A transmitter (sometimes abbreviated XMTR) is an electronic device which with the aid of an antenna propagates an electromagnetic signal such as radio, television, or other telecommunications. A transmitter usually has a power supply, an oscillator, a modulator, and amplifiers for audio frequency (AF) and radio frequency (RF). The modulator is the device which piggybacks (or modulates) the signal information onto the carrier frequency, which is then broadcast. Sometimes a device (for example, a cell phone) contains both a transmitter and a radio receiver, with the combined unit referred to as a transceiver. More generally and in communications and information processing, a “transmitter” is any object (source) which sends information to an observer (receiver). When used in this more general sense, vocal cords may also be considered an example of a “transmitter”. In industrial process control a “transmitter” is any device which converts measurements from a sensor into a signal to be received, usually sent via wires, by some display or control device located a distance away. Typically in process control applications the “transmitter” will output a 4-20 mA current loop or digital protocol to represent a measured variable within a range. For example, a pressure transmitter might use 4 ma as a representation for 50 psig of pressure and 20 ma as 1000 psig of pressure and any value in between proportionately ranged between 50 and 1000 psig. Older technology transmitters used pneumatic pressure typically ranged between 3 to 15 psig (20 to 100 kPa) to represent a process variable.
In radio terminology, a receiver is an electronic circuit that receives a radio signal from an antenna and decodes the signal for use as sound, pictures, navigational-position information, etc. Radio and radio receiver are often used specifically for receivers whose output consists only of sound, although other types of receivers,such as television receivers, are technically radio receivers as well. A radio receiver is a real world example of a receiver in the information theoretic sense. As an audio appliance, “receiver” refers to 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 undistingushed 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.
Most older receivers also came with a loudspeaker (see photo). 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 tape decks and video components like video-cassette recorders, DVD players, 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 convienience 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 connections are also common today. 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. Self-powered radios (clockwork radio) with a hand-cranked generator are used in developing nations or as part of an emergency/disaster preparedness kit.
Electromagnetism is the physics of the electromagnetic field; a field encompassing all of space which exerts a force on particles that possess the property of electric charge, and is in turn affected by the presence and motion of those particles. It is often convenient to understand the electromagnetic field in terms of two separate fields: the electric field and the magnetic field. The electric field is produced by the presence of electrically charged particles, and causes the electric force. Electric force is the force observed as static electricity, and causes the flow of electric charge (electric current) in electrical conductors. The magnetic field is produced by the motion of electric charges, i.e. electric current. The magnetic field causes the magnetic force associated with magnets. The term “electromagnetism” comes from the fact that electrical and magnetic forces are involved simultaneously. A changing magnetic field produces an electric field (this is the phenomenon of electromagnetic induction, which provides for the operation of electrical generators, induction motors, and transformers). Similarly, a changing electric field generates a magnetic field. Because of this interdependence of the electric and magnetic fields, it makes sense to consider them as a single coherent entity — the electromagnetic field. This unification, which was completed by James Clerk Maxwell, is one of the triumphs of 19th century physics. It had far-reaching consequences, one of which was the understanding of the nature of light. As it turns out, what is thought of as “light” is actually a propagating oscillatory disturbance in the electromagnetic field, i.e., an electromagnetic wave. Different frequencies of oscillation give rise to the different forms of electromagnetic radiation, from radio waves at the lowest frequencies, to visible light at intermediate frequencies, to gamma rays at the highest frequencies.
The theoretical implications of electromagnetism led to the development of special relativity by Albert Einstein in 1905.
In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying quantity. Signals are often scalar-valued functions of time (waveforms), but may be vector valued and may be functions of any other relevant independent variable. The concept is broad, and hard to define precisely. Definitions specific to subfields are common. For example, in information theory, a signal is a codified message, ie, the sequence of states in a communications channel that encodes a message. In a communications system, a transmitter encodes a message into a signal, which is carried to a receiver by the communications channel. For example, the words “Mary had a little lamb” might be the message spoken into a telephone. The telephone transmitter converts the sounds into an electrical voltage signal. The signal is transmitted to the receiving telephone by wires; and at the receiver it is reconverted into sounds.
Signals can be categorized in various ways. The most common distinction is between discrete and continuous spaces that the functions are defined over, for example discrete and continuous time domains. Discrete-time signals are often referred to as time series in other fields. Continuous-time signals are often referred to as continuous signals even when the signal functions are not continuous; an example is a square-wave signal. A second important distinction is between discrete-valued and continuous-valued. Digital signals are discrete-valued, but are often derived from an underlying continuous-valued physical process.
In common use the word noise means unwanted sound or noise pollution. In electronics noise can refer to the electronic signal corresponding to acoustic noise (in an audio system) or the electronic signal corresponding to the (visual) noise commonly seen as ’snow’ on a degraded television or video image. In signal processing or computing it can be considered data without meaning; that is, data that is not being used to transmit a signal, but is simply produced as an unwanted by-product of other activities. In Information Theory, however, noise is still considered to be information. In a broader sense the film grain or even advertisements in web pages can be considered noise. Radio noise is interference with radio transmissions caused either by thermal noise from receiver input circuits or by radiated electromagnetic noise picked up by the receiver’s antenna. If no noise was picked up with radio signals, even weak transmissions could be received at virtually any distance by making a radio receiver that was sensitive enough. In practice this doesn’t work, and a point is reached where the only way to extend the range of a transmission is to increase the transmitter power. Thermal noise can be made lower by cooling the circuits, but this is only usually worthwhile on radio telescopes. In other applications the limiting noise source depends on the frequency range in use. At low freqencies (longwave or mediumwave) and at high frequencies (shortwave), interference caused by lightning or by nearby electrical impulses in electrical switches, motors, vehicle ignition circuits, computers, and other man-made sources tends to swamp transmissions with thermal noise. These noises are often referred to as static. At very high frequency and ultra high frequency these sources can still be important, but at a much lower level, such that thermal noise is usually the limiting factor. In the microwave region, cosmic background noise may be relevant.
Electromagnetic noise can interfere with electronic equipment in general, causing malfunction, and in recent years standards have been laid down for the levels of electromagnetic radiation that electronic equipment is permitted to radiate. These standards are aimed at ensuring what is referred to as electromagnetic compatibility, or EMC.
In telecommunication, a mixer is a nonlinear circuit or device that accepts as its input two different frequencies and presents at its output (a) a signal equal in frequency to the sum of the frequencies of the input signals, (b) a signal equal in frequency to the difference between the frequencies of the input signals, and, if they are not filtered out, (c) the original input frequencies.
An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. A low-frequency oscillator (or LFO) is an electronic oscillator that generates an AC waveform between 0.1 Hz and 10 Hz. This term is typically used in the field of audio synthesizers, to distinguish it from an audio frequency oscillator.
Shortwave radio operates between the frequencies of 2,310 kHz and 30 MHz (30,000 kHz)  and came to be referred to as such in the early days of radio because the wavelengths associated with this frequency range were shorter than those commonly in use at that time. An alternate name is HF or high frequency radio. Short wavelengths are associated with high frequencies because there is an inverse relationship between frequency and wavelength.
The Longwave radio broadcasting band are those frequencies between 153 – 279 kHz, which correspond to wavelengths longer than 600 meters. This range is included within the low frequency band (but the low frequency band extends above and below longwave signals). Longwave signals have the property of following the curvature of the earth, making them ideal for continuous, continental communications. Unlike shortwave radio, longwave signals do not reflect or refract using the ionosphere, so there are fewer interference-caused fadeouts. Instead, the D-layer of the ionosphere and the surface of the earth serve as a waveguide directing the signal.
The earliest radio transmitters were all longwave transmitters, because propagation of radio waves of higher frequency was not yet understood. Radio alternator or spark-gap transmitters were commonly used to generate the radio frequency carrier wave.
Mediumwave (MW) radio transmissions serves as the most common band for broadcasting. The standard AM broadcast band is 525 kHz to 1715 kHz in North America, but remains only up to 1615 kHz elsewhere. In most of the Americas, mediumwave stations are separated by 10 kHz and have two sidebands of ±5 kHz. In the rest of the world, the separation is 9 kHz, with sidebands of ±4.5 kHz. Both provide adequate audio quality for voice, but are insufficient for high-fidelity broadcasting, which is common on the VHF FM bands. In the US the maximum transmitter power is restricted to 50 kilowatts, while in Europe there are medium wave stations with transmitter power up to 2.5 megawatts.
Mediumwave signals have the property of following the curvature of the earth (the groundwave) at all times, and also reflecting off the ionosphere at night (skywave). This makes this frequency band ideal for both local and continent-wide service, depending on the time of day. For example, during the day a radio receiver in the state of Maryland is able to receive reliable but weak signals from high-power stations WFAN, 660 kHz, and WOR, 710 kHz, 400 km away in New York City, due to groundwave propagation. The effectiveness of groundwave signals largely depends on ground conductivity—higher conductivity results in better propagation. At night, the same receiver picks up signals as far away as Mexico City and Chicago reliably. Many North American stations are required to shut down or reduce power at night in order to make way for clear channel stations that can then be received over a wider range.
In Europe, each country is allocated a number of frequencies on which high power (up to 2.5 MW) can be used; the maximum power is also subject to international agreement. Other countries may only operate low-powered transmitters on the same frequency, again subject to agreement. For example, Russia operates a high-powered transmitter, located in its Kaliningrad exclave and used for external broadcasting, on 1323 kHz. The same frequency is also used by low-powered local radio stations in England; other parts of England can still receive the Russian broadcast. International mediumwave broadcasting in Europe has decreased markedly with the end of the Cold War and the increased availability of satellite and Internet TV and radio, although the cross-border reception of neighbouring countries’ broadcasts by expatriates and other interested listeners still takes place. Due to the high demand for frequencies in Europe, many countries operate single frequency networks; in Britain, BBC Radio 5 broadcasts from various transmitters on either 693 or 909 kHz. These transmitters are carefully synchronised to minimise interference from more distant transmitters on the same frequency.
The decibel (dB) is a measure of the ratio between two quantities, and is used in a wide variety of measurements in acoustics, physics and electronics. While originally only used for power and intensity ratios, it has come to be used more generally in engineering. The decibel is widely used in measurements of the loudness of sound. It is a “dimensionless unit” like percent. Decibels are useful because they allow even very large or small ratios to be represented with a conveniently small number (similar to scientific notation). This is achieved by using a logarithm.
An antenna or aerial is an electrical device designed to transmit or receive radio waves or, more generally, any electromagnetic waves. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, radar, and space exploration. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies. Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Some antenna devices (parabola, horn antenna) just adapt the free space to another type of antenna. Antennas were used for the first time, in 1889, by Heinrich Hertz (1857-1894) to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. He even placed the emitter dipole in the focal point of a parabolic reflector.
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