clear

Fs = 500000;                    % Sampling frequency
T = 1/Fs;                       % Sample time
L = 3000;                       % Length of signal
t = (0:L-1)*T;                  % Time vector

 a=0.5;
 f1=5000;
 f2=100000;
 f3=60000;
 w1=2*pi*f1;
 w2=2*pi*f2;
 w3=2*pi*f3;
s=(1-a*sin(w1*t)).*sin(w2*t).*sin(w3*t);
for i=1:length(s)
    if s(i)>0
        y(i)=s(i);
    else
        y(i)=0;
    end
end
plot(Fs*t,y)
title('Signal détecté')
xlabel('time (milliseconds)')

 
 
NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(y,NFFT)/L;
Z=  fft(s,NFFT)/L;
f = Fs/2*linspace(0,1,NFFT/2+1);
% 
% % Plot single-sided amplitude spectrum.*
figure(2)
subplot(2,1,1)
plot(f,2*abs(Z(1:NFFT/2+1))) 
 title('Spectre du signal avant détection')
 xlabel('Frequency (Hz)')
 ylabel('|Y(f)|')
 grid
 
subplot(2,1,2)
plot(f,2*abs(Y(1:NFFT/2+1))) 
 title('Spectre du signal détecté')
 xlabel('Frequency (Hz)')
 ylabel('|Y(f)|')
 grid