Rescaling the x-axis of FFT transformed histogram

[eberry]

The answer can be found in the fftw documentation:
fftw.org/doc/The-1d-Discrete … _0029.html

Here is what I believe the answer is:

• Say your input histogram has a sampling frequency of “f”, where f = 1 / (input histogram bin width)
• Say your output histogram has “nFFT” bins
• Then for k <= (nFFT / 2), the kth bin of your output histogram corresponds to the frequency k * f / nFFT
• If you only want to plot positive frequencies, then you’re done. Just plot the first nFFT / 2 bins and ignore the following bins of the FFT output.
• If you do want to plot negative frequencies, the values corresponding to the negative frequencies are stored in backwards order in the second half of the output [k > (nFFT / 2)].

As an aside: in addition to my question on this topic, I noticed a few very similar questions on this forum that had not been answered. Here are just two examples:

Axis rescale in FFT.C

Hello, I was playing with the FFT.C and tried to extract the frequency of the input fuction. The totorial says: Unfortunatelly I couldn’t figure out how this is done. Any advice? Regards, Christian

FFT Help

Hi, I am new to root and having problems with FFT. I want to perform the FFT on captured time domain signals captured in the lab. I have for example a 4 MHz square wave with 37,500 sample points. The code I am using is below. I would really appreciate if someone with more experience could help me answer these questions. Should I use the same amount of time-domain points for the FFT? On my output FFT graph , I am getting negative amplitudes? How is that possible if I am only getting the re…

Is this feature simply no longer supported? That would be a shame.

Best,
Edmund