jeffrey.kissel@LIGO.ORG - posted 12:00, Thursday 16 October 2025 - last comment - 14:08, Thursday 16 October 2025(87511)
All QUADs L2 to R0 Damping, PITCH/ROLL (Before vs. After L2 OSEM PD Sat Amp Whitening Upgrade)
J. Kissel This past Tuesday (Oct 14 2025), we upgraded the L2 (PUM) OSEM PD Sat Amps to improve their whitening filter, per ECR E2400330; see LHO:87469. These QUAD L2 PUM OSEMs (in which the flags are on the QUADs main chain PUM, and the LED & PD are on the reaction chain PUM [aka the "PenRe"]) are used in feedback. This "damping" of a "3.3" Hz reaction chain resonance is different ITMs and ETMs (ITM Install = LHO:65728; ETMs Install = LHO:68012 --- commissioning of both = LHO:68200); the ITM has resonances at 3.28 / 3.31 Hz and the ETM has modes at 3.18 Hz / 3.42 Hz. Because the improved sat amps come with better compensation of the whitening filter frequency response, we know that the frequency dependent gain on each channel -- especially around 3.3 Hz may change by as much as ~10%. In the aLOG (and comments collection) I report (a) That we *did* see a change in the open loop gain TFs of these loops, but (after ~2 days of observing since the upgrade) the IFO doesn't seem to mind the changes. (b) We were able to get better coherence on the open loop gain TFs in the 0.1 to 10 Hz region because of the sat amp whitening improvement (c) A comparison between all four QUAD's PITCH and YAW damping, so that we might understand them better and potentially improve them. In this main aLOG I show the comparison between the PITCH and ROLL open loop gain TFs and loop suppression for all four QUADs. Rushed commentary: - The surprising thing to me is that we're getting gain peaking and/or some suppression of other modes below 1 Hz, which I'm confident was not the design intent. - Even though the filters and EPICs gains are the same for EX and EY, ETMX has a lot more open loop gain below 1 Hz. - Also though the ETM loop suppression is about the same on these 3.18 / 3.42 Hz, the ITM loops have pretty different amount of gain peaking. - I'm guessing we *don't* want this much sharp gain peaking just *off* the "3.3" Hz mode.
Images attached to this report
Comments related to this report
Here's the EPICs configurations, gains, and frequency response for L2DAMP filters. In foton, I added a temporary filter that multiplied in the EPICs gain in use (including the sign) for plotting/comparison purposes. As shown, the ITM filter and overall gain design is identical between ITMX and ITMY, and the ETM filter design and overall gain is identical between ETMX and ETMY.
Images attached to this comment
ITMX Before vs. After Sat Amp Swap Open Loop Gain and Loop Suppression plots for PITCH and ROLL.
As advertised -- no substantial change. The overall magnitude of PITCH open loop gain dropped by 18% between 0.5 and 3 Hz, but I suspect this is actually a clean-up of the compensation or frequency dependent calibration rather than a "real" gain drop. Since this all happens below the intended functional frequency, there's little to no change in functional loop suppression, save a little bit *less* gain peaking at 3.3 Hz.
The templates for taking this this before vs. after data lives in
/ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ITMX/SAGR0/Data/
2025-10-14_1830UTC_H1SUSITMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P.xml
2025-10-14_1830UTC_H1SUSITMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R.xml
and to make the comparison in the main aLOG entry, I exported the results to
/ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ITMX/SAGR0/Data/
2025-10-14_1830UTC_H1SUSITMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P_LoopSuppression_tf.txt
2025-10-14_1830UTC_H1SUSITMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P_OpenLoopGain_tf.txt
2025-10-14_1830UTC_H1SUSITMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R_LoopSuppression_tf.txt
2025-10-14_1830UTC_H1SUSITMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R_OpenLoopGain_tf.txt
Images attached to this comment
ITMY Before vs. After Sat Amp Swap Open Loop Gain and Loop Suppression plots for PITCH and ROLL.
Even less change than ITMX -- arguably none at all to within the precision [i.e. coherence] of the measurement.
The templates for taking this this before vs. after data lives in
/ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ITMY/SAGR0/Data/
2025-10-14_1830UTC_H1SUSITMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P.xml
2025-10-14_1830UTC_H1SUSITMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R.xml
and to make the comparison in the main aLOG entry, I exported the results to
/ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ITMY/SAGR0/Data/
2025-10-14_1830UTC_H1SUSITMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P_LoopSuppression_tf.txt
2025-10-14_1830UTC_H1SUSITMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P_OpenLoopGain_tf.txt
2025-10-14_1830UTC_H1SUSITMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R_LoopSuppression_tf.txt
2025-10-14_1830UTC_H1SUSITMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R_OpenLoopGain_tf.txt
Images attached to this comment
ETMX Before vs. After Sat Amp Swap Open Loop Gain and Loop Suppression plots for PITCH and ROLL.
Arguably no change -- and the measurement coherence is better *after* in the 0.5 to 5 Hz band because of the improved whitening on the L2 OSEM PDs.
The templates for taking this this before vs. after data lives in
/ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ETMX/SAGR0/Data/
2025-10-14_1830UTC_H1SUSETMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P.xml
2025-10-14_1830UTC_H1SUSETMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R.xml
and to make the comparison in the main aLOG entry, I exported the results to
/ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ETMX/SAGR0/Data/
2025-10-14_1830UTC_H1SUSETMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P_LoopSuppression_tf.txt
2025-10-14_1830UTC_H1SUSETMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P_OpenLoopGain_tf.txt
2025-10-14_1830UTC_H1SUSETMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R_LoopSuppression_tf.txt
2025-10-14_1830UTC_H1SUSETMX_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R_OpenLoopGain_tf.txt
Images attached to this comment
ETMY Before vs. After Sat Amp Swap Open Loop Gain and Loop Suppression plots for PITCH and ROLL.
Similar to ETMX -- arguably no change at all in open loop gain or loop suppression to within the measurement precision i.e. coherence.
The templates for taking this this before vs. after data lives in
/ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ETMY/SAGR0/Data/
2025-10-14_1830UTC_H1SUSETMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P.xml
2025-10-14_1830UTC_H1SUSETMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R.xml
and to make the comparison in the main aLOG entry, I exported the results to
/ligo/svncommon/SusSVN/sus/trunk/QUAD/H1/ETMY/SAGR0/Data/
2025-10-14_1830UTC_H1SUSETMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P_LoopSuppression_tf.txt
2025-10-14_1830UTC_H1SUSETMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_P_OpenLoopGain_tf.txt
2025-10-14_1830UTC_H1SUSETMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R_LoopSuppression_tf.txt
2025-10-14_1830UTC_H1SUSETMY_R0_WhiteNoise_0p02to50Hz_L2DAMP_OLGTF_R_OpenLoopGain_tf.txt
Images attached to this comment
