[time-nuts] Allan variance Vs Plain Old Accuracy

[Martyn Smith]

Hi All,

This comment is bound to get you all going.

Maybe I'm being stupid, but why does everyone use Allan Variance and not
plan old accuracy?

I am very familiar with David Allan's full article on Allan Variance.
However Allan Variance isn't the same as accuracy.

Accuracy is what is important to most people.  And that's not RMS but peak
to peak, e.g worse case.  And not averaged over 24 hours but averaged over 1
second or less.

Although I sell GPSDO using OXCO's, I don't find their accuracy as good as
my rubidium disciplined oscillators.

I have an article on my web site where I compare a OXCO based unit versus my
rubidium's unit.  I won't say what OXCO unit it is, but its one of the well
known ones on the market that is talked about in this forum.

www.ptsyst.com/AppNote2.pdf

Also a friend of mine measured one of the current leading OXCO based units
(from a very big manufacturer).  He found its accuracy was +3.93E-10
and -4023E-10 over a 10.5 hour period.  Over the same time the unit
displayed a worse case error of 1.4E-12.  It tended to drift in one
direction for a long time and then drop.  So its Allan Var was still very
good, but its accuracy  was poor.  That's another one of my gripes.  The
frequency accuracy displayed by many units has little bearing on actual
frequency accuracy.

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

And now that I can sell you a Rubidium with GPS locking built in for under
$600.00 why buy an OXCO!!!!!

Ok, I do need an order for > 5000 pieces.

Best Regards

Martyn