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COMMUNICATION CIRCUITS

AND

PRINCIPL

ES

PRACTICAL

FILE



INDEX1. TO STUDY AND IMPLEMENT AM MODULATION

2. TO STUDY AND IMPLEMENT AM DEMODULATION

3. TO STUDY AND IMPLEMENT DSBSC MODULATION

4. TO STUDY AND IMPLEMENT DSBSC DEMODULATION

5. TO STUDY AND IMPLEMENT SSB MODULATION

6. TO STUDY AND IMPLEMENT SSB DEMODULATION

EXPERIMENT 1 AIM

To study amplitude modulation

SOFTWARE REQUIRED: MATLAB

THEORY

Amplitude modulation (AM) is a technique used in electronic communication, most commonly

for transmitting information via a radio carrier wave. AM works by varying the strength of thetransmitted signal in relation to the information being sent

In amplitude modulation the modulation voltage whose frequency is lower than that of the

carrier frequency varies the amplitude of a carrier signal. The complex envelope of an AM signal

is given by

)](1[)( t m At g c+=



Ac indicates the power level of AM and m(t ) is the Modulating Signal. Hence the amplitude

modulated signal is given by

s(t)=g(t)*cos(wct)

Energy spectrum of the AM modulated message signal.

The percentage of overall modulation is

If the amplitude of the message signal and that of the carrier signal are equal then the

modulation index is 100%.

AM SIGNAL WAVEFORM

Amax = 1.5Ac

Amin = 0.5 Ac

% Positive modulation= 50%

% Negative modulation =50%

Overall Modulation = 50%

WAVEFORMS:

[ ] [ ]max min max ( ) min ( )% Modulation 100 1002 2

c

m t m t A A

A

−−= × = ×

]0[i.e.,modulationof absencein theenvelopeAMof Level-

)](1[of valueMinimum-

)](1[of valueMaximum-

min

max

=

+

+

m(t) A

t m A A

t m A A

c

c

c



0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 - 1

0

1

t im e

m

e s s a g e

s i g n a l

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 - 1

0

1

t im e

c a r r i e r s i g n a l

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 - 2

0

2

t im e

m

o d u l a t e d

s i g n a l

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 0

2 0 0

4 0 0

6 0 0

t im e

f f t m

o d u l a t e d s

i g n a l

MATLAB CODE:

% AMPLITUDE MODULATION

m=.7;

pi=3.14;t=0:1000;

fm=10;

fc=100;

mt=cos(2*pi*fm*t);

ct=cos(2*pi*fc*t);

md=(1+m.*cos(2*pi*fm*t)).*cos(2*pi*fc*t);

ft3=abs(fft(md));

subplot(4,1,1)

plot(t,mt),xlabel('time'),ylabel('message signal');

subplot(4,1,2)

plot(t,ct),xlabel('time'),ylabel('carrier signal');

subplot(4,1,3)

plot(t,md),xlabel('time'),ylabel('modulated signal');subplot(4,1,4)

stem(t,ft3),xlabel('time'),ylabel('fft modulated signal');

PRECAUTIONS

1. The carrier frequency should be sufficiently larger than the modulating signal frequency.

Experiment no.2



AIM

To study amplitude demodulation

SOFTWARE REQUIRED: MATLAB

THEORY

The simplest form of product detector multiplies an incoming signal by its carrier, to produce a

copy of the original message, and another AM signal at twice the original carrier frequency. This

high-frequency component can then be filtered out, leaving the original signal

If m(t ) is the original message, the AM signal can be shown to be

Multiplying the AM signal x

(t ) by an oscillator at the same frequency as and in phase with the carrier yields

which can be re-written as

After filtering out the high-frequency component based around cos(2ωt ) and the DC component C , the originalmessage will be recovered.

Although this simple detector works, it has two major drawbacks:

• The frequency of the local oscillator must be the same as the frequency of the carrier, or else the output

message will fade in and out in the case of AM, or be frequency shifted in the case of SSB

• Once the frequency is matched, the phase of carrier must be obtained, or else the demodulated messagewill be attenuated, but the noise will not be.



0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0-1

0

1

t i m e

m

e s s a g e

s i g n a l

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0-1

0

1

t i m e

c a r r i e

r s i g n a l

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 00

0 . 5

1

t i m e

d e m

o d u l a t e d

s i g n a l

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0

0

1 0 0

2 0 0

3 0 0

t i m e

f f t s i g n a l

MATLAB CODE:

% AMPLITUDE DEMODULATION

m=.7;

pi=3.14;

t=0:500;

fm=10;

fc=100;

mt=cos(2*pi*fm*t);

ct=cos(2*pi*fc*t);

md=(1+m.*cos(2*pi*fm*t)).*cos(2*pi*fc*t);

dt=md.*cos(2*pi*fc*t);

[b,a]=fir1(100,.003,'DC-1');

st=filter(b,a,dt);

ft3=abs(fft(st));

subplot(4,1,1)


subplot(4,1,2)


subplot(4,1,3)

plot(t,st),xlabel('time'),ylabel('demodulated signal');subplot(4,1,4)

stem(t,ft3),xlabel('time'),ylabel('fft signal');



EXPERIMENT 3

AIM

To study Double Side Band- Suppressed Carrier Modulation using MATLAB.

SOFTWARE REQUIRED

MATLAB

THEORY

Double-sideband suppressed-carrier transmission (DSB-SC): transmission in which (a) frequencies

produced by amplitude modulation are symmetrically spaced above and below the carrier frequency and (b) the

carrier level is reduced to the lowest practical level, ideally completely suppressed.

In the double-sideband suppressed-carrier transmission (DSB-SC) modulation, unlike AM, the wave carrier isnot transmitted; thus, a great percentage of power that is dedicated to it is distributed between the sidebands,

which implies an increase of the cover in DSB-SC, compared to AM, for the same power used.

DSB-SC equation is of the form

It is obtained by eliminating the carrier component from AM signal. Its disadvantage is that it

needs a coherent carrier detector at the receiver.

This is used for RDS (Radio Data System) because it is difficult to decouple.

t t m At s ccω

cos)()(=

http://en.wikipedia.org/wiki/File:DSBSC_Modulation.svg



WAVEFORMS:

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0-1 0

0

1 0

t i m e

m

e s s a g e

s i g n a l

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0

-1 0

0

1 0

t i m e


0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0-1 0 0

0

1 0 0

t i m e

m

o d u l a t e d

s i g n a l

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 00

2

4x 1 0

4

t i m e

f f t m

o d u l a t e d

s i g n a l

MATLAB CODE:

% DSBSC MODULATION

fm=10;

fc=100;

am=8;

ac=10;

t=0:2000;

mt=am*cos(2*pi*fm*t);

ct=ac*cos(2*pi*fc*t);st=mt.*ct;

ft3=abs(fft(st));

subplot(4,1,1)


subplot(4,1,2)


subplot(4,1,3)

plot(t,st),xlabel('time'),ylabel('modulated signal');

subplot(4,1,4)

stem(t,ft3),xlabel('time'),ylabel('fft modulated signal');



PRECAUTIONS


EXPERIMENT 4

AIM

To study Double Side Band- Suppressed Carrier demodulation using MATLAB.

SOFTWARE REQUIRED

MATLAB

THEORY

DSB-SC demodulation is done by coherent demodulator. The local oscillator present in

demodulator generates a carrier which has same frequency and phase as that of carrier in

demodulated signal.



For demodulation the audio frequency and the carrier frequency must be exact otherwise we get

distortion

MATLAB CODE:

% DSBSC DEMODULATION

fm=10;

fc=100;

am=8;

ac=10;

t=0:1000;

mt=am*cos(2*pi*fm*t);

ct=ac*cos(2*pi*fc*t);

md=mt.*ct;

dt=md.*cos(2*pi*fc*t);

[b,a]=fir1(100,.003,'DC-1');

st=filter(b,a,dt);

ft3=abs(fft(st));

subplot(4,1,1)

plot(t,mt),xlabel('time'),ylabel('message signal');subplot(4,1,2)


subplot(4,1,3)

plot(t,st),xlabel('time'),ylabel('demodulated signal');

subplot(4,1,4)

stem(t,ft3),xlabel('time'),ylabel('fft demodulated signal');

http://en.wikipedia.org/wiki/File:Distortion.jpg



WAVEFORMS:

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0- 1 0

0

1 0

t i m e

m

e s s a g e

s i g n a l

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0- 1 0

0

1 0

t i m e


0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0

- 5 0

0

5 0

t i m e

d e m

o d u l a t e d

s i g n a l

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 00

1

2x 1 0

4

t i m e f f t d e m

o d u l a t e d

s i g n a l

PRECAUTIONS




EXPERIMENT 5 AIM

To study SSB Modulation using MATLAB.

SOFTWARE REQUIRED

MATLAB

THEORY

Single-sideband modulation (SSB) is a refinement of amplitude modulation that more

efficiently uses electrical power and bandwidth. It is closely related to vestigial sideband

modulation (VSB). Amplitude modulation produces a modulated output signal that has twice the

bandwidth of the original baseband signal. Single-sideband modulation avoids this bandwidth

doubling, and the power wasted on a carrier, at the cost of somewhat increased device

complexity.SSB was also used over long distance telephone lines, as part of a technique known

as frequency-division multiplexing (FDM).

Only a single side band of the DSB-SC modulated wave is transmitted.



SSB signal is represented by :

φSSB (t) = m(t) cos(ωc t) ± m^(t) sin(ωc t)

where

φUSB (t) = m(t) cos(ωc t) + m^(t) sin(ωc t)

and

φSSB (t) = m(t) cos(ωc t) - m^(t) sin(ωc t)



Generation of SSB Signal

Waveforms

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 00

1 0

2 0

3 0

4 0

5 0

t i m e

u p p e r s i d e

b a n d

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 00

1 0

2 0

3 0

4 0

5 0

t i m e

l o w e r s i d e

b a n d

MATLAB CODE:

%ssb modulation

fm=.02;

fc=.2;

t=0:100;

mt=cos(2*pi*fm*t);

mht=sin(2*pi*fm*t);

upp=mt.*cos(2*pi*fc*t)-mht.*sin(2*pi*fc*t);

low=mt.*cos(2*pi*fc*t)+mht.*sin(2*pi*fc*t);

ffu=abs(fft(upp));

ffl=abs(fft(low));



subplot(2,1,1)

stem(t,ffu),xlabel('time'),ylabel('upper side band');

subplot(2,1,2)

stem(t,ffl),xlabel('time'),ylabel('lower side band');

PRECAUTIONS




EXPERIMENT 6 AIM

To study SSB demodulation using MATLAB.

SOFTWARE REQUIRED

MATLAB

THEORY

The front end of an SSB receiver is similar to that of an AM or FM receiver. To recover the origina

signal from the IF SSB signal, the single sideband must be frequency-shifted down to its original

range of baseband frequencies, by using a product detector which mixes it with the output of a

beat frequency oscillator (BFO)For this to work, the BFO frequency must be accurately adjusted.If the BFO is mis-adjusted, the output signal will be frequency-shifted,



WAVEFORMS:



0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0 - 1

-0 . 5

0

0 . 5

1

t im e

m e s s a g e

s i g n a l

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0 -0 . 5

0

0 . 5

t im e

d e m

o d u l a t e d

s i g n a l

MATLAB CODE:

%ssb demodulation

fm=10;

fc=100;

t=0:2000;

mt=cos(2*pi*fm*t);

ct=cos(2*pi*fc*t);

mht=sin(2*pi*fm*t);

upp=mt.*cos(2*pi*fc*t)-mht.*sin(2*pi*fc*t);

st=upp.*ct;

[b,a]=fir1(100,.003,'DC-1');

dt=filter(b,a,st);

subplot(2,1,1)


subplot(2,1,2)plot(t,dt),xlabel('time'),ylabel('demodulated signal');

PRECAUTIONS


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