electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

, a local oscillator (LO) is an

electronic oscillator An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave or a triangle wave. Oscillators convert direct current (DC) from a power supply to an alternating ...

used with a

mixer Mixer may refer to: Electronics * DJ mixer, a type of audio mixing console used by disc jockeys * Electronic mixer, electrical circuit for adding signal voltages * Frequency mixer, electrical circuit that creates new frequencies from two signals ...

to change the frequency of a signal. This frequency conversion process, also called heterodyning, produces the sum and difference frequencies from the frequency of the local oscillator and frequency of the input signal. Processing a signal at a fixed frequency gives a radio receiver improved performance. In many receivers, the function of local oscillator and mixer is combined in one stage called a " converter" - this reduces the space, cost, and power consumption by combining both functions into one active device.

Applications

Local oscillators are used in the

superheterodyne receiver A superheterodyne receiver, often shortened to superhet, is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original car ...

, the most common type of

radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...

circuit. They are also used in many other communications circuits such as

modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by modulating one or more c ...

cable television Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with bro ...

set top box A set-top box (STB), also colloquially known as a cable box and historically television decoder, is an information appliance device that generally contains a TV-tuner input and displays output to a television set and an external source of si ...

es, frequency division multiplexing systems used in telephone trunklines, microwave relay systems,

telemetry Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', "remote", an ...

systems,

atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...

radio telescope A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...

s, and military

electronic countermeasure An electronic countermeasure (ECM) is an electrical or electronic device designed to trick or deceive radar, sonar, or other detection systems, like infrared (IR) or lasers. It may be used both offensively and defensively to deny targeting info ...

(antijamming) systems. In

satellite television Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite orbiting the Earth directly to the viewer's location. The signals are received via an outdoor parabolic antenna com ...

reception, the

microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...

frequencies used from the satellite down to the receiving antenna are converted to lower frequencies by a local oscillator and mixer mounted at the antenna. This allows the received signals to be sent over a length of cable that would otherwise have unacceptable signal loss at the original reception frequency. In this application, the local oscillator is of a fixed frequency and the down-converted signal frequency is variable.

Performance requirements

Application of local oscillators in a receiver design requires care to ensure no spurious signals are radiated. Such signals can cause interference in the operation of other receivers. The performance of a signal processing system depends on the characteristics of the local oscillator. The local oscillator must produce a stable frequency with low harmonics. Stability must take into account temperature, voltage, and mechanical drift as factors. The oscillator must produce enough output power to effectively drive subsequent stages of circuitry, such as mixers or frequency multipliers. It must have low

phase noise In signal processing, phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity ( jitter). Generally speaking, radio-frequency eng ...

where the timing of the signal is critical.Bowick, Christopher; Blyler, John; Ajluni, Cheryl: ''RF Circuit Design (2nd Edition)''. Elsevier 2008 pp. 190–191. In a channelized receiver system, the precision of tuning of the frequency synthesizer must be compatible with the channel spacing of the desired signals.

Types of LO

crystal oscillator A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. The oscillator frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable clock ...

is one common type of local oscillator that provides good stability and performance at relatively low cost, but its frequency is fixed, so changing frequencies requires changing the crystal. Tuning to different frequencies requires a

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 transmitter or receiver that works by the superheterodyne principl ...

which leads to a 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.

Unintended LO emissions

Detection of local oscillator radiation may disclose the presence of the receiver, such as in detection of automotive radar detectors, or detection of unlicensed television broadcast receivers in some countries. During

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...

, Allied soldiers were not allowed to have superheterodyne receivers because the Axis soldiers had equipment which could detect the local oscillator emissions. This led to soldiers creating what is now known as a foxhole radio, a simple improvised radio receiver which has no local oscillator.

References

{{DEFAULTSORT:Local Oscillator Electronic oscillators Radio electronics Feedback

Applications

Performance requirements

Types of LO

Unintended LO emissions

See also

References