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Entry  Fri Apr 24 03:57:30 2009, Yoichi, Update, Locking, DARM demod phase 
    Reply  Fri Apr 24 04:38:49 2009, Yoichi, Update, Locking, DARM demod phase 
       Reply  Fri Apr 24 11:34:32 2009, rob, Update, Locking, DARM demod phase darm_phase_rotate.png
          Reply  Fri Apr 24 17:26:57 2009, Yoichi, Update, Locking, DARM demod phase 
Message ID: 1516     Entry time: Fri Apr 24 11:34:32 2009     In reply to: 1515     Reply to this: 1519
Author: rob 
Type: Update 
Category: Locking 
Subject: DARM demod phase 

Quote:

Quote:
Tonight, I was able to go up to arm power = 40 by tweaking the DARM demodulation phase.
I think the DARM loop became unstable because the demodulation phase was not right and the error signal contained some junk from I-phase.
I did not do any sophisticated demodulation phase optimization. Rather I just tweaked the phase so that the dark port image becomes stable.
I will do more careful demodulation phase tuning next time.


In the next try, I was actually able to go up to arm power = 70 stably.
At this power level we are ready for the RF CARM hand off.


There's actually code in place in the LSC to dynamically adjust the demod phase for AS1. I've never made much use of it, because it's possible to get around the problem with some gain tweaking if you start at the right phase, or because I did the DC readout handoff earlier.

Attached is a cartoon showing how the demod phase at the dark port changes as the CARM offset is decreased.
Attachment 1: darm_phase_rotate.png  41 kB  | Hide | Hide all
darm_phase_rotate.png
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