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 Forum index » DIY Hardware and Software » Developers' Corner
Temp compensation resistor for VCO expo converter
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m.o



Joined: Jul 05, 2014
Posts: 8
Location: Sweden

PostPosted: Sat Jul 12, 2014 5:41 pm    Post subject: Temp compensation resistor for VCO expo converter Reply with quote  Mark this post and the followings unread

Edit: Sorry I realize now that these are not the same - I'll go back in my corner now
Sad

Hi guys,
Ahem, I thought I had this tempco business figured out but reality once again has slapped me down to earth Smile

So I'm working on a VCO design, pretty much standard thing with two transistors for expo conversion, op-amp integrator going into an op-amp comparator that empties the integrator cap with a transistor.

For expo conversion I'm using BCM847BS which is a pair of matched transistors in one package.

Not my problems come with the tempco resistor before the expo transistors.
As I understand it I need to compensate the voltage going into the base of the expo by +3300ppm/C (0,33% increase per degree C temp).

My implementation right now is that I have the CV input buffered to unity by an op-amp, then I have a voltage divider;
At the "top" I have a 10kOhm NTC with -3430ppm/C (close enough I hope), then a 100 Ohm trimpot with the wiper going to the base of the expo tranny, and under that a 100 ohm resistor to ground.

* The NTC is a SMD mounted on a small adapter sub-board together with (besides) the transistor package, I'm planning to put a gob of epoxy covering the NTC + transistors, but I haven't yet.

I was under the impression that this would be pretty much equal to what René Schmitz shows in figure 5 in his "Whats that exp and tempco stuff? " -page (http://www.schmitzbits.de/expo_tutorial/index.html).

However when I try it, my oscillator drifts a lot (from an initial 300Hz it drops to ~120Hz over about 30-50 minutes), and If I open my window it starts climbing.

What am I doing wrong here?

Did I get it all backwards?
I tried simulating the voltage divider as well as my whole VCO in LTSpice and it looks decent there, could be doing it wrong though?

Attached picture is my way of simulating the temp dependent resistor...


tempco sim.jpg
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tempco sim.jpg


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