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Entry  Fri Dec 20 01:24:11 2013, Den, Update, LSC, high bandwidth loop achieved for yarm CM_OL_meas.pdfcm_ol_sim.pdfCM_slow_fast_cross.pdf
    Reply  Fri Dec 20 03:31:07 2013, Koji, Update, LSC, lock acquisition path for the CM servo 
       Reply  Tue Apr 22 02:14:55 2014, ericq, Update, LSC, lock acquisition path for the CM servo 
    Reply  Fri Dec 20 03:34:40 2013, Koji, Update, LSC, high bandwidth loop achieved for yarm 
       Reply  Fri Dec 20 22:45:02 2013, Koji, Update, LSC, high bandwidth loop achieved for yarm 
Message ID: 9840     Entry time: Tue Apr 22 02:14:55 2014     In reply to: 9500
Author: ericq 
Type: Update 
Category: LSC 
Subject: lock acquisition path for the CM servo 

In an effort to familiarize myself with the analog CM servo, I've begun to replicate Koji and Den's work from the ELOG post that this is a reply to.

I hooked POX11Q into the IN1 of the CM board. (POX is rotated by ~86 degrees in the CDS, meaning analog Q is almost perfect.)

While there, I took out the too-long delay cables Jenne introduced for REFLs 11 and 55. (Also note: when we do cable-based analog phasing in the future, we should do it on the LO side, instead of the PD input side.) I also heard a dangerously crinkly sound from the short SMA cable for REFL11, so I replaced it with a beefy looking new one I found on the SP table.

I messed with the gain and offset in the CM_SLOW input filter to get it to look just like POX11_I_ERR, and was able to lock the arm on it without an issue. I then put the SR560 between the CM and MC (30k pole, but also AC coupled, because I figure the digital loop should be doing the work down there, and don't want to kick the AO with an offset), and was able to turn on the AO path with a gain of 8dB on the CM board and 10dB on the MC board, as detailed in Koji's procedures.

I wasn't able to increase the AO gain to 9dB without breaking lock, but maybe this is ok, because by judging by the LSC filter gains, POX11 might be about 3 times bigger than POY, so maybe 8dB AO gain on POX ~ =18dB AO gain on POY? I was able to put the CM servo offset at 0, but turning on boosts promptly kicked the MC out of lock.

I'm stopping for the night; but tomorrow I'll bust out a spectrum analyzer to see if I actually have won some bandwidth with the CM servo, and check out the situation with the offsets and boosts.

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