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Entry  Mon Jan 12 18:11:25 2015, ericq, Update, ASC, Test Mass -> Transmon QPD TFs measured Jan12_singleArmSensing.pdfJan12_singleArmSensing.xml.zip
    Reply  Thu Jan 15 18:57:41 2015, ericq, Update, ASC, Transmon QPD loops live loopDesign.pdf
       Reply  Fri Jan 16 04:14:38 2015, ericq, Update, CDS, Unexpected CDS behavior digitalProblem.pdf
Message ID: 10912     Entry time: Fri Jan 16 04:14:38 2015     In reply to: 10908
Author: ericq 
Type: Update 
Category: CDS 
Subject: Unexpected CDS behavior 

EDIT: Sleepy Eric doesn't understand loops. The conditions for this observation included active oplev loops. Thus, obviously, looking at the in-loop signal after the ASC signl joins the oplev signal will produce this kind of behavior. 

After some talking with Rana, I set out on making an even better-er QPD loop. I made some progress on this, but a new mystery halted my progress. 

I sought to have a more physical undertanding of the plant TF I had measured. Earlier, I had assumed that the 4Hz plant features I had measured for the QPD loops were coming from the oplev-modified pendulum response, but this isn't actually consistent with the loop algebra of the oplev servos. I had seen this feature in both the oplev and qpd error signals when pushing an excitation from the ASC-XARM_PIT (and so forth) FMs. 

However, when exciting via the SUS-ETMX-OLPIT FMs (and so forth), this feature would not appear in either the QPD or oplev error signals. That's weird. The outputs of these two FMs should just be summed, right before the coil matrix. 

I started looking at the TF from ASC-YARM_PIT_OUT to SUS-ETMY_TO_COIL_1_2, which should be a purely digital signal routing of unity, and saw it exhibit the phase shape at 4Hz that I had seen in earlier measurements. Here it is:

I am very puzzled by all of this. indecision Needs more investigation.

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