[time-nuts] Aging and Thermal Correction in Holdover

Attila Kinali attila at kinali.ch
Wed Oct 26 14:28:39 EDT 2016

On Wed, 26 Oct 2016 17:01:17 +0000 (UTC)
Bob Stewart <bob at evoria.net> wrote:

> I'm working on the Aging and Thermal Correction algorithms in my GPSDO.  At 
> this point, it's pretty simple: For aging, every X seconds I either 
> increment or decrement the DAC value by 1 as determined by the unit's 
> performance history.  For thermal, every X seconds (user settable) I check 
> to see if the thermistor has increased or decreased and step the DAC one 
> step in that direction.
> So, now I'm wondering if there's any correlation between the temperature and 
> aging.  IOW, if the ambient temperature in the case goes up a degree F, does 
> that change the aging rate by some small fractional value?  I recognize that 
> there is a point of diminishing returns, and I may have already hit that.

Yes, there is a coupling between temperature and aging, but how large it
is and into which direction depends highly on your system. It is also
very likely to change over time. But in general it will be drowned by
all other effects and uncertainties.

In general you don't want to set those parameters by hand, but let the control
loop estimate them. This is called adaptive control and the most common
way to do it is to use a Kalman Filter. There are many tutorials on the
net how to build a simple Kalman Filter. As a system model I suggest to use
the simplest that you can imagine that captures enough of the effects
to reach the results you want. E.g. in your case, I would go for a linear
or quadratic temperature dependence and a linear time aging. If you add
more parameters and make the system more complex, run into the risk that
your control system is unable to get good guesses for the parameters and
thus becomes unstable

			Attila Kinali

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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