THIS QUESTION PAPER CONSISTS OF 10 PAGES

UNIVERSITI TUN HUSSEIN ONN MALAYSIA

SECOND TEST SEMESTER I

SESSION 2012/2013

SUBJECT NAME : ELECTRONIC COMMUNICATION SYSTEM

SUBJECT CODE : BEB 31803 DURATION : 1 HOUR INSTRUCTION : ANSWER ALL QUESTIONS.

NAME : ___________________________

Qtn. No. Marks

MATRIC ID : ___________________________ 1

SECTION : ___________________________ 2

LECTURER : Total

Q1. a) What are the three (3) most predominant modulation schemes used in

digital radio system? Answer Amplitude Shift Keying (1) Frequency Shift Keying (1) Phase Shift Keying (1) (3 marks)

(b) The simplest digital modulation technology is Amplitude Shift Keying

(ASK), where a binary information signal directly modulates the amplitude modulation except there are only two output amplitudes possible. Mathematically, ASK is denoted as

[ ] ⎥⎦⎤

⎢⎣⎡+= tAtVtV cmask ωcos

2)(1)()( (Equation 1)

Where

frequencyradianCarrier

VoltsamplitudeCarrierAVoltssignaldigitaltV

ASKwavetV

c

m

ask

=

=

==

ω

)(2

)()()(

)(tVm is a normalized binary waveforms where +1 V= logic 1 and

-1 V= logic 0. Simplified Equation 1 for logic 1 and 0, and sketch the ASK waveform for binary input 110010100.

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Logic 1= +1 V

[ ]

tA

tA

tAtVtV

c

c

cmask

ω

ω

ω

cos

]cos2

][11[

cos2

)(1)()(

=

+=

⎥⎦⎤

⎢⎣⎡+=

(2.5)

Logic 0 = -1 V

[ ]

0

]cos2

][11[

cos2

)(1)()(

=

−=

⎥⎦⎤

⎢⎣⎡+=

tA

tAtVtV

c

cmask

ω

ω

(2.5)

Waveform 110010100

(2) (7 marks)

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(c) Encode the binary input 11010100011 using the following encoding technique

i) NRZ-L ii) RZ (Unipolar) iii) RZ (AMI) iv) Biphase L

1 1 0 1 0 1 0 0 0 1NRZ-L (2.5)

RZ- Unipolar (2.5)

RZ- AMI (2.5)

Bi- phase-L (2.5)

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(10 marks)

Q2 (a) A significant reduction in the performance of a communication system, can be attributed by correlated and uncorrelated noise. With the aid of a diagram, briefly discuss the sources of both noise factors. Solution:

a) Sources of both noise factors are shown in the figure given below:

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Correlated noise: This type of noise exists only when signal is present.

Uncorrelated noise: This type of noise exists all the time, whether there is any signal or not. There are two broad categories of uncorrelated noise which are internal noise and external noise.

External noise: Generated outside the device or circuit. Three primary sources are atmospheric, extraterrestrial and man made.

• Atmospheric noise: o Naturally occurring electrical disturbance originate within Earth’s

atmosphere o Commonly called static electricity o Most static electricity is naturally occurring electrical conditions, such as

lighting o In the form of impulse, spread energy through wide range of frequency o Insignificant at frequency above 30 MHz

• Extraterrestrial noise: o Consists of electrical signals that originate from outside earth atmosphere,

deep‐space noise. Divide further into two, solar noise and cosmic noise. i. Solar noise:

• Generated directly from sun’s heat. There are 2 parts to solar noise.

• Quite condition when constant radiation intensity exist and high intensity.

• Sporadic disturbance caused by sun spot activities and solar flare‐ups which occur every 11 years.

ii. Cosmic noise:

• Continuously distributed throughout the galaxies, small noise intensity because the sources of galactic noise are

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located much further away from sun. It's also often called as black‐body noise.

• Man‐made noise: o Source – spark‐producing mechanism such as from commutators in electric

motors, automobile ignition etc o Impulsive in nature, contains wide range of frequency that propagate

through space the same manner as radio waves o Most intense in populated metropolitan and industrial areas and is therefore

sometimes called industrial noise.

• Impulse noise: o High amplitude peaks of short duration in the total noise spectrum.

Consists of sudden burst of irregularly shaped pulses. o More devastating on digital data, o Produce from electromechanical switches, electric motor etc.

• Interference: o External noise o Signal from one source interfere with another signal. o It occurs when harmonics or cross product frequencies from one source fall

into the pass‐band of the neighboring channel. o Usually occurs in radio‐frequency spectrum

Internal noise:

Generated within a device or circuit. 3 primary kinds, shot noise, transit-time noise and thermal noise

• Shot noise: o Caused by random arrival of carriers (hole and electron) at the output

element of an electronic device such as diode, field effect transistor or bipolar transistor.

o The currents carriers (ac and dc) are not moving in a continuous, steady flow, as the distance they travel varies because of their random paths of motion.

o Shot noise randomly varying and is superimposed onto any signal present. o When amplified, shot noise sounds similar to metal pellets falling on a tin

roof. o Sometimes called transistor noise

• Transit‐time noise (Ttn): o Any modification to a stream of carriers as they pass from the input to the

output of a device produces irregular, random variation (emitter to the collector in transistor).

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o Time it takes for a carrier to propagate through a device is an appreciable part of the time of one cycle of the signal , the noise become noticeable.

o Ttn is transistors are determined by carrier mobility, bias voltage, and transistor construction.

o Carriers traveling from emitter to collector suffer from emitter delay, base Ttn,and collector recombination‐time and propagation time delays.

o If transmit delays are excessive at high frequencies, the device may add more noise than amplification of the signal.

• Thermal noise: o Due to rapid and random movement of electrons within a conductor due to

thermal agitation o Present in all electronic components and communication system. o Uniformly distributed across the entire electromagnetic frequency

spectrum, often referred as white noise. o Form of additive noise, meaning that it cannot be eliminated, and it

increases in intensity with the number of devices and circuit length. o Set as upper bound on the performance of communication system. o Temperature dependent, random and continuous and occurs at all

frequencies.

(5 marks)

(b) A miniaturized electronic device is designed with a bandwidth

performance of 25 MHz operating at a room temperature. Given a load resistance of 100 Ω, determine:

i. The thermal noise power in dBW.

ii. The noise voltage if there is an additional resistor connected in parallel to the existing one. Assume R2 = 75 Ω .

iii. The signal to noise ratio (DB) if the signal voltage is 25 V.

Solution: i. B = 25 MHz Load resistance = R = 100 Ω

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Thermal noise power = NP kTB=

Where 231.38 10k −= × And T at room temperature is equal to 290K Therefore:

23 61.38 10 290 25 10NP −= × × × ×

BW

131 10NP W−= ×

Whereas power in dBW can be calculated as:

1310 log1 10NP W−= ×

130NP d= −

ii. Now noise voltage can be calculated as:

4NV RkT= B Given that is an additional resistor R2 = 75 Ω connected in parallel to the existing one therefore for parallel combination of resistors we have:

100 75100 75

R ×=

+

7500175

R =

42.8R = Ω

So 134 42.8 1 10NV −= × × ×

4.1NV Vμ=

iii. Now signal voltage is given as 25V therefore signal to noise ratio can be calculated as:

( ) 10 log S

N

VS dBN V

=

25( ) 10log

4.1S VdBN Vμ

=

9

( ) 67.8S dB dBN

= (15 marks)

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