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Entry  Mon Dec 2 00:27:20 2019, gautam, Update, ALS, Green ALS resurrection ALSnoiseComparison.pdfALS_TR_RIN.pdfGreemAM.pdf
    Reply  Mon Dec 2 18:20:29 2019, gautam, Update, ALS, EY uPDH post mixer LPF EY_uPDH_LPF.pdfEY_uPDH_OLTF.pdf
       Reply  Mon Dec 2 20:18:18 2019, rana, Update, ALS, EY uPDH post mixer LPF 
          Reply  Tue Dec 3 18:15:42 2019, gautam, Update, ALS, EY uPDH post mixer LPF 
             Reply  Tue Dec 3 21:28:24 2019, gautam, Update, ALS, EY uPDH post mixer LPF uPDH.pdf
       Reply  Tue Dec 3 00:10:15 2019, Koji, Update, ALS, EY uPDH post mixer LPF 
          Reply  Tue Dec 3 00:51:25 2019, gautam, Update, ALS, EY uPDH post mixer LPF 
Message ID: 15068     Entry time: Tue Dec 3 21:28:24 2019     In reply to: 15066
Author: gautam 
Type: Update 
Category: ALS 
Subject: EY uPDH post mixer LPF 

Here are some loop transfer functions. I basically followed the decomposition of the end PDH loop as was done in the multi-color metrology paper. There is no post-mixer low pass filter at the moment (in my model), but already you can see that the top of the phase bubble is at ~10 kHz. Probably there is still sufficient phase available at 30 kHz, even after we add an LPF. In any case, I'll use this model and set up a cost function minimization problem and see what comes out of it. For the PZT discriminant, I used 5 MHz/V, and for the PDH discriminant, I used 40 uV/Hz, which are numbers that should be close to what's the reality at EY.

(i) Note that there could be some uncertainty in the overall gain (VGA stage in the servo).

(ii) For the cavity pole, I assumed the single pole response, which Rana points out isn't really valid at ~1 MHz, which is close to the next FSR

(ii) The PZT response is approximated as a simple LPF whereas there are likely to be several sharp features which may add/eat phase. 

Quote:

 I'll work on preparing a notebook.

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