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Continuous wave

From Wikipedia, the free encyclopedia

A continuous wave or continuous waveform (CW) is anelectromagnetic wave of constant amplitude and frequency;

and in mathematical analysis, of infinite duration.Continuous wave is also the name given to an early methodof radio transmission, in which a carrier wave is switched onand off. Information is carried in the varying duration of theon and off periods of the signal. In radio transmission, CWwaves are also known as "undamped waves", to distinguishthis method from damped wave transmission.

Radio

Very early radio transmitters used a spark gap to produce radio-frequency oscillations in thetransmitting antenna; the signals produced by these spark-gap transmitters had a characteristic

rapidly damped amplitude during each pulse of radiated energy. When alternators and laterelectronic oscillators became available, the signal strength remained constant during each codeelement, 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 carrieron and off produces a finite bandwidth relating to the transmission rate. Strictly speaking, a keyedcarrier may be referred to as "ICW" for "Interrupted continuous wave" but the necessity of keying isusually understood.

Early radio transmitters were incapable of handling thecomplexity of actual audio and therefore CW was the

only form of communication available. CW stillremained a viable form of radio communication formany years after voice transmission was perfected,because simple transmitters could be used. The lowbandwidth of the code signal, due in part to lowinformation transmission rate, allowed very selectivefilters to be used in the receiver which blocked out muchof the atmospheric noise that would otherwise reduce theintelligibility of the signal.

Continuous-wave radio was called radiotelegraphybecause like the telegraph, it worked by means of asimple switch to transmit Morse code. However, insteadof controlling the electricity in a cross-country wire, the switch controlled the power sent to a radiotransmitter. This mode is still in common use by amateur radio operators.

Modulation techniques

Analog modulation

AM SSB QAM FM PM SM

Digital modulation

FSK ASK OOK PSK QAMMSK CPM PPM TCM OFDM

Spread spectrum

CSS DSSS FHSS THSS

See also: Demodulation, modem

Contents

1 Radio 1.1 Key clicks

2 Laser physics 3 See also 4 References

A commercially manufactured paddle foruse with electronic keyer to generate

Morse code

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A continuous-wave radar system is one where a continuous wave is transmitted by one aerial while asecond aerial receives the reflected radio energy.

In military communications and amateur radio, the terms "CW" and "Morse code" are often usedinterchangeably, despite the distinctions between the two. Morse code may be sent using directcurrent 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 duringeach code element. At the receiver, the received signal is mixed with a heterodyne signal from aBFO (Beat Frequency Oscillator) to change the radio freqeuncy impulses to sound. Though mostcommercial traffic has now ceased operation using that mode, it is still popular with amateur radiooperators.

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 istransmitted in the extra bandwidth used by a transmitter that turns on/off more abruptly is known askey clicks. Certain types of power amplifiers used in transmission may increase the effect of keyclicks.

Laser physics

In laser physics and engineering the term "continuous wave" or "CW" refers to a laser whichproduces a continuous output beam, sometimes referred to as 'free-running'. This is as opposed to aq-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-EighthEdition , (1991), ARRL, Newington CT USA ISBN 0-87259-168-9

CW Bandwidth Described

Retrieved from "http://en.wikipedia.org/wiki/Continuous_wave"Categories: Lasers | Radio modulation modes

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