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Entry  Mon Dec 5 17:44:41 2011, kiwamu, Update, General, my plan tonight 
    Reply  Mon Dec 5 19:21:55 2011, kiwamu, Update, LSC, coarse beat note signal : ADC limited above 30 Hz Yarm_ALS_coarse.png
       Reply  Tue Dec 6 00:26:00 2011, kiwamu, Update, LSC, ALS became robust : UGF = 100 Hz 
Message ID: 6072     Entry time: Mon Dec 5 19:21:55 2011     In reply to: 6071     Reply to this: 6074
Author: kiwamu 
Type: Update 
Category: LSC 
Subject: coarse beat note signal : ADC limited above 30 Hz 

The signal observed by the coarse frequency discriminator was actually dominated by the ADC noise above 30 Hz.

It means that once increasing the UGF more than 30 Hz the servo will feed the ADC noise to the test mass and shake it unnecessarily.

I guess this could be one of the reasons of the unstable behavior in the Y end PDH lock (#6071).

(But still it doesn't fully explain the instability).


 To improve the situation I am going to do the following actions:

   (1) Installation of a whitening filter (probably use of SR560s)

   (2) Redesign of the servo filter


Here is a brief noise budget of the coarse sensor.


Gray curve: free running noise when no servo is applied

Green curve : in-loop noise when the ALS loop is closed with the coarse frequency-discriminator. The UGF was at 30 Hz.

Red curve : ADC noise of the coarse discriminator

Quote from #6071

 So far I still kept failing to increase the UGF of the ALS servo for some reason (see #6024).

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