[time-nuts] GPS ADEV?

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 7 06:49:00 EDT 2007

Tom Van Baak wrote:
> John,
> I like the questions you ask. Here's an updated plot:
> http://www.leapsecond.com/pages/3gps/gps-adev.gif
> Note that the tau 1 second Allan deviation is essentially
> the standard deviation of the sawtooth: in the graph the
> VP is about 55 ns, CNS about 15 ns, CNS2 about 3 ns.
> (CNS is an M12+; the CNS2 adds sawtooth correction).
> The plot was made from about 12 hours of 1PPS data
> for each of the three Motorola receivers.
> Note also that for each decade the relative stability gets
> 10x better (since the slope is -1; white noise) and that
> the ratio among these three receivers stays the same
> regardless of the tau; the distance between the lines is
> pretty much fixed (about 6 dB). This directly relates to
> their relative jitter (sawtooth error).
> Now, at much larger tau I would expect the three lines
> to merge as something else becomes the limiting factor,
> but that's well over tau 10^6 seconds.
> I also overlaid the earlier longer-term CNS2 data (pink)
> to the plot; the same -1 slope continues to a week.
> Note the TIC resolution (150 ps for a 53132A) is not the
> limiting factor for any of the plots here. However, if one
> were to use a TIC with resolution of several ns instead
> of sub-ns you'd see the CNS2 plot (only) shifted up a bit.
> For contrast I added a (well-engineered) Datum 2000 to
> the plot (black line). It was measured with a TSC 5110A
> at 5 MHz rather than a hp 53132A at 1PPS.
> What GPS receiver boards do you have handy, so that
> you could repro this plot? It looks OK to me but having
> a "second source" would make me feel better.
> /tvb
> http://www.LeapSecond.com
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Is there anyway that you can derive the same plot for an M12+ receiver 
with post facto (software) sawtooth correction?
It should be marginally better than a CNS II. Hardware correction always 
adds some noise. Some of the Dallas programmable delay chips used in the 
CNS II are noisier than others.


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