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Entry  Fri Feb 20 19:09:48 2015, ericq, Update, ASC, QPD frontend code unified 
    Reply  Mon Feb 23 08:06:59 2015, rana, Update, ASC, QPD frontend code unified 
    Reply  Thu Mar 5 20:44:30 2015, Jenne, Update, SUS, damprestore script updated 
    Reply  Mon Mar 9 18:10:59 2015, Jenne, Update, ASC, Oplev filters re-copied 
    Reply  Sat Mar 14 00:12:18 2015, rana, Update, SUS, Oplevs huh? burp.png
       Reply  Mon Mar 23 13:27:33 2015, ericq, Update, SUS, ITMX oplev quadrant gains unbalanced olsum.png
          Reply  Fri Apr 3 14:57:38 2015, manasa, Update, SUS, BS oplev BS_oplev_Apr3.png
             Reply  Fri Apr 3 15:15:55 2015, Steve, Update, SUS, BS oplev 
                Reply  Fri Apr 3 19:35:14 2015, Jenne, Update, SUS, BS oplev centered 
Message ID: 11160     Entry time: Mon Mar 23 13:27:33 2015     In reply to: 11142     Reply to this: 11199
Author: ericq 
Type: Update 
Category: SUS 
Subject: ITMX oplev quadrant gains unbalanced 

I've been poking around the oplev situation. One thing I came across regarding ITMX was that the gain on segment 4 seems to be about higher than the other segments. I was led to believe this by steering the optic around, and looking at the counts on each quadrant when the other 3 were dark.

Putting a gain of 0.86 (the ratio of the other segments' max counts over segment 4's max counts) in the segment 4 FM flattens the 1 Hz peak in the ITMX_OL_SUM spectrum, as well as significantly reducing the sub-Hz coherence of the sum with the individual quandrant counts. This is what I would expect from reducing the coupling of angular motion due to quadrant gain mismatch into the sum. 

Here are the ITMX_OL_SUM spectra before and after (oplev servos are off).

The "burps" and control filter saturations are still unexplained. Investigations continue...

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