Performing a robust fft of a sinus
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Hi,
I want to perform a fourier transform near the Nyquist frequency. I plot in a loop the maximum of the FFT for an increasing length of the sinus.
However the result is really unstable, regarding to both the length of the sinus and the sinus frequency.
Fe = 400; %sampling frequency, Hz
F = 163; %sinus frequency, Hz
A = 26; %amplitude
for i=10 : 100
clear sinus, clear vect_temp, clear FFT, clear FFT_prime;
N = i*100;
vect_temp = (1/Fe:1/Fe:N/Fe); %s
sinus = A*sin(2*pi*F*vect_temp);
FFT = abs(fft(sinus))/ length(sinus); %FFT normalized
NFFT = 2^nextpow2(length(sinus));
FFT_prime = abs(fft(sinus,NFFT)) / length(sinus); %FFT normalized
hold on,
plot(N,max(FFT),'+');
plot(N,max(FFT_prime),'*r');
end
Any idea what's the problem?
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