Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

10 FREQUENCY MODULATION, Volume Ithe detector will be the same as that at the receiver output and willbe as portrayed by the rectangle OCEH.The spectra of Fig. 3 show the manner in which the frequency modulationreceiver produces a greater signal-noise ratio than the amplitudemodulation receiver. The noise at the output of the detector ofthe frequency modulation receiver consists of frequencies which extendout to an audio frequency equal to one half the intermediate-frequencychannel width, and the amplitudes of these components are proportionalto their audio frequency. Hence in passing through the audiochannel the noise is reduced not only in range of frequencies, but alsoin amplitude. On the other hand, the components of the signal waveare properly disposed to produce detected signal frequencies whichfit into the audio channel. In the case of the amplitude modulationreceiver, the amplitude of the components at the output of the audioFig. 3—Amplitude and frequency modulation receiver noise spectra. OBA =frequency modulation detector output. ODH = frequency modulation receiveroutput. OCEH= amplitude modulation receiver output.channel is the same as that at the output of-the detector since the spectrumis rectangular. Thus the frequency modulation signal-noise ratiois greater than the amplitude modulation signal-noise ratio by a factorwhich depends upon the relative magnitudes of the spectra OCLEHand ODH. The magnitudes which are of concern are the root-meansquareand peak values of the voltage due to the spectra.Root-Mean-Square Noise ConsiderationsThe average noise power or average root-mean-square voltage ratiobetween the rectangular amplitude modulation spectrum OCEH andthe trangular frequency modulation spectrum ODH, of Fig. 3, may befound by a comparison of the squared-ordinate areas of the two spectra.Thus,W a _ area OCEH (ordinates) 2W, area ODH (ordinates) 2