202 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



ments in the manufacturing processes, including greater accuracy in the 

 subdivision of the core winding-space, more uniform winding-arrangements 

 in layer formation by automatic winding-machines, and the use of external 

 series-inductance adjustments by means of preselected, miniature, inductance 

 coils, which resulted in much smaller residual unbalances than could be 

 obtained by the nearest full-turn adjustments. More precise crosstalk 

 measurement-circuits were developed, and the practice started of making 

 the crosstalk adjustments and measurements in test circuits having, at the 

 test frequency, impedance characteristics approximating those of the lines 

 in which the coils under test are used. (Previously a relatively insensitive 

 test-circuit having "compromise" impedance characteristics had been used 

 for all types of side and phantom coils.) Also, the factory crosstalk adjust- 

 ments were concentrated on minimizing near-end crosstalk in loading units 

 intended for two-wire circuits, since in such circuits the far-end crosstalk is 

 relatively unimportant. In loading apparatus intended for use on four-wire 

 circuits, the crosstalk adjustments were made to minimize far-end crosstalk 

 because the associated phantom-group circuits are always used in the same 

 direction of transmission and the one-way repeaters associated with them 

 block the propagation of near-end crosstalk. The external inductance ad- 

 justments, above referred to, were especially designed for the reduction of 

 phantom-to-side crosstalk. Other adjustments and assembly processes con- 

 trolled crosstalk between associated side circuits. 



As a result of the various improvements above referred to, the average 

 phantom-to-side crosstalk in the loading apparatus was reduced to about 

 one-fourth of the earlier values. These reduced values were of the same order 

 as the cable crosstalk in the individual loading sections after the completion 

 of the capacitance-unbalance test-splicing. 



In the late 1920's and early 1930's, when the need for improved trans- 

 mission systems on two-wire repeatered circuits resulted in the development 

 of the H88-50 and B88-50 facilities previously described, another develop- 

 ment campaign was started to improve loading apparatus crosstalk-per- 

 formance. This resulted in the reduction of the average loading coil crosstalk 

 to values much smaller than those caused by the cable residual capacitance- 

 unbalances after test splicing, so that the effective repeater-section crosstalk 

 is not significantly larger than that which would result if it were possible to 

 manufacture perfectly balanced loading coils. Important factors in this 

 improved performance were the use of series, external, resistance adjust- 

 ments in particular loading units where worth-while crosstalk reduction 

 could be thus obtained, and the use of a new series of inductance elements 

 having closer gradations in their inductance values to more closely approach 

 the theoretical optimum adjustments. More compact assembly-arrangements 

 of the phantom loading units in individual shielding containers also con- 



