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Message ID: 9170     Entry time: Fri Sep 27 16:02:23 2013
Author: manasa 
Type: Update 
Category: Green Locking 
Subject: Y arm ALS phase tracking loop gain changed 

[Masayuki, Manasa]

While trying to lock the arms using ALS we found that the locks were not very stable and the in-loop noise was higher than seen before.

I looked into things and checked the out-of loop noise for ALS and found that the Y arm ALS noise (rms) was higher than the X arm.

To troubleshoot, I measured the OLTF of the phase tracking loop. While X arm was healthy, things weren't looking good for the Y arm. Sadly, the Y phase tracking loop gain was set too high with a phase margin of -2 degrees. We brought down the gain from 300 to 150 and set the phase margin close to ~55 degrees.

X arm Phase tracker loop:
UGF = 1.8 K Hz
Phase margin = 50 degrees

Y arm Phase tracker loop:
UGF = 1.6 KHz
Phase margin = 55 degrees

Attachment 1: outofloop.pdf  68 kB  Uploaded Fri Sep 27 17:05:04 2013  | Hide | Hide all
Attachment 2: PTX_OLTF.pdf  18 kB  Uploaded Fri Sep 27 17:09:39 2013  | Hide | Hide all
Attachment 3: YPT_OLTF_after1.pdf  18 kB  Uploaded Fri Sep 27 17:12:15 2013  | Hide | Hide all
Attachment 4: YPT_OLTF_before.pdf  18 kB  Uploaded Fri Sep 27 17:13:02 2013  | Hide | Hide all
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