[sdiy] YuSynth vco tracking

[David G. Dixon]

> Two points, two trimmer adjustments

...I should probably mention that, when I do this, typical tracking below CV
= 0V is roughly as follows (absolute and relative errors in parentheses):

 0V: 1000     ( 0   ,  0.00%)
-1V:  500.3   (+0.3 , +0.06%)
-2V:  250.2   (+0.2 , +0.08%)
-3V:  125.0   ( 0.0 ,  0.00%)
-4V:   62.37  (-0.13, -0.21%)
-5V:   31.05  (-0.20, -0.64%)

Hence, the LF tracking is actually a sigmoid rather than a line.
 
Of course, the error at the lowest octave is audible.  Perhaps it would be
better to start at 31.25 or 62.5 and perfect the octave there.  However,
this would send 500 closer to 501.  I can improve things if I drop the
frequency at CV = 0V back to 500Hz (change a resistor), but then the HF
(above 4kHz) goes flat.  Of course, the frequency errors up there are not
really audible; only the beating of one VCO against another, which requires
tracking precision from one VCO to another rather than accuracy.

Do other VCOs lose tracking accuracy at low frequencies?  (I suppose I could
improve things by using opamps with lower offsets, but I'm not sure that is
really the problem; I think it is an artifact of 2164).