Re: Low distortion sine wave VCO's - state of the art?

In article <hOWdnWIc8NHxVLvZnZ2dnUVZ_v6dnZ2d@xxxxxxxxxxxx>,
John Popelish <jpopelish@xxxxxxxx> wrote:

I just realized that if you replace the two integrator input
resistors with operational transconductance amplifiers, you turn
this into a VCO (Well, a current controlled oscillator anyway.
Adding a couple resistors to the gain control inputs would fix
that.) A pair of OTA's from the same pack should track well
enough for the AGC to keep the output amplitude quite constant.
With low distortion opamps, I think the second integrator (from
the variable feedback point ) might well be able to hit the .01%
distortion spec.

Yes you are spot on. I have a 1980-ish design doing
exactly that...... fast frequency slewing, with no
bounce. 5x multipliers instead of OTA's though.

2x multipliers in series with the integration R's
for the voltage control of frequency, as above.
2x multipliers squaring Sin and Cos, outputs added
together to produce a fast-response rectification without
the need for a smoothing cap. Vref was subtracted at the
same time to produce the error voltage for the
multiplier that increased/decreased the oscillation
gain.

I got about 0.005% THD in the 1980 prototype, which
was at the limit of the 400Hz notch filter I had at
that time. That was with 2x low THD multipliers in
the signal path (AD634?), using the correct low THD
X or Y input for the AC, and Vpk held down to no
more than about 7v (4-5Vrms).

--
Tony Williams.
.

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