SV: Re: SV: [sdiy] QVCO schemo up

[karl dalen]

Forget the Ad663 stuff, Ians cirquit works gerat! :)
Also, trow away SPICE, digital simulations just give you voltage offsets! ;-)

Reg
KD

--- "JH." <jhaible at debitel.net> skrev:

> > 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.
> 
>