1080 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



1-4 



CODE; 1 



CODE COMBINATIONS 



' juiriji 



2 jirL.._.._n_rL.._.. 



3 iirL.A..A.._.. 



- _rL..A..jL._n... 



1 2 3 4 5 6 



NUMBER OF PULSE - BLANI<, COMBINATIONS IN EACH CODE GROUP 



Fig. 9 — Repeater errors as a function of pulse distribution in code. 



are followed by a space in the particular code. The codes used for various 

 points on the abscissa are shown on the graph. The omission error curves 

 plotted in this manner are linear. These data demonstrate that the 

 presence of a pulse modifies the trigger level in the next timing interval. 

 This is largely due to the negative excursion of the damped cosine volt- 

 age from base to ground in the blocking oscillator. On Fig. 10(a) is 

 shown the circuit of the single shot blocking oscillator used in the 

 repeater. With no timing an incoming signal must overcome bias V dc 

 to trigger the repeater. The solid curve on Fig. 10(b) shows the dc bias 

 with the timing wave added at the blocking oscillator emitter. Fig. 10(c) 

 shows the base voltage when a pulse is produced in the first timing inter- 

 val. The pulse begins at U and ends at U . As previously mentioned the 

 sudden rise of the base and collector impedance coupled with the fall 

 of the current in the transformer windings, produces an inductive voltage 

 surge across transformer Tz at h . The decay of this voltage surge can 

 be controlled by the inductance of the transformer and the damping 

 resistor Rf, . This positive decay voltage across the base will inhibit the 

 blocking oscillator from triggering. It is essential that this decay be 

 adjusted so it will inhibit triggering until the following time slot. If 



