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?
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).
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)
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