2-31Figure 2-27C.—Overmodulation of a carrier. NONSINUSOIDAL MODULATING WAVE.Observe the modulating square wave in figure 2-28. Remember that it contains an infinite number ofodd harmonics in addition to its fundamental frequency. Assume that a carrier has a frequency of 1megahertz. The fundamental frequency of the modulating square wave is 1 kilohertz. When these signalsheterodyne, two new frequencies will be produced: a sum frequency of 1.001 megahertz and a differencefrequency of 0.999 megahertz. The fundamental frequency heterodynes with the carrier. This is also trueof all harmonics contained in the square wave. Side frequencies associated with those harmonics will beproduced as a result of this process. For example, the third harmonic of the square wave heterodynes withthe carrier and produces sideband frequencies at 1.003 and 0.997 megahertz. Another set will be producedby the fifth, seventh, ninth, eleventh, thirteenth, fifteenth, seventeenth, and nineteenth harmonics of thesquare wave, and so on to infinity.Figure 2-28.—Spectrum distribution when modulating with a square wave.Look at figure 2-28 and observe the relative amplitudes of the sidebands as they relate to theamplitudes of the harmonics contained in the square wave. Note that the first set of sidebands is directlyrelated to the amplitude of the square wave. The second set of sidebands is related to the third harmoniccontent of the square wave and is 1/3 the amplitude of the first set. The third set is related to the amplitudeof the first set of sidebands and is 115 the amplitude of the first set. This relationship will apply to eachadditional set of sidebands.

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