There is a broad noise peak at 48 Hz and its harmonic (96 Hz) that comes and goes in DARM. We do not know the cause, and I am posting this in hopes that DetChar can look into it. The attached pdf shows the DARM spectrum over ~10 minutes, but the spectrogram shows that the broad spectral feature is actually a narrower line that is wandering in frequency by several Hertz, at a time scale of 5-10 seconds. We know that these peaks can appear with or without the squeezer injection.
Hi Peter, this is what we once found about 48Hz. We can check tomorrow if this is still the situation.
The tank motion mechanism reported back then by Jeff doesn’t seem to be related to the current 48 Hz and 96Hz frequency-modulated lines. The frequency track of the fast moving ~48 Hz line resembles alignment signals such as DHARD Y but we haven’t exhaustively checked other signals. We also don’t have a good clue for the origin of the line and Bruco didn’t find coherence. Fig 1 shows a strong zoom of the line, it oscillates in frequency with a roughly 10s period and 4 HzPkPk amplitude. Fig 2 shows the same with DHARD Y overlayed. Fig 3 shows Peter’s time from above with DHARD Y.
I am wondering whether we can explain this as a whistle with untypically small frequency motion. Do we happen to have VCO and RF frequencies ~48Hz apart?