[sdiy] VCO design question

[harrybissell at wowway.com]

http://www.till.com/articles/VariableSaw/index.html

This is Don Tillman's idea to do this. There are others...

You might want to consider that while the idea is appealing... the sonic
effect of the saw / triangle wave morph is a little weak. Its more of a
subdued version of PWM (imho) and might be underwhelming as an audio VCO.

There will be other discussions in the archives a bout this

H^) harry



On Tue, 02 Dec 2008 00:04:46 -0800, David G. Dixon wrote
> 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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Harry Bissell & Nora Abdullah 4eva