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Entry  Tue Mar 24 05:05:09 2015, ericq, Update, LSC, AO Path engaged CARM_DARM_AOengaged_23March2015.pdfloops.png
    Reply  Tue Mar 24 18:22:11 2015, ericq, Update, LSC, AO Path engaged IMC_ao_Mar242015.pngIMC_olgs_Mar242015.png
    Reply  Tue Mar 24 18:47:10 2015, ericq, Update, LSC, AO Path engaged 
Message ID: 11167     Entry time: Tue Mar 24 18:22:11 2015     In reply to: 11163
Author: ericq 
Type: Update 
Category: LSC 
Subject: AO Path engaged 

For increased flatness of the AO response, and thus less gain peaking in the CARM loop, I reccomend turning down the MC servo VCO gain to 22dB, -6dB of the current setting. 

From there, we should be able to up the overall CARM gain by another 10dB, and turn on a super boost. 

I measured the IN1/IN2 response of the IMC loop with the aglient analyzer providing the IN2 excitation, to see the transfer function of the AO acutation. The hump in the TF explains the flattening out of the CARM OLTF we saw last night. Turning down the gain by 6dB flattens this bump, and more importantly, has around 10dB less gain when the phase goes through -180, meaning more gain margin for the CARM loop. 

Oddly, when I back out the MC OLG from these measurements, the loop shape is different than what Koji and Rana measured in December (ELOG 10841). Specifically, there is some new flattening of the loop shape around 300-400kHz that lowers the frequency where the phase hits -180. What could have caused this???

The -6dB that I mentioned was determined by putting the MC UGF at about 100kHz, at the peak of the phase bubble. This should allow us to safely have a CARM UGF of 40kHz since the MC loop has around +10dB loop gain there, which Rana once quoted as a rule of thumb for these loops. At that UGF, at least one CM board super boost should be fine, based on the loop shapes measured last night. 

Lastly, I also checked out whether the 3 MC super boosts were limiting the AO shape; I did not observe any diffrence of the AO TF when turning off one super boost. It's likely totally fine. 

Attachment 1: IMC_ao_Mar242015.png  125 kB  | Show | Show all
Attachment 2: IMC_olgs_Mar242015.png  126 kB  | Show | Show all
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