I modified filiters for LSC_MICH and LSC_PRCL again to cope with power recycling gain fluctuation.
After some more alignment, power recycling gain increased (but still ~3.7). It fluctuates more than a factor of 2, and I began to see glitches again. So I needed more gain margin, as Koji pointed out.
I played around with filters, but I couldn't remove all the glitches. Gain margin now look OK in principle.
It looks like PRM motion is related. Since PRM doesn't have oplev now, I will see PRM oplev tomorrow.
New openloop transfer function:
LSC_MICH
UGF ~100 Hz, phase margin ~ 50 deg
no phase flip in less than factor of ~5 gain change
550 usec delay
LSC_PRCL
UGF ~100 Hz, phase margin ~ 70 deg (phase bump at UGF)
no phase flip in less than factor of ~5 gain change
550 usec delay
 
Power recylcing gain:
It is now ~3.7. It fluctuates pretty much. See time series data below when I locked PRMI. MICH and PRCL locks at the same time.
G = (1600-244)/(266-244)*0.06 = 3.7

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