how to do Fourier transform with current data

Hello,

I have a set of data as shown in the attactment, in which the "t_spectrum" shows the time and "y_spectrum" shows the electric field amplitude. And from this I want to do Fourier transform from these data. I have tried fft function but it seems that the spectrum is not reasonable.

So could anyone please recommend some approaches to that?

Thank you very much

Answer 1

[Star Strider]

Try this:

E = dlmread('y_spectrum.txt',' ',1,0);
t = dlmread('t_spectrum.txt',' ',1,0);
Ts = mean(diff(t));
Fs = 1/Ts;
Tstd = std(diff(t));
Fs = 12E+15;                                            % Sampling Frequency
[Er,tr] = resample(E,t,Fs);                             % Resample To Regularly-Spaced Intervals

figure
plot(t,E,'.b')
hold on
plot(tr,Er,'-r')
hold off
grid
legend('Original E', 'Resampled E')

Fn = Fs/2;                                              % Nyquist Frequency
L = numel(tr);                                          % Vector Lengths
Erm = Er - mean(Er);
FT_Erm = fft(Er)/L;
Fv = linspace(0, 1, fix(L/2)+1)*Fn;
Iv = 1:numel(Fv);

[pks,locs] = findpeaks(abs(FT_Erm(Iv))*2, 'MinPeakHeight',3E+10);

figure
plot(Fv, abs(FT_Erm(Iv))*2)
hold on
plot(Fv(locs), pks, '.r')
hold off
grid
text(1.5E+15, 7.5E+10, sprintf('Peak %10.3E  Frequency %10.3E\n', [pks Fv(locs).'].'))

producing:

I am not certain what does not appear to be ‘reasonable’ about it. It looks good to me.

Answer 2

[dang khoa tran]

Thank you very much.

Your answer is very complete. I mention "not reasonable" since the spectrum is very ugly. And the reason is that the resampling is not included in my code.

Anyway, thank you again.

[Star Strider]

As always, my pleasure!