We have been taking some ASC measurements while locked on RF DARM for DCPD whitening investigations. 

It seems that lowering the DHARD P gain by a factor of 2 (47232 ) was causing a 1 Hz instability yesterday and Friday night.  We made some measurements of the DHARD loop in the low noise ASC state, the attached screenshot shows the measurement with the 1 Hz resG engaged, the gain at -42 and no low pass engaged.  We still have the non-minimum phase at 1 Hz, and if we were to reduce the gain to -21 as we did on friday this would be very close to unstable. Instead of trying to reduce the gain, we decided to just modify the low pass filter.  We modified the ELP10b filter (FM1) to be a 20Hz low pass which gives us a bit more roll off that the ELP17Hz which was for this measurement of the noise in DARM (both are shown in the second attachment).  The guardian will now leave the gain of DHARD Y at -42, and turn on the new 20Hz low pass.  This seems to be a sort of OK compromise between gain peaking at 5 Hz and at 1 Hz.  

For DHARD Y, the filter gain hasn't changed at high frequency, but the combination of 2 low pass filters that we were using wasn't very good.  The forth attachment shows the new ELP20 filter that I swapped out for the old cut off, it gives us back about 10 degrees of phase at the 4Hz ugf and gives us about a factor of 3 less control signal around 35Hz. In the guardian I have swapped FM2+8 for FM9.  

We haven't been able to test these changes.