THE TYPE PI CARRIER SYSTEM 



365 



The input section to the control amphfier is either a flat bridging pad 

 for the case where all carriers are always present on the line or a pilot 

 pick-off filter and its associated single transistor amplifier where carriers 

 are turned on and off for supervision. The latter extra amplifier is neces- 

 sary because the pilot power is 20 db below the power in each normal 

 carrier. 



The regulator stiffness provided by the repeater regulator results in 

 a variation of 1 db in output carrier power for a 10-db variation in re- 

 ceived total carrier or pilot power. 



4. COMPONENTS 



Development of the passive components of the PI carrier system, 

 including the various filters and other networks, were influenced by 

 three major considerations. The manufacturing cost had to be as low as 

 possible consistent with the traditional standards of Bell System service 

 life. The components had to lend themselves to maximum utilization of 

 the printed wiring techniques to be used as the basic equipment method. 

 And lastly, advantage wherever possible was to be taken of the fact that 

 transistors are low" power devices. 



4.1 Filters 



Component-wise, filters are the most important single assembly de- 

 termining the first cost of a carrier system employing frequency division 

 multiplexing and frequency separation for obtaining equivalent four-wire 



FROM 



TRANSMITTER 



VARIOLOSSER 



REPEATER 

 AMPLIFIER TO 



TRANSMITTER 



REFERENCE 

 BIAS AND DC 

 DIFFERENTIAL 



AMPLIFIER 



LEVEL 

 ■ADJUSTMENT 



CONTROL 

 AMPLIFIER 



RECTIFIER 



PILOT PICK-OFF 



FILTER AND 



TRANSISTOR 



AMPLIFIER 



OR PAD 



Fig. 10 — PI Repeater regulator block schematic. 



