Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

228 FREQUENCY MODULATION, Volume Icompare the results of different deviations under actual service conditions.ConclusionsFrom the above considerations, it seems apparent that if fluctuationnoise is the primary limitation, a frequency-modulation broadcastingsystem using a deviation of 20 kilocycles will produce a signalnoiseratio greater than the values normally considered acceptable,for all distances out to the distance at which the improvement thresholdoccurs. If the limitation is impulse noise a similar relationship exists,but it is felt that further field work is desirable to study the resultsunder actual service conditions.The effect of increasing the deviation of the system is to reduce thedistance at which the threshold effect is realized, but at the same timethe signal-noise ratio at distances equal to or less than that distanceisimproved.The interests of band width conservation are, of course, best servedby a choice of the lowest value of deviation that will yield an acceptablesignal-noise ratio out to the threshold distance. It would alsoseem that the same choice will yield the desired service at the lowestcost.In order to evaluate carefully these effects under a large numberof actual service conditions, a series of field tests are being undertaken,using receivers designed for optimum performance at each of thedifferent values of deviation under consideration.AcknowledgmentThe guidance and helpful suggestions of Mr. H. 0. Peterson, andthe assistance of Mr. R. E. Schock in the experimental work of thispaper, are gratefully acknowledged.References1Murray G. Crosby, Frequency-Modulation Propagation Characteristics,Proe. I.R.E., Vol. 24, No. 6; June (1936).- Murray G. Crosby, Freauency-Modulation Noise Characteristics, Proc.I.R.E., Vol. 25, No. 4; April (1937).3H. H. Beverage, Some Notes on Ultra-High-Frequency Propagation,RCA Review, Vol. 1, No. 3, January (1937).4K. G. MacLean and G. S. Wickizer, Notes on the Random Fading of50-Megacycle Signals Over Nonoptical Paths, Proc. I.R.E., Vol. 27, No. 8;August (1939).