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

[Bob kb8tq]

Hi

This all comes back to *having* the ADEV / HDEV / Phase Noise data
on your steered device and GPS to work from. In a lot of cases, people
do not have that information ….. They are looking for a procedure to 
truly do this “blind”. 

Bob

> On Mar 8, 2020, at 5:52 AM, Magnus Danielson <magnus at rubidium.se> wrote:
> 
> H
> 
> On 2020-03-08 07:38, Poul-Henning Kamp wrote:
>> --------
>> In message <5583E434-4C72-4A4F-A60B-75A4204EF2D5 at n1k.org>, Bob kb8tq writes:
>> 
>>> Backing up a bit …. the objective is not to minimize overshoot or 
>>> keep the loop from oscillating. The issue here is optimizing the noise
>>> output of the combination of GPS + OCXO when combined via
>>> the control loop. It’s a very different objective ….
>> Yes, but a PI loop is still the best mathematical tool for it,
>> you just need the PI loop to have adjustable parameters.
>> 
>> Adjusting those parameters after the initial capture is the hard
>> part, because the signal you are looking for is in the "wander"
>> domain.
> 
> First, a PID PLL degenerates into a PI loop. The P and D steering ends
> up achieving the same thing as P in the PI, do there is no benefit of D.
> I have shown such derivations in the past. Not too hard to do.
> 
> Second, a PI loop has trivial dimensioning from damping factor and
> frequency, and the damping factor we know we need to keep high enough,
> so say 3 or higher, so we end up only having the loop frequency, which
> is the reciprocal of time-constant. These rules is easy to do, a page of
> paper is enough or a whiteboard.
> 
> So, these basic facts is just the rule of the game.
> 
>> The best I have come up with, is to average the measured phase error
>> to get rid of the GPS jitter/sawtooth, and adjust the PI loop
>> parameters based on the time between sign-changes of that averaged
>> signal.
> Third, the averaging filter needs a limitation on how low it frequency
> can go, before it starts to affect the pole-pair of the PI-loop, at
> which time stability cannot be guaranteed. Corrections needs to be
> performed to ensure stability and performance as it comes closer.
>> If you plot the histogram of the time between sign-changes, you want
>> the peak below the supposed "allan-intercept" and if you get time
>> intervals more than double the "allan-intercept" you have probably
>> tightend too much.
> 
> 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.
> 
> Cheers,
> Magnus
> 
> 
> 
> 
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