[time-nuts] Aging and Thermal Correction in Holdover

Attila Kinali attila at kinali.ch
Thu Oct 27 14:45:47 EDT 2016

On Thu, 27 Oct 2016 16:13:06 +0000 (UTC)
Bob Stewart <bob at evoria.net> wrote:

> 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.

It is _probably_ minimal. How large it is depends on your setup.
But from what I know of OCXOs the change is mostly dominated by
the non-linear time dependence. I don't know of any measurements
of the temperature dependend aging for long running crystals.
Probably because aging for old crystals becomes much lower than
temperature effects.

> 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.

I.e. you have a very simple, linear temperature dependence and aging model?

> 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.

I guess you are measuring the temperature of the OCXO outside of the can.
Keep in mind that the temperature outside is lower than inside. I.e. you
have a temperature gradient. If the temperature changes, then the gradient
changes as well. As long as the rate of temperature change is low, the
gradient will stay linear (ie the temperature inside the OCXO increases
linearly with the distance from the outer can). If the rate of temperature
change is fast, then the gradient will start to bend. Which in turn means
that the heat transport between inside and outside of the OCXO is not a
linear function of the outside temperature anymore. Or in other words:
your simplistic system model does not capture the dynamic behavior of
the OCXO correctly, which results in discrepances between expected and
observed behaviour.

Depending on what you are actually trying to do, I recommend that you
get a book on control theory and read up on some of the basics.
One of the books I like is "Feedback Control of Dynamic Systems"
by Gene Franklin. It's reasonably simple and has lots of examples.

And if I may, please talk about energy (or rather power in this
context), not BTU. The former is a physical term, while the latter
is an outdated unit that shouldn't be used anymore and has no clear

			Attila Kinali

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

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