% EE-202 Lab Experiment 6  H. Saadat, Fall 2002
% AM Modulation and envelope detection of AM
f0 = input('Enter the message signal frequency in Hz, f_o = '); % The message signal 
fc = input('Enter the carrier frequency in Hz, f_c = '); % Carrier signal 
R=100000; 
CU = input('Enter capacitance in micro Farad, C = ');
C = CU*10^-6;
tau=R*C;
fs=20*fc; 
Ts=1/fs;  
t=0:Ts:2/f0;
%t=[1:2^13]/fs;

x = cos(2*pi*f0*t); 
cs= cos(2*pi*fc*t); 
x_AM = (1+0.7*x).*cs;                 % Amplitude modulated signal
n=length(x_AM);
x_AMRect = x_AM;
for k=1:n
   
    if x_AMRect(k) < 0
       x_AMRect(k) = 0;
   else, end
end

figure('Units','normalized', ...
	'Color',[0.8 0.8 0.8], ...
	'Position',[0.09 0.1 0.8 0.8], ...
	'Tag','EE202 Envelop Detector');

subplot(3,1,1), plot(t, cs,'b', t,x,'r'), grid on
title(['Message frequency f_0 = ', num2str(f0),...
       ' Hz,  Carrier frequency f_c = ', num2str(fc),...
       ' Hz, R = ', num2str(R/1000), ' K\Omega, C = ', num2str(CU),' \muF']) 
legend('Carrier','Message', 4)
subplot(3,1,2), plot(t, x_AM, 'm'), grid on 
title('AM Signal');
xlabel('t, s'), ylabel('x_{AM}') 

% Envelope detection of AM
x_o=x_AM*0;
for k=2:length(x_o)
   x_o(k)=x_o(k-1)-(x_o(k-1)/(fs*R*C));

   if (x_o(k) < x_AM(k))
      x_o(k)=x_AM(k);
   end;
end;

subplot(3,1,3), plot(t,x_AMRect,'m',t,x_o,'b'); grid on
xlabel('Time (sec)')
title('Envelope Detector Output and Rectified AM Signal');
legend('Rectified AM signal', 'Envelope detector output', 4)
axis([0 2/f0 -2 2])