[sdiy] Sine-wave VCOs?

[James Patchell]

You can make a sine wave oscillator using a dual OTA in a state variable 
type filter connection.  I used a zener diode pair to make the AGC to 
control the output amplitude.  Trouble is, this is a somewhat more 
complicated circuit, plus, it doesn't perform very well at very low 
frequencies.  But, you do get sin/cos out of it.  However, because it is 
OTA based, the THD of the sine wave is about 0.4%, and you can get just 
about the same performance using an OTA to wave shape a triangle.  So, it 
is about the same distortion, higher performance, etc...so, at least in the 
analog world, that is why I personally prefer the wave shaper methode.

Now, in the digital world, things are much simpler...but, lets not go there :-)


At 08:15 AM 5/3/2004 -0700, Metrophage wrote:
>I can't help but notice that practically every synth circuit I see
>which has a sine output was achieved by shaping another wave, typically
>the triangle. Is it really so hard to make sine VCOs, or are they just
>not desired? Of course, many schematics I have seen are just "function
>generators", probably precise but with no voltage control. How does a
>keyboard tracking VCF which self-resonates compare? I am not so much
>into subtractive synthesis, but it seems to me that filters might
>suggest ways to build sine VCOs
>CJ
>
>
>
>
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         -Jim
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