Chapter Noise

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • wavelengths of most broadcast and communications signals, which had wavelengthsin the kilometer range.Anyone can build or obtain a shortwave or general-coverage radio receiver, installa modest outdoor antenna, and listen to signals from all around the world. This hobbyis called shortwave listening or SWLing. In the United States, the proliferation ofcomputers and on-line communications has, to some extent, overshadowed SWLing,and many young people grow up today ignorant of a realm of broadcasting and com-munications that still predominates in much of the world. But some people are still fas-cinated by the idea that people can contact each other by wireless devices without theneed for any human-made infrastructure other than an antenna at the source and an-other antenna at the destination. The ionosphere returns shortwave signals to theearth’s surface and allows reliable global broadcasting and communication to take placetoday, just as it always has, and just as it always will.There are various commercially manufactured shortwave receivers on the markettoday, and some of these are inexpensive. An outdoor wire antenna costs practicallynothing. Most electronics stores carry one or more models of shortwave receiver, alongwith antenna equipment, for a complete installation. One problem with low-pricedshortwave receivers is that they usually lack the mode flexibility, selectivity, and sensi-tivity necessary to engage in serious SWLing. If you are interested in this hobby andwant to obtain high-end equipment, shop around in consumer electronics and amateurradio magazines. Most electronics and book stores carry periodicals and books for thebeginner as well as the experienced SWLer. A library can also be a good source of in-formation, especially if you are interested in “antique” shortwave receivers, some ofwhich can be found at amateur-radio conventions and flea markets.NoiseIn a wireless system, the term noise refers to an electromagnetic field that usuallyhas large bandwidth; that is, it occurs over a wide range of frequencies and wave-lengths. Noise does not convey information. It can be either natural or human-made.It’s never good newsNoise never helps, and often degrades, the performance of a wireless system. It is aconcern in any device or system in which data is sent from one place to another. Thehigher the noise level, the stronger a signal must be if it is to be received error-free.At any given signal power level, higher noise levels translate into more errors and re-duced communications range.Figure 32-9 is a spectral display of signals and noise, with amplitude as a functionof frequency. The device that generates this display is called a spectrum analyzer. Thehorizontal axis shows frequency; the vertical axis shows amplitude. The backgroundnoise level is called the noise floor. Signals above the noise floor appear in the displayand can be received. The strongest signals are received with the fewest errors; weaksignals are subject to the most errors. Signals below the noise floor are not displayedand cannot be retrieved unless a more sophisticated receiving system is used, or thetransmitter power output is increased, or both.Noise619