[time-nuts] Does adjusting ocxo's degrade adev / madev ?

[Charles Steinmetz]

Mark wrote:

>I've long had a nagging suspicion that OCXO's that are not adjusted 
>will in practice have lower ADEV than ones that are tweaked 
>regularly.   Several days ago I noticed that one of my 10811's was 
>performing quite well (I believe this is the first time any of 
>10811's have delivered adev / madev numbers in the 13's) and this 
>was sustained for approx 2 days.   After trimming the frequency and 
>letting it sit for a number of hours  I noticed the adev was notably worse.

Every time a quartz oscillator is disturbed in any way, its stability 
is very likely to go down for a time until it settles back in.  The 
disturbance could be adjusting its frequency (either mechanically or 
via EFC), interrupting the power, changing the crystal temperature, 
physically bumping the unit, or anything else that changes its 
operation.  How long it takes to settle back into "normally stable" 
operation depends on the crystal itself and on how violent the 
disturbance was.  I would never expect a quartz oscillator to be back 
to normal stability for at least some days after a macro frequency 
adjustment.  [Tiny, tiny adjustments such as done by ongoing GPS 
discipline do not seem to have a large effect on the stability of 
quartz oscillators.]

But I note that you say an oscillator that had never delivered 
stability in the e-13's had a couple of very good days, and then 
didn't go back to the e-13's in a number of hours.  First, you 
probably shouldn't expect it to go back to the "couple of good days" 
level -- it's likely to go back to its "normal" level.  And second, 
you shouldn't expect it to get there for at least a few days.

Best regards,

Charles