S. Ballmer, J. Kissel

Using the data from Marc's independent bench-top measurements of the new OMC DCPD Whitening Chassis (LHO aLOG 42361; upgraded to swap out the 2nd whitening stage for a low-pass filter), I've finally fit*** the response of the three stages and updated the digital compensation. Recall there are high frequency poles (> 5 kHz) that we do not compensate (but do have non-negligible phase impact below 1 kHz, so we need to compensate for them in the a-causal low-latency pipeline). I did not formally fit these.

Check out the last three plots of the attached for how much the compensation changed, but the message is that at ~100 Hz, depending on the stage and channel, there's as much as a 1-3 % change per stage.
Remember, we've typically only been using the 1st stage, so the biggest change is the 3% decrease in DCPD A's compensation. This (I believe; math to follow) means a ~3% decrease in BNS range.

We should:
(a) Now repeat Stefan's measurement of the DCPD balance, updating the DCPD balance matrix, (see LHO aLOG 45734)
(b) Remeasure this whitening chassis in-situ in order to 
    (i) improve the data quality on the DCPD measurement, and 
    (ii) better fit (especially the uncompensated) the poles and zeros, similar to Kiwamu's methods in LHO aLOG 28087.

*** The data quality of some of the measurements are poor (maybe some sort of export bug), so I just quickly fit the TFs by hand, and ignored learning new or shaking off the bit rot of old fitting code. The data analysis script lives here:
    /ligo/svncommon/CalSVN/aligocalibration/trunk/Common/Electronics/H1/Scripts/plot_omcdcpdwhiteningmods_20180604.m

Design strings for the new compensation:
DCPD A
    ST1: zpk([10.42],[0.993],0.975,"n")
    ST2: zpk([52.7;  35.448 + 35.263i; 35.448 - 35.263i; 353.55 + 353.55i; 353.55 - 353.55i], 
             [531.0; 35.355 + 35.355i; 35.355 - 35.355i; 354.48 + 352.63i; 354.48 - 352.63i],-0.9995,"n")
    ST3: zpk([10.33],[0.894],1.1,"n")

DCPD
    ST1: zpk([10.135],[0.966],0.975,"n")
    ST2: zpk([52.32; 35.448 + 35.263i; 35.448 - 35.263i; 353.55 + 353.55i; 353.55 - 353.55i], 
             [527.2; 35.355 + 35.355i; 35.355 - 35.355i; 354.48 + 352.63i; 354.48 - 352.63i],-1.0004,"n")
    ST3: zpk([10.445],[0.896],1.11,"n")
The extra complex pairs of poles and zeros in the design for the ST2 (new low passes) are to take out a small wiggle at about 50 and 500 Hz that the single pole compensation couldn't fix. Also remember that this low pass dubiously does in fact have a sign flip, which is compensated for here.