Impedance canceller circuit
AssigneeTellabs Oy
InventorCharles W. Chambers, Jr.
Life of this patent
Estimating the term…
Abstract
An improved impedance canceller circuit, for use with two-wire transmission systems of the type encountered in telephony, is adapted to be interfaced with the wires and operates in a manner to effectively eliminate or cancel an unwanted impedance across the wires. The impedance canceller includes a current source, and essentially supplies on the wires all of the current that would otherwise be required by the unwanted impedance in response to a metallic voltage applied across the wires by a signal source, so that the signal source does not supply such current and the unwanted impedance appears as an open circuit to the metallic voltage. At the same time, the impedance canceller provides common mode rejection to longitudinal voltages on the wires.
Claims11
An impedance canceller for interfacing with a two-wire transmission system, for example with the tip and ring leads of a telephone pair, for generating a controlled current flow through the system to cancel an metallic impedance in the system and for providing common mode rejection to longitudinal voltages on the system, said impedance canceller having first and second outputs each for connection with a separate one of the wires and comprising first and second voltage amplifier means each having an input, an output and an output impedance connected between the amplifier means output and a separate one of said impedance canceller outputs, said first voltage amplifier means output being connected through its output impedance with said impedance canceller first output;
- means for connecting said first amplifier means output with said second amplifier means input so that voltage signals at said first amplifier means input control the voltages generated at each of said amplifier means outputs;
- feedback means coupled between said impedance canceller first output and said first amplifier means input;
- and circuit means coupled between said impedance canceller outputs and said first amplifier means input, said feedback means and said circuit means coupling signals at said impedance canceller outputs from the wires on the transmission system to said first amplifier means input, said feedback means and said circuit means in response to equal longitudinal voltages on the wires coupling to said first voltage amplifier means input signals which cancel to zero at said first voltage amplifier means input so that said first and second voltage amplifier means do not generate voltages at their outputs in response to equal longitudinal voltages on the wires, whereby said impedance canceller has common mode rejection to equal longitudinal voltages on the wires, wherein said first and second voltage amplifier means each generate zero volts or reference potential at their outputs in response to equal longitudinal voltages on the wires and substantially equal value and opposite direction current flows through their output impedances, and therefore at said impedance canceller outputs, in response to and having a value in accordance with noncancelling signals at said first amplifier means input, wherein the two-wire transmission system is of the type having a metallic impedance and a metallic voltage source connected across the wires, said impedance canceller including sensing circuit means for connection with said wires and said metallic impedance for sensing the current that would flow through the metallic impedance in response to the metallic voltage and for generating a control voltage having a value representative thereof;
- and means for coupling said control voltage with said feedback means and therefore with said first voltage amplifier means input to operate said first and second amplifier means to generate at said impedance canceller outputs a current flow having a value equal to that of the current that would flow through the metallic impedance in response to the metallic voltage, whereby said impedance canceller generates the current that would be provided to the metallic impedance by the metallic voltage source so that the metallic impedance is effectively cancelled with respect to and appears as an open circuit to the metallic voltage.
An impedance canceller for interfacing with a two-wire transmission system, for example with the tip and ring leads of a telephone pair, for generating a controlled current flow through the system to cancel an metallic impedance in the system and for providing common mode rejection to longitudinal voltages on the system, said impedance canceller having first and second outputs each for connection with a separate one of the wires and comprising first and second voltage amplifier means each having an input, an output and an output impedance connected between the amplifier means output and a separate one of said impedance canceller outputs, said first voltage amplifier means output being connected through its output impedance with said impedance canceller first output;
- means for connecting said first amplifier means output with said second amplifier means input so that voltage signals at said first amplifier means input control the voltages generated at each of said amplifier means outputs;
- feedback means coupled between said impedance canceller first output and said first amplifier means input;
- and circuit means coupled between said impedance canceller outputs and said first amplifier means input, said feedback means and said circuit means coupling signals at said impedance canceller outputs from the wires on the transmission system to said first amplifier means input, said feedback means and said circuit means in response to equal longitudinal voltages on the wires coupling to said first voltage amplifier means input signals which cancel to zero at said first voltage amplifier means input so that said first and second voltage amplifier means do not generate voltages at their outputs in response to equal longitudinal voltages on the wires, whereby said impedance canceller has common mode rejection to equal longitudinal voltages on the wires, wherein said first and second voltage amplifier means each generate zero volts or reference potential at their outputs in response to equal longitudinal voltages on the wires and substantially equal value and opposite direction current flows through their output impedances, and therefore at said current source outputs, in response to and having a value in accordance with noncancelling signals at said first amplifier means input, wherein the two-wire transmission system is of the type having a metallic impedance and a metallic voltage source connected across the wires, said impedance canceller including sensing circuit means for connection with said impedance canceller outputs and with the metallic impedance for sensing the metallic voltage across the wires and the current flow that would occur through the metallic impedance in response to the metallic voltage, said sensing means having an output coupled with said feedback means and therefore with said first amplifier means input and generating at its output a control signal to operate said first and second amplifier means so that the same generate at said impedance canceller outputs the same current flow that would occur through the metallic impedance in response to the metallic voltage, whereby said impedance canceller supplies such current and the metallic impedance is effectively cancelled with respect to the metallic voltage.
An impedance canceller for interfacing with a two-wire transmission system of a type having a metallic impedance and voltage source connected in series across the wires, said impedance canceller having first and second outputs each for connection with a separate one of the wires for generating through the wires and the metallic impedance a controlled current flow and for exhibiting common mode rejection to longitudinal voltages on the wires, said impedance canceller comprising a first operational amplifier (op amp) A1 connected at a noninverting input thereto with reference or ground potential;
- a resistor R1 connected at one end to an output from said op amp A1 and coupled at an opposite end with said impedance canceller first output;
- a second op amp A2 connected at a noninverting input thereto with reference potential;
- a resistor R3 connected between the inverting input to and an output from said op amp A2;
- a resistor R4 connected between the output from said op amp A2 and an inverting input to said op amp A1;
- a resistor R5 connected between the inverting input to and the output from said op amp A1;
- a resistor R6 connected between said opposite end of said resistor R1 and the inverting input to said op amp A2;
- an op amp A3 connected at a noninverting input thereto with reference or ground potential;
- a resistor R8 connected between the output from said op amp A1 and an inverting input to said op amp A3;
- a resistor R9 connected between the inverting input to and an output from said op amp A3;
- a resistor R10 connected at one end with the output from said op amp A3 and coupled at an opposite end with said impedance canceller second output;
- a resistor R11;
- a resistor R12 connected in series with said resistor R11 between said opposite ends of said resistors R1 and R10 and at a juncture therebetween with the inverting input to said op amp A1;
- and a resistor R13 connected between said opposite end of said resistor R10 and ground, wherein the two-wire transmission system is of the type having a metallic impedance across the wires thereof through which a current I flows in response to a metallic voltage across the wires, said impedance canceller including sensing circuit means for connection with said wires and with said metallic impedance for sensing the current flow I through the metallic impedance in response to the metallic voltage and for generating at an output from said sensing means a voltage V4 having a value in accordance with the value of the current flow, and a resistor R16 connected between said sensing circuit means output and the inverting input to said op amp A2, the value of said output voltage V4 in response to the current I being such that, and the values of the resistors R1, R3-R6, R8-R13 and R16 being selected so that, the output voltage from said op amp A1 generates a current flow at said impedance canceller outputs which is equal to the current I through the metallic impedance in response to the metallic voltage, whereby said impedance canceller generates on the wires and through the metallic impedance the same current that would be provided by the metallic voltage so that the metallic impedance is effectively cancelled with respect to the metallic voltage and appears as an open circuit thereto.
An impedance canceller for a two-wire transmission system of the type having a metallic impedance between and a metallic voltage across the wires, in which the metallic voltage generates a current flow through the metallic impedance, comprising a current source having an input and first and second outputs, each output being connectable with a separate one of the wires, said current source generating at its outputs and through the wires and metallic impedance a current flow having a value in accordance with the value of a signal at its input;
- monitoring circuit means at an input to said impedance canceller for sensing the current that would flow through the metallic impedance in response to the metallic voltage and for generating a control signal having a value in accordance therewith;
- and means for coupling said control signal with said current source input, said current source in response to said control signal generating at its outputs and through the metallic impedance the same current flow that would occur through the metallic impedance in response to the metallic voltage, whereby the metallic impedance is effectively cancelled and appears as an open circuit with respect to the metallic voltage.
Dependencies are inferred from the claim text.