[time-nuts] Another "atomic" clock question

Bob Camp lists at rtty.us
Mon Mar 3 07:59:55 EST 2014


The simple approach is to use an op amp, a thermistor, and a couple of resistors. No need for anything digital. You can easily get all the gain possible (before oscillation) out of a very simple circuit. 

The net result will be about a 1C stability when you run it over temperature (say 0 to 50 C) in a lab chamber. You can tweak it a bit to get the thermal gain up to a few hundred if you have a chamber to give you feedback on your changes. 


On Mar 3, 2014, at 5:18 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.
> 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.
> 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.
> My straw man would be a thermistor and OP-Amp feeding into the ADC on your 
> favorite uProc.  Maybe the other side of a bridge would be adjustable.
> How much power do you need to keep things warm?  I'm assuming something like 
> a watt or 2 with something like a PWM from the uProc.
> -- 
> These are my opinions.  I hate spam.
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