Attenuation and termination circuit using impedance synthesis

1. In a connecting device, a circuit for isolating and attenuating a selected network signal transmitted on a communication network having a wire pair capable of connecting with the connecting device, the circuit comprising:

an attenuating circuit connected between the connecting device and the communication network, wherein the attenuating circuit synthesizes an attenuating impedance having a predetermined magnitude such that the selected network signal is attenuated by a predetermined amount, the attenuating circuit including:

a detection circuit that detects the magnitude of a line voltage potential across the wire pair and that outputs a corresponding control signal; and

a current source that produces a current having a magnitude related to the control signal and that provides the attenuating impedance having the predetermined magnitude; and

a resistive circuit connected to substantially isolate the attenuating circuit from the wire pair.

2. The circuit as in claim 1 , wherein the resistor has a resistive value that is relatively large in comparison to the characteristic impedance of the communication network.

3. The circuit as in claim 1 , wherein the detection circuit comprises:

an analog to digital converter circuit that provides a digital representation of the magnitude of a line voltage potential across the wire pair;

a digital signal processing circuit that provides a control signal having a magnitude that is calculated based upon the magnitude of the line voltage and a desired attenuating impedance; and

wherein the current source produces a current having a magnitude that is substantially equal to the line voltage divided by the desired attenuating impedance.

4. The circuit as in claim 3 , wherein the attenuating circuit further comprises a digital to analog converter circuit that provides a control voltage for the current source based in part on the control signal received from the digital signal processing circuits.

5. The circuit as in claim 1 , wherein the attenuating circuit attenuates the signal at an input of a receiver amplifier of the connecting device.

6. The circuit as in claim 1 , further comprising a termination circuit that provides a synthesized termination impedance having a magnitude that is substantially equal to the characteristic impedance of the communication network.

7. The circuit as in claim 1 , wherein the network signal is a metering pulse signal.

8. In a modem capable of connecting to a wire pair of a telephone network, a circuit for attenuating a network signal present on the wire pair, the circuit comprising:

a resistor having a value that is relatively large in comparison to a characteristic impedance of the telephone network; and

an impedance synthesis circuit capable of synthesizing a desired impedance magnitude that attenuates the network signal by a predetermined amount the impedance synthesis circuit comprising:

a generator that detects a voltage measured across the wire pair, and that calculates a current value based upon the voltage and the desired impedance; and

a current source which produces a current having the calculated current value so as to provide the desired synthesized impedance capable of attenuating the network signal.

9. The circuit as in claim 8 , further comprising:

an impedance synthesis circuit capable of synthesizing a desired characteristic termination impedance such that signal reflection is minimized and power transfer is maximized.

10. The circuit as in claim 9 , wherein the resister is at least about 10,000 Ohms and the termination is on the order of 600 Ohms.

11. In a device capable of receiving and sending signals over a wire pair of a telephone network, a circuit for isolating and attenuating a specified network signal by a predetermined amount, the circuit comprising:

a termination circuit capable of selectively synthesizing a termination impedance having a predetermining value that substantially matches a characteristic impedance of the telephone network; and

an attenuation circuit capable of selectively synthesizing an attenuation impedance that is capable of attenuating the specified network signal by a predetermined amount, the attenuation circuit comprising:

a resistor having a value that is relatively large in comparison to a characteristic termination impedance of the telephone network; and

an impedance synthesis circuit capable of synthesizing a desired impedance that attenuates the network signal by a predetermined amount.

12. The circuit as in claim 11 , wherein the impedance synthesis circuit comprises:

a generator that deletes a voltage measured across the wire pair, that calculates a current value based upon the voltage and the desired impedance, and that generates a control signal based at least in part on the calculated current value; and

a current source that produces a current having the calculated current value according to the control signal, so as to provide the desired impedance capable of attenuating the network signal.

13. The circuit as in claim 12 , wherein the generator comprises:

an analog to digital conversion circuit that detects a voltage measured across the wire pair and converts it to a representative digital signal;

a digital signal processing circuit that receives the representative digital signal, calculates a current value necessary to generate the desired impedance, and generates a digital control signal; and

a digital to analog conversion circuit that converts the digital control signal into the control signal based at least in part on the calculated current value.