[time-nuts] Caveats on Allan Deviation with ultra stable oscillators

John Miles john at miles.io
Fri May 30 20:31:51 UTC 2014

> I usually don't use drift removal as I want to see the effects of drift!
> The effects of oscillators locking together are very apparent on both the
> phase and AD plots when using a DMTD system. There was no indications of
> such locking!
> My point was that if you are measuring ultrastable Quartz oscillators
> against each other the AD at the higher Tau will not reflect the true
> stability of the oscillators.

Well, it will reflect the true stability if they're not exposed to the same
drift stimuli, right?  Neglecting shared long-term aging trends, they should
both end up in random-walk territory over the long run.   Otherwise, if you
run the two oscillators in open air in the same room or even the same
building, then they will respond similarly to HVAC cycles, diurnal cycles,
and whatever other environmental changes are common to both.  That will make
their ADEV look better than reality when you measure them against each

It doesn't mean the long-term ADEV is necessarily invalid as a statistic,
just that you probably haven't eliminated all of the common-mode influences.
To the extent the two oscillators drift independently, the ADEV measurement
is valid.

For that matter, the isolation amps in your DMTD are also exposed to the
same environment.  Their residual phase tempco should be much better than
any quartz oscillator, but if they're worse for some reason, they may
dominate the long-term measurement.   And of course, if you don't have
enough isolation, you could be injection-locking the OCXOs in a really low
bandwidth (days, perhaps).  

Shared power supply leads can also induce entrainment -- or even separate
power supplies, if you run the leads right next to each other.  Some of the
Wenzel ULNs seem to be susceptible to this if you don't add bypassing at
their power terminals.
> Only a Maser or high performance Rubidium (HP5065A) will reveal the true
> behavior.

True, because they aren't as sensitive to environmental effects.  But if you
benchmark two nearby 5065As or masers carefully enough, their long-term ADEV
will also look better than it really is, and for the same reasons.  (Some
people have even reported similar behavior with cesium standards, although I
don't see how that could happen.  There aren't supposed to be any
first-order temperature effects in a CBT, and I'd think that any lower-order
effects would be way beneath the tube's flicker floor...)

-- john, KE5FX
Miles Design LLC

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