Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

40 FREQUENCY MODULATION, Volume Ical plates of the oscilloscope. A bias proportional to the frequencychange of the heterodyning voltage was applied to the other set ofoscilloscope plates. Consequently the spectra obtained are those whichwould be produced by the combination of a single noise component ofvariable frequency and the carrier. At the higher carrier-noise ratios,the spectrum is rectangular for amplitude modulation and triangularfor frequency modulation. The dip in the middle of the amplitudemodulation spectrum is where the audio output is near zero beat. Asthe carrier-noise ratio is decreased, the frequency modulation spectrumdeviates from its triangular shape and the wave form of the receiveroutput has increased harmonic content at the lower audio frequencieswhere the audio selectivity does not eliminate the harmonics.The amplitude modulation spectra of Fig. 16 also show the presenceof added harmonic distortion on the lower modulation frequenciesand lower carrier-noise ratios. However, the effect is so small that it isof little consequence.The spectra of Fig. 16 allow a better understanding of the situationwhich is theoretically portrayed by (7) of the theory.Experimental ConclusionsIt can be concluded that the experimental data in general confirmthe theory and point out the following additional information:The improvement threshold starts at a carrier-noise ratioaboutthree or four decibels above equality of peak carrier and noise in thefrequency modulation intermediate-frequency channel. Hence the fullfrequency modulation improvement may be obtained down to a peakcarrier-noise ratio in the frequency modulation receiver of three or fourdecibels.The root-mean-square fluctuation noise characteristics differ fromthe peak fluctuation noise characteristics for carrier-noise ratios belowthe improvement threshold. The improvement threshold starts atabout the same peak carrier-noise ratio, but the improvement does notfall off as sharply as it does for peak signal-noise ratios. Thus, for carrier-noiseratios below the improvement threshold the energy contentof the frequency modulation noise is reduced, but the peak characteristicsare approximately the same as those of the amplitude modulationreceiver. The characteristics are not exactly the same due to the frequencylimiting which allows the noise peaks to depress the signal, butdoes not allow them to rise above the signal.The crest factor of the fluctuation noise at the outputs of the frequencyand amplitude modulation receivers is about thirteen decibelsor 4.5 to one for the strong-carrier condition. The crest factor of ampli-