[time-nuts] Allan variance Vs Plain Old Accuracy

[SAIDJACK at aol.com]

Hello Martyn,
 
>Rubidium's oscillators usually stay within 1E-10 accuracy, about five  times
>better than any OXCO unit I've measured.

yes, your data  shows this. This is actually also reflected in your ADEV 
measurements, your data  shows that at 100s and 500s the Rb has about 10x better 
ADEV performance  than the OCXO, and looking at your 3600s plot the OCXO 
wanders around in about  that time frame, so there is some correlation here.
 
So the ADEV numbers are not all that unrelated to frequency accuracy  :)
 
Please see Toms' ADEV measurements for four GPSDO's that he posted a couple  
of days ago, the ADEV numbers for those units are a bit, to  significantly 
better than your ADEV measurement numbers, I think the reason  is that the units 
he tested very likely all had DOCXO's.
 
>From your stability measurements it looks like the OCXO was a  single oven 
unit. A double oven unit's stability spec versus temp is  usually 10x to 100x 
better than a single oven, so should perform much better at  a marginal cost 
increase.
 
Frequency errors in GPSDO's are primarily caused by two  sources:
 
   1) The accuracy of the GPS. Any GPS errors especially from  200s and 
longer will of course affect the frequency accuracy since the OCXO  follows the GPS 
tightly at averaging times longer than 1000s (typically). Rb's  are usually 
locked with much larger averaging times, so the OCXO has to be very  good in 
the interim.
 
   2) The stability of the OCXO. The more inherently stable the  OCXO, the 
less error correction control the loop has to do, the better the  frequency 
accuracy (and ADEV), and the further-out the time constant for the GPS  locking 
can be pushed.
 
Many OCXO vendors claim their DOCXO units to be Rb replacements. I think  
with the aid of a good GPS receiver, this can actually be accomplished for most  
applications, or at least GPSDO's can get close to the same performance.
 
But it does require very good OCXO's and GPS performance, and you are right  
Rb's are on average more accurate due to this requirement.
 
GPSDO's still have some advantages: 
 
   * a good Rb with a built-in GPS is still more (to a lot more)  expensive 
than a very good DOCXO-based GPSDO, but the accuracy differences  may actually 
be small
 
   * the Rb requires much higher power consumption
 
   * The Rb generates more RF noise (due to all of the RF  related 
frequencies and switching regulators etc inside the unit)
 
   * an Rb has a lamp-design-related limited lifetime, and  their MTBF is 
lower than typical GPSDO's
 
   * some commercial GPSDO's can operate up to  +85C which most commercial 
Rb's cannot do (SRS PRS-10 is limited to 65C on it's  baseplate for example, 
meaning the ambient has to be much <<65C)
 
   * GPSDO's are typically smaller and lighter than  GPS-disciplined RB's
 
I think the choice is still not that clearly differentiated.
 
bye,
Said
 
 
 





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