Jeff B, Ed, Kiwamu,

Today we have changed two parameters in the OMC_LOCK guardian as follows.

• Changed the scan starting voltage back to -24 V (probably this was unnecessary).
• Changed the lower threshold for the peak ratio of the upper and lower rf sidebands to -25%.

The guardian codes are reloaded and checked into the SVN. We tested once and worked fine.

We had an issue with the OMC_LOCK guardian where it did not succeed in finding a carrier resonance. Although this may sound familiar to some of us (for example, alog 23543, alog 23823), the symptom was different this time. The upper and lower 45 MHz rf sidebands had different peak power by approximately 11%. As the OMC_LOCK guardian stringently checks whether the power ratio is within [+25%, -10%], it was unfortunately exceeding the lower threshold and hence the failures. I have no idea why the power ratio now exceeds the power threshold. I briefly checked the TCSs but none of them seemed suspicious. Furthermore, since the interferometer was able to fully lock with the RF readout at a lower power, the change we are trying to identify may be a subtle thing.

After all, we have relaxed the lower threshold to -25% so that it matches the upper one. Also, during the investigation, we have reverted the start voltage for scanning OMC back to -24 V which had been -30 V in the past few days (alog 23823). This change might be unnecessary, but since it works fine, we will leave it as it is.