Lee, Kentaro, Sheila, Georgia

I've been tracking an regular glich that appears in the EOMRMSMON of the squeezer TTFSS. This glitch also corresponds to real phase noise seen in the beatnote between the seed and LO light on the homodyne. When the squeezer LO-loop is ON the glitch disappears.

SQZ_TTFSS_EOMRMS_Spikes.png shows the EOMRMSMMON channel while the LO loop is OFF. The glitch occurs near a voltage crossing of the PSL VCO. When the LO loop is enabled, the EOMRMS stays low, but you can see in SQZ_VCO_tunephase.png. That the SQZ VCO is tracking the PSL vco due to the LO loop. This was supposed to be prevented by the reconfiguration of the ALS fiber pickoff in 52381. Georgia helped us track the signals driving the ALS distribution AOM and it appears to be sourced by the PSL VCO OUT2 (split to also enter RF Patch Panel #11 on ISC-R1) rather than being sourced from a static REF 79.4MHz from the distribution source.

Currently, I think the squeezer is effectively still locked to the refcavity frequency, so we cannot easily measure seeding or backscatter, and the CARM-FF channel is needed until we reconfigure the ALS-distribution-AOM source.

I still do not understand the glitching though. It is appearing in the EOM monitor and doesn't correspond to any channels than the VCO frequency (not the PZT offset, slow offset or SQZ frequency counters). This suggests that it is a drifting whistle-like beatnote showing in the FSS PFD moving in and out of the control band and being driven into real phase noise by the servo (at a high enough frequency to be visible in the RMS monitor). While it is fortunate that it lessens when the LO loop is active, it is strange that it goes away only when the SQZ VCO moves with the PSL VCO. This suggests some underlying cause that could be injecting some phase noise through the TTFSS servo at other times, even if it is not so obvious as these glitches. We'll keep tracking it.