[sdiy] Dual oscillator cores?

[David G. Dixon]

> With my new 2164-expo VCO, I'm thinking that a triangle-core VCO could be
> done very easily simply by switching the polarity of the
> reference-current-determining voltage with an opamp and a JFET wired as a
> buffer/inverter fed by a schmitt trigger at +/-5V.  I don't think that the
> switching speed would be as critical as it is in a saw-core VCO.  However,
> the gain-determining resistors on the opamp should probably be hand-
> matched to 0.1%, which is usually no big deal with a batch of good 1%
> resistors.

Yes, I just tried it in simulation, and it really is that easy!  A 1k
trimmer + 5.6k ground resistor with a 10k positive feedback resistor on the
schmitt trigger ensures perfect +/-5V triangles.  However, the
high-frequency tracking appears to be even more of an issue than with the
saw-core VCO, so I don't really see an advantage.  If I used a faster
schmitt trigger than TL074 wired up as such, that might make a difference,
as does a small capacitor in the positive feedback loop.  It's probably not
the JFET, as it's not actually draining anything in this case.