VCA and Op Amps Form Simple VCO/Function Generator

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mediatechnology
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VCA and Op Amps Form Simple VCO/Function Generator

Post by mediatechnology »

I've been toying with the idea of using THAT2180 and THAT2181 series VCAs as exponentially-controlled current sources and sinks. By placing a reference current into the audio input of a THAT VCA and controlling Ec, one can produce a current output that is exponentially related to Ec. This has a lot of potential uses.

One of the first things I wanted to try was a simple VCO "function generator" capable of producing triangle and squarewaves. The following is not a precision instrument but only a crude proof-of-concept. By inserting a VCA between a comparator and integrator the operating frequency can be varied over a fairly wide range at low cost. Accurate scaling, temperature compensation and precision were not initial objectives.

Image

U1B and the 2181C VCA form a voltage-controlled integrator. U1A, configured as a comparator with hysteresis, detects the zero crossings of the integrator output. The output polarity of U1A determines the polarity of the reference current to the VCA and whether or not it sources, or sinks, current into the virtual ground input of U1B. This causes the feedback capacitor to either charge or discharge. As the output of U1B crosses zero, the polarity of U1A's output reverses. In this configuration the THAT VCA inverts polarity. (The VCA in this circuit has no output I-V converter op amp to re-invert the internal OTA feeding the log/antilog gain cell.) Due to the inversion, the inputs of U1A are reversed compared to typical two-op amp circuits where the control element is a resistor.

With the values shown the hysteresis band around U1A is very narrow. Lowering the 1M to 100K produces a larger triangle wave output and lowers the maximum operating frequency.

Due to the linear dB properties of the THAT VCA, the voltage at input Ec has an exponential relationship to the output current. The sign of the output current is determined by the sign of the input current.

As mentioned this is a proof-of-concept and it works quite well. There are obvious improvements to be made in the comparator, reference stability (currently set by U1A's clip points), scaling accuracy etc. But, it works. I think it might, with suitable improvements and scaling, make a good LFO.
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JR.
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Re: VCA and Op Amps Form Simple VCO/Function Generator

Post by JR. »

A precision VCA may be overkill for a simple square wave generator, but a pair of VCAs in a state variable configuration might make a nice adjustable sinewave.

I don't know but maybe using a pair of THAT rms detectors driven by two phases off the SVF might offer some ripple cancellation. It might be interesting to look at that.

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mediatechnology
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Re: VCA and Op Amps Form Simple VCO/Function Generator

Post by mediatechnology »

I don't know but maybe using a pair of THAT rms detectors driven by two phases off the SVF might offer some ripple cancellation. It might be interesting to look at that.
Thanks JR. That was just mentioned to me and it was done in the late 70s at dbx. It's worth a do-over.

I think the triangle, not the squarewave, is the objective for this one. I was thinking more electronic music where triangle and square are more useful. For those guys shaped sinewave is good enough.

BTW LTC has a nice sinewave generator app note I need to find.
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Re: VCA and Op Amps Form Simple VCO/Function Generator

Post by JR. »

Well if you're talking analog synth you might want to add some asymmetry to make sawtooths, etc.I don't know if you can bias the VCA up to DC.

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Re: VCA and Op Amps Form Simple VCO/Function Generator

Post by mediatechnology »

JR - I think the Reference input to the VCA, when the polarity is switched at what would normally be the "audio" input, could be set at different potentials (currents) for each polarity. This would make the charging ramps have different slopes.

National's AN-263 has that option: http://www.national.com/an/AN/AN-263.pdf#page=10

The distortion trim before the FET switches sets the symmetry. It would seem that large differences between Ref and -Ref would permit sawtooth. Another sawtooth option would use integrator reset. The folks at electro-music.com provided this cite which also uses a VCA as current source:

http://www.sowa.synth.net/modular/vco4m.gif

In my crude proof-of-concept there's a little asymmetry already due to the uncontrolled and uneven clipping points of the TL084.

One poster at electro-music commented that triangle was more useful to him than sawtooth though I think the ability to alter the duty cycle would be useful.
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