[Jenne, Jason, Daniel]
Since Daniel has seen so much coherence with REFL port RIN (alog 70611), we thought we'd try increasing the ISS gain in case it's due to our being gain limited at high frequencies.
We didn't have much phase margin at all in the second loop (Measured last week in alog 70684), but we did in the first loop (last measured Feb 2022, alog 61826).
So, we increased the first loop by 6 dB (first attachment), now UGF is 46kHz with 39 deg phase margin. The gain slider is now at 11 dB. We also had to adjust the output offset from 5 to 4.3. We can turn on / off the first loop at input power of 2W just fine.
We increased the second loop by 4 dB (second attachment), now UGF measured at the -20dB line is 37 kHz with 32 deg phase margin. We had tried increasing the second loop by a further 2 dB (third attachment), but we decided that 41 kHz UGF wasn't worth having only 23 deg phase margin. These measurements were both taken with the input power at our current nominal 60W, but PRM and both ITMs misaligned. For the current lock (as well as for all these measurements) we just changed the gain after the ISS second loop was closed and DC coupled.
Still to do: At low power / DOWN in IMC_LOCK, the ISS second loop gain slider should be -2 dB (that's where all the offsets were most recently tuned). Then, in CLOSE_ISS, once the ISS is closed, the gain slider should be increased to +2 dB.
Also to do - make sure we know / remember how to get the SR785 connected to the wifi so we can download the data to the control room workstations.
The IMC_LOCK guardian's DOWN state already looks at and uses the lscparams.ISS_acquisition_gain setting of -2.
I have added some logic to the OPEN_ISS state of IMC_LOCK to also reset the second loop gain slider to -2 (to prepare for re-closing it).
I have modified the logic in CLOSE_ISS to increase the second loop gain. The final gain value for H1:PSL-ISS_SECONDLOOP_GAIN when we get to Observing should be +2.
Since we're in Observing, I have not loaded the IMC_LOCK guardian. I'll leave a sticky not for the operator to do that when we're next out of Observe.
If, when we get to Observing, the H1:PSL-ISS_SECONDLOOP_GAIN is not at +2, it is okay (for tonight, until I debug / fix why guardian didn't do it right) to change the slider in units of 1 (which is the default slider click value) until it is +2.
If there are any troubles with this this evening, please give me a call.
