[sdiy] TH XR2206 VCO tune pot

[David G. Dixon]

> Oscar asked if I knew the 0V frequency of the oscillator--in fact I
> don't because I haven't started the tuning process yet. I wanted to get
> the range under control first.

You "should" be able to calculate the 0V frequency of the VCO.  I'm not sure
how to do this with XR2206, but it's fairly straightforward with a typical
saw-core VCO.  You simply need to know the reference expo current, the raw
ramp voltage, and the timing cap size.

> So why would having control over the 0V frequency be important
> otherwise? So that your pot starts in the middle of the audible range....?

It's not a matter of "controlling" the 0V frequency; just knowing what it
is.  Disconnect all the tuning pots (if you can), alligator-clip the
1V/octave input to ground, and measure the frequency.

> In option b), the 100K summing resistor is *replaced* by the 300K
> resistor, correct?

Yes.  This reduces the current by a factor of three, which reduces the
summing amp gain by a factor of three.  Hence, a 30V tuning range (pot
connected directly to rails) with a 300k input resistor is equivalent to a
10V tuning range (pot with 200k of extra resistance on the rails) with a
100k input resistor.

If you understand how voltage dividers work, and how inverting summers work,
you can design these sorts of changes yourself with absolute confidence!