Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

BAND WIDTH AND READABILITY 59The reason for this superiority of a system using a low frequencydeviation over one using a high frequency deviation, when maximumdistance is the consideration, can best be shown by a study of thecurves of Figures 1 and 2. These curves compare the peak and rootmean-squaresignal-to-noise ratio characteristics of two frequencymodulation systems having maximum frequency deviations* of 620 kilocycles, respectively. The curves were taken by varying thecarrier strength of the frequency-modulated signal generator andmeasuring the signal-to-noise ratio at the output of each receiver forthe full frequency deviation that each receiver was capable of. Theradio-frequency input circuit of the two receivers was common sothat both receivers were on an equal basis as far as carrier strengthand input noise were concerned. The noise consisted of the thermalagitation and tube hiss within the receiver. For the curves of Figure1, the signal and noise in the receiver output were measured by meansof an oscilloscope which indicates peak voltages. For those of Figure2, an ordinary rectifier-type meter was used so that root-meansquaresignal-to-noise ratios were obtained.From these curves, it can be seen that, for carriers below a certainvalue, the low-deviation system produces a greater signal-to-noise ratioand is therefore more capable of "reaching down" into the noise toreceive a signal. It will be noted that both systems give a signal-tonoiseratio which is approximately proportional to the carrier strengthdown to a certain carrier strength. Below that strength there is arather sudden drop-off in signal-to-noise ratio. This drop-off is due toa phenomena which is peculiar to a frequency modulation system 1 andwhich is called the "improvement threshold" effect. Any frequencymodulation system has an improvement threshold above which the frequencymodulation gain or improvement is realized and below whichthe signal becomes submerged in the noise. The threshold occurs whenthe peak voltage of the carrier is equal to the peak voltage of the noisein the intermediate-frequency channel of the receiver. The full frequencymodulation improvement is not realized until the carrier isabout twice as .strong as the noise.andIt will be noted that the improvement threshold for the widersystems of Figures 1and 2 occurs at a stronger carrier strength thanthat for the narrower system. Hence, other things being equal, thewider system requires more transmitter power to produce a signalwhich will be above the threshold. The reason for this will be apparentwhen it is realized that the wider receiver must have a wider* "Deviation" in this paper refers to the amount of frequency shiftto one side of the carrier.1 Murray G. Crosby, "Frequency Modulation Noise Characteristics",Proc. of I.R.E., Vol. 25, No. 4, April 1937.