sheila.dwyer@LIGO.ORG - posted 18:25, Thursday 27 October 2016 - last comment - 12:00, Friday 28 October 2016(30947)
PR2 roll mode notched
The PR2 Roll mode has been reduced in the DARM spectrum by adding 1Hz wide bandstops (centered at 40.9Hz) to all the top mass damping loops for PR2.
Once these were on the line in DARM at 40.934Hz is gone. There are other HSTS roll modes which now show up, 40.918 (MC2 or SR2 maybe) and something smaller at about 40.937Hz.
For the record, Sheila saved the modified filter file on Oct 27 2016 at 16:55 PDT, or Oct 27 2016 23:55:00 UTC. She says she hit load on the filter banks individually "pretty much right after." I've now subsequently loaded the entire filter file from the GDS_TP screen on Fri Oct 28 2016 at 10:38:53 PDT (Oct 28 2016 17:38:53 UTC) The filter design string is ellip("BandStop",4,1,60,40.4,41.3)gain(1.12202) and lives in FM9. Also, I attach a comparison between calibrated DARM ASDs from last night's long lock stretch against one from several nights ago. The BW is a delightful 0.3 [mHz] -- hooray for IFO stability! The conclusion is a little different from Sheila's above conclusion: the PR2 R3 mode @ 40.93 Hz is not *gone*, but it has definitely been reduced by an order of magnitude. Interestingly, the 27.41 [Hz] V3 mode also is a reduced by a factor of 0.70. Indeed, there are other R3 modes now exposed at 40.8687, 40.8775, 40.9171, in addition to the residual of the PR2 mode at 40.9356 [Hz]. That being said, it looks like given the density of modes around 40.9, it'll be fine to use the same notch design for all HSTS. We'll successively notch the R3 mode out of every HSTS, and see what disappears. By looking at my 2014 comparison between LLO, LHO, and a Low Noise damping loop design, G1401291, at first glance since the filter only distorts the phase at 20 Hz by 2 [deg], we should have plenty phase margin to notch out this mode in every suspension. But do not that LHO still has a complete mess of a design -- pretty much all the SUS have different gains and filter designs. So, we'll do so with caution. Even more so if we want to notch the V3 mode.