Probably this is a readback problem and thus not that important, but there's a bump at around 10kHz in dark noise of IOP channels of LSC-REFL_A_RF9_I_IN1 and Q_IN1, coherence is large between these two channels, and both the shape of the bump and the coherence are dependent on whitening gain.
In the attached, TP_CH28 and 29 correspond to RF9_Q_IN1 and I_IN1 respectively.
Red/blue is with 12dB of whitening gain, green/brown 45dB, pink/cyan 0dB, all with two stages of whitening filters. I don't understand this.
It seems to be a small effect and these digital signals are not used for control, but we need to check if this is present in channels that are actually used for control and on the same whitening board (H1:IOP-LSC0_MADC1_TP24=POP_A_RF9_Q, TP25=I, TP26=POP_A_RF45_Q, TP27=I, TP30=REFL_A_RF45_Q, TP31=I).
When MC unlocked for whatever reason, I misaligned MC2 and PRM and measured the IOP channels that are on the same whitening chassis as LSC-REFL_A_RF9 (1st attachment), and nothing looks too crazy except REFL_A_RF9 channels (28 and 29). POP_A_RF9_I and Q (24 and 25) have a bit of a bump at around 4.2kHz or so, which shows up in POP_A_RF9_Q in-lock (2nd attachment) but it looks harmless there.
For IOP-frontend channel name relation see top left of the 1st attached.
I'll look at the analog signal of REFL_A_RF9 again on the floor, but if I find nothing I won't investigate further.
