Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

SERVICE RANGE OF FREQUENCY MODULATION 225since root-mean-square values have little significance due to the veryhigh and variable crest factor.At distances beyond the improvement threshold for impulse noise,the curves are plotted for the condition of full modulation in whichthe improvement threshold manifests itself. Under this condition, thesignal-noise ratio is limited to a value which is equal to the deviationratio when pre-emphasis is not being used.It will be remembered thatthis ratio is what might be termed the "silencing" signal-noise ratiosince the noise tends to punch holes inthe signal, but when the noisepulses have a relatively slow rate of recurrence, this condition is quite6,7 . .60//V:.MI£SJFig. 9—Peak signal-noise ratio versus distance. Impulsenoise = 100 peak microvolts per meter.Curve I = amplitude modulation, 500 watts radiated.Curve II = frequency modulation, 1000 watts radiated,maximum frequency deviation = 20 kilocycles.Curve III = frequency modulation, 1000 watts radiated,maximum frequency deviation = 75 kilocycles.tolerable. On the other hand, if the pulses have a high rate of recurrence,the noise tends to smother the signal and service is limited tothe improvement threshold distance as in the case of fluctuation noise.When pre-emphasis is being used, a gain is added to this limited signalnoiseratio which isequal to the pre-emphasis gain of 7.5 decibels foramplitude-modulation noise. The pre-emphasis gain for amplitudemodulation noise is used in this case since the noise in the silencingcondition has lost its triangular spectrum characteristic of the frequency-modulationnoise received above the improvement threshold.Thus, for the system with a 20-kilocycle deviation, the signal-noiseratio in the silencing condition beyond the improvement threshold dis-