Variable frequency oscillator

A variable frequency oscillator (VFO) in electronics is an oscillator whose frequency can be tuned (i.e. varied) over some range. It is a necessary component in any tunable radio receiver or transmitter that works by the superheterodyne principle, and controls the frequency to which the apparatus is tuned.

In a simple superhet radio receiver, the incoming radio frequency signal (at frequency ${\displaystyle f_{IN}}$) from the antenna is mixed with the VFO output signal tuned to ${\displaystyle f_{LO}}$, producing an intermediate frequency (IF) signal that can be processed downstream to extract the modulated information. The IF signal frequency is chosen to be either the sum of the two frequencies at the mixer inputs (up-conversion), ${\displaystyle f_{IN}+f_{LO}}$ or more commonly, the difference frequency (down-conversion), ${\displaystyle f_{IN}-f_{LO}}$, depending on the receiver design.

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