To increase the effective signaling and supervisory range of a central office, a loop extender has tip side and ring side booster circuits, each of which comprise a pair of oppositely polarized floating booster voltage supplies and a loop polarity detector for selectively switching one of such supplies into series with the central office battery. Time constant circuits are associated with each of the loop polarity detectors so that they follow changes in the d. c. polarization of the central office ring and tip leads relative to one another and to ground, but not the a.c. swings of any ringing voltage that may be present. Provision is made, however, for overriding the time constant circuit for the tip side polarity detector in the absence of a ringing voltage so that it can then follow short time constant changes in the d. c. polarization of the tip lead. The booster voltage supplies are secondary supplies which are energized by power from a primary supply only in the presence of a control signal in one or both of the central office tip and ring leads, whereby the power drained by the loop extender is minimized. Further, the maximum current that can be drawn from the primary supply is limited, thereby protecting the loop extender and other central office equipment against being damaged in the event that the output of the loop extender is inadvertently short circuited or coupled to a low impedance subscriber's loop.
A telephone loop extending circuit for aiding central office battery. Each voltage insertion circuit includes the following: First and second pairs of terminals, first and second voltage sources, first and second transistors having base electrodes and emitter-collector electrode circuits. First sides of the first and second voltage sources, of opposite voltage polarity, are coupled to a first terminal of the corresponding pair. One side of the emitter-collector electrode circuit of each of the transistors is coupled to a second terminal of the corresponding pair. The other sides of the emitter-collector electrode circuits of the first and second transistors are individually coupled to second sides of, respectively, the first and second voltage sources. First and second optical coupling means are provided for the first and second transistors, respectively, each for sensing current flow between terminals of a pair and each being responsive to current flow in one of two opposite directions between terminals for enabling the corresponding transistor to couple the corresponding voltage source to the second terminal of the corresponding pair. A bypass capacitor is coupled from one terminal to the other of the corresponding pair for passing selected signals around the transistors and voltage sources from terminal to terminal. A resistor is coupled from the base electrodes of both transistors to the first sides of the voltage sources and is operative in combination with at least one of the transistors for providing a discharge path for the bypass capacitor after an externally applied signal is removed from terminals of the corresponding pair.
A loop extender for adding a series-aiding boost voltage to a subscriber loop of a telephone system and having a d.c. boost voltage supply, an amplifier circiit having an active device connecting the voltage supply in series with a conductor of said loop for applying to the loop a magnitude of boost voltage that is dependent upon the gain of the amplifier circuit, and a further circuit, including a feedback for the amplifier circuit, for controlling the gain of said amplifier circuit in such a way that said gain is increased to cause an increase in the applied boost voltage in response to a rise in the magnitude of the loop current to a first predetermined threshold and/or in such a way that said gain is decreased to cause a decrease in the applied boost voltage in response to a decrease in the magnitude of the loop current to a second predetermined threshold.
Transmission and signaling range of a telephone transmission line is extended by controllably inserting boost potentials into the line to series aid an applied central office battery. Insertion of the potentials is realized by sensing the direction of current flow in the line and switching a corresponding potential into the line. Optical isolators, each including a light emitting diode and phototransistor, are advantageously employed as line current sensors and switching elements. Insertion of boost potential into the line is controllably delayed in response to changes in the direction of current flow caused by apparent reversals in the polarity of the applied central office battery.
A telephone loop extending circuit for aiding central office battery. There are first and second pairs of input/output terminals and first and second voltage insertion circuits corresponding to the first and second pairs of terminals. Each insertion circuit has a path for current flow coupled between the corresponding pair of terminals. Each insertion circuit has a voltage source, a controllable switch operative when enabled for switching the voltage source between terminals of the corresponding terminal pair in first and second directions of polarity and a sensing circuit for sensing an applied current flow through the corresponding path in first and second directions and responsive to such sensing for enabling the controllable switches to switch the voltage source, respectively, in the first direction or in the second direction of polarity. Additionally included is a circuit operative in response to a rapid reversal in polarity of an applied voltage between two terminals, one from each of the terminal pairs, while the voltage source is coupled in one direction of polarity for enabling the controllable switch in either of the insertion circuits to reverse the polarity in which the corresponding voltage source is coupled in between terminals of the corresponding terminal pair.
A telephone transmission system has repeaters spaced along both the transmit and receive legs of the transmission line, with a power supply for the repeaters including a current regulator having positive and negative supply lines connecting the power supply to the two legs of the transmission line. Current surge protectors are connected from the two legs of the transmission line to ground. A pair of current sensors responsive to the current levels in the positive and negative supply lines, between the power supply and the current surge protectors closest to the power supply, produce output signals representative of the respective current levels. A control signal generating means receives the output signals from the current sensors and produces a control signal in response to a deviation of the higher current in the supply lines from a preselected reference level. The control signal is supplied to the current regulator to maintain the higher current level in the supply lines at the preselected reference level.
