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

Richard (Rick) Karlquist richard at karlquist.com
Mon Dec 1 13:11:11 EST 2014

On 11/30/2014 11:09 PM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
> 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?

What HP did with the 10811 was to make a few special crystals that
were 500 Hz off frequency and build them into oscillators.  These
oscillators were mixed with the DUT and the 500 Hz beat note was then
squared up and its ADEV measured with a frequency counter.  After
measuring a bunch of production line oscillators, they could establish
a minimum ADEV that would be attributed to the offset oscillator.  If
this level of performance wasn't good enough, other offset crystals
could be tried until a "golden" crystal was found.

> > 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.

In theory this makes sense, however, it would require a high offset 
crystal and a low offset crystal to do a 3 way round robin.  There
wasn't enough need to go to the trouble of having 2 crystal designs.

There is an NBS paper written maybe 40 years ago explaining the magic
of the beat note method.

Rick Karlquist N6RK

> 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?
> Dave
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