VCO temp compensation ideas

[Rene Schmitz]

Hi all!

During the last days I thought about temperature compensating VCOs.
There are several methods which all have drawbacks, such as the 
heating of the diff pair, which uses a "high" current, or the
tempco resistor which only reduces the temperature coefficient.
(3300ppm/K vs. 3500ppm/K, 200ppm remaining) 

I started with the Idea of changing the reference current into the 
diff pair with temperature, which soon turned out not to work. :-(

Then I had the idea to replace the resistor /tempco resistor voltage
divider by a VCA, that is controlled by a thermo sensing transistor.
Again this doesn't work, since I'm multipling where I should be 
dividing. :-{   (old trap!!)

Then I had the Idea of using a divider, consisting of a multiplier
in the feedback-loop of an opamp. It is obvious that if you use a 
two quadrant multiplier consisting of a differential transistor pair,
and an differential opamp amplifier, you get the same annoying 3300ppm
term, right? 
Now in the feedback-loop of the opamp this turns into 1/3300ppm, hmmm!?
Using two of the three "spare" transistors in a CA8046/86 makes a fine 
differential amplifier on the same temperature like the expo. converter 
transistors. This all combined looks like temperature compensation could 
be archeived by introducing a divider (preset to divide by one) into the 
CV path between the summing amplifier and the V/oct trimpot.
The parts count is pretty much the same as with the heater approach,
it uses two opamps and maybe ten resistors, but doesn't need as much current.

And it should work at the whole temperature range. The heater just can't 
make the chip cooler than it is, I mean when you preset the chip temperature
to 40°C what do you do when the ambient temperature is higher??
(Anyone that takes a synth to the beach?! :-)

A further advantage is that one could add a temperature dependant voltage
(remember there is still one trannie left in the CA3046/86)
at the multiplier pair to over-compensate, maybe this can compensate the
temperature drift in other components as caps, discharge transistor...
One would have a trimpot to set compensation.

I have not tried it, because I first wanted to know what you experts think
of it, so...
What do you think?

Bye 
 Mr. Am-I-right-or-am-I-wrong Rene