Sampling freq in fft, what to put?

Hi!

I have a voltage signal vs. time. I want to have the fourier transform of it and plot it against the frequency axis. I have read the fft help, but I am lost as I don't know what to put for my sampling frequency!

Answer 1

[Matt J]

Frequency sampling obeys

 frequencysampling=1/N/timesampling

where N is the signal length and timesampling the time sample spacing. The frequency axis to plot against is then,

frequencyAxis = (  (0:N-1) -ceil((N-1)/2)  )/N/timesampling;

This is the centered axis, to be used after you've applied fftshift() to your spectrum.

Answer 2

[Youssef Khmou]

There are many tutorials on how to perform this task, understanding the methodology is by learning an example, the things that must be know about your data is the N length and the Fs sampling frequency, let us fast example :

 t=0:0.01:10; % Fs=100 Hz
 Fs=100;
 N=length(t);
 NFFT=512; % number of points for computing the spectrum
 y=sin(2*pi*t*35);
 fy=fft(y,NFFT)/NFFT;
 frequency=(0:NFFT-1)*Fs/NFFT;
 figure, plot(frequency(1:end/2),abs(fy(1:end/2));