Local oscillator

In electronics, the term local oscillator (LO) refers to an electronic oscillator when used in conjunction with a mixer 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 to the mixer. 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.

The term local refers to the fact that the frequency is generated within the circuit and is not reliant on any external signals, although the frequency of the oscillator may be tuned according to external signals.

Local oscillators are used in the superheterodyne receiver, the most common type of radio receiver circuit. In this application, the frequency of the local oscillator (LO) is chosen to be similar to the radio frequency (RF) received on the antenna, such that the difference between them is much smaller than the RF. Either high-side injection (where the LO frequency is greater than the RF) or low-side injection (where the LO frequency is less than the RF) may be employed. The difference can then be filtered from the sum to extract the intermediate frequency (IF).

• For high-side injection, ${\displaystyle f_{\mathrm {IF} }=f_{\mathrm {LO} }-f_{\mathrm {RF} }}$.
• For low-side injection, ${\displaystyle f_{\mathrm {IF} }=f_{\mathrm {RF} }-f_{\mathrm {LO} }}$.

So the frequency chosen for the local oscillator should be ${\displaystyle f_{\mathrm {LO} }=f_{\mathrm {RF} }\pm f_{\mathrm {IF} }}$.

They are also used in many other communications circuits such as modems, cable televisionset 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. In satellite television reception, the microwave 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.

The performance of a signal processing system depends, amongst other factors, on the characteristics of the local oscillator.

• Emissions - receiver design requires care to ensure no spurious signals are radiated by the local oscillator. Such signals can cause interference in the operation of other receivers.
• Stability - the local oscillator must produce a stable frequency with low harmonics. ^[2] Stability must take into account temperature, voltage, and mechanical drift as factors.
• Power - the oscillator must produce enough output power to effectively drive subsequent stages of circuitry, such as mixers or frequency multipliers.
• Noise - it must have low phase noise where the timing of the signal is critical.^[3][4]
• Precision - in a channelized receiver system, the precision of tuning of the frequency synthesizer must be compatible with the channel spacing of the desired signals.

Un oscilador de cristal es un tipo común de oscilador local que proporciona buena estabilidad y rendimiento a un coste relativamente bajo, pero su frecuencia es fija, por lo que cambiarla requiere cambiar el cristal. La sintonización a diferentes frecuencias requiere un oscilador de frecuencia variable , lo que implica un compromiso entre estabilidad y capacidad de sintonización. Con la llegada de la microelectrónica digital de alta velocidad, los sistemas modernos pueden utilizar sintetizadores de frecuencia para obtener un oscilador local estable y sintonizable, pero aún se debe tener cuidado de mantener características de ruido adecuadas en el resultado. Los bucles de enganche de fase son un medio alternativo para generar frecuencias LO precisas. Al sincronizarse con otra fuente de frecuencia, este método garantiza un alto nivel de precisión y estabilidad. ^{[ 5 ]}

La detección de la radiación del oscilador local puede revelar la presencia del receptor, como en la detección de detectores de radar en automóviles o la detección de receptores de transmisión de televisión sin licencia en algunos países. Durante la Segunda Guerra Mundial , a los soldados aliados no se les permitía tener receptores superheterodinos porque los soldados del Eje contaban con equipos capaces de detectar las emisiones del oscilador local. Esto llevó a que los soldados crearan lo que hoy se conoce como radio de trinchera , un receptor de radio simple e improvisado que carece de oscilador local.

Los mejores receptores de comunicaciones militares de la Segunda Guerra Mundial fueron diseñados para suprimir las emisiones del oscilador local. Por ejemplo, el famoso RCA AR-88 cuenta con un excelente blindaje. Además, utiliza dos etapas de RF de pentodo sintonizadas antes del mezclador superheterodino. Los tubos de pentodo tienen una ganancia inversa prácticamente nula, por lo que las emisiones del oscilador local no podían propagarse a través de la antena.

• Receptor de conversión directa
• detección homodina
• detección heterodina
• detección heterodina óptica
• NE612 , oscilador y mezclador multiplicador de celda Gilbert.