[sdiy] VCO design question

[David G. Dixon]

I would like to design a VCO which can morph smoothly from up-ramp to
triangle to down-ramp, preferably under voltage control.  At this point I
see two options for doing this:

One option takes its inspiration from the Korg MS-20 Modulation Generator,
and uses two FET switches connecting the expo current to either side of a
pot, with the wiper of the pot feeding the summing junction of the
integrator op-amp.  The switches are controlled by the output of a
comparator (the output of which also provides a square wave with PWM) and an
inverter after the comparator.

By my reckoning, if the up-ramp has the duration R1*C, and the down-ramp has
the duration R2*C, then the total triangle period should be (R1+R2)*C, and
since (R1+R2) = the total R of the potentiometer (which is fixed) the
frequency should not depend on the symmetry of the triangle wave.  The
challenge here is to give voltage control to the pot.  I'm thinking of an
NTE7072 (a dual IC-based DC controlled potentiometer), but I'm not sure that
it would work, and I'm a bit out of my depth.  I'd like to avoid digital
pots if I could, although this may be the simplest and cheapest option.  I'm
also wondering if an LM13700 (dual OTA) could be rigged up to act as a VC
pot (by somehow using each as a VC resistor and somehow rigging it so that
their total resistance is constant -- how, again I have no idea).

Here's the Korg MS-20 schematic.  The Modulation Generator is somewhere near
the centre of the picture:
http://www.korganalogue.net/korgms/images/service/ms20/circ1.gif

The other option is to build something like what is discussed in this paper:
http://synth.stromeko.net/diy/SawWM.pdf.  This is much more complicated, and
I'm not sure my (very nascent) circuit design skills are up to the challenge
of fleshing this conceptual design out with actual component values.

I note that Analogue Systems has a VCO which does what I describe -- see
http://www.analoguesystems.co.uk/pdf/RS%2095.pdf.  I have no idea how they
achieve it, but I'm guessing it's akin to option 1 above.

Has anyone else on the list tried to do this?  Is there an existing
schematic somewhere that I'm not aware of?  Am I nuts for thinking I can do
this?  Any suggestions welcome!

David G. Dixon
Professor (but NOT of electronics!!)
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C.  V6T 1Z4
Canada
 
Tel 1-604-822-3679
Fax 1-604-822-3619
 
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