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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: 9499     Entry time: Fri Dec 20 01:24:11 2013     Reply to this: 9500   9501
Author: Den 
Type: Update 
Category: LSC 
Subject: high bandwidth loop achieved for yarm 

Koji, Den

CM Servo with POY11 successfully engaged. UGF: ~15kHz.


Tonight we decided to repeat one arm locking using high-bandwidth CM servo. We low-passed AO signal to avoid saturations of the EOM. We tried different configurations that compromise between noise and loop phase margin and ended up with a pole at 30kHz. SR560 is used as a low-pass filter.

Another problem that we faced was big (~2.6V) electronic offset at the input of 40:4000 BOOST. Once engaged, cavity would be kicked out of lock. We calibrated this offset to be almost half linewidth of the cavity (~300pm). To avoid lock loss during engaging the boost we increased common mode gain to maximum (31 dB).

Measured OL is attached. UGF is 15kHz, phase margin is 60 degrees. We have also simulated evolution of loop shape during bringing AO path. Plot is attached.

The final procedure is

  • set common gain up to 31dB, AO gain to 8dB, MC IN2 gain 10dB, CM offset 0.7V
  • lock arm with CM slow path with bandwidth of 200 Hz
  • enable AO path, gradually increase slow and fast gains by 12 dB
  • enable boost
Attachment 1: CM_OL_meas.pdf  15 kB  | Hide | Hide all | Show all
CM_OL_meas.pdf
Attachment 2: cm_ol_sim.pdf  223 kB  | Hide | Hide all | Show all
cm_ol_sim.pdf
Attachment 3: CM_slow_fast_cross.pdf  9 kB  | Show | Hide all | Show all
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