SHORT-HAUL CARRIER TELEGRAPH 



G75 



tative loop. As is well known, the asymmetrical wave shape causes 

 positive signal bias. 



(b) shows an "effective polar" circuit along with the wave shapes it 

 delivers. This is the circuit con\'cntionally used to drive subscriber loops. 

 It presents a constant low iniiKHJauce to the loop and might therefore 

 be considered a "close-and-close" circuit. 



(c) shows the electronic loop circuit. The driving tetrodes are operated 

 in their high-impedance region, above the knee of the plate-current 

 pUite-voltage curve. They deliver a highly symmetrical rectangular 

 wave to the loop and little or no bias results. This circuit presents a 

 nearly constant high impedance to the loop and might be considered 

 an "open-and-open" circuit. 



Although the rectangular wave is inferior to a peaked wave in that 

 less a^'erage power is delivered for the same values of steady-state cur- 

 rent and voltage, it provides entirely acceptable transmission for 19- 

 gauge cable loops up to about 20 to 25 miles in length. Inasmuch as 80 

 volts potential is absorbed in the electron tube plate circuits, this is 

 almost the maximum length over which 62.5 ma can be supplied when 

 loop battery of 260 volts is iLsed. 



M_t_ 



(1) 



S* 



1 _L 



IC 



(2)n 



;-; 



CURRENT AT 

 (1) 



CURRENT AT 

 (2) 



(a) "OPEN AND CLOSE" CIRCUIT 



T (b) "close and close" CIRCUIT 



"T" T (c) "open and open" circuit 



Fig. 5 — Explanation of electronic loop circuit. 



