2-11Figure 2-7B.—Capacitance change in an oscillator circuit during modulation. AT A FREQUENCY TWICE THATOF (A), THE CAPACITY CHANGES THE SAME AMOUNT, BUT TWICE AS OFTEN.Figure 2-8 shows how the frequency shift of an fm signal goes through the same variations as doesthe modulating signal. In this figure the dimension of the constant amplitude is omitted. (As theseremaining waveforms are presented, be sure you take plenty of time to study and digest what the figurestell you. Look each one over carefully, noting everything you can about them. Doing this will help youunderstand this material.) If the maximum frequency deviation is set at 75 kilohertz above and below thecarrier, the audio amplitude of the modulating wave must be so adjusted that its peaks drive the frequencyonly between these limits. This can then be referred to as 100-PERCENT MODULATION, although theterm is only remotely applicable to fm. Projections along the vertical axis represent deviations infrequency from the resting frequency (carrier) in terms of audio amplitude. Projections along thehorizontal axis represent time. The distance between A and B represents 0.001 second. This means thatcarrier deviations from the resting frequency to plus 75 kilohertz, then to minus 75 kilohertz, and finallyback to rest would occur 1,000 times per second. This would equate to an audio frequency of 1,000 hertz.Since the carrier deviation for this period (A to B) extends to the full allowable limits of plus and minus75 kilohertz, the wave is fully modulated. The distance from C to D is the same as that from A to B, sothe time interval and frequency are the same as before. Notice, however, that the amplitude of themodulating wave has been decreased so that the carrier is driven to only plus and minus 37.5 kilohertz,one-half the allowable deviation. This would correspond to only 50-percent modulation if the systemwere AM instead of fm. Between E and F, the interval is reduced to 0.0005 second. This indicates anincrease in frequency of the modulating signal to 2,000 hertz. The amplitude has returned to its maximumallowable value, as indicated by the deviation of the carrier to plus and minus 75 kilohertz. IntervalG toHrepresents the same frequency at a lower modulation amplitude (66 percent). Notice the GUARDBANDS between plus and minus 75 kilohertz and plus and minus 100 kilohertz. These bands isolate themodulation extremes of this particular channel from that of adjacent channels.

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