[time-nuts] First success with very simple, very low cost GPSDO

Magnus Danielson magnus at rubidium.dyndns.org
Sat Apr 12 17:12:02 UTC 2014

On 10/04/14 20:28, Tom Van Baak wrote:
> I agree with Charles. Further, you don't even have to wait a predetermined amount of time (this would be oscillator or environment dependent). Instead simply monitor the rate of frequency change. When the drift rate drops to the level where your PID is known to be able to track, then start the PID.
> Realize that just 2 seconds after power-up you have your first frequency measurement. By 3 seconds you have your first drift measurement. Just wait until it falls to however few ppm/second or ppb/second you need for your loop to smoothly track. This avoids special case PID startup or wind-up code. Although you can argue it merely replaces it with special case drift rate code.
> I'm suspicious of fast/slow tracking loops. If you want to vary the tracking, perhaps it is best to continuously, transparently, smoothly vary loop parameters according to drift rate rather than use a hardcoded fast/slow algorithm binary switch. I'm sure there's deep theory on this, which I have not read yet.

This is where many spend their time building a heuristics.

My favorite solution is to derive the phase detector and let the result 
feed into the integrator through a scaling factor. This input to the 
integrator is in parallel to the I factor input. Code-wise we get 
something like this:

Vdf = Vdp - Vdp_pre
Vdp_pre = Vdp
Vi = Vi + I*Vdp + F*Vdf
Vf = Vi + P*Vdp

For far-out frequency errors, the PI PLL is weak, due to Bessel 
coefficient, so the FLL dominates and the F factor steers the loop gain 
of the FLL. It steers the Vi state of the integrator until it becomes 
close, with an exponential decay into zero frequency error. When it 
does, the Bessel coefficient becomes stronger and stronger and then PI 
PLL starts to take over. When the gain of the PLL surpasses that of the 
decaying FLL the PLL just takes over... by design. When the PLL has 
locked the frequency, the FLL part just doesn't have gain, but adds a 
little noise.

The benefit of this approach is that the frequency correction is kept in 
the Vi state, and depending on the Vi state either the FLL or PLL 
dominates, there is no hand-over, there is no external intelligence to 
chose mode, it is always steered by the need from frequency-error 
needing to be corrected and the current Vi, or if you so like, the 
current Vi error.

Simple, relatively easy to analyse and completely linear regulation 


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