Continuous wave
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robot Removing: [[tr:CW]]
{{Mergefrom|Morse codewave|date=March 2008}}
{{Modulation techniques}}
{{dablink | For the amateur radio mode called "continuous wave" (CW) see [[morse code]] }}
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. In radio transmission, CW waves are also known as "undamped waves", to distinguish this method from [[damped wave]] transmission.
== Radio ==
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 had a characteristic rapidly damped amplitude during each pulse of radiated energy. When [[alternator#Radio alternators|alternator]]s and later electronic [[oscillator]]s became available, the signal strength remained constant during each code element, leading to the description of this technique as "continuous" waves.
An unmodulated carrier has no bandwidth and conveys no information; the act of keying the carrier on and off produces a finite bandwidth relating to the transmission rate. Strictly speaking, a keyed carrier may be referred to as "ICW" for "Interrupted continuous wave" but the necessity of keying is usually understood.
[[Image:Bencher paddle.jpg|thumb|250px|A commercially manufactured paddle for use with electronic keyer to generate Morse code]]
Early radio transmitters were incapable of handling the complexity of actual audio and therefore CW 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.
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.
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.
In [[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.
===Key clicks===
In morse (on off carrier keying), if the carrier wave is turned on or off rapidly, the bandwidth will be large; if the carrier turns on and off more slowly the bandwidth will be smaller. The problem of excessive bandwidth used by a [[morse code|morse]] [[transmitter]] which turns on/off too quickly is known as [[key clicks]]. Certain types of power amplifiers used in a transmitter may increase the problematic effects of key clicks.
==Laser physics==
In [[laser physics]] and engineering the term "continuous wave" or "CW" refers to a [[laser]] which produces a continuous output beam, sometimes referred to as 'free-running'. This is as opposed to a [[q-switched]], [[gain-switching|gain-switched]] or [[modelocking|modelocked]] laser, which produces pulses of light.
== See also ==
* [[Damped wave]]
* [[On-off keying]]
* [[Amplitude modulation]]
* [[Types of radio emissions]]
==References==
*Larry D. Wolfgang et. al, (ed), ''The ARRL Handbook for Radio Amateurs, Sixty-Eighth Edition '', (1991), ARRL, Newington CT USA ISBN 0-87259-168-9
*[http://www.w8ji.com/cw_bandwidth_described.htm CW Bandwidth Described]
{{Morse code}}
[[Category:Lasers]]
[[Category:Radio modulation modes]]
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