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Entry  Thu Jul 16 03:01:57 2015, ericq, Update, LSC, Old beatbox hooked back up beat_comparison.pngaLIGO_vs_beatbox.xml.zip
    Reply  Thu Jul 23 03:09:05 2015, ericq, Update, LSC, aLIGO demod board lives! beatbox_vs_aLIGO.png
       Reply  Thu Jul 30 14:42:03 2015, ericq, Update, LSC, aLIGO demod board lives! partialALSbudget.pngdemodDriveLevels.pngALScoherences.pngpartialALSbudget.zip
          Reply  Thu Jul 30 15:58:00 2015, ericq, Update, LSC, Beat note Alignment fluctuation effects measured RF_RIN.pngRF_RINspec.pngRFampCoh.xml.zip
             Reply  Tue Aug 4 03:02:30 2015, ericq, Update, LSC, Beat note Alignment fluctuation effects measured coherences.pnglinX.png
                Reply  Tue Aug 11 02:40:29 2015, ericq, Update, LSC, 50m delay lines - Rough calibrations 
Message ID: 11470     Entry time: Thu Jul 30 15:58:00 2015     In reply to: 11468     Reply to this: 11478
Author: ericq 
Type: Update 
Category: LSC 
Subject: Beat note Alignment fluctuation effects measured 

However, I wonder how much of the low frequency noise can be explained by instability of the beat alignement on the PSL table, and how this might be quantified. 

I followed my hunch, and the truth comes out.

I recalled that the aLIGO demod board has a handy DB9 output on the back panel for the detected power at the RF and LO inputs. I hooked this up into the BEATY ADC channels while checking the ALSX spectrum in IR lock. 

This is assuredly the limiting factor in our ALS sensitivity.

Note: I'm calling the fluctuations of the beatnote amplitude "RF Amplitude RIN," to put things in reasonble units. I haven't looked up the board's conversion of dBm to V, but the LO should be around 0dBm in this measurement. 

The coherence between the phase tracker output and the LO amplitude is significant over a broad range, mostly dipping where real cavity motion peeks up into the spectrum. 

Also, the feature from 10-100Hz in the RIN spectrum is one I've often seen directly in ALS spectra when beatnote alignement is bad or the beatnote frequency is high, convincing me further that this is what's to blame. 

So: what do we do? Is there anything we can do to make the green alignment more stable?

Attachment 1: RF_RIN.png  171 kB  | Hide | Hide all
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Attachment 2: RF_RINspec.png  107 kB  | Hide | Hide all
RF_RINspec.png
Attachment 3: RFampCoh.xml.zip  2.499 MB  Uploaded Fri Jul 31 12:19:03 2015
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