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 varying duration of the 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.

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 transmitters consisted of brief pulses of radio frequency oscillations which died out rapidly to zero, called damped waves. The disadvantage of damped waves was that they produced electromagnetic interference that spread over the transmissions of stations at other frequencies.

This motivated efforts to produce radio frequency oscillations that decayed more slowly. The ideal was an oscillating signal that didn't decrease to zero at all but continued at a constant strength (amplitude) during each code element, this was described as a "continuous wave". This type of signal has all its energy concentrated at one frequency, and doesn't interfere at all with signals at other frequencies, allowing many radio stations to share a given band of frequencies. The first transmitters capable of producing continuous waves, the Alexanderson alternator and vacuum tube oscillators, became widely available after World War 1.

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.

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 aerial while a second aerial receives the reflected radio energy.

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 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 that mode, it is still popular with amateur radio operators.

Older non-directional beacons used in air navigation use CW to transmit their identifier.

Key clicks

In any form of on-off carrier keying, if the carrier wave is turned on or off abruptly, the bandwidth will be large; if the carrier turns on and off more gradually, the bandwidth will be smaller. 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.

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-switched or 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 [Amazon-US | Amazon-UK]
• CW Bandwidth Described

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