BACKGROUND OF THE INVENTION: Typically, a line interface circuit forms part of a line interface card and connects customer telephone equipment in a telephone system to a switch or similar equipment. When customer telephone equipment is in use, an associated line interface circuit provides an energizing direct current to the telephone equipment by way of a subscriber loop, emanating from tip and ring terminals of the line interface circuit.

The energizing direct current is typically provided by a battery, also connected to the line interface circuit. The battery is continually recharged so that a current source is available in the event of a power failure.

Often, a transformer forming part of the line interface circuit isolates the subscriber loop from other low voltage equipment, including low voltage interface circuitry used to provide telephony signals from and to an interconnected switch or similar equipment. Such a transformer makes conventional line interface circuits electrically robust, and protects low voltage line interface circuit components and interconnected switches and equipment from high voltage surges, such as those that might be caused by lightning.

Line interface circuits usually also provide a suitable terminating impedance to an associated subscriber loop and customer equipment. Although a terminating impedance that precisely matches the subscriber loop is desirable, it has been impractical as the impedance of subscriber loops varies from subscriber loop to subscriber loop. Nevertheless, telephony standards prescribe set terminating impedances for line interface circuits.

Numerous solutions to provide line interface circuits having suitable terminating impedances have been proposed.

For example, U. S. patent no. 5,333,192, the contents of which are hereby incorporated by reference, proposes the use of a feedback network to provide a selected terminating impedance on a line interface circuit having a transformer.

However, as the transformer is an integral part of the proposed line interface circuit, such a feedback network is typically designed with a particular transformer in mind.

Variations in the transformer may lead to instability of conventional feedback designs. Moreover, such a conventional design does not easily lend itself for use in association with digital filters proposed in modern circuit design.

Accordingly, an improved line circuit interface allowing for the flexible yet robust selection of a terminating impedance is desirable.

SUMMARY OF THE INVENTION: It is therefore an object of the present invention to provide a line interface circuit that uses feedback to achieve a desired terminating impedance, while allowing use of a wide

range of transformers or similar components.

In accordance with the invention, two feedback networks are used to provide a desired terminating impedance of a loop driver circuit within a line interface circuit. A second feedback network urges the terminating impedance to partially approach the desired terminating impedance. A first feedback network completes the job by urging the terminating impedance to closely approximate the desired terminating impedance.

In accordance with an aspect of the invention, a line interface circuit includes first and second terminals for interconnecting the line interface circuit with a subscriber loop. A loop driver circuit is electrically coupled to the terminals to provide telephony signals to an interconnected subscriber loop. A first feedback network is in communication with the terminals and takes as inputs a signals representative of sensed voltage across the terminals and sensed current provided to an interconnected subscriber loop by the terminals. A second feedback network is in communication with the terminals and coupled to the loop driver circuit. The second feedback network also takes as inputs a signals representative of the sensed voltage and sensed current. The second feedback network drives the loop driver circuit to urge a ratio of the sensed voltage and sensed current to at least. partially approach a desired terminating impedance at the terminals. The first feedback network has an output coupled to at least one of the second feedback network and the loop driver circuit to urge a pre- selected ratio between the sensed voltage and the sensed current, and thereby provide the desired terminating impedance at the terminals.

In accordance with another aspect of the invention, a

line interface circuit includes first and second terminals for interconnecting the line interface circuit with a subscriber loop. A loop driver circuit is electrically coupled to the terminals to provide telephony signals to an interconnected subscriber loop. A second feedback networks is in communication with the terminals and coupled to the loop driver circuit, driving the loop driver circuit to urge a terminating impedance between the terminals to at least partially approach a desired terminating impedance. A first feedback network is in communication with the terminals and is coupled to at least one of the second feedback network and the loop driver circuit to urge the terminating impedance to closely approach the desired terminating impedance.

Other aspects and features of the present invention will become apparent to those of ordinary skill in the art, upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING: In figures which illustrate, by way of example only, preferred embodiments of the invention, FIG. 1 is a block diagram of a conventional line interface circuit; FIG. 2 is a block diagram of a feedback network forming part of the line interface circuit of FIG. 1; FIG. 3 is a block diagram of an improved line interface circuit, exemplary of an embodiment of the present invention; and FIG. 4 is a block schematic of a line card including an improved line interface circuit, exemplary of an

embodiment of the present invention.

DETAILED DESCRIPTION: FIG. 1 illustrates, in block diagram, a conventional line interface circuit 10. Line interface circuit 10 is typically formed as part of a line card, used to interface a telephony switch to a subscriber loop. The subscriber loop may be interconnected to line interface circuit 10 by way of terminals 12 and 14, which are often referred to as"tip"and "ring"terminals. Tip and ring terminals 12 and 14 emanate from a loop driver circuit 16. Loop driver circuit 16 includes two switch interconnect terminals 20 and 22, which couple a hybrid (not shown) and a coder/decoder (CODEC) (not shown) that are in communication with a telephony switch, to provide telephony payload signals to line interface circuit 10. These telephony payload signals are ultimately converted to analog signals modulated onto a subscriber loop interconnected with terminals 12 and 14.

Power terminals 24 and 26 interconnect line interface circuit 10 with a power supply (not shown), such as a battery, that provides current used to drive subscriber equipment interconnected with tip and ring terminals 12 and 14.

Loop driver circuit 16 also typically includes a conventional multi-winding transformer 28 that electrically isolates tip and ring terminals 12 and 14 from interconnected low voltage circuitry, such as an interconnected hybrid or CODEC. As illustrated, transformer 28 preferably includes three coupled windings. Two of the three windings are connected in series between two resistors 15a and 15b that feed current to interconnected subscriber equipment from the

power supply (not shown). The third winding may be driven by a signal source. As the third winding is coupled to the remaining windings, signals driving this winding are presented across the remaining two windings. As such, signals to be modulated on an interconnected subscriber loop, or demodulated from the loop may be present across this third winding.

In order to properly drive telephony equipment interconnected with tip and ring terminals 12 and 14, line interface circuit 10 is designed to present a fixed terminating impedance between terminals 12 and 14. In North America, most operating telephone companies require a terminating impedance equivalent to 900 ohms in series with 2.16 microfarads.

In order to provide the required terminating impedance at terminals 12 and 14, a feedback network 18 may be interconnected between terminals 12 and 14 and across feed resistors 15a and 15b, and coupled to the third winding of transformer of loop driver circuit 16.

As will be appreciated by those of ordinary skill in the art, feedback techniques and in particular negative feedback is often used to control a desired system output. Typically, a system input, representing the desired system output, and an actual system output are compared. Feedback strives to reduce the difference (often referred to as error) between the input and output, thereby controlling the system to have the desired output. If the output and system input are not equal, a properly designed system including feedback will cause the output of the system to respond to reduce the error. Of course, a feedback controlled system will only function properly if it is controllable. Often the nature of inputs,

outputs, the system and any feedback network will cause instability preventing the system from being controlled.

Feedback is described more generally in Sedra, Adel S. and Smith, Kenneth C., Micro-Electronic Circuits, Holt, Rinehart and Winston, 1982.

In FIG. 1, feedback network 18 is interconnected between the terminals 12 and 14 (system outputs) and transformer 28 (a control input) to control the voltage to current ratio at terminals 12 and 14. Specifically, feedback network 18 measures current and voltage at terminals 12 and 14, and drives the transformer 28 of loop driver circuit 16 to maintain a voltage to current ratio closely approaching the desired terminating impedance between terminals 12 and 14.

Voltage may be measured across terminals 12 and 14. Current may be measured through use of a bridge (not illustrated) that measures current provided to an interconnected subscriber loop, attributable to differential signals through these resistors, as for example described in a U. S. Patent application entitled CURRENT SENSING CIRCUIT AND TELEPHONE LINE INTERFACE CARD WITH ENHANCED CURRENT SENSING CIRCUIT, filed concurrently herewith assigned to Nortel Networks naming Robert Bisson, Scott McGinn and Martin Handforth as inventors, and incorporated herein by reference. A specific line interface circuit using such a described feedback network 18 is disclosed in U. S. Patent No. 5,333,192.

A conventional feedback network 18 that may be used in a line interface circuit is illustrated in block diagram in FIG.

2 and includes loop current sensor 32 and a loop voltage sensor 34 used to sense voltages and currents representative of voltages and current at and through terminals 12 and 14.

The feedback network 18 typically includes at least one

scaling filter 36. Filter 36 preferably scales (ie. multiplies or divides), as required, one. of the voltage and current signals by a factor proportional to the desired (typically complex valued) terminating impedance, Zin. As illustrated, filter 36 may multiply the current sensed by current sensor 32 by the desired terminating impedance Zin. A summer 38 takes as its inputs the sensed voltage or current signal and the filtered signal and subtracts these at its output. Output of summer 38 may then be amplified by an amplifier 30 whose output may be used to drive loop driver circuit 16 to reduce and ultimately eliminate the output of summer 38, thereby providing the desired terminating impedance.

Conventional line interface circuit 10, while having a highly controllable terminating impedance set by the parameters of filter 36, may be prone to instability manifested by the presence of oscillations and ultimately resulting in an inability to control the desired input impedance. Such instability may be attributed, at least in part, to the contribution of the impedance of the loop driver circuit 16 and other unpredictable imperfections of components used to form interface circuit 10 (often referred to as a "parasitic impedance"), to the feedback loop formed from the combination of feedback network 18 and loop driver circuit 16 and the gain required by feedback network 18. Instability of line interface circuit 10 may be reduced through the introduction of compensating components, such as a damping network between tip and ring terminals 12 and 14. However, the parasitic impedance of loop driver circuit 16 may be highly dependent on components used to form loop driver circuit 16. In particular, the parasitic impedance of loop driver circuit 16 may be highly dependent on the

characteristics of any transformer forming part of loop driver circuit 16.

Moreover, stability of line interface circuit 10 may be further reduced by the nature of feedback network 18. For example, if filter 36 is implemented as a simple digital filter on a digital signal processor, filter 36 may introduce delays, equivalent to a phase shift between the input and output of feedback network 18. In the presence of such delays, the feedback loop may become unstable and not be able to provide the necessary terminating impedance at tip and ring terminals 12 and 14.

Accordingly, an improved line interface circuit 42, exemplary of an embodiment of the present invention is illustrated in FIG. 3. Line interface circuit 42 includes a loop driver circuit 16'substantially similar to conventional loop driver circuit 16 of FIG. 1. Loop driver circuit 16'has tip and ring terminals 12'and 14' ; power terminals 24'and 26'interconnecting line interface circuit 42 with a power supply (not shown); interconnect terminals 20'and 22' interconnecting interface circuit 42 to a switch or similar equipment (not shown); transformer 28' ; feed resistors 15a' and 15b', all substantially similar to corresponding terminals and inputs of loop driver circuit 16 of FIG. 1.

Line interface circuit 42 further includes first and second feedback networks 44 and 46, that are preferably nested. First and second feedback networks 44 and 46 drive loop driver circuit 16'so that the overall terminating impedance at terminals 12'and 14'of line interface circuit 42 is equal to a desired (often complex) terminating

impedance, Zin. Second feedback network 46 drives loop driver circuit 16'to partially approach the desired terminating impedance. This, in turn, reduces the gain required from the signal of first feedback network 44.

A specific exemplary implementation of a line card 40 including a line interface circuit 42 is illustrated in FIG.

4. As illustrated, a loop driver circuit 16'preferably includes a multi-winding transformer 28', substantially similar to transformer 28. Accordingly, transformer 28' preferably includes three coupled windings. Inputs to the first of two windings are interconnected to a power supply such as a battery; the other preferably to ground. Outputs of these winding are interconnected with feed resistors 15a'and 15b'. Feed resistors 15a'and 15b'extend to tip and ring terminals 12'and 14'. A third winding of transformer 28'is interconnected between ground and the output of a second feedback network 46, to be driven by second feedback network 46. While the interconnection of second feedback network 46 to transformer 28', has been illustrated as a conventional connection of second feedback network 46 to ground by way of the third winding of transformer 28', a person skilled in the art will appreciate that second feedback network 46 may drive the third winding of transformer 28'in any number of ways, including, for example, by way of push-pull interconnection.

In the illustrated embodiment, second feedback network 46 includes a current sensor 50, and a voltage sensor 52, interconnected with tip and ring terminals 12'and 14'to sense voltage (VL) and current (IL) at these terminals.

Voltage sensor 52 may, for example, be a high impedance amplifier taking as inputs sensed voltage directly at tip and

ring terminals 12'and 14'. Current sensor 50 may include a high impedance amplifier interconnected by way of a bridge to feed resistors 15'a or 15'b to sense current through terminals 12'and 14'to an interconnected subscriber loop. Such amplifiers may be formed using conventional operational transistor, or other amplifiers known to those of ordinary skill in the art. As well, as will be appreciated by those of ordinary skill in the art, current and voltage sensors 50 and 52 may be formed in many other ways. For example, current sensor 50 could include a hall effect device sensitive to differential currents through tip and ring terminals 12'and 14'. Current and voltage sensors 50 and 52 may introduce gain by scaling sensed voltages and currents by factors of KI and Kv for reasons that will become apparent.

Second feedback network 46 further preferably includes a summer 60 that takes as its inputs the output of current sensor 50 multiplied or divided by filter 62, which preferably acts as a scaling circuit, and voltage sensor 52. In second feedback network 46, filter 62 scales (ie. multiplies or divides) by a scalar the output of current sensor 50 in a conventional manner. Filter 62 may be formed from active or passive components and could be formed from a conventional scaling circuit such as an operational amplifier configured to amplify its input by a factor of K1. As will become apparent, filter 62 need not scale the output of current sensor 50 by a scalar value. Instead filter 62 could scale sensed current by a complex impedance. Alternatively filter 62 could scale sensed voltage provided to summer 60.

Conveniently, the gain factors of current and voltage sensors 50 and 52, KI and Kv, are chosen to limit the expected

dynamic range of voltage and current signals at terminals 12' and 14'so that these may conveniently be summed by summer 60.

The output of summer 60 is provided to an amplifier 64 that acts as a gain stage. The output of amplifier 64 is connected to a third winding of transformer 28'of loop driver circuit 16'. Signals presented on this third winding are coupled to tip and ring terminals 12'and 14'. This, in turn, completes a feedback loop formed by second feedback network 46 and loop driver circuit 16'. Assuming this feedback loop is stable, summer 60 will strive to reduce the difference at its inputs.

The gain of amplifier 64 will accordingly depend on electrical characteristics of loop driver circuit 16', and will usually be chosen so a feedback loop formed by feedback network 46 is stable.

Absent any additional feedback network (such as feedback network 44, described below), the feedback loop formed by loop driver circuit 16'and second feedback network 46 will cause the ratio of the sensed voltage to sensed current at tip and ring terminals 12'and 14'to approach a value determined by K (ie. proportional to K1-summer 60 will urge Kv*V-KI**Kl*I to approach 0). As should not be apparent, if filter 60 has a complex transfer function, second feedback network 46 would urge the input impedance at tip and ring terminals 12 and 14 to mirror this complex transfer function. Similarly, if filter 62 were interconnected with the output of voltage sensor 52, second feedback network 46 would urge the impedance at tip and ring terminals 12 and 14 to approach a valued determined by K1 (ie. proportional to 1/K>.

As noted, KI and Kv are conveniently chosen to scale sensed voltage and sensed current to limit the dynamic range

of the signals summed by summer 60.

Now, first feedback network 44 preferably takes as its inputs sensed voltages and currents at tip and ring terminals 12'and 14'. Conveniently, the voltage and current sensors 50 and 52 may be shared by feedback networks 44 and 46. Thus, outputs of sensors 50 and 52 may also be provided to first feedback network 44. As should be appreciated, first and second feedback networks 44 and 46 could alternatively each include their own current and voltage sensors.

Within first feedback network 44, the output of current sensor 50 is provided to a digital signal processing block 54 ("DSP") that acts as a filter. Preferably, DSP multiplies the output of current sensor 50 by a factor proportional to the desired terminating impedance, Zin, at tip and ring terminals 12'and 14'. So, in North America, the filter implemented by DSP 54 would have a transfer function equivalent to a 2.16 RF capacitor in series with a 900 D resistor. The outputs of DSP 54 and voltage sensor 52 are provided to regular and inverted inputs of a summer 56, respectively. The output of summer 56 is provided to an amplifier 58 that amplifies the difference signal at the output of summer 56 by a gain factor of A2. The output of amplifier 58 is preferably provided to a third inverting summing input of summer 60 of feedback network 46, as illustrated. This connection closes a first feedback loop formed by first feedback network 44 providing feedback to the second loop formed by loop driver circuit 16'and feedback network 46. The value of A2 will depend primarily on the characteristics of loop driver circuit 16', so that the feedback loop closed by first feedback network 44 will be stable.

DSP 54 may conveniently also be used as the CODEC for the line card 40. As well, signals to be modulated onto an interconnected subscriber loop originating with interconnect terminals 20'and 22'may be presented at DSP 54.

As noted, summer 60 of second feedback network 46, and summer 56 of first feedback network 52 strive for zero outputs. Thus, for stable operation, Kv*V-KI*IL*Ki+A2* (Kv*VL- KI*IL*Zin) [ie. the output of summer 60] strives for zero AND Kv*VL-KI*IL*Zin [ie. the output of summer 56] strives for zero.

Thus, choice of the filter forming part of DSP 54, and hence Zin and K1 should be complementary, so that both outputs of summer 60 and 56 may stably achieve the desired outputs. As will be appreciated, not all combinations values of Kl and Zin, will allow stable operation of interface circuit 42. In the preferred embodiment K1 is chosen to be proportional to the resistive component of the desired Zin. As such, second feedback network 46 urges the terminating impedance of loop driver circuit 16'to partially approach the desired input impedance Zin and compensates for the parasitic impedance of loop driver circuit 16'and particularly transformer 28'. Put another way, second feedback network 46 compensates for the parasitic impedance of loop driver circuit 16'so that this parasitic impedance no longer materially affects first feedback network 44. First feedback network 44, urges loop driver circuit 16'to closely approach the desired input impedance Zin.

As such, both first and second feedback network 44 and 46 co-operate and are easy to stabilize. Moreover, the feedback loop formed as a result of the second feedback network 46 may

therefore be operated with lower open-loop gain (ie. lower A1) as the outer feedback loop formed by first feedback network 44 may compensate for any residual errors not eliminated by the loop formed by second feedback network 46. As should now be appreciated, the parasitic impedance loop driver circuit 16' including that of transformer 28'as seen by first feedback network 44 has been compensated by second feedback network 46.

Thus, circuit 42 is basically independent of the impedance of transformer 28'and thus facilitates replacement of transformer 28'by a solid state circuit, such as for example an operational amplifier. First feedback network 44 does not have to be designed or tuned with any particular transformer in mind. As well, the filter of first feedback network 44 may be implemented with a relatively low power DSP, that introduces its own delays.

Conveniently, the filter of first feedback network 44 may be formed as part of a DSP. As such, the desired terminating impedance Zin may be easily reprogrammed, as required. So, line interface circuit 42 could easily be adapted to adaptively select a desired terminating impedance.

Alternatively, the desired terminating impedance could be reprogrammed as line interface circuit 42 is used in various regions, having different terminating impedance requirements.

As should be appreciated by a person of ordinary skill in the art, the above described embodiments are susceptible to modifications in many ways without departing from the present invention. For example, it might be possible to interconnect both feedback networks 44 and 46 directly with loop driver circuit 16'to achieve the desired terminating impedance. Of course, output signals of the two feedback networks would need to be suitably combined. As well, either sensed voltage or

current could be filtered (ie. multiplied or divided, as required) in either feedback network 44 or 46 in order to achieve the desired result. Polarities of various summers and amplifier could be reversed. Filter 62 could be implemented in any one of a number of known ways. DSP 54 could be replaced with a substantially similar analog filter. Any number of components could be added to achieve substantially equivalent interconnections of components as illustrated.

The above described embodiments, are intended to be illustrative only and in no way limiting. The described embodiments are susceptible to many modifications of form, size, arrangement of parts, and details of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.