The VCO – the beginnings of a new era in synthesis
The voltage controlled oscillator is a very common circuit often used in conjunction with a phase locked loop circuit. VCOs often use a varactor diode to control the output frequency of the oscillator. They are cheap to build and are often used in modular synthesiser designs.
This page is not necessarily a continuation of the building blocks in a synthesiser as the VCO in it’s own right makes for a very versatile piece of test equipment.
The VCO can have a control voltage which is fixed to get a single tone or the control voltage can be that from a modulation input which causes shift in pitch synched with the input control.
The VCO in a synthesiser is normally tuned to one of either:
- Volts per Octave, the common standard used in Moog where e.g. in sequence each volt equals one octave. This is currently the more popular configuration;
- Hertz per Volt, commonly found in Yamaha where each octave equals double or half one Volt.
The VCO is very versatile and in modular form will have a multitude of outputs, usually being ramp (sawtooth), triangular, sine and square. VCOs can be connected together through the control voltage, the outputs can be wave-shaped and of course, modulated from an external (or internal source).
One of my favourite circuits is brought to you by Analogue Designs, using the AD654
Some circuits of the VCO
Schematic of popular but ingenious VCO by Thomas Henry on Birth of a Synth – Scott Stites.
Nuts and Volts, as always, gives a very interesting view of OTAs (Operational Transconductance Amplifiers – Part II the LM13700). Article by Ray Marston.
Onsemi MC74HC4046A – VCO/PLL
Muff Wiggler – Wave Shape Transition (beyond the scope of a beginner but very interesting, thanks to “Highcooley”).
Experimentalists Anonymous – Oscillators, Minimoog VCO etc