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

[cdelect at juno.com]

Recently I have been testing some Quartz oscillators with
exceptional stability. (low parts in 10-13th)

I've noticed something that jumped out at these performance levels!
Normally I test oscillators on a DMTD system with either
an ultra stable Quartz or a Hydrogen Maser reference.

On these latest oscillators at the longer Tau (>100sec.) 
the Quartz versus Quartz data shows much better performance 
than the Quartz versus Maser!

What is happening (I think) in this case is that both Quartz units
have exceptionally low and similar ageing. 
This similar ageing rate masks the true stability performance at longer
Tau.
The Maser data (with no effective ageing in this case) 
reveals the true stability at the longer Tau!

Cheers!

Corby Dawson