Sheila, Louis, Elenna, Daniel

Between maintence day and mode hopping troubles, we did some troubleshooting of ALS/TR CARM handoff today.  

• Based on Daniel’s suggestion we checked the IMC OLG.  We have nearly the same settings as described in 89438, (5dB IN1 gain both times, 6dB fast gain in March, 5dB fast gain now)  but the ugf is 65 kHz now compared to 38kHz in March, so the gain is nearly doubled for reasons that we don’t understand. (instructions for using SR785: 76751 ) We turned the IMC servo IN1 gain to 1 (reduced from 5dB), and Elenna remeasured, the ugf is now 38kHz (link plot).  I edited the IMC_REFL1 gain in lscparams.  The UGF is now 38 kHz. 
• We measured the MCL crossover again, (without the gain reduction we were doing yesterday). It is now closer to the reference, which makes sense with the lower IMC gain. (screenshot)
• We measured the ALS COMM OLG by injecting into the common mode board (plot), this shows a ugf just barely above the MCL crossover. For comparison we looked at figure 5.23 in  Staley Thesis, this OLG was then 5.3kHz.  We changed the H1:LSC-REFL_SERVO_IN2GAIN from 17dB to 27dB, and remeasured (add plot) (this gain increase is not added to the guardian).  
• Elenna and I measured the COMM PLL OLG, it is around 8kHz (photo).  Comparing this to figure 5.15 in Alexa’s thesis, the gain is very low.  We found an alog from Stefan in 2018, 44150 where this was lowered.  It seems like this is probably OK. 

The gain increase didn't allow the TR_CARM handoff to go smoothly.