Under request from commissioning coordinator, reporting monitoring needs coming from squeezing team, we moved the line on PStab to roughly 2468Hz. Amplitude has been left unchanged.

Figure 1. Comparing PSTAB_PD2_AC_MONIT and PSTAB_PD2_AC_MONIT_PRE, the PSTAB loop has a gain of ~1500 at 3kHz, this seems reasonable as the PSTAB loop gain at 3kHz in the AdV TDR is 1000.

Figure 2. The line injection allows to measure the PSTAB coupling to the dark fringe. In blue is data with the line injection and in purple data with the PSTAB loop open from December 16 2019. Surprisingly the coupling now is a factor 6 higher than back in December. But is is only a factor 1.6 higher than what was measured back in July 2019.

Figure 3 shows the projection of PSTAB noise (both sensing and sideband RAM contamination) using the July 2019 transfer function scaled to match the observed coupling at the 2961Hz line.

Figure 4 shows the same but using the PSTAB transfer function measured on Dec 16 (which was much lower, hence the low coherence between 100Hz and 1kHz). Above 100Hz the two projections agree, but at low frequency the Dec 16 transfer function show a much higher coupling. This might be just a symptom that scaling the transfer function using the line at 3kHz for a noise below 100Hz is the wrong thing to do. I.e. the changes in the PSTAB coupling at high frequency are not the same as changes in the coupling function at low frequency.

In any case, the PSTAB projection above 100Hz is about a factor 10 below the measured sensitivity.

During daily meeting I saw that the line was no longer in the data. At 14:31 UTC I switched off the function generator and unplugged the cable from perturb input of the PStab rampeauto.

Useful data with this injection lasted from 11:11 UTC to 13:00 UTC.

In order to look at the time dependence of the coupling from PSTAB to B1 I have looked at the height of the line at 2961Hz over time in both PSTAB and B1 over a few hours.

Figure 1, shows the time series for 20 minutes of data, the coulping fluctuates by up to a factor 3.

Figure 2, shows the spectrum of that coupling time series, the fluctuations are mostly below 200mHz, but there is no clear structure between 1mHz and 200mHz, maybe there is some higher lines at ~25mHz and ~40mHz, but it is hard to say if it is real or just a statistical fluctuation.

Figure 3, looking at two consecutive periods of 3 hours, the peak at 25mHz looks significant, but not the one at 40mHz.

users/mwas/detchar/PSTABcoupling_20200116/TFrms.m