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Entry  Tue Feb 17 00:24:47 2015, rana, jenne, Update, LSC, ALS Fool filter updated for more cancellation ALSfoo_150216.png15.png
    Reply  Wed Feb 18 01:58:51 2015, ericq, Update, LSC, ALS Fool single arm performance  ALSfool_spectra.pngALSfool_kick.pngALSfool_LoopShapes.pngALSfool_Feb182015.zip
       Reply  Wed Feb 18 17:51:40 2015, ericq, Update, LSC, ALS Fool impulse response ALSfool_fasterKick.png
          Reply  Wed Feb 18 19:06:40 2015, Koji, Update, LSC, ALS Fool impulse response latex-image-1.png
Message ID: 11047     Entry time: Wed Feb 18 17:51:40 2015     In reply to: 11046     Reply to this: 11048
Author: ericq 
Type: Update 
Category: LSC 
Subject: ALS Fool impulse response 

Koji raised a good question about the step response I wrote about. Namely, if the UGF of the ALS servo is around 100Hz, we would expect it to settle with a characteristic time on the order of tens of milliseconds, not seconds, as was seen in the plot I posted. 

I claim that the reason for the slow response was the fact that the feedforward FM stayed on after the kick, despite the MC filter bank being triggered off. Since it has filters that look like a oscillator at 1Hz, the ringdown or exponential decay of this filter's output in response to the large impulsive output of the PDH control signal just before being triggered down would slowly push the ALS error signal around through the feedforward path. 

Given this reasoning, this should be helped by adding output triggering to the FF filter that uses the MC trigger matrix row, as I wanted to do anyways. I have now added this into the LSC model (as well as DQ at 2kHz for the MC_CTRL_FF_OUT), and the impulse response is indeed much quicker. 

In the following plot, I hit ETMY with a five sample, 5000 amplitude, impulse (rather than a step, as I did yesterday). The system comes back to PDH lock after ~40ms. 

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