154 



BELL SYSTEM TECHNICAL JOURNAL 



In the criss-cross scheme (Fig. 11) the side-to-side combinations, 

 which are those marked 1/2, 3/4, 5/6, etc., appear twice, i.e., along the 

 left and right edges of the panel. Advantage of this is taken by instal- 

 ling balancing coils at both locations. This is done because one side- 

 to-side coil of about 1.3 microhenries may not be large enough in all 

 cases in spite of the fact that the mean side-to-side crosstalk has been 

 reduced 9 db by poling. 



15 BALANCING PANELS FOR 5 GROUPS NOS. 1,2,3,4 AND 5 AS INDICATED - 

 (EACH GROUP 20 PAIRS, 100 PAIRS TOTAL) 



I AGAINST 4 



I AGAINST 3 



3 AGAINST 5 



2 AGAINST 3 



3 AGAINST 4 



4 AGAINST 5 



I AGAINST 5 



1 AGAINST 2 



2 AGAINST 5 



2 AGAINST 4 



Fig. 12 — Allocation of balancing panels designed to maintain phase equalization 

 of coils at all stages. Panels with suitable cross-connections between them are in- 

 stalled in following order. 



For first group — Install 1 



Add second group — Add 2, and 1 against 2 



Add third group — Add 3, 1 against 3, and 2 against 3 



Etc. 



Balancing Procedure 



As stated above, the far-end crosstalk in a repeater section can not 

 be balanced out completely over the frequency range with a single bal- 

 ancing unit. To determine the balanceable as distinct from the non- 

 balanceable crosstalk, involves crosstalk measurements in phase and 

 magnitude at a number of frequencies, using each pair of a two-pair 

 combination as a disturbing circuit in turn. The balanceable crosstalk 

 may then be separated from the non-balanceable crosstalk by computa- 

 tion. Balancing by this method would be impracticable because of the 

 time required. As a practical scheme, it has been shown that balancing 

 at a frequency of about 40 kc. will produce satisfactory results over the 

 type K range even though part of the non-balanceable crosstalk may 



