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BELL SYSTEM TECHNICAL JOURNAL 



up of certain oxides which have a very large negative temperature coefficient of 

 resistance. The regulator is "back-acting" and maintains a substantially constant 

 output voltage at the pilot frequency over a range of about 9 db in input voltage. 

 The feedback circuit of the amplifier is so designed that the changes in the resist- 

 ance of the thermistor produce changes in gain over the entire frequency band in 

 such a way as to compensate for the changes in loss in the coaxial conductors, as 

 illustrated in Fig. 7. Changes there shown are for ±70° F., which is about the 

 maximum which is expected in a repeater section, even though the cable is of the 

 aerial type. 



In a long system, it has not been feasible to make the accuracy of equalization 

 and regulation in each 5-mile section sufficient to give the desired overall uni- 

 formity of transmission. Hence, certain supplementary adjustment is required. 

 Devices for such adjustment have been installed at 50-mile points on the Stevens 

 Point-Minneapolis S3'stem with satisfactory results. Also, two additional pilot 



60 

 50 



40 

 to 



I 30 



O 

 UJ 

 Q 



z 20 



10 



50 60 80 100 200 300 500 1000 2000 3000 5000 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 7 — Regulation hot and cold vs. frequency 



channels have been provided, one at 64 kc and one at 3096 kc. These serve to 

 indicate the circuit performance and the need for manual adjustment. These 

 pilots could be used to actuate automatic regulators if desired. For longer sys- 

 tems, it is expected that additional, and necessarily more compHcated, supple- 

 mentary devices will be required at intervals of perhaps 200 to 500 miles. 



Perjormance 



A complete repeater containing amplifiers for each direction of transmission, 

 automatic regulators, equalizers, power supply and various automatic alarm 

 features is mounted in a box about 2 x 2 x 1 ft. as shown in a photograph (Fig. 8). 

 Measurements on the overall performance of systems with many such repeaters 

 in tandem indicate a high degree of transmission stability and freedom from noise. 

 In the neighborhood of the pilot frequency the transmission variations are in the 

 order of .1 db. At other frequencies there are slow drifts due to aging of tubes 

 which, when they reach a few db, will require readjustment. These changes are 

 now effected manually at the attended stations. 



