The PMC error signals have some weird broadened oscillations in them:
Above, pink is North and green is South PMC Error Signal taken from Mixer Out port.
- I took the spectrum using SR785 of the Mixer Out port of the two PMC loops.
- Next, I measured the cross-spectrum density of the two error signals.
- I wasn't sure though if the peak in the cross-spectrum is just because higher magnitude signal on both error signals, so I divided it by the magnitude of measured FFT of the two signals to get a unitless cross spectrum in the third plot.
- I'm not sure though how to interpret the cross-spectrum in either case.
- The oscillations could be a manifestation of some common-mode noise on the table (maybe some mechanical oscillation).
- Or there could be a systematic bad UGF around 480 Hz.
- In any case, can we actually relate the bump seen in beatnote noise near 400 Hz to these oscillations?