Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

12 FREQUENCY MODULATION, Volume Inoise, this equation also holds true for the case where modulation ispresent.Peak Noise ConsiderationsIn the ultimate application of signal-noise ratios, peak voltages areof prime importance since it is the peak of the noise voltage whichseems to produce the annoyance. This is especially true in the caseof impulse noise such as ignition where the crest factor of the noisemay be very high. Thus the energy content of a short duration impulsemight be very small in comparison with the energy content of thesignal, but the peak voltage of the impulse might exceed the signal peakvoltage and become very annoying. The degree of this annoyancewould of course depend upon the type of service and will not be goneinto here. In view of this importance of peak noise considerations, thefinal judgment in the comparison between the systems of frequencyand amplitude modulation treated here will be based upon peak signalnoiseratios.When the peak voltage or current ratio of the frequency and amplitudemodulation spectra is to be determined, the characteristics of thedifferent types of noise must be taken into consideration. There seemto be two general types of noise which require consideration. The firstof these is fluctuation noise, such as thermal agitation and shot effect,which is characterized by a random relation between the various frequenciesin the spectrum. The second is impulse noise, such as ignitionor any other type of noise having a spectrum produced by a suddenrise of voltage, which is characterized by an orderly phase and amplituderelation between the individual frequencies in the spectrum.Experimental data taken by the author have shown that the fluctuationnoise crest factor is constant, independent of band width, when thecarrier is strong compared to the noise. Thus the peak voltage of fluctuationnoise varies with band width in the same manner that rootmean-squarevoltage does, namely, as the square-root of the bandwidth ratio. Consequently, for the strong-carrier condition, the peakvoltage characteristics of fluctuation noise may be determined by applyingthe experimentally determined crest factor to the root-meansquarecharacteristics. Hence, in the case of fluctuation noise, (15) appliesfor peak noise improvement as well as for average root-meansquarenoise improvement.Impulse Noise CharacteristicsA simple way of visualizing the manner in which impulse noiseproduces its peak radio-frequency voltage is to consider the case of a