[time-nuts] How can one measure ADEV of a good oscillator?
kb8tq at n1k.org
Mon Dec 1 19:08:46 EST 2014
> On Dec 1, 2014, at 1:11 PM, Richard (Rick) Karlquist <richard at karlquist.com> 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.
Others did a similar thing by simply taking production OCXO’s to the limit of their EFC range. That let you do a coarse sort to find some number of “likely” units. Next step was to pop a few of them open and short this or that out to get a reasonable beat note. Numbers in the 10 to 20 Hz range were pretty common. After that a single mixer setup followed by a simple limiter got things good enough to screen the production lots.
>> > 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?
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