MATLAB: How to plot fft outputs in frequency domain

fast fourier transformfftfrequency domainMATLABplotplotting

Hi all,

I have attached the m-file for information about my question.

I have time-history acceleration data named BodyAccel_y in the workspace. I converted it to frequency domain by using fft in MATLAB.

However, as I noticed, the outputs are complex numbers.

I would like to plot the output data on Y-axis against their individual frequencies in X-axis.

I am new to this subject. I would appreciate any guidance/suggestion/instruction provided for this matter.

Thank you.

Best Answer

You read here....https://in.mathworks.com/help/matlab/ref/fft.html

You have to generate the frequency values based on the sampling you have and use abs of the result of fft.

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Hello,

You have used your absolute values to plot your image. Now, to reverse the operation you need the full fft results which is a complex matrix for regenerating your input image. Otherwise, you get something else!

Each fft component has value and phase. You shouldn't ignore the phase component which is ignored when you only use absolute values.

This is what you should do:

w = hanning(434)*hanning(362)';
FFT = fftshift(fft2(w)); % complex matrix
FFT_abs = abs(FFT); % absolut values of fft
imagesc(1+log10(FFT_abs)) % show fft results
w_new = ifft2(ifftshift(FFT)); % you need fft not fft_abs

Hope it helps,

Mona

Best Answer

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