[time-nuts] GPS Disciplined Oscillators

SAIDJACK at aol.com SAIDJACK at aol.com
Wed Apr 25 14:18:23 EDT 2007


 
In a message dated 4/25/2007 09:50:38 Pacific Daylight Time,  
mfeher at eozinc.com writes:

>So,  to
>reiterate the question, if I was clear enough, what kind of  frequency
>excursions should I anticipate to see amongst my three  disciplined
>oscillators in lets say 24 hours, or in a month. Assume GPS  disciplining was
>working all of that time (can I even assume that with  aging?). BTW, how is
>my assumption regarding the oscillators  aging?


Hello Mike,
 
from my experience, good oscillators will age parts in 1E-08 per day or  
less. Bad ones can age much faster, some parts in 1E-06 in the first days even.  
That's a pretty significant variation in EFC voltage required to compensate  
that.
 
We have seen units that age significantly in the first 2 - 3 days, then  slow 
down, and after 6 months or so almost have no aging. We have seen units  
where the aging actually accelerates over time.
 
Aging is a very slow changing process after the crystal has run some days  
(second derivative is small), so GPS locking will usually easily compensate this 
 error.
 
There are oscillators that have popcorn noise (frequency jumps) that can be  
really annoying, and this effect is similar to rapid aging (from one second to 
 another): almost sudden the phase/frequency of the OCXO changes radically 
from  one second to another. This is usually caused by either dust particles 
leaving,  or landing on the crystal, the crystal "cracking" in it's holders, or  
radioactive particles hitting the crystal.
 
Also, thermal effects will usually require much larger shifts in EFC  voltage 
than aging, unless you are using a double-oven high quality OCXO.  Thermal 
effects on the required EFC voltage usually swamp aging effects and are  harder 
to deal with because they are more or less random, and not a nice  (almost) 
straight line on the EFC voltage.
 
bye,
Said 



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