craig.cahillane@LIGO.ORG - posted 21:17, Monday 08 October 2018 - last comment - 15:51, Tuesday 09 October 2018(44422)
Change power-up gain slider from LSC IN1 to IMC FAST to reduce glitch size
Craig, Sheila We have glitches in CARM associated with the discrete stepping of our analog gain sliders while we power up. These are unavoidable, since we must compensate for the increase in optical gain by decreasing the analog gain by an equal amount, and we cannot use digital power normalization like for other LSC. In order to reduce the size of the glitches, we have moved the stepping stage downstream, from LSC-REFL_SERVO_IN1GAIN to IMC-REFL_SERVO_FASTGAIN and LSC-MCL_GAIN. Our current gain settings are not optimal, but the glitches are lower than before: Sheila will post a follow-up confirming this. We did this by changing what sliders theISC_library.pyfunctionIMC_power_adjust_func()adjusts. I also adjusted IMC_LOCK so the IMC locks with IMC IN1 at 25 dB now (this is increased by 3 dB from usual, another alog is in the works). In any case, the guardian works now for both locking the IMC and powering up. There are a total of 7 relevant gain sliders in the analog CARM path, along with two compensation filters (from the awiki pole/zero -- IMC: 40 Hz / 17 kHz, LSC: 40 Hz / 4 kHz ) and one boost (1 kHz / 20 kHz). Assuming these filters have a gain of 1 at AC, then at DC we get gains of: IMC Comp DC Gain = 52.56 dB IMC Boost Gain = 26.0 dB LSC Comp DC Gain = 40 dBCARM Gain Slider Values at 2 watts (at the end of Guardian states RESONANCE, CARM_TO_ANALOG, and ADJUST_POWER [at 10 watts]) ISC_LOCK State SUM NODE A IN2 LSC IN1 LSC MCL GAIN LSC FAST IMC IN1 IMC IN2 IMC FAST | CARM FAST OVERALL CARM SLOW OVERALL ------------------------------------------------------------------------------------------------------------------------------------------------------- RESONANCE 2 dB -16 dB 240 (47.6 dB) 7 dB 25 dB -6 dB 0 dB | -13 dB 33.6 dB CARM_TO_ANALOG -12 dB 11 dB 53.8 (34.6 dB) 0 dB 25 dB -12 dB 0 dB | -13 dB 33.6 dB ADJUST_POWER [10 W] -12 dB 11 dB 10.2 (20.2 dB) 0 dB 25 dB -12 dB -14 dB | -27 dB 19.2 dBCARM FAST OVERALL = SUM NODE A IN2 + LSC IN1 + LSC FAST + IMC IN2 + IMC FASTCARM SLOW OVERALL = SUM NODE A IN2 + LSC IN1 + LSC MCL GAINNeither "OVERALL" gain includes the gain of the filters.
The two attached lockloss plots show an attempt at powering up earlier today where every other gain step in the common mode board made a large glitch in the CHARD loops, and one of these caused a lockloss. The second plot shows the same hard asc channels during a power up after this change, there is no glitch. This was causing us to occasionally loose lock while trying to power up, but it looks like this fixed the problem.
Gain slider combinations of -12dB followed by +11dB look like unity gain, but effectively loose 16dB of SNR.