The band limited RMS (BLRMS) (H1:OMC-BLRMS_32_BAND#_BP for #={1 to 7}) bands that monitor DARM (Actually OMC DCPD A) frequencies in the range 2.5 kHz through to 32 kHz were being used in the lockloss monitoring tools to assess if a lockloss was due to PI.

These were not working correctly, so I poked my head in this morning.

The BLRMS used Elliptical filters, which have a flat response down the DC, and we were seeing a large DC value in the outputs that was "saturating" any high frequency signal we might care about.

These BLRMS have been altered to use Butterworth filters instead of Elliptical. Butterworth filters have the highly valuable property of actually being AC coupled so that should do the trick.