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    2.
    发明专利
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    公开(公告)号：JPH05300278A

    公开(公告)日：1993-11-12

    申请号：JP10308991

    申请日：1991-04-08

    申请人： NORTHERN TELECOM LTD

    发明人： SUTANRII DANIERU ROOZENBAUMU

    IPC分类号： H04B3/03 ,  H04M19/00 ,  H04Q3/00

    摘要： PURPOSE: To obtain a simple circuit which can monitor a subscriber's telephone set by remarkably reducing common mode noise caused by longitudinal currents in a line interface device and providing a DC power source terminating resistance and an AC power source terminating impedance which are desirable for a communication line. CONSTITUTION: A line interface device is provided a primary winding and a secondary winding and is constituted in such a way that the longitudinal current flowing through the primary winding is offset so as to remarkably reduce common mode noise. A DC power source terminating resistance is given to the device by the actions of a chip/ring feeding resistance, a control circuit, a DC feedback circuit, and a chip/ring amplifier circuit. In addition, an AC current signal component from a communication line is inductively coupled with the secondary wiring of a transformer from the primary winding of the transformer and an AC power source terminating impedance is given to the device by the actions of the control circuit, an AC feedback circuit, and the chip/ring amplifier circuit. The monitoring of a subscriber's telephone set become possible when a chip/ring voltage divider is directly connected to the control circuit through resistance.