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Entry  Tue Nov 25 23:11:41 2014, Dmass, Noise Hunting, Cryo ISS, Anomalous intensity noise CavHz2RIN.png
    Reply  Tue Dec 2 16:05:06 2014, Dmass, Noise Hunting, Cryo ISS, ISS loops almost work 
Message ID: 1170     Entry time: Tue Dec 2 16:05:06 2014     In reply to: 1169
Author: Dmass 
Type: Noise Hunting 
Category: Cryo ISS 
Subject: ISS loops almost work 

[Dmass, Nic]

We resurrected the intensity loops on Friday. They almost work. 

Why is it difficult?

  • The AOM driver we have is only linear over a narrow range of input voltages, and requires an offset:
  • Approximately 500 mV linear range with offsets of +/- 600 mV (the driver is symmetric and does roughly the same thing with + or - inputs at the back
  • Outside of these regions we see major harmonic distortion / clipping
  • The ISS servo board has a fixed input offset (the voltage on transmission PD which it tries to to via the AOM) - can tune the power / make a voltage divider
  • The ISS servo board does NOT have any way of independently tuning the output offset
  • We resorted to fine tuning the power at transmission via the input power in order to set the offset to exactly what we wanted:
  • We drove the AOM driver in its linear range with the cavities locked, plugged the transmission into the ISS servo box, and looked at the signal coming out of the ISS servo box - we tuned it close to the correct offset for the input of the AOM driver (with a negative sign and lots of gain) by eye/scope
  • We closed the loop and gently fiddled with the input power, and were able to lock the ISS
  • IT WAS EXTREMELY FINNICKY (re: there was a very narrow range of input power that it would lock for, the spectrum was nonstationary/bouncy, and it would break after a minute or so at max)
  • We have ~5% fluctuation in RIN at transmission due to the platform swinging
  • We could re-lock by fine tuning the input power again and get another couple minutes.
  • The boosts worked.

Does it work?

  • Almost / sort of. We could get fast measurements done with it, but it will require constant babysitting and quick spectra collection
  • It does not have enough range to make it into the "healthily working" category.

 

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