516 



BELL SYSTEM TECHNICAL JOURNAL 



consistent with a satisfactory margin over crosstalk from adjacent channels 

 furnishes a practical method of discriminating against the influence of noise 

 components near the top of the baseband where the noise spectrum is 

 strongest. The exact shape of the gate is not very critical. ^ The curves 

 have been calculated for a rectangular gate coincident with the channel 

 allotment time, which is just possible without crosstalk in the case of non- 



-80 ^ 



O 



-85 "^ 



-90 ^ 



-95 



10 20 40 60 80 100 200 400 600 1000 2000 4000 10,000 



RADIO SIGNAL BANDWIDTH IN MEGACYCLES PER SECOND 



Fig. 13 — PAM-FM; performance with respect to fluctuation nuse. Relations between 

 bandwidth, power, and audio signa!-to-noise ratio for 1000 4-kc channels. 



overlapping sinusoidal pulses. A somewhat shorter rectangular gate or a 

 gate of sinusoidal shape leads to very nearly the same results. The ad- 

 vantage of gating as compared to instantaneous sampling is approximately 

 8 db. Calculation of the gated noise is a straightforward process if based on 

 the concept of the FM noise spectrum acting as signal on a product demodu- 

 lator in which the carrier consists of the harmonics of the gating function. 

 Each harmonic demodulates the spectrum centered about the harmonic 



