[time-nuts] New Subscriber, DIY GPSDO project (yes, another one)

[Magnus Danielson]

Hi,

On 2020-03-08 16:15, jimlux wrote:
> On 3/8/20 1:52 AM, Magnus Danielson wrote:
>>
>> The Allan intercept is really where the cut-over from reference Allan
>> plot to the steered oscillator plot. The concept of Allan intercept is
>> actually not perfect science, but a concept. The actual physics would
>> make the cut-over analysis on the phase-noise plots make more sense, but
>> for the time-constants we talk, that's where the Allan deviation plot
>> has taken over typically. Actually doing the cut-over in Allan deviation
>> form carries with it biases values, making the Allan intercept value
>> biased. It gets you to the right neighborhood, sure, but do expect a few
>> trims for optimum stability.
>>
>
>
> The conceptual idea being similar to setting a PLL Loop filter
> bandwidth such that the reference noise (multiplied up) and the VCO
> noise are the same at that point?
>
Yes, that's where the term intercept comes from.
> Of course, it's "easy-ish" for a crystal oscillator (flat noise
> spectrum at crossover) and VCO (steadily declining spectrum at
> crossover) and those noise spectra remain the same (ish).
In theory simple yes.
>
> I think the "art" comes in picking the right gains and bandwidths,
> because of things like GPS has diurnal variations, temperature
> variations (also diurnal, but also faster with HVAC turning on and off)

Which is the reality with a number of systematic disturbances which is
not random noise. Such concerns takes over, and the Allan deviation
intercept is even further from modeling that correctly.

Cheers,
Magnus