[time-nuts] Low cost synchronization

[Mike Ciholas]

On Thu, 18 Aug 2005, John Day wrote:

> You can actually get very good tempco's by running two 
> oscillators either 500kHz or 1MHz apart and mixing them. 
> Mounted in a thermal mass but without an oven they will oven 
> give better than TCXO and moderate oven type performance.

Curious idea.  I guess the assumption is that the two crystals 
experience the same environment and that has the same effect on 
them.

I wonder if you did the same thing with two 32.768KHz crystals?  
You would learn their initial calibration at the factory, then 
track the time so that the difference is maintained.  I'd have to 
sit down and do the math to see if this makes sense as this would 
be only a few ppm typically.  The assumption here is that the two 
crystals, being of the same type in the same environment, would 
age and change at the same rate.  An experiment can be performed 
to check this.

Quick example: One crystal is 1ppm faster than the other.  Then 
the two counters will "slip" 1 count every 30.5 seconds.  You 
count the "slips" to record time, the slips per unit time being 
measured at the factory.  My intuition says there is something 
mathematically wrong here, the slip rate probably won't be stable 
to <1ppm.

I guess this would generalize to more than 2 crystals.  We could 
put a dozen in there and average the results to reduce the error.  
Seems like it would reach diminishing returns fairly quickly, 
though.

Random question: Any good books on the finer details of crystals?  
I'm looking for something that goes into all the gory details of 
how they vary, age, drift, etc.

> Real time temperature compensation is actually quite common. In 
> several radio applications I have designed systems using two 
> techniques.
> 
> 1.. Have the manufacturer supply AT crystals with carefully 
> controlled cut angles - for a given cut angle the tempco will 
> not vary much. Then use a simple compensation table based on 
> angle.
> 
> 2.. Learn the crystal characteristics. In a couple of chamber 
> runs measure the characteristics of each crystal. You probably 
> only need to do 20 temperatures to cover -10 to +60C. Work out 
> the slope and using some nice interpolation algorithm work out 
> a correction table. Then voltage control the Xtal osc to keep 
> the frequency on target.

I can get a start from the chart in the datasheet on the crystal:

http://www.ecsxtal.com/pdf/ecs-3x8.pdf

I don't actually need to adjust the crystal itself, all I need to 
do is accumulate the error mathematically and "fix" the answer 
past the counter output.  So no need for additional circuits.

> #2 works well enough that a pretty standard crystal will 
> deliver TCXO or better performance because very few TCXO's use 
> active compensation.

Are we well under +/- 1ppm?  If we can get down to +/- 0.1 or 0.2 
ppm, I think that would do it, but 1ppm isn't enough.

> But really, can we fit all of this ins key fob? Nope!

I was thinking a tiny microcontroller, a small watch crystal, and 
a thermistor would easily fit into a key fob.  The question is 
whether the embedded algorithm in the device can be good enough 
to keep it on time.

Still, the WWVB approach seems like the best bet so far but the 
crystal only approach would be so nice.

-- 
Mike Ciholas                            (812) 476-2721 x101
CIHOLAS Enterprises                     (812) 476-2881 fax
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