After the CAL team changed the OMC digital phase in 82413, the SQZ at the start of the lock went through an excursion much larger than usual, all the way to 5dB of ASQZ before settling around -3.5dB of SQZ, plot. We often have a SQZ excursion at the start of the lock but it rarely gets above 0dB. We are unsure if it makes sense that the OMC change would effect the SQZ like this or we were just unlucky.
I paused SQZ_MANGER and took SQZ_ANG_ADF to DOWN. I think tuned H1:SQZ-CLF_REFL_RF6_PHASE_PHASEDEG for the best SQZ and adjusted H1:SQZ-ADF_OMC_TRANS_PHASE to get H1:SQZ-ADF_OMC_TRANS_SQZ_ANG to zero, effectively setting the setpoint of the servo. Adjusted from -138 to -128 sdf. We may need to change this again or revert before going back to observing if the SQZ isn't good with a more thermalized IFO.
Also used this time to reduce the HAM7 rejected SHG power. Enabling (not moving) the picos unlocked SQZ which is surprising.
The supposition of *how* the change in digital anti-aliasing might have impacted the ADF / SQZ loop is as follows: - The actuation / excitation of the ADF / SQZ tuning system is a single line driven into the ADF's VCXO at 322.0 Hz. - That ADF field beats against the IFO field, which is influenced by the DARM control loop. - The new digital AA filter causes -4.5 deg phase loss (and an increase in amplitude of 0.1%) at 322.0 Hz, and changes the DARM open loop gain in that same way. - Thus the 322 Hz DEMOD of the IFO vs. SQZ beatnote field that's picked off at 3.125 MHz needs a phase adjustment. If that (I don't really know what I'm talking about) model of what's happening is happening, I would have expected the ADF SQZ DEMOD phase to need exactly 4.5 [deg] of phase change, much like the OMC LSC SERVO's DEMOD phase needed changing at it's modulation frequency (see LHO:82413). Camilla's main log here suggests a (-138) - (-128) = 10 [deg] phase shift. I very much welcome other mechanisms/models of what has happened if this change in SQZ behavior is a result of the OMC DCPD digital filter change.
Once we'd been locked for 4 hours, I again tweaked the SQZ angle by finding the angle ether side of good SQZ and going to the middle the adjusting ADF phase, diff attached, so that the total phase change since the OMC changes is the 4deg that Jeff predicted.
SQZ at 1kHz+ still isn't as good as I remember so we touched the OPO temp while in observing as in 80461, no change needed.
Reverted back to -128deg ready for reverting CAL team to revert thier changes.