CROSSTALK AND NOISE FEATURES 



141 



pedances matching the average Hne impedance sufficiently well so that 

 the effectiveness of balancing is reduced only slightly. 



Nature of Far-End Crosstalk Coupling 

 The coupling between two cable pairs in a short length may be 

 represented by a mutual admittance and a mutual impedance. The 

 former is due almost entirely to capacitance unbalance, which varies 

 but little with frequency, so that its effect could be practically balanced 

 out by means of a simple condenser. The latter, however, involves a 

 complex mutual inductance of the form Ma + jMb, because of the 

 proximity effect of the wires of a pair and of other cable conductors.^ 

 As shown on Fig. 2, both components vary considerably with fre- 



1.0 



; 0.8 

 (0 



U 0.6 



I 

 o 



0.4 

 ) 



i 0.2 



< 



a: 







-0.2 



5 10 15 20 25 30 35 40 45 50 55 60 65 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 2 — Mutual inductance between cable pairs in terms of value 

 for Ma at 10 kilocycles. 



quency; Ma on the average decreasing as the frequency increases while 

 Mh in the general case is of negative sign and reaches a maximum value 

 at 56 kilocycles. 



Type of Balancing 



To obtain maximum reduction in crosstalk it would be necessary to 



use a condenser for balancing the mutual admittance and an inductance 



coil for balancing the mutual impedance or to use some equivalent 



complex network. Experimental balancing in a particular cable using 



the coil-condenser method reduced the mean crosstalk over the type K 



1 "Cable Crosstalk — Effect of Non-Uniform Current Distribution in the Wires," 

 R. N. Hunter and R. P. Booth, Bell System Technical Journal, April 1935. 



