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Figure 3-12 shows the AM wave resulting when a square wave modulates a sinewave carrier. In Fig. 3-12(a), the percentage of modulation is 50; in Fig. 3-12(b), it is100. In this case, when the square wave goes negative, it drives the carrier amplitude tozero. Amplitude modulation by square waves or rectangular binary pulses is referred toas amplitude-shift keying (ASK).ASK is used in some types of data communication whenbinary information is to be transmitted.Another crude type of amplitude modulation can be achieved by simply turning thecarrier off and on. An example is the transmitting of Morse code by using dots and dashes.Amplitude Modulation Fundamentals103Figure 3-10Frequency spectrum of AM broadcast band.540 kHz10 kHzchannel535 kHz1550 kHz2560 kHz31590 kHz1061600 kHz1605 kHz107Pulse modulationSeventhharmonicFifthharmonicThirdharmonicSidebandsproduced by thefundamental andits harmonicsFundamentalfcCarrier Figure 3-11Frequency spectrum of an AM signal modulated by a square wave.Amplitude-shift keying (ASK)
104Chapter 3Figure 3-12Amplitude modulation of a sine wave carrier by a pulse or rectangular waveis called amplitude-shift keying. (a) Fifty percent modulation. (b