A METHOD OF IMPEDANCE CORRECTION 835 



circuit are still more firmly related. Thus for example the attenuation 

 of the structure beyond its operating range results chiefly from the 

 readily computed departure of the resistance or conductance char- 

 acteristic of the network from that of the generator. It is also pro- 

 duced, in part, however, by the failure of the reactance or susceptance 

 correcting network to annul in this range the imaginary component 

 furnished by the resistance or inductance controlling network and the 

 effect of this factor is less easy to determine. In the modified analysis 

 it is often possible to do away with the distinction between the two 

 types of networks. The complete insertion loss characteristic is then 

 embodied in a single polynomial expression. In the modified form, 

 moreover, the analysis may often be used to determine the phase as 

 well as the attenuation of the circuit. 



Granted these results, it is but a short step to the conclusion that 

 the impedance correcting analysis offers a possible approach to the 

 design of filters. While it is usually true that the networks will 

 attenuate frequencies beyond the region in which impedance require- 

 ments have been set, the amount of the mismatch which produces 

 this attenuation, since it depends upon the impedance correcting 

 parameters, is still more or less under our control. By suitable 

 adjustments of the correcting network, therefore, we can design a 

 structure to meet attenuation as well as impedance requirements. A 

 particularly interesting situation occurs when the load impedance is a 

 constant pure resistance.^" As we have already seen, a load impedance 

 of this type satisfies our mathematical specification and it can therefore 

 be used with a ladder network. Since a perfect impedance match 

 already exists in the circuit an inserted network can be called an 

 impedance correcting device only by courtesy. Unless the network 

 contains so many branches that the mathematical complexity of the 

 problem is overwhelming, however, it is possible to so manipulate the 

 impedance correcting parameters that the network impedance matches 

 the generator impedance approximately over a certain frequency band 

 but is very poor outside this range. It follows from our previous 

 discussion that the network will transmit frequencies lying within this 

 band efficiently, but will attenuate other frequencies. Networks 

 designed in accordance with this method therefore function as filters. 

 They differ from conventional filters in several respects, however. 

 For example they are non-recurrent, they cannot be divided into 

 discrete sections with matched image impedances, and they do not 

 possess definite cutoffs. 



" This circuit arrangement was first investigated by E. L. Norton and W. R. 

 Bennett, who developed a complete analysis for a number of particular cases. 



