Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

320 FREQUENCY MODULATION, Volume Iwhen frequency modulation was used there was less interference producedby two stations operating at the same frequency than for the correspondingcase of amplitude modulation, and that less power was required tocover a given area.It was also found that when the ratio of the carriervoltage to the noise voltage is high, the signal-to-noise ratio improvementdue to frequency modulation is considerable. As the interfering noisevoltage is increased with respect to the desired carrier-wave voltage,the improved noise suppression is obtained as long as the desired signalis several times as strong as the noise.When a definite carrier-to-noise voltage ratio is reached (a ratio of 2 or 3for wide-band frequency modulation) the amount of distortion in theaudio output increases rapidly. When the noise voltage exceeds the signalvoltage during all parts of the audio cycle, the noise eliminates the desiredsignal. This means that when frequency modulation is used the signal iseither good or bad; there is only a small range for the ratio of carriervoltage to noise voltage that gives a noisy, but tolerable, signal.Multipath transmission occurs when two or more interfering signalscome from the same transmitter, but one is delayed with respect to theothers because of a longer transmission path.been observed when multipath transmission occurs inConsiderable distortion hasfrequency-modulatedbroadcasting and fairly complete discussions of this problem areavailable. 10-13 If the second wave comes from a different station thanthe desired wave, the result is common- or adjacent-channel interferenceaccording to whether the two carrier frequencies are nearly the same orare separated by the width of one channel.There is not much information available on the amount of interferenceto be expected in the new frequency modulation band. The effects to bedescribed were observed on the old 42-50 megacycle band and on the30-42 megacycle police bands. The frequency of occurrence and themagnitude of these effects will not be known for the new 88-108 megacycleband until a reasonable number of transmitters with normal powerand antenna gains are in operation.analysis given here will be applicable.If such interference does occur, the7Herbert J. Reich, "Interference Suppression in A-M and P-M," Communications,Vol. 22, No. 8, pp. 7, 16, 19, 20; August, 1942.8Robert N. Johnson, "Interference in F-M Receivers," Electronics, Vol. 18,No. 9, pp. 129-131; September, 1945.8 1. R. Weir, "Field Tests of Frequency- and Amplitude-Modulation WithUltra-High-Frequency Waves," Gen. Elec. Rev., Vol. 42, Nos. 5 and 6, pp. 188-191, May, 1939; pp. 270-273, June, 1939.10Murray G. Crosby, "Observations of Frequency-Modulation Propagationon 26 Megacycles," Proc. I.R.E., Vol. 29, No. 7, pp. 398-403; July, 1941.11A. D. Mayo and Charles W. Sumner, "F.M. Distortion in MountainousTerrain," Q.S.T., Vol. 28, No. 3, pp. 34-36; March, 1944.18Murlan S. Corrington, "Frequency-Modulation Distortion Caused byMultipath Transmission," Proc. I.R.E., Vol. 33, No. 12, pp. 878-891; Dec, 1945.18 S. T. Meyers, "Nonlinearity in frequency-modulation radio systems dueto multipath propagation," Proc. I.R.E., Vol. 34, No. 5, pp. 256-265; May, 1946.