How to plot properly a 2d fft of data?

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mm99 2020-1-4

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/498914-how-to-plot-properly-a-2d-fft-of-data

评论： Star Strider 2020-1-5

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?

thanks in advance

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Answer 1

Star Strider 2020-1-4

2
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https://ww2.mathworks.cn/matlabcentral/answers/498914-how-to-plot-properly-a-2d-fft-of-data#answer_408684

在 MATLAB Online 中打开

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:

D = load('matlab.mat');
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')