Evan, Sheila

In response to alog 17555 we have changed the L1/L3 crossover for ETMY, similar to what was described in LLO alog 14428.  

We adjusted the plant inversion a bit, increasing the Q of the 3.6 Hz zeros from 6 to 12, added a lead filter (2 Hz zero, 8 Hz pole, this is currently split into two filter banks but can be combined next time we loose lock.)  We were able to push to crossover from  about 0.7 Hz as measured in 17625 to about 2.5 Hz, with about 30 degrees of phase margin.  We also added a boost, with zeros at 0.3 Hz with a Q of 1.5.  (the second lead filter and the boost are in the L2 bank, and are not currently turned on by the guardian.  I plan to move them to the L1 bank before adding them.)

The first screenshot shows two measurements of the crossover, one before the plant inversion was tuned and the second lead was added, and the red one is the configuration now.  The second screenshot shows that the rms ESD drive was reduced by a factor of 2,gain peaking around 3 Hz accounts for about half the rms.  LLO was able to get the rms drive lower, but this is good enough to turn off the ESD linearization.  

We left the IFO alone in this configuration (linearization off) for a few minutes at a time: 8:55 UTC April 7 to 9:03 UTC, and again from 9:41 UTC to 9:57 UTC.  We are going to leave the IFO in single bounce for the vent tomorow, so there won't be any long stretches of data to check for glitches until after the vent.