Following up on the discovery that the 3.4 Hz instability is due to the PRM offload, I redesigned the M1/M3 offload for PRM. The old offload has a crossover at 0.5 Hz, which I believe was too large. I targeted a cross-over at 0.1 Hz. This mean that we the new crossover, the PRM equivalent actuator will have less gain at low frequency, but there's plenty of room there.

The new design is shown in the first attached figure, showing the target crossover of 100 mHz. The second plot compares a model of the new crossover design with the old crossover. The model of the old situation reproduces very well the measured PRM actuation. The thrid plot compare sthe old filter with the new filter (without including the band stops).

I implemented the new crossover in the H1:SUS-PRM_M1_LOCK_L filter bank FM7. This is to replace FM4+FM9 (I don't think we need the 10 Hz elliptic low pass).

We tested the new crossover during lock. It works fine and as expected:

• the PRCL 3.4 Hz peak is disappeared
• the PRCL error signal has less suppression below 0.5 Hz (we can fix it with a boost in the PRCL loop if desired)
• the PRCL control signal is larger below 0.5 Hz to compensate for the reduced actuation gain
• with the IFO down, the measured PRM actuation shows the effect of the new offloading.

We'll automate this new configuration.

I suspect that SRM needs a similar treatment, to reduce the 3.5 Hz peak, and maybe also to tackle that huge 1 Hz peak.