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Hello,

I would like to know which frequencies are hidden in my velocity signal (=velocity vector).

I started straight forward: I took the "fft" of the vector and plotted its amplitude against the frequencies, but this does not give me the desired result, i.e. no clear frequency peaks were observed.

Since the velocity vector is non-periodic, i tried to smoothen the beginning and end of the signal by means of a hanning window. Unfortunately, it did not work.

I attached all the data in the zip-file it contains:

- UpV_irreg.mat (velocity vector)
- t_irreg.mat (corresponding time vector)
- fft_test.m (my code)

Any help would be greatly appreciated.

Star Strider
on 1 Feb 2019

Star Strider
on 17 Feb 2019

The amplitude in the frequency domain and time domain are the same in the code I posted. Note that MATLAB computes a two-sided Fourier transform, so the energy is equally divided between the positive and negative frequencies. It is therefore necessary to multiply the amplitude by 2 in the plotted one-sided Fourier transform to display the amplitude accurately:

plot(Fv, abs(FT_D(Iv))*2)

The discrete nature of the fft usually results in some ‘leakage’ of signal energy to near-by frequencies, so the peak amplitude in the fft may not always match (may be less than) the amplitude of the time-domain signal.

If you want to calculate the time-domain signal from the frequency-domain signal, you need to use the original two-sided Fourier transform results, without the amplitude scaling necessary for the one-sided Fourier transform.

I do not understand your reason for doing this, however. If you want to do frequency-selective filtering of your signal, do that in the time domain with some of the filter designs that the Signal Processing Toolbox (among others) allows you to create.

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