J. Kissel, A. Pele

While Seb suggests that moving the HAM3 blends to the "250mHz" configuration is a good configuration to "solve" the 0.6 [Hz] feature (see LHO aLOG 16100), we argue that it would be better to live with a 0.6 [Hz], digitally-sharp, peak than to increase the X motion by the factor of several. OR we should find a different set of blend filters for RY that *don't* increase the low frequency motion.

I attach 2 supporting documents:
(1) 16196_20150121183131_H1-SEI_QUIET_3C14C7_SPECTRUM-1105747216-86400.png
The performance comparison between all HAM platforms from the Jan 20th summary pages. On this day, HAMs 2, 4, 5,and 6 are in the "nominal" LHO configuration, all using "01_28" blends on all degrees of freedom, with sensor correction ON for all DOFs, GS13s in high-gain mode, with newly improved isolation loops. HAM3 however, was in Seb's suggested configuration -- the same as the other HAMs *except* for the RY blend filter, which has been changed from "01_28" to "250mHz."

(2) 2015-01-21_H1vL1_BlendFilter_Comparison.pdf
A comparison between the H1 "01_28," H1 "250 mHz", and L1 "400" RY blend filters, as well as the H1 "01_28" and L1 "250" X blends. This shows that 
   (a) In RY, 
       (i) the H1 "01_28" and L1 "400" are virtually identical, as expected. To be precise, for some reason, the L1 GS13 filters are 8% higher in gain, but otherwise identical.
       (ii) The H1 "250mHz" blend is actually *lower* in blend frequency than the H1 "01_28" filters. This means the low-frequency roll-off of the GS13 high pass is much slower for the "250mHz" than for the "01_28".
   (b) In X, 
       (i) there is a good bit of difference in the 0.2 - 4 [Hz] region. We suspect this is because the H1 "01_28" filters were copied over from LLO in Oct 2013, before Ryan had made further tweaks to these blend filters to improve performance around the SUS resonances. The performance is most different at 0.75 [Hz], where the displacement sensor low-pass is lower by a factor of 10.
       (ii) it's concerning that the GS13 high-pass -- though 19% different in overall gain -- does *not* differ between the two site's versions of the filter. This must be what Ryan means when he suggests his filters are "almost" complementary.

While all points are interesting with respect to the sociology and history of the SEI commissioning, point (2)(a)(ii) is the key for HAM3. Because the RY GS13 filter rolls off slower and lower for the "250mHz" blends, the differential vertical GS13 noise is reinjected into the RY loop. This excess residual RY motion couples to X via tilt-horizontal coupling, which degrades the low-frequency noise performance in the same frequency region. Bad. We'd discovered this nasty cross-coupling path as far back as The Stone Ages.

We should find/design an RY filter for HAM3 that *increases* the blend frequency from the H1 "01_28" filters, if we intend to run for a while in such an odd configuration. We may try copying over a few other L1 blend filters. This is all still lower priority compared to the *rest* of the to-do list, but Arnaud and I are worried that the excess low-frequency motion in HAM3 alone might be causing problems with cavity motion.