Continuous wave
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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, in which a carrier wave is switched on and off. Information is carried in the rhythm and spacing with which the signal is sent. CW is thus is a form of on-off keying (OOK). In radio transmission, CW waves are also known as "undamped waves", to distinguish this method from damped wave transmission.
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Radio
Very early radio transmitters used a spark gap to produce radio-frequency oscillations in the transmitting antenna; these signals had a characteristic rapidly damped amplitude during each pulse of radiated energy. When alternators and later electronic oscillators 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
While early radio transmission and reception were incapable of handling the complexity of actual audio and therefore CW was the only form of communication available, it still remained a viable form of radio communication for many years after voice transmission was perfected, because of the low bandwidth of the signal which allows CW to cut through bad atmospheric conditions where AM and voice modulations would get lost. A simple half watt CW transmitter could transmit thousands of miles at night because of this bandwidth.
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 due to its simplicity and reliability CW is the basis of the continuous-wave radar system, where a continuous wave is transmitted by one aerial while a second aerial receives the reflected radio energy.
In amateur radio parlance, the terms "CW" and "Morse code" are often used interchangeably, despite the distinctions between the two (Morse code may be sent using sound and 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 very large; if the carrier turns on and off more slowly the bandwidth will be smaller. The problem of excessive bandwidth used by a morse transmitter which turns on/off too sharply is known as key clicks. If a perfectly made morse (CW) transmitter's output is fed into a series of cascaded class C stages, then the output will likely suffer from key clicks, since the output power of a class C stage increases greatly when input power is increased slightly.
Laser physics
In laser physics and engineering the term "continuous wave" or "CW" refers to a laser which produces a continuous output beam. This is as opposed to a q-switched or modelocked laser, which produces pulses of light.
References
Larry D. Wolfgang et. al, (ed), The ARRL Handbook for Radio Amateurs, Sixty-Eighth Edition , (1991), ARRL, Newington CT USA ISBN 0872591689