Uncertainty using FFT function.

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Andrejus
Andrejus on 4 Nov 2014
Commented: Harry on 4 Nov 2014
Hello all,
I am trying to get the spectral shape of my time domain signal. For this I use FFT. However, the results I get are not as expected and I cannot understand why. My code for FFT is as follows:
NFFT=length(eta);
Fs=2;
transf_length=pow2(nextpow2(NFFT));
transforma = fft(eta,transf_length);
frequ = (0:transf_length-1)*(Fs/transf_length)*2*pi; % to make in radians/s
power = transforma.*conj(transforma)/transf_length;
Everything seems to be fine, but the magnitude of the spectrum when compared to the expected spectrum is not the same (approximately 30 times larger). The first figure shows the FFT vs. the expected spectrum. The second figure shows the FFT power/30 vs. the expected spectrum.
Can someone explain if there is something wrong with my code, or I am just misunderstanding something?
Thanks!
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Andrejus
Andrejus on 4 Nov 2014
Yes, but in the code I have applied the division by 30 in order to get values close to expected ones... if you remove division then the first figure in my questions will pop up.
Harry
Harry on 4 Nov 2014
I think you need to strip this problem down to what you are having trouble with. Asking people to install an obscure library to run your code will possibly mean you get less help. If your problem is with the FFT, then you should be able to describe the problem with just a few lines of code.
Maybe plot the FFT of a simple signal, explain why that is correct... then plot the FFT of your signal and describe how it is not as expected.

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