[time-nuts] Designing an embedded precision GPS time

tnuts at joshreply.com tnuts at joshreply.com
Wed Nov 1 13:11:00 EDT 2017

>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?



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