Programmable plain old telephone line impedance matching circuits

Abstract

Programmable plain old telephone line impedance matching circuits that provide an accurate, integrated programmable termination and source impedance for a telephone line interface circuit (SLIC). The accurate matching of this programmable termination and source impedance with the termination impedance of the 2 Wire connection eliminates signal reflections on this connection and therefore removes the echo, which echo can lead to reduced signal quality in packetized VOIP systems. The programmable integrated impedance matching circuits of this invention use a programmable time continuous amplifier preferably in combination with a double sampling programmable switched capacitor feedback stage (sampling twice per clock cycle by sampling on the positive and negative phases of a non-overlapping clock cycle). Use of a switched capacitor circuit to synthesize a resistance allows the realization of the relatively long time constants needed, generally, thereby allowing the realization of the present invention programmable impedance matching circuit as an integrated circuit without external discrete resistors or capacitors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the three types of characteristic telephone line impedances found throughout the global telephony administrations.

FIG. 2 illustrates the typical arrangement of a prior art solid-state impedance matching circuit.

FIG. 3 is equivalent circuit block diagram for the impedance matching circuit of FIG. 2.

FIG. 4 illustrates the maximum achievable trans hybrid return loss for various delays.

FIG. 5 illustrates the typical arrangement of the programmable integrated impedance matching circuits of the present invention.

FIG. 6 presents an equivalent simplified single ended diagram for the typical arrangement of the impedance matching circuits of the present invention.

FIG. 7 illustrates the typical arrangement of the programmable integrated impedance matching circuits of the present invention further incorporating a Teletax (metering pulse) feature.

FIG. 8 presents a simplified single ended diagram for the metering pulse section of FIG. 7.

FIG. 9 illustrates the typical arrangement of the impedance matching circuit of the present invention as modified in order to add a compensation network for parasitic components at the telephone line terminals.

FIG. 10 illustrates an actual implementation of the impedance matching circuit of the embodiment of FIG. 9 of the present invention, including the Teletax feature.

FIG. 11 illustrates how the capacitors are programmed.

FIG. 12 illustrates how the resistors are programmed.

FIG. 13 illustrates the PLL and the internal non-overlapping clock generator as referenced to an external reference.

FIGS. 14 a and 14b illustrate detailed implementations for the two switched capacitor configurations controlled by register C.

FIG. 15 illustrates the circuit of the present invention as configured to add a compensation network for parasitic components at the telephone line terminals due to wiring and surge protection circuitry.

FIG. 16 illustrates how the exemplary circuit of the present invention can compensate for a parallel parasitic capacitance Cp.

Claims

1. A programmable telephone line impedance matching circuit for coupling to a telephone line through an operational transconductance amplifier, the matching circuit having AC feedback from the telephone line, comprising an integrated circuit having: analog circuitry coupled between a matching circuit input and a matching circuit output, the analog circuitry providing time continuous amplification between the matching circuit input and the matching circuit output;the time continuous amplification having a programmable feedback resistance programmable to cause an output impedance at the operational transconductance amplifier output to match the plain series resistance of any of a plurality of characteristic telephone line impedances;a feedback amplifier having a capacitance in its feedback circuit, an output of the analog circuitry being coupled through a first switched capacitor circuit to an input of the feedback amplifier, a second switched capacitor circuit being coupled in parallel with the capacitance in the feedback circuit, an output of the feedback amplifier being coupleable to an input of the analog circuitry, the switched capacitor circuits synthesizing resistances.

2. The matching circuit of claim 1 wherein the switched capacitor circuits are double sampling switched capacitor circuits.

3. The matching circuit of claim 2 wherein the capacitance in the switched capacitor circuits is programmable.

4. The matching circuit of claim 1 wherein a characteristic telephone line impedance of a simple resistance is matched by the output impedance at the transconductance amplifier output by selection of an appropriate programmable feedback resistance and decoupling the output of the feedback amplifier from the input of the analog circuitry.

5. The matching circuit of claim 1 wherein a characteristic telephone line impedance of a simple resistance in series with a simple capacitance is matched by the output impedance at the transconductance amplifier output by selection of an appropriate programmable feedback resistance, causing the first switched capacitor circuit to simulate a corresponding resistance, causing the second switched capacitor circuit to simulate an infinite resistance and coupling the output of the feedback amplifier to the input of the analog circuitry.

6. The matching circuit of claim 1 wherein a characteristic telephone line impedance of a simple resistance in series with a parallel combination of a resistance and a capacitance is matched by the output impedance at the transconductance amplifier output by selection of an appropriate programmable feedback resistance, causing the first switched capacitor circuit to simulate a corresponding resistance, causing the second switched capacitor circuit to simulate an appropriate resistance and coupling the output of the feedback amplifier to the input of the analog circuitry.

7. The matching circuit of claim 1 wherein the analog circuitry further comprises circuitry for receiving a metering pulse and providing the matching circuit output matching a characteristic telephone line impedance at a predetermined frequency.

8. The matching circuitry of claim 1 further comprising a network to compensate for parasitic components at the telephone line terminals due to wiring and surge protection circuitry.

9. The matching circuitry of claim 1 further comprised of circuitry for compensating for a parallel parasitic capacitance on the telephone line.

10. The matching circuitry of claim 1 further comprised of feedback capacitance coupled in parallel with the programmable feedback resistance to provide anti-aliasing filtering for an input signal and smoothing filtering from the switched capacitor circuits.

11. A programmable telephone line impedance matching circuit for coupling to a telephone line through a transconductance amplifier, the matching circuit having AC feedback from the telephone line, comprising an integrated circuit having: analog circuitry coupled between a differential matching circuit input and a differential matching circuit output, the analog circuitry providing time continuous amplification between the differential matching circuit input and the differential matching circuit output;the time continuous amplification having feedback resistances programmable to cause an output impedance at the transconductance amplifier output to match the plain series resistance of any of a plurality of characteristic telephone line impedances;a feedback amplifier having capacitances in its feedback circuit, a differential output of the analog circuitry being coupled through first switched capacitor circuits to a differential input of the feedback amplifier, second switched capacitor circuits being coupled in parallel with the capacitances in the feedback circuit, a differential output of the feedback amplifier being coupleable to a differential input of the analog circuitry, the switched capacitor circuits synthesizing resistances.

12. The matching circuit of claim 11 wherein the switched capacitor circuits are double sampling switched capacitor circuits.

13. The matching circuit of claim 12 wherein the capacitance in the switched capacitor circuits is programmable.

14. The matching circuit of claim 11 wherein a characteristic telephone line impedance of a simple resistance is matched by the output impedance at the transconductance amplifier output by selection of an appropriate programmable feedback resistances and decoupling the differential output of the feedback amplifier from the differential input of the analog circuitry.

15. The matching circuit of claim 11 wherein a characteristic telephone line impedance of a simple resistance in series with a simple capacitance is matched by the output impedance at the transconductance amplifier output by selection of appropriate programmable feedback resistances, causing the first switched capacitor circuits to simulate corresponding resistances, causing the second switched capacitor circuits to simulate an infinite resistance and coupling the differential output of the feedback amplifier to the differential input of the analog circuitry.

16. The matching circuit of claim 11 wherein a characteristic telephone line impedance of a simple resistance in series with a parallel combination of a resistance and a capacitance is matched by the output impedance at the transconductance amplifier output by selection of appropriate programmable feedback resistances, causing the first switched capacitor circuits to simulate corresponding resistances, causing the second switched capacitor circuits to simulate appropriate resistances and coupling the differential output of the feedback amplifier to the differential input of the analog circuitry.

17. The matching circuit of claim 11 wherein the analog circuitry further comprises circuitry for receiving a metering pulse and providing the differential output of the matching circuit having a characteristic telephone line impedance at a predetermined frequency.

18. The matching circuitry of claim 11 further comprising a network to compensate for parasitic components at the telephone line terminals due to wiring and surge protection circuitry.

19. The matching circuitry of claim 11 further comprised of circuitry for compensating for a parallel parasitic capacitance on the telephone line.

20. The matching circuitry of claim 11 further comprised of feedback capacitors coupled in parallel with the programmable feedback resistances to provide anti-aliasing filtering for an input signal and smoothing filtering for the switched capacitor circuits.