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BELL SYSTEM TECHNICAL JOURNAL 



returning circuit at the amplifier input, a point where the sideband 

 level is very low. To put the near-end crosstalk on the same basis 

 as the far end, the output directional filter must introduce enough 

 attenuation in its non-transmitting range to make up this level 

 difference. This attenuation is increased until the near-end crosstalk 

 due to this cause is appreciably less than the far end. 



The output current of one amplifier may be 30 TU or more stronger 

 than the input current to the amplifier for the opposite direction, 

 and the directional filter at the input of this second amplifier must 

 offer sufficient attenuation to the output currents of the first so that 

 they will not contribute materially to its load. 



I ? 3 1 5 6 7 « 9 10 II 12 13 M 15 lo 1/ IS 19 ZO^TrzFlJTrisTTFTMTsld 



FREQUEIXY KILOCYCLES 



Figure 40 — Typical directional filter characteristics 



Figure 40 shows the selectivity characteristics of the two directional 

 filters. 



A pair of filters having important functions is the line filter set 



