To fix the problem of accidental PUM/ESD crossovers near the violin mode resonances, the PUM is now rolled off more aggressively above the crossover (see attachment of EY L2 LOCK FM6 vs FM7, where grey is old and purple new).
Previously, the PUM had an f2 plant inversion out to about 600 Hz. It is now more like 300 Hz. There is also a broad notch around the first violin mode just to make sure that we do not have an accidental crossover there.
DARM OLTF attached (blue and red are essentially part of the same measurement). The high end of the phase bubble has flattened out a bit.
The PUM/ESD crossover was remeasured and was found to be satisfactory (attachment). Additionally, the rms drives to the three stages seem to be acceptable as well (attachment), although these were taken during low seismic activity.
Taking the data from the time in the spectra posted above, I looked at what is using up the ESD range. It looks like it should be fine to engage two stages of low pass filter in the LVLN driver.
The first attached plot shows the ESD MASTER_OUT LL channel as well as the expected signal level after applying the compensation filter for the two (50:2.2) analog filters.
The RMS goes from 1450 to 30000 cts after switching the filter.
Most of the RMS increase would come from a few CAL lines around 540 Hz which are not accurately notched by the DARM filter bank. These filters should be modified when the line frequencies are changed. Also, the line ampltiudes are too large. Probably the line amplitudes should be set by determining what physical parameter we need to estimate with what SNR, instead of some ad-hoc amplitude based on the power spectra.
Notching out the CAL lines would reduce the DAC signal from an RMS of 30000 cts (un-tenable) to 3000 cts (reasonable).
