I've got this unusual spectrogram and I am really curious what these circles could mean. Has anyone ever seen anything like this? What could be the source of the signal?
In Understanding FFT Overlap Processing, p. 10, it is suggested that uniform of constant windows, for certain hop patterns, with repeated pulses, could generate such artifacts.
I'm not able to comment yet but could it be some kind of FM Synthesis? Like Olli Niemitalo wrote, it looks like Moiré pattern. I found a spectrogram which looks a little bit similar: