I have a setup with a pair of high-frequency (200k) ultrasonic microphones which I am trying to run from a Raspberry Pi Zero. The microphones are attached to a USB hub, and both the hub and the Pi draw power from an Adafruit Powerboost regulator. When I record the ambient noise in my office, I see the following:
When I record me snapping my fingers, I see the following:
This is one channel of a stereo recording, taken at 200Khz with a block size of 8192. The band at 25k is from a known source (some kind of tone generator in a neighboring lab), but the fuzzy bands directly above it I cannot account for. What I find most confusing are the periodic bursts of noise (roughly every 2ms) centered on the 25k tone and appearing around the signal, as if the recording is somehow being smeared across the spectrum into what appear as vertical bands. All of this looks like some kind of sampling error on the Pi's end, but I'd like to know if it's characteristic of anything obvious and possibly eliminate analog noise in the pipeline as a possibility. This is the second USB adapter I have tried, with the same issue. Thanks for whatever help you can provide!