[sdiy] VCO Tuning goals

[Ian Fritz]

>My question was really meant to be more to the general
>"best" way to approach tuning to get the most musically useful VCO.

First, we have to understand that if the method used for high-freq tracking 
compensation could provide exact compensation, then it wouldn't matter what 
procedure was used to dial it in.  However the compensation scheme 
generally doesn't have the correct mathematical form for exact 
compensation, so there will be second order errors.  The nature of these 
will depend on the particular design.

There are many ways to set up a VCO tracking, and you will probably get 
lots of ideas here.  My method (as described on my dial-a-tempco page) is 
as follows.  I have a little box with a pushbutton that puts out either 
zero or 1.0000V.  I connect this to the V/Oct input of the VCO.  Starting 
near the low end, I set the VCO to ~100 Hz and adjust the V/Oct trimmer for 
a correct octave (using the box).  Then I go to a higher frequency and set 
the HF Track to get a correct octave there.  Then back to readjust the100 
Hz octave, then up to the HF octave again, etc, until both octaves are 
correct.  Then I measure all the octaves from 25 Hz up to 20 kHz and look 
at the deviation in tuning at each point.

The tricky part is deciding what to use for the upper frequency.  This you 
might have to experiment with.  2 kHz or 5 kHz are reasonable places to 
start.

Musically, what is objectionable about mistracking between VCOs is the 
beats they make against each other.  A 1 Hz beat frequency is (of course) 
1% mistuning at 100 Hz and 0.1% mistuning at 1 kHz.  So tracking is most 
critical at the top end of the musically useful pitch range.  So there is a 
good argument for getting the best HF tracking at the 1kHz / 2kHz octave or 
the next one and forgetting about errors at higher frequencies.  OTOH if 
the oscillator is so good that it only requires a small amount of 
compensation, then it works fine to adjust the HF Track at a much higher point.

HTH

  Ian