Virgo Logbook

As a quick tests, I added some temporary signals to LSC_Acl for demodulating the PRCL line in B2_18MHz. We tried a few times to make a scan of the phase B2_18MHz_phi0, but the interferometer could not stay locked long enough with PR aligned. The same was therefore done using the DARM line in B7_6MHz (with ITF_LOCK in state LOCKED_SSFS), which allowed us to do a full scan. At first, the demodulation was done with a low-pass filter of 2 Hz, but the signals were too noisy. Lowering the frequency to 20 mHz, the signals started to make sense. Note that are two demodulations: first the photodiode signal is demodulated at 6MHz yielding the usual two signals. These are then both demodulated at the line frequency of 41.1 Hz. This gives four signals II, IQ, QI and QQ, which are plotted in figure 1. During the scan, the vector of the demodulated line in B7_6MHz_I stays relatively stable, at an angle that is determined by the closed-loop transfer function of the loop at the frequency of the line. The vector of the line demodulated in B7_6MHz_Q passes trough the origin during the scan of the demodulation phase.

Some basic math shows that the error in the demodulation phase is given by dphi = atan((II*QI + IQ*QQ) / (II^2 + IQ^2)). An offline calculation is shown in figure 2, the scan is well visible and the slope seems correct. The vertical offset between phi0 and the measured phase might hint that the demodulation phase was mistuned by about 7 degrees.

I have ACL code ready for calculating this online. This might be added for B2_6MHz and B2_18MHz using the PRCL line, and for B7/B8_6MHz using the DARM line. To do something similar for B4, we would need a permanent line on the SSFS (maybe the old 1111 Hz), but it is not clear what would be the best ACL process to do this.