IMPEDANCE CORRECTION OF WAVE FILTERS 775 



full length line. Moreover the components of this type of crosstalk due 

 to magnetic and capacitative coupling are nearly out of phase and so 

 one tends to neutralize the other. As a consequence of this equal effect 

 of the line characteristic on all the components which reach the receiver 

 at B" the resultant crosstalk can theoretically be eliminated at all 

 frequencies when only two circuits are present by a single transposition 

 (crossing the wires) in the center of either line. 



Crosstalk currents of the fourth type, "reflected near-end crosstalk" 

 following such paths as A'efB'B" and AghB'B", cannot, however, be 

 disposed of so easily. The length of line traversed by the component 

 currents which make up the resultant crosstalk depends upon the point 

 at which they cross from one line to the other, and they will therefore 

 be affected in various fashions by the line attenuation and phase shift. 

 The transposition scheme required to eliminate crosstalk resulting 

 from these currents will consequently depend, at any frequency, upon 

 the length of the line and upon its phase and attenuation characteristics 

 at that frequency. Complete elimination of crosstalk of the fourth 

 type cannot be secured, even for two circuits over a finite frequency 

 band, from a finite number of transpositions. 



When other lines are adjacent to the two we have considered the 

 problem of reducing "far-end" and "near-end" crosstalk by trans- 

 positions is still more complicated. With a number of lines it is no 

 longer even theoretically possible to eliminate far-end crosstalk by a 

 single transposition. It is, in general true, however, that the cost of a 

 transposition scheme adequate for far-end crosstalk is still much less 

 than that of the elaborate system of transposition required to reduce 

 near-end crosstalk to tolerable values. From an economic standpoint 

 therefore, the cost of transpositions required for near-end crosstalk is 

 usually the main feature to be considered. 



Impedance Correction an Economic Means of Controlling Crosstalk 



Another method of reducing this near-end crosstalk, and one which 

 experience has shown to be much cheaper than an elaborate transposi- 

 tion scheme is found in the reduction of the reflection coefficient be- 

 tween the line and the repeaters. Obviously the magnitude of the 

 reflected near-end crosstalk depends upon the amount of the impedance 

 mismatch at the junction between the line and the terminal equipment 

 (e.g. at B' in Fig. 3). It can be made as small as we please, even with 

 a very simple transposition scheme, if the reflection coefficient at line- 

 repeater junctions can be sufficiently reduced. No serious mismatches 

 would occur if the impedances of repeaters and terminal equipment 

 were that of the modulators or amplifiers, since at carrier frequencies 



