While the laser was down this morning, I took a quick measurement of the ETMX pit oplev damping loop. It has about 5 dB of damping in a narrow range around 0.46Hz, and has zero phase and -10dB of gain at 1.5 Hz with no light in the arm.
Because the noise from these loops is limiting DARM at 10 Hz (alog 34735), I made a slightly more aggressive cut off (LP8 is intended as a replacement for LP10) which can be tried next time we have a chance. This is the reason there are unloaded changes in the filter files for all 4 quads right now.
I used this measurement to update the quad model with gain of the electronics and software between the PUM pitch actuators and the optical lever. See CSWG log 11209.
The gain was found simply by comparing the measured loop gain to the modeled loop gain with the installed filter, and scaling the model until matched the measurement. The gain applied to the model to make them match is 1/1500.
More measurements like this for the other suspensions will be useful for telling us if this gain factor is the same for all suspensions, or at least the same across sites.
Thanks for including the measured data, that made the analysis a lot easier.