Local Oscillator

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.

Several local oscillators can be strung in series to form a local oscillator chain (LO chain).

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.

Other articles related to "local oscillator":

Detector (radio) - Amplitude Modulation Detectors - Product Detector
... product of the modulated signal and a local oscillator, hence the name ... it multiplies the signal by the output of the local oscillator ... in some type of nonlinear device, with a signal from the local oscillator, to produce an intermediate frequency, referred to as the beat frequency, from which the ...
Synchrodyne - History and Applications
... The design suffered from the thermal drift of the local oscillator which changed its frequency over time ... To counteract this drift, the frequency of the local oscillator was compared with the broadcast input signal by a phase detector ... This produced a correction voltage which would vary the local oscillator frequency keeping it in lock with the wanted signal ...
Radio Transmitter Design - EMC Matters - Spurious Emissions - Local Oscillators and Unwanted Mixing Products
... If the local oscillator signal was to enter the power amplifier and not be adequately suppressed then it could be radiated ... Normally with good choice of the intermediate and local oscillator frequencies this type of trouble can be avoided, but one potentially bad situation is in ...
Homodyne Detection
... the local oscillator) is derived from the same source as the signal before the modulating process ... One is the local oscillator and the other is sent to the system to be probed ... The scattered light is then mixed with the local oscillator on the detector ...

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