Modern radio receivers and transmitters usually use some form of digital frequency synthesis to generate their VFO signal. Varactors have a number of disadvantages including temperature drift and aging, electronic noise, low Q factor and non-linearity. The varactor bias voltage may be generated in a number of ways and there may need to be no significant moving parts in the final design. Since the width of its non-conducting depletion region depends on the magnitude of the reverse bias voltage, this voltage can be used to control the junction capacitance. See also: varactor and voltage controlled oscillatorĪ reversed-biased semiconductor diode exhibits capacitance. Adjustment of this capacitor is sometimes facilitated by a mechanical step-down gearbox to achieve fine tuning. The variable capacitor is a mechanical device in which the separation of a series of interleaved metal plates is physically altered to vary its capacitance. ![]() The passive component whose value is adjustable is usually a capacitor, but could be a variable inductor. There are two main types of VFO in use: analog and digital.Īn analog VFO is an electronic oscillator where the value of at least one of the passive components is adjustable under user control so as to alter its output frequency. Other uses include chirp generators for radar systems where the VFO is swept rapidly through a range of frequencies, timing signal generation for oscilloscopes and time domain reflectometers, and variable frequency audio generators used in musical instruments and audio test equipment. In a radio frequency (RF) transmitter, VFOs are often used to tune the frequency of the output signal, often indirectly through a heterodyning process similar to that described above. ![]() Demodulation takes place at baseband using low-pass filters and amplifiers. ![]() In a simple superheterodyne receiver, the incoming radio frequency signal (at frequency f I N Hz. It is a necessary component in any tunable radio transmitter or receiver that works by the superheterodyne principle, and controls the frequency to which the apparatus is tuned. A Heathkit amateur radio transmitter circa 1969, with external VFOĪ variable frequency oscillator ( VFO) in electronics is an oscillator whose frequency can be tuned (i.e., varied) over some range.
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