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> > > > > I'm looking for a waveform generator that outputs two sine waves of
> > > > > the same frequency with 90 degree phase difference (sine and cosine).
> > > > > I need a variable frequency between 0.05 Hz and 10 Hz. Is there an
> > > > > analog design that uses a single potentiometer or perhaps is voltage
> > > > > controlled ? Low distortion is not a requirement.
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> > > > > Steve
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> > > > With only a little trickiness, you can get two variable gains from one
> > > > pot. You ground the wiper and use the two ends as variable
> > > > resistances.
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> > > > Th classic 3 op-amp "state variable" oscillator naturally makes two
> > > > outputs at 90 degrees. The tricky bit is making their amplitudes
> > > > equal. For this you need two equal variable gains. I think I have
> > > > seen a circuit that does this but I can't remember it right now.
>
> > > You are thinking of the state variable filter where you get LP, BP and
> > > HP at the same time. With a 3 op amp filter, you can scale the
> > > amplitude at each op amp output. This is known as dynamic range
> > > adjustment.
>
> > No I'm thinking of the oscillator of the same basic topology and
> > offering it as a solution to the OP's problem.
>
> > Making two gains that depend on one pot would be the tricky part.
>
> > If the OP is willing to use a ganged pot, the state variable
> > oscillator may be exactly what he wants. It would be nicer to use a
> > 10 turn pot because of the 200:1 range that the OP wants.
>
> I was thinking of the state variable as a phase shift network (narrow
> band Hilbert transformer). Wrong application obviously. I suppose the
> same network could be used as an oscillator with positive feedback.
Yes, a very small positive feedback makes it into a stable oscillator. It is a very good oscillator for doing variable frequency oscillators because the frequency is linear with the gains. This means that with a ganged pot, you can easily make the dial linear.
Because op-amps come in quads, you can use the 4th op-amp as the positive feedback and amplitude control.
My ascii art isn't working so I will explain:
Make a simple inverting op-amp circuit.
Change the feedback resistor into two resistors in series.
Pick off the center point of the two resistors as the output.
Apply a small signal and the op-amp inverts as expected.
Apply a large signal and the op-amp hits the rails. The inverting input of the op-amp will no longer stay at ground and thus the output shows a non-inverted signal.
The extra op-amp and 4 resistors gives you the amplitude stablization circuit.
Or
> you could make a sine wave oscillator, then feed it to the state
> variable filter to make quadrature signal. This could have the
> additional feature of filtering the sine wave oscillator.