[time-nuts] How can one measure ADEV of a good oscillator?

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Mon Dec 1 02:09:14 EST 2014

I think I have a flaw in my understanding of this.

How can something like an SR620 measure the ADEV of an oscillator,  if the
oscillator is of a similar or better than the reference fed into the SR620?

I see plots of ADEV  for hydrogen masers, but I can't understand how this
can be measured from the phase data unless the reference is better than the
DUT, which is not going to be possible with a good hydrogen maser.

I was thinking it might be possible if one has 3 oscillators and 3 time
interval counters to perhaps solve 3 simultaneous equations. I can't prove
that, but it seems intuitively correct.

I must be missing something!

Also I have seen graphs of both Allan variance and Allen deviation.  Both
are typically 10^-12 for a decent oscillator, but given the variance and
standard deviation are related by a square root, they can't both be around
10^-12.  I would expect to see values of 10^-6 or 10^-24, but I don't see
such dramatic differences from 10^-12.

If I see numbers around 10^-12 on an OCXO,  is that the Allen variance or
Allen Deviation?


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