% MATLAB Script for Amplitude Modulation

% Although it is possible to modulate any signal over a sinusoid, however I

% will use a low frequency sinusoid to modulate a high frequency sinusoid

% without the loss of generality.

format long;

% Clear all previuosly used variables and close all figures

clear all;

close all;

% Amplitude, Frequency and Phase Shift for Modulating Signal

A1 = 2; f1 = 5; p1 = 0;

% Amplitude, Frequency and Phase Shift for Carrier Signal

A2 = 4; f2 = 20; p2 = 0;

% Sample Rate - This will define the resolution

fs = 1000;

% Time Line. Longer the signal, better will be the fft

t = 0: 1/fs : 1;

% Generate the message signal

s1 = A1*sin(2*pi*f1*t + p1);

% Plot the message signal

figure(1);

plot(t,s1);

xlabel('Time (sec)');

ylabel('Amplitude');

title(['Message Signal with frequency = ',num2str(f1),' Hz']);

grid on;

% Generate the Carrier wave

s2 = A2*sin(2*pi*f2*t + p2);

% Plot the carrier wave

figure(2);

plot(t,s2);

xlabel('Time (sec)');

ylabel('Amplitude');

title(['Carrier Signal with frequency = ',num2str(f2),' Hz']);

grid on;

% Finally the Modulation

% Ref. Modern Analogue and Digital Communication Systems - B. P. Lathi

% Amplitude Modulation with Suppressed Carrier

% Double Sideband with Suppressed Carrier (DSB-SC)

s3 = s1.*s2;

% Generate the Envelope

s3_01 = A1*A2*(sin(2*pi*f1*t));

s3_02 = -A1*A2*(sin(2*pi*f1*t));

% Amplitude Modulation with Large Carrier

% Double Sideband with Large Carrier (DSB - LC)

s4 = (A2 + s1).*sin(2*pi*f2*t);

% Generate the Envelope

s4_01 = A2 + s1;

s4_02 = -A2 - s1;

% ----------------------------------------------------------

% Let's Check out the frequency content of the two Modulations

% Number of FFT points. N should be greater than Carrier Frequency

% Larger the better

N = 2^nextpow2(length(t));

f = fs * (0 : N/2) / N;

% Find FFT

s3_f = (2/N)*abs(fft(s3,N));

s4_f = (2/N)*abs(fft(s4,N));

%-------------------------------------------------------------

% Plot the two Modulations

% Plot the DSB-SC Signal

figure(3);

subplot(2,1,1);

plot(t,s3);

hold on;

plot(t,s3_01,'r');

hold on;

plot(t,s3_02,'g');

xlabel('Time (sec)');

ylabel('Amplitude');

title('Double Sideband with Suppressed Carrier');

grid on;

subplot(2,1,2);

plot(f(1:100),s3_f(1:100));

xlabel('Frequency (Hz)');

ylabel('| Amplitude |');

title('Spectral Anaalysis (Single Sided PSD)');

grid on;

% Plot the DSB-LC Signal

figure(4);

subplot(2,1,1);

plot(t,s4);

hold on;

plot(t,s4_01,'r');

hold on;

plot(t,s4_02,'g');

xlabel('Time (sec)');

ylabel('Amplitude');

title('Double Sideband with Large Carrier');

grid on;

subplot(2,1,2);

plot(f(1:100),s4_f(1:100));

xlabel('Frequency (Hz)');

ylabel('| Amplitude |');

title('Spectral Anaalysis (Single Sided PSD)');

grid on;

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