2-9of times per second that the instantaneous frequency is varied from the average (carrier frequency) iscontrolled by the frequency of the modulating signal. The amount by which the frequency departs fromthe average is controlled by the amplitude of the modulating signal. This variation is referred to as theFREQUENCY DEVIATION of the frequency-modulated wave. We can now establish two clear-cut rulesfor frequency deviation rate and amplitude in frequency modulation:Figure 2-5.—Effect of frequency modulation on an rf carrier.AMOUNTOF FREQUENCY SHIFT IS PROPORTIONAL TO THE AMPLITUDE OF THEMODULATING SIGNAL(This rule simply means that if a 10-volt signal causes a frequency shift of 20 kilohertz, then a20-volt signal will cause a frequency shift of 40 kilohertz.)RATE OF FREQUENCY SHIFT IS PROPORTIONAL TO THE FREQUENCY OF THEMODULATING SIGNAL(This second rule means that if the carrier is modulated with a 1-kilohertz tone, then the carrier ischanging frequency 1,000 times each second.)Figure 2-6 illustrates a simple oscillator circuit with the addition of a condenser microphone (M) inshunt with the oscillator tank circuit. Although the condenser microphone capacitance is actually verylow, the capacitance of this microphone will be considered near that of the tuning capacitor (C). Thefrequency of oscillation in this circuit is, of course, determined by the LC product of all elements of thecircuit; but, the product of the inductance (L) and the combined capacitance of C and M are the primaryfrequency components. When no sound waves strike M, the frequency is the rf carrier frequency. Anyexcitation of M will alter its capacitance and, therefore, the frequency of the oscillator circuit. Figure 2-7illustrates what happens to the capacitance of the microphone during excitation. In view (A), the audio-frequency wave has three levels of intensity, shown as X, a whisper; Y, a normal voice; and Z, a loudvoice. In view (B), the same conditions of intensity are repeated, but this time at a frequency twice that ofview (A). Note in each case that the capacitance changes both positively and negatively; thus thefrequency of oscillation alternates both above and below the resting frequency. The amount of change isdetermined by the change in capacitance of the microphone. The change is caused by the amplitude of thesound wave exciting the microphone. The rate at which the change in frequency occurs is determined by

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