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| Analog modulation |
| Digital modulation |
| Hierarchical modulation |
| Spread spectrum |
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Acontinuous wave orcontinuous waveform (CW) is anelectromagnetic wave of constantamplitude andfrequency, typically asine wave, that formathematical analysis is considered to be of infinite duration.[1] It may refer to e.g. alaser orparticle accelerator having a continuous output, as opposed to apulsed output.
By extension, the termcontinuous wave also refers to an early method ofradiotransmission in which a sinusoidalcarrier wave is switched on and off. This is more precisely calledinterrupted continuous wave (ICW).[2]Information is carried in the varying duration of theon and off periods of the signal, for example byMorse code in early radio. In earlywireless telegraphy radio transmission, CW waves were also known as "undamped waves", to distinguish this method fromdamped wave signals produced by earlierspark gap type transmitters.
Very early radio transmitters used aspark gap to produce radio-frequency oscillations in the transmitting antenna. The signals produced by thesespark-gap transmitters consisted of strings of brief pulses ofsinusoidal radio frequency oscillations which died out rapidly to zero, calleddamped waves. The disadvantage of damped waves was that their energy was spread over an extremely wide band offrequencies; they had widebandwidth. As a result, they producedelectromagnetic interference (RFI) that spread over the transmissions of stations at other frequencies.
This motivated efforts to produce radio frequency oscillations that decayed more slowly; had less damping. There is a direct relation between the rate of decay (the reciprocal of thetime constant) of a damped wave and its bandwidth; the longer the damped waves take to decay toward zero, the narrower the frequency band the radio signal occupies, so the less it interferes with other transmissions. As more transmitters began crowding the radio spectrum, reducing the frequency spacing between transmissions, government regulations began to limit the maximum damping or "decrement" a radio transmitter could have. Manufacturers produced spark transmitters which generated long "ringing" waves with minimal damping.
It was realized that the ideal radio wave forradiotelegraphic communication would be a sine wave with zero damping, acontinuous wave. An unbroken continuous sine wave theoretically has no bandwidth; all its energy is concentrated at a single frequency, so it doesn't interfere with transmissions on other frequencies. Continuous waves could not be produced with an electric spark, but were achieved with thevacuum tubeelectronic oscillator, invented around 1913 byEdwin Armstrong andAlexander Meissner. AfterWorld War I, transmitters capable of producing continuous wave, theAlexanderson alternator andvacuum tubeoscillators, became widely available.
Damped wave spark transmitters were replaced by continuous wave vacuum tube transmitters around 1920, and damped wave transmissions were finally outlawed in 1934.
In order to transmit information, the continuous wave must be turned off and on with atelegraph key to produce the different length pulses, "dots" and "dashes", that spell out text messages inMorse code, so a "continuous wave" radiotelegraphy signal consists of pulses of sine waves with a constant amplitude interspersed with gaps of no signal.
In on-off carrier keying, if the carrier wave is turned on or off abruptly,communications theory can show that thebandwidth 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:where is the necessary bandwidth in hertz, is the keying rate in signal changes per second (baud rate),and 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 ormultipath propagation is expected.[3]
The spurious noise emitted by atransmitter which abruptly switches a carrier on and off is calledkey clicks. The noise occurs in the part of the signal bandwidth further above and below the carrier than required for normal, less abrupt switching. The solution to the problem for CW is to make the transition between on and off to be more gradual, making the edges of pulsessoft, appearing more rounded, or to use other modulation methods (e.g.phase modulation). Certain types of power amplifiers used in transmission may aggravate the effect of key clicks.
Early radio transmitters could not bemodulated to transmit speech, and so CW radio telegraphy was the only form of communication available. CW still remains a viable form of radio communication many years after voice transmission was perfected, because simple, robust transmitters can be used, and because its signals are the simplest of the forms ofmodulation able to penetrate interference. The low bandwidth of the code signal, due in part to low information transmission rate, allows very selective filters to be used in the receiver, which block out much of the radio noise that would otherwise reduce the intelligibility of the signal.
Continuous-wave radio was calledradiotelegraphy because like thetelegraph, it worked by means of a simple switch to transmitMorse code. However, instead of controlling the electricity in a cross-country wire, the switch controlled the power sent to a radiotransmitter. This mode is still in common use byamateur radio operators due to its narrow bandwidth and highsignal-to-noise ratio compared to other modes of communication.
In military communications andamateur radio the terms "CW" and "Morse code" are often used interchangeably, despite the distinctions between the two. Aside from radio signals, Morse code may be sent usingdirect current in wires, sound, or light, for example. For radio signals, a carrier wave is keyed on and off to represent the dots and dashes of the code elements. The carrier's amplitude and frequency remainconstant during each code element. At the receiver, the received signal is mixed with aheterodyne signal from a BFO (beat frequency oscillator) to change the radio frequency impulses to sound. Almost all commercial traffic has now ceased operation using Morse, but it is still used by amateur radio operators.Non-directional beacons (NDB) andVHF omnidirectional radio range (VOR) used in air navigation use Morse to transmit their identifier.
Morse code is all but extinct outside the amateur service, so in non-amateur contexts the term CW usually refers to acontinuous-wave radar system, as opposed to one transmitting short pulses. Somemonostatic (single antenna) CW radars transmit and receive a single (non-swept) frequency, often using the transmitted signal as thelocal oscillator for the return; examples include police speed radars and microwave-type motion detectors and automatic door openers. This type of radar is effectively "blinded" by its own transmitted signal to stationary targets; they must move toward or away from the radar quickly enough to create a Doppler shift sufficient to allow the radar to isolate the outbound and return signal frequencies. This kind of CW radar can measurerange rate but notrange (distance).
Other CW radars linearly or pseudo-randomly "chirp" (frequency modulate) their transmitters rapidly enough to avoid self-interference with returns from objects beyond some minimum distance; this kind of radar can detect and range static targets. This approach is commonly used inradar altimeters, inmeteorology and in oceanic and atmospheric research. Thelanding radar on theApollo Lunar Module combined both CW radar types.
CWbistatic radars use physically separate transmit and receive antennas to lessen the self-interference problems inherent in monostatic CW radars.
Inlaser physics and engineering, "continuous wave" or "CW" refers to alaser that produces a continuous output beam, sometimes referred to as "free-running," as opposed to aq-switched,gain-switched ormodelocked laser, which has a pulsed output beam.
The continuous wavesemiconductor laser was invented by Japanese physicistIzuo Hayashi in 1970.[citation needed] It led directly to the light sources infiber-optic communication,laser printers,barcode readers, andoptical disc drives, commercialized by Japanese entrepreneurs,[4] and opened up the field ofoptical communication, playing an important role in futurecommunication networks.[5] Optical communication in turn provided the hardware basis forinternet technology, laying the foundations for theDigital Revolution andInformation Age.[6]
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