[sdiy] quadrature sine wave oscillator question

[synth at charlielamm.com]

I was going to mess around with this:
http://focus.ti.com/docs/apps/catalog/resources/appnoteabstract.jhtml?abstractName=sbfa013

It's 3 phase not 4;

Getting the VCO to scale (using an OTA for the Freq resistor?  Vartec?
Don't know) might be tricky

Also it sounds like the amplitude of the output may be hard to makie
consistent over a wide freq. range?


On Tue, 25 Mar 2003, Czech Martin wrote:

> has anybody dealt whith those?
>
> This looks like a state var filter, but the Q
> regulation is set up so that a sine wave can build
> up.
>
> To prevent heavy saturation and distortion a amplitude
> regulation is needed, like in all low distortion designs
> be it Wien bridge or else.
>
> Unfortunately the gain/phase characteristics change
> a lot if the frequency is changed a lot, so
> the regulator must work fast.
>
> Usually this is done via saturation devices (zeners etc.).
> This will cause some distortion.
> Or by some peak detector and very slow lowpass filter.
>
> If the quadrature outputs are squared und summed up,
> this term will be DC (ideal) and can be taken for
> instanteneous amplitude regulation:
>
> SQR(sin(x)) + SQR(cos(x)) = 1
>
> I know there was a single chip or hybride package for
> this. Has anybody tried to build an audio range quadrature
> VCO using this method?
>
> Cheers
> Martin Czech
>
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