Voltage controlled oscillator
A voltage controlled oscillator (VCO) is an electronic oscillator that produces cyclical waveforms with frequency controlled by a voltage input. Usually sine, triangle, square and sawtooth waveforms are available, although sometimes the tri/saw/ramp outputs are merely waveshaped alternatives of the same output controlled manually or by control voltage.
The oscillator frequency is varied by an applied cv, which though not necessarily different to any other cv, will generally be of a standard designed to produce accurate pitch-tracking across a 12 semitone per octave musical scale.
- 1V/octave – Used by Roland, Moog, Sequential Circuits, Oberheim and ARP and in Eurorack.
- 1.2V/octave - Used by Buchla.
- Volts/Hertz – Used in most but not all Korg and Yamaha synthesizers.
Usually modulating signals may also be fed into the VCO to cause frequency modulation (FM), pulse width modulation (PWM) and forms of synchronisation. It is worth noting some VCOs can act as low frequency oscillators and may have built in waveshaping, ring modulation, sub-oscillators, etc.
Sawtooth core VCOs work by converting the control voltage to a current and using this to charge a small capacitor. A comparator compares the voltage across this capacitor with some fixed voltage (E.G. 10 V). When that threshold is reached, a FET shorts the capacitor, bringing the voltage across back to 0 and starting a new cycle.
A VCO can go out of tune because:
- The control voltage drifts with temperature. This is easily taken care of by using op amps with low offset and low temperature coefficient.
- The exponential voltage to current converter drifts with temperature. This is by far the worst offender. The fix is usually to use a tempco (a positive temperature coefficient thermistor), with a temperature coefficient of about +3350 PPM/K. Or doing the exponential conversion with two quarters of an SSM2164/V2164 (one to do the exponential conversion, one to provide the temperature compensation).
- The value of the capacitor drifts with temperature. For this (and for other reasons) use a polystyrene or NP0/C0G ceramic cap.
- The threshold voltage drifts with temperature or power supply load. This is avoided by using a proper voltage reference instead of a resistive divider.
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- Wikipedia:Voltage-controlled oscillator
- Synthesizer Projects and Schematics by Ian Fritz, has pages about VCOs and circuits to compensate for temperature drift effects
- On Oscillator Linearity and Musicality by Neil Johnson