[time-nuts] Another "atomic" clock question

[Chris Albertson]

On Mon, Mar 3, 2014 at 2:18 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>
>>> Junk crystals are good thermometers.  Ballpark is 1 ppm/degree-C
>
> albertson.chris at gmail.com said:
>> So does this mean I can epoxy a sandstone power resister to a junk crystal
>> and keep the frequency exactly perfect by varying the power in the resister?
>
> Sure, for some values of "perfect" and such.

There is only one value of "perfect".  The goal is to keep the
frequency spot-on the marked 10MHz  If this system works the crystal
never moves off it's design value.

We are not using het to push or pull the crystal off it's fundamental
design point but maybe we say to "push or pull it back to center.

>
> I've occasionally thought about building something like this, just for the
> hell of it to see what happens and/or what I learn, and or how good I/we can
> get on a low budget.

That is my goal too.  I'm never impressed that people with unlimited
budget do good work.  But doing the same with recycled junk parts
really is impressive.

>
> I think there are two problem areas.  One is sensors and control algorithms.
> The other is board layout.
>
> Where is the sweet spot on complexity vs accuracy?  I'm looking for
> science-fair level of goodness rather than super-expensive to get another 0
> or two.
>
> What's the best low-cost way to measure temperature?  Many of the obvious
> choices are only good to 0.1 C.  That's great if you are trying to measure
> room temperature or or want to keep your CPU from melting, but it's probably
> leaving a lot on the table if you are interested in the frequency from a
> crystal.

Why are you measuring temperature.  Just let it be "whatever".  You
measure the frequent and then adult the current in the heater to keep
the frequency constant.  I assume that if the crystal really is a good
thermometer then frequency is all you need to measure.

One can make the control easier by adding some thermal mass.  A big
chunk of metal would add some stability.
>




-- 

Chris Albertson
Redondo Beach, California