Today we were struggling to lock ALS_COMM.  This has been an issue for a while, but has not been a showstopper until today.  

First off, COMM was causing the IMC to lose lock.  Yesterday I lowered the IMC gain by 6 dB.  This seems to have enhanced the ALS_COMM locking issue by increasing the relative additive offset on the IMC servo board, i.e. I decreased the IMC IN1 gain but not the IN2 gain.  

I lowered IMC IN2 by 6 dB (from -6 to -12 dB) in the guardian, this helped alleviate the additive offset issue.

I spent some time trying to understand the COMM locklosses we've been seeing.  One place we were losing lock in HANDOFF_PART2 when trying to turn up the CARM Servo Board IN2 gain from -31 to 17 dB, basically where we transition to COMM control.  I believe this was caused by the COMM VCO railing while we are trying to transition.  I altered the ALS_COMM guardian to check if the VCO is railed in the PREP_FOR_HANDOFF state, and if it is, set the VCO frequency to whatever the current beatnote is and try again.  

This has helped some, but not all of our COMM locklosses.  In HANDOFF_PART1 we are still seeing locklosses directly after engaging the IN2 input on both servo boards.  This was because the COMM output to Sum Node B IN1 was railed.  This is mysterious because there was no monitor within the COMM PLL itself which indicated anything is railed.  In any case, resetting the COMM VCO solves the issue, so I coded it up into guardian.  

The biggest unsolved mystery: COMM PLL -> DIFF noise, even when COMM is not locked. 

After these hacks, LOCKING_ALS has not yet caused a lockloss after seven attempts.  This is like putting vodka in your gas tank... it'll work, but who knows for how long.