AMPLITUDE MODULATION USING MATLAB

In this modulation technique the frequency and the phase of the message signal remains constant and the value of amplitude gets modulated. Here, the carrier signal is taken as a very high frequency signal.  The amount of modulation depends on the MODULATION INDEX ‘m’ and the CARRIER FREQUENCY ‘fc’.

MATLAB CODE FOR MODULATION INDEX 0.5.

a=input('ENTER THE AMPLITUDE OF SIGNAL');

b=input('ENTER THE AMPLITUDE OF CARRIER SIGNAL');

fm=input('ENTER THE FREQUENCY OF MESSAGE SIGNAL');

fc=input('ENTER THE FREQUENCY OF CARRIER SIGNAL');

x = 0:0.001:1;

m=0.5;

sm=a*sin(2*pi*fm*x);

sc=b*sin(2*pi*fc*x);

smod=(a+m*sm).*sin(2*pi*fc*x)

subplot(3,1,1),plot(x,sm)

title('MESSAGE SIGNAL');

subplot(3,1,2),plot(x,sc)

title('CARRIER SIGNAL');

subplot(3,1,3),plot(x,smod)

title('AMPLITUDE MODULATED SIGNAL');

OUTPUT

CASE 1:

AMPLITUDE OF SIGNAL  - 5

AMPLITUDE OF CARRIER SIGNAL - 5

FREQUENCY OF MESSAGE SIGNAL - 5

FREQUENCY OF CARRIER SIGNAL – 50

CASE 2:

AMPLITUDE OF SIGNAL - 5

AMPLITUDE OF CARRIER SIGNAL - 10

FREQUENCY OF MESSAGE SIGNAL - 5

FREQUENCY OF CARRIER SIGNAL – 50

MATLAB CODE FOR VARIOUS MODULATION INDEX.

Here we have to find the AMPLITUDE MODULATED signal for different MODULATION INDEXES and plot them in the same graph using ‘subplot’ command. The modulation indexes are 0.15, 0.3, 0.5 and 0.8.

a=input('ENTER THE AMPLITUDE OF SIGNAL');

b=input('ENTER THE AMPLITUDE OF CARRIER SIGNAL');

fm=input('ENTER THE FREQUENCY OF MESSAGE SIGNAL');

fc=input('ENTER THE FREQUENCY OF CARRIER SIGNAL');

x = 0:0.001:1;

m1=0.15;

m2=0.3;

m3=0.5;

m4=0.8;

sm=a*sin(2*pi*fm*x);

sc=b*sin(2*pi*fc*x);

smod1=(a+m1*sm).*sin(2*pi*fc*x)

smod2=(a+m2*sm).*sin(2*pi*fc*x)

smod3=(a+m3*sm).*sin(2*pi*fc*x)

smod4=(a+m4*sm).*sin(2*pi*fc*x)

subplot(4,1,1),plot(x,smod1)

title('MODULATION INDEX 0.15');

subplot(4,1,2),plot(x,smod2)

title('MODULATION INDEX 0.3');

subplot(4,1,3),plot(x,smod3)

title('MODULATION INDEX 0.5');

subplot(4,1,4),plot(x,smod4)

title('MODULATION INDEX 0.8');

OUTPUT

CASE 3:

AMPLITUDE OF SIGNAL - 5

AMPLITUDE OF CARRIER SIGNAL - 10

FREQUENCY OF MESSAGE SIGNAL – 5Hz

FREQUENCY OF CARRIER SIGNAL – 50Hz

CASE 4:

AMPLITUDE OF SIGNAL - 5

AMPLITUDE OF CARRIER SIGNAL - 10

FREQUENCY OF MESSAGE SIGNAL – 7Hz

FREQUENCY OF CARRIER SIGNAL – 70Hz

APPLICATION

Amplitude modulation is utilized in many services such as television, standard broadcasting, aids to navigation, telemetering, radar, facsimile etc. Although the message content may vary widely for these, the mechanism of combining the message and the message carrier at the sending terminal is basically the same.