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Department of Electronics Engineering,
Communication Systems Laboratory
Laboratory Manual
for
B. Tech. (Electronics), III Year (V – Semester)
Lab Course EL 392 ( Communication Lab. – I)
LIST OF EXPERIMENTS
1.
Wave-form Analysis
2.
AM & Demodulation
3.
FM – VCO
4.
FM Demodulation - PLL
5.
Transmission Lines
6.
Envelop Detector
7.
PWM
8.
Sampling & Re-construction
9.
SSB – Modulation & De-modulation
-----------------------Compiled by
M. Hadi Ali Khan
B. Sc. Engg (AMU)., Ex-MIEEE (USA), Ex-MIETE (India), Ex-MSSI (India)
Electronics Engineer,
Department of Electronics Engineering,
AMU, Aligarh – 202 002
Email:- h a d i a l i k h a n @ g m a i l . c o m
Note:- This laboratory manual is also available on the internet at the following URL :-
http://www.geocities.com/hak786in/lab1.pdf
MHAK/230706
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Experiment # 01 (WF Analysis)
Object:- (a) Generate a symmetrical Square wave of lowest frequency using wave-form
generator section of the kit supplied. Measure its Amplitude & Frequency.
(b) Determine the amplitudes & frequencies of the first eight harmonics of the wave-form
generated in part (a) above, using the same kit.
Apparatus / Kit used:- 1. Sigma Harmonic Analyzer Trainer Kit Model COM-118.
2. Dual Trace CRO
Procedure :- Refer to pages 6, 7 & 8 of the manual of the Sigma Harmonic Analyzer trainer
Kit COM-118
Observations:- Wave-form generated:- Symmetrical Square wave
Amplitude of the symmetrical Square wave = 5.4 Volts p-p (adjusted)
Frequency of the symmetrical Square wave = 150 Hz (adjusted)
S.No.
1.
2.
3.
4.
5.
6.
7.
8.
frequency of Sinewave (Hz)
150
300
450
600
750
900
1050
1200
Rela t iv e A mp lit u de
o f the H a r mo n ic
( mV)
1.8 x 0.5 = 900 mv
0
300 mv
0
200 mv
Remarks
1st harmonic present
2nd harmonic absent
3rd harmonic present
4th harmonic absent
5th harmonic present
6th harmonic absent
7th harmonic present
8th harmonic absent
Plot the Spectrum of the above signal.
Experimental setup:-
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Experiment # 02 (AM & Demodulation)
Object:- (a) Draw the modulation characteristics (m vs A) of the AM section of the kit
supplied (AQUILA AM Demonstrator Model AET – 14)
(b) Use demodulator section of the kit to recover the message signal.
Observations:Carrier Signal (RF signal): Ac = 300 mVp-p (adjusted)
fc = 1.0 MHz (fixed)
Modulating Signal (AF signal): fm = 800 Hz
S. No.
Am
(voltsp-p)
Modulation Index (m)
A
B
Modulation Index, m (%)
1.
2.
3.
4.
5.
6.
7.
8.
Sample Calculation:The modulation index, m = [ (A - B ) / (A + B ) ] x 100 %
A curve between m versus Am gives the modulation characteristics of the AM.
For Recovered Message Signal (at demodulator’s o/p),
Amplitude =
Frequency =
volts p-p
Hz
Experimental Setup:-
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Experiment # 03 (FM - VCO)
Object:- (a) Determine the parameters (free-running frequency & the Control voltage) of the
given VCO kit.
(b) Plot the modulation characteristics ( ∆f vs ∆V) and justify that the given ckt is
working as the frequency modulator.
Observations:-
1.
2.
3.
4.
S. No.
Time-period of the VCO output, To = (4.2 x 2.0) µs;
Amplitude of the VCO output, Am = 11.8 V
Free – running frequency of the VCO (fo) = (1/ To) = 119 KHz,
Control voltage, Vc (built-in DC voltage) = + 9.6 V
Biasing supply voltage, Vcc
= +/- 12 Volts (symmetrical dual supply)
Readings for modulation Characteristics :DC Volts
(Vi)
Time-period
(Ti)
fi = 1/Ti
∆f = ( fi – fo)
∆V = (Vi – Vc)
Vi = a variable dc voltage (+5 volts to 11.8 volts) to be applied at the control input (Vc) of the VCO;
fi = the corresponding frequency of the VCO output (Vo)
A curve between ∆f versus ∆V gives the modulation characteristics of the VCO.
Theoretically, fo = {2 (Vcc – Vc)}/ R1C1Vcc ;
V+
Vc
Com
VCO
NE 566
V-
VCO Output
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Experiment # 04 (FM Demodulation - PLL)
Object:(a) Determine the free-running frequency, the lock-range and the Capture-range of the
given PLL kit.
(b) (b) Plot the demodulation characteristics ( VD vs ∆f ) and verify that the given PLL
ckt is working as a FM Demodulator.
Apparatus Used :-
Observations:-
1. The PLL Kit,
2. 1 MHz Function generator with digital frequency display
3. 20 MHz Dual Trace CRO
(a) Free-running frequency, fo = 119 KHz ; (fo = 1/To, )
f1 = ………………KHz ;
( L -- UL )
f2 = …………… KHz ;
( UL -- L )
f3 = ………………KHz ;
( L -- UL )
f4 = ………………KHz ;
( UL -- L )
(b) Lock-range (fL)
= ( f1 – f3) =
(c) Capture-range (fc) = ( f2 – f4 ) =
KHz
KHz ; (Verify that fL > fc )
(d) Readings for Demodulation Characteristics:
S. No.
Incoming signal frequency (fi)
DC Level of the
Differential output
(VD)
∆f = ( fi – fo)
Vi = a sinusoidal signal having variable frequency & amplitude around 8 volts p-p (obtained
from an external signal generator) to be connected at input of the PLL marked Vi ; for
the purpose of determination of the lock-range, capture-range & the Demodulation
characteristics of the PLL circuit.
NB:- A curve between VD versus ∆f shows the Demodulation characteristics of the PLL.
MHAK/230706
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Experiment # 05 (Transmission Lines)
Object:- (a) Determine the velocity of propagation of HF pulses through the given
Transmission line. Calculate the inductance per meter (L) and the capacitance
per meter length (C) of the given line.
(b) Trace the reflections observed at the other end of the given line terminated into
different loads.
Observations:Part (a) :- Propagation Characteristics of the “ open-circuited “ transmission line:Specifications of the HF pulses transmitted:- PRR =
MHz; Amplitude =
volts,
Pulse-duration = 0.2 µs
Specifications of the transmission line used:- Length ( l ) = 30 meters; Zo = 50 Ω
Time-delay between the transmitted & reflected pulses, td = (1.6/5)µs = 0.32 µs
Vp = (2l)/ td = 1.8 x 108 m/s
L = Zo / Vp = ………… micro-Henry/meter;
C = 1 / Zo Vp = ………… µF/meter
Part (b) :- Reflections due to different load terminations (a load connected to the line)
Use three tracing papers only to trace the reflections in the following way:Use single tracing paper to trace the Reflections corresponding to “Resistive load terminations”
of 0 Ω, 50 Ω and 100 Ω ; ( show the reflected pulses by the dotted lines) ;
Use one tracing paper to trace the Reflections corresponding to any one “capacitive load
termination” and use one tracing paper to trace the Reflections corresponding to “Inductive
load termination”
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Experiment # 06 (Envelop Detector)
Object:- (a) Determine the typical values of the parameters of the AM signal properly
detected by the given Envelop Detector Circuit.
(b) Determine its Detection Characteristics (max. m at a constant fm & max. fm at
constant m).
Observations:Part (a): Specifications of the AM signal for its proper detection (without distortion) :AM Level,
Amax = 1.0 Vp-p
Modulation frequency,
fm
= 800 Hz
Modulation Index,
m
= 30 %
Carrier frequency,
fc
= 460 KHz
Part (b): Detection Characteristics:(a) Max. m (at fm = 500 Hz) = 90 %
(b) Max fm (at m = 30 %) = 18 KHz
-------------------------------Experimental Setup :-
(1) Maximum permissible value of the modulation frequency of the AM signal detected by the
given Envelop Detector without any distortion is governed by the expression:max fm =
For m = 30 %, R = 2.7 K and C = 0.01 µF;
max fm = 5.8(m-2 – 1)1/2 = 18.44 KHz;
(2) Maximum permissible value of the modulation index of the AM signal detected by the
given Envelop Detector without any distortion is governed by the expression:-
mmax =
For fm = 1 KHz; mmax =
= 92 %
% error =
m
20 %
30 %
fm
28 KHz
18.7 KHz
m
40 %
50 %
fm
13.5 KHz
10 KHz
m
80 %
90 %
fm
4 KHz
1.5 KHz
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Experiment # 07 (PWM)
Object:- Generate a PWM signal using the given kit of PWM and plot its modulation
characteristics (
τ
Apparatus Used :-
vs VD).
1. A PWM unit,
2. A pulse generator,
3. A Dual Trace CRO
4. A 5-volts DC supply and a variable DC supply (Aplab model 7711)
Observations:1. Specifications of the pulses used:-
Amplitude = 4.5 Vp-p
PRR = 2.0 KHz, Duty Cycle = 20 %
( T = 500 µs, & τ = 100 µs)
2. Magnitude of the built-in DC voltage at modulating-signal input = 2.0 V DC
3. Corresponding pulse-width at PWM output = 100 µs
4. Measurements for modulation characteristics:-
S. No.
1.
2.
3.
4.
5.
6.
7.
8.
DC Volts at modulation Input (volts)
0.5
1.0
1.5
2.0
2.4
2.8
3.0
3.5
Pulse-duration at PWM output (µs)
60
70
80
100
120
160
280
340
Experimental Setup for PWM :1. Obtain a train of pulses from external pulse generator having A = 4.5 V, f = 2 KHz and
τ = 100µs and apply it at the Carrier input of the PWM unit.
2. Observe the PWM output on the CRO screen. Measure the duration of the pulses appearing
on the CRO.
3. Connect a variable dc voltage (0.2 volt to 3.4 volts) obtained from an external variable dc
supply at the modulating signal input of the PWM unit.
4. Note the effect of variable dc voltage on the pulse-width at the PWM output.
The modulation Characteristics of the PWM is the curve plotted between the Pulse-duration versus
DC voltage.
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Experiment # 08 (Sampling and Re-construction of Signals)
Object:- (a) Trace the samples of a sinusoidal signal obtained using the Sampling Trainer kit
Model ST – 2101 at any three different sampling frequencies and take
observations to verify Nyqu i st C ri t eri a .
(b) Re-construct the sampled signal using 2nd order and the 4th order LowPass filters and trace the wave-shapes of the re-constructed signals.
Note- For the procedure t o be followed for taking observations , refer to the
pages 24 to 30 of the work-book of the kit ST-2101 .
Observations:- frequency of the sinusoidal signal = 1.0 KHz
Sampling frequency
= 32 KHz
Minimum Sampling frequency
=
2 KHz
Take observations at sampling frequencies of 4 KHz, 8 KHz and 16 KHz
Cut-off frequency of 2nd order Low-Pass filter =
KHz
Cut-off frequency of 4th order Low-Pass filter =
KHz
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Experiment # 09 (SSB Modulation & Demodulation)
Object:- (a) Study the SSB Modulation using the given Trainer kit Model ST – 2201.
(b) Study the SSB Demodulation using the given Trainer kit Model ST – 2202.
Note:- Read Work-books of the kits ST-2201 & ST-2202 and follow the procedure
described therein and trace the wave-shapes of SSB modulated and de- modulated signals.
Measure their amplitudes and frequencies.
*************
MHAK/230706