Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

COMMON- AND ADJACENT-CHANNEL INTERFERENCE 337To find the amplitudes of the various harmonics produced duringcommon-channel interference, compute the value of the desired C— or S —function from equations 20 and 21, and multiply by the proper factor,which is shown by the above equation 19 for the audio output. A specialtable of Bessel functions has been prepared for this purpose. 17The effect of a de-emphasis network following the discriminator, andof a low-pass audio filter, can be determined by computing the amplitudeof each harmonic that falls within the working range, correcting each onefor amplitude and phase changes in the audio amplifier and filters, andthen recombining them by superposition.If the signal-noise ratio is defined as the desired audio output withno interfering carrier present, divided by the peak noise (i.e., the maximumdeparture from the desired audio output when no interference ispresent), then, as shown by Figures 17 and 18, the signal-noise ratioindependent of the modulation index, but depends only on the ratio of thetwo voltages, x. This assumes a perfect limiter, adequate band width inthe amplifiers and discriminator, and linear-phase-shift circuits.If a de-emphasis network and a low-pass audio filter are used, many ofthe harmonics will be attentuated or removed, and the nonlinear phaseshift will prevent the remaining harmonics from coming into phase all atthe same time. The peaks of noise are therefore reduced considerably.When the modulation index, D/[x, is large, the noise beat-note peaks comevery rapidly. This means that the harmonics will be of high order andthey will be reduced or removed by the audio selectivity.isThis accountsfor the observed noise reduction with wide-band frequency modulationand shows that it is very important to use a de-emphasis network and lowpassfilter.Common-Channel Interference, Both Signals Modulated. The precedingcases have described the interference produced by an unmodulated carrieron the same channel as the desired signal, and the effect of detuning theinterfering carrier.This section is a discussion of the case when both thedesired and undesired signals are modulated sinusoidally, and of theresultant distortion, which is even more complicated.In order to illustrate this form of interference,conditions:assume the followingDi/y.i = 10, D 2 /'M = 5, Di = 4D 2 , (M = 2[A 2 ,x = E2/E1For example, Di = 60 kc, [Xi = 6 kc, Di = 15 kc, (a2 = 3 kc, x = 0.5 and0.9 could be one set of numerical values.17Murlan S. Corrington and William Miehle, "Tables of Bessel FunctionsJ„(x) for Large Arguments," Jour. Math. Phys., Vol. 24, No. 1, pp. 30-50;Feb., 1945.