Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

FREQUENCY MODULATION PROPAGATIONCHARACTERISTICS^ByMurray G. CrosbyRCA Communications, Inc., Riverhead, N. Y.Summary— Early work on frequency modulation is described whereinthe propagation characteristics of frequency modulation were determinedfor frequencies between 9000 and 18,000 kilocycles. Oscilloscopic wave formand aural program observations, taken on a circuit between California andNew York, showed that frequency modulation is much more distorted by theeffects of multipath transmission than is amplitude modulation. The distortionis greatest at the lower modulation frequencies and higher depthsof modulation where the side frequencies are most numerous.Oscilloscopic observations of the Lissajou figures formed by placing theoutputs of receivers connected to spaced antennas on opposite oscilloscopeplates showed that the diversity characteristics of frequency modulation aresimilar to those of amplitude modulation. . That is, the detected outputs ofthe receivers tend to remain in phase for the lower modulation frequenciesand become more phase random as the modulation frequency is increased.However, this tendency is almost obliterated on the lower modulation frequenciesof frequency modulation by the presence of unequal harmonic distortionin the two receiver outputs.Theory is given analyzing the distortion encountered in a two-pathtransmission medium under various path amplitude and phase relation conditions.The theory explains phenomena observed in the tests and pointsout the extreme distortion that can be encountered.INIntroductionTHE annals of radio, most of the work done on frequency modulationhas been of a theoretical nature 1 - 2 - 3and very little has beenoffered as a result of any experimental development.Considerable theory has also been presented in the consideration ofthis type of modulation as a defect of amplitude modulation. 3 - 4 - 5 - 6 - 7* Decimal classification: R630.ll.f Reprinted from Proc. I.R.E., June, (1936).John R. Carson, "Notes on the theory of modulation," Proc. I.R.E.,1vol. 10, pp. 57-64; February, (1922).2Balth van der Pol, "Frequency modulation," Proc. I.R.E., vol. 18, pp.1194-1205; July, (1930).3 Hans Roder, "Amplitude, phase, and frequency modulation," Proc.I.R.E., vol. 19, pp. 2145-2176; December, (1931).R. Bown, D. K. Martin, and R. K. Potter, "Some studies in radiobroadcast transmission," Proc. I.R.E., vol. 14, pp. 57-131; February, (1926).5 R. K. Potter, "Transmission characteristics of a short-wave telephonecircuit," Proc. I.R.E., vol. 18, pp. 633-648; April, (1930).J. C. Schelleng, "Some problems in short-wave telephone transmission,"Proc. I.R.E., vol. 18, pp. 933-937; June, (1930).7T. L. Eckersley, "Frequency modulation and distortion," ExperimentalWireless and The Wireless Engineer, vol. 7, pp. 482-484; September, (1930).A