160 



BELL SYSTEM TECHNICAL JOURNAL 



figure, N represents a source of carrier frequency voltage in a repeater 

 office, connected to a voice frequency pair which transmits this voltage 

 into the outside cable where it is induced on the carrier pairs. These 

 voltages are reduced by inserting suppression coils in the longitudinal 

 voice frequency paths at the junction between the ofhce and the outside 

 cable connected to carrier inputs. 



The design of coils giving the requisite carrier frequency suppression 

 without appreciably affecting voice frequency transmission on the 

 circuits in which they are connected was difificult. One coil is used for 

 each phantom group. Each coil has sixteen windings, four for each 

 line wire. These windings are so paired and disposed about the core 



80,000 



70,000 5 



Q 30 



O 25 



60,000 : 



50,000 < 



40,000 - 



30,000 5 



15 20 25 30 35 40 45 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 18 — Longitudinal impedance and suppression of noise suppression coils. 



as to make possible very small side-to-side and phantom-to-side cross- 

 talk between line windings. They also permit obtaining very small 

 leakage flux in both the sides and the phantoms; hence the coils intro- 

 duce very small transmission loss in their voice frequency circuits. 

 The leakage impedance of the coils plus the impedance of the cable stub 

 used to connect them into the circuit is held down so that the efTect 

 on repeater singing and echoes in the voice circuits is very small. 

 The coils are so wound that their longitudinal inductance is in anti- 

 resonance with their distributed longitudinal capacitance at ap- 

 proximately the top cable carrier frequency, resulting in a large increase 

 in their suppression in this critical frequency range. The longitudinal 



