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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: 11681     Entry time: Fri Oct 9 16:23:25 2015     In reply to: 11680     Reply to this: 11686
Author: ericq 
Type: Update 
Category: LSC 
Subject: ALS plant shape 

Ah, I understand it now! Since the additive offset path keeps the post-cavity frequency TF flat, the pre-cavity frequency must grow above the cavity pole, which is why ALS sees a zero. 

Ok, so this means we want to apply two lowpasses to the ALS signal for use as fast CARM control, if we want it to be capable of scalar blending with REFL11: one at ~120Hz to imitate the CARM coupled cavity pole present in REFL11, and one at ~3.8kHz to undo the "IMC cavity zero" present in ALS. 

At this point, I'm starting to prefer an active circuit to do this lowpassing; using LISO to check designs for two cascaded passive LPFs it looks like the ALS signal would have to be attenuated by a factor of ~20 at DC if we don't use resistors smaller than 1k, given the low input impedence of the CM board. 

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