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Message ID: 2399     Entry time: Fri Aug 23 18:15:49 2019
Author: anchal 
Type: DailyProgress 
Category: BEAT 
Subject: Beatnote after a while 

The cavity temperature control (aftter the last fixes by Andrew) seem to be working good actually now that the Vacuum Can temperature is stabilized nicely. SO I didn't want to interfere with the PID's job which it seems is trying to reach to the set point almost critically. However, today, the beatnote came below 125 MHz, so we were in range with New Focus 1811 to take the spectrum. So I did it.

Two measurements

I used the coupled output from 20 dB coupler to feed the moku and use it's phase meter along with SR785 witht he previous PLL setup. Since the beatnote was still drifitng by around 10 kHz/24 sec, I took spectrum with linewidth of 1 Hz and used 20 averages to catch the PLL frequency noise in between its jumps. Simultaneously (almost), I took measurements with moku also to see if we can reliably switch over to moku. Good thing about moku is that it is faster in adjusting it's carrier frequency to lock to the signal and hence the jumps are unnoticeable. The attached plots are the measurements.

Uncertainty in moku's ASD plots!

Scott and I have written a modified PSD calculation function, which does everything same as a normal weltch function would do, but on top of it, it provides 15.865% and 84.135% percentile of all the individual segments the function used to calculate PSD. Also, the reported value is median and not mean. Further, this function implements welch function with different sizes of npersegment to ensure more averaging at higher frequencies and equal number of points in each decade. All this is done in mokuReadFreqNoise.py which uses modeifiedPSD.py. Linear detrending of data is also used before calculating the PSDs from the timeseries data provided by moku.


  • I think we can safely switch over to using Moku for measuring beatnote frequency noise, given it is available.
  • Beatnote obviously doesn't look so good. But at the point, the FSS aren't working as expected.
  • There is a weird peak at around 500 Hz which wasn't there before.
  • I'll add the noisebudget with new calculations using different values of Shear and Bulk Loss Angles soon. It is kind of difficult to get these values though.
  • My plan is to keep this functioning state all the time. I'll make sure the cavities are locked with good mode matching and near the desired beatnote frequency.
  • Then I'll focus on the known issues and sources of error and will keep monitoring the beatnote changes every 2-3 days.

Code and Data

Attachment 1: ComparisonOfMokuandSR785BeatnoteSpectrum.pdf  215 kB  Uploaded Fri Aug 23 19:34:15 2019  | Hide | Hide all
ComparisonOfMokuandSR785BeatnoteSpectrum.pdf ComparisonOfMokuandSR785BeatnoteSpectrum.pdf ComparisonOfMokuandSR785BeatnoteSpectrum.pdf ComparisonOfMokuandSR785BeatnoteSpectrum.pdf ComparisonOfMokuandSR785BeatnoteSpectrum.pdf
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