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Message ID: 2529     Entry time: Tue Feb 11 18:21:17 2020
Author: anchal 
Type: DailyProgress 
Category: PMC 
Subject: Can't keep my mind of this; Why PMC Error signal has these oscillations? 

The PMC error signals have some weird broadened oscillations in them:

Above, pink is North and green is South PMC Error Signal taken from Mixer Out port.

Spectrum measurement:

  • I took the spectrum using SR785 of the Mixer Out port of the two PMC loops.
  • Next, I measured the cross-spectrum density of the two error signals.
  • I wasn't sure though if the peak in the cross-spectrum is just because higher magnitude signal on both error signals, so I divided it by the magnitude of measured FFT of the two signals to get a unitless cross spectrum in the third plot.
  • I'm not sure though how to interpret the cross-spectrum in either case.
  • The oscillations could be a manifestation of some common-mode noise on the table (maybe some mechanical oscillation).
  • Or there could be a systematic bad UGF around 480 Hz.
  • In any case, can we actually relate the bump seen in beatnote noise near 400 Hz to these oscillations?


Attachment 2: PMC_Error_Signal_Analysis.pdf  74 kB  | Show | Show all
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