Northern Telecom Limited

Zdenek J Holy

Stanley D Rosenbaum

Calvin Plett

Reinhard W Rosch

A typical active impedance line feed circuit includes tip and ring amplifiers being controlled in response to signals at tip and ring voltage taps by control circuit to exhibit a.c. impedance and d.c. resistance characteristics for the purpose of supplying energizing current via tip and ring terminals for tip and ring leads of a two wire communication line. In one example of the active impedance line feed circuit, improved operational tolerance of longitudinal interference and of a ground fault condition is provided by a d.c. amplifier being responsive to voltages at the tip and ring terminals. The circuit is characterized in that an inverting input of the d.c. amplifier is connected via resistors to the tip and ring voltage taps. In operation, the d.c. amplifier provides complete cancellation of all common mode signals at the tip and ring voltage taps and a compensation circuit is also connected between the tip and ring voltage taps and is responsive to the output of the control circuit for compensating for non-symmetrical current, the non-symmetrical current being introduced between the tip and ring voltage taps by normal operation of the control circuit, thereby, a threshold of ground fault current limiting action is substantially constant under all operating conditions. The d.c. amplifier is conveniently manufacturable in integrated circuit technology along with the remaining amplifiers in the active impedance line feed circuit.

Active impedance line feed circuit

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.

Active impedance transformer assisted line feed circuit with supervision

Normal operating range of an active impedance line feed circuit is extended to at least that of a prior art passive resistance line feed circuit. The active impedance line feed circuit includes tip and ring feed resistors in series with outputs of tip and ring amplifiers for coupling a.c. signals to a two wire communication line with an effective source impedance as defined by an a.c. feedback network in response to differential currents in the communication line. Effective output direct current incremental resistances of the tip and ring amplifiers are controlled by a d.c. control circuit connected across the tip amplifier. In an instant of an OFF HOOK communication line of less than a certain resistance a predetermined constant direct current is supplied by controlling the output direct current incremental resistances to be high. In an instant of the communication line resistance being too high, as in the case of a longer line, the output direct current incremental resistances of the tip and ring amplifiers are controlled to approach a few ohms such that at least a minimum energizing direct current is suppliable from the active impedance line feed circuit over an extended range of communication line.

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Nepean, Canada

Zdenek J Holy