[time-nuts] Conditioning Rubidium Oscillators

Jim Harman j99harman at gmail.com
Mon Mar 14 19:56:42 EDT 2016


On Mon, Mar 14, 2016 at 2:37 PM, Lars Walenius <lars.walenius at hotmail.com>
wrote:

> I read this but couldnĀ“t understand why this is superior to the PI-loop
> with a pre-filter?
>
>
> http://ptfinc.com/wp-content/uploads/2016/03/App_37_RubContol-Rubidium-Control-%E2%80%93-A-Different-Approach.pdf
>
> Anybody can say why? Even if regression is very useful the limitation of
> the GPS and ionosphere will be the problem?How much better is it reasonable
> to get??
>

I think the potential benefit of this approach is that it continuously
predicts the long term drift of the oscillator and attempts to compensate
for it. If the drift is reasonably linear, this means that you can use a
larger time constant in the control loop and thus be less sensitive to
short term GPS timing variations, while keeping the phase error close to
zero

Of course if the oscillator drift is not predictable, this won't help and
might even make things worse.

I have done some experiments with an OCXO and a controller design similar
to the one Lars posted some time ago. I plotted the trend in the 3-hour
average DAC values over many days and used Excel to do a least-squares fit
to that data. As long as the oscillator is powered on continuously, this
gives an R^2 of over 90%, so the linearity of the drift is very good. If I
use this slope as a correction factor, i.e. adding X DAC counts per day to
the output of the PI control algorithm, it significantly reduces the
average TIC error at long time constants


-- 

--Jim Harman


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