how to do FFT

Hi, I have an assignment in wich i need to apply FFT in matlab. The problem is that i don't have any function or frequency because all of my data came out from an accelerometer. although i have sampling rate, sampling time interval and number of samples i don't know the frequency or the function of the sine wave formed by my data. Can you please enlighten me how to do my FFT? any help would be greatly appreciated

Answer 1

[Star Strider]

See the documentation for the R2015a version of fft. The code between the top two plot figures is what you need to do.

[Vasilis Papadopoulos]

if you mean this code:

Y = fft(y,251);
Pyy = Y.*conj(Y)/251;
f = 1000/251*(0:127);
plot(f,Pyy(1:128))
title('Power spectral density')
xlabel('Frequency (Hz)')

can you please explain to me what is 251 represends cause I don't understand this example

[Star Strider]

The ‘251’ is the number of points of ‘y’ used to calculate the fft. If the length of ‘y’ is shorter than that, then it is zero-padded to that lenght, if longer, it is truncated to a length of ‘251’. That refers to the length of a vector or the row-size of a matrix.

The rest of the code squares the elements of ‘Y’, creating the power spectral density. Only the first 128 samples are plotted because this is a one-sided fft (although I would have chosen 126 instead of 128, since that is half the length of ‘Y’+1).

[Vasilis Papadopoulos]

thank you Sir

[Star Strider]

My pleasure.