2-10the rate at which the capacitance of the microphone changes. This rate of change is caused by thefrequency of the sound wave. For example, suppose a 1,000-hertz tone of a certain loudness strikes themicrophone. The frequency of the carrier will then shift by a certain amount, say plus and minus 40kilohertz. The carrier will be shifted 1,000 times per second. Now assume that with its loudnessunchanged, the frequency of the tone is changed to 4,000 hertz. The carrier frequency will still shift plusand minus 40 kilohertz; but now it will shift at a rate of 4,000 times per second. Likewise, assume that atthe same loudness, the tone is reduced to 200 hertz. The carrier will continue to shift plus and minus 40kilohertz, but now at a rate of 200 times per second. If the loudness of any of these modulating tones isreduced by one-half, the frequency of the carrier will be shifted plus and minus 20 kilohertz. The carrierwill then shift at the same rate as before. This fulfills all requirements for frequency modulation. Both thefrequency and the amplitude of the modulating signal are translated into variations in the frequency of therf carrier.Figure 2-6.—Oscillator circuit illustrating frequency modulation.Figure 2-7A.—Capacitance change in an oscillator circuit during modulation. CHANGE IN INTENSITY OFSOUND WAVES CHANGES CAPACITY.

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