[sdiy] VCO Tuning goals

[David Ingebretsen]

Thanks Ian. Very useful information. I'll re-read your "dial-a-tempco"
article, too.

David

~~ -----Original Message-----
~~ From: Ian Fritz [mailto:ijfritz at comcast.net]
~~ Sent: Friday, October 02, 2009 2:52 AM
~~ To: David Ingebretsen; synth-diy at dropmix.xs4all.nl
~~ Subject: RE: [sdiy] VCO Tuning goals
~~ 
~~ 
~~ >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