Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

50 FREQUENCY MODULATION, Volume Iamount of phase modulation produces a frequency excursion or deviationof only 45 cycles per second. 2 Since the output of the frequencymodulationdetector of a frequency-modulated wave receiver is dependentonly upon the deviation, or excursion, from the mean frequency, itcan be seen that the output will be 150 times greater when the carriersare separated by 15,000 cycles than when the separation is only 100cycles. It is for this reason that the beat note between two frequencymodulatedcarrier wave stations becomes inaudible as the frequenciesare brought closer and closer together.V. Frequency-Modulation MethodsA frequency-modulated wave transmitter should emit a wave ofCAPBienOUTPUT OFi/M/rs/r STAGEFig. 5unvarying amplitude with a frequency deviation about its mean(carrier) frequency in perfect conformity with the speech or programmodulation. This neglects the matter of high-frequency pre-emphasiswhich will be discussed shortly.There are two basic methods of producing this desired result. Ineither case, however, and unlike amplitude-modulated wave transmitters,the modulation takes place at the source of the transmitterfrequency-determining circuits. Thereafter, the original oscillatorfrequencyenergy may be amplified, multiplied in frequency, orheterodyned to any other frequency without altering the modulatingcomponents except to the extent that any multiplication of the source2Actually when a single side component "phase modulates" an existingcarrier, as in this ca"se, the frequency modulation is not symmetrical. However,with a two to one ratio of original amplitude the "peak to peak"frequency shift is nearly equal to the values which the above figures wouldindicate.