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Message ID: 1897     Entry time: Tue Jan 26 11:47:12 2021
Author: Paco 
Type: DailyProgress 
Category: General 
Subject: low quality PDH error signal 

After getting what looked like a decent cavity reflection signal, installed RFPD yesterday. For this, removed the lens that was right before the PD because the RFPD area is large enough, but keep ND filter in place. Powered with +- 18 VDC and monitor DC out on the scope, and RF out is sent to the IF of the mixer in the PDH box. For the LO, split the Marconi RF output and connected the demodulated signal into Ch2 of the scope in hopes that there was an error signal.

A hint of the error signal is present (blue trace below), although deeply buried in line noise (and harmonics up to ~180 Hz) so there really are two things to optimize now -->

  1. Line noise (hunting for ground loops or equipment, e.g. power supplies, analyze LO spectrum before/after splitters, mixers, etc...)
  2. Mode matching (this was the first reaction, as improving the cav refl SNR by means of mode matching makes a better pdh err signal)

Other things attempted so far -->

  • Switched mixers, splitter, and RF cables
  • Bypass the phase shifter completely
  • Scan LO phase
  • Floated RFPD power supply
  • Floated PDH box power supply (really only affecting the phase shifter if anything, though unlikely to matter at this point)
Attachment 1: poor_PDH_err.jpg  3.804 MB  | Hide | Hide all
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