Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

:100 FREQUENCY MODULATION, Volume Isingle path, and where echo is small when it is present, 11 fair successwould be possible with a frequency modulation circuit where the modulationfrequencies were not higher than about 5000 cycles. However,the logical place for frequency modulation is on the ultra-high frequencieswhere the only multipath transmission that exists 15 is thatdue to the direct ray and the ray reflected from the ground and othernear-by objects. On the other hand, even this multipath transmissionon ultra-short waves will introduce appreciable distortion to modulationfrequencies of the order used in television. This distortion wouldbe particularly severe in the case where the transmitter and receiverare both at rather high elevations, but would have the possibility ofelimination by the use of directivity to discriminate against one ofthe rays.The specific conclusions of the tests may be summarized as followsThe distortion is most severe on the lower modulation frequencies andon the lower radiation frequencies. The distortion was equally severewith or without limiting. The diversity tests showed a tendency forthe two receivers to stay in phase at the lower modulation frequencies,but this tendency was almost obliterated by unequal harmonic distortionon the two receivers. The higher modulation frequencies and thelower radiation frequencies showed the most random phase characteristics.The specific conclusions of the theory concerning the two-pathcase may be summarized as follows : When the two paths are exactlyequal in amplitude, provided conditions are such that the resultantamplitude does not go to zero, the effect is a change in the effectivefrequency deviation with no harmonic distortion. When the strengthof one path is great compared to the other, odd harmonics are introducedwhen the phase difference between the carriers is zero and evenharmonics are introduced when the phase difference is ninety degrees.The most destructive distortion occurs when the amplitudes of thepaths are not quite equal. Under this condition the distortion is greatestfor the higher values of F d /F m and for phase relations such that thephase differences between the modulation frequencies and betweenthe carrier frequencies are both 180 degrees.AcknowledgmentThe author is indebted to Messrs. H. H. Beverage and H. O. Petersonunder whose guidance the work of this paper was carried out.14T. L. Eckersley, "Multiple signals in short-wave transmission," Pboc.I.R.E., vol. 18, pp. 106-122; January, (1930).16Bertram Trevor and P. S. Carter, "Notes on propagation of waves belowten meters in length," Pboc. I.R.E., vol. 21, pp. 387-426; March, (1933).