Following up on Kiwamu's LSC model change, and in preparation for closing the ISS 3rd loop, I made a loop filter.

The idea is to make an ISS loop which sends the arm power signals (TRX and TRY) to the ISS 1st loop error point to prevent radiation pressure driven instabilities.  This loop only need to have gain around 0.5Hz (near the suspention pitch mode), so the open loop TF can be a peak around 0.5Hz with upper and lower UGFs a factor of 5 or so away (see figure 1).  The assumed plant transfer function is the interferometer coupled cavity pole aroud 0.7Hz, and the (yet to be realized) anti-aliasing filter (SR560 AC-coupled and with a pole at 10Hz, see figure 2).  The control CDS filter is shown in figure 3.