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A voltage-controlled oscillator (VCO) is an electronic oscillator whose output frequency changes in response to an applied control voltage. VCOs provide tunable signals in devices and circuits such as phase-locked loops (PLLs), frequency synthesizers, radio systems and clock generators.
What is a voltage-controlled oscillator?
An oscillator produces a repeating alternating-current signal. A VCO lets a control voltage adjust the signal’s frequency: changing the voltage changes how quickly the output cycles. The exact voltage-to-frequency relationship depends on the circuit and component; a VCO has a specified tuning range, and its behavior outside that range should not be assumed.
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The relationship between input voltage and output frequency is called the tuning characteristic. Its slope, often called tuning sensitivity or VCO gain, describes how much the frequency changes for a given voltage change and is commonly expressed in hertz per volt.
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How does a VCO work?
Common LC implementation
In an LC oscillator, an inductor and capacitor exchange energy, producing a resonant frequency determined by their effective inductance and capacitance. A common RF design uses a varactor diode as a voltage-controlled capacitor. Changing its reverse-bias voltage changes its capacitance, which shifts the resonant frequency and therefore the oscillator’s output frequency.
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This is one common VCO architecture, not the definition of every VCO. Other implementations may work differently, so the tuning mechanism and performance depend on the particular design.
Tuning range and gain
A real VCO operates over a defined tuning-voltage and output-frequency range. Tuning gain matters because a higher gain means a smaller voltage change produces a larger frequency change. That can make voltage noise at the tuning input more consequential for phase noise.
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What does a VCO do in a PLL?
A phase-locked loop uses a VCO as an adjustable output source. The loop changes the tuning voltage to move the VCO frequency toward the required relationship with a reference signal. VCOs also serve in frequency synthesizers, radio and telecommunications equipment, clock-generation systems, and circuits that use frequency or phase modulation.
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Which VCO specifications matter?
No single specification determines whether a VCO is suitable. Compare the parameters against the signal, circuit and operating conditions you need:
- Tuning range: The minimum and maximum output frequencies available across the permitted tuning-voltage range.
- Tuning gain or sensitivity: The frequency change per volt, commonly stated in hertz per volt. Consider its interaction with voltage noise at the tuning input.
- Phase noise: A measure relevant to signal purity and timing performance. In common varactor-tuned designs, a wider tuning range and low phase noise can be competing goals.
- Output and power: Check output level, current consumption, harmonics and spurious emissions for the application.
- Operating limits: Verify supply and tuning-voltage limits, including whether the rest of the circuit can meet the tuning requirements.
How VCOs are characterized
A typical characterization setup holds the supply voltage fixed, varies the tuning voltage and measures the RF output. Depending on the test, measurements can include tuning range, tuning sensitivity, frequency pushing and pulling, output power, current, harmonic power, phase noise, AM noise, settling time and spurious emissions. Use measurements taken under conditions relevant to the intended application.
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VCO versus VCXO
A voltage-controlled crystal oscillator (VCXO) is a related type of oscillator whose crystal-based frequency can be adjusted by voltage, for example through a varactor. The terms are not interchangeable when the use of a crystal reference element matters to the application.
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