[time-nuts] uC ADC resolution (was: Poor man's oven)

Bob kb8tq kb8tq at n1k.org
Wed Jun 7 17:25:59 EDT 2017


> On Jun 7, 2017, at 4:40 PM, Attila Kinali <attila at kinali.ch> wrote:
> On Wed, 7 Jun 2017 15:25:38 -0400
> Charles Steinmetz <csteinmetz at yandex.com> wrote:
>> Chris wrote:
>>> Assume me goal is to regulate temperer of an aluminum block to within
>>> 0.1C, how good must my ADC be?   Is an effective 6-bits good enough?
>>>     *   *   *If I scale the input to the ADC such that it os 1.0C from 0 to 63
>>> counts then each cunt is 1/64 C which is about 6 times better then my
>>> allowed error of 0.6 C.
>> The required number of bits depends on the range it represents -- it is 
>> all about scale, as your second paragraph above seems to recognize.
>> You are right, 6 bits is enough to represent a 1C span to the nearest 
>> 1/64C.  But that is not the whole answer.  This representation will be 
>> mapped to 64 steps of oven output.  If the oven has very little power, 
>> these can be reasonably fine steps -- but recovery from large errors 
>> will be painfully slow.  On the other hand, if the oven has normal-ish 
>> power, the steps will be much too large to control the oven temperature 
>> finely.
> Addedum to what Charles wrote:
> If you want to build a temperature control for something similar
> like an quartz oven, just get one of the modern delta-sigma ADCs.
> You'll pay €10 for one, but it's really a hassle free way to
> precisely measure temperature. As most of these have a large
> number of channels, you can measure multiple sensors as well
> at no additional cost (beside the thermistor).
> Additionally: if I would set out to build my own OCXO today, I would
> go and buy one of those lunch thermos flasks to house everything. Their
> isolation is higher than anything you can easily build yourself,
> especially at that size. I would place the (inner) oven at the bottom,
> probably using a puck design similar to the E1938, place the electronics
> on top of it and close the lid using an aluminium plate which forms
> the outer oven. 
> Such a design allows to have low temperature gradients within the flask
> (due to the metal walls). The outer oven allows to optimize the inner
> oven for stability without the need to deal with large temperature ranges.
> And all together it is still quite cheap.

I certainly do not disagree with the idea of using a thermos or a cheap eBay
dewar in a home brew OCXO project. 

It’s not all a free lunch. You still have issues. One somewhat counter intuitive one
is the gradient / cooling issue. The same super duper insulation that helps
you also hurts you. Your internal circuitry will have a finite power requirement.
Yes it can be pretty low, but it’s still there. You have very high thermal resistance.
Even small power sources will generate noticeable temperature rise(s).

No that observation is not original to me. It’s been passed down over the 
generations of OCXO designers. I’d love to identify just who passed it on.
Unfortunately it’s been way to long for me to remember exactly who.


> One drawback, though, is the large size of the flask. But for a hobby
> project that does not need to fit into another product, this is fine.
> 			Attila Kinali
> -- 
> You know, the very powerful and the very stupid have one thing in common.
> They don't alters their views to fit the facts, they alter the facts to
> fit the views, which can be uncomfortable if you happen to be one of the
> facts that needs altering.  -- The Doctor
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