[sdiy] VCO design question

[Paul Maddox]

Hi,

  I seem to recall that the ETI Trancendent thingy oscillators did this.
At the risk of being flamed, there's a third option, use a digital  
oscillator with wavetables and make life a lot less complicated :-)

Paul

On 2 Dec 2008, at 08:04, 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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