[sdiy] 2164 Expo VCO tracking

[David G. Dixon]

I spent much of today (when not watching the final round of the Masters)
slaving over the latest incarnation of the 2164 Expo VCO.  I'm still not
entirely happy with the tracking, but I would like something else to compare
it to.  I can't seem to find any independent VCO tracking data on the web,
except for the writeup of the MOTM 300 on the Synthtech website which
alludes to 0.05% tracking over a very wide range.

Here is the best tracking I've been able to achieve with my latest PCB, by
messing around with a couple of resistor values (target values and relative
errors in parentheses):

-4V:    27.48 (   27.50  -0.07%)
-3V:    55.00 (   55.00   0.00%)
-2V:   110.0  (  110.0    0.00%)
-1V:   220.0  (  220.0    0.00%)
 0V:   440.0  (  440.0    0.00%)
 1V:   880.8  (  880.0   +0.09%)
 2V:  1760    ( 1760      0.00%)
 3V:  3520    ( 3520      0.00%)
 4V:  7025    ( 7040     -0.21%)
 5V: 13755    (14080     -2.31%)

Note that the VCO was set to oscillate at 440.0Hz at a CV of 0V by adjusting
the panel pots, the CV summing amp feedback trimmer was adjusted to optimize
LF tracking, and then the HF trimpot was adjusted to eliminate error at
3520Hz.  The octave voltages were generated by the six-octave selector
switch on my custom scanning keyboard, which selects from a ladder of
hand-matched 100k 1% resistors.  I went from the lowest A to the highest A
on my keyboard above 880Hz and readjusted the 440.0Hz point to read the
values above 440Hz.

Obviously, the tracking falls apart much above 7kHz, but you can't really
discern pitches above that.  The tracking is more or less perfect within the
musically useful range, except for the octave around 1V, which is always a
bit sharp (I don't know why).

Tracking was near-perfect on the solderless breadboard up to 16kHz and
beyond, but is less so on PCB.  I'm still trying to figure out why.  I think
it must have been a fortuitous combination of stray capacitances, etc.

So, would this be considered "good" tracking?  I presume it doesn't compare
favourably with the MOTM 300, but I'm curious how it stacks up to other
VCOs.  If anyone here has any tracking data of their own to share, I'd
really love to see it.