244 BELL SYSTEM TECHNICAL JOURNAL 



tional to Zi and three fixed branches proportional to a second general 

 impedance Zi. The change from a variable resistance to a variable 

 impedance makes little difference in the analysis. It is merely neces- 

 sary to replace each R and i?o by a Z or Zo in every equation. So 

 long as equation (3), with the appropriate modification, is satisfied, 

 as it is in this structure, the resulting family of attenuation character- 

 istics will have the same general symmetrical form as those obtained 

 from the resistance controlled devices. A set of curves illustrating 

 this point is shown at the bottom of Fig. 12. They are drawn on the 

 assumption that the Zi and Zt impedances are respectively inductances 

 and resistances. 



The structure of Fig. 12 will still function satisfactorily as a variable 

 equalizer if it is turned inside out so that the Zi/2 impedances become 

 the terminations and the central shunt branch becomes the variable. 

 In this event the present variable impedance must be set at its nominal 

 value. The resulting structure is essentially the inverse of the network 

 of Fig. 12. In the same way, of course, each of the other configurations 

 which have been described can be replaced by its inverse. 



