Voltage controlled oscillators are commonly abbreviated as VCO. The VCO’s are electrical circuits that yield an oscillatory output voltage. A VCO is an oscillator whose output frequency is proportional to the applied input voltage. The parts of a VCO circuit has a LC tank circuit with an inductor(L) and a capacitor(C) along with one or two transistors accompanied by a buffer amplifier. A VCO gives a periodic output signal where the output signal parameter is directly related to level of input control voltage. The center frequency of a VCO is the frequency of the periodic output signal formed by the VCO when the input control voltage is set to a nominal level. The voltage-controlled oscillator has a characteristic gain, which often is expressed as a ratio of the VCO output frequency to the VCO input voltage.
VCO’s often utilize a variable control voltage input to produce a frequency output. The control voltage input typically may be tuned so that the VCO produces a desired, operational frequency output. The input control voltage is then adjusted up or down to control the frequency of the periodic output signal. A voltage controlled oscillator is capable of changing an oscillating frequency in response to a change in control voltages. A VCO typically employs one or more variable capacitors commonly called as varactors to allow for adjustment of the frequency of oscillation for the VCO. The tuning range of the VCO refers to the range of oscillation frequencies attained by varying the varactors.
Two important parameters in VCO design are sweep range and linearity. Linearity correlates the change in frequency or the VCO output to the change in the control voltage. The sweep range is the range of possible frequencies produced by VCO control voltage. Various types of VCO’s have been discovered so far. VCO’s comprised of bipolar junction transistors have been used to generate output ranging from 5 to 10MHz.
Voltage controlled oscillators are basic building blocks of many electronic systems especially phase-locked loops(PLL) and may be found in computer disk drives, wireless electronic equipment such as cellular telephones, and other systems in which oscillation frequency is controlled by an applied tuning voltage. The voltage oscillator components are almost an inevitable part of all digital communication equipments. VCO’s are used for producing local oscillator signals (LO) which are in turn received by the transmitter and the receiver systems for the frequency up conversion and the down conversion respectively. Wireless subscriber communication units such as the GSM use voltage oscillator circuits for generating radio frequency signals. The VCO’s are also employed in many synthesizer and tuner circuits and one best example for that is Television. A high frequency VCO is used in applications like processor clock distribution and generation, system synchronization and frequency synthesis.
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