We're having some mysterious lock losses as we move from large CARM offset to less large offset. With the DRMI locked, the ALS starts bringing the arms in and the ALS / IMC lose lock.

Suspecting the recent FSS tunings, we looked at the FSS screen. The FAST gain was down at +5 dB. Also, the EOM Drive readback was up at +3V. The attached plot shows the EOM readback (PC_MON) as the FAST gain is tuned.

I have left it at 22.2 dB, where the EOM drive is minimized. IN mid-April, there are a series of entries from Rick and Peter where the loop is tuned up, but the fast gain is turned down incrementally from 21 to 5 dB. Why so??

Also, it seems like we should aim for a ~250 kHz UGF for the FSS to avoid the peaking at 1.8 MHz where the notch is not getting all of the EOM resonance.