[time-nuts] Allan variance by sine-wave fitting

Tom Van Baak tvb at LeapSecond.com
Mon Feb 26 05:43:04 EST 2018

This note is a follow-up to Ralph Devoe's ADEV posting earlier this month.

It's a long story but last week I was in the Bay Area with a car full of batteries, BVA and cesium references, hp counters, and a TimePod. I was able to double check Ralph's Digilent-based ADEV device [1] and also to independently measure various frequency standards, including the actual 5065A and 5071A that he used in his experiment.

For the range of tau where we overlap, his ADEV measurements closely match my ADEV measurements. So that's very good news. His Digilent plot [2] and my TimePod / TimeLab plot are attached.

Note that his Digilent+Python setup isn't currently set up for continuous or short-tau measurement intervals -- plus I didn't have my isolation amplifiers -- so we didn't try a *concurrent* Digilent and TimePod measurement.

I'm sure more will come of his project over time and I hope to make additional measurements using a wider variety of stable / unstable sources, either down there in CA with another clock trip or up here in WA with a clone of his prototype. It would be nice to further validate this wave fitting technique, perhaps uncover and quantify subtle biases that depend on power law noise (or ADC resolution, or sample rate, or sample size, etc.), and also to explore environmental stability of the instrument.

I can tell Ralph put a lot of work into this project and I'm pleased he chose to share his results with time nuts. I mean, it's not every day that national lab or university level projects embrace our little community.


[1] http://aip.scitation.org/doi/pdf/10.1063/1.5010140 (PDF)
[2] https://www.febo.com/pipermail/time-nuts/2018-February/108857.html

----- Original Message ----- 
From: "Ralph Devoe" <rgdevoe at gmail.com>
To: <time-nuts at febo.com>
Sent: Monday, February 12, 2018 1:52 PM
Subject: [time-nuts] Allan variance by sine-wave fitting

> I've been comparing the ADEV produced by the sine-wave fitter (discussed
> last November) with that of a counter-based system. See the enclosed plot.
> We ran the test in Leo Hollberg's lab at Stanford, using a 5071a Cesium
> standard and my own 5065a Rubidium standard. Leo first measured the ADEV
> using a Keysight 53230a counter with a 100 second gate time. We then
> substituted the fitter for the counter and took 10,000 points at 20 second
> intervals. The two systems produce the same ADEV all the way down to the
> 5x10(-14) level where (presumably) temperature and pressure variations make
> the Rb wobble around a bit.
> The revised paper has been published online at Rev. Sci. Instr.

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