Stefan, Evan

Summary

Adding an offset to the error point of the ASB36I→SRM yaw loop reduces the SRCL→DARM coupling by 10 dB. It seemingly has the added benefit of fixing the slow drift in POP90.

Details

SRM yaw loop offsets

After several days of trying to do SRCL feedforward on a moving target, we decided to try attacking the SRCL→DARM coupling itself.

We started by applying offsets to the dETM loops, since Gabriele's simulation seemed to indicate that this could cause an increase in the high-frequency coupling. We applied offsets of several hundred counts (both signs) to dETM pitch and yaw, and then injected broadband noise in SRCL. We saw a reshaping of the noise (see attachments), but no overall improvement.

Then we moved on to offsets in the SRM WFS yaw loop. Here we saw a more promising improvement with a positive offset at the error point (see attachment). In the end we found that an offset of 2100 ct works best in terms of reducing the coupling at high frequencies. This also seems to coincide with minimal power in POP90.

POP90 stabilization

With this yaw offset engaged, it seems that the drift in POP90 mostly went away when the SRM yaw offset was engaged around 09:14:00 (attachment). There is still some drifting, but the timescale is much longer. Perhaps we are closer to optimal coupling of the 45 MHz sidebands into the SRC. This particular lock lasted for about 2.5 hours (it broke for unrelated reasons) and showed no sign of the POP90/ASB36I instability that we've seen this past week.

DARM offset and calibration

For some additional reduction in SRCL coupling, we tried locking with a smaller DARM offset (14 pm, not 7 pm as I said before) and then updated the calibration (FM10 in the sensing inversion of CAL-CS). However, I have reverted these changes. This means that in general you should not believe the recorded inspiral range for most of the night.

Guardian changes, other configuration changes

If LSC_FF is requested, the Guardian will proceed from DC_READOUT to LOWNOISE_ESD_ETMY to LSC_FF. If desired, one can make a pit stop at COIL_DRIVERS before going through LSC_FF.

I had wanted to add some Guardian code that steps down the TRX/TRY QPD whitening gains by 6 dB before the ASC comes on (and then adjusts digital gains accordingly), since the QPDs will saturate when powering up beyond 10 W. But it seems this makes the ITM loops unstable somehow. So I've left some commented-out code in DRMI_ON_POP and in the DOWN state.

In the ITM M0 lock filters, I moved the integrators from FM2 to FM4 and stuck +20 dB gains in FM2. Now we have some room to adjust the offloading speed.