SV: [sdiy] QVCO schemo up

[JH.]

> I SPICE simulated it, already. It works well
> as advertized.  DO NOT try it with a 3080
> it will generate quasi-triangle waves. You need
> the LM13700 (or 13600 I presume) with the diode
> linearization.
> 
> I did get offset on the two waves in opposite
> directions. I have not 'trimmed' the simulation.

That's what I get with _every_ QVCO based on
OTAs in simulation. Must be the offset voltage.
I _hope_ the THAT VCAs would solve this problems
(if they work in this application, that is.)


> It takes hundreds of cycles to come up to stable
> amplitude. I suspect this will be similar to an old
> Wien-bridge oscillator which takes a long time to
> stabilize after a frequency shift, but I didn't
> simulate that, yet.

Ok, this might be the catch then. I hope it's not,
but it makes sense. Very weak gain correction,
low distortion, but long reaction time.
If it only has to react to gain drift, no problem.
If for some reason it has to react to a frequency
change (why would it, btw?), then FM on such
a VCO would be a problem.

I've posted a drawing (simulated, but not tested) of a
possible version of the QVCO from Tietze/Schenk
here:
http://www.oldcrows.net/~jhaible/matrix_fx/jh_ts_qvco_idea.pdf
They have drawn it with analog computer building blocks,.
which I changed to AD633s and THAT VCAs.
They advertise this circuit for being frequency modulated
without amplitude effects, as opposed to FM'ing LC
oscillator.

It's not as elegant as Ian's original circuit. It could be
quite small, too (compared to the waveshaping stuff
I presented some days ago), but quite expensive.

JH.