1060 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 195() 



common to all pulse systems is the large bandwidth required for trans 

 mission. 



On wire linos this large transmission band will create a number of 

 problems. The phase-loss ^'ariations, crosstalk and temperature effects 

 will be greatly increased over the transmission band as compared to that 

 of the more conventional systems. It can be shown however that if the 

 repeater spans are made sufficiently short these problems will largely 

 disappear. Only rough equalization will be needed, crosstalk and tem- 

 perature effects become negligible. Furthermore the repeater power 

 requirements will be small and the circuitry comparatively simple, 

 since only partial regeneration will be required. The problem remains 

 to build a regenerative repeater so simple that it will be economically 

 sound to use on short spans of line. The development of the transistor 

 with its small size and low power requirements has made such a repeater 

 feasible. | 



1.1 Pulse Distortion Caused by Low Frequency Cutoff 



Since the frequency spectrum of a binary pulse train will extend down 

 to and include dc, the ideal repeater should be able to handle the complete 

 frequency band to avoid signal distortion. This would preclude the use 

 of coupling transformers and condensers which attenuate the low fre- 

 quencies and remove the dc. Practical considerations however dictate 

 the use of these elements which means that the repeater will have a 

 low frequency cutoff. The distortion of a binary pulse train produced by 

 low frequency cutoff presents one of the most vexing problems the 

 designer of a regenerative repeater must cope with. It produces what is 

 probably the most potent source of intersymbol interference found in an 

 average binary pulse communication system. This interference consists 

 of a transient response whose effect may be appreciable far beyond the 

 end of the pulse itself. 



When a train of ideal fiat top pulses with infinitely steep sides is applied 

 to a load through a condenser or a transformer, the transient response 

 persisting beyond the end of the pulse is an exponential and may be 

 expressed as 



T = kPoe~'' (1) 



The time t, is measured from the end of the pulse and the damping co- 

 efficient 6 is a function of the low frequency cutoff.* Po is the amplitude 



* The value of b may be approximated by 



b = 27r/o 



\vhere/o is the frequent-y in cycles/sec at which the low fre(iuency loss characteris- 

 tic of the transformer is 6 db above that of the pass band. 



