Virgo Logbook

Digging further, it seems that a phase jump in the phase of the LNFS at the same moment that the phase-camera process is restarted was not an isolated case, a similar coincidence happened around 2018-03-28-14h19m18-UTC. The EPRB_PC process was restarted a few times, the first of which triggers a jump in the phase, see figure 1. In another case, the phase jumps during the night in coincidence with some missing samples and a jump in the CfgChange counters, see fig 2.

Note that the EPRB_PC and LNFS_Demod processes run both on rtpc6. My guess is that for some reason the real-time processes are overloaded, which triggers a reset of the sine generator used for the phase derotation in LNFS_Demod, which changes the demodulation phase of all photodiodes ...

It would be good to understand this issue, but one possible way to avoid this problem entirely is to remove the phase derotation step, and directly use the measured phase of the LNFS (which then rotates by the frequency difference between the nominal and actual demod frequency) to derotate the signals of the photodiodes.