J. Kissel

Here're some spectra that are the same as what Vincent had posted, but focused on the LF and RT OSEMs, and calibrated into [um/rtHz]. I show four configurations (each config has the ISI/HPI in its best performance state):
(1) As we discovered the problem, with all SUS damping loops closed (shows giant feature)
(2) With only V loop turned off (still shows giant feature)
(3) With V and R loops turned off (does NOT show giant feature)
(4) With V and R loops turned off, and with a broad-band 1000 [ct] excitation sent out of the RT COILOUTF bank (does NOT show giant feature).

Note, I also measured 
(5) With only R loop turned off (still showed giant feature), 
(6) With V and R loops turned off, and with a broad-band 1000 [ct] excitation sent out of LF COILOUTF bank (still did NOT show giant feature), 
(6) With V and R loops turned off, and with a broad-band 100 [ct] excitation sent out of LF COILOUTF bank (still did NOT show giant feature), 
(6) With V and R loops turned off, and with a broad-band 10 [ct] excitation sent out of LF COILOUTF bank (still did NOT show giant feature), 
but they were redundant with the above information, and would have over-crowded an already crowded plot.

First page of the attachment shows configurations (1)-(4). Second page shows the ASD of the 1000 [ct] excitation, meant to look roughly like a damping loop signal. 

Given that we only see this feature with the damping loops that involve LF and RT ON, and we don't see it by turning the damping loops OFF and driving an artificial, non-OSEM-sensor, signal AND we know it's not a loop instability, I think we can narrow down the source to something in the analog sensing chain that is oscillating.

We should replace the M0LFRT, R0LFRT satamp tomorrow, and see if this feature goes away. I blame the satamp, because the satamps have plagued use wih a terrible history of oscilliations.

I bet $1.00 USD.

If anyone is betting against, please please please, shoot me more ideas on how to narrow down the source further.