[time-nuts] Re: Does cm accurate GPS via RTK give more accurate time?

[Poul-Henning Kamp]

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Pavel Kořenský via time-nuts writes:


> I'm currently using this module in my prototype GPSDO based on a cesium 
> clock, [...]
>
> Now, regarding how to measure which clock is "better": that's a tough 
> question. If you have only two clocks, you can't really tell which one 
> is more accurate — you need a third, better reference.  […]

But that is not really what you want to know, is it ?

What you want to know is that your PLL has the right time-constant,
so that you derive maximum benefit from both your two sources.

This is what Dave Mills called "The Allan Intercept"

When your PLL tracks near the Allan Intercept, the error output of
the phase detector will have a flat-ish frequency spectrum.

If your PLL is too loose, the spectrum will slope up towards higher
frequencies, if it is too stiff, lower frequencies will dominate.

But be aware that with a Cs-VCO, the signal is in constant peril
from rounding and truncation in floating point math.

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