CROSSTALK AND NOISE FEATURES 



145 



In most existing toll cables the side-to-side capacitance coupling was 

 reduced when the cables were installed, by means of test-splicing within 

 the 6000-foot sections. Obviously, for poling to be effective it is 

 necessary to operate mainly on the inductance component. The poling 

 measurements, therefore, are made at about 1 kc. where an approximate 

 measure of the inductance component can be obtained directly since 

 the capacitance and inductance components of the crosstalk are at an 

 angle of almost 90° at this frequency. Figure 3 shows the crosstalk 

 results obtained by means of 1-kc. poling on 14 repeater sections. It 

 has been shown that this 9 db reduction is within 2 to 3 db of the 



,-50 



O -55 



-60 



-65 



X -70 



-75 



-80 



10 



15 20 25 30 35 40 45 50 



FREQUENCY IN KILOCYCLES PER SECOND 



55 



60 65 



Fig. 3 — R.M.S. side-to-side far-end crosstalk per repeater section from measurements 



on 14 repeater sections. 



maximum reduction possible with much more complicated poling 

 involving measurement and consideration of both components at 

 carrier frequencies. 



After side-to-side poling, coil balancing cannot be expected to give 

 as much as 16 db reduction in crosstalk. This is unimportant, how- 

 ever, as long as the required reduction can be obtained more economic- 

 ally by the combined methods rather than by balancing alone. 



Crosstalk Balancing Coil 



Since the voltage which causes the crosstalk current in the disturbed 

 circuit may be in either a clockwise or counter-clockwise direction, the 

 balancing device, for flexibility reasons, should be capable of establish- 

 ing voltages in either direction. A balancing coil was developed, 

 therefore, which in operation may be likened to that of two separate 

 transformers with simultaneously movable cores. The primary wind- 



