I am recording some data for scientific purposes (it is neurological data), and the data is corrupted by some very strange noise. At multiples of about 700 Hz (slightly differing on different days) there are box like peaks in the multitaper spectrum. The width of each box is proportional to its frequency. The highest peak is at 5 times the base frequency, and every other repetition of the box decreases in power, the rest has the same power. I have attached an idealized image of how the spectrum of the noise looks below (in reality the width of the boxes is much smaller).
I am wondering what could cause this kind of noise. I would be interested in both practical ideas as to what device may be causing this (btw the data is recorded in a hospital) as well as theoretical insights into how this may happen (modulation, filtering etc etc).
Thank you and best regards!
Edit: Adding a few more details about the real data.
The data is recorded with implanted electrodes (256 channels) which connect to a so called headstage. During the recording the headstage is connected to two recording devices (128 ch each). I should mention that some channels are significantly more affected by this noise, and in general the channels from the first device are affected much more strongly. These devices are connected with a BNC cable which somehow ensures synchronization of the internal clocks. Collaborators of mine have suggested that the noise is due to this synchronization, but it is also present without this cable, so i do not think that this is necessarily the case. Unfortunately I do not know much more about the specific inner workings of this recording set-up. I can currently not play around with the set-up but will be able to do so in the near future, so suggestions what to try then are also welcome.
This is a plot of the real multitaper spectrum, where the base frequency of the peaks was 668. The 1/f shape is what is expected in this kind of data. In the image it appears as if the noise is narrow spikes, but they are in fact boxes with a width proportional to their frequency, as described above. The sharp spike at 10000Hz is some (separate?) source of noise that I also do not know the origin of. I thought it best to focus on one problem at a time.
Edit 2: Additional images to show the box shape and width of boxes. These are from another day where the box shape is particularly pronounced, not all days are that boxy. Here the base frequency of the noise as around 699Hz.