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VCO in synthesisers

The VCO or Voltage Controlled Oscillator



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.

VCO in synthesisers
Rick Wakeman – the king of Moog (photo credit above, Aurelio Moraes, live, Sao Paulo, Brazil)

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

the VCO - analog designs AD654
Analogue Designs – AD654 – Technical Information

 

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

Further Reading:

Muff Wiggler – Wave Shape Transition (beyond the scope of a beginner but very interesting, thanks to “Highcooley”).

Experimentalists Anonymous – Oscillators, Minimoog VCO etc

Vintage Synthesisers using Ring Bridge Modulation

Oscillators for the Newbie – Positive Feedback

The 741 Operational Amplifier

FM in Music Synthesizers

 

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