Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

222 FREQUENCY MODULATION, Volume Iin such a system, the increase in power would be two-to-one.The usualfactor of four-to-one assumes the more inefficient types of low-levelmodulation.The values of signal-noise ratio obtained for all of the curves werecorrected by subtracting one-half the maximum fading range as portrayedby Figure 2. Formula (4) was used to calculate the averagesignal-noise ratio for amplitude modulation and the fading correctionwas applied to obtain the minimum signal-noise ratio.It was assumedthat the transmission conditions of the fading correction curve werenear enough to the conditions of this example to allow this directcorrection without interpolation.Where the propagation curves of the corresponding amplitude-modulationsystem were to be. determined as well as those of the frequencymodulationsystems, which was the case in this example, the simplestprocedure was to draw the amplitude-modulation curves and then constructthe frequency-modulation curves which follow lines parallel tothe amplitude-modulation curve, but at higher signal-noise ratio levelsfor distances within the improvement threshold distances. To do thisthe average amplitude-modulation signal-noise ratios for fluctuationnoise were calculated from (4). These were corrected for fading andthe pre-emphasis gain of 5.6 decibels was added. For the low-deviationfrequency-modulation system, the fluctuation noise gain is equal to 1.73times the deviation ratio, p. (/* = 20/15 = 1.33) or 7.2 decibels. Theuse of pre-emphasis adds another 7.4 decibels to the frequency-modulationgain as compared to the amplitude-modulation system with preemphasissince the total gain due to pre-emphasis on the frequencymodulationsystem is 13 decibels. The transmitter power gain addsanother 3 decibels. Hence the curve for frequency modulation with adeviation of 20 kilocycles is 7.2 + 7.4 + 3 = 17.6 decibels higher thanthecorresponding amplitude-modulation curve for the region withinthe threshold of improvement distance of the frequency-modulationsystem. The frequency-modulation system with a 75-kilocycle deviationhas a larger deviation ratio (/a = 75/15 = 5) so that its total gainover the amplitude-modulation system with pre-emphasis is 18.7 + 7.4+ 3 = 29.1 decibels.For the case of impulse noise, the frequency-modulation gains areequal to twice the deviation ratio and the pre-emphasis gains are alsodifferent. For amplitude modulation, the pre-emphasis gain is 7.5 decibelsand for frequency modulation it is 4.5 decibels more or a total of12 decibels. Hence, the gain of the frequency-modulation system usinga 20-kilocycle deviation is 8.5 + 4.5 + 3 = 16 decibels. The correspondinggain for the system using a deviation of 75 kilocycles is 20 + 4.5+ 3 = 27.5 decibels.