BAND WIDTH AND TRANSMISSION PERFORMANCE 



557 



occurring in the repeaters is cumulative. In microwave repeaters second- 

 order distortion products fall outside the band and third-order distortion 

 is likely to be predominant. We assume in what follows that the distortion 

 arises solely from a cubic term. \\'hen the low-level gains of the repeaters 

 are maintained equal to the preceding span losses, it can be shown that the 

 single-frequency compression characteristic at the end of n spans is approxi- 



O[f,UJ40 



,9 30 



\^0 10 



-10 -8 -6-4-2 2 •" 



INPUT TO RADIO REPEATER IN DECIBELS 



15 



40 



30 



25 20 15 10 5 



INPUT OF ONE GROUP 

 IN DECIBELS (BELOW Pq) 



20 25 30 35 



RATIO OF PULSE POWER 



TO NOISE POWER 



IN RECEIVING FILTER 



(DECIBELS) 



Fig. 24 — Noise threshold and overload ceiling in frequency divided PCM groups, 



mately the same as for one span but occurs at a power level 10 log n db 

 lower. This approximation becomes more exact as the over-all distortion 

 involved becomes less (as by lower input power). Fig. 25 shows a third 

 order compression curve for one span and the resulting curves, obtained 

 graphically, for 2, 4 and 10 spans. Examination of these curves shows that 

 the curves are substantially the same as for the single span but displaced, 

 3, 6 and 10 db respectively. This is illustrated by line A, which intersects 

 all of the curves at the same compression value (1.7 db). The points 



