[time-nuts] Low cost synchronization

David Forbes dforbes at dakotacom.net
Thu Aug 18 14:18:44 EDT 2005

David Andersen wrote:
> Local stable crystal:  Actually, you could make it more than stable  
> enough, but it would exceed your power requirements, because you'd  
> probably fall back to an oven controlled oscillator.  There goes your  
> battery.  But why did you try your initial experiments with 32.768Khz  
> watch crystals?  You're much more likely to find a good, solid 10Mhz  
> reference with an SC cut TCXO.  For instance, that maxim IC you  
> mentioned has +- 2ppm, which is really quite awful by instrumentation  
> standards.  Compare to this one:
>    http://www.bdelectronic.com/frequency/oscillatorTCXO.html
> .3ppm tempco, +- 1ppm/year.  They don't show their overall allen  
> deviation curves, but you get the idea - it'll be within 1ppm by the  
> end of the year, and since that aging will probably happen over time,  
> I'd guess it would probably get you something like 10 seconds within  a 
> year.  Or something like:
>   http://www.vectron.com/products/tcxo/tc140.pdf
> (... which is probably expensive, but which you can get in 0.2 ppm  
> accuracy vs. temperature and <2ppm/10 years).
>  -Dave


The requirement that you seem to have missed is the 18 month battery 
lifetime. A 10 MHz oscillator is a couple milliapmeres, so it won't do 
the trick. The watch crystal needs only about 10 microamperes to 


The 32K crystal may be usable, but you'd have to put some effort into 
the design to get the temp compensation tuned to the particular 
crystal, and you'd have to grade the crystals for tempco in the mfg 
stage. That might be doable in quantity, if you come up with the right 
sort of computerized test fixture in an oven.

I have built a few nixie tube wristwatches using the cheap 32KHz 
crystals, so I have direct experience in this matter. (Has anyone else 
on this list built an electronic wristwatch?) Getting the crystal 
adjusted to 1ppm is not too hard. You'd have to temperature compensate 
it to get to 0.1 ppm, and that would be limited to perhaps 10C-30C 
temperature range.

It's a lot easier to compensate the crystal if it's worn on the wrist 
rather than sitting in a car, since a person's wrist is essentially an 
oven. The real world has ridiculous temperature extremes - don't even 
think about stabilizing a crystal used outdoors unless it's thermally 
connected to a human.

You should be able to evaluate the feasibility of using a compensated 
crystal based on the above.

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