Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FREQUENCY MODULATION 47band pass of our receiver were just sufficiently wide to pass onlythose side components at 4- and — 10 kc, we would have the conditionshown in Figure 3A. In this figure the first three side bands areshown with their relative amplitudes when the peak frequency excursiondivided by the modulating frequency is equal to unity. The theoreticallyperfect pass band is shown just including the carrier and thefirst two side bands. The resultant wave is as represented vectoriallyin Figure 3B, with the upper and lower side bands revolving about thepoint P, first causing a phase displacement to point P' and then to P".ACC£PTAVCE_Fig. 3Under these conditions it can be seen that the resultant instantaneousscalar magnitude of the carrier will vary between .78 and 1.16. Inother words, amplitude modulation at twice the modulating frequencyhas resulted from deletion of all but the first side bands. This amplitudemodulation contains no components at the 10,000-cycle frequency,ischiefly second harmonic, and contains small amounts of other evenharmonics.If now this wave shown emerging from our perfect 20-kc bandpass filter is passed through a limiter stage which denudes the carrierof any amplitude modulation, the vector A will oscillate between thepoint A' and A", which means that additional side bands have beenrecreated to cancel the amplitude modulation. It so happens that allof the even harmonic side bands are reinserted by a limiter stage, butthat the odd harmonic side bands are not so reinserted, and thus a