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Message ID: 9492     Entry time: Thu Dec 19 03:29:34 2013
Author: Den 
Type: Update 
Category: LSC 
Subject: CM servo test using yarm is complete 

Koji, Den

Procedure:

  • lock yarm on IR, wire POY to CM input
  • transition arm to CM length path by actuating on IMC
  • increase AO gain for a stable crossover
  • engage CM boosts

Result:

  • arm can be kept on resonance and even acquired on MC2
  • stable length / AO crossover is achieved
  • high bandwidth loop can not be engaged because POY signal is too noisy and EOM is running out of range

We spent some time tuning CM slow servo such that fast path would be stable in the AO gain range -32db -> 29dB (UGF=20kHz) when all boosts are turned off and common gain is 25dB. Current filters that we use for locking are not good enough - AO can not be engaged due to oscillations around 1kHz. This is clearly seen from slow path closed loop transfer function. I will attach servo shapes tomorrow.

Attached plot "EOM" shows EOM rms voltage while changing AO gain from -10dB to 4dB. For UGF of 20kHz we need AO gain of 29dB.

It seems we can start using CM servo for CARM offset but the sensor should be at least factor of 30 better than POY. Add another factor of 10 if we would like to use BOOST 2 and BOOST 3.

Attachment 1: EOM.png  12 kB  | Hide | Hide all
EOM.png
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