Attenuation and termination circuit using impedance synthesis

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

an attenuating circuit connected between the connecting device and the wire pair of 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; and

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

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

3. The circuit as in claim 1 , wherein the attenuating 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

a current source that 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 a characteristic impedance of the communication network.

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

8. A circuit for attenuating a metering pulse signal generated by a telephone network, the circuit comprising:

an attenuating circuit to attenuate the metering pulse signal using an impedance that is synthesized to a predetermined value; and

an isolating circuit to allow the attenuating circuit to receive the metering pulse signal from the telephone network when the metering pulse signal is present, and to isolate the attenuating circuit from the telephone network when the metering pulse signal is absent.

9. The circuit as in claim 8 , wherein the isolating circuit comprises a resistor connected across a wire pair of the telephone network.

10. The circuit as in claim 8 , further comprising an impedance circuit to provide a termination impedance, wherein the termination impedance is synthesized to a predetermined value that substantially matches a characteristic impedance of the telephone network.

11. A circuit for isolating and attenuating a specified network signal over a wire pair of a telephone network by a predetermined amount, the circuit comprising:

a termination circuit to synthesize a termination impedance having a predetermining value that substantially matches a characteristic impedance of the telephone network;

an attenuation circuit to attenuate the specified network signal by a predetermined amount; and

a resistor connected between the attenuation circuit and the wire pair of the telephone network, wherein the resistor has a value that is relatively large in comparison to the characteristic impedance of the telephone network.

12. The circuit of claim 1 , wherein the resistive circuit is to substantially isolate the attenuating circuit from the wire pair of the telephone network when the selected network signal is absent on the wire pair.