# Harmonics in Quantal Histogram

I'm studying what I believe to be a system that produces a (noisy) quantal response (i.e. there should be a minimum increment of response). To do this, I've plotted a histogram of some ~6000 amplitude values from the system into small bins, and performed an FFT on the histogram, expecting that a common quantal value should appear as the lowest frequency "spike". However, my spectral response: looks odd to me. What I see are what look like harmonics.

Why does this occur in the quantal release problem? Is this lowest frequency actually my quantal response, or is it a typical electrical response/artefact from the FFT of a signal with so many discontinuities? I thought that odd-numbered harmonics are the most powerful, but here they seem to be linearly declining. Why am I seeing other smaller spikes appear 1/3rd of the way through each of the "main" harmonic bands?

• Can you talk a little bit more about the application itself? If you FFT the histogram, you get back the harmonics that make up the histogram "waveform". Unless the histogram was periodic (in other words, multimodal at regular intervals), the FFT would not return anything "sensible". Furthermore, if the histogram is "fine" to the extent of having zeros in between the bars it will present a set of spikes to the FFT. Yes, what you see there looks like a square pulse. Can you post the histogram that was used as the input to this too?
– A_A
Jun 26 '18 at 11:04
• Exactly, I'm looking for a common periodicity in the histogram. Ideally, quantized responses should show up in the histogram (larger numbers at 5, 10, 15, etc). However I'm expecting a bit of noise (some responses might be 7, 12, 17, etc). I can edit the original post to include the histogram.
– Mark
Jun 27 '18 at 0:20
– A_A
Jun 29 '18 at 15:39