Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

::SERVICE RANGE OF FREQUENCY MODULATION 215noise-determining ratios and, therefore, must be expressed in terms ofequivalent channel widths. The equivalent channel widths are differentfor the two types of noise; hence, a separate factor is used for eachtype of noise. For fluctuation noise, the equivalent channel width isdetermined by dividing the area under the energy response curve (theselectivity curve plotted with the ordinates squared) by the height ofthe curve at resonance. This gives the width of the rectangular channelwhich would be equivalent to the actual round-topped channel. Forimpulse noise, the equivalent channel width is determined by dividingthe area under the amplitude response curve by the height of the curveat resonance.A further simplification of the ratios, Z fand Z u may be effected byexpressing them in terms of the deviation ratio, fi. Thus,Z f=K f n(fluctuation noise) (10)Zi — KiH (impulse noise) (11)where K f= F n /2F d (fluctuation noise) (10a)and K t=F fi/2F d(impulse noise) (11a)in which F d= maximum applied frequency deviation.The factors K fand Ktexpress the ratio between the equivalent bandwidths of the intermediate-frequency channel and the total plus andminus peak frequency deviation of the frequency-modulation system.In other words they specify how far out on the intermediate-frequencyselectivity curve the frequency deviation may be carried.In order to determine the formula for the carrier-noise ratio inthe frequency-modulation intermediate-frequency channel for the caseof fluctuation noise, (4) and (10) may be substituted in (8) whichgivesCN0.0067 \ZWahfD,«-N D n VK^i(fluctuation noise, peak values) (12)Likewise, for impulse noise (4) and (11) may be substituted in (9)which givesC„ 0.0067 VWahfD,"-*= (impulse noise, peak values) (13)N K, (i D» N(12) and (13) give the carrier-noise ratio in the intermediatefrequencychannel of the frequency-modulation receiver for the twotypes of noise. Since it is known that the improvement threshold