[time-nuts] Designing an embedded precision GPS time

Attila Kinali attila at kinali.ch
Sun Nov 12 10:10:49 EST 2017

On Wed, 1 Nov 2017 13:11:00 -0400
<tnuts at joshreply.com> wrote:

> I am trying to beat existing products like the Dallas DS3231 and Micro 
> Crystal RV-8803-C7-32.768kHz-3PPM-TA-QC, which use (I think) a similar 
> strategy. I’m hoping I can beat them by using more accurate temp tensing, 
> longer and more exhaustive calibration effort, and anything else possible! 
> Can you give a quick explanation (or point to reference material) covering 
> the fundamental limits to XTAL compensation accuracy, and how to get there?

You probably can get better, but is it worth the effort?
The problem with temperaure compensation is, that you have to measure
the crystal temperature with very high accuracy. Measuring something
close by means you have to model the crystals actual temperature using
the temperature measurements you made before. The more parameters you
add to your system to be identified, the more problems you get accurately
identifying them, as they are not completely independent and cannot be
directly measured.

Directly measuring the crystal temperature has been done and one quite
popular approach is MCXO. [1] and [2] are two early papers that describe
the approach in quite some detail. There are a few problems with this.
One is that you need a special crystal that is designed for dual mode
operation and the other is that you need quite a bit of electronics
around it to work. Oh.. and if you are living in the US, please be
aware that MCXO are covered by ITAR.

For general modeling of crystals, I recommend reading John Vigs
Oscillator Tutorial. It covers most of what we know about crystal
oscillators and gives lot of references.

				Attila Kinali

[1] "A Microcomputer-Compensated Crystal Oscillator Using a
Dual-Mode Resonator", by Benjaminson, 1989

[2] "Resonator Self-Temperature-Sensing Using a Dual-Harmonic-Mode
Crystal Oscillator", Schodowski, 1989

You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

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