Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

168 FREQUENCY MODULATION, Volume Iunit that would also have excellent frequency stability. The frequencycorrection should be effected independent of the modulation process,and, if possible, provision should be made to correct frequency manuallyif there should be a failure of the automatic frequency control. A goodcrystal-controlled oscillator should be used as a standard referencefrequency source for the automatic frequency-control circuit.One type of frequency modulation exciter (illustrated by the blockdiagram of Figure 4) beats the master oscillator frequency down toone megacycle, using a crystal oscillator as the beat-frequency source.The one-megacycle beat frequency acts on a discriminator to provide adirect current control potential which is applied to a reactance modulatortube grid so as to compensate any drift of the master oscillatorfrequency. The time constant of the correction is of sufficient durationto prevent it responding to modulation.