214 



BELL SYSTEM TECHNICAL JOURNAL 



discussed here is 0.1 db. From the roots obtained from the above equations 

 of 7th degree and 4th degree complexity, the open and short-circuit im- 

 pedances are determined which in turn lead to the element values. The 

 complete development of the process resulted in the filter portion of the 

 network shown on Fig. 10. 



The remainder of the schematic shows the equalizer which corrects the 

 rounding of the filter characterisic near the cut-ofifs due to dissipation. 

 The equalizer is of conventional bridged "7" design with constant "i?" 

 impedance in tandem with a simple series section. 



60 



o 



!2 30 



25 30 35 40 45 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 12 — Insertion loss-frequency characteristic of the band elimination filter. 



Repeating coils are required as shown because the filter was designed at 

 a 600-ohm level to give commercial elements whereas it is required to oper- 

 ate between 135 ohm resistances. In the schematic a resistance will be 

 noted in series with one termination. This is^eeded because the "insertion" 

 design with inverse impedance terminations as shown here requires unequal 

 terminations to produce the specified loss characteristic. Usually this 

 would be taken care of by proper design of the repeating coil but, in this 

 case, economic reasons dictated the use of the same repeating coil at both 

 ends of the structure. The termination was therefore built out with a 



