Plotting FFT when the time steps are not equidistant

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).

[Youssef Khmou]

Neil, the file is not attached

[fendertunes]

Its attached now. Thanks

[Image Analyst]

So, did you try interp1 like I suggested in my answer?

Answer 1

[Image Analyst]

Yes. Try interp1() to get uniformly spaced samples.

[fendertunes]

That interp1() worked like a charm! Thanks so much.

I have a related question. So after interp1(), I was able to plot/stem an FFT response of the attached txt file. Is there a way to see or output the first 100 harmonics and it's corresponding amplitudes for the FFT? I mean I can see them on the stem plot, but is there a function to extract the first 100 and the amplitudes for it?

[Image Analyst]

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]

I am looking for first 100 harmonic amplitudes, real part.

I will try - FFTResult(1:100) and seeing if that does it.

Answer 2

[Walter Roberson]

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