Voltage controlled oscillator (VCO)
A voltage-controlled oscillator (VCO) is a frequency-varying oscillation circuit that changes its output frequency according to an external voltage. The varying range of the frequency is determined according to its purposes. A typical voltage controlled oscillator (VCO) generates an output signal oscillating at a frequency determined in accordance with a voltage supplied from an external unit. A VCO employs one or more variable capacitors (varactors) to allow for adjustment of the frequency of oscillation for the VCO. Voltage controlled oscillators mainly produce high frequency signals. The frequency of these high frequency signals generally depends upon the capacitive value of a resonant circuit. This capacitive value is modified as a function of a control voltage applied to the varactor. In the case of use in a frequency synthesiser, this filtered control voltage originates from a phase detector of a phase lock loop. In general, voltage controlled oscillators for high-frequency signals include a resonant circuit and an active circuit for amplifying a signal output from the resonant circuit, and the resonant circuit includes a resonator having a predetermined resonant frequency, and a variable capacitance diode connected to the resonator for shifting the resonant frequency according to a control voltage. Voltage controlled oscillators are commonly employed in a variety of applications, including communication and timing circuitry. In particular, voltage controlled oscillators are commonly used in phase locked loop (PLL) control systems. A phase locked loop (PLL) is used to generate a clock signal that is synchronized with a reference pulse signal. The PLL includes a phase comparator for comparing the clock signal generated by the PLL with the reference pulse signal, a low pass filter for generating DC voltage in accordance with the comparison result of the phase comparator, and a voltage-controlled oscillator (VCO) for generating the clock signal from a control voltage. Phase locked loop circuitry is frequently used for such purposes as helping to retrieve data from a signal that may vary in frequency and/or phase. The PLL may be in relatively general-purpose circuitry such as a programmable logic device (PLD) that may need to support signaling at any frequency in a wide range of frequencies. PLL circuits are implemented in a wide variety of electronic devices including frequency synthesizers for transceivers in radio communication devices. PLL circuits are attractive in modulation applications due to their combination of controllable modulation and stable and adjustable carrier frequency.
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