[time-nuts] OCXO Phase Noise Measurement in Primitive Conditions

Poul-Henning Kamp phk at phk.freebsd.dk
Wed Aug 27 13:32:27 EDT 2014

In message <1409158879.13035.YahooMailNeo at web142706.mail.bf1.yahoo.com>, Bob St
ewart writes:

So here is a pretty interesting way to optimize a GPSDO that I've
been playing with for some years.  I don't have a formal mathematical
formulation of it.

It is somewhat related to what Dave Mills calls "the Allan intercept"
except this you can actually measure and not just estimate.

You run several (long!) test-series with different timeconstants
in your PLL, and you record the resultant EFC and phase offset
as a function of time.

If your timeconstant is too short, you will have a lot of
high-frequency signal in the EFC, too long and you get too
much high-frequency signal in the phase offset.

The optimal timeconstant is where you have the least sum of
spectral power where the two curves cross each other.

My experience so far is that the curve around the optimum is
very flat, getting the timeconstant  wrong by a factor of two
hardly changes the resultant performance.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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