Strange Result from FFT using Scipy

Question

I'm trying to do an FFT of some data (a Gaussian pulse), but I'm finding a strange result. The real and imaginary components of the resultant FFT alternate in sign every index of the array. The absolute values of the arrays, however, are continuous. So, I get something that looks like this:

FFT of Gaussian pulse

Does anybody have an idea on what is causing this? Thanks!

Linking this more or less same question here: [Strange phase for gaussian beam 2D](https://stackoverflow.com/questions/55556913/strange-phase-for-gaussian-beam-2d) — Cris Luengo, Jun 03 '23 at 14:02

score 6 · Answer 1 · edited Jun 03 '23 at 14:04

6

Alternating signs in the frequency domain corresponds to an exp(j*pi*n) complex factor which by the shift theorem corresponds to a time domain circular shift of N/2 samples. Looking at your time domain Gaussian pulse you should notice that the peak indeed appears at N/2 instead of index 0.

Shifting back your time domain Gaussian pulse with ifftshift should give you a pulse centered at 0 whose frequency domain representation does not have this sign alternation.

edited Jun 03 '23 at 14:04

Cris Luengo

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answered May 27 '16 at 16:23

SleuthEye

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You should use `iffshift` not `fftshift` (I know, confusing nomenclature) – David Hoffman Oct 26 '16 at 05:01

Strange Result from FFT using Scipy

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