Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

22 FREQUENCY MODULATION, Volume 1to the carrier divides the over-all transmission of the receiver intocomponents due to the carrier and each side frequency. The amplitudesof these components are proportional to the frequency difference betweenthe noise voltage and the side frequency producing the component.The frequency of the audio noise voltage in each one of thesecomponent spectra is equal to the difference between the side frequencyand the noise radio frequency. Thus the application of the modulationchanges the noise from a single triangular spectrum due to the carrier,into a summation of triangular spectra due to the carrier and sidefrequencies. In the absence of selectivity, the total root-mean-squarenoise would be unchanged by the application of the modulation sincethe root-mean-square summation of the frequency modulation carrierand side frequencies is constant; hence the root-mean-square summationof noise spectra whose amplitudes are proportional to the strengthof the carrier and side frequencies would be constant. However, sinceselectivity is present, the noise is reduced somewhat. This can be seenby considering the noise spectrum associated with one of the higherside frequencies. The noise spectrum associated with this side frequency,which acts as a "carrier" for its noise spectrum, is curtailedatthe high-frequency end by the upper cutoff of the intermediatefrequencychannel. The region of noise below the side frequency iscorrespondingly increased in range, but yields high-frequency noisevoltages which are eliminated by the audio-frequency selectivity.Consequently when modulation is applied, the noise is slightly reduced.The amount of this reduction may be calculated by a root-mean-squaresummation of the individual noise spectra due to the carrier and sidefrequencies. For the case of a deviation ratio of unity, an actual summationof the various spectra for full applied modulation has shownthe root-mean-square reduction to be between two and three decibelsdepending upon the audio frequency of the noise. The same sort ofsummations also shows that the reduction becomes less as the deviationratio is increased.The weak-carrier root-mean-square noise characteristics in thepresence of applied frequency modulation do not lend themselves tosuch straightforward calculation as the corresponding strong-carriercharacteristics and will not be gone into here. The same can be saidfor the peak-noise characteristics in the presence of applied frequencymodulation.Transmitter Frequency Modulation Power GainThe above considerations, which are based upon the equivalentconditions of equal carrier amplitude at the input of the amplitude