Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

34 FREQUENCY MODULATION, Volume IThe point where the crest factor of the noise increases, which occursat the frequency modulation improvement threshold, has a rather distinctivesound to the ear. When fluctuation noise is being observed,as this point is approached the quality of the hiss takes on a moreintermittent character, somewhat like that of ignition. This point hasbeen termed by the author the "sputter point," and since it coincideswith the improvement threshold it is a good indicator for locating theimprovement threshold. It is caused by'the fact that the fluctuationnoise voltage has a highly variable instantaneous peak voltage so thatthere are certain intervals during which the instantaneous peak voltageof the noise is higher than it is during other intervals. Consequently,asthe maximum peak value of the noise approaches the peak valueof the signal, the higher instantaneous peaks will have their crest factorincreased to a greater degree than the lower instantaneous peaks.Fig. 12 shows oscillograms taken on the fluctuation noise output of theFig.FA12— Wave form of the fluctuation noise output at unity carrier-noise ratioin the frequency modulation receiver. F = frequency modulation receiver.A = amplitude modulation receiver.frequency and amplitude modulation receivers with the 1600-cycle lowpassfilter in the audio circuit and with the signal-noise ratio adjustedto the sputter point. These oscillograms also tend to show how thefrequency modulation signal would sound "cleaner" than the amplitudemodulation signal when the carrier-noise ratio is below the improvementthreshold.Data were also taken to show the fluctuation noise characteristicsas frequency modulation is applied. These data were taken by insertinglow-pass or high-pass filters in the audio system and then applying amodulation frequency to the frequency modulated oscillator whichwould fall outside the pass band of the filters. The low-pass filter cutoff at 1600 cycles so that modulating frequencies higher than 1600cycles were applied. The output of the filtercontained only noise inthe range from zero to 1600 cycles and the change of noise versus frequencydeviation of the applied modulation could be measured. Thehigh-pass filter also cut off at 1600 cycles so that measurements of thelioise in the range from 1600 to 6500 cycles were made while applying