Trouble with MODE17 (ITMX 15542 Hz) could be due to instability from large phase changes with slight frequency drift in our damping loop. Mechanical mode frequency at beginning of a cold lock sits around 15541.875 Hz and has drifted to 15541.95 Hz after about three hours. The previous bandpass filter carried ~40 deg phase change over that time. In an effort to fix this, I've made two BP filters that are shifted versions of one another to be turned on as the frequency migrates. The guardian uses the PLL frequency monitor to track the average frequency change of the mode peak and make the filter change switch at the appropriate time. Fall offs and notches in the filters are such that center of BP --> switching point ~10 deg. I've watched the guardian successfuly make this change.
Proof of concept appears to have worked; Mode17 was kept stable throughout frequency drift well beyond the previous 3 hourish mark. Lockloss occured after ~6 hours from the partner Mode25 who saw a phase change of ~140 deg over this time, eventually probably driving up this usually easily dampable mode. Attachment shows freq drift over last night's lock. I've added stepping bandpasses to both modes (controlled with the guardian) now to accomodate frequency drift over much longer locks.