The 3.4 Hz peak is a dominat feature in many ASC signals and is dominating the RMS of the PRCL loop.

I ran a script to look at all DQ'ed channels (>6000) and look for a peak at 3.45 Hz. The criterion to determine if a signal has a peak is twofold. EIther 1) the spectrum at 3.45 Hz is at least 5 times larger than the value at 3.45 Hz +- 0.3 Hz or 2) the signal has coherence larger than 0.5 with LSC-PRCL_IN1_DQ, which is dominated by the 3.4 Hz peak

The attached file lists all signals where a peak was found according to the criteria above.

Some interesting observations:

• we find the usual ASC CHARD and CSOFT loops, and REFL WFS or QPD
• we see it in the ETM and ITM drives, not suprisignly since the peak is in ASC and DARM
• we see it in the ETM R0 damping, again no suprise since it's getting there through DARM
• there is a peak at 3.4 Hz in MC1 / MC2, but it is not coherent, so maybe it's just a similar suspension resonance

The most interesting observation is that the only other not expected place where the peaks is visible is in PRM suspension signals, both M3 and M1 drives and witness sensors