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A local oscillator is an electronic oscillator used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the frequency of the local oscillator and frequency of the input signal of interest. These are the beat frequencies. Normally the beat frequency is associated with the lower sideband, the difference between the two.
Local oscillators are used in the superheterodyne receiver, the most common type of radio receiver circuit. They are also used in many other communications circuits such as modems, cable television set top boxes, frequency division multiplexing systems used in telephone trunklines, microwave relay systems, telemetry systems, atomic clocks, radio telescopes, and military electronic countermeasure (antijamming) systems.
The performance of a signal processing system depends on the characteristics of the local oscillator. The local oscillator must remain stable in frequency. It must produce enough output power to effectively drive subsequent stages such as mixers or frequency multipliers. It must have low phase noise where the timing of the signal is critical.   In a channelized receiver system, the precision of tuning of the frequency synthesizer must be compatlble with the channel spacing of the desired signals.
A crystal oscillator is one common type of local oscillator that provides good stability and performance at relatively low cost, but changing the frequency then requires changing the crystal. Tuning to different frequencies requires a variable-frequency oscillator which requires compromise between stability and tunability. With the advent of high-speed digital microelectronics, modern systems can use frequency synthesizers to obtain a stable tunable local oscillator, but care must still be taken to maintain adequate noise characteristics in the result.