[sdiy] Temperature stable lin-exp converter with a CA3086 or CA3046

[patchell]

    Yes, a very well known technique.  I bought a VCO kit a long time ago
(~1978), the Tau 1050 (http://www.silcom.com/~patchell/TAU1010/TAU1050.PDF) that
does this.  It is a very effective way of stabalizing an expo pair.

    It is not without its problems (no matter what you do, there are problems,
so you have to trade one problem off for another).  The biggest one is that the
warmer it gets, the more difficult it is for the temp controller to maintain
temperature.  Also, just because of the way a temp controller works, as the
temperature changes outside, the temperature of the chip will change, but not as
much.  You basically divide the outside temp change by the loop gain of the
control loop.  Also, you need to insulate the CA3046 from drafts (put a piece of
styrofoam on it).  And the biggest problem is the added power consumption.  It
does take a significant amount of power to heat the chip.

Rob Mantel wrote:

> Hi all,
>
> In this book I have here, they use a CA3086 (or CA3046) to build a lin-exp
> convertor for a VCO. Two of the transistors inside do the real lin-exp
> conversion, two others are used to control the temperature, one as 'sensor',
> one as 'heater'. From what I understand of it this should lead to a constant
> temperature of the ca3086 (kind of like the old ua726 got its constant
> temperature from its oven), so no external temperature compensation is
> needed anymore.
>
> Is this a well known thing to do? If so, why doesn't everyone do it this
> way, it is a very cheap solution, no expensive parts involved at all. Does
> anyone have any experience with the temperature stability of VCO's built
> this way?
>
> Rob (who floods the list with questions today, sorry)

--
 -Jim
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