CROSSTALK AND NOISE FEATURES 143 



As the net result, capacitance coupling is no longer all important, 

 inductance coupling at 60 kc. actually predominating by a factor of 

 about 3 to 1 in existing cables. Capacitance balancing should, there- 

 fore, be less effective than balancing designed to reduce the inductance 

 coupling. Tests have shown that capacitance balancing alone gives a 

 crosstalk reduction of about 11 db while inductance balancing alone 

 gives a reduction of about 16 db. Since the latter reduction is suffi- 

 cient, except possibly for small cables or special cases, the type K 

 balancing has been designed on this basis. Far-end crosstalk currents 

 due to the two kinds of coupling have phase relations not differing from 

 zero or 180 degrees by more than about 15 to 40 degrees, depending on 

 whether the upper or lower type K frequencies are considered. There 

 is, therefore, a tendency for either type of balancing unit to annul both 

 kinds of coupling. 



To obtain as much as 16 db reduction it is necessary that the fre- 

 quency characteristic of the balancing coil simulate that of the cable 

 (Figure 2). This was found practicable, as discussed later, by shunting 

 the primary (or secondary) of the coil by a properly designed impedance. 



Size of Balancing Coil 



To meet the crosstalk requirement it is necessary to balance each 

 carrier pair against every other carrier pair. If 50 carrier pairs were 

 used, there would be 49 balancing coils connected to each pair for 

 balancing to all the other pairs, a total of 1225 coils. For convenience, 

 adjustable coils having the same mutual inductance range and the 

 same self-inductance are used. Hence, the insertion loss per coil, 

 resulting from the self-inductance and resistance of the coils, must be 

 kept small. In addition, the self-inductance of the coil presents a 

 problem from the impedance standpoint. To keep the impedance at 

 any point in the balancing panel as nearly like the average cable im- 

 pedance as practicable, the self-inductance of a series of coils must be 

 neutralized by capacitances shunted at suitable intervals. It is very 

 desirable, therefore, to use coils whose self and mutual inductances are 

 no larger than actually essential. Consequently, an attempt has been 

 made to keep the maximum crosstalk before balancing low. 



Due to special measures, described below, it appeared that the 

 maximum inductance unbalance per repeater section could be kept be- 

 low about 1.3 to 1.5 microhenries, with the possible exception of side- 

 to-side unbalances, and trial balancing coils were designed accordingly. 



Crosstalk Reduction Before Balancing 

 Changes in the original splicing are made at approximately 6000-foot 

 intervals, i.e., at points where voice frequency loading coils must be 



