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Entry  Sun Feb 16 17:40:11 2014, Koji, Update, LSC, Friday Night ALS 
    Reply  Tue Feb 18 18:52:08 2014, Jenne, Update, LSC, ALS not locking with LSC  ALSX_POXvsBeatnote.pdfALSY_POYvsBeatnote.pdfBeatNoteSpectra_ArmsLockedWithIR_28Feb2014.pdfALSvsLSC_LockingFilters.pdf
       Reply  Tue Feb 18 20:31:29 2014, Koji, Update, LSC, ALS not locking with LSC  
          Reply  Tue Feb 18 23:27:14 2014, Jenne, Update, LSC, ALS not locking with LSC  ALSX_POXvsBeatnote_withEXCtfs.pdf
Message ID: 9646     Entry time: Tue Feb 18 18:52:08 2014     In reply to: 9641     Reply to this: 9647
Author: Jenne 
Type: Update 
Category: LSC 
Subject: ALS not locking with LSC  

Koji mentioned to me (and elogged) that he was unsuccessful locking the ALS using the LSC servos.  He suggested I look into this.

So, rather than just looking at the transfer function between POX or POY and the green beatnotes at a single frequency, I did a whole transfer function.  The point was to see if the TF is flat, and if we get any significant phase lag in the transfer from c1als to c1lsc.  (c1als is running on the IOO machine, so an RFM connection is involved in getting it over to the LSC machine.)

In the first figure, I have plotted POX vs. Beatnote_PHASE_OUT (ALS error signal, still in the c1als model), and POX vs. ALSX_IN1 (the ALS error signal, after transfer over to the c1lsc model).  You can see that we have a little phase lead in the blue transfer function, and fairly significant phase lag in the red (red is after transfer over to the lsc model).  In the grand scheme of things, the magnitude is fairly flat, however that is not perfectly true - the peaks seen near 50 Hz and 300Hz are repeatable.  The relative phase lag between the "BEATX" version of the signal in the ALS model, and the "ALSX" version of the signal in the LSC model is 15 degrees at 200 Hz, which corresponds to 33 usec.   


The second figure is the same as the first, except for the Yarm.  The relative phase lag between the ALS version of the error signal and the LSC version is 16 degrees at 200 Hz, which is about 35 usec.


As a side note, before trying any ALS locking, I took a spectrum of the beatnote (in the ALS model) while the arms were locked with IR:


To check things, I made sure that I could lock the Xarm ALS using the old ALS system - I was able to do so.  (Has someone put the "watch" script as a constantly-on thing?  It's kind of nice not to have to turn it on, although we'll need to change it to turn off the LSC versions of the servos eventually). 

Then, I tried locking the Xarm using the LSC system (using only FM5 of the regular LSC-XARM filter bank).  Like Koji, I was not able to acquire lock.  As a next step, I copied all of the LSC-XARM filters into an empty filter module, LSC-XXXDC (the first one on the list underneath LSC-XARM), and copied over the ALS Xarm filters to the LSC Xarm filter bank.  I then tried to acquire lock, but am unable to get it to stay.  Using the ALS system, when you put in a small gain, the beatnote starts to settle down, and as you increase the gain, the beatnote stops moving (as seen on the spectrum analyzer) almost completely.  However, using the LSC system, the beatnote never really stops moving or settles down.  And if I increase the gain, I push the ETM hard enough that I lose green lock.  I have put the regular LSC filters back for now.

Here is a plot from Foton comparing the FM5 filter modules from the LSC-XARM (regular IR locking) and the ALS-XARM servo.  They are pretty different, and have 10 degrees of phase difference at 200 Hz, because 2 of the 3 poles are complex in the LSC version, while the ALS version is just a single real pole.


Anyhow, I am declaring it to be dinnertime, and I plan to return in a few hours. Since I put the regular LSC filters back (since I'm going to have to realign after dinner anyway), the IFO should be in its nominal state if anyone wants to come in and play with it.

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