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Entry  Thu Apr 2 04:11:20 2015, ericq, Update, LSC, Not much locking, Xover measurement CARMxOver_Apr3.pngApr2_Xover.xml.zip
    Reply  Thu Apr 2 15:34:34 2015, ericq, Update, LSC, Not much locking, Xover measurement xOverModel.pngloopModel.png
       Reply  Thu Apr 2 17:11:28 2015, ericq, Update, LSC, Not much locking, Xover measurement xOverModel.pngloopModel.png
Message ID: 11194     Entry time: Thu Apr 2 04:11:20 2015     Reply to this: 11195
Author: ericq 
Type: Update 
Category: LSC 
Subject: Not much locking, Xover measurement 

A paltry two locks tonight, but not entirely useless. I had some issues keeping the PRMI locked, which some additional boosts helped with. But, my feeling was that our crossover process is not tuned well. 

At full lock, both sub-loops have high gain around the crossover region, so the usual DTT loop transfer function measurement produces a meausrement of Gdigital/G_aopath (or minus that. I.e. I'm not currently 100% which is the bad phase in this plot, though it intuitive looks like 0 ). Thus, we can directly look at the crossover frequency and the effect of the different filters there. (I've also been working on an up-to-date CARM loop model today, so this will help inform that). 

Below, the black traces are the crossover at the end of the script when using the 120:500 "helper," and purple is without it. As we turn up the AO path gain, the trace "falls" from above, which explains why we can see instabilities around the violin filter. 

Having the helper on definitely made the probability of surviving the first overall CARM gain ramp higher, but it's not currently intuitively clear to me why that is the case. Afterwards, we can turn the helper off, to keep the shallower crossover shape. This is what I've put in to the up script for now. I also added a few seconds delay for when the script wants to switch DARM to RF only; I found it was maybe speeding too fast through this point.

DTT xml attached

Attachment 1: CARMxOver_Apr3.png  275 kB  | Hide | Hide all
Attachment 2: Apr2_Xover.xml.zip  36 kB  Uploaded Thu Apr 2 05:11:53 2015
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