Following Arnaud's work on the SRM M2 stage (alog 13558), I made a couple of modifications on the L2P filters that he implemented in this past August. It is now engaged, but I did not get a chance to evaluate the performance of it yet.

(Merging the two filters into one)

First, as he mentioned in the alog, a filter in FM2 that he installed was numerically unstable. It seemed that the FM2 filter had been meant for the inverse of the P2P response which grew up at high frequencies as f⁴. Therefore the filter was accidentally designed to return incredibly high values at the Nyquist frequency. In order to fix this issue, I decided to combine this filter with the other one, i.e. L2P such that the high frequency response becomes flat. Even though this idea of combining two or multiple aggressive filters into a moderate one is generally good, foton did not allow me to do this due to too many numbers of poles and zeros this time. I could have split the filters again into two moderate ones, but instead I decided to drop off some pairs of poles and zeros which are so similar to each other that dropping them did not change the overall response so much. Also, I took out a 2nd order zero at 20-ish Hz in order to let the L2P decoupling filter roll off at high frequencies because I was worried about saturation in DACs especially for the 3f locking. The attached is the transfer function of before and after the modification. Of course, now the filter is not unstable any more.

As seen in the attached, the modified version is accurate until 3 Hz or so and it completely deviates at high frequencies. I am hoping that this is OK because the M2-M3 cross-over is now as low as 4.5 Hz and therefore the discrepancy at high frequencies does not matter. I did not get a chance to see the performance of this filter yet.