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Entry  Wed Jun 10 03:19:58 2015, ericq, Update, LSC, AUX PDH error measured in CDS AUXspectra.png
    Reply  Fri Jun 12 17:09:58 2015, ericq, Update, LSC, Beatbox needs whitening gain beatGain.png
       Reply  Sat Jun 13 18:37:46 2015, ericq, Update, LSC, Beatbox needs whitening gain ADCdiagnosis.png
          Reply  Sat Jun 13 23:52:14 2015, ericq, Update, LSC, Beatbox needs whitening gain 
Message ID: 11355     Entry time: Fri Jun 12 17:09:58 2015     In reply to: 11351     Reply to this: 11356
Author: ericq 
Type: Update 
Category: LSC 
Subject: Beatbox needs whitening gain 

Short entry just to preview a new development; more detail about this investigation will soon follow. 

The beatbox I and Q signals are too small at the ADC! I was able to reduce the RMS out of loop ALS sensitivity (arm locked on POX) by 300Hz with G=10 SR560s between the beatbox output and the ADC whitening chassis input. Increasing the beat note amplitude via RF amplifier had no positive effect. 

There is still a reasonable gap between this and the beatbox's noise levels, as measured with a marconi. There may be additional headroom for whitening gain; the SR560 maximum output range is smaller than the ADC input range. 

The high frequency noise has >0.5 coherence with the PDH error signal above a kHz or so, but not much below that. 

We should probably either modify the output whitening of the beatbox, or introduce some (variable?) whitening gain in a seperate circuit. 

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