Quadrature VCO schematic posted

[Magnus Danielson]

From: Chris MacDonald <christopher_macdonald at yahoo.com>
Subject: Re: Quadrature VCO schematic posted
Date: Mon, 9 Oct 2000 10:15:30 -0700 (PDT)

> Hi Magnus!

Hi Chris,

> > Ah, nice. But being a bit picky, this is just an quadrature
> > oscillator core, but I don't see any CV input. I personally see
> > exactly what treatment which would be appropriate to have a CV input,
> > but for other less oriented in the field would be confused by having
> > this called an "VCO".
> 
> I see what you are saying, I just thought that the inclusion of the
> frequency potentiometer at the top of the drawing was enough to suggest
> "VCOness".  I have amended the drawing and link to say "Variable
> Frequency Quadrature Sinewave Oscillator". 

Ah, that's better ;)

> Since I haven't expanded the frequency control beyond what is seen in
> the schematic, I didn't want to add any circuitry to the drawing that I
> hadn't actually tested.  Sorry to disappoint, I guess the addition of a
> full-featured CV section is "left as an exercise for the reader."  :)

Right. A minor note. You could use the expo converter from the ASM-1
VCF, but I would not count on it to be as stable as the VCOs.

> > In essence this is just an state-variable filter brougth up to self
> > oscillation and the diode setup ensures the stabilization of the
> > amplitude. If you compare this to the SEM/ASM-1 filter you are close.
> 
> Exactly.  Sorry, that's what I was trying to say in my first post.  In
> fact, Mini Modular owners could probably add a "quadrature oscillator
> mod" to their filter by adding a DPDT switch, an opamp, five diodes,
> three resistors, and a trimmer.  

Right. Consider this as an answer to the answer to the debate on the
possibility to get an state variable type of filter to self-oscillate.
You can.

> Here's a link to the Bode patent if you are interested (thanks to Mike
> I. for mentioning the patent). 
> 
> http://www.delphion.com/details?pn=US04145670__

Aha, worth looking at.

Cheers,
Magnus