J. Kissel

I'm reviewing the state of suspensions' collection of damping loop open loop gain / loop suppression / closed loop gain TF measurements, because we'd like to use the loop suppression to accurately estimate the OSEM sensor noise left over after regressing out the ISI input displacement (much like Valera has done for PR3 in LHO:84277, but he didn't invert the loop suppression). We're doing this so we have "the best we can" metric for when we start to implement ECR E2400330 and improve the whitening within the sat amps to improve the sensor noise between 0.05 and 10 Hz (see CSWG:11249 for a model of the noise improvement). 

I've found that SRM's collection (taken back in 2022 during the upgrade to Level 2.0 filters; LHO:65310 ) had not been updated after the Aug 2023 commissioning efforts that reduced the gain in all DOFs by half, from -1.0 to -0.5 (LHO:72106).

So, I've gathered a new set of open loop gain TFs this morning (from which the loop suppression and closed loop gain TFs come "for free" if you gather the right channels).

The data set has been committed to 
    /ligo/svncommon/SusSVN/sus/trunk/HSTS/H1/SRM/SAGM1/Data/
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_L_0p01to100Hz_OpenLoopGainTF.xml
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_P_0p01to100Hz_OpenLoopGainTF.xml
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_R_0p01to100Hz_OpenLoopGainTF.xml
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_T_0p01to100Hz_OpenLoopGainTF.xml
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_V_0p01to100Hz_OpenLoopGainTF.xml
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_Y_0p01to100Hz_OpenLoopGainTF.xml

The references in these templates were updated before each new measurement was taken, so it's a direct comparison between "before" vs. "after" gain reduction on the level 2.0 filters.
It's not terribly surprising that the magnitude of the the open loop gain dropped, but due to the complexity of the unity gain crossings, phase evolution beneath it and the confirmed MIMO nature of the loops, it's never obvious what effect this has on the loop suppression or closed loop gain in the 0.1 to 10 Hz region, thus the remeasure.
For comparison of the loop suppressed plant, P / (1 + G), before vs. after the gain reduction -- see LHO:85277 (note: technically that data also has a switch of basis for transverse, but the consequence on the loop is nil; per that aLOG). 

And for the ease of use, I've exported the loop suppression (A: EXC, B: IN2, to show IN2/EXC = 1 / (1 + G) ), and committed to 
    /ligo/svncommon/SusSVN/sus/trunk/HSTS/H1/SRM/SAGM1/Data/
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_L_0p01to100Hz_OLGTF_LoopSuppression_tf.txt
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_P_0p01to100Hz_OLGTF_LoopSuppression_tf.txt
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_R_0p01to100Hz_OLGTF_LoopSuppression_tf.txt
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_T_0p01to100Hz_OLGTF_LoopSuppression_tf.txt
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_V_0p01to100Hz_OLGTF_LoopSuppression_tf.txt
        2025-06-24_1704_H1SUSSRM_M1_WhiteNoise_Y_0p01to100Hz_OLGTF_LoopSuppression_tf.txt