Rana, Evan
Yesterday, Rana made a low-pass filter in order to remove EX ESD saturations when ALS DIFF is used as a DARM sensor. However, today we continued to see some locklosses around the time that DARM is handed off from ALS DIFF to AS45Q.
Therefore, we looked at FM8 and FM10 in the DARM filter bank, which are low-pass filters used with DIFF only. We found that we could relax the amount of rolloff in order to win some phase at 10 Hz and below (see attached foton plot). There is not nearly as much suppression above 500 Hz, but it doesn't really matter since most of the rms drive to the EX ESD is accumulated below 100 Hz. The attached spectrum shows the EX UIM and ESD drives both before (blue) and after (red) retuning. There is no increase in the the rms drive to the ESD (part of this is also due to Rana's retuning of the UIM/ESD crossover).
Finally, the attached transfer function shows the ALS DIFF OLTF before (blue) and after (red) retuning. The "before" OLTF was taken yesterday and has 4 dB less gain than the red transfer function (because we added 4 dB of missing gain in the LSC-DARM violin bandstop filters).
Additionally, we switched FM1 and FM7 in LSC-DARM. FM7 was a boost that would cause the OMC transmission to flutter when first engaged. FM1 was the suspension compensation, and since it is always on there is no point in having it in such prime real estate. So we've switched them and updated the appropriate settings in the ISC_LOCK and ALS_DIFF guardians.
We were also getting some transitions during this transition due to some unsuppressed 0.2 Hz microseism in the ESD drive. Ideally, all of the low frequency force is taken care of by the upper stages, but this was not the case here:
- L1/L3 crossover near 1 Hz with 30 deg phase margin.
- Two integrators in L1. 0.1:0 and 0.3:0. Unfortunately, the 0.3 Hz integrator had a 10 second ramp. This is not allowed when turning on integrators. Use zero history, immediate turn on.
- These two boosts didn't do much to reduce the microseism, but did have a lot of gain below 0.1 Hz, where we didn't need it so much.
The new filters have a no-ramp integrator and a RG at 0.2 Hz which is Q-matched to the microseism bump. The L3 signal has much less 0.2 Hz signal and our ESD saturations are somewhat reduced. This seems to work fine on both ETMx and ETMy all through the low noise state.
Guardian scripts updated to reflect the new filters. Now the ETMX / ETMY switcheroo for flipping to low noise ESD state can be done by gain ramping alone -- no filter switching to match actuators.