Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

60 FREQUENCY MODULATION, Volume I-intermediate-frequency channel which inherently accepts a widerspectrum of the noise. The wider spectrum of noise has a larger peakvoltage so that the improvement threshold distance for the wide systemoccurs at a higher carrier strength. Thus the threshold for the widersystem of Figure 1 occurs at a carrier strength which is about twicethe corresponding strength for the narrower system.The differenee between the shapes of the curves of Figures 1 and2 is due to differences in crest factor (the ratio between the peak androot-mean-square voltage) of the noise. For carrier strengths abovethe improvement threshold, the crest factor is constant at a ratio ofabout 4.5. When the carrier is on the improvement threshold, the crestfactor is higher by an amount which depends upon the deviation ratio(the ratio between the maximum frequency deviation and the maximumaudio frequency of the system) of the frequency modulationreceiver. This increase in crest factor is caused by the fact that thehigher peaks of the noise approach equality with the carrier so thatthese peaks are at the threshold while the lower peaks produce acarrier-to-noise ratio which is above the threshold. When the peakvoltages of the carrier and noise approach equality, the effective frequencyvariation of the resultant wave rises to very high values andthe stronger of the two voltages assumes control of the receiver. Thus,if the noise is stronger than the signal the noise assumes control anddepresses the signal. Hence as the carrier is lowered towards thethreshold, the effective frequency deviation of the noise rises untilthe higher peaks begin punching holes in the signal. As the carrier isweakened still further, the weaker noise peaks also punch holes in thesignal so that it is submerged in the noise. The point at which thehighest peaks of the noise just begin to reach equality with the carrierpeak voltage, produces a "sputtering" type of noise which changes thecharacter of tube hiss or thermal agitation so that the improvementthreshold is easily recognized. Figures 3A and B show the wave formof the noise at the threshold (which has been called the "sputterpoint") and above the threshold respectively.When the noise is ignition or similar man-made noise the situationis similar to that shown by the peak signal-noise ratio curves ofFigure 1. In addition, the frequency modulation receiver has inherentto it a noise-silencing action which is at least as effective if not betterthan the best amplitude-modulation noise silencer. This noise-silencingaction is self-adjusting and is automatically adjusted for best operationas soon as the signal is tuned in. More detailed description of thisaction is considered elsewhere. 1 -8Murray G. Crosby, "The Service Range of Frequency Modulation'RCA Review, Vol. 4, No. 3, January 1940.