[sdiy] exponential vs linear vco cores

[David G. Dixon]

> Tempco class discussion based on Rene's website, with a side note that if
> you really want to go down the rabbit hole, you should check out Ian
> Fritz's website.

Didn't want to go down the hole, or even near the hole.  That's why the
2164-based approach was so appealing to me.  No gluing of expensive and hard
to find parts (tempcos) to other expensive and hard to find parts
(super-matched pairs)!