TJ, Oli
Starting around 02/20 at 11:30 UTC, the range had a step down, where it stayed for the rest of the lock. After this step down, the range was also much noisier than it had been before the step (ndscope1). Jane Glanzer ran Lasso for us during this lock stretch (lasso), and the top channel that came back with the highest correlation to this range drop was H1:SUS-PI_PROC_COMPUTE_MODE8_NORMLOG10RMSMON, with the other channels all having much lower correlation coefficients. This was weird to us because we bandpass and downconvert to monitor mode 8 but we don't monitor or damp it, and we don't even turn on its PLL. When you plot the correlated channel along with related PLL channels (SUS-PI_PROC_COMPUTE_MODE8_PLL_AMP_FILT_OUTPUT, SUS-PI_PROC_COMPUTE_MODE8_PLL_LOCK_ST, and SUS-PI_PROC_COMPUTE_MODE8_PLL_ERROR_SIG), you can see there was some weird noise in multiple of these channels that started when the range dropped (ndscope2).
I tried plotting a series of PI_PROC_COMPUTE_MODE channels for every mode (there are 32 in total), and out of all of them, only mode 8 and mode 24 showed any change in any of their respective channels around the time of the range drop(ndscope3). It only being these two channels is very interesting. PI mode 8 comes from the TRX QPD and has a bandpass between 10 and 10.8 kHz. Like I mentioned earlier, we do not actively do anything to this channel. Mode 24 on the other hand, we definitely monitor and are damping it a lot of the time. Mode 24 is read in from the DCPDs and has a bandpass of, and the PI is centered around 10.431 kHz. It is damped by using the ESD on ETMY. Mode 24 actually has more channels that correlate better and have larger amplitudes than mode 8, but Mode 8 NORMLOG10RMSMON correlated better with lasso over the entire lock stretch.
Zooming into when the range drop started yesterday, we actually see that the large drop in range happened about 12 minutes before we see the huge error signals in the mode 24 PLL, and it is at the very beginning of the rise in mode 8 and 24 NORMLOG10RMSMON (ndscope4).
Zooming out, this excess noise in those PI channels seems to have started early on February 5th, aka after relocking after the February 4th maintenance day(ndscope5). On other days, the range doesn't seem to have been affected by this noise though, at least nowhere near the amount that it was affected yesterday. Sometimes during these periods of noise, the range won't be very good (below 155), but other times we'll see this noise in modes 8 and 24 and still be right around or above 160.
Since it looks like the range drop yesterday started before the PI channels really started changing, we're pretty sure the issue is somewhere else and is just bleeding into these two downconverted channels. Because these two PI channels go through bandpasses in the 10kHz regime, there might be something in that frequency range. It is interesting though that although another PI channel we actively monitor, Mode 31, is also in the same frequency area (centered at 10.428 kHz), is read from the OMC DCPDs, and is damped using ETMY, all just like PI24, there doesn't seem to be any coupling into its channels.
Either way, pretty good leads were made this morning towards finding the actual cause of this range drop. TJ and Camilla looked into the many glitches that appeared then at 60 and 48 Hz, as well as noting that the line at 46.1 Hz had grown, which is a known PR3 roll mode (82924).
