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

Magnus Danielson magnus at rubidium.dyndns.org
Wed Aug 27 16:28:07 EDT 2014


Poul-Henning,

On 08/27/2014 07:32 PM, Poul-Henning Kamp wrote:
> --------
> 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.

I followed the paper-trail back to an article by Judah Levine, that 
describes it in the context of the ACTS system. It's translation over to 
NTP is somewhat unfortunate, due to the difference in noise characteristics.

> 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.

The "Allan intercept" is rather the intercept point between the Allan 
variance of the reference signal and that of the local oscillator. In 
the original paper separate measurements made the intercept point more 
or less a static value for those assumptions.

The noise of a packet network and the noise of a modem line of that time 
is quite different in characteristics.

> 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.

I agree. As long as you are in the right neighborhood, you do fairly 
well. You just need good enough models to properly model the 
neighborhood you're in.

I have yet to seen a thorough ADEV/AVAR analysis of that cross-over 
point for optimum performance, all I have seen is translation of the 
phase-noise type rule-of-thumb analysis.

Cheers,
Magnus



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