In the process of investigating the locklosses due to FSS glitching and working on spare chassis for the FSS in the PSL, we compared the power spectrum of the PZT monitor between H1 and L1. We found some difference in the power spectrum, plot attached.
I discovered today that LLO is indeed doing some additional digital filtering to their FSS_FASTMON channel, which would very likely explain the difference in spectra Marc shows above. Just looking at the MEDM screen for the filter bank, it shows three filters in use (called "cts2V', "NPRO", and "toMHz") while LHO is using none; parameters of these are attached in a screenshot. I'm not entirely sure what the purpose of these are, but from what I can tell there is an additional pole at 10Hz, which would explain the 1/f-looking drop in noise towards higher frequencies.
