jeffrey.kissel@LIGO.ORG - posted 10:32, Tuesday 22 September 2015 - last comment - 18:13, Tuesday 22 September 2015(21788)
Actuation Delay Cycles changed to 7 [clock cycles] to better Approximate Super-Nyquist Corrections
J. Kissel I've increased the actuation delay before the sum of the the RESIDUAL (sensing) and CTRL (actuation) paths in the CAL-CS reproduction of H1:CAL-CS_DARM_DELTAL_EXTERNAL from four 16 [kHz] clock cycles (244 [us]) to seven 16 [kHz] clock cycles (427 [us]). This is done by changing the H1:CAL-CS_DARM_CTRL_DELAY_CYCLES EPICs record. The change is motived in LHO aLOG 21746. Recall this only affects the reproduction of the DARM displacement signal H1:CAL-CS_DARM_DELTAL_EXTERNAL (and therefore its ASD projected on the wall). I attach screenshots of the before and after. Note that the ASDs were taken ~30 minutes apart, so I don't expect the detailed structure to be the same. However, the change in phase at the sensing / actuation cross over causes overall shap change in the bucket. Also note that the transfer function used to remove the systematics from the DELTAL EXTERNAL channel will now have to be updated (documented in LHO aLOG 20481), to avoid double counting the high frequency affects. In the attached ASD, those corrections have *not* been changed, so we are likely double counting the corrections. More on that after I reconcile what Kiwamu had done and what Peter suggests. I've captured the updated setting in both the OBSERVE.snap and SAFE.snap SDF files, and committed each to the repository.
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I've updated the slide I'd made in the PreER7 -era that elucidates all of the time delays and approximations that we've made to come up with the seven 16 [kHz] clock-cycle delay between the actuation path and sensing path. The ER7 version is pg 7 of G1500750. See attached. In summary, the total delay *between* the inverse sensing and actuation chains, if we approximate all high-frequency frequency response as delays that are in addition to the "true" delays from computer exchanges, if we weren't limited by 16 [kHz] clock cycles should be 442.6 [us]. Because we are limited to 16 [kHz] clock cycles, we've chosen 7, which is a delay of 427.3 [us] -- a 15.3 [us] systematic error. You should also remember, that having this delay between the chains means that CAL-CS_DELTAL_EXTERNAL has an overall delay or "latency" equivalent to that of the inverse sensing function advance, which is 213.6 [us]. Note that these numbers are LHO-centric -- the approximation for the OMC DCPD signal chain of 40 [us] assumes the pole frequencies of the H1 OMC DCPD chain, and the estimation of the systematic error in phase uses H1's DARM unity gain frequency of 40.65 [Hz]. For LLO, the details should be redone if a precise answer is needed.
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