[time-nuts] Designing an embedded precision GPS time

Adrian Godwin artgodwin at gmail.com
Wed Nov 1 15:13:11 EDT 2017

How do those compare with vectron's part : ?


There's also this patent.


I don't really know if that's valid - it seems to propose something similar
to the numerically-compensated oscillator in my rather old PM6666 frequency

On Wed, Nov 1, 2017 at 5:11 PM, <tnuts at joshreply.com> wrote:

> >While crystal curves are indeed cubic, there are higher order terms in
> >the curve. The “why” is something people get to write papers on. If you
> >are trying to compensate to tight specs, you will see all sorts of
> >stuff. It is not at all uncommon to see >9th order curves residual
> curves. Indeed some of that is from residuals in the compensation circuit
> as well as from the crystal.
> I’ve been trying to research this very topic!
> Can you point to some of these papers?
> I am trying to build the most accurate fee running, low power time base I
> can. I am using an MCU, 32768Khz watch crystals, 0.5C accuracy temp sensor,
> lots of thermal bringing between them, and mass around them. The idea is to
> measure the frequency shift at all temps in the range, and even in both
> directions (hopefully to capture some hysteresis) for each unit and then
> use that database to compensate in software once the system is free running.
> 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?
> That is, if I had an infinitely precise temp sensor and an infinite amount
> of time to characterize an XTAL, what would be limits to how accurately I
> could temp compensate it?
> Also, what are the limits of characterizing and compensating for aging?
> What other sources of inaccuracy would I need to consider?
> Thanks!!!
> -josh
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