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Message ID: 289     Entry time: Wed Aug 18 00:07:37 2010
Author: Frank 
Type: DailyProgress 
Subject: noise level of beat signal 

i tried a couple of things today to figure out what determines the low frequency noise level of the beat measurement.
I did the following steps, nevertheless the noise level didn't change:

  • re-aligned everything starting with the PMC
  • coupling into the cavities is >90%
  • matched power levels in both paths to match power at RF photodetector
  • re-positioned the RF photodiodes for locking the caviries
  • added beam dumps for beams reflected from RF photodiodes
  • optimized overlap of both beams on RF photodiode (beat)
  • changed mixer from 7dbm model to 13dbm model
  • removed cable going to DAQ from DC output of RCAV RF photodiode as it is causing ~36MHz oscillation of RF PD (even with no light)
  • changed gains for both loops, ACAV and RCAV
  • added first QPD at pick-off right in front of periscope into chamber for RCAV to check pointing

things to be checked tomorrow:

  • power fluctuations of beams into both cavities
  • power fluctuations of transmitted beam
  • pointing of both beams (should also show up in power noise spectrum in transmission)
  • measure TF of all loops with current setting for noise estimation

other things to change:

  • move temp readout to VME based stuff
  • rename VCO input monitor signal channel name
  • remove PSL RT stuff from fb0 to see if networking problems are caused by that
  • replace current cables for temp readout due to loose connection somewhere on table
  • check ACAV RF photodiode
  • change PID controller variable names in order to add other software loops like feedback from VCO input signal to RCAV temp setpoint. Right now we have already 4 software loops:
  1. feedback to laser for RCAV using FAST actuator signal
  2. temp ctrl for RCAV
  3. temp ctrl for ACAV
  4. feedback to laser temp for ACAV using VCO input signal (used to track ACAV temp tuning to match RCAV resonance)


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