[time-nuts] GPSDO three cornered comparisons

Tom Van Baak tvb at LeapSecond.com
Sun Oct 21 20:39:01 EDT 2012


This is problematic, since the Z38xx software is not really measuring the output of the GPSDO. Instead it's taking the internally reported PLL time error measurements from the disciplining loop and pretending they are a measure of real performance as measured against a real frequency standard.

However, this PLL data can be used to see how well the loop is working; that in itself can be interesting. For example, differences in an ADEV plot of the PLL error term can be used to reveal the time constant used by the PLL. So one can still use the ADEV-style calculation on the phase error of a closed PLL; but don't confuse this with the actual performance of the GPSDO as a time/frequency standard. Does this make sense, or shall I explain more?

Or, if you have some raw data you can send me off-line I'll take a look at it.


----- Original Message ----- 
From: "David Hooke" <dhooke at gmail.com>
To: <time-nuts at febo.com>
Sent: Sunday, October 21, 2012 4:44 PM
Subject: Re: [time-nuts] GPSDO three cornered comparisons

> Tom,
> The ADEVs I quoted are from the Z83xx software, so I assume it's 
> comparing time from the GPS system to it's own oscillator's output, in 
> the same way that LH does. I've been monitoring the basic stuff you 
> suggested.
> It's because I don't have a known reference to compare the units with 
> that I want to use the 3-corner method to start sorting the wheat from 
> the chaff.
> david
>>> I have an HP 58503A and a Symmetricom 58503B which are behaving quite
>>> differently to one another using a common antenna. Using Z38xx, there's
>>> almost an order of magnitude difference between the reported ADEVs at
>>> 40k seconds (HP:7e-13, Sym:5e-12). Both have 10811 oscillators and are
>> Where are you getting the "reported ADEVs"? What counter, what reference, what software?
>> Did you happen to periodically log any SCPI data during the runs, so that you can compare SV counts, signal strengths, TI averages, and so on? In general it might be good to have a quick look at this mundane data first.
>> /tvb

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