IMPEDANCE CORRECTION OF WAVE FILTERS 773 



physical association, transmission is the only consideration. Conse- 

 quently relatively large reflection coefficients are not objectionable in 

 such circuits. Curve I of Fig. 2 shows the reflection coefficient of an 

 early carrier system which was not intended to work with other systems 

 of the same type. Large as these reflections appear relative to present 

 day standards they did not seriously impair the transmission of the 

 system. It is also true that appreciably better results could not have 

 been obtained with the design technique available when the filters for 

 the system were developed. 



As the increase in the demand for long distance traffic made it 

 necessary to associate systems on the same pole line the situation 

 became radically different through the introduction of a new factor 

 crosstalk between systems. Crosstalk between systems at carrier 

 frequencies is inherently large and the methods of reducing it expensive. 

 The reflections due to the mismatching in systems increase the cross- 

 talk between them by introducing a type of interference known as 

 "reflected near-end crosstalk." This type of crosstalk can be made 

 negligible only by making the impedance mismatching in the two 

 systems very small. Since "near-end crosstalk" contributes heavily 

 to the cost of the arrangements for reducing crosstalk between carrier 

 systems the substantial elimination of impedance irregularities between 

 lines and the filters and associated apparatus composing the terminals 

 of systems becomes of great economic importance. 



As this need for reducing and ultimately eliminating these irregular- 

 ities appeared a series of improvements in design technique have been 

 developed, each better than its predecessor, which have culminated in 

 a technique which appears to be adequate for the purpose. It 

 differs in many essential features from the others and leads to a new 

 type of filter section which is not of the standard recurrent ladder type. 

 It is the purpose of this paper to give some idea of the relation of cross- 

 talk to impedance mismatching, show how the successive stages of the 

 filter development have grown out of the system requirements and 

 finally to present an outline of the final technique. The accompanying 

 paper. "A Method of Impedance Correction." by H. W. Bode gives 

 this technique in detail. 



Impedance Irregularities and Crosstalk 



The ultimate relation between reflection and crosstalk between two 

 lines which are associated with a number of others on telephone poles is 

 extremely complicated. An idea of the principles which underlie the 

 relationship may be obtained by considering only two of the circuits 

 and assuming that the others have been temporarily removed.* If these 



* Since these two circuits consist of two pairs of wires, there is a potential phantom 



