superheterodyne receiver

superheterodyne receiver
Name	Superheterodyne Receiver
Caption	Block diagram of a typical superheterodyne circuit.
Invented	1918
Inventor	Edwin Howard Armstrong

superheterodyne receiver is a radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original carrier frequency. This revolutionary architecture, patented by Edwin Howard Armstrong in 1918, became the dominant design for virtually all radio, radar, and television receivers throughout the 20th century. Its superior selectivity and sensitivity over earlier designs like the tuned radio frequency receiver allowed for clearer reception and the practical development of new communication technologies.

Principle of operation

The core principle involves mixing the incoming radio frequency signal from the antenna with a signal from a local oscillator within the receiver. This process, called heterodyning, produces sum and difference frequencies. The difference frequency, known as the intermediate frequency, is selectively amplified by fixed-tuned IF amplifier stages. A key component, the mixer, performs this frequency conversion. The carefully chosen IF, such as 455 kHz for AM broadcasting or 10.7 MHz for FM broadcasting, allows for optimal filtering and amplification before final detection. This two-step process effectively separates the tasks of frequency selection and signal amplification.

History

The superheterodyne concept was invented and patented by American electrical engineer Edwin Howard Armstrong in 1918 during his service in the United States Army Signal Corps in World War I. Its initial development was intended for a direction-finding apparatus. After the war, Armstrong sold the patent to the Westinghouse Electric Corporation. The design saw limited early use due to the expense and instability of early vacuum tubes required for the local oscillator. Widespread commercial adoption began in the mid-1930s after improvements in tube technology and circuit design by companies like RCA Victor and Philips. It became the standard for consumer radios, critical for the success of broadcast networks like the BBC and CBS.

Design and components

A classic superheterodyne receiver comprises several key stages. The RF amplifier provides initial gain and selectivity. The local oscillator, often a variable-frequency oscillator, is tuned in tandem with the RF stage. The mixer combines their outputs. The resulting IF signal passes through one or more IF amplifier stages, which provide the majority of the receiver's gain and adjacent-channel selectivity, typically using crystal filters or ceramic filters. The detector, such as a diode detector for AM or a ratio detector for FM, extracts the audio or video information. This is followed by an audio amplifier driving a loudspeaker. Additional circuits like automatic gain control and squelch are often incorporated.

Advantages and disadvantages

The primary advantage is exceptional selectivity, allowing rejection of unwanted signals close to the desired frequency. It also offers high sensitivity and stable gain due to the fixed-frequency IF amplifiers. The design simplifies the production of multi-band receivers, such as those covering the shortwave bands. A significant disadvantage is the potential for image response, an unwanted signal that mixes to the same IF, which must be suppressed by the RF front-end. It can also suffer from intermodulation and requires more complex circuitry than a simple crystal radio or regenerative receiver.

Applications

The architecture is ubiquitous in wireless communication. It is the foundational design for nearly all AM radio and FM radio broadcast receivers. It is used in television tuners, both for analog television and digital television standards like ATSC. It is essential in two-way radio systems used by public services, in cellular network base stations and handsets, and in satellite communication receivers. The principle is also applied in radar systems, radio astronomy equipment like that used at the Arecibo Observatory, and in software-defined radio platforms.

Variations and related concepts

A common variation is the double-conversion superheterodyne, which uses two IFs to better reject image frequencies, prevalent in communications receivers and satellite television receivers. The related homodyne receiver or direct-conversion receiver mixes the signal directly to baseband. The upconversion receiver uses a higher first IF to mitigate image issues. The superheterodyne transmitter applies the same mixing principle in reverse. The fundamental heterodyne principle is also critical in other fields, such as in the operation of a theremin and in optical heterodyne detection for LIDAR systems.

Category:Radio electronics Category:American inventions Category:Radio technology