Matlab script that simulates the modulation and demodulation of an AM wave

AM3_25.m

%% Author: Ajay Thampi
%% Problem: 3.25

% Modulation Process
Fc = 20;                        % carrier frequency of 20 kHz
Fs = 160;                       % sampling rate of 160 samples per ms
Fm = 0.4;                       % modulating frequency of 0.4 kHz
t = 0:1/Fs:10;                  % t of 10 seconds
c = cos(2*pi*Fc*t);             % carrier signal
m = cos(2*pi*Fm*t);             % modulating signal
s = c + 0.25*cos(2*pi*(Fc+Fm)*t) + 0.25*cos(2*pi*(Fc-Fm)*t); % FCAM 50% mod
dsb_sc = ammod(m,Fc,Fs);        % DSB-SC signal
figure;
subplot(4,1,1);
plot(t, m);
title('Modulating Signal'); xlabel('time (s)'); ylabel('amplitude');
subplot(4,1,2);
plot(t, c);
title('Carrier Signal'); xlabel('time (s)'); ylabel('amplitude');
subplot(4,1,3);
plot(t, s);
title('FCAM Signal'); xlabel('time (s)'); ylabel('amplitude');
subplot(4,1,4);
plot(t, dsb_sc);
title('DSB-SC Signal'); xlabel('time (s)'); ylabel('amplitude');

% Non-Coherent Detection Step 1: Envelope Detection
Vc(1) = 0;                              % initial capacitor voltage
for i = 2:length(s)
    if s(i) > Vc(i-1)                   % diode on (charging)
        Vc(i) = s(i);
    else                                % diode off (discharging)
        Vc(i) = Vc(i-1) - 0.023*Vc(i-1);
    end
end
Vd(1) = 0;
for i = 2:length(dsb_sc)
    if dsb_sc(i) > Vd(i-1)              % diode on (charging)
        Vd(i) = dsb_sc(i);
    else                                % diode off (discharging)
        Vd(i) = Vd(i-1) - 0.023*Vd(i-1);
    end
end

% Non-Coherent Detection Step 2: Low Pass RC Filter
h = fir1(100, 0.0125, 'low');            % 1 kHz cut-off frequency
foutputc = filter(h,1,Vc);
figure;
subplot(3,1,1);
plot(t, Vc);
title('Envelope detector output of FCAM signal'); xlabel('time (s)'); ylabel('amplitude');
subplot(3,1,2);
plot(t, Vd);
title('Envelope detector output of DSB-SC signal'); xlabel('time (s)'); ylabel('amplitude');
subplot(3,1,3);
plot(t, foutputc);
title('Non-coherent demodulated FCAM signal'); xlabel('time (s)'); ylabel('amplitude');