Attached is a plot of the residual (in-lock) DARM spectrum, calibrated in meters according to the calibration factor that is used in OAF: 2.86e-7 m/cnt for DARM_IN1. The residual is about 7e-14 m-rms. The residual is dominated by the 0.45 Hz quad pendulum mode. We should suppress this peak further. There is a resonant gain filter in DARM, but it is a little too low (0.43 Hz vs 0.45), and just is not enough. However, there is also a dip in the SUScomp filter at 0.45 Hz, which reduces the gain at that frequency by 10-12 dB. This compensates the damped pendulum response -- but what is the point of including here? It's not like we're going to try and have a ~1 Hz bandwidth for this loop. Seems like the thing to do is to remove this feature (some complex pole and zero pairs around 0.4-0.6 Hz) from the SUScomp filter, which will increase the gain at 0.45 Hz by 10-12 dB.

Next would be further squashing the noise from 1-5 Hz. The L1 DARM boosts have more gain in this band, so we could adopt that design. First we should make sure we are not injecting noise at these frequencies from residual OpLev feedback, or quad local (BOSEM) damping.