VCO

[Grant Richter]

> 
> In both cases the integrator are LF353 instead.
> Please, feel free to suggest another OPamp for
> high freq usage as a integrator.
> Do you feel that the input impedance of the integrator
> may do several degration to the high frequency upper limit?
> 
> All of you who have made a VCDO and not using aCEM3340
> tell me about the design of yours an what kind of components
> you use to make the driver VCO for the wave table.
> I'm not talking about a PLL but rather a high(100Khz and above)
> freq VCO,temp stable,and high linear scale VCO for 1V/Oct use?
> If you have a clever PLL design tell me about it to.
> (Not to many components)
> Does any one have a good ide'a on this issue, so lets hear it now!
> 
The VCO's in the phase locked loop IC's (CD4049, 74ACT4049) work very well
as high frequency oscillators. Even if you don't use the phase locked
portion. A straight CD type with a 10pf cap runs up to ~5Mhz and my
frequency counter showed the ACT type running up to ~43Mhz. Since these are
linear VCOs you could use an exponential volt to volts converter (like an
EML101) to make a 1V/Oct. unit. I don't know if they are linear enough to
pull this off over an extended range.

The phase locked loop has the disadvantage of producing audible portamento
artifacts on steps (although small and independent of step size). It does
have the advantage of allowing you to lock the wavetable to a regular VCO.

My current project is to replace the PLL/Counter circuit with a high speed
DAC run off a sawtooth to generate the address bits. This is bound to be
imperfect but then again it's music not space shots. This has the advantage
of allowing different sweep types depending on the input waveform. This
also makes the wavetable board truely an add-on waveshaper.

Grant Richter
http://www.wiard.com/