BAND WIDTH AND TRANSMISSION PEIUORMANCE 



529 



signal-to-noise ratio equal to the ratio of unmodulated carrier power to 

 the noise power in the double width radio band. Since we keep the FM 

 power marginal for all bandwidths an additional banflwidth imjirovcment 



» 

 of 10 log -=r accrues. Substituting H = 92 mc, F,, = 4 mc, and li = 



2Fb 

 92 — 8 = 84 mc, will show that the above gains total 31 db. 



75 



70 



UJ 65 

 u 



2 



- 55 

 O 



< 50 



a 



z 



UJ 



cr 40 



LU 



LL 



S35 



z 



T 30 



O 



^^25 

 < 

 z 



O 20 

 in 



Q 15 



Q 

 D 



< 10 



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



RADIO SIGNAL BANDWIDTH IN MEGACYCLES PER SECOND 

 Fig. 20 — FDM-FM; performance with respect to CW interference. Relations between 

 Ixandwidth and audio signal-to-interference ratio for marginal ratio of FM wave to inter- 

 ference; 4-kc channels. 



Fig. 20— FDM-FM, CW Ixterferknck 



The disturbance produced by CW is most readil}- e\-aluated when all 

 channels are idle for then we have only the frequency error produced by a 

 sine wave of relatively small amplitude superimposed on the steady sinusoi- 

 dal carrier wave. To a first approximation (see Appendi.x II) the error has 

 a frequency equal to the difTerence between the carrier and CW frequencies 

 and an amplitude equal to this frequency difTerence multiplied by the ratio 

 of the CW to the carrier amiilitude. The error thus increases linearly with 

 the frequency of the channel in which it falls but, since the channel levels 



