lab manual - srmcem

LAB MANUAL

COMMUNICATION ENGINEERING LAB (KEC – 451)

DEPARTMENT

OF

ELECTRONICS & COMMUNICATION ENGINEERING

SHRI RAM SWAROOP MEMORIAL GROUP OF PROFESSIONAL COLLEGES

LUCKNOW U.P.

AFFILIATED TO

DR. A.P.J. ABDUL KALAM TECHNICAL UNIVERSITY

SHRI RAM SWAROOP MEMORIAL GROUP OF PROFESSIONAL COLLEGES

B.Tech (EC) III Sem. (2020-21) COMMUNICATION ENGINEERING LAB (KEC – 451)

INDEX

S.No. Name of Experiment CO Page No.

1. To Study DSB Modulation and Determine its Modulation Factor and power in sidebands.

CO1 1-6

2. To study Amplitude Demodulation by Linear Diode Detector.

CO1 6-11

3. To Study Frequency Modulation & determine its modulation factor.

CO1 11-15

4. To study Pulse Amplitude Modulation by Sample and Hold Circuit.

CO2 15-19

5. To demodulate the obtained PAM Signal by 2nd Order LPF.

CO2 19-22

6. To study Pulse Width Modulation and Pulse Position Modulation.

CO2 22-25

7. To Study pulse code modulation and demodulation technique.

CO2 25-28

8. To study delta modulation & demodulation technique. CO3 28-32

9. Study of Amplitude Shift Keying Modulator & Demodulator.

CO3 32-36

10. Study of Phase shift Keying modulator and demodulator. CO4 36-39

Course Outcomes (CO): At the end of this course students will demonstrate the ability to CO1 : Analyze and compare different analog modulation schemes for their modulation

factor and power. CO2 : Study pulse amplitude modulation CO3 : Analyze different digital modulation schemes and can compute the bit error

performance. CO4 : Study and simulate the Phase shift keying. CO5 : Design a front end BPSK modulator and demodulator.

Communication Engineering Lab (KEC-451)

ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 1 of 39

EXPERIMENT NO.-1

OBJECT:- To Study DSB Modulation and Determine its Modulation

Factor and power in sidebands.

APPARATUS USED:-

S.NO. APPARATUS NAME SPECIFICATION QUANTITY

1. Training Kit Kit (ST-2201) 1

2. CRO 20 MHz 1

3. Connecting probes - 7-8

THEORY:-

AMPLITUDE MODULATION:- In amplitude modulation, amplitude of

carrier wave is varied in accordance with the amplitude of the modulating signal

or message signal, but the frequency of carrier remains constant.

AM wave is given by following expression.

EAM = (Ec+Emsin(ωmt)).sinωct

A Sine wave carrier is of the form ec=Acsin (ωct)

A Sine wave modulation signal is of the form em=Amsin (ωmt)

Modulation Factor:-The amount by which amplitude of the carrier wave

increases or decreases depends on the amplitude of information signal and is

called modulation factor.

Modulation factor = 𝑉𝑚𝑎𝑥 −𝑉𝑚𝑖𝑛

𝑉𝑚𝑎𝑥 +𝑉𝑚𝑖𝑛

This modulation factor can also be defined as the ratio of the modulation signal

amplitude to the carrier amplitude.

mAM = 𝐸𝑚

𝐸𝑐 where 0 ≤ mAM ≤ 1


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 2 of 39

DIAGRAM:-

Fig 1.1 Message, Carrier and modulated wave

PROCEDURE :-

This experiment investigates the generation of double sideband amplitude

modulated (AM) waveforms, using the ST2201 module. By removing the carrier

form such an AM waveforms, the generation of double sideband suppressed

carrier (DSBSC) AM is also investigated. To avoid unnecessary loading of

monitored signals, X10 oscilloscope probes should be used throughout this

experiment.

1. Ensure that the following initial conditions exist on the board.

a. Audio input select switch should be in INT position.

b. Mode switches in DSB position.

c. Output amplifiers gain potentiometer in full clock wise position.

d. Speakers switch in off position.

2. Turn on power to the ST2201 board.

3. Turn the audio oscillator blocks amplitude port to its full clock wise (MAX)

Position, and examine the blocks output (TP14) on an oscilloscope. This is the

audio frequency sine wave which will be as our modulating signal. Note that the


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 3 of 39

sine waves frequency can be adjusted from about 300Hz to approximately 3.4

KHz, by adjusting the audio oscillator‟s frequency potentiometer.

Note also that the amplitude of this audio modulating signal can be reduced

to zero, by turning the audio oscillator‟s amplitude present to its fully counter

clock wise (min.) position. Return the amplitude present to its maximum

position.

4. Turn the balance port, in the balance modulator and band pass filter circuit 1

block, to its fully clock wise position. It is this block that we will use to perform

double-side band amplitude modulation.

5. Monitor, in turn, the two inputs to the balanced modulator and band pass filter

1 block at TP1and TP9 note that:

a. The signal at TP1 is the audio frequency sine wave from the audio

oscillator block. This is the modulating input to our double –sideband

modulator.

b. Test point 9 carries a sine wave of 1MHZ Frequency and amplitude 120

mVpp approximate. This is the carrier input to our double-sideband modulator.

6. Next, examine the output of the balanced modulator & band pass filter circuit

I block (at TP3) together with modulating signal at TP1 Trigger the oscilloscope

on the TP signal.

Check that the waveforms as shown in figure1.1

Amplitude

CARREIR

LOWER SIDEBAND UPPER SIDEBAND

(1MHZ – fm) 1MHZ (1MHZ + fm) Frequency

Fig 1.2 Amplitude Vs. Frequency Graph for AM wave

Percentage Modulation:-To determine the depth of modulation, measure the

maximum amplitude (Vmax) and the minimum amplitude (Vmin) of the AM

waveform at TP3, and uses the following formula:


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 4 of 39

% Modulation Factor = 𝑉𝑚𝑎𝑥 −𝑉𝑚𝑖𝑛

𝑉𝑚𝑎𝑥 +𝑉𝑚𝑖𝑛×100

Where Vmax and Vmin are the maximum and minimum amplitudes of modulated

wave.

OBSERVATION:-

S NO. Vmax Vmin Volt/Div % mAM

CALUCATION:-

Power in both side bands is given by,

PSB= Pc m2/2 = Ac

2 mAM

2/4

Where Ac= Peak Amplitude of Carrier Signal

Pc=Power in Carrier signal and Pc=1/2Ac2

2Ac=Peak to Peak voltage of carrier

Pc = Ac2/2

RESULT :-

The modulation factor is_________ and power in side bands is___________.

PRECAUTIONS :-

1. The initial condition must be ensured on the board before starting the

experiment.

2. The CRO must be adjusted for proper operation and vision before

recording the result.

3. Reading should be taken very carefully.

Short Answer Type Questions Q1.What are the methods of DSB-SC generation.

Q2. Mention advantages of DSB-SC over DSB-FC.

Q3. What is the BW of DSB-SC for a single tone modulating signal with

frequency w?

Q4. What happens in case of over modulation?

Q5. What is the range of audio frequencies?


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 5 of 39


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 6 of 39

EXPERIMENT NO.-02

OBJECT:- To study Amplitude Demodulation by Linear Diode Detector.

APPARATUS USED:-

S. NO. APPARATUS NAME SPECIFICATION QUANTITY

1. Training kit ST-2201 Scientism 1

2. Received Kit ST-2202 Scientism 2

3. CRO 20 MHz Scientism 3

4. Connecting Probes - 7-8

THEORY:-

Envelope Detector:- Consider the case of an AM wave in which the carrier

frequency is much larger than the message frequency and the AM wave looks

like the modulating signal. Thus desired modulation can be accomplished by

extracting the envelope of the resultant AM wave.

Principle of Operation:- On the positive half cycle of the input signal, the

device is forward biased and the capacitor C charges rapidly to the peak of the

i/p signal, where the input signal falls below this value the diode becomes

reverse biased and capacitor C discharges through the load Resistor RL , The

discharge process continues until the next positive half cycle.

When the input signal becomes greater than the voltage across the capacitor the

diode conducts again and the process is repeated. We assume the diode to be

ideal in the sense that it presents zero impedance to the current, in the forward

biased region and the infinite impedance in the reversed bias region. We also

assume that the AM wave has been applied to the envelope detector from a

voltage source having a series resistor Rs and the time constant Rs C much short

compared with the carrier time period RsC <<1/fc, So that the capacitor charges

rapidly and there by follows the applied voltage when the diodes are conducting.

On the other hand discharging time constant RLC must be long enough to ensure

that the capacitor discharges slowly through the RL between the positive peak of

the carrier wave but not so long that the capacitor voltage does not discharges at

the maximum rate of change of the modulating wave i.e. 1/fc ≤ RLC ≤≤ 1/ω.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 7 of 39

Where ω is the message bandwidth. The result is that the capacitor voltage,

which is the detected output, is nearly same as the envelope of the AM wave. In

the detected output some ripples may be present which may be finally removed

by using a low pass filter.

Diagonal Peak Clipping Effect:- When the discharge time constant RLC is too

large and when the envelope of the modulating signal is falling, the diode will

not conduct even around the peak of the carrier cycle and the detected output

voltage will therefore fail to follow the envelope of the input signal during this

period. The detected output will thus only be a distorted version of the envelope

of the input signal. This type of distortion is called ‟Diagonal Clipping‟. For the

given time constant RLC and the given modulating signal frequency fm the

maximum permissible depth of modulation mmax that will ensure absence of

diagonal clipping is given by,

mmax=1/(1+ ωm2RL

2C

2)1/2

In most of the practical applications mmax is nearly 0.6 i.e. 60%.

The second type of distortion which occurs at modulation index greater than

100% is the negative peak clipping which occurs as the results of the

overlapping of two envelopes which takes place when the percentage

modulation is more than 100%. Such as AM wave when given to the diode

detector will be clipped at the negative peak because the detector cannot follow

the negative peak of the envelope as the diode for all values negative peak will

be reversed biased.

Modulation Factor:- The amount by which the amplitude of the carrier wave

increases or decreases depends on the amplitude of information signal and is

called modulation factor.

Modulation factor = 𝑉𝑚𝑎𝑥 −𝑉𝑚𝑖𝑛

𝑉𝑚𝑎𝑥 +𝑉𝑚𝑖𝑛

This modulation factor can also be defined as the ratio of modulation signal

amplitude of the carrier amplitude.

mAM = Em/Ec where 0 ≤ mAM ≤ 1


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 8 of 39

A sine wave carrier is of the form, ec=Ecsin(ωct)

A sine wave modulation signal is of the form, em=Emsin(ωmt)

DIAGRAM:-

Fig. 2.1 Envelope Detector

PROCEDURE:-

1. Ensure that the following initial conditions exist on the board.

a. Audio input select switch should be in INT position.

b. Mode switches in DSB position.

c. Output amplifiers gain potentiometer in full clockwise position.

d. Speakers switch in OFF position.

2. Turn on power to the ST2201 board.

3. Turn the audio oscillator blocks amplitude pot to its full clockwise (MAX)

position and examine the blocks output (TP14) on an oscilloscope. This is the

audio frequency sine wave which will be as our modulating signal. Note that

the sine waves frequency can be adjusted from about 300 Hz to

approximately 3.4 KHz, by adjusting the audio oscillator‟s frequency

potentiometer.

4. Note also that the amplitude of this audio modulating signal can be reduced

to zero, by turning the Audio oscillators amplitude present to its fully counter

clock wise (MIN) position, Return the amplitude present to its max position.

5. Monitor, in turn, the two inputs to the balanced modulator & band pass filter

1 block, at TP and TP9. Note that.

a. The signal at TP1 is the audio frequency sine wave from the audio

oscillator block. This is the modulating input to our double sideband

modulator.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 9 of 39

b. Test Point 9 carries a sine wave of 1MHz frequency and amplitude 120 m

Vpp approximate. This is the carrier input to our double sideband

modulator.

6. Next; examine the output of the balance modulator & band pass filter circuit 1

block (at TP3), together with the modulating signal at TP1 Trigger the

oscilloscope on the TP1 signal.

7. The output from the balanced modulator & band pass filter circuit 1 block (at

TP3) is a double sideband AM waveform, which has been formed by amplitude

modulating the 1MHz carrier sin wave with the audio frequency sin wave form

the audio oscillator.

8. Now apply the modulated wave form to the Y input of the oscilloscope and

the modulating signal to the X input.

9. Connect and make the setting as per connection diagram as shown in fig. 2.2

10. Observe the signal flow from the input of diode detector to anode of diode

D6, at its cathode, after and at the output at TP31.

11. Vary the preset R45 in the diode detector block while observing the output

of diode detector.

12. You can see the variation in the detected output when you change the RC

time constant of the filter formed by R45 and C32.

OBSERVATION:- The Demodulate AM Wave has been observed on the

CRO.

RESULT:-We Studied envelop detector and we get the demodulating signal at

receiver and very similar to the modulating signal as transmitted and the

comparison is trace on paper.

PRECAUTIONS:-

1. Check all the connection before switching on the supply.

2. Try to avoid the noise in the signal.

3. Tracing must be taken very carefully.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 10 of 39

Short Answer Type Questions

Q1. What is Modulation and Demodulation?

Q2. State the techniques of demodulation?

Q3. What is the difference between detector and demodulator?

Q4. What is diagonal clipping?

Q5. What should be the range of modulation index?


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 11 of 39

EXPERIMENT NO.-3

OBJECT:- To Study Frequency Modulation & determine its modulation

factor.

APPARATUS REQUIRED:-

S.No. APPARATUS NAME SPECIFICATION QUANTITY

1. Training Kit Scientech ST-2107K 1

2. CRO 20 MHz Scientech 1

3. Connecting probes ----------- 6-7

4. DC Power Supply +5V,-5V and +12V,-12V

from external source or

from ST-2612 analog lab

-

THEORY:-

Frequency Modulation:- Frequency modulation (FM) is that form of angle

modulation in which the instantaneous frequency of carrier is varied linearly

with respect to message signal.

fi=fc+kf.m(t)

fi = instantaneous frequency

fc = frequency of carrier wave.

Kf = frequency sensitivity of modulation in Hz/V

m(t) = voltage signal of message signal

m(t) =Am Cos{2πfmt}

FM signal, s(t) = Ac cos{2πfct+2πKf }

FM Transmitter:- FM wave contains an infinite number of side band

frequencies for sinusoidal modulation therefore the required bandwidth for

transmitting such signal is also of infinite extent.

Generation of FM waves:- There are basically two methods of generation of

FM signal.

1. Indirect method

2. Direct method


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 12 of 39

1. Indirect Method:-Narrow band frequency modulated wave (NBFM) is first

produced be modulating the carrier wave by base band signal m(t). The resulting

narrow band signal is than multiplied to have desired frequency deviation of ∆f.

2. Direct Method:-The carrier frequency is directly changed in accordance with

the input base band signal by means of VCO (voltage controlled oscillator).

Advantages of FM:-

1. The amplitude of FM wave is constant. It is thus independent of modulation

depth and thus more efficient.

2. FM receiver can be fitted with the amplitude limiter to remove amplitude

variation caused by noise.

3. FM broadcast is used in upper UHF&GHF frequency ranges at which there

happens to be less noise than in the MF&HF range occupied by AM broadcast.

DIAGRAM:-

Fig. 3.1 Waveform of Frequency Modulated Signal

PROCEDURE:-

Connect 0-9Vrms from ST-2612 or from any external source (transformer o/p or

function generator) to sockets m and c of AB09 board respectively using 2mm

patch cords.

1. Now set the amplitude of AC Source signal at 1V peak to peak by varying

amplitude pot of AC Source.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 13 of 39

2. Connect patch cord from output of AC Source to input of DC Source/Level

Shifter.

3. Connect patch cord from output of DC Source/Level Shifter to input of FM

modulator.

4. Connect CH1 of Oscilloscope to output of FM modulator and CH2 to output

of DC source/Level shifter; observe the waveform of frequency modulation

by varying frequency of AC Source.

5. Observe the effect of variation of the DC level to FM modulator output by

varying pot of DC source/level shifter.

OBSERVATION:-

1. Signal provided by AC Source at input of FM modulator generates frequency

modulated wave at its output.

2. Modulated wave observed on Oscilloscope varies its frequency according to

amplitude of input signal.

RESULT:-

The frequency modulated waveform is generated and trace is obtained using

CRO.

PRECAUTIONS:-

1. Check all the connecting wires before switching on the kit.

2. All the connections should be tight.

3. We should keep the amplitude of audio o/p to approximate a frequency of

2KHz.


Q1. What is frequency deviation in FM?

Q2.What is the useful parameter for determination of bandwidth?

Q3. Which one is better in terms of noise immunity AM or FM?

Q4. What is basic principle of FM detection?

Q5. What is the function of amplitude limiter?


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 14 of 39


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 15 of 39

EXPERIMENT NO.-4

OBJECT:-To study Pulse Amplitude Modulation by Sample and Hold

Circuit.

APPARATUS REQUIRED:-

S. NO. APPARATUS NAME SPCIFICATION QUANTITY

1. Training Kit Scientech ST-2110 1


3. Connecting Probes 7-8

THEORY:-

The pulse Amplitude modulation (PAM) system is where the train of pulse

corresponding to the samples of each signal is modulated in amplitude in

accordance with the signal itself I.e. the height of the transmitted pulse very with

the amplitude of the message.

Sample and Hold Output:-If the pulse width of carrier pulse train used in

natural sampling is made very short compared to the pulse period, the natural

PAM is referred to as instantaneous PAM. As it has been discussed, shorter

pulse is desirous for allowing many signals to be included in TDMA format but

the pulse can be highly corrupted by noise due to lesser signal power one way to

maintain reasonable pulse energy is to hold the sample value until the next

sample is taken. This technique is formed as sample and hold techniques now,

the area under the curve (which is equivalent to the signal power) is greater and

so the filter output amplitude and quality of reproduced signal is improved.

The “hold” facility can be provided by a capacitor when the switch connects the

capacitor to PAM output it charges to the instantaneous value. A buffered

sample and Hold circuit consists of unit gain buffers proceeding and succeeding

the charging capacitor the high input impedance of the proceeding buffer

prevents the loading of the message source and also ensures that the capacitor

charges by a constant rate irrespective of the source impedance. The high input

impedance of the succeeding buffers prevents the charge form the capacitor due

to loading and hence the capacitor can hold the charge for infinite time, at least


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 16 of 39

theoretically. However small leakage current through the capacitor dielectric

into positive input of second buffer is always present which causes gradual

charge loss. The rate of change of voltage w.r.t. time dv/dt is called as drop rate

and is important parameter in sample and hold circuit design.

DIAGRAM:-

Fig 4.1 Generation of PAM signal using Sample and Hold circuit

PROCEDURE:-

1. Connect the circuit as per connection diagram.

a) Output of sine wave to modulation signal IN of pulse Amplitude modulation.

b) 8 kHz pulse output to pulse IN of pulse Amplitude modulation block.

c) Keep the frequency selector switch in 1 KHz position.

2. Switch ON the power supply.

3. Monitor the output of sample & hold circuit at TP4.

4. Vary the amplitude of input sine wave &its frequency by the frequency

change over switch to 2 KHz.

5. Also vary the input‟s frequency by connecting the pulse input to different

Square wave frequency available on-board viz 16, 32, & 64 KHz.

6. Switch ON faults NO 1, 2, & 3 one by one and observe their effects on pulse

amplitude modulation output, and try to explain reason behind them.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 17 of 39

7. Switch OFF the power supply.

8. Try to locate the fault and explain the reason behind them.


OBSERVATION:- The pulse amplitude modulation waveform is generated

and traces it obtained by CRO.

RESULT :-The sampled waveform of pulse amplitude modulation is traced on

the trace paper.

PRECAUTIONS:-

1. Connection must be proper.

2. Traces must be taken with care.

3. Calibration must be proper.


Q1. What is pulse amplitude modulation?

Q2. What is Sample & Hold circuit?

Q3. Define Nyquist rate.

Q4. What are the components of a sample & hold circuit?

Q5. List some applications of sample & hold circuit.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 18 of 39


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 19 of 39

EXPERIMENT NO.-5

OBJECT:-To demodulate the obtained PAM Signal by 2nd

Order LPF.

APPARTUS USED :-

S.No. APPARATUS NAME SPECIFCATION QUANTITY

1. Training Kit ST-2110 SCIENTECH 1



THEROY:-

Pulse Amplitude Modulation:-It may be that type of modulation in which the

amplitude of regularly spaced rectangular pulses is varied according to

instantaneous value of the modulating or message signal input. The pulses of

PAM signal may be of flat top type or ideal type. Out of PAM, PPM & PWM,

PAM is widely used because during the transmission the noise interferes with

the top of the transmitted pulses on this noise can be easily removed if PAM

pulse has flat top.

Demodulation of PAM Signal:- Demodulation is the reverse process of

modulation in which the modulating signal is recovered back from a modulated

signal. PAM demodulation is done using a sample and hold circuit and 2nd

order

low pass filter.

PROCEDURE :-


a) Output of sine wave to modulation signal IN in PAM block keeping the

switch in 1 KHz Position.

b) 8 KHz pulse output to pulse IN.

c) Connect the sample output to low pass filter input.

d) Output of low pass filter to input of AC amplifier, Keep the gain pot in AC

amplifier block in max position.

2. Monitor the output of AC amplifier. It should be a pure sin wave similar to

input.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 20 of 39

3. Try varying the amplitude of input, the amplitude of output will vary.

4. Similarly connect the sample & hold & flat top output to LOW PASS

FILTTER and see the demodulated waveform at the output of AC Amplifier.

5. Switch ON the switched faults NO, 1, 2, 3, 4, 5 & 8 one by one and see their

effects on output.

6. Try to locate the fault and explain the reason behind them.


DIAGRAM:-

Fig. 5.1 Demodulation of PAM signal

OBSERVATION :-

The modulated & demodulated PAM signals are observed on CRO.

RESULT :-The waveform of PAM signal & the demodulated signal shown on

CRO are traced on trace paper.

PERCAUTIONS :-

1. Connections must be proper.

2. Traces must be taken with care.

3. Calibration must be proper.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 21 of 39


Q1. Define PAM.

Q2. What is the advantage of PAM over PWM?

Q3. What are the drawbacks of PAM system?

Q4. What is Sampling?

Q5. What is under sampling?

EXPERIMENT NO.-6


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 22 of 39

EXPERIMENT NO.-6

OBJECT:-To study Pulse Width Modulation and Pulse Position

Modulation.

APPARATUS REQUIRED :-

S. No. APPARATUS NAME SPECIFICATION QUANTITY

1. Training Kit Scientech ST-2110 1



THEORY :-

Pulse Width Modulation:- In this type of modulation, the width of the pulse of

carrier signal is varied in accordance with the instantaneous value of amplitude

message signal. In PWM the bandwidth of transmission channel depends on rise

time of the pulse and the instantaneous power of the transmitter varies. The main

advantage of PWM is that power loss in the switching devices is very low.

Pulse Position Modulation:-In this modulation, the position of the pulse of

carrier signal is varied in accordance with the instantaneous value of message or

modulating signal. The final signal is called pulse position modulated signal. In

PPM the bandwidth of the transmission channel depends on rising time of the

pulse. In PPM the noise interference is minimum & the instantaneous power of

the transmitter remains constant.

PROCEDURE:-


a) Sine wave output of function Generator block to Modulation input.

b) 64 KHz Square wave output to pulse input.

2. Switch ON the power supply.

3. Observe the output of pulse width modulation block.

4. Vary the amplitude of sine wave and see its effect. On pulse output.

5. Vary the sine wave frequency by switching the frequency selector switching

to 2 KHz.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 23 of 39

6. Also change the frequency of the pulse by connecting the pulse input to

different pulse frequency viz 8, 16, 32 KHz and see the variations in the

pulse width modulation output.

7. Switching ON fault no 1.2&5 one by one & observe their effect on pulse

width Modulation output and try to locate them.

8. Switching OFF the power supply.

DIAGRAM:-

Fig 6.1 PWM & PPM Waveforms

OBSERVATION :-

The modulated waveforms of PWM & PPM are observed on CRO.

RESULT :-

The waveform of PWM and PPM are traced paper.

PRECAUTIONS:-

1. Connection should be right & tight.

2. Switch ON when circuit is correct.

3. Trace should be observed clearly.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 24 of 39


Q1. What is PWM?

Q2. What are the disadvantages of PWM?

Q3. What is the advantage of PPM over PWM and PAM?

Q4. What is pulse position modulation?

Q5. What are the advantages of pulse position modulation?


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 25 of 39

EXPERIMENT NO.-7

OBJECT:- To Study pulse code modulation and demodulation technique.

APPARATUS USED:-


01. Training kit ST-2103/ Scientech

ST-2104

1

02. CRO 20MHz Scientech 1

03. Connecting probes As required

THEORY:-

In PCM system, the amplitude of the sampled waveform at definite time interval

is represented as a binary code. The analog signal is first sampled according to

the Nyquist criteria. It states that for faithful reproduction of the limited signal,

the sampling rate must be at least twice the highest frequency component

present in the signal. Sampling Frequency ≥ 2fm

The sampled value is the allocated binary codes, which define a narrow range of

amplitude value. Each binary word defines particular amplitude level. The

sampled value is then approximated to the nearest amplitude level. The sample

is then assigned a code corresponding to the amplitude level, which is then

transmitted. The process is called as Quantization and it is generally carried out

by A/D converter.

Fig.7.1 Block Diagram of PCM


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 26 of 39

PROCEDURE:-

1. Check and connect the signal from function generator at ST- 2103.

2. Any of the signals from function generator given to the either CH1&CH2.

3. Check the signal at point sample 0& sample 1.

4. Check the signal at TP-45&TP-46 before and after the buffer amplifier.

5. Check the condition of A/D converter.

6. All switch of switch fault to the off position & error code selector switch to

00 positions. Set the MODE switch of transmitter timing logic to the fast

condition.

7. Set the PSEDUDO RANDOM SYNC. Code Generator to the off position.

8. Check the PCM output logic. Connect the TX CLOCK & TX SYNC of

transmitter timing logic to RX CLOCK &RX SYNC of receiver timing logic.

9. Set all the switch of ST-2103 Kit

10. Connect the PCM O/P to the PCM data I/P of RX & check the bit position of

a D/A converter.

11. Finally check the signal of CH1 & CH2.

RESULT:- The pulse code modulation & demodulation has been studied.

PRECAUTIONS:-

1. Don‟t make a loose connection.

2. Check the connections before power on the supply.

3. Check the switch faults carefully.


Q1. Which modulation technique is used to convert analog signal into digital

signal?

Q2. What type of noise does affect PCM?

Q3. Mention one of the disadvantages of PCM.

Q4. What is the sequence of operations in which PCM is done?

Q5. What is Quantization?


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 27 of 39


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 28 of 39

EXPERIMENT NO.-8

OBJECT:- To study delta modulation & demodulation technique.

APPARATUS USED:-


01. Training kit ST-2105 Scientech 1

02. CRO 20MHz Scientech 1

03. Connecting Probes As required

THEORY:-

DELTA MODULATION:-

Delta modulation is a process of digital modulation technique developed after

pulse code modulation. In this scheme, at each sampling time say the kth

sampling time & (k-1)th

the difference b/w the sample value at sampling time k

and the sampling time (k-1) is encoded into just a single bit i.e. at each sampling

time.

OPERATION OF DELTA MODULATOR:–

The analog signal which is to be encoded into digital data is applied to the input

of the voltage comparator which compares it with its previous value & assigns a

single bit 1 or 0 according to incoming signal.

DISADVANTAGE OF DELTA MODULATION:- Delta modulation is

limited due to following 2 drawbacks:-

NOISE:-

Noise is defined as average unwanted unpredictable random wave form

accompanying signal wherever the signal is received at the receiver by any

communication channel or medium which is always accompanied by noise.

DISTORTION:–

Distortion means that the receiver receives the O/P which is not the exact replica

of the analog I/P signal transmitted at the transmitter.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 29 of 39

DISTORTION IN DELTA MODULATION OCCURS DUE TO

FOLLOWING CASES: -

As it has been discussed that, when the analog signal is greater than the

integrator o/p the integrator ramps up to meet the analog signal. The ramping

rate of integrator change of analog i/p is faster than the ramping rate of the

modulator which is unable to catch up with information signal, this causes a

large disparity b/w the info signal & its quantized approximation. This

phenomenon is known as “Slope-Overloading” and causes the loss of rapidly

changing information.

Fig. 8.1 Delta Modulation and Demodulation

Fig. 8.2 Delta Modulated Waveform


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 30 of 39

PROCEDURE :-

1. Connect the comparator O/P node 11 of transmitter to the I/P node 12 of

bistable circuit.

2. Connect the comparator I/P node 10 to O/P node 17 of integrator.

3. Connect the O/ P node 15 of bipolar convertor to I/P node 16 of integrator.

4. Connect the TX clock O/P node 2 of clock generator to clock I/P node13 of

bi-stable circuit.

5. Connect I/P node 31 of bi-stable circuit of receiver to O/P node 14 of bi-

stable circuit of transmitter.

6. Connect the receiver clock O/P node 3 to input node 32 of bi-stable circuit

of receiver.

7. Connect the I/P node 46 of integrator of receiver to bipolar O/P node 34 of

bipolar converter of receiver.

8. Connect the O/P node 47 of integrator of receiver to I/P node 50 of low

pass filter.

9. Connect the CRO to the I/P node 10 of comparator of transmitter and trace

the waveform.

10. Connect the CRO at O/P node of integrator & trace the waveform.

11. Connect the CRO at O/P node of bi-stable circuit and trace the waveform.

12. Connect the CRO at O/P node of LPF and trace the waveform.

RESULT:-

Delta modulation & demodulation has been studied & effect of slope overload

has been observed.

PRECAUTIONS:-

1. Connection should be tight.

2. Make the connection after switch off the power supply.

3. Tracing of the waveforms should be done carefully.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 31 of 39


Q1. In digital transmission, which modulation technique requires minimum

bandwidth?

Q2. Name the digital modulation scheme in which the step size is not fixed.

Q3. What are the factors that cause quantizing error in delta modulation?

Q4. Mention the case when Granular noise occurs

Q5. Mention the case when slope overload distortion occurs.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 32 of 39

EXPERIMENT NO.-9

OBJECT:- Study of Amplitude Shift Keying Modulator & Demodulator.

APPARATUS USED :-

S.NO. APPARATUS USED SPECIFICTION QUANTITY

01. Training kit ST-2106, ST-2107

Scientech 1, 1


03. 8 bit variable data

generator ST-2111Scientech 1

THEORY:-

To transmit digital data from one place to another, we have to choose the

transmission medium. It can neither be directly sent through the transmission

medium nor over the antenna because the antenna of practical size works on

very high frequencies much higher than our data transmission rate. To be able to

transmit the data over antenna, we have to modulate the carrier signal‟s phase or

frequency or amplitude which is varied in accordance with the digital data. At

the receiver side, we separate the signal from modulated signal by the process of

„demodulation‟. Modulation also allows different data streams to be transmitted

over same channel. This process is called as „multplexing‟ & result is a

considerable saving of available bandwidth. Some of the basic digital

modulation techniques are ASK, PSK & FSK.

ASK (AMPLITUDE SHIFT KEYING):- The simplest method of modulating

a carrier with a data stream is to change the amplitude of the carrier wave at

every time the data changes. This modulation technique is known as “Amplitude

Shift Keying”. The simplest way of achieving amplitude shift keying is by

switching on the carrier whenever the data bit is „1‟ & switching off whenever

the data bit is „0‟. This technique is known as „On-Off Keying‟. Thus for

Data=1, carrier is transmitted and Data=0, carrier is suppressed.

The ASK wave from is generated by balanced modulator circuit which is also

known as a linear multiplier. We apply the digital data stream and carrier signal


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 33 of 39

to the inputs of linear multiplier. To demodulate the ASK waveform, we pass it

through the filter. Amplitude shift keying is less efficient because the inherent

noise in the transmission channel can deteriorate the signal so much that the

amplitude changes in the modulated carrier wave due to noise addition. This

may lead to the incorrect decoding at the receiver.

DIAGRAM:-

Fig 9.1Message signal and ASK Modulated signal

Fig. 9.2 ASK Waveform Generator


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 34 of 39

.PROCEDURE:-

1. Connect the ground, clock, data nodes of 8 bit variable data generator (ST-

2111) to clock, data input & ground nodes of data formatting.

2. Connect the output node of NRZ (L) available at node 5 to modulation I/P

node of carrier modulation circuit available at node 27.

3. Connect the node 16 available at carrier generator circuit to node 26 available

at carrier generator circuit to node 26 available at carrier modulation circuit.

4. Do proper adjustments via carrier offset, gain and modulation offset nodes.

5. Connect the output node of carrier modulation circuit available at node 28 to

CRO & trace out the ASK waveform.

6. For Demodulation, connect the output of carrier modulation circuit available

at node 28 to the ASK demodulator available at node 21.

7. Connect the output node 22 of ASK demodulator to the input of LPS

available at node 27.

8. Connect CRO at LPS output available at node 28 &trace out the demodulated

ASK waveform.

RESULT:- The study of ASK modulation & demodulation is completed.

PRECAUTIONS :-

1. Don‟t make loose connection.

2. Check the connection before switch ON the power supply.

3. Check the all switch fault carefully.


Q1. What do you mean by ASK?

Q2. What are types of ASK demodulation techniques?

Q3. What does Asynchronous ASK demodulator consist of?

Q4. What does Synchronous ASK demodulator consist of?

Q5. What is the bandwidth of ASK modulated signal?


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 35 of 39


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 36 of 39

EXPERIMENT NO.-10

OBJECT :-. Study of Phase shift Keying modulator and demodulator.

APPARATUS USED :-

S.NO. APPARATUS USED SPECIFICTION QUANTITY

01. Training kit ST-2106, ST-2107

Scientech 1, 1


03. 8 bit variable data

generator ST-2111Scientech 1

THEORY :-

Phase Shift keying involves the phase change of the carrier sine wave between

00 and 180

0 in accordance with the data stream to be transmitted. PSK is also

known as phase reversal keying (PRK). PSK modulator is very similar to ASK

modulator. Both use balanced modulator to multiply the carrier with the

modulating signal. But in contrast to ASK technique, the line coding technique

applied to the input sequence is bipolar. When the modulating signal is positive,

the output of modulator is a sine wave in phase with the carrier. When input is

negative voltage, the output of modulator will be sine wave shifted out of phase

by 1800 from the carrier input. The unipolar to bipolar converter converts the

unipolar data stream to bipolar data.

At receiver side, the square loop detector circuit is used to demodulate the PSK

signal. The incoming PSK signal with 00 & 180

0changes is first fed to the square

law device which multiplies the input by itself. The output of this block is a

signal of twice the frequency received. After that this signal is passed through a

BPF which blocks DC signal and passes higher frequency only. After that a

frequency divider circuit divides frequency and we get signal with same

frequency as received. This signal is applied to multiplier and then coherent

detection takes place.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 37 of 39

DIAGRAM:-

Fig. 10.1 Message, Carrier and PSK Modulated waveform

PROCEDURE :-

Connect the ground, clock, data nodes of 8 bit variable data generator (ST-2111)

to clock, data input & ground nodes of data formatting Circuit.

1. Connect the output node of NRZ (L) available at node 5 to modulation I/P

node of carrier modulation circuit available at node 27.

2. Connect the node 16 available at carrier generator circuit to node 26 available

at carrier modulation circuit.

3. Do proper adjustments via carrier offset, gain and modulation offset nodes.

4. Connect the output node of NRZ-M available at node 6 to input of uniform

bipolar convertor available at node 20.

5. Connect the output of uniform bipolar convertor available at node 21 to input

of carrier modulation circuit available at node 27.


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 38 of 39

6. Connect the 960 KHz carrier frequency available at node 17 to the input of

carrier modulation circuit available at node 26.

7. Connect the CRO to the output node of carrier modulation circuit available at

node 28 & trace out the PSK waveform.

8. For Demodulation connect the output of carrier modulation circuit available at

node 28 to the input of PSK demodulator available at node 10.

9. Connect the output of PSK demodulator available at node 15 to the input of

LPS available at node 23.

10. Connect the CRO to the output of LPS available at node 24 & trace out the

PSK demodulated waveform.

RESULT:- The study of PSK modulator & demodulator circuit is done & the

respective wave forms are shown.

PRECAUTIONS :-

1. Don‟t make loose connection.

2. Check the connection before switch ON the power supply.

3. Check the all switch fault carefully.


Q 1. What do you mean by PSK?

Q 2. What are types of PSK demodulation techniques?

Q 3. What is Bandwidth Efficiency of PSK signal?

Q 4. What does Synchronous PSK demodulator consist of?

Q 5. What is the bandwidth of PSK modulated signal?


ECE Department,

SRMGPC, Lko

Prepared By

Faculty In-charge

Checked By

Head of Department

Page 39 of 39

lab manual - srmcem

Documents