It provides simultaneous voice-data and text transmission between individual desktop

,video conferencing and group video conferencing systems.

Channels in ISDN:

B-channel are used for data and D-channels are intended for signaling and control but can be used for data

also.

Access in ISDN:

There are two types of access to ISDN.

1. Basic Rate Interface (BRI): Here BRI consists of 2B channels with a B.W of 64Kbps and one D channel

with a B.W of 16 Kbps. So BRI contains 2B+D

2. Primary Rate Interface (PRI): here PRI consists of large no of B channels and one D channel. PRI

consists of 23B+D

Digital Subscriber-Line (DSL):

DSL is replacement of ordinary modem over same copper cable. So it is a family of technologies that

provides digital data transmission over local telephone network. Down load speed ranges from 128 kbps to

24000kbps. It is a very high speed connection that uses same wire as regular telephone network.

Advantages of DSL Modem:

1. In this case one can leave interconnection open and can still use phone line for voice call.

2. Speed of DSL is higher than that of normal modem

3. DSL modem does not require new wiring and can use same phone line for internet connection.

Disadvantage of DSL modem:

1. DSL connection is better when you are closer to provider‘s central office

2. Connection is faster for receiving data than for sending it over the internet

3. Service is not available every where

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ADSL-(Asymmetric Digital Subscriber Line):

Here data flow in 1direction is higher than data flow in other direction .Here down load speed is higher than

upload speed. ADSL uses two frequency band

1. 25.875KHz-138KHz is used for upstream

2. 138KHz-1104KHz is used for down load speed.

VDSL-Very high bit rate DSL: It is used for high bit rate DSL

SDSL- Symmetric DSL: It is used mainly by business man. Receiving and sending data rate is same for

both down load speed.

RADSL- Rate Adaptive DSL:

Variation of ADSL modem can adjust speed of connection depending upon length and quality of line.

Mechatronics:

It is combination of mechanical-engineering, electronics-engineering& software- engineering. Mechatronics

is related with self operating machine eg.robot.

Internet&World Wide Web:

Internet is a collection of interconnected computer networks linked by copper wires&fiber optics cables.

While web is a collection of interconnected documents linked by hyper links and URL and is accessible

using internet. It is publically accessible worldwide system of interconnected computer network which

transmits data by packet switching using a standardized Internet protocol (IP). Internet is based upon packet

switching.

Internet was started as ARPANET in 1965 by DARPA (Defence Advance Research project Agency) which

later grew as internet. ARPANET is Advanced Research Project Agency Network.

Internet protocol (IP):

These are certain rules which govern communication of data across a packet switched network.

IP-Address:

It is a unique number that devices use in order to identify and communicate with each other on a network

utilizing internet protocol standard. Number currently used in IP address ranges from 1.0.0.0 to

255.255.255.255. Internet protocol knows each logical host interface by a number, the IP address on any

given network.

I-Mode:

It is a wireless internet service which is very popular in Japan. It was inspired by WAP developed in USA. I-

mode was developed as an expensive method of packet switched high speed communication.

WAP :( Wireless Application Protocol)

It is an open international standard for application that uses wireless communication eg. Internet access from

a mobile phone. It is known protocol used for majority of world‘s mobile internet sites.

POTS service: (Post office Telephone service)

ACARS :( Aircraft Communication Addressing and Reporting system):

It is a digital data link system for transmission of small message between aircraft and ground station via

radio or satellite. It was protocol defined in 1970 and used telex format.

Mobiles phone are not allowed in aircraft because mobile phone could interfere with sensitive equipment on

aircraft. Level of interference depends upon phone system used and phone components in plane. Older

analog phones transmit more power and so more interference.

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Blue tooth:

It is a way to connect and exchange information between two devices like mobiles Mobile jammer:

,PC‘s, laptops, printers and digital camera etc.It is available everywhere and i.e. why called as globally

available short range network. Both devices in this network must be blue tooth enabled.

General application of blue tooth:

It transfers file between 2 mobiles, laptop to laptop, 2 LANS can also be constructed keyboard to PC and

between desk top and mice.

Mobile Jammer:

By use of mobile jammer mobile can‘t transmit and receive signal from/to BTS. It is basically used for

avoiding disturbances at temple, church etc. It works on very simple concept that is interference. This

mobile jammer sends same frequency which mobile phone uses and this causes enough interference for

mobile signal which make mobile signal very weak and finally not able to communicate with respective

BTS.

E-mail:

For sending e-mail from one place to another important protocols used are SMTP and POP-3.

Working of E-mail:

Step-1: sender writes e-mail and his MUA (Mail-User-Agent)formats the message in internet e-mail format

and a protocol SMTP sends it to local MTA(Mail-Transfer-Agent). Mail-Transfer-Agent is a computer

program that transfer e-mail message from one place to another place MTA is same as Mail-Exchange-

Server.

Step-2: MTA looks up this domain name in DNS to find another MTA accepting message for that domain

this another MTA is nothing but another Mail-Exchange-Server.

Step-3: Now message goes to User‘s inbox from this MTA by use of SMTP protocol. So mail is reached in

inbox from MTA by use of SMTP.

Step-4: Now user gets this e-mail by use of Post-Office-Protocol (POP-3)

Modem and Codec:

Modem: For long distance transmission data should be in analog form because digital data can‘t be sent for

long distance. In case of digital repeater, transformer can‘t be used efficiently. But in general modulation

used is Digital so to avoid this problem one will use Modem which will convert digital to analog on TX side

and analog to digital on Rx side. So by use of modem transmission is made easier over transmission channel.

So Modem means modulation and demodulation both.

Codec: It means coding/decoding or compression/decompression.

Difference between communication/Telecommunication: Exchanging information between 2entities is

known as communication if it is at long distance then it is known as telecommunication. If receiver is

listening and understands everything effectively then it is called as effective communication

Various channel Access method:

FDMA: Frequency Division Multiplexing

TDMA: Time Division Multiplexing

SSMA: Spread Spectrum Multiplexing

SDMA: Space Division Multiplexing

WDMA: wave length Division Multiplexing

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CSMA: Carrier Sense Multiplexing

FDMA: In this case total bandwidth is divided into many bands of frequency and each subdivision/band has

its own carrier frequency.

TDMA: In this case same frequency is divided into different time slots. This is generally used in mobile

telephony.

GSM uses FDMA to prevent interference between outward and inward signals while TDMA is used to allow

multiple handsets to work on a single cells.

SSMA: In this case energy generated at a single frequency is deliberately spread over a wide band of

frequency. This concept is generally used in mobile jammers.

SDMA: Here power is distributed in a particular direction which has more number of users. So directivity

of antenna will be high in a particular direction and low in another direction. So space division is according

to situation or availability of users.

WDMA: when multiple optical carrier signals are multiplexed on a single optical fiber by use of different

wavelengths of LASER light to carry different signals.

Note: WDM is applied to optical carrier while FDM is applied to radio carrier. Both Radio and light are both

forms of electromagnetic radiation.

CSMA: Transmitter listens for carrier wave before trying to send it, it tries to detect the presence of an

encoded signal from another station before attempting to transmit. Multiple accesses means multiple nodes

send and receive on one medium.

Line code: it is a code chosen for use within a communication system for transmission purposes. This line

code must not contain DC component because transmission of DC component is not possible for long

distance so line code is preferred for long distance communication. Example of line code are Uni-polar,

Polar, Bi-polar and Manchester.

Manchester coding is one in which each bit of data is signified by at-least on transition. It is known as self

clocking i.e. accurate synchronization of data stream is possible.

Pulse Code Modulation (PCM): It is a standard form of digital audio

&video and is used in Compact Disc (CD). PCM is a digital representation of an analog signal where

magnitude of signal is sampled regularly at uniform intervals and then quantized to a series of symbols in

digital code.

MP-3 player: MPEG-1 audio layer-3 is a lossy compression format and it provides representation of PCM

audio data in much smaller size by compressing redundant terms. So MP-3 player is nothing but compressed

form of PCM based digital audio.

Dolby sound: It is a trade mark for audio noise reduction system and other systems that improves

performance and fidelity of audio recording.

FM&PM: Frequency modulation and Phase modulation are two modulation techniques. FM is generally

used at VHF radio frequency for high fidelity broadcasts of music and speech. Advantage of FM is that it is

more robust against noise and interference and i.e. why it is a high fidelity radio transmission. PM is not

preferred because it requires more complex receiving hardware and there can be ambiguity problem of

phase.

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TV/Car remote control:

IR (Infra-red) transmission is used for this type of remote control. IR transmission is used for short range

communication. Remote control contains LED which transmits IR light and TV/car contains Si-Photo-diode.

IR light from remote control falls on the photo diode of TV and this photo diode responds to IR light.

Generally this photo diode responds only to rapidly pulsing signal created by transmitter.

IR radiations are useful for indoor use in areas of high population density and it cannot penetrate walls &so

does not interfere with other devices in adjoining room.

I-MAX: It is a film projection system created by company called I MAX of Canada. It is 22 meter wide and

16 meter high.

Global-Positioning System (GPS): it is a satellite navigation system which is used to determine position of

a target by use of a satellite constellation of 24 satellite in intermediate circular orbit 20200Km. It is used for

locating a target, vehicle and also used for surveying.

Vehicle tracking by GPS/GPS tracking:

Every vehicle will contain GPS receiver and calculates current position using process of trilateration after

measuring distance to at least 4 satellites by comparing satellite coded time signal transmission. Here

receiver calculates the orbit of each satellite based on information encoded in their radio signals and

measures distance to each satellite. So this data is recorded in this unit,this recorded data can be stored

within tracking unit or it may be transmitted to a central location form where info can be send back to user

after making it in user friendly form.

Attenuation&distortion:

Attenuation means loss of signal and generally repeaters are used for compensating attenuation or loss. But

repeaters amplify the noise along with the signal resulting in a poor signal to noise ratio.

Distortion means inaccurate reproduction of a signal caused by changes in signal‘s waveform,either

amplitude or frequency. To avoid distortion equalizers are used and one type of equaliser used in analog

environment is the load coil. By use of load coils frequency response is flatten.

Performance parameters in digital transmission:

In digital transmission system the quality of communication is mainly assessed by two factors:

1. Bit-Error-Rate

2. Jitter

BER: The BER is the measure of error bits with respect to total number of bits transmitted in a given time.

The total number of bits transmitted can be known from the bit rate of digital signal. For quality

communication the requirement is not more than one error bit in one million bits.

Short term variation of the significant instances of digital signal from their corresponding reference positions

is referred to as jitter.