Plotting FFT when the time steps are not equidistant

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fendertunes
fendertunes 2014-1-5
评论： fendertunes 2014-1-6
I have a .txt file with time and associated voltages. I am trying to import this in Matlab and look at the frequency response. The problem is the time steps are not equidistant. FFT need equidistant time points. Is there a way I can somehow linspace or extrapolate timesteps to create equidistant points? I have attached the txt file (first column is timestep, second is voltage).

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Image Analyst
Image Analyst 2014-1-5
Yes. Try interp1() to get uniformly spaced samples.
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Image Analyst
Image Analyst 2014-1-6
编辑：Image Analyst 2014-1-6
The fft is the amplitude. The magnitude of the fft gives the relative amplitude, energy, power in the various frequencies, I think (Wayne King would know for sure). You can extract the first 100 by taking theFFt(1:100). Then pass into abs() to get the magnitude, or take the real part with real(). Not sure what you're looking for. Do you want to plot 100 singe waves, each with its own amplitude? It would probably be best if Wayne answered this question for you because he knows the fine details about what frequencies the elements correspond to and what the values correspond to. But if I answered your original question, can you mark it as "Accepted"?
fendertunes
fendertunes 2014-1-6
I am looking for first 100 harmonic amplitudes, real part.
I will try - FFTResult(1:100) and seeing if that does it.

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Walter Roberson
Walter Roberson 2014-1-6
You should be using a Non-Uniform Discrete Fourier Transform. Mathworks does not provide a routine for this, but you can use a Type I nufft from the File Exchange contribution http://www.mathworks.com/matlabcentral/fileexchange/25135-nufft-nfft-usfft

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