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| {{Modulation techniques}}
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| A '''continuous wave''' or '''continuous waveform''' ('''CW''') is an [[electromagnetic wave]] of constant [[amplitude]] and [[frequency]]; and in [[mathematical analysis]], of infinite duration. Continuous wave is also the name given to an early method of [[radio]] [[transmission (telecommunications)|transmission]], in which a [[carrier wave]] is switched on and off. [[Information]] is carried in the varying duration of the [[on-off keying|on and off periods]] of the signal, for example by [[Morse code]] in early radio. In early [[wireless telegraphy]] radio transmission, CW waves were also known as "undamped waves", to distinguish this method from [[damped wave]] transmission.
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| == Radio ==
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| Very early radio transmitters used a [[spark gap]] to produce radio-frequency oscillations in the transmitting antenna. The signals produced by these [[spark-gap transmitter]]s consisted of brief pulses of radio frequency oscillations which died out rapidly to zero, called [[damped wave]]s. The disadvantage of damped waves was that they produced [[electromagnetic interference]] that spread over the transmissions of stations at other [[Frequency|frequencies]]. Mathematically, the extremely wideband excitation provided by the spark gap was bandpass filtered by the self-oscillating antenna side circuit, which, because of its simple construction, also had a relatively broad and poorly controlled filter characteristic.
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| This motivated efforts to produce radio frequency oscillations that decayed more slowly. Strictly speaking, an unmodulated continuous carrier has no bandwidth and cannot interfere with signals at other frequencies, but conveys no information either. Thus it is commonly understood that the act of keying the carrier on and off is necessary. However, in order to bring the bandwidth of the resulting signal under control, the buildup and decay of the radio frequency envelope needs to be slower than that of the early spark gap implementations.
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| When this is done, the spectrum of the signal approaches that of a continuous sinusoidal oscillation, while temporally its [[amplitude]] varies between zero and full carrier strength. As such, the resulting narrower bandwidth mode of operation is to this day described as "continuous wave". The resulting signal allows many radio stations to share a given band of frequencies without noticeable mutual interference.
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| In on-off carrier keying, if the carrier wave is turned on or off abruptly, [[Shannon–Hartley theorem | communications theory]] can show that the [[Bandwidth (signal processing) |bandwidth]] will be large; if the carrier turns on and off more gradually, the bandwidth will be smaller. The bandwidth of an on-off keyed signal is related to the data transmission rate as:
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| <math> B_n = B K </math>
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| where <math>B_n</math> is the necessary bandwidth in hertz,
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| <math>B </math> is the keying rate in signal changes per second [[baud]],
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| and <math> K </math> is a constant related to the expected radio propagation conditions; K=1 is difficult for a human ear to decode, K=3 or K=5 is used when fading or [[multipath propagation]] is expected. <ref> L. D. Wolfgang, C. L. Hutchinson (ed) ''The ARRL Handbook for Radio Amateurs, Sixty Eighth Edition'', ([[ARRL]], 1991) ISBN 0-87259-168-9, pages 9-8, 9-9 </ref>What is transmitted in the extra bandwidth used by a [[transmitter]] that turns on/off more abruptly is known as ''key clicks''. Certain types of power amplifiers used in transmission may increase the effect of key clicks.
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| The first transmitters capable of producing continuous wave, the [[Alexanderson alternator]] and [[vacuum tube]] [[oscillator]]s, became widely available after [[World War I]].
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| [[File:Bencher paddle.jpg|thumb|250px|A commercially manufactured paddle for use with electronic keyer to generate Morse code]]
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| Early radio transmitters could not be modulated to transmit speech, and so CW radio telegraphy was the only form of communication available. CW still remained a viable form of radio communication for many years after voice transmission was perfected, because simple transmitters could be used. The low bandwidth of the code signal, due in part to low information transmission rate, allowed very selective filters to be used in the receiver which blocked out much of the atmospheric noise that would otherwise reduce the intelligibility of the signal.
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| Continuous-wave radio was called [[radiotelegraphy]] because like the [[telegraph]], it worked by means of a simple switch to transmit [[Morse code]]. However, instead of controlling the electricity in a cross-country wire, the switch controlled the power sent to a radio [[transmitter]]. This mode is still in common use by [[amateur radio]] operators.
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| A [[continuous-wave radar]] system is one where a continuous wave is transmitted by one [[Antenna (radio)|aerial]] while a second aerial receives the reflected [[radio]] energy.
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| In military communications and [[amateur radio]], the terms "CW" and "Morse code" are often used interchangeably, despite the distinctions between the two. Morse code may be sent using [[direct current]] in wires, sound, or light, for example. A carrier wave is keyed on and off to represent the dots and dashes of the code elements. The carrier's amplitude and frequency remains [[constant envelope | constant]] during each code element. At the receiver, the received signal is mixed with a [[heterodyne]] signal from a BFO ([[beat frequency oscillator]]) to change the radio frequency impulses to sound. Though most commercial traffic has now ceased operation using Morse it is still popular with amateur radio operators. [[Non-directional beacon]]s used in air navigation use Morse to transmit their identifier.
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| ==Laser physics==
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| In [[laser physics]] and engineering, "continuous wave" or "CW" refers to a [[laser]] that produces a continuous output beam, sometimes referred to as "free-running," as opposed to a [[q-switched]], [[gain-switching|gain-switched]] or [[modelocking|modelocked]] laser, which has a pulsed output beam.
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| == See also ==
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| * [[Amplitude modulation]]
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| * [[The CW Operators' Club]]
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| * [[Damped wave]]
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| * [[On-off keying]]
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| * [[Tikker]]
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| * [[Types of radio emissions]]
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| ==References==
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| <references/>
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| *[http://www.w8ji.com/cw_bandwidth_described.htm CW Bandwidth Described]
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| {{Morse code}}
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| [[Category:Radio modulation modes]]
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| [[Category:Amateur radio]]
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| [[Category:Laser science]]
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