Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

52 FREQUENCY MODULATION, Volume IThe amplitude modulation has little or no significance since it isalways "limited" off in subsequent stages, but it is interesting to notethat at least with sine-wave modulation, all amplitude modulation isat even harmonics of the modulation frequency without any of thefundamental being present. This is shown by the fact that the vectorrepresenting the carrier maximizes at each end of its phase excursionor twice each modulating cycle.The developed phase modulation can be seen to be a function of theamplitudes of the horizontal vectors in Figure 6, which have beenderived from the balanced modulator. As long as the angle, whosetangent is equal to the sum of the modulating vectors divided by thecarrier vector, does not exceed 30° (the range through which theFROM BALANCEDMODULATORJXMS£RTEOCAPP/eGFig. 6tangent is approximately equal to the angle)the phase modulation canbe said to vary linearly with the modulating components. 3The peak-frequency deviation of a frequency-modulated wave isrelated to the peak-phase deviation by the expression4,-.3602ttFrequency DeviationX'Modulating Frequencyin which is the peak-phase deviation in degrees.If it is desired to produce a frequency deviation of ± 75,000 cycleswith a 50-cycle modulating frequency, the phase displacement wouldamount to ± 86,000 degrees. If the assumption of 30 degrees for the3 Actually 30° phase modulation by this process results in nearly 10 percent harmonic distortion, but since the resulting frequency modulation isa function of the product of the phase deviation and the modulating frequencythe maximum phase displacement is required only for full modulationat the lowest modulating frequency.