366 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1957 





Fig. 11 — Miniaturized inductor for PI carrier. 



operation on the line. Past experience has shown that one-quarter or 

 more of the first cost is often chargeable to the various filter. The decision 

 to employ double sideband modulation was largely based on the knowl- 

 edge that when frequency space is available the double sideband channel 

 filters are generallj' the least expensive. 



With this decision made, every effort was directed toward the achieve- 

 ment of channel filter designs of maximum efficiency in element utiliza- 

 tion. Inexpensive wide-limit capacitors were used, and the desired per- 

 formance achieved through the use of an adjustable ferrite inductor 

 expressly developed for the PI system. The filters are rapidly adjusted in 

 the manufacturing process using visual display testing circuits. 



4.2 Inductors 



The inductor which makes this possible is shown in Fig. 11. It is de- 

 signed for printed wiring use and provides a wide range of inductance 

 while maintaining excellent "Q" performance in the carrier and voice 

 range. This is accomphshed in a single basic design by so selecting the 

 winding for particular nominal inductances that the air-gap adjustment 

 remains at or near its most efficient setting. Inductors of this tj^pe were 



