how to obtain the frequency when doing the fourier trasform

Hi,

I am applying the fast fourier trasform to a timeseries. When I plot the transform (or better its absolute value) I would like to have on the x axis a range of frequency but I am not sure how to calculate that.

I am working with data that are each minute and are registered in a timetable.

Thanks in advance,

Giacomo

Answer 1

[Star Strider]

Assuming that the data are regularly sampled, so that the sampling intervals are the same for all of them, I usually do something like this:

y = ...;                                                    % Signal (Vector Or Matrix)
t = ...;                                                    % Time Vector 
Fs = 1/mean(diff(t));                                       % Sampling Frequency
Fn = Fs/2;                                                  % Nyquist Frequency
ym = y - mean(y);                                           % Subtract Column Means From All Columns (Eliminates D-C Offset Effect)
L = numel(t);                                               % Signal Lengths
FTy = fft(ym)/L;                                            % Fourier Transform (Scaled For Length)
Fv = linspace(0, 1, fix(L/2)+1)*Fn;                         % Frequency Vector
Iv = 1:numel(Fv);                                           % Index Vector

figure
plot(Fv, abs(FTy(Iv,:))*2)
grid

This calculates and plots a one-sided Fourier transform. You will need to supply ‘t’ and ‘y’. The code should be reasonably robust.

.

[giacomo labbri]

Thanks for your detailed answer!

[Star Strider]

As always, my pleasure!