Sam and I have been investigating a ~40 Hz peak in the HAM ISIs and PEM accelerometers, first noticed by Camilla and Sheila in alogs 79001 and 78879. Jim also investigated and suggested the peak may be caused by gain peaking from the isolation loops (alog 79145). The peak is strongest in HAMs 2, 6, and the LVEAFLOOR YCRYO Z accelerometer. We tracked the first appearance of the peak to May 20th. We've found there are actually two peaks roughly 0.2 Hz apart, which drift a bit in frequency from ~39.45 Hz (20/05/24 16:00 UTC) to ~40.2 Hz (20/05/24 22:00 UTC), measured from the point between the two peaks (see 1st attachment). The peaks consistently remain about 0.2 Hz apart in frequency but they do not always have the same magnitude ratio, even between channels measured at the same time (see 2nd attachment). Overall, the peaks vary in magnitude, occasionally disappearing altogether, but we have yet to find a time when one peak disappears and the other doesn't (see 3rd & 4th attachments for peaks disappearing and reappearing, respectively). We'll continue looking into surrounding peaks and the coherence with DARM.
The extra drive on HAM2 is likely from a very thin phase margin on the HAM2 X feedback loop. I measured this yester day and found that it had something like 5 deg of phase margin with a 32-ish hz ugf. I backed this off quite a bit, phase margin is now around 20deg and the ugf is more like 27 hz. Might be worth doing some quick checks on these for all the ISIs. Measurements of the X ugf and phase margin are attached. Red is the modified loop I installed yesterday, blue is the loop with the filters that have been running up to that point, not sure when the last measruements were run, it's been a while.
