THE TYPE PI CARRIER SYSTEM 



363 



32 

 30 



10 28 



UJ 



26 

 24 



o 



LU 

 Q 



z 



Z 22 

 < 



^ 20 



18 



16 



10 20 30 40 50 60 70 



FREQUENCY IN KILOCYCLES PER SECOND 



80 



90 



100 



Fig. 8 — PI repeater gain-frequency characteristic. 



proper value for the nonregulated repeater and to adjust the power of 

 the received carriers to the center of the regulator range for a regulated 

 repeater. The output span pads are used where it is necessary to adjust 

 the repeater output power in order to equalize levels between a PI sys- 

 tem and other PI systems or other types of carrier systems operating on 

 the same open wire line. These pads provide attenuation in 2 db steps 

 up to 30 db. 



The repeater amplifiers were designed to have a wide enough frequency 

 band to cover both high and low groups of frequencies, so that each re- 

 peater contains two identical amplifiers. Each amplifier has three transis- 

 tors with each stage connected as a common emitter. Western Electric 

 Company PNP 7B and 6B transistors are used in the first and last stages, 

 respectively, and a NPN type 4C transistor is used in the second stage. 

 Local feedback is required around each transistor to reduce the gain 

 spread and phase variations among units. Overall feedback is obtained 

 around the three transistors with hybrid coils at input and output. 



The repeater equalizer characteristic represents a compromise for 

 several types of transmission facilities generally encountered in the rural 

 plant. The equalizer design also covers both the high and low freciuency 

 groups so that identical equalizers are used for both the high group and 

 low group sides of the repeater. 



A preUminary characteristic for the overall repeater gain is shown in 

 Fig. 8 plotted against the design objective. There is a significant depar- 

 ture in shape only at the cut-apart frequencies. This will be corrected 

 sufficiently to permit as many as four repeaters to be used in tandem. 

 As the design objective is a compromise of the loss of several types of 

 lines that may be encountered in the use of this system, the departures 



