Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

16 FREQUENCY MODULATION, Volume Ienough to have its harmonics eliminated by the audio selectivity, butbecomes more peaked as the frequency is made lower so that moreharmonics are included. This effect tends to increase the peak voltageof the low-frequency noise voltages which have a large number ofharmonics present. Thus, as the carrier-noise ratio approaches unity,the triangular audio spectrum is distorted by an increase in the amplitudeof the lower noise frequencies.The above gives a qualitative and partially quantitative descriptionof the noise spectrum which results at the lower carrier-noise ratios.Further development would undoubtedly make possible the exact calculationof the peak and root-mean-square signal-noise ratioreceiver output when the carrier-noise ratioat theat the receiver input isclose to unity, but, because of the laborious nature of the calculationsinvolved in evaluating the terms of (18), and pressure of other work,the author is relying upon experimental determinations for these data.Noise Crest Factor CharacteristicsThe crest factor characteristics of the noise can be studied to anapproximate extent by a study of (6). This equation portrays theresultant peak frequency deviation of the wave at the output of thelimiter. From it,the crest factor characteristics of the output of thedetector may be determined since in the frequency modulation receiverfrequency deviations are linearly converted into detector output voltages.However, the crest factor characteristics of the receiver outputare different from those at the detector output due to the effect of theselectivity of the audio channel. This isespecially true in the case ofthe frequency modulation receiver with a deviation ratio greater thanunity, that is, where the audio channel is less than one half the intermediate-frequencychannel. Consequently, in order to obtain the finalresults, the effect of the application of the audio selectivity must beapplied to the results determined from a study of (6).From the curves of Fig. 4, it can be seen that the peak frequencydeviation of the wave given by (7) occurs at a phase angle equal to 180degrees. From the similarity of (6) and (7), it can be seen that thepeak frequency deviation of (6) would also occur at a phase angle of180 degrees. At this phase angle the noise peak frequency deviationfrom (6) is/i„ (peak) =/ 1 d(t)\(F na- F d sin pt)\ 2x dt )(C N) - 1(N/C) - 1