[time-nuts] TBolt and Z3801A performance data

[John Ackermann N8UR]

Yes, all were made with GPS antenna connected (and running from the same
antenna via an 8-port splitter).

What I was mainly commenting on is the TBolt's pronounced "hump" that
shows up in the measurements against the FTS-1050A but largely
disappears in the pairwise measurement.

The longer tau readings are interesting mainly to see how the GPS loop
kicks in; tau below about 1000 seconds seem related to OCXO performance
on the TBolts, while the Z3801A seems to have a longer time constant,
and somewhat more gradual attack.  (By the way -- the TBolts were
running with the loop parameters they were set to when I got them).

John
----
John Miles said the following on 08/09/2008 05:41 PM:
> 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
> 
> 
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