This morning I measured and fit again the SRCL feed-forward. Measurements attached.

The new feed-forward filter is effective in reducing the SRCL coupling (indeed the old feed-forward was amplifying the SRCL noise in DARM between 10 and 20 Hz) (figure 2)

To improve the feed-forward between 10 and 30 Hz I designed a new AC coupling, with less phase rotation at 10 Hz, and with a band-stop between 0.8 and 1.2 Hz. Using this AC coupling I could fit a good filter down to 10 Hz (figure 1).

The band-stop reduced the feed-forward output at around 1 Hz well below the previous version (figure 4).

Unfortunately, the new feed-forward triggered again an instability that was slowly growing in many ASC signals. This time the instability is not at 1.1 Hz (the BS pitch mode) but at 1.2 Hz, which is exactly the corner frequency of the band-stop. So it seems this instability is not due to a too large control signal exciting a mode, but due to a spurious unstable loop. This makes it much harder to understand whether a given feed-forward is stable or not.