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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1957 



determined with the aid of the cumulation factor given by (6.22). For 

 example, if the rms deviation at the output of repeater iV is assumed the 

 same as at the input of the first repeater, the permissible number of 

 repeaters in tandem is less than given by (6.26) by the factor [(1 — m"), 

 (I + nOf, m = b,;fp . When the fixed phase shift is 30°, m ^ 0.5 and 

 N ^ 300. 



6.5 Self- Starting of Self-Timed Repeaters 



With self-timing it is necessary that repeaters be self-starting if the 

 timing wave should be absent for any reason. If each repeater is self- 

 starting, this will also be the case for a repeater chain, since starting will 

 be progressive along the chain. Initially, before the timing wave has 

 reached the appropriate amplitude at all repeaters, there will be a high 

 rate of digital errors. 



With timing from the recei\-ed pulse train, the resonant circuit will be 

 excited by every pulse and the timing wave will reach its normal ampli- 

 tude in about n ^ Q pulses. With timing from the regenerated pulse 



PULSE 

 TRAIN 



TIMING: 

 WAVE ■ 



Fig. 9 — Progression of repeater .starting in absence of timing wave when 



timing is derived from regenerated pulse train. 



O Triggering points with timing wave absent. Xoise prevents triggering at cer- 

 tain points, n. Timing wave reaches fraction of normal value, Ri . 



A Triggering i^oints with timing wave Ri. Timing wave increases to norma! am- 

 plitude R. 



• Triggering points witli normal timing wave. 



