The current configuration of the BSC-ISI's does not use any high gain ST2 RX/RY isolation loops because we dont have a blend filter design that rolls off fast enough at low frequency to avoid injecting GS-13 noise back into the X/Y dofs. However, above 1 hz the suspension point motion is dominated by ST2 RX/RY motion. One way we can improve this is by using relatively low gain AC coupled loops on ST2 RX/RY. With RichM's help I've tested this some on a couple ISI's and it seems to work, but there are trade-offs. First attached plot shows the improvement in ETMX longitudinal motion I've managed to get. Green and brown are the ground for both measurements, pink traces are RY loops on, light blues is off. In the 2-10 hz band I can get about a factor of about 2.5 reduction (I've seen up to ~5) , but at about 1 hz the motion is worse by about a factor of 2. Not spectacular, and I'm not sure that the improvement at a few hz is worth the loss at 1 hz. I've looked at the oplev (second plot, pink is on, light blues is off) and the oplev doesn't seem to see a substation difference at 1 hz.

One reason to do this would be to speed recovery after a lock loss. The ISI RX/RY loops get rung up by the impulse from the suspension, and it seems plausible that the ISI would settle down quicker is the RX/RY loops had more gain.

The designs for the ETMs RX/RY loops are included in the attached pdf. The lugfs are around 1 hz, uugfs are around 10-20 hz, phase margins are around 40-50 degrees. Higher gain loops didn't seem to provide much more isolation, but made the motion around 1hz worse.