Sheila and I examined the new ASC subbudget today (see 72245) and we did not like the shape of CHARD Y above 20 Hz. This new shape is a result of Gabriele's adjustment to CHARD Y to give more low frequency suppression (71866, 71927).

Since this feature appears above 20 Hz and a recent measurement of CHARD Y shows that the 3.3 Hz UGF has a 44 deg phase margin, I figure we can design some 20 Hz cutoff filter that removes this feature with minimal effect on the loop stability.

Attached is a screenshot of the new design (in red) compared with the old design (in gold). I first turned off "JLP200", which is a very high frequency low pass that I think is unnecessary. Then I constructed a low Q elliptical low pass at 20 Hz with 10 dB of suppression. The result is 6 deg less of phase margin at 3.3 Hz. I minimized any passband ripple so that the gain increase around 10 Hz is only 1 dB.

I updated the guardian. It will no longer use FM10 (200 Hz low pass), and will turn on FM8 (20 Hz low pass) along with Gabriele's filters. No gain change is required.

This filter has been successfully tested.