Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

PROPAGATION CHARACTERISTICS 91The effect due to limiting seemed to be the indication that the distortionwas caused by a change in the instantaneous frequency deviationof the wave and not due to an added amplitude modulation. Thiswas evidenced by the fact that the limiter effectively removed all amplitudemodulation introduced by fading, but the extreme distortionremained with little or no perceptible difference attributable to thelimiter.The general result of the use of different radiation frequencies overthe circuit was to indicate that the quality of modulation was most impairedon the frequencies where the selective fading was most marked.Of the three frequencies observed, 18,020 kilocycles showed the leastamount of selective fading, 9010 the greatest, and 13,690 a mean betweenthe other two. Consequently, the quality of frequency modulationon 18,020 kilocycles was only occasionally inferior to that on amplitudemodulation. On 9010 kilocycles there was scarcely an intervalof good quality on the frequency modulation, and the amplitude modulationwas also considerably distorted. 13,690 kilocycles was considereda representative frequency for the observations, since it presentedperiods of extremely poor quality along with periods of fairly goodquality. Consequently, the oscillograms were taken on the 13,680-kilocycle radiations.Fig. 6 gives the oscillograms showing the effects of distortion fadingon wave form. From a comparison of the 300- and 1000-cycle oscillogramstaken on frequency modulation, it is apparent that the amountof distortion on 300 cycles is much greater than that on 1000 cycles.At the lower modulation frequencies on frequency modulation, theharmonics were comparable to and stronger than the fundamental fora large percentage of the time. As the modulation frequency was increased,the amount of distortion decreased until at modulation frequenciesabove about 3000 cycles, little or no harmonic distortion wasencountered. One of the reasons for this absence of harmonic distortionon the higher frequencies is probably the fact that the pass band of thereceiver intermediate-frequency amplifier was twelve kilocycles andthat of the audio amplifier six kilocycles. Hence, for modulation frequenciesabove 3000, the side bands above the first were eliminated bythe intermediate-frequency filter and the harmonics were eliminatedby the audio system. The reason for the excessive distortion at thelower modulation frequencies of frequency modulation is no doubt dueto the effects of multipath transmission in disturbing the phase relationsbetween the many side frequencies produced by these lower modulationfrequencies.Fig. 6J shows a wave form taken on amplitude modulation when