Last night, at 2W, we adjusted gains on the common mode board to achieve a carm UGF of about 15 kHz with 56 deg of phase margin, which we thought was close enough to our target (92306). I searched back in the alog and found a time when Sheila and Matt measured the CARM OLG at 2 W during O4 (May 2025), 84644. Matt reported a ugf of 11 kHz with 41 deg of phase margin.
Below is a table comparison of the common mode board gains between last night and this time in May 2025.
| Date | LSC REFL_SUM_A_IN2GAIN | LSC REFL_SERVO_IN1GAIN | LSC REFL_SERVO_FASTGAIN | IMC REFL_SERVO_IN2GAIN | IMC REFL_SERVO_IN1GAIN | IMC REFL_SERVO_FASTGAIN | LSC REFL_SERVO_COMCOMP | Overall CMB gain | UGF/Phase Margin |
| May 2025 | 12 | 0 | 16 | -22 | 3 | 5 | ON | 6 | 11 kHz, 41 deg |
| Now | 3 | 0 | 7 | 0 | 2 | 5 | OFF | 10 | 15 kHz, 56 deg |
This means that we have overall increased the common mode board gain by 4 dB, and maybe held on to more phase by not engaging the common comp boost. Perhaps we could drop the gain by 4 dB and have more room to turn on the comcomp.
Last night, we were worried that we might have lost some optical gain in CARM. However, this comparison indicates that actually we probably are the same as before.