Receiver (radio)

A radio receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc.

In consumer electronics, the terms radio and radio receiver are often used specifically for receivers designed for the sound signals transmitted by radio broadcasting services – historically the first mass-market radio application.

Types of radio receivers
Various types of radio receivers may include:


 * Consumer audio and high fidelity audio receivers and AV receivers used by home stereo listeners and audio and home theatre system enthusiasts.


 * Communications receivers, used as a component of a radio communication link, characterized by high stability and reliability of performance.


 * Simple crystal radio receivers (also known as a crystal set) which operate using the power received from radio waves.


 * Satellite television receivers, used to receive television programming from communication satellites in geosynchronous orbit.


 * Specialized-use receivers such as telemetry receivers that allow the remote measurement and reporting of information.


 * Measuring receivers (also: measurement receivers) are calibrated laboratory-grade devices that are used to measure the signal strength of broadcasting stations, the electromagnetic interference radiation emitted by electrical products, as well as to calibrate RF attenuators and signal generators.

Consumer audio receivers
In the context of home audio systems, the term "receiver" often refers to a combination of a tuner, a preamplifier, and a power amplifier all on the same chassis. Audiophiles will refer to such a device as an integrated receiver, while a single chassis that implements only one of the three component functions is called a discrete component. Some audio purists still prefer three discreet units - tuner, preamplifier and power amplifier - but the integrated receiver has, for some years, been the mainstream choice for music listening. The first integrated stereo receiver was made by the Harman Kardon company, and came onto the market in 1958. It had undistinguished performance, but it represented a breakthrough to the "all in one" concept of a receiver, and rapidly improving designs gradually made the receiver the mainstay of the marketplace. Many radio receivers also include a loudspeaker.

Hi-Fi / Home theater
Today AV receivers are a common component in a high-fidelity or home-theatre system. The receiver is generally the nerve centre of a sophisticated home-theatre system providing selectable inputs for a number of different audio components like turntables, compact-disc players and recorders, and tape decks ( like video-cassette recorders) and video components (DVD players and recorders, video-game systems, and televisions).

With the decline of vinyl discs, modern receivers tend to omit inputs for turntables, which have separate requirements of their own. All other common audio/visual components can use any of the identical line-level inputs on the receiver for playback, regardless of how they are marked (the "name" on each input is mostly for the convenience of the user.) For instance, a second CD player can be plugged into an "Aux" input, and will work the same as it will in the "CD" input jacks.

Some receivers can also provide signal processors to give a more realistic illusion of listening in a concert hall. Digital audio S/PDIF and USB connections are also common today. The home theater receiver, in the vocabulary of consumer electronics, comprises both the 'radio receiver' and other functions, such as control, sound processing, and power amplification. The standalone radio receiver is usually known in consumer electronics as a tuner.

Some modern integrated receivers can send audio out to seven loudspeakers and an additional channel for a subwoofer and often include connections for headphones. Receivers vary greatly in price, and support stereophonic or surround sound. A high-quality receiver for dedicated audio-only listening (two channel stereo) can be relatively inexpensive; excellent ones can be purchased for $300 US or less. Because modern receivers are purely electronic devices with no moving parts unlike electromechanical devices like turntables and cassette decks, they tend to offer many years of trouble-free service. In recent years, the home theater in a box has become common, which often integrates a surround-capable receiver with a DVD player. The user simply connects it to a television, perhaps other components, and a set of loudspeakers.

Portable radios
Portable radios include simple transistor radios that are typically monoaural and receive the AM, FM, and/or short wave broadcast bands. FM, and often AM, radios are sometimes included as a feature of portable CD, MP3 CD, and USB key players, as well as cassette player/recorders.

A Boombox (or Boom-box) – also sometimes known as a Ghettoblaster or a Jambox, or (in parts of Europe) as a "radio-cassette" – is a name given to larger portable stereo systems capable of playing radio stations and recorded music.

Self-powered portable radios, such as clockwork radios are used in developing nations or as part of an emergency preparedness kit.

Early development
While James Clerk Maxwell was the first person to prove electromagnetic waves existed, a German named Heinrich Hertz demonstrated these new waves by using spark gap equipment to transmit and receive radio or "Hertzian waves", as they were first called.

The world’s first radio receiver (thunderstorm register) was designed by Alexander Stepanovich Popov, and it was first seen at the All-Russia exhibition 1896. He was the first to demonstrate the practical application of electromagnetic (radio) waves, although he did not care to apply for a patent for his invention.

A device called a coherer became the basis for receiving radio signals. The first person to use the device to detect radio waves was a Frenchman named Edouard Branly, and Oliver Lodge popularised it when he gave a lecture in 1898 in honour of Hertz. Lodge also made improvements to the coherer. Guglielmo Marconi believed that these new waves could be used to communicate over great distances and made significant improvements to both radio receiving and transmitting apparatus. In 1895 Marconi demonstrated the first viable radio system, leading to transatlantic radio communication in December 1901.

John Ambrose Fleming's development of an early thermionic valve to help detect radio waves was based upon a discovery of Thomas Edison's (called "The Edison effect", which essentially modified an early light bulb). Fleming called it his "oscillation valve" because it acted in the same way as water valve in only allowing flow in one direction. While Fleming's valve was a great stride forward it would take some years before thermionic, or vacuum tube technology was fully adopted.

Around this time work on other types of detectors started to be undertaken and it resulted in what was later known as the cat's whisker. It consisted of a crystal of a material such as galena with a small springy piece of wire brought up against it. The detector was constructed so that the wire contact could be moved to different points on the crystal, and thereby obtain the best point for rectifying the signal and the best detection. They were never very reliable as the "whisker" needed to be moved periodically to enable it to detect the signal properly.

Valves
An American named Lee de Forest, a competitor to Marconi, set about to develop receiver technology that did not infringe any patents to which Marconi had access. He took out a number of patents in the period between 1905 and 1907 covering a variety of developments that culminated in the form of the triode valve in which there was a third electrode called a grid. He called this an audion tube. One of the first areas in which valves were used was in the manufacture of telephone repeaters, and although the performance was poor, they gave significant improvement in long distance telephone receiving circuits.

With the discovery that triode valves could amplify signals it was soon noticed that they would also oscillate, a fact that was exploited in generating signals. Once the triode was established as an amplifier it made a tremendous difference to radio receiver performance as it allowed the incoming signals to be amplified. One way that proved very successful was introduced in 1913 and involved the use of positive feedback in the form of a regenerative detector. This gave significant improvements in the levels of gain that could be achieved, greatly increasing selectivity, enabling this type of receiver to outperform all other types of the era. With the outbreak of the First World War, there was a great impetus to develop radio receiving technology further. An American named Irving Langmuir helped introduce a new generation of totally air-evacuated "hard" valves. H. J. Round undertook some work on this and in 1916 he produced a number valves with the grid connection taken out of the top of the envelope away from the anode connection.

Autodyne and superheterodyne
By the 1920s, the tuned radio frequency receiver (TRF) represented a major improvement in performance over what had been available before, it still fell short of the needs for some of the new applications. To enable receiver technology to meet the needs placed upon it a number of new ideas started to surface. One of these was a new form of direct conversion receiver. Here an internal or local oscillator was used to beat with the incoming signal to produce and audible signal that could be amplified by an audio amplifier.

H. J. Round developed a receiver he called an autodyne in which the same valve was used as a mixer and an oscillator, Whilst the set used fewer valves it was difficult to optimise the circuit for both the mixer and oscillator functions.

The next leap forward in receiver technology was a new type of receiver known as the superheterodyne, or supersonic heterodyne receiver. A Frenchman named Lucien Levy was investigating ways in which receiver selectivity could be improved and in doing this he devised a system whereby the signals were converted down to a lower frequency where the filter bandwidths could be made narrower. A further advantage was that the gain of valves was considerably greater at the lower frequencies used after the frequency conversion, and there were fewer problems with the circuits bursting into oscillation.

The idea for developing a receiver with a fixed intermediate frequency amplifier and filter is credited to Edwin Armstrong. Working for the American Expeditionary Force in Europe in 1918, Armstrong thought that if the incoming signals were mixed with a variable frequency oscillator, a low frequency fix tuned amplifier could be used. Armstrong's original receiver consisted of a total of eight valves. Several tuned circuits could be cascaded to improve selectivity, and being on a fixed frequency they did not all need to be changed in line with one another. The filters could be preset and left correctly tuned. Armstrong was not the only person working on the idea of a superhet. Alexander Meissner in Germany took out a patent for the idea six months before Armstrong, but as Meissner did not prove the idea in practice and did not build a superhet radio, the idea is credited to Armstrong.

The need for the increased performance of the superhet receiver was first felt in America, and by the late 1920s most sets were superhets. However in Europe the number of broadcast stations did not start to rise as rapidly until later. Even so by the mid 1930s virtually all receiving sets in Europe as well were using the superhet principle. In 1926 the tetrode valve was introduced, and enabled further improvements in performance.

War and postwar developments
In 1939 the outbreak of war gave a new impetus to receiver development. During this time a number of classic communications receivers were designed. Some like the National HRO are still sought by enthusiasts today and although they are relatively large by today's standards, they can still give a good account of themselves under current crowded band conditions. In the late 1940s the transistor was discovered. Initially the devices were not widely used because of their expense, and the fact that valves were being made smaller, and performed better. However by the early 1960s portable transistor broadcast receivers (transistor radios) were hitting the market place. These radios were ideal for broadcast reception on the long and medium wave bands. They were much smaller than their valve equivalents, they were portable and could be powered from batteries. Although some valve portable receivers were available, batteries for these were expensive and did not last for long. The power requirements for transistor radios were very much less, resulting in batteries lasting for much longer and being considerably cheaper.

Semiconductors
Further developments in semiconductor technology lead to the introduction of the integrated circuit in the late 1950s. This enabled radio receiver technology to move forward even further. Integrated circuits enabled high performance circuits to be built for less cost, and significant amounts of space could be saved.

As a result of these developments new techniques could be introduced. One of these was the frequency synthesizer that was used to generate the local oscillator signal for the receiver. By using a synthesizer it was possible to generate a very accurate and stable local oscillator signal. Also the ability of synthesizers to be controlled by microprocessors meant that many new facilities could be introduced apart from the significant performance improvements offered by synthesizers.

Digital technologies
Receiver technology is still moving forward. Digital signal processing where many of the functions performed by an analog intermediate frequency stage can be performed digitally by converting the signal to a digital stream that is manipulated mathematically is now widespread. The new digital audio broadcasting standard being introduced can only be used when the receiver can manipulate the signal digitally.

While today's radios are miracles of modern technology, filled with low power high performance integrated circuits crammed into the smallest spaces, the basic principle of the radio is usually the superhet, the same idea which was developed by Edwin Armstrong back in 1918.

Measurement receivers

 * Agilent
 * Dynamic Sciences
 * Electro-Metrics
 * Rohde & Schwarz