[time-nuts] New algorithm, better ADEV

Tom Van Baak tvb at LeapSecond.com
Sun Jan 18 08:44:41 EST 2015

>    I'm doing a longtime test with this counter. The DUT and Reference
> are from the same X72 Rb oscillator.The X72 is powered by a R&S linear
> supply, the counter is powered by laptop USB port. I started the test
> 1 hour after the Rb is locked.
>    I have uploaded the test data about 2 days to
> http://www.qsl.net/b/bi7lnq//freqcntv4/test/20150118/ . I don't know
> how to interprete this kind of chart. :(

Li Ang,

Nice data. Thanks for sharing.

In a "self test" like this, try using a phase plot ('p') or TDEV plot ('t'). As you see, the X72 ADEV or MDEV plot is sort of boring, showing mostly a -1 slope to forever.

A phase plot gives you an idea of the instrument rms and peak-to-peak noise. It also reveals experimental setup or run errors. For example, you appear to have a problem (unusual phase jump) near second #86760.

If you have a clean data set, a TDEV plot gives you a view of the time resolution of the instrument. Because it removes the -1 slope of MDEV it more clearly shows resolution as a function of time. When it rises at larger tau, then you have evidence of environmental sensitivity in your instrument.

Also, in the case of a long run, try using TDEV and the TimeLab "trace history" feature ('e' -> Trace History). Setting it to 5 or 10 gives you a better feel of normal vs. outlier vs. best case performance. This is one way to see how much or little any glitches in your data affect your results.

Now, the next step is using two different, but very stable, references that have a known frequency offset. The problem with using the same REF/DUT is that you do not get exposure across the entire 100 ns period of the 10 MHz frequency. An alternative is to try the experiment a few times, with say, a 10, 20, and 50 ns cable delay in one of the paths.


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