Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

94 FREQUENCY MODULATION, Volume Itained on frequency modulation with a 300-cycle modulation frequency,given in Fig. 7C, E, F, G, and H, obtain their grotesque distortionsfrom the presence of different harmonic distortions in the two receiveroutputs. Thus one receiver might be receiving a signal producing a purefundamental tone only, whereas the other receiver might be receivinga signal rich in harmonics; consequently, the diagram is distorted accordingto the difference in the amounts of harmonic content presentat the instant of exposure. The effect produced by this unequal harmoniccontent on the two sets of oscilloscope plates can be seen from astudy of the Lissajou figures given in Fig. 5 f, g, h, i, and j. Thus thediagram of Fig. 5 f might be formed by the condition of fundamentaltone from one receiver and second harmonic from the other, or thediagram of Fig. 5i by second harmonic from one receiver and thirdharmonic from the other. The rapid shift from one harmonic relationto another together with the presence of complex instead of simplewave forms on the two sets of plates makes possible erratic and grotesquepatterns. 12 This effect was apparent to such a degree on frequencymodulation that difficulty was experienced in determining thephase tendency of the fundamental tones. The effect was only occasionallynoticeable on amplitude modulation.The diagrams given in Fig. 7 I and J for a modulation frequency of1000 cycles, show how the higher modulation frequencies on amplitudemodulation are marked by phase differences which are not present onthe lower tones. The higher tones on frequency modulation given inFig. 7 K, L, M, and N for a modulation frequency of 1000 cycles,and in Fig. 7 O and P for a modulation frequency of 5000 cycles, alsoindicate marked phase differences and only a slight indication of differentharmonic distortions. Amplitude and frequency modulationshowed about the same amount of phase distortion on the higher modulationfrequencies.TheoryIt is well known that the cause of fading distortion, taking place inthe range of frequencies used in these tests, is due to multipath transmissionin which the signal is coin-eyed to the receiving antenna bymeans of more than one path. These paths consist of refractions orreflections from the conducting layers of the ionosphere. Since theseconducting layers vary in height and since the signal may follow acourse consisting of various numbers of ricochets between the ionosphereand the earth, the signal traveling over the longer path is given12 A more complete set of these Lissajou figures may be found in the book,"High-Frequency Measurements," by A. Hund, pp. 71-75; McGraw-Hill,(1933).