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Message ID: 2292     Entry time: Wed Jan 23 18:13:48 2019
Author: anchal and awade 
Type: DailyProgress 
Category: FSS 
Subject: FSS Gain and Offset tuning 

Today we checked FSS Common path and Fast path transfer functions to see unity gain frequencies and any possible optimizations.


North Path:

  • With new photo detector at 36 MHz in, we found that we are able to maximize the common gain setting the voltage control to +10V giving us ~160 kHz unity gain frequency. This was around 30 kHz before.
  • For the fast path, I set the gain voltage control to 1V to keep unity gain frequency around 7-9 kHz.
  • I tuned the offset to bring DC offset at OUT1 of common path to zero using a multimeter.
  • RMS error of the mixer channel (just at the output of common gain stage) is found to be ~12 mV (averaged for 10s with an oscilloscope)

South Path:

  • Here, we found that the FSS loop was completely screwed up. The error signal wasn't high enough and the transfer function had a weird drop between 3kH to 6kHz.
  • I optimized input polarization to the EOM and faraday isolator again and optimized the phase delay cable by adding a short cable as well.
  • I still couldn't manage to get PDH error signal peak to peak more than ~180 mV. With about 1.1 mW light coming to the photo detector which has transimpedance of ~1.5 kOhm, this would be just 0.07 rad of modulation depth.
  • I checked RF monitor port of the EOM driver which had a signal strength of -4.5 dBm. If we use 155 coupling factor as mentioned in DCC document, this corresponds to ~38.5 dBm power going into EOM and it requires 36 dBm only for 0.3 rad modulation depth.
  • So something is not right in our calculation and/or EOM phase modulation happening.
  • However, after doing these optimizations, I was able to get a good transfer function at the common gain stage and was able to maximize common gain setting the voltage control to 10V giving us ~ 77 kHz of unity gain frequency.
  • For the fast path, I was unable to get a good transfer function due to poor signal to noise ratio.
  • I tuned the offset to bring DC offset at OUT1 of common path to zero using a multimeter.
  • RMS error of the mixer channel for this path is surprisingly lower than North path, at around 6 mV (averaged over 10s with an oscilloscope).

I have switched off auto gain and offset tuner services. I'll start a beat note measurement today to see if our changes in FSS loop and reattaching wideband photo detector changed anything.

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