[time-nuts] Aging and Thermal Correction in Holdover

Bob Stewart bob at evoria.net
Thu Oct 27 12:13:06 EDT 2016

Hi Attila,
Thanks for confirming that the thermal impact on the aging rate is minimal.  I had suspected that that was so, but had no easy way to tell.  Since I want to keep this as simple as possible, I'll probably stay with a simple division of the DAC delta over 3.5 days.  And since the thermal impact seems to be 1:1 to the DAC delta under most circumstances, I may experiment with modifying the beginning and end points of the calculation by the temperature.  But, hopefully with a long enough averaging period, it won't make much of a difference.
The odd thing about the thermal impact is that if the thermal change delta is too high, it's no longer 1:1.  I suspect that may be because it's an easy thing for the OCXO's heater to add BTUs, but it can only subtract them passively.  And shedding BTUs into a rising temperature environment is problematic.


      From: Attila Kinali <attila at kinali.ch>
 To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
 Sent: Wednesday, October 26, 2016 1:28 PM
 Subject: Re: [time-nuts] Aging and Thermal Correction in Holdover
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.


More information about the time-nuts mailing list