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Entry  Fri Sep 18 13:33:17 2015, ericq, Update, LSC, Fast ALS troubles - Noise at 36kHz IRlock_noises.pdf
    Reply  Fri Sep 18 16:08:41 2015, ericq, Update, LSC, Fast ALS troubles - Noise at 36kHz PRMIlock_REFLspectra.pdfIMCspectrum.pdf
       Reply  Fri Sep 18 19:15:35 2015, rana, Update, LSC, Fast ALS troubles - Noise at 36kHz 
    Reply  Tue Sep 29 16:52:49 2015, ericq, Update, LSC, Fast ALS troubles - unknown zero OLG_fastALS.pdf
       Reply  Tue Sep 29 18:03:11 2015, rana, Update, LSC, use LISO Screen_Shot_2015-09-29_at_5.55.34_PM.png
          Reply  Wed Sep 30 10:00:02 2015, ericq, Update, LSC, used LISO 
       Reply  Fri Oct 2 03:29:16 2015, ericq, Update, LSC, Fast ALS progress - AO path crossed over, but no high BW crossedover.pdf
          Reply  Fri Oct 9 13:31:21 2015, ericq, Update, LSC, ALS plant shape xarm_plantTFs.pdf
             Reply  Fri Oct 9 14:50:18 2015, Koji, Update, LSC, ALS plant shape 
                Reply  Fri Oct 9 16:23:25 2015, ericq, Update, LSC, ALS plant shape 
                   Reply  Tue Oct 13 16:28:21 2015, ericq, Update, LSC, Fast ALS pomona liso_lp.zip2LPfit.pdf
Message ID: 11648     Entry time: Tue Sep 29 16:52:49 2015     In reply to: 11620     Reply to this: 11649   11658
Author: ericq 
Type: Update 
Category: LSC 
Subject: Fast ALS troubles - unknown zero 

Fast ALS control continues to elude me. 

I fixed my LPF to take the input impedance of the CM board input into account; this unfortunately results in about -12dB DC gain of the ALS signal due to voltage-divider-y things, but by my estimation, this still puts the DFD noise above the input-referred voltage noise of the input AD829 on the CM board, so it'll do for now. The 120Hz pole shows up as expected when comparing the usual digital channels and the CM_SLOW output, and is digitally compensated with a zero at 120Hz (with a digital pole at 5k so nothing blows up). 

However, there seems to be some zero in the analog path somewhere that spoils the loop shape for the AO path. Here's a measurement of the X arm OLG from 10-100kHz, when the digital control is happening with ~100Hz UGF via ALS X I -> CM IN2 -> CM_SLOW -> LSC_CARM -> ETMX, and there is some AO action via ALS X I -> CM IN2 -> IMC IN2

The peak is recognizable as the gain peaking in the IMC servo (and changes predictably with changes to the IMC crossover and loop gains), which is expected. However, one can see that the magnitude is roughly flat before the peak, and the phase is around 0. With the 1/f LPF, we should see some downward slope and phase starting around -90. 

Thus, there must be some zero in the fast or common path, maybe at a few kHz where the digital loop wouldn't really see its effect. I'm not sure what it could be at this point in time.

One thought I had is that I never really checked the TF of DFD response to frequency modulation of the RF beat. I used an SR785 to drive the external FM input of a Fluke 1061A synthesizer, and saw it to be totally flat from 1-100kHz with carriers from 30-100MHz, so that should be fine. (For a little while I was confused by what seemed to be some heavy high-passing going on, but it turns out that the Fluke just can't push much low frequency FM; the manual says -3dB at 20Hz.)

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