Giacomo, Lisa, Matt

While yesterday's "MC2-M2 as a low frequency offload path for M3" approach worked for signals well below 100mHz, it did little to save M3 from saturation due to signals around the microseism.  It was also not sufficient for ISI testing.

Through more measurements we found that the MC2 M2-M3 cross-over had a small region of stablity between 10 and 20Hz with only the old 100:1 filter engaged.  The maximum phase margin in this region was about 10dg, which really doesn't sound like enough to be reliable, so I started another filter design cycle.  The result is shown in the attached plot: unconditionally stable up to 20Hz, but not really optimized in terms of gain (we could have a lot more if we are willing to invert the plant features).  This was sufficient to keep the M3 drive RMS below 10k counts (at LOCK filter output, so 2.5k at the DAC), which is about a factor of 10 better than before and about a factor of 40 from saturation.