86 



BELL SYSTEM TECHNICAL JOURNAL 



could extract one channel from the line, while permitting others to pass 

 through it without disturbance. Several of these networks were then placed 

 in cascade to make up the required filter. 



The Hybrid Channel-Dropping Unit 



After several possibilities were considered the constant resistance channel- 

 dropping circuit illustrated schematically in Fig. 3 was selected.^ This 

 circuit is made up of two hybrid junctions, two identical channel reflection 

 filters tuned to the dropped channel, and two quarter wavelength sections of 

 line. 



In order to understand the operation of this circuit, the properties of a 

 hybrid circuit, Fig. 4(a), must first be understood. This circuit, which has 



INPUT 



DROPPED CHANNEL 



OTHER 

 CHANNELS 



^ (90° PHASE SHIFT) 

 4 



T 



advantages: 



ISOLATION OF DESIGN 



PROBLEMS. 

 CIRCUIT FLEXIBILITY 



Fig. 3 — Possible constant impedance channel dropping filter. 



been embodied at voice frequencies as the hybrid coil and at microwaves as 

 the hybrid junction (E-H plane T junction)^ can be represented schematically 

 as in Fig. 4(b). Let us connect four transmission lines. A, B, C, D, each 

 terminated in its characteristic impedance to the four pairs of terminals of 

 the hybrid circuit. Then if each of these lines is matched to the pair of ter- 

 minals to which it is connected, the following characteristics will result. 

 Power in transmission line C flowing towards the junction will divide equally 

 into lines A and B, flow away from the junction and be absorbed in loads A 

 and B. None of this power will appear in line D or be reflected back into 



* Lumped element networks 4vith properties similar to those of this circuit have been 

 devised by Vos and Laurent, U. S. Patent 1,920,041, and Bobis, U. S. Patent 2,044,047. 

 A. G. Fox of these laboratories has independently devised similar microwave circuits. 



» W.A.Tyrrell, Hybrid Circuits for Microwaves, Proc. I.R.E., Vol. 35, pp. 1294- 

 1306, November 1947. 



