[time-nuts] Low cost synchronization

Mike Ciholas mikec at ciholas.com
Thu Aug 18 15:34:05 EDT 2005

On Thu, 18 Aug 2005, David Andersen wrote:

> I assume that the devices of which you're speaking are 
> standalone items? Something like a sensor deployment, possibly 
> networked?  Knowing more about how you actually plan on using 
> these would help a bit.

Yes, it would.  The devices are part of a social experiment and 
are meant to be carried by humans.  The devices, at preprogrammed 
times, signal the human with an audio signal.  We want all the 
devices to create this signal "simultaneously", which means 
something on the order of a few seconds variation among all the 
units anywhere in the world.  There is no network, no display, no 
user interface, zip.  A key fob would be ideal and low cost is 

> Most cheap WWVB watches and clocks don't work very well on the 
> east coast, from my experience.  I poked around at a few in my 
> old lab in Boston, and they were a no-go.

Yes, this is my worry.  Switching power supplies in the 
vicinity seem to be the determining factor in how well these 
systems work.

> 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.

It is important to note that we don't need an *accurate* time 
reference, we need s *stable* one.  That means we're happy to 
correct, in software, a crystal that is running 10ppm slow *if* 
it holds that error throughout it's life.  We can build 
adjustments into the software, so no need to pull, tweak, or PLL 
to some arbitrary frequency.

> But why did you try your initial experiments with 32.768Khz 
> watch crystals?

Power.  A CR1620 battery is 75mAH and a CR2032 is 225 mAH.  For 
18 months, this is 6uA or 17uA average current draw.  The only 
thing that can meet that is a low KHz crystal.

> 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.

One idea is to keep the TCXO off most of the time, turn it on and 
let it stabilize, then compare it to the low power crystal.  Make 
an adjustment based on that.  But, I suspect the constant on/off 
will affect the TCXO poorly, and you miss most of the low power 
crystal variations (temperature, shock, etc).  So this doesn't 
seem possible.  I'd also guess these TCXOs are >$10 in qty.

Lastly, +/- 1ppm is +/- 30 seconds per year which is still not 
really good enough.

> Another option you may have just eliminated on principle:  
> Internet synchronization.

Technically, this works really well.  The concept we had for this 
was a USB key fob.  Plug it in to an Internet connected computer 
and capture the time.  The device could also have a super cap on 
board for power, charge it from the USB slot, and you don't need 
a battery.  I was thinking this would be an ideal concept with all 
sorts of flexibility.  The sync operation would not need to be 
done too often, perhaps once every 90 days, so limited access is 
okay as long as it occurs often enough.

However, there are issues.  Namely that the USB device would need 
software on the computer to make the connection and that the 
experiment wants to be as accessible to people as possible, even 
those which have no access to a computer.

A possible compromise was suggested: build a GPS time module with 
USB ports on it that serves the role of the time provider.  This 
would replace or augment the PC.  This complicated the experiment 
as well but provided a way for unconnected or distant people to 
sync up.  It would only work well if people using the device are 
"grouped" which may not be compatible with the experiment's 

> WWVB and its kin may well be your best bet, _if_ you can hear 
> them enough places.

Yes, so far that is the leading candidate.  Maybe if we all 
chipped in a $0.01 each ($3M total), they can get the power up to 
1MW?  That would probably get world wide coverage (and cook 
nearby hot dogs... :-).

Mike Ciholas                            (812) 476-2721 x101
CIHOLAS Enterprises                     (812) 476-2881 fax
255 S. Garvin St, Suite B               mikec at ciholas.com
Evansville, IN 47713                    http://www.ciholas.com

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