Active impedance transformer assisted line feed circuit with supervision filtering

摘要

A supervision filtering function is included in a line circuit wherein tip and ring voltage amplifiers supply energizing current to a two wire communication line via tip and ring primary windings of a transformer and tip and ring feed resistors. The tip and ring amplifiers operate in response to a control circuit which is differentially responsive to resistively coupled voltage signals corresponding to current flows in the tip and ring feed resistors, and which is also differentially responsive to a.c. signals from a secondary winding of the transformer being coupled via a resistive capacitive network to an amplifier in the control circuit. An output of the amplifier provides a supervisory signal. A feed switch is operable by a ring signal to conduct ringing currents to the communication line via the tip and ring feed resistors to the exclusion of the tip and ring primary windings. A control switch is also operated at the same time by the ring signal to open circuit the secondary winding of the transformer such that a resistive capacitive network causes the amplifier to function as a low pass filter to reduce a.c. signal components in the supervisory signal.