%信號直接相乘后求FFT
for nn=1:5
clear

k=5E+10;  %調頻斜率
fo=10E+6;%載頻
T=30E-6;%脈沖寬度
n=4000;%抽樣點數
fs=40E+6;%采樣頻率
ts=1/fs;%采樣時間間隔
t=ts:ts:n*ts;

p=zeros(1,n);
p(500+1:500+T*fs)=ones(1,T*fs);
signal=1/sqrt(T)*cos(2*pi*(fo*t+k*t.^2/2)).*p;
signal_spect=abs(fft(signal));

signal_power=sum(signal.^2);
snr=-7;
noise_power=signal_power*10^(-snr/10);
a=sqrt(noise_power/n);
%a=0;
noise=a*randn(1,n);
s=signal+noise;
s_spect=abs(fft(s));

delay=500;
mix=s(1:n-delay).*s(delay+1:n);
mix_spect=abs(fft(mix));

figure
subplot(2,1,1)
plot(mix)
subplot(2,1,2)
plot(mix_spect)

value_max=max(mix_spect(2:length(mix_spect)-1));
position=find(mix_spect==value_max);
k_detect=(position(1)-1)*fs/(n-delay)/(delay*ts);

ratio=abs((k_detect-k)/k)

end