[time-nuts] TBolt and Z3801A performance data

[John Miles]

These tests were made with the GPS antenna connected?  At t >> 100 seconds,
they should all look about the same, because that's where GPS disciplining
comes in, no?  They should not be uncorrelated in the long run.

To the extent one Z3801 looks worse than the other at large values of tau,
I'd expect there to be a good reason, like better GPS reception on one of
them, or a much-worse OCXO.

Measuring phase noise by comparing the two against each other should be
fine, though, since their short-term drift isn't being corrected at that
timescale.  Your noise floor is a couple dB worse than the one I tested, but
they're otherwise about the same.

- john, KE5FX


> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of John Ackermann N8UR
> Sent: Saturday, August 09, 2008 1:28 PM
> To: Discussion of precise time and frequency measurement
> Subject: [time-nuts] TBolt and Z3801A performance data
>
>
> I just put the results of some tests of 2 "Time-Nuts Special"
> Thunderbolts, as well as 2 Z3801As, at
> http://www.febo.com/pages/gpsdo_comparison/
>
> I learned an interesting (and important) lesson in doing these
> measurement.  I initially measured the pairs of GPSDO against each other
> (e.g., one TBolt as "reference" and the other TBolt as "DUT").  In
> theory, if the two oscillators are identical, and if their noise is
> uncorrelated, the results of the pair can be used to deduce the results
> of the individual units.
>
> However, in this case doing so gave a very optimistic view -- the TBolts
> were better than the pair of Z3801As!  Another set of measurements
> comparing the GPSDOs against an independent reference revealed that the
> first measurement was a lie.
>
> I guess you can think of it like this.  Picture two OCXOs that both age
> at the same rate and in the same direction.  Because they drift
> together, measuring their relative phase hides their actual drift and
> makes them look better than they are.  On the other hand, if they were
> drifting in opposite directions, they would look worse than they are.
> An identical aging trend is a "correlation" even though it's external to
> the things we usually think of.
>
> John