Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

26 FREQUENCY MODULATION, Volume Iwith the signal at the output of the detector and is then reduced inpeak voltage by the audio selectivity. The amount of this reductionfor impulse noise would be a ratio equal to the deviation ratio or, inthis case, twelve decibels. In the case of fluctuation noise, the reductionof the noise, which is present in the absence of modulation, would beequal to the square root of the deviation ratio, or six decibels, and thecorresponding curve is shown by the line v-w. However, these lines donot portray the actual signal-noise ratio characteristics since the noisedepresses the signal when the carrier-noise ratios go below the improvementthreshold. In the case of fluctuation noise this signal depressioncauses the signal to become smothered in the noise as the carrier-noiseratio is lowered below the improvement threshold. On the other hand,with impulse noise such as ignition, where the pulses are short andrelatively infrequent, carrier-noise ratios below the improvementthreshold will present an output signal which is depressed by the noiseimpulses, but which is quite usable due to the small percentage of timethat the impulse exists.The curves of Fig. 6, which take into account the frequency modulationtransmitter gain, utilize (27) and (28) to obtain the frequencymodulation improvements at the high carrier-noise ratios. Thesecurves assume a carrier at the frequency modulation receiver inputswhich is twice the strengtb-of that present at the amplitude modulationreceiver input. The frequency modulation improvements are thereforeincreased by sixdecibels and the improvement thresholds occur atsignal-noise ratios in the amplitude receiver which are six decibels belowthe corresponding ratios for the case where the transmitter gainis not taken into account.Further conclusions of the theory are as follows: For the high carrier-noiseratios, the application of modulation does not increase theroot-mean-square value of the noise above its unmodulated value. Also,in the case of the low deviation ratio receivers, the root-mean-squarevalue of the noise will be slightly reduced as the modulation is applied.ExperimentIn the experimental work it was desired to obtain a set of datafrom which curves could be plotted showing the frequency modulationcharacteristics in the same manner as the theoretical curves of Fig/ 5.To do this it was necessary to have an amplitude modulation referencesystem and frequency modulation receivers with deviation ratios ofunity and greater than unity. Equal carrier voltages and noise spectracould then be fed to these receivers and the output signal-noiseratios measured while the carrier-noise ratio was varied. Since it was