Urgent: How do I perform FFT on a time signal

I have a vibration time signal am working on. I am finding it difficult to write the code to perform fft on it. Could anyone help with it please? The length of the signal is 40000 while the sampling frequency is 20000.

Additionally, how are the length of the signal and sampling frequency arrived at?

[Youssef Khmou]

try this submission :

http://www.mathworks.com/matlabcentral/fileexchange/40002-psd-power-spectral-density-and-amplitude-spectrum-with-adjusted-fft

[Jan]

While this question might be "urgent" for you, it is not "urgent" for all readers. Therefore it is recommended toavoid such pushing terms.

Answer 1

[dpb]

doc fft

Sampling frequency is set by the hardware, time then determines the length.

Answer 2

[Youssef Khmou]

hi, given your signal X,

 Fs=2e+4;
 Resolution=512;
 Ts=1/Fs;
 N=4e+4;
 T=N*Ts;
 Fx=abs(fft(X,Resolution));
 Frequency=(0:Resolution-1)*Fs/Resolution;
 figure, plot(Frequency(1:end/2),Fx(1:end/2)); % two sided to one side

Answer 3

[Roule]

Hi, just use the following code:

fs=20000; % sampling frequency

N=40000;  % length of signal

dF=Fs/N;

f=-Fs/2:dF:Fs/2-dF;   %remember nyquist frequency?

plot(f,abs(fftshift(fft(<your signal>)/N)));

Hope this helps!