[time-nuts] GPS Locked and Unlocked Performance Comparison

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Feb 12 21:28:05 EST 2008


John Miles wrote:
>> To me, the interesting part is that both the Thunderbolt and the Miller
>> designs appear to degrade the performance of the OCXO in locked mode at
>> short Tau compared to unlocked. The Z3801 does very well < 300sec, but
>> degrades the ADEV at Tau farther out. The Fury does best, never
>> worse locked
>> than unlocked. The practical difference between the Fury and
>> Z3801 at short
>> Tau looks to be mostly in the OCXO, the Fury is the one that actually has
>> the least degradation of the OCXO performance overall.
>>
>> Performance at short Tau cannot be better locked than unlocked, so it is
>> driven by the OCXO and software, but the fact that two designs (we will
>> forgive the Miller design, which looks to be mated to a very good
>> OCXO
>>     
>
> The interesting thing about the Miller example is that if you click on the
> link that goes to his page, the OCXO that he shows is just a simple, cheap
> Temex(?) part.  I bought 5 or 6 of these, or something that looks very much
> like them, for about $20 each not long ago from a UK seller.  Nothing
> special about it.
>
> Tom, was this graph taken with his Isotemp 10.0 MHz OCXO, or the cheaper
> one?  What was your reference... one of the masers?
>
> -- john, KE5FX
>   
Tom

Both the Thunderbolt and Miller designs appear to measure at least 10x
better than the most optimistic expectations.

In the case of the Miller design either the GPS receiver 10KHz output is
extraordinarily stable with no phase jerking every second or something
is confounding the measurements.
One possibility is injection locking of the OCXO via its EFC due to
inadequate shielding or via its output due to its buffer amplifier
having inadequate isolation.

What cable types did you use to connect the GPSDO's and the reference
signal to the Allen deviation measurement instrument?

Is there any way to measure the Allen deviation of the 10kHz GPS
receiver output, or at least verify that the 10KHz phase is jerked every
second.
The jerk may be done by adjusting the length of the 10kHz cycle just
before the second marker.

Bruce



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