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Entry  Tue May 26 02:15:36 2020, gautam, Update, LSC, Lock acquisition portal entry 
    Reply  Tue May 26 02:31:00 2020, gautam, Update, LSC, Lock acquisition sequence PRFPMIlock_1274418200_1274418550.pdf
       Reply  Wed May 27 20:14:51 2020, Koji, Update, LSC, Lock acquisition sequence 
    Reply  Tue May 26 02:37:19 2020, gautam, Update, LSC, DARM loop measurement and fitting DARM_TF.pdfDARM_TF_breakdown.pdf
    Reply  Tue May 26 03:01:35 2020, gautam, Update, LSC, CARM loop  CARM_OLTF.pdf
       Reply  Wed Jun 3 01:46:14 2020, gautam, Update, LSC, CARM loop  CM_loop_topology.pdfCARM_TFs.pdfCARM_OLTF.pdfCARM_xover.pdfCARM_OLG_evolution.pdf
    Reply  Tue May 26 03:06:59 2020, gautam, Update, LSC, PRFPMI sensing matrix PRFPMI_20200524sensMat.pdfPRFPMI_20200524sensMatHistograms.pdf
    Reply  Tue May 26 03:26:58 2020, gautam, Update, LSC, Preliminary noise budget PRFPMI_NB.pdf
    Reply  Tue May 26 14:32:44 2020, gautam, Update, LSC, Arm transmission RIN armRIN.pdf
       Reply  Wed May 27 19:36:33 2020, Koji, Update, LSC, Arm transmission RIN 
          Reply  Thu May 28 18:36:45 2020, gautam, Update, LSC, Arm transmission RIN PRFPMIcorner_ASC_PIT_1274419354_1274419654.pdfPRFPMIcorner_ASC_YAW_1274419354_1274419654.pdfPRFPMIcorner_ASC_coherence_1274419354_1274419654.pdf
             Reply  Fri May 29 00:34:57 2020, rana, Update, LSC, Arm transmission RIN 
             Reply  Wed Jun 3 02:14:32 2020, gautam, Update, ASC, PRC ASC improves arm transmission RIN PRC_ASCsignals.pdfarmRIN_PRC_ASC.pdfPRFPMIcorner_ASC_PIT_1275190251_1275190551.pdfPRFPMIcorner_ASC_YAW_1275190251_1275190551.pdfPRFPMIcorner_ASC_coherence_1275190251_1275190551.pdf
                Reply  Fri Jun 19 16:30:09 2020, gautam, Update, ASC, Some thoughts about ASC sensingResponse.pdfsensingResponse_torque.pdf
                   Reply  Tue Jul 7 14:06:10 2020, gautam, Update, ASC, Some more thoughts about ASC ITM_OL_DCcoupling.png
    Reply  Tue May 26 16:00:06 2020, gautam, Update, LSC, Power buildup diagnostics PRFPMIcorner_DC_1274419354_1274419654.pdfPRFPMIcorner_SB_1274419354_1274419654.pdf
       Reply  Wed May 27 17:41:57 2020, Koji, Update, LSC, Power buildup diagnostics 
          Reply  Wed Jun 3 02:08:00 2020, gautam, Update, LSC, Power buildup diagnostics PRFPMIcorner_DC_1275190251_1275190551.pdfPRFPMIcorner_SB_1275190251_1275190551.pdf
    Reply  Wed Jun 3 01:34:53 2020, gautam, Update, LSC, Lock acquisition update portal 
       Reply  Wed Jun 3 03:29:26 2020, Koji, Update, LSC, Lock acquisition update portal 
          Reply  Wed Jun 3 11:40:56 2020, gautam, Update, LSC, Lock acquisition update portal 
             Reply  Wed Jun 3 18:49:47 2020, gautam, Update, LSC, PRG and CARM signal sign armCavReflectivities.pdfIFOreflectivities.pdfPDHerrSigs.pdfPRGvsLoss_finesse.pdf
Message ID: 15366     Entry time: Wed Jun 3 01:46:14 2020     In reply to: 15351
Author: gautam 
Type: Update 
Category: LSC 
Subject: CARM loop  

Summary:

The CARM loop now has a UGF of ~12 kHz with a phase margin of ~60 degrees. These values of conventional stability indicators are good. The CARM optical gain that best fits the measurements is 9 MW/m.

I've been working on understanding the loop better, here are the notes.

Details:

Attachment #1 shows a block diagram of the loop topology.

  • The "crossover" measurement made at the digital CARM error point, and the OLG measurement at the CM board error point are shown.
  • I've tried to include all the pieces in the loop, and yet, I had to introduce a fudge gain in the digital path to get the model to line up with the measurement (see below).

Attachment #2 shows the OLGs of the two actuation paths.

  • Aforementioned fudge factor for the digital path is included.
  • For the AO path, I assumed a value of the PDH discriminant at the IMC error point to be 13 kHz/V, per my earlier measurement. 
  • I trawled the elog for the most up-to-date info about the IMC servo (elog9457, elog13696, elog15044) and CM board, to build up the model. 

Attachment #3 and #4 show the model, overlaid with measurements of the loop OLG and crossover TF respectively.

  • No fitting is done yet - the next step would be to add the delay of the CDS system for the digital path, and the analog electronics for the AO path. Though these are likely only small corrections.
  • For the crossover TF - I've divided out the digital filters in the CARM_B filter bank, because the injection is made downstream of it (see Attachment #1).
  • There is reasonably good agreement between model and measurement.
  • I think the biggest source of error is the assumed model for the IMC OLTF.

Attachment #5 shows the evolution of the CARM OLG at a few points in the lock acquisition sequence.

  • "Before handoff" corresponds to the state where the primary control is still done by the ALS leg, but the REFL11 signal has begun to enter the picture via the CARM_B path.
  • "IN2 ramped" corresponds to the state where the AO path gain (=IN2 gain on the IMC servo board) has been ramped up to its final value (+0 dB), but the overall loop gain (=IN1 gain on the CM board) is still low. So this is preparation for high bandwidth control. Typically, the arm powers will have stabilized in this state, but ALS control is still on.
  • "Pre-boost" corresponds to an intermediate state - ALS control is off, but the low frequency boosts have not yet been enabled. I typically first engage some ASC to stabilize things somewhat, and then turn on the boosts.
  • "Final" - self explanatory.

Next steps:

Now the I have a model I believe, I need to think about whether there is any benefit to changing some of these loop shapes. I've already raised the possibility of changing the shape of the boosts on the CM board, with which we could get a bit more suppression in the 100 Hz - 1kHz region (noise budget of laser frequency noise --> DARM required to see if this is necessary). 

Attachment 1: CM_loop_topology.pdf  67 kB  Uploaded Wed Jun 3 03:04:40 2020  | Hide | Hide all
CM_loop_topology.pdf
Attachment 2: CARM_TFs.pdf  108 kB  Uploaded Wed Jun 3 03:04:50 2020  | Hide | Hide all
CARM_TFs.pdf
Attachment 3: CARM_OLTF.pdf  91 kB  Uploaded Wed Jun 3 03:05:06 2020  | Hide | Hide all
CARM_OLTF.pdf
Attachment 4: CARM_xover.pdf  97 kB  Uploaded Wed Jun 3 03:05:14 2020  | Hide | Hide all
CARM_xover.pdf
Attachment 5: CARM_OLG_evolution.pdf  121 kB  Uploaded Wed Jun 3 03:05:21 2020  | Hide | Hide all
CARM_OLG_evolution.pdf
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