780 



BELL SYSTEM TECHNICAL JOURNAL 



A modification of m-type sections, leading to the so called x"-termin- 

 ations,^ is used when filters must be connected in parallel. The modifi- 

 cation consists essentially in the elimination of the final shunt branches 

 of the m-derived sections at the paralleling junction. Their places are 

 taken in the transmitting band of either filter by the impedance of the 



(mm 



Fig. 8 — Schematic of x-terminated filters — showing the way in which the parameter 

 "x" determines the impedance which is added to each filter. 



attenuating filter. A simple combination of low-pass and high-pass 

 filters, having x- terminations at their common junction and m-type 

 sections facing their load impedances is shown in Fig. 8. The termin- 

 ating network for the low-pass filter consists of the impedance AB and 

 that of the high-pass filter while the network for the high-pass filter 



2 



Ai 

 U 



a. 



ZLU 



go- 



UJ 



-I 

 u. 



LU 



12 16 20 



FREaUENCY IN KILOCYCLES PER SECOND 



24 



28 



Fig. 9 — Reflection coefficient characteristic of parallel low-pass and high-pass filters 

 from the type " C " carrier telephone system. 

 I — Using x-terminations. 

 II — After the addition of a simple correcting network to the x-terminated filters. 



is the impedance CD and that of the low-pass filter. The reflection 

 coefficient of a typical pair of low-pass and high-pass filters from the 

 Type C carrier telephone system, terminated similarly to the filters 

 of Fig. 8, is shown by Curve I of Fig. 9. 



These methods were supplemented by a number of more or less 

 empirical schemes. For example, x-terminations, as Zobel described 



3 See U. S. Patent No. 1557230, issued to O. J. Zobel. 



