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Entry  Thu May 1 03:15:03 2014, Jenne, Update, LSC, Yarm locking with CM board Yarm_CMlocking_spectra_30Apr2014_copy.pdfYarm_CMlocking_TFs_30Apr2014_copy.pdf
    Reply  Thu May 1 03:23:07 2014, ericq, Update, LSC, Yarm locking with CM board 
    Reply  Thu May 1 13:03:34 2014, Jenne, Update, LSC, MC board pulled for AC coupling 
       Reply  Thu May 1 14:45:44 2014, Jenne, Update, LSC, MC board back in IMG_1394.JPGIMG_1395.JPGIMG_1398.JPGIMG_1402.JPG
    Reply  Thu May 1 16:41:35 2014, ericq, Update, LSC, Yarm locking with CM board AOsignsSame.pdfAOsignsOpposite.pdfloopModeling.m
       Reply  Thu May 1 17:00:05 2014, rana, Update, LSC, Yarm locking with CM board 
Message ID: 9893     Entry time: Thu May 1 16:41:35 2014     In reply to: 9888     Reply to this: 9894
Author: ericq 
Type: Update 
Category: LSC 
Subject: Yarm locking with CM board 

 (Edited this post; Forgot to account for the FMs other than 4 and 5... it now agrees better!)

I did some quick MATLAB simulation of the relevant loops to try and understand what was going on. I put the digital UGF around 200Hz, and then brought in the AO path with both signs. 

In these plots, blue is digital only, green is AO+digital with the crossover happening at the UGF, and red is the AO gain set to five times of what it was in the green curve. 


Based on the phase curves in the loop measurements, I would be inclined to say the pink -AO case corresponds to the opposite sign plot, and the +AO case to the same sign plot. 

This correspondence also holds for the appearance of the peaks in the noise curves, the Opposite sign case has a dip in loop gain at ~50Hz (pink curve, -AO), same sign around ~30Hz (brown curve, +AO). 

However, both of these look like they become unstable at some point in the transition! This agrees with our experience last night...

I'll fiddle around and try to come up with some compensating digital filter that will make the Opposite sign scenario work. 

The MATLAB code I used to make these plots is attached. 

Attachment 3: loopModeling.m  2 kB  Uploaded Thu May 1 17:58:18 2014  | Hide | Hide all
clear all

cycleT = 60e-6;

% AI, AA shapes from http://nodus.ligo.caltech.edu:8080/40m/8555
[z,p,k] = ellip(4,4,60,2*pi*7570,'s');
AI = zpk(z,p,k*10^(4/20)) * zpk([],-2*pi*13e3,2*pi*13e3);
AI.OutputDelay = 1*cycleT;

[z,p,k] = ellip(8,0.001,80,2*pi*7570,'s');
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