Voltage controlled oscillators
An oscillator is an inherently unstable system that switches from one unstable state to another (and back to the original state) at a given rate. A voltage-controlled oscillator (VCO) uses a voltage to vary the rate of oscillation.
An example of a VCO is a 2nd-order LC (Inductor-Capacitor) tuned tank (filter) circuit, which oscillates at the resonant frequency of the tank. If the fixed capacitor is replaced by a varactor (or much less often, a variable inductor), then the VCO frequency can be changed by changing the voltage across the varactor. Examples of VCO topologies include the Armstrong Oscillator, Clapp Oscillator, Colpitts Oscillator, Relaxation Oscillator, Hartley Oscillator, and Vackar Oscillator.
Voltage-Controlled Oscillators are not typically used on their own (free-running mode) since their frequency is not stable. In conjunction with a phase-locked loop, VCO frequency can be made stable to the level of parts per million.
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