A few filter changes for the MICH / BS tonight:

1. Moved the Butterfly stop band filter from the M2 COIL filter banks into the M2 LOCK, so that we only have to use 1 filter. I think there's no danger of the  mode getting rung up by the OL loops.
2.  Removed the 300 Hz notch for the BS Violin and replaced it with a 10 Hz wide 80 dB stopband filter. There is no appreciable phase lag at the MICH UGF of 10 Hz. This catches not only the 299,5 Hz BS violin mode (which was ringing up and saturating tonight), but also a couple peaks at 302 and 303 Hz which were dominating the BS DAC range when the DRMI was locked on the 1f signals.
3. The MICH loop phase margin is only ~25 deg, due to the low freq boosts and the elliptic low pass at 40 Hz. I have made a RLP65 to replace the ELP40. This has approximately the same phase lag, but better attenuation around 100 Hz. During the 3f lock the RMS at the DAC was ~33000 counts (out of 131000). This was ~8-10x more than during the 1f locks and mostly comes from the low SNR of the BBPD used to acquire the REFL 3F signals.

My guess from the noise that we hear in the speakers is that the BS coil driver DAC channels has been lightly saturating whenever we reduce the CARM offset. Its pretty close to the rails before we start and the signals are only noisier when we're on the side of the CARM fringe.

The first attachment compares the ELP40 with the RLP65.

The second attachment compares the M2 LOCK signal with 1f lock (PURPLE), 3f lock (BROWN), and 3f lock with the new filter (BLUE). The new filter gives us a 3-4x reduction in the RMS. I think this should give us a more smooth 3f lock and a smoother CARM reduction.