Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

NOISE CHARACTERISTICS 112G-)F1 aJ. {^ x T-) dF -\2Fj(12)where W a /W/ is the ratio between the amplitude modulation averagenoise power and the frequency modulation average noise power at thereceiver outputs. The root-mean-square noise voltage ratio will beN a /WZ - Fi— - (r-m-s fluctuation) = A/ — - = \/3 —-•N, V W f 2F a(13)Equation (13)gives the root-mean-square noise voltage ratio forequal carriers applied to the two receivers. The modulation factor ofthe frequency modulated signal due to the applied frequency deviation,F d , is, from (3), equal to 2F d /F { . The modulation factor of theamplitude modulated signal may be designated by M and has a maximumvalue of 1.0. Thus the ratio between the two signals will be givenbyS a FiM Fi— (peak or r-m-s values) = —— = (for M = 1.0). (14)Sf 2Fd 2FdDividing (13) by (14), to find the ratio between the signal-noise ratiosat the outputs of the two receivers, givesSt/Nf ,,_ F d——— (r-m-s values) = V3 —•Oa/NaF a(15)It is apparent that the ratio between the frequency deviation andthe audio channel, F d /F n , is an important factor in determining thesignal-noise ratio gain effected by frequency modulation. A convenientterm for this ratio is the "deviation ratio" and it will be designated assuch hereinafter.Equation (15) gives the factor by which the amplitude modulationroot-mean-square signal-noise ratio ismultiplied in order to find theequivalent frequency modulation signal-noise ratio. Since this factor isused so frequently hereinafter, it will be designated by the word "improvement."The improvement given by (15) has been developedunder the assumption of zero applied frequency deviation (no modulation)and a carrier which is strong compared to the noise. However, aswillbe shown later, as long as the carrier is strong compared to the