Georgia, Adam, Keita, Marie, Sheila, Jenne

This afternoon we found that the SRC2P setting that made DHARD P stable (probably just barely) last night didn't work today.  The first attached screenshot is a measurement of the DHARD P loop with higher resolution than last night, showing that even with the SRC2P gain reduced by a factor of 15 it still has an impact on the DHARD P loop at 1 Hz.  (first screenshot)

Qualitatively, since the AS_C QPD is sensitive to DHARD when there is a DARM offset, because of the interference between the carrier 00 and the carrier 01 mode, the SRC2P closed loop response has an impact on the DHARD P loop (because the ASQ sensor is also sensitive to SR2 motion because of the DARM offset). 

We then spent some time measuring the SRC2 P loop, both in DRMI and in full lock.  With the settings we used last night (green traces in 2nd and 3rd attachment), with a gain of 1 and a compensation filter for the HSTS engaged, the closed loop response of the SRC2P loop has a wiggle in phase at 1 Hz, although the open loop gain is around -20dB at 1 Hz.  We changed the loop to have a ugf around 0.04 Hz, removing the HSTS compensation and adding a low pass at 0.7 Hz to reduce the phase wiggle in the open loop response at 1 Hz.  

We have not put these new settings in the guardian, but we would like to try to engage the SRC2 P loop with the gain of 10, and FM1,3,4,5,7 to see what the impact of this on DHARD is.  

This may not stabilize DHARD, but at least it will remove one cross coupling that is causing the loop shape to change around the zero phase margin frequency.