VCO hacks, tweaks

[Joachim Verghese]

> I'm glad you caught the 3300ppm/K vs. 3500ppm/K problem, I missed that one
> completely.

I fear this issue is even more complicated if you opt for really
accurate compensation.

The problem is that the temp coefficient of the scale factor is a
non-linear function of temperature. 3300ppm/K is accurate only around
30 deg(C) (303K), since

(303 + 0.5) / (303 - 0.5)  =  1.003306

At 40 deg(C) (313K), which perhaps is closer to the operating temperature
of an enlosed circuit, the required compensation coefficient is

(313 + 0.5) / (313 - 0.5)  =  1.003200, i.e. 3200ppm/K

In other words, using a (linear) tempco resistor isn't the most accurate
compensation method, although it works well enough for most purposes.

-joachim