CARRIER SYSTEMS ON TELEPHONE LINES 



621 



frequencies in the carrier range this margin must be still greater to 

 prevent distortion, which becomes objectionable when circulating 

 currents of any size are allowed to exist. This "feed-back" effect 



iO 21 ZZ S.3 ^-f Z5 Z6 



FREQUENCY- KILOCYCLES 

 Figure 39 — Typical band filter characteristic 



will also affect the repeater input impedance, and because of the 

 necessity for closely controlling this characteristic the margin between 

 gain and attenuation is not permitted to be less than 25 T. U. at any 

 frequency used for transmission in either direction. The impedance 

 of these filters on the line side must match the line impedance closely 

 in order that no considerable reflection of the carrier currents can 

 take place at this junction point. 



As was mentioned previously, the output of an amplifier contains, 

 due to modulation, other frequencies in addition to those which 

 compose the input, so that crosstalk is to be expected between some 

 of the channels. The amount of this crosstalk, which will appear 

 at the far end, depends on the ratio of the sideband currents to the 

 interfering currents produced in the amplifier, the measurement being 

 made at the repeater output. The near-end crosstalk, however, is 

 dependent on the level difference between the strong output of the 

 one amplifier and the weak input to the other. Those frequencies 

 which may give trouble in the channels at the near end enter the 



