# How to plot properly a 2d fft of data?

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mm99 on 4 Jan 2020
Commented: Star Strider on 5 Jan 2020
Hi!
Ive generated a wave in a numerical model using a chirp signal and collected the responses of the model in chosen points. How can I plot properly the 2d fft of this data. The responses are in attached mat.
Here is the chirp signal:
t2=0:dt:(2e-4)-dt;
signal2 = chirp(t2,0,9.99900000000000e-05,1e6,'linear');
Is the attached image correct?

Star Strider on 4 Jan 2020
Not entirely. The posted image is the plot of the two-sided Fourier transform after using the fftshift function. The result is that the frequency axis is not correct. (Note that a 2D fft (fft2) is usually applied to images and similarly-constructed matrices. The 1D fft is correct here.)
Try this:
mat = D.mat;
t = 0:numel(mat)-1; % Time Vector (Units: Time Units)
Ts = mean(diff(t)); % Sampling Time Interval
Fs = 1/Ts; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
L = numel(t); % Signal Length
FTmat = fft(mat)/L; % Fourier Transform
Fv1 = linspace(0, 1, fix(L/2)+1)*Fn; % Frequency Vector - One-Sided Fourier Transform (Units: Cycles/Time Unit)
Iv = 1:numel(Fv1); % Index Vector
figure
plot(Fv1, abs(FTmat(Iv))*2)
grid
title('One-sided Fourier Transform')
xlabel('Frequency')
ylabel('Amplitude')
Fv2 = linspace(-Fn, +Fn, L); % Frequency Vector - Two-Sided Fourier Transform (Units: Cycles/Time Unit)
figure
plot(Fv2, fftshift(abs(FTmat)))
grid
title('Two-Sided Fourier Transform')
xlabel('Frequency')
ylabel('Amplitude')
producing: Star Strider on 5 Jan 2020
As always, my pleasure!