Attached are spectra comparing the SUS ETM L3 LV ESD channels from lock segments before (Jan 24, 2016 03:35:00 UTC) and after (Feb 1, 2016 01:50:00 UTC) the ESD Driver update last Tuesday alog 25175 (and the subsequent ESD fixes on Wed 25204). Before the install, the channels looked like ADC noise, while they look live now. The ETMx plot is included, but of course the ETMx ESD is not ON during either lock stretch, so all the plot really says is that something happened to the channels. Whether or not the ETMy channels look like they actually should, according to various ECRs, is to be determined.
Keita helped me make more useful plots to evalute the new LV chassis mods. See attached.
The alog that motivated all of these is alog 22199.
In Betsy's new plots, reference traces are with the old hardware, current traces are new, both in full low noise lock. In summary, this is good.
The spectrum/coherence plot shows that the new whitening is useful, the monitor is actually monitoring what we send rather than just ADC noise. (It used to be totally useless for f>7Hz or so as you can see from the reference coherence.) You can also see that there's a small coherence dip at around 30Hz, and there are deeper dip at around various band stop filters, but otherwise it's actually very good now.
In the second plot, you see Y channel spectrum together with X. Since we don't send anything to X during low noise lock, X is just the sensing noise. When you compare the signal (red) and the sensing noise (light green), we can see that the signal is larger than the noise across the entire frequency range except the stopbands.
At around 30Hz (where we saw a tiny coherence dip in the first plot) the noise is only a factor of 3 or 4 below the signal. We expect the higher frequency drive above f=100Hz to drop as we increase the power, so the signal/noise ratio there might drop in the future. There's still a large headroom before we rail the ADC (RMS is 600 counts), so if necessary I'm sure we can make some improvement, but this is already very good for now.
The only thing is, what's these lines even when we don't send anything (X)?
It seems as if 57Hz and harmonics, whatever they are, in the non-driven channel are at least as large as in the driven channel.
57Hz was the HWS at the end stations.
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=25383