CROSSTALK AND NOISE FEATURES 



155 



be neutralized at this frequency. This is theoretically undesirable 

 since the crosstalk reduction at other frequencies is impaired. 



To prevent undue interference into operating carrier circuits when 

 balancing, a frequency falling between the transmitted bands must be 

 used. For this reason, the balancing coils are adjusted at a test 

 frequency of 39.85 kc. and a measurement to check the suitability of 

 the adjustment is made at 28.15 kc. Figure 13 shows the crosstalk 

 vs. frequency before and after coil balancing by this method on three 

 repeater sections. 



Additional Crosstalk Remedial Measures 



Although poling as well as balancing is done to reduce side-to-side 

 crosstalk, this crosstalk is still considerably greater than the pair-to- 

 pair crosstalk. For this reason, side-to-side crosstalk is diluted among 



'P. -70 



Z -75 



z 



J -80 



-85 



-90 



-95 



15 20 25 30 35 40 45 50 



FREQUENCY IN KILOCYCLES PER SECOND 



60 65 



13 — R.M.S. far-end crosstalk per repeater section from measurements on 3 

 repeater sections. 



the pair-to-pair combinations by a system of quad-splitting at repeater 

 points. 



The crosstalk after balancing (Fig. 13) is considerably higher at the 

 upper end of the frequency band than at the lower end. Consequently, 

 if circuits were set up to use the same channel throughout, the crosstalk 

 in the upper-frequency channels would be materially greater than that 

 in the lower-frequency channels. In order that all circuits may be 

 equally satisfactory from the crosstalk standpoint, a system of special 

 channel assignments in successive intervals, say 500 to 1000 miles, can 

 be used. This will tend to equalize both the crosstalk and the noise 

 on all circuits, thus permitting a somewhat cheaper design then if 

 each channel had to meet the crosstalk and noise limits by itself. 



