286 



BELL SYSTEM TECHNICAL JOURNAL 



is illustrated by Fig. 16 in which the breaking point with a 100-cycle shift 

 occurs only 2 db before that with a 400-cycle shift although the difference in 

 actual signal amplitude is 12 db. Figure 23 shows the effect on signal-to- 

 noise ratio of progressively varying the frequency shift while the bandwidth 



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 5z 



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 V£ 

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 Sa 



ou 



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0.2 0.4 0.6 0.8 I.O 1.2 1.4 1.6 

 FREQUENCY SHIFT IN KILOCYCLES PER SECOND 



Fig. 23. — Effect of magnitude of frequency shift on distortion produced by thermal 

 noise — 80-cycle cutoff low-pass filter. 



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-400 -300 -200 -100 100 200 300 400 

 FREQUENCY DRIFT IN CYCLES PER SECOND 



Fig. 24. — Effect of frequency drift on distortion produced by thermal noise in FS 

 transmission — 740-cycle band and 80-cycle cutoff low-pass filter. 



is kept constant. There is a fairly broad region in which the signal-to-noise 

 ratio is little affected by the amount of frequency shift. For optimum 

 results a frequency shift of 50% to 6()% of the band width is indicated, and 

 thus has been used in most of the experimental results given herein. 



Frequency Instabilities 



Frequency drift of the carrier input to the receiving terminal in general 

 causes biased signals and if severe enough results in failure of the system. 



