40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  40m Log  Not logged in ELOG logo
Entry  Mon Sep 7 23:49:14 2020, gautam, Update, BHD, A first look at RF44 scheme 
    Reply  Tue Sep 8 01:31:43 2020, Koji, Update, BHD, A first look at RF44 scheme 
    Reply  Wed Sep 9 00:05:18 2020, gautam, Update, BHD, More notes on the RF44 scheme IMG_3397.JPGphaseNoisePSD.pdfmagnitudeHist.pdfLOpowerDrift.png
       Reply  Tue Sep 15 22:11:52 2020, gautam, Update, BHD, More notes on the RF44 scheme RF44.pdf
Message ID: 15575     Entry time: Tue Sep 15 22:11:52 2020     In reply to: 15565
Author: gautam 
Type: Update 
Category: BHD 
Subject: More notes on the RF44 scheme 


After more trials, I think the phase tracker part used to provide the error signal for this scheme needs some modification for this servo to work.


Attachment #1 shows a block diagram of the control scheme.

I was using the "standard" phase tracker part used in our ALS model - but unlike the ALS case, the magnitude of the RF signal is squished to (nearly) zero by the servo. But the phase tracker, which is responsible for keeping the error signal in one (demodulated) quadrature (since our servo is a SISO system) has a UGF that is dependent on the magnitude of the RF signal. So, I think what is happening here is that the "plant" we are trying to control is substantially different in the acquisition phase (where the RF signal magnitude is large) and once the lock is enabled (where the RF signal magnitude becomes comparitively tiny).

I believe this can be fixed by dynamically normalizing the gain of the digital phase tracking loop by the magnitude of the signal = sqrt(I^2 + Q^2). I have made a modified CDS block that I think will do the job but am opting against a model reboot tonight - I will try this in the daytime tomorrow. 

I'm also wondering how to confirm that the loop is doing something good - any ideas for an out-of-loop monitor? I suppose I could use the DCPD - once the homodyne phase loop is successfully engaged, I should be able to drive a line in MICH and check for drift by comparing line heights in the DCPD signal and RF signal. This will requrie some modification of the wiring arrangement at 1Y2 but shouldn't be too difficult...

The HEPAs, on the PSL table and near ITMY, were dialled down  / turned off respectively, at ~8pm at the start of this work. They will be returned to their previous states before I leave the lab tonight.

Attachment 1: RF44.pdf  118 kB  Uploaded Wed Sep 16 10:40:34 2020  | Hide | Hide all
ELOG V3.1.3-