Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

90 FREQUENCY MODULATION, Volume IFor the diversity experiments, an arrangement as shown in Fig. 4was used. Two complete frequency modulation receivers were fed bytwo harmonic wire antennas spaced 450 feet apart. The detected outputsof the tone modulation were fed to the opposite oscilloscope plates.Hence, by noting whether the oscilloscope diagram was a line in thesecond and fourth quadrants, a circle,or a line in the first and thirdquadrants, the relative phase relation of the two detected outputs couldbe determined as being in phase, ninety degrees out of phase, or 180degrees out of phase, respectively. (See Fig. 5a, b, c, d, and e.)For the wave form observations, the output of a single receiver wasapplied to one set of oscilloscope plates and a timing voltage to theother. Quality distortion was also checked by the aural observation ofmusic which was transmitted by the frequency and amplitude modulationtransmitters./ o o ^Fig.5—Lissajou figures obtained with various phase relations and frequencyratios between voltages applied to opposite oscilloscope plates.Wave Form DataDay and night observations were made using music and tone modulationon frequencies of 18,020, 13,690, and 9010 kilocycles. The generalconclusions derived were that frequency modulation is far more susceptibleto the effects of selective fading than amplitude modulation.On music, the bass and lower notes were distorted and jumbled togetherwhile the violin and the other higher solo instruments came throughfairly well. The intelligibility of speech was very low; in some cases asignal which gave fair intelligibility on amplitude modulation was practicallyunintelligible on frequency modulation. The distortion seemedto be proportional to the depth of modulation, since a sort of blastingeffect was observed on the modulation peaks, and transmissions withlowered deviation came through better than those with higher deviation.The distortion also seemed to be greatest during fading minimum^at which time "bursts" of distortion would appear; this condition wasalso present to a lesser extent on amplitude modulation.