A FILTER AND A SWITCHING CIRCUIT

Field of the Invention

The present invention broadly relates to a filter, such as an in-line filter or a central filter for coupling voice frequency equipment to a DSL data and telephone transmission line. The present invention also relates to a switching circuit, such as a bidirectional switching circuit for an in-line filter.

Background of the Invention

Almost every telephone in a household is linked to a telephone exchange via a pair of twisted copper wires. A comprehensive network of such pairs of twisted copper wires is available and it is commercially attractive to use this network also for modern high frequency data communication such as Digital Subscriber Line (DSL) systems. Most conveniently conventional voice frequency equipment (VFE) , such as telephones, or ISDN equipment (ISDN) should operate simultaneously with the DSL systems on the same pair of twisted copper wire. However, as DSL systems operate at high frequencies, it is important that the VFE does not provide a low impedance for the high frequencies, does not itself generate high frequency signals and does not receive high frequency signals which could have an impact on the satisfactory operation of the VFE .

For this purpose a low-pass filter may be positioned between the VFE and the pair of twisted copper wires.

However, often a number of VPEs are parallel-connected to the same pair of copper twisted wires. The single incoming pair of twisted copper wires itself often cannot be conveniently accessed and installation of a central filter often is not possible. Consequently, it is usually most convenient to install such a filter for each VFE and in the proximity of each VFE .

However, as a number of the VFEs typically are connected in parallel to the same pair of twisted copper wires, the corresponding filters together would result in an unsatisfactory change to the impedance of the line degrading the performance of the VFE . Other telecommunications applications, such as ISDN services, also have a similar requirement for isolation of DSL signals from existing services.

In order to overcome this problem, so called in-line filters (distributed or micro filters) typically are connected between the VFE and the twisted pair of copper wires. Such filters are arranged so that filter elements, such as low-pass filter elements, are switched on or off as a function of a DC current to the VFE (telephone on or off the hook) so that low total impedance is avoided. Such in-line filters typically are rather sensitive and are activated if a few mA of dc current is conducted through the filter. When the filter is activated (for example, if a respective VFE is in use) , greater high frequency attenuation for both current directions of the filter is implemented to provide maximum isolation between the VFE and the high frequency DSL system. When no, or only a very small, dc current is conducted, less high frequency attenuation and correspondingly less loading of

the line is implemented to facilitate correct operation of other VFE when in use .

Maximum attenuation is advantageous when the VFE is not active so that the DSL equipment connected to the line side is protected from any noise in the DSL band generated on the inactive VFE side of the filter. However, a minimum loading needs to be maintained on the line side to maintain the correct impedance for operation of VFE. To achieve both objectives in a satisfactory manner is still a challenge and there is a need for technological advancement .

A further problem arises with conventional in-line filters such as in-line filter 10 shown in Figure 1. Conventional in-line filters are designed to provide high levels of DSL frequency attenuation, such as that required by the European ETSI TS101-952-1-5 and AS/ACIF S041 requirements, and usually comprise four-stage ^' balanced circuit including two-switched bridging impedances which are commonly comprise high voltage capacitors. The in-line filter 10 is connected to a pair of twisted cooper wires, which carries telephone signals and high frequency DSL data, on the left side allowing the throughput of telephone signals to a VDE only on the right side. Inductors 11, HA and capacitors 12, 14 are series connected when switch 16 is open and provide initial 2 ^nd order attenuation to the high frequency DSL signal for the off-line state. Tank circuits 18 and 18A provide an impedance for the voice frequency impedance matching and therefore increased return loss. Tank circuits 20, 2OA, 22, 22A in combination with capacitor 24, 26 (when the switch 28 is open) provide further low

- A - pass attenuation and a sharp cut-off at the bottom of the DSL frequency band.

A DSL system transmits peak voltages which might exceed the breakdown or protection voltage level in one or both directions across switch 16 or 28 (which may be transistor switches) when these switches are in an open circuit position (filter is off-line) . The result is that nonlinear signals may be generated, which would generate noise at the LINE/DSL port and/or the VDE. The detrimental impact on the quality of VDE signal, data telephone or data transmission is increased if a number of such in-line filters are parallel connected. An increased DSL signal level, allowed under ADSL2 and ADSL2+ standards, enhances these problems.

Further, in-line filters known to date have the disadvantage that their bi-directional current sensing switches are configured so that they are not easily modified to have different properties for different DC current directions. However, to facilitate the requirements associated with a wide range of characteristics of telephone twisted pair lines between telephone exchanges and VFE, it would be advantageous to have a filter which could have different characteristics for different directions of DC current flow. Consequently there is a need for technological advancement.

Summary of the Invention

The present invention provides in a first aspect a filter for coupling a voice frequency equipment (VFE) to a transmission line that can be used for transmission of DSL

data and telephone signals, the filter comprising: first and second conductive paths, each conductive path comprising a series of first and second tank circuits (Tl and T2) , each first tank circuit Tl being arranged for VFE frequency impedance matching and each second tank circuit being arranged for band-stop or low-pass filtering, first and second connecting paths coupling the first and the second conductive paths, first and second capacitors positioned along the first and the second connecting paths respectively, at least one switch being positioned in the second connecting path for switching the second capacitor in or out of the second connecting path, a first inductor that forms together with the first capacitor an attenuating filter component arranged so that a high frequency signal will experience an attenuation before reaching the or each switch; and circuit elements comprising a second inductor and a third capacitor being connected in parallel with the, or a respective, switch, the second inductor and the third capacitor having an inductance and a capacitance, respectively, that is selected so that signal attenuation at a predetermined frequency range is provided when the, or the respective, switch is in an off-line position.

The third capacitor and the second inductor typically are series connected and the second inductor may be parallel connected with a resistor so that an LCR circuit is formed .

The present invention provides in a second aspect a filter for coupling a voice frequency equipment (VFE) to a

transmission line that can be used for transmission of DSL data and telephone signals, the filter comprising: first and second conductive paths, each conductive path comprising a series of first and second tank circuits (Tl and T2) , each first tank circuit Tl being arranged for VFE frequency impedance matching and each second tank circuit T2 being arranged for band-stop or low-pass filtering, first and a second connecting paths coupling the first and the second conductive paths, first and second capacitors positioned along the first and the second connecting paths respectively, one switch, the switch being positioned in the second connecting path for switching the second capacitor in or out of the second connecting path, a first inductor that forms together with the or each first capacitor an attenuating filter component arranged so that a high frequency signal will experience an attenuation before reaching the one switch.

As in the filter according to the first or second aspect of the present invention the high frequency signal experiences an attenuation before reaching the or each switch, the likelihood for high peak voltages at the or each switch is reduced. Consequently, it is less likely that an open switch will experience a break-through voltage .

The first inductor of the filter according to the first or second aspect of the present invention typically is one of at least two first inductors . At least one of the first inductors typically is positioned on each conductive path in a manner so that the high frequency signal is conducted

through the first inductor before passing through any one of the tank circuits.

The filter according to the first or second aspect of the present invention typically also comprises at least one damping impedance that includes a damping resistor incorporated into at least one of the tank circuits T2. The or each damping impedance typically is arranged to improve the longitudinal balance performance of the in- line filter and also improves the higher frequency attenuation of the in-line filter which is of advantage for higher speed DSL services over longer distances.

In one specific embodiment each tank circuit T2 comprises a damping resistor connected in series with a capacitor of a respective tank circuit T2.

The or each switch of the filter according to the first or second aspect of the present invention, which typically is a balanced in-line filter, typically is a transistor-based switch.

Each second tank circuit T2 of the filter according to the first or second aspect of the present invention typically has a cut-off frequency at or around the lower frequencies of the DSL system in question. Each series of tank circuits typically comprises three tank circuits and each series typically is formed by the tank circuits Tl, T2 , T2 in that order. The second connecting path typically is coupled to the series between T2 and T2. Alternatively, each series may be formed by the tank circuits T2 , Tl and T2 in that order. In this case the second connecting path typically is coupled to the series between the tank

circuits Tl and T2. In a further variation, each series may be formed by the tank circuits T2 , T2 and Tl in that order. In this case the second connecting path typically is coupled to the series between the tank circuits T2 and Tl.

The present invention provides in a third aspect a filter for coupling a voice frequency equipment (VFE) to a transmission line that can be used for transmission of DSL data and telephone signals, the filter comprising: first and second conductive paths, each conductive path comprising a series of first and second tank circuits (Tl and T2) , each first tank circuit Tl being arranged for VFE frequency impedance matching and each second tank circuit T2 being arranged for band stop or low-pass filtering, at least one connecting path coupling the first and the second conductive paths at positions between adjacent tank circuits, at least one connecting impedance positioned along the connecting path, at least one switch, the or each switch being arranged for switching the, or at least one of the, impedance element in or out of the or each connecting path, wherein one of the tank circuits T2 is positioned on a respective conductive path in a manner so that a signal from the transmission line will pass through that tank T2 circuit before passing through any other tank circuit on the conductive path.

The filter typically also comprises at least one first inductor, which typically is positioned on the, or a

respective, conductive path in a manner so that the high frequency signal will be conducted through the first inductor before passing through any one of the tank circuits .

The filter according to the third aspect of the present invention typically comprises a second inductor and a capacitor being connected in parallel with the, or a respective, switch, the second inductor and the third capacitor having an inductance and a capacitance, respectively, that is selected so that attenuation at a predetermined frequency range is provided when the, or the respective, switch is in a off-line position.

The capacitor and the second inductor of the filter according to the third aspect of the present invention typically are series connected and the second inductor may be parallel connected with a resistor so that a LCR circuit is formed.

The or each connecting impedance of the, or a respective, connecting path typically is a capacitor.

In one specific embodiment of the present invention the filter, which typically is a balanced in-line filter, comprises two connecting paths coupling the first and the second conductive paths at positions between adjacent tank circuits and at least one connecting impedance is positioned along each connecting path. Further, the filter comprises in this embodiment at least two switches, each positioned in a respective connecting path in manner such that each switch is arranged for switching at least one of the connecting impedance in or out of a respective

connecting path.

The filter according to the third aspect of the present invention may also comprise a third connecting path which couples the first and the second conductive path and a further capacitor being positioned along the third connecting path. In this variation the further capacitor, typically together with the first inductor, is arranged so that the high frequency signal will experience an attenuation before reaching one of the series of tank circuits .

The filter according to the third aspect of the present invention typically comprises at least one damping impedance that includes a damping resistor incorporated into at least one of the tank circuits T2. In one specific embodiment each tank circuit T2 comprises a damping resistor connected in series with a capacitor of the tank circuit T2.

The or each switch of the filter, which typically is a balanced in-line filter, typically is a transistor-based switch.

Each second tank circuit T2 typically has a cut-off frequency at or around the lower DSL frequency limit.

Each series of tank circuits typically comprises three tank circuits and each series typically is formed by tank circuits T2 , Tl, T2 in that order. The first connecting path typically is coupled to the series between tank circuits T2 and Tl and the second connecting path

typically is coupled to the series between tank circuits Tl and T2.

Alternatively, each series may be formed by the tank circuits T2 , T2 and Tl in that order. In this case the first connecting path typically is coupled to the series between the tank circuits T2 and T2 and the second connecting path typically is coupled to the series between the tank circuits T2 and Tl .

The present invention provides in a fourth aspect a filter for coupling a voice frequency equipment (VFE) to a transmission line that can be used for transmission of DSL data and telephone signals, the filter comprising: first and second conductive paths, each conductive path comprising a series of first and second tank circuits (Tl and T2) , each first tank circuit being arranged for VFE frequency impedance matching and each second tank circuit being arranged for band-stop or low-pass filtering, at least one connecting path coupling the first and the second conductive paths, an impedance element positioned along the or each connecting path and at least one damping impedance comprising a damping resistor connected in series with the capacitor forming a part of at least one of the tank circuits T2.

The or each damping impedance improves the longitudinal balance performance of the in-line filter and also improves the higher frequency attenuation of the filter which is of advantage for higher speed DSL services over longer distances. Further, the or each damping impedance

improves the attenuation of the filter when the VFE is online .

The filter typically also comprises at least one switch positioned along a respective connecting path, and arranged for the switching the impedance element, or at least one of the impedance elements, in or out of a respective connecting path. The switch may be a transistor-based switch.

In one specific embodiment of the fourth aspect of the present invention the switch, or at least one of the switches, is positioned along the second connecting path and the filter also comprises a first capacitor, positioned along the first connecting path, and a first inductor. The first capacitor and the first inductor typically are located so that an attenuating filter component is formed and a high frequency signal will experience an attenuation before passing through any one of the tank circuits.

The filter typically also comprises a second inductor and a second capacitor being connected in parallel with the or each switch and having an inductance and a capacitance, respectively, that is selected so that attenuation at a predetermined frequency range is provided when the, or the respective, switch is in an off-line position.

The second capacitor and the second inductor typically are series connected and the second inductor may be parallel connected with a resistor so that a LCR circuit is formed.

The or each connecting impedance of the, or a respective, connecting path typically is a capacitor.

The filter according to the fourth aspect of the present invention typically is a balanced in-line filter. In one specific embodiment each tank circuit T2 comprises a damping resistor connected in series with a capacitor of the tank circuit T2.

Further, the filter according to the first, second, third or fourth aspect of the present invention may comprise a current sensor. The current sensor may comprise a current dependent impedance for damping a signal conducted through the filter in a manner such that damping increases below a dc threshold current. The current sensor typically comprises non-linear semiconductor components such as back-to-back diodes in parallel with a resistance. This typically provides two advantages for smaller DC currents associated with inactive VFE: Firstly, the damping impedance of the current sensor increases as the DC current decreases which improves the longitudinal balance of an inactive connection. Secondly, the increased damping improves further the high frequency attenuation of the filter.

The current sensor may also be coupled to the or each switch and may be used to control switching of the or each switch. The current sensor comprise any number of back-to- back diodes in parallel with the resistor, base-to-emitter junctions of transistors connected in parallel with a resistor, a resistor in series with any of the above, or a combination of the above and/or other equivalent nonlinear electronic components.

The present invention provides in a fifth aspect a switching circuit comprising: a first, second and third terminal, a first transistor for switching an electrical pathway between the first and the third terminal , a second transistor for switching an electrical pathway between the second and the third terminal, a first impedance element for generating a first voltage associated with a switching current through the first impedance element and between the first and second terminals, the first impedance element being coupled to the first transistor so that at least a portion of the first voltage is applied to the first transistor for switching the electrical pathway between the first and the third terminal, and a second impedance element for generating a second voltage associated with a switching current through the second impedance element and between the second and first terminals, the second impedance element being coupled to the second transistor so that at least a portion of a second voltage is applied to the second transistor for switching the electrical pathway between the second and the third terminal, wherein one of the transistors is arranged for switching the electrical pathway between the first and the third terminal for a switching current in a first direction and the other transistor is arranged for switching the electrical pathway between the second and the third terminal for a switching current in the reverse direction.

The above-defined switching circuit is bidirectional and has the advantage that the switching properties for a current from the first terminal to the second terminal are independent from the switching properties for a current in the reverse direction. For example, such a switch may be incorporated in an in-line DSL filter positioned between VFE and a pair of twisted copper wires for usage of the VFE and DSL systems via the same pair of twisted copper wires. Further, components of the switching circuit typically comprise semi-conducting materials such as silicon based materials and consequently the switching circuit may be fabricated at relatively low cost.

The first and the second impedance elements of the switching circuit typically comprise an ohmic resistive component and/or an ac impedance component . The first and the second impedance elements may also comprise diodes and/or transistors. A first diode of the first impedance element may be connected in parallel with a first impedance of the first impedance element. A second diode of the second impedance element may be connected in parallel with a resistor or ac impedance of the second impedance element . The first and the second diodes may be general purpose diodes and typically are connected in a manner such that the first and the second transistors are protected from large currents. Alternatively, the first and the second diodes may be Schottky diodes which have the advantage of a relatively low voltage drop.

In a specific embodiment the first and the second , impedance elements each comprise at least two diodes that are connected in parallel with a resistor or a ac impedance. For example, each impedance element may

comprise a general purpose diode and a Schottky diode which typically are parallel connected with the resistor or ac impedance of each impedance element . In this case the Schottky diode of the first impedance element typically is connected so that it limits the reverse voltage across the first transistor and the Schottky diode of the second impedance element typically is connected so that it limits the reverse voltage across the second transistor. The general purpose diode of the first impedance element typically is arranged for determining, together with the resistor or ac impedance of the first impedance element, the voltage that is generated at the first impedance element and that is at least in part applied to the first transistor. In the same manner the general purpose diode of the second impedance element typically is arranged for determining, together with the resistor or ac impedance of the second impedance element, the voltage that is at least in part applied to the second transistor. In this embodiment the Schottky diodes and the general purpose diodes of each impedance element typically are connected in a "back-to-back" manner.

In a further embodiment of the present invention the first and second impedance elements each comprise a third and fourth transistor respectively, which may replace the Schottky diodes. In this case the first and second impedances typically also comprise resistors, such as bias resistors. The third and fourth transistors and resistors of the first and second impedance elements typically provide for an even lower forward voltage drop and better reverse protection than the Schottky diodes.

The third transistor of the first impedance element typically is connected with collector to emitter current path in parallel with a resistor or ac impedance of the first impedance element and the fourth transistor of the second impedance element typically is connected with collector to emitter current path in parallel with a resistor or ac impedance of the second impedance element .

The switching circuit typically comprises a third impedance element connected in series between the third terminal and the first transistor, for setting a particular filter characteristic when the first transistor is switched on for DC current in one direction.

Further, the switching circuit typically comprises a fourth impedance element connected in series between the third terminal and the second transistor, for setting a particular filter characteristic when the second transistor is switched on for DC current in the reverse direction. The fourth impedance element may also be connected in series between the third terminal and both the first and second transistors .

The third and fourth impedance elements typically are ac impedances. Should it be required to have a filter with uniform characteristics for both directions of DC current then the third and fourth impedance elements may have the same value or may be omitted.

The first and second transistors typically are of the same type and may be both PNP or NPN transistors. The third and fourth transistors are also typically of the same type and may be both NPN or PNP.

The present invention provides in a sixth aspect an inline filter, such as a DSL filter, comprising the above defined switching circuit.

The in-line filter typically comprises at least one impedance element having a resonance circuit comprising a capacitor and an inductor. Each resonance circuit typically also comprises a damping resistor connected in series with the capacitor. This improves the longitudinal balance performance of the in-line filter and also improves the higher frequency attenuation of the in-line filter which is of advantage for higher speed DSL services over longer distances.

The invention will be more fully understood from the following description of specific embodiments of the invention. The description is provided with reference to the accompanying drawings .

Brief Description of the Drawings

Figure 1 shows a diagram of an in-line filter according to the prior art,

Figure 2 shows a diagram of an in-line filter according to a first embodiment of the present invention,

Figure 3 shows a diagram of an in-line filter according to a second embodiment of the present invention, Figures 4 (a) , (b) and (c) show diagrams of in-line filters according to a third embodiment of the present invention,

Figure 5 (a) and (b) show diagrams of an in-line filter according to a further embodiment of the present invention,

Figure 6 shows a portion of a diagram of a filter according to an embodiment of the present invention

Figure 7 (a) and (b) show portions of diagrams of a filter according to another embodiment of the present invention

Figure 8 shows a portion of a diagram of a filter according to a further embodiment of the present invention, Figure 9 shows a portion of a diagram of a filter according to yet another embodiment of the present invention,

Figure 10 shows a portion of a diagram of a filter according to another embodiment of the present invention, Figure 11 shows a diagram of a switching circuit according to an embodiment of the present invention,

Figure 12 shows a diagram of a switching circuit according to an embodiment of the present invention,

Figure 13 shows a diagram of a switching circuit according to an embodiment of the present invention, and

Figure 14 shows a diagram of a switching circuit according to an embodiment of the present invention.

Detailed Description of Specific Embodiments

Referring initially to Figure 2, an in-line filter according to a first embodiment of the present invention is now described. The in-line filter 200 may for example be connected between a pair of twisted copper wires for conduction of telephone signals and voice frequency equipment (VFE) . For example, the VFE may be a telephone. In this embodiment the in-line filter 200 is a DSL in-line filter for low-pass-filtering which is typically required

to make such a conventional VFE - line also suitable for DSL data transmission. The in-line filter 200 comprises a number of impedances 201 to 214 and switches 216, 218 which are used to switch connections between lines 220 and 222 via impedances 213 and 214. In the described example switching is conducted as a function of currents conducted through the lines 220 and 222 (VFE in use or not in use) and to ensure that incorrect impedances for VFE are avoided .

In this embodiment the switches 216 and 218 are solid state switching circuits and include transistors. Further, the filter 200 comprises a current sensing arrangement (not shown) which is arranged to switch the transistors as a function of a DC current conducted through the lines 220 and 222.

Two of the impedances on each line of the in-line filter 200 typically are second-order parallel resonant elements that have resonant frequencies at or around the lower frequencies of the DSL system in question. For example, impedances 202, 206, 208 and 212 may be such second order filter elements. In the present embodiment the performance of the second order filter elements is improved by the presence of damping resistors connected in series with the capacitors of each second order filter element 202, 206, 208 and 212. This results in an expected sharp cut-off frequency of the low-pass filter characteristics of the in-line filter 200. In addition this improves the longitudinal balance performance of the in-line filter and improves higher frequency attenuation of the in-line filter which is of advantage for higher speed DSL services over longer distances.

Figure 3 shows a balanced in-line filter 300 according to another embodiment of the present invention. The filter 300 is arranged for connection to a pair of twisted copper wires, which carries telephone signals and high frequency DSL data, on the left side and to a VFE on the right side. Inductors 302 and 302A and capacitor 306, together with tank circuits 308/308A provide initial attenuation to the high frequency DSL signal. Tank circuits 308/308A are connected on the VFE side of capacitor 306 and together are arranged for providing an attenuation at the lower end of the DSL frequency band where peak signals are most likely to interfere. The resultant possible peak voltage appearing across capacitors 312 and 314 (when switch 320 is open) and across the switch 320 therefore is substantially reduced compared with the possible peak voltage across capacitors 12 and 14 in the prior art inline filter shown in Figure 1. The result is that the possibility of non-linear signals, noise occurring at the LINE/DSL port and/or the VDE due to breakdown of switch 320 or 322 is substantially reduced.

In this embodiment each second tank circuit 308, 308A, 324 and 324A comprises a damping resistor connected in series with a capacitor of the tank circuit T2.

The switches 320 and 322 typically are solid state switching circuits having transistors. In this embodiment switch 322 is parallel connected with inductance 317, capacitor 316 and resistor 319 where the inductance 317 is parallel connected with resistor 319 and series connected with capacitor 316. It is typically required to achieve a particular overall filter loss for each of the on-state or

off-state (switch 322 closed or switch 322 open) conditions. This embodiment provides the advantage of configuring an off-state filter characteristic for the case of switch 322 being open that does not influence the on-state filter characteristics for the case of the switch 322 being closed.

The filter 300 typically comprises a current sensing arrangement (not shown) , such as back-to-back diodes in parallel with a resistor, and controls the switching as a function of a dc current conducted through the filter.

Tank circuits 308 and 308A have the dual function of providing relatively sharp low pass cut-off filter characteristics, in combination with tank circuits 324 and 324 A, as well as attenuation in combination with capacitor 306 and inductors 302 and 302A to reduce peak signal levels across an open switch 320 typically to levels below any breakdown levels for the switch 320.

In this embodiment, the tank circuits 310 and 310A are arranged for VFE impedance matching and the tank circuits 308, 308A, 324 and 324A are arranged for band stop/ low pass filtering having a cut-off frequency at or around the lower frequencies of the DSL system in question. In this example, the capacitances of capacitor 306 and 312 together have approximately the same capacitance as capacitor 12 of the prior art in-line filter 10 shown in Figure 1.

It is to be appreciated that variations of the filter 300 described above may not necessarily comprise the capacitor 316 and the resistor 319.

Figure 4 (a) shows a balanced in-line filter 400 according to a further embodiment of the present invention. In this embodiment the filter comprises only one switch 322 and, compared with the filter 300, the capacitors 312 and 314 and the switch 320 are removed. This filter design is particularly advantageous as it is simplified and therefore of reduced production cost. As in filter 300 described above, the capacitor 306 together with the inductances 302 and 302A provide initial attenuation of a high frequency signal before the high frequency signal reaches the tank circuits and the switch 322. Again, in this embodiment switch 322 is parallel connected with inductance 317, capacitor 316 and resistor 319 where the inductance 317 is parallel connected with resistor 319 and series connected with capacitor 316. It is typically required to achieve a particular overall filter loss for each of the on-state or off-state (switch 322 closed or switch 322 open) conditions. This embodiment provides the advantage of configuring an off-state filter characteristic for the case of switch 322 being open that does not influence the on-state filter characteristics for the case of the switch 322 being closed.

It is to be appreciated that variations of the filter 400 described above may not necessarily comprise the capacitor 316 and the resistor 319.

Figure 4 (b) shows circuit diagram of a balanced in-line filter that provides advantages similar to those of the above-described filter 400, but the capacitor 306 is replaced by two capacitors 306 and 307.

Figure 4 (c) shows circuit diagram of a balanced in-line filter that provides advantages similar to those of the above-described filter 400, but the capacitor 307 provides for initial attenuation via inductors 302 and tank circuits 308.

Figure 5 (a) shows a circuit diagram for a balanced inline filter 500 according to a further embodiment of the present invention. The filter 500 is related to the filter 400 described above. In the filter 500, however, the positions of the tank circuits 308 and 310 (and 308A and 310A) have been interchanged.

Figure 5 (b) shows a circuit diagram for a balanced in- line filter 550, which is very similar to that of filter 500. In this embodiment, however, the tank circuits 308, 308A, 324 and 324A do not include damping resistors. The fixed capacitor 306 may be split into 2 capacitors and positioned in a similar way to those in figure 4a or replaced with capacitor 307 placed in a similar position to capacitor 307 in figure 4b.

It is to be appreciated that variations of the filters 500 and 550 may not necessarily comprise the capacitor 316 and the resistor 319.

Figure 6 shows portion of a diagram of a filter 600 which comprises two conductive path 602 and 604 and at least two tank circuits 606 and 608 which are of the same type as tank circuits 308, 308A, 324 and 324A described above. In this embodiment, the damping resistors in series with the capacitor of the tank circuits are connected directly across the tank circuit inductor. The filter 600 also

comprises current sensing arrangements 610 and 612. The current sensing elements 610 and 612 are arranged to control the switching of the switch 614 as a function of a dc voltage conducted through the filter 400. For example, each current sensing arrangement 610 and 612 may comprise back-to-back diodes which, together an additional impedance such as an additional resistor, determine a voltage applied to the switch 614, which typically is a transistor switch and which is arranged for switching above a threshold voltage (and consequently above a dc threshold current conducted through the filter) .

Figure 7 (a) shows a portion of a diagram of a filter 700. The filter 700 is related to the filler 600, the main difference being that in this example the filter 700 comprises at least two tank circuits 702 and 704, which are of the same type as the tank circuits 606 and 608, but which have damping resistors connected to the tank circuit inductors via the current sensing arrangement 611 and 613. The current sensing arrangements, together with the damping resistors of the respective tank circuits, control the damping as a function of the level of DC current flowing through the current sensing arrangement. This provides two advantages for smaller DC currents associated with inactive VFE: Firstly, a dynamic resistance of the current sensing arrangement of the transistor-based switching circuit typically increases as the DC current decreases which increases the damping of the tank circuit thereby improving the longitudinal balance of an inactive connection. Secondly, the additional series impedance associated with the damping resistor impedance improves further the high frequency attenuation of the filter.

Figure 7 (b) shows the same circuit as figure 7 (a) except that all of the damping of tank circuits 702 and 704 are provided by the current sensing elements 610 and 612 respectively.

Figure 8 shows a portion of a diagram of another example of a filter. The filter 800 is related to the filter 700, but comprises two additional tank circuits 802 and 804 which are of the same type as tank circuits 606 and 608 but comprise damping resistors.

Figure 9 shows a portion of a diagram of a filter 900 which is related to the filter 800, the only difference being that tank circuit 802 incorporates damping with current sensing dynamic impedance and tank circuit 704 does not .

Figure 10 shows a portion of a diagram of a filter 1000 which in this embodiment comprises two switches 316 and 320. This embodiment is related to filter 300 shown in Figure 3.

The filters 600, 700, 800, 900 and 1000 also comprise tanks circuits of the same type as tank circuit 310 and which are not shown in Figures 6 to 10.

Although the filter has been described with reference to particular examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. For example, in filter 300 the tank circuits 324 and 324A may exchange positions with the tank circuits 310 and 310A respectively. In addition, in filter 300 capacitor 306 may not be present and capacitors 312,

314, 316 and/or 318 may then have respective adjusted capacitances. Also in filters 400 and 500 capacitor 306 may be split into two capacitors 306 and 307 or may be replaced by capacitor 307.

The current sensing arrangement of the filter 600 comprises two back-to-back diodes and an additional impedance. However, it will be appreciated by a person skilled in the art that in variations of the described embodiments the current sensing arrangement may comprise any number of back-to-back diodes in parallel with the resistor, base-to-emitter junctions of transistors in parallel with the resistor, a resistor in series with any of the above, or a combination of the above and/or other equivalent non-linear electronic components.

Further, the current sensing arrangement of the filter 700, 800, 900 or 1000 comprise two back-to-back diodes and a damping resistor. It will be appreciated that in variations of the described embodiments the current sensing arrangement may comprise any number of back-to- back diodes in parallel with the damping resistor, base- to-emitter junctions of transistors connected in parallel ^■ with the damping resistor, a resistor in series with any of the above, or a combination of the above and/or other equivalent non-linear electronic components.

Referring now to Figures 11 and 12, switching circuits according to specific embodiments of the present invention are now described. Figure 11 shows a switching circuit

1000 having a first terminal 1002, a second terminal 1004 and a third terminal 1006. The switching circuit 1000

comprises impedances 1008 and 1010, transistors 1012 and 1014 and impedances 1013 and 1015.

When a current flows between the first terminal 1002 and the second terminal 1004, a voltage is generated across the impedances 1008 and 1010. In this embodiment the impedances 1008 and 1010 are connected to respective emitter and base contacts of the transistors 1012 and 1014. For a current from the first terminal to the second terminal a voltage generated at the impedance 1008 is applied to the transistor 1012 between base and emitter. Because of the reverse orientation of the transistor 114, a voltage generated at the impedance 1010 is applied to the transistor 1014 in a reverse direction. Consequently, for a current over a predetermined threshold, one of the transistors 1012 and 1014 will be switched and for the reverse current direction the other transistor will be switched. As the transistors 1012 and 1014 control the electrical pathways between the third terminal 1006, the third impedance 1013 and the first terminal 1002 and between the third terminal 1006, the fourth impedance 1015 and the second terminal 1004, respectively, these electrical pathways can be switched by currents between the first terminal 1002 and the second terminal 1004.

The switching thresholds and properties of the electrical pathways depend on the impedances and the transistors used in the switching circuit 1000 and can be controlled independent from each other. Consequently the switch 1000 provides a bi-directional switch in which the switching properties in each current direction are independent from one another .

The impedances 1008 and 1010 may be any suitable impedances and typically comprise ohmic resistors. In this example the transistors 1012 and 1014 are NPN bipolar transistors but PNP bipolar transistors- can also be used.

Figure 12 shows a related switching circuit 1100 in which the transistors 1012 and 1014 are connected in a reverse manner and consequently the operation of the switching circuit 1100 is reversed compared with that of the switching circuit 1000.

Figure 13 shows a switching circuit 1200 according to a further specific embodiment of the present invention. Again, the switching circuit 1200 comprises transistors 1012 and 1014 and impedances 1008 and 1010. Further, , diodes 1016 and 1018 are connected in parallel with the impedances 1008 and 1010. In addition, diodes 1020 and 1022 are connected in parallel with the impedances 1008 and 1010. For a flow of a small current from the second terminal 1004 to the first terminal 1002 most of the current will flow through the impedances 1008 and 1010. However, for larger currents the voltage generated at the impedances and applied to the diodes will increase and diode 1020, connected in a forward direction for that current, will provide a significant path for larger currents. The voltage across the impedance 1008 and across the diode 1020 is dependent on the current between the second and the first terminal and on the properties of these components. It is possible to choose the electrical properties of the impedance 1008 and the diode 1020 so that a predetermined current results in a selected voltage which is then applied to resistor 1024 and between base and emitter of transistor 1012 to switch the transistor

1012. In the same manner the impedance 1010 and the diode 1022 are selected to generate a voltage which is applied to impedance 1026 and between the base and emitter of transistor 1014 for switching transistor 1014 if a threshold current is reached in a reverse direction.

The diodes 1020 and 1022 may be any type of suitable diode but typically are general purpose silicon diodes. In addition, diode 1016 is connected in parallel with the impedance 1008 and the diode 1020. Diode 1018 is connected in parallel with the impedance 1010 and the diode 1022. The diodes 1016 and 1018 have dual functions, firstly limiting the reverse voltage across the transistors 1012 and 1014 and secondly providing a low voltage drop and the DC current path around the inactive transistor. The diodes 1016 and 1018 may be of any suitable type, but typically are Schottky diodes.

In this embodiment the diodes 1016 and 1020 and the diodes 1018 and 1022 are connected in a back-to-back manner which provides robust protection against large transient voltages and currents.

In this embodiment the switching circuit 1200 also comprises further impedances 1027 and 1028 and further

Zener diodes 1029 and 1030. The impedances 1027 and 1028 provide a small DC bias for the transistors 1012 and 1014 and the Zener diodes 1029 and 1030 provide voltage protection for the transistors 1012 and 1014. The switching circuit 1200 comprises further impedances 1013,

1015 and 1032 which provide independent loads for currents to and from the third port 1006. Should uniform filter characteristics be required for DC currents in either

direction then impedances 1013, 1015, 1027 as well as Zener diode 1029 may be omitted and the collectors of transistors 1012 and 1014 would then be joined and connected to impedance 1032.

It is to be appreciated that in an alternative embodiment the transistors and the diodes may all have reverse orientations. It is also to be appreciated that the independent nature of the circuit allows the left hand side transistors and impedances to be placed in the circuit of a filter at a different location to the right hand side thus achieving an even greater degree of flexibility with respect to direction of DC current flow and filter characteristics.

Figure 14 shows a switching circuit according to a further specific embodiment of the present invention. Operation of the circuit is similar to that of the above described switching circuit 1200 except that transistor 1034 with associated impedance 1040 and transistor 1036 with associated impedance 1038 are included to reduce the voltage drop across the transistors 1012 or 1014 respectively when they are inactive. This embodiment is more suitable to integrate onto a silicon chip and has the advantage of lower overall DC voltage drop in the DC current path.

The filter 200, illustrated in Figure 2, includes switches 216 and 218, which may be provided in the form of switching circuits 1000 or 1100 as illustrated in Figures 11 and 12. In this case, the first and second terminals of the switching circuits 1000 or 1100 are connected between impedances 202 and 204 and between impedances 210

and 212, respectively. The third terminal is connected to impedance 213 and 214 respectively.

In a further embodiment the switches 216 and 218 are switches of the type of switching circuit 1200 shown in

Figure 13 or switching circuit 1300 shown in Figure 14 and illustrated above. In this case the switches are connected in the same manner and typically the impedances 213 and 214 are provided by the impedance 1013 and 1032 or 1015 and 1032 of the switch 1200 or 1300. In either variation the in-line DSL filter 200 activates switches 216 and 218 as a function of currents in the lines 220 and 222.

Although the switching circuit has been described with reference to particular examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. For example, the impedance elements 1013 and 1015 shown in Figure 11 may be replaced by a single impedance element connected in series between the third terminal and both the transistors 1012 and 1014.