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

[Magnus Danielson]

Rick,

On 12/01/2014 07:11 PM, Richard (Rick) Karlquist wrote:
>
>
> 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 remember that HP had some simpler mixer-squarer box that could be 
used. Essentially the same as the NBS receiver chain.

>
>> > 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.

Thinking of the NBS phase-noise set or Dave Allans DTMF paper?

Cheers,
Magnus