[sdiy] Tempco adjuster idea

[Ian Fritz]

Hi Magnus --

At 09:30 AM 4/27/2003, Magnus Danielson wrote:
> >
> > Actually, the kT comes from classical statistical mechanics
> > (thermodynamics), but thanks for the support!  :-)
>
>Yes, it does... and to show why they invented the quantum mechanics, since 
>it prooved
>to be part of quantum mechanic behaviour, where statistical mechanics is 
>mearly an
>extention to that field (even if the areas where originally invested the 
>other way
>around).

Well, sure, but I thought you were saying that the semiconductor energy 
levels were coming into play.  Those go into the I_es prefactor.

> > That's what I want to get at ultimately.  With the correct tempco at room
> > temperature how do the remaining nonlinearities and drift stack up vs.
> > whatever errors you get with active compensation (like the spectacular
> > Patchell result)?
>
>That takes carefull measurements. I have two good DMMs, but no means to 
>measure
>temperature very accurately. It seems nobody really seems to give out the 
>details on
>that. Any proposals?
>
>Ease of calibration is my concern. (Ice-water and boiling water in my lab 
>doesn't
>*really* feel like a good idea.)

I just use an LM335, which directly reads absolute T at 10 mV/K.  You can 
correct the readout with a single resistor determined from a single T 
calibration point if you want, but so far I haven't bothered.

For the next level of accuracy and precision you can use a platinum (Pt) 
sensor -- an RTD element -- in a Wheatstone bridge circuit.  You can buy 
these elements precalibrated.

For a calibration point you can use the melting point of ice if you are 
careful.  The ice from your freezer will be below the freezing point, so 
you need to crush it up real fine and make a slurry with some water inside 
a thermos bottle.  Then immerse your sensor in oil inside a test tube, 
making sure it doesn't touch the glass, then put that assembly into the 
ice-water slurry.  This is a standard lab procedure for making a 
thermocouple reference junction.   To be super-fussy, use distilled or 
deionized water for both the water and the ice.

Boiling water is not recommended for calibration, for several reasons 
including lack of homogeneity, dependence on atmospheric pressure and 
dependence on impurities.  It's OK for rough estimates, though.

The biggest problem with measuring drift of a VCO is keeping the 
temperature uniform across the circuit board.  Gradients and drafts just 
kill you.  I put my boards in a plastic container with crushed-up newspaper 
to prevent drafts, and I try to heat the container uniformly.  But it is 
not easy to measure drifts below about 200 ppm/K.

> > Agreed. And I know Jim is working on simplifying his design. Now if I 
> could
> > just understand how it works!
>
>Ah! Actually, I looked at Jim's design and considered an alternative 
>design. Jim used
>up a complete OTA for the inverse properties, where as I worked on how to 
>do things
>properly with the use of linearizing diodes. I put it away, but I felt 
>quite confident
>that reductions of Jim's curcuit could be done.

Jim tells me he looked at the linearity of several configurations and 
picked the best one.  I think linearity is the biggest issue with the 
active compensation scheme.  In the double OTA scheme Jim uses, the 
nonlinearities cancel because the two inputs are at the same voltage.

>The whole trick where to do a "bandgap" temperature reference and multiply the
>incomming CV with that reference, that was the idea. This multiplication 
>naturally
>needs to be very linear... and you end up having some problems in another 
>corner.
>However, such things can be solved. Once you know that general plan, it's the
>execusion of the plan which should form the next layer of confusion.

What I meant about not understanding Jim's circuit is that I can't get the 
numbers to come out right.  The circuit analysis seems straightforward, but 
I get the wrong scale factor.

>The comparator seems to be forgotten all the time BTW. I've never seen an 
>analys of
>its sensitivity from external sources (temperature included) into the 
>frequency of the
>oscillator.

Agreed -- this should be looked at more.  Any ideas on what to measure?

>So you are saying that I should not heat things to 1500 K unless I take 
>out the
>FET-opamps? ;O)

Or just use them as fuses.  :-)

   Ian