[sdiy] Cheap frequency counter for oscillator calibration

[David G. Dixon]

Well, I've just learned by experience that trying to get two VCOs in tune
with each other by tuning each one separately with a frequency counter is
hopeless.

Instead, I adjusted one 2164 Expo VCO using the frequency counter to attain
best degree of 1V/octave tracking I could muster (bearing in mind that my
keyboard scanner circuit, which is the source of the CV, must also be
trimmed to give 1.000V per octave, and my digital multimeter loses a decimal
place after 3.999, so there is a tiny bit of error there as well).  Then I
tuned a second 2164 Expo VCO to the first by feeding sine waves from each
VCO to my o'scope set to X-Y mode.  The two VCOs are in sync when the
spinning circle stands still.

The two VCOs have identical components, but they don't track identically.
However, with a bit of doing, I was able to get the circle to stand very
nearly still over 8 octaves.  There are one or two octaves where there is
slight movement, but overall I'm very happy with this outcome.  This is the
way I will tune VCOs from now on.