1064 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



output pulses of .1/ and N are identical, then Pm = Fj^ and Icm = Id^ 

 and eq. (6) becomes 



Tm - QTs = e-^'huPMioc - 8e"') (8) 



This expression can be made equal to zero if 



8 = ae"^'' (9) 



By this means zero wander produced in one repeater can be eliminated 

 at the input of the next repeater. The low frecjuency distortion of one 

 repeater corrects for the corresponding distortion produced in the pre- 

 vious repeater. 



If the electrical characteristics of uU the repeater output transformers 

 are identical it is possible to completely remove the effects of the tran- 

 sient tails due to low frequency cutoff.* It is important however that t\ 

 should not be so large that the feedback pulse occupies the next timing 

 interval. W. R. Bennett has shown that a similar cancellation of tran- 

 sients can be accomplished for more complicated types of low frequency 

 cutoff characteristics. In this case the transient tails ^^ ill be the sum of a 

 number of exponentials having different amplitudes and damping co- 

 efficients. Here the ciuantized feedback must be provided by multiple 

 loops, of greater complexity. 



It may be disturbing at first to observe the resultant sum of the incom- 

 ing signal and feedback as shown on Fig. 1(e). It should be noted how- 

 ever that the signal is not changed in any way until the repeater has 

 triggered the regenerated pulse, and at the next time slot the tails have 

 been cancelled, so that when the next pulse arrives it too will begin at 

 the zero axis. Tails may also be produced by high fre(|uency phase-loss 

 characteristics. These however, may be removed by proper equalization. 



1.3 Timing In a Regenerative Repeater 



The binary regenerative repeater must not only regenerate the shape 

 and amplitude of each individual pulse but it must also keep them in 

 proper time seciuence with other signal pulses. To accomplish this a suit- 

 able timing wave must be provided. This timing wave may be trans- 

 mitted over separate pairs of wires or it may be derived from the signal. 

 In the past it has been connnon to obtain a sine wave of the repetition 



* It can be shown that, with reasonable differences in damping coefficients, 

 quantized f(!edback will fjreatly reduce interyyml)()l interference even when con- 

 sidei'ing a single pulse. If the coiil ril)utions from all the transients of an infinite 

 train of random pulses are summed, the resvUt;int interference is further reduced 

 and can be considered negligible. 



