MATLAB: FFT, Periodogram difference, Power Spectral Density Representation and Effect of Hilbert Trasnform to PSD

fftfourierhilbertperiodogrampower spectral densitypsd

Hi,
My questions are;
1)Generally,FFT and periodogram do not show same thing! I want to get Power Spectral Density. If signal is real data and in time.
M=1533;fs=27; freqVector = (-(M-1)/2:(M-1)/2)/M*fs;
 figure;plot(freqVector,20*log10(abs(fftshift(fft(hilbert(signal))))));
figure;periodogram((hilbert(signal)),[],'onesided',[],27*10^9,'power') why this above line needs abs of it ? why not support complex ?
figure;periodogram((hilbert(back_remvd(1:150,30))),[],'twosided',[],27*10^9,'power') why complex is ok for twosided ?
and the amplitude values btw fft and periodogram representation are not same. Why ?
Can you show me the right PSD and spectrum representation please ?
thanks in advance.

Best Answer

hans, If you are treating narrow band signals then FFT or pwelch as said by Andrew is sufficient, but for wideband signals, you need to use periodogram.
Periodogram of narrowband signal will show you a narrow line of fundamental frequency along the time axis, for FFT try this function :
http://www.mathworks.com/matlabcentral/fileexchange/40002-psd-power-spectral-density-and-amplitude-spectrum-with-adjusted-fft
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