A voltage controlled oscillator (VCO) produces an oscillation frequency based on an input voltage, which can be fixed or variable. VCOs can produce sine or triangular waveforms and use different methods to control frequency. Harmonic VCOs have better frequency stability, while relaxation VCOs can be tuned to a wider range of frequencies and are suitable for integrated circuits. Voltage controlled capacitors are commonly used to control high frequency VCOs, while low frequency VCOs use voltage controlled electric current sources.
A voltage controlled oscillator (VCO) uses an input voltage to determine its oscillation frequency. This can include direct current (DC) input with a fixed voltage to produce an oscillation with a fixed frequency. A voltage controlled oscillator can also receive a variable voltage to produce an oscillation with a modulated frequency. Some oscillators of this type have a digital pulse output capable of producing amplitude-modulated pulses. A VCO could be classified as a harmonic oscillator or a relaxation oscillator, depending on the type of waveform it produces.
Harmonic oscillators produce waveforms with sine shapes. This type of voltage controlled oscillator must include an amplifier to send the signal to the input voltage source, which creates a resonant frequency which produces positive gain around the feedback loop and creates the sine waveform. A harmonic oscillator might also include a varactor diode which provides some of the capacitance for the resonant circuit. The oscillation frequency can then be varied based on the diode’s input voltage.
A relaxation oscillator produces a triangular shaped waveform. This type of voltage-controlled oscillator contains three subtypes, including an emitter-coupled oscillator, a grounded capacitor oscillator, and a delay-based ring oscillator. The amount of time it takes to charge the capacitor determines the oscillation frequency for the emitter-coupled oscillator and the capacitor-grounded oscillator. The oscillation frequency of the delayed coupled oscillator depends on the amount of time it takes for the oscillator to reach each gain increase.
Harmonic voltage controlled oscillators have greater frequency stability than relaxation VCOs with respect to changes in noise, power, and temperature. Frequency control is also better in a harmonic VCO, as this type uses a separate circuit to control frequency. The primary benefit of a relaxation VCO is its ability to be tuned to a wider range of frequencies than the harmonic type. A relaxation VCO is also more suitable for use in integrated circuits.
A voltage controlled oscillator often uses a voltage controlled capacitor to control the output frequency. This type of capacitor contains a semiconductor diode with a capacitance that depends on its voltage. The output frequency of the oscillator can then be controlled by changing the input voltage of the diode. A voltage controlled capacitor is a convenient method of controlling the output frequency of a high frequency VCO because voltage controlled capacitors are readily available in a wide range of capacities. A low frequency VCO typically uses a voltage controlled electric current source to vary the output frequency.
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