Data transmission

Chapter Data transmission

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

  • 26CHAPTERData transmissionTO CONVEY DATA, SOME ASPECT OF A SIGNAL MUST BE VARIED. THERE ARE several different characteristics of a signal that can be made to change in a controlledway, so that data is “imprinted” on it. Modulation is the process of imprinting data ontoan electric current or radio wave.Modulation can be accomplished by varying the amplitude, the frequency, or thephase of a wave. Another method is to transmit a series of pulses, whose duration, am-plitude, or spacing is made to change in accordance with the data to be conveyed.The carrier waveThe “heart” of most communications signals is a sine wave, usually of a frequency wellabove the range of human hearing. This is called a carrier or carrier wave. The lowestcarrier frequency used for radio communications is 9 kHz. The highest frequency is inthe hundreds of gigahertz.For modulation to work effectively, the carrier must have a frequency manytimes the highest frequency of the modulating signal. For example, if you want tomodulate a radio wave with hi-fi music, which has a frequency range from a few hertzup to 20 kHz or so, the carrier wave must have a frequency well above 20 kHz. A goodrule is that the carrier must have a frequency of at least 10 times the highest modu-lating frequency. So for good hi-fi music transmission, a radio carrier should be at200 kHz or higher.This rule holds for all kinds of modulation, whether it be of the amplitude, phase, orfrequency. If the rule is violated, the efficiency of transmission will be degraded, result-ing in less-than-optimum data transfer.474Copyright © 2002, 1997, 1993 by The McGraw-Hill Companies. Click here for terms of use.