Fourier transform of a function

Hello everyone, I come to you with a question on the fourier transform of a function.

I am trying to obtain the fourier transform of the signal of the position of an object depending on the time.

For now here is my signal (on the two image the top graph) and the fourier transform I obtain from the code I write below the images (taken from a previous question found in this site, quoted from Star Strider's answer).

function fourier_transform(z,h)
    %z is 10 000
    %h changes a lot depending on the result
    figure;
    subplot(211);
    plot(z,h);
    
    
    
    n = length(z);
    Ts = 1;                                         % Sampling Interval (one measure every one seconds for 10 000 seconds)
    Fs = 1/Ts;                                      % Sampling Frequency (samples)
    Fn = Fs/2;                                      % Nyquist Frequency
    nfft = 2^nextpow2(n);
    Hn = h-mean(h);                                 % Remove Constant Offset > might not be necessary
    ft = fft(Hn,nfft)/n;
    Fv = linspace(0, 1, fix(n/2)+1)*Fn;             % Frequency Vector
    Iv = 1:length(Fv);                              % Index Vector
    subplot(212);
    plot(Fv, abs(ft(Iv))*2)

The result obtained seem really wrong. Did I make a mistake in my code?

Thank you in advance for your help

JdC

Answer 1

[Star Strider]

The only problem I see is in the ‘Fv’ calculation.

It should be:

Fv = linspace(0, 1, (nfft/2)+1)*Fn; % Frequency Vector

Apparently, my original code (that I thank you for quoting) did not use ‘nfft’ as calculated here. (If it did, not calculating ‘Fv’ correctly was my error, and I apologise for it.)

.

[JdC]

Thank you very much for your anwer, indeed I forgot to quote you on my post, and changed it now.

It does indeed work now.

Thanks again

JdC

[Star Strider]

As always, my pleasure!

No worries — I just recognised my code (there’s a lot of it out there). No specific citation necessary, however I like to be cerrtain that my posted code works correctly.