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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: 11478     Entry time: Tue Aug 4 03:02:30 2015     In reply to: 11470     Reply to this: 11490
Author: ericq 
Type: Update 
Category: LSC 
Subject: Beat note Alignment fluctuation effects measured 

Notes from tonight's work:

  • PMC alignment tweaked. Not much gained
  • WFS/MC2 offsets tweaked after recentering beams on WFS and some hand alignment. 
  • Vertex oplevs realigned for the first time in forever
  • With an RF coupler, measured the X green beatnote to be +5dBm into the splitter. This resulted in -33dBm at the control room analyzer. 
  • Switched the ALS demod board inputs, from piping the delayed signal to the RF input, to sendingit to the LO input. This was motivated by wanting the mixer closer to compression, hopefully to reduce beatnote amplitude fluctuation sensitivity. This won some noise >100Hz.
    • This led to record ALS noise levels - X:217Hz, Y:203Hz yes
    • +2dBm into the board still leaves us some headroom for futher amplification. Board schematic lists +10dBm LO as "nominal," but maybe this isn't worth it... 
  • PRFPMI locking is still stalled at bringing in the RF signals. Debugging continues.
  • Some beatnote amplitude fluctuation investigations (see below)
  • Note to self: demod board schematics include an unspecified RF lowpass. Check out what got stuffed in there. 

I've explored the beatnote fluctuations a bit further. 

First, I realized that we already had a channel than functions much like an RF level monitor: the phase tracker Q output. I verified that indeed, the Q signal agrees with the RF monitor signals from the demod board within the phase tracker bandwidth. This simplifies things a little.

I also found that the Y beat suffers a fair bit less from these effects; which isn't too surprising given our experience with the alignment stability.

One possible caveat to my earlier conclusions is that the beatnote amplitude could be fluctuating due to real RIN of the green light transmitted through the cavity. In fact, this effect is indeed present, but can't explain all of the coherence. If it did, we would expect the DC green PDs (ALS-TR[X/Y]) to show the same coherence profile as the RF monitors, which they don't.  

The next thing I was interested was whether the noise level predicted via coherence was realistic. 

To this end, I implemented a least-squares subtraction of the RF level signal from the phase tracker output. I included a quadratic term of the RF power, but this turned out to be insiginficant. 

Indeed, using the right gain, it is possible to subtract some noise, reproducing nearly the same spectrum as the coherence based estimate. The discrepency at 1Hz is possible from 1Hz cavity RIN, as suggested by the presence of some coherence with TRX. 

However, this is actually kind of weird. In reality, I would've expected the coupling of RF level fluctuations to be more like a bilinear coupling; changing the gain of the mixer, rather than directly introducing a linearly added noise component. Maybe I just discovered the linear part, and the bilinear coupling is the left over low frequency noise... I need to think this over a little more.  

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