AU2006100159A6 - A filter - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant(s): Advanced Circuits and Systems Pty Ltd Invention Title: A FILTER The following statement is a full description of this invention, including the best method of performing it known to me/us: -2- A FILTER Field of the Invention The present invention broadly relates to a filter, such as an in-line filter or a central filter, for coupling a voice frequency equipment to a DSL data and telephone transmission line.

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 E) 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 equipment 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 equipment and the pair of twisted copper wires. However, often a number of VFE equipment is 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.

H:\lisaf\keep\Speci\retype\P60058 .doc 3 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 VFE's 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 equipment. 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 equipment 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 equipment (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 VFE equipment 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 equipment 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 equipment side of the filter. However, H:\lisaf\keep\Speci\retype\P60058 .doc 4 a minimum loading needs to be maintained on the line side to maintain the correct impedance for operation of VFE equipment. 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 designed to provide high levels of DSL frequency attenuation, such as required by the European ETSI TS101-952-1-5 and AS/ACIF S041, usually comprise a four-stage balanced circuit including two switched bridging impedances which are commonly 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 only telephone signals to a VDE on the right side. Inductors 11, 11A 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 means for the voice frequency impedance matching and therefore increased return loss. Tank circuits 22, 22A in combination with capacitor 24, 26 (when the switch 28 is open) provide further low 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 open circuit (filter is off-line). The result is that non-linear signals may be generated and add noise to the LINE/DSL port and/or the VDE. The detrimental impact on the quality H:\lisaf\keep\Speci\retype\P60058 .doc 5 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.

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 (TI and T2), each first tank circuit T1 being arranged for VFE frequency impedance matching and each second tank circuit being arranged for band-stop/ low pass filtering, first and a second connecting path coupling the first and the second conductive paths, first and second capacitors positioned along the first and the second connecting paths respectively, one switching means, the switching means being positioned in the second connecting path for switching the second capacitor in or out of the second connecting path, wherein the or each first capacitor, together with an inductance means, form an attenuating filter component arranged so that a high frequency signal will experience an attenuation before reaching the switching means.

The attenuating filter component typically is arranged so that a high frequency signal will experience an attenuation before reaching one of the series of tank circuits and the switching means.

As in the above-defined in-line filter the high H:\lisaf\keep\Speci\retype\P60058 .doc 6 frequency signal experiences an attenuation before reaching the or each tank circuits and the switching means, the likelihood for high peak voltages at the switching means is reduced. Consequently, it is less likely that an open switching means will experience a break-through voltage.

The inductance means typically is one of at least two inductance means. At least one inductance means typically is positioned on each conductive path in a manner so that the high frequency signal is conducted through the inductance means before passing through any one of the tank circuits. The inductances and the or each first capacitor typically form attenuating filter for the high frequency signal.

The filter 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 inline 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 the tank circuit T2.

The switching means of the filter, which typically is a balanced in-line filter, typically is a transistor-based switching means.

Further, the filter typically comprises a current sensing means. The current sensing means may comprise a current dependent impedance. The current dependent impedance typically is incorporated into the filter so that the damping impedance is controlled by an amount of H:\lisaf\keep\Speci\retype\P60058.doc 7 DC current flowing through at least one conducting path.

The current sensing means typically comprises non-linear semiconductor devices such as back to back diodes in parallel with a resistance. In one embodiment the current sensing means forms a part of the damping impedance with or without additional damping resistance. 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 damping impedance improves further the high frequency attenuation of the filter.

Each second tank circuit T2, 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 T1, 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 T2, T1 and T2 in that order. In this case the second connecting path typically is coupled to the series between T1 and T2.

In a further variation, each series may be formed by T2, T2 and T1 in that order. In this case the second connecting path typically is coupled to the series between T2 and T1.

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 H:\lisaf\keep\Speci\retype\P60058 .doc 8 data and telephone signals, the filter comprising: first and second conductive paths, each conductive path comprising a series of first and second tank circuits (T1 and T2), each first tank circuit T1 being arranged for VFE frequency impedance matching and each second tank circuit T2 being arranged for band stop and/or low pass filtering, at least one connecting conductive path coupling the first and the second conductive paths at positions between adjacent tank circuits, at least one connecting impedance means positioned along the connecting path, at least one switching means, the or each switching means being arranged for switching the, or at least one of the, impedance means 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 two inductance means. At least one inductance means typically is positioned on each conductive path in a manner so that the high frequency signal will be conducted through the inductance means before passing through any one of the tank circuits.

The or each connecting impedance means of the connecting paths typically are capacitors.

In one specific embodiment of the present invention the filter, which typically is a balanced in-line filter, comprises two connecting conductive paths coupling the H:\lisaf\keep\Speci\retype\P60058 .doc 9 first and the second conductive paths at positions between adjacent tank circuits and at least one connecting impedance means positioned along each connecting path.

Further, the filter comprises in this embodiment at least two switching means positioned in respective connecting paths in manner such that each switching means is arranged for switching at least one of the connecting impedance means in or out of a respective connecting path.

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

The filter 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 switching means of the filter, which typically is a balanced in-line filter, typically is a transistor-based switching means.

Further, the filter typically comprises a current sensing means. The current sensing means may comprise a current dependent impedance. The current dependent impedance typically is incorporated into the filter so that the damping impedance is controlled by an amount of DC current flowing through at least one conducting path.

The current sensing means typically comprises non-linear semiconductor devices such as back to back diodes in parallel with a resistance. In one embodiment the current H:\lisaf\keep\Speci\retype\P60058.doc 10 sensing means forms a part of the damping impedance with or without additional damping resistance.

Each second tank circuit T2, 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 T2, TI, T2 in that order. The first connecting path typically is coupled to the series between T2 and T1 and the second connecting path typically is coupled to the series between T1 and T2.

Alternatively, each series may be formed by T2, T2 and T1 in that order. In this case the first connecting path typically is coupled to the series between T2 and T2 and the second connecting path typically is coupled to the series between T2 and Ti.

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 (Ti and T2), each first tank circuit being arranged for VFE frequency impedance matching and each second tank circuit being arranged for band-stop/ low pass filtering, at least one connecting path coupling the first and the second conductive paths, impedance means positioned along the or each connecting path, at least one damping impedance comprising a damping resistor connected in series with the capacitor forming a H:\lisaf\keep\Speci\retype\P60058 .doc 11 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 inline 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 on-line.

The filter typically also comprises at least one switching means arranged for the switching the, or at least one of the, impedance means in or out of the or each connecting path. The switching means may be a transistorbased switching means.

Further, the filter typically comprises a current sensing means. The current sensing means may comprise a current dependent impedance. The current dependent impedance typically is incorporated into the filter so that the damping impedance is controlled by an amount of DC current flowing through at least one conducting path.

The current sensing means typically comprises non-linear semiconductor devices such as back to back diodes in parallel with a resistance. In one embodiment the current sensing means forms a part of the damping impedance with or without additional damping resistance..

This typically provides two advantages for smaller DC currents associated with inactive VFE: Firstly, an impedance of the current sensing means of the 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 increased damping improves further the high frequency attenuation of the filter.

H:\lisaf\keep\Speci\retype\P60058 .doc 12 The filter typically also comprises at least two inductance means. At least one inductance means typically is positioned on each conductive path in a manner so that the high frequency signal will be conducted through the inductance means before passing through any one of the tank circuits.

The connecting impedance means of the connecting paths typically are capacitors.

The filter 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.

In any one of the above described examples and embodiments of the first, second and third aspect of the present invention each current sensing means may comprise: any number of back-to-back diodes in parallel with a 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 non-linear electronic components.

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, H:\lisaf\keep\Speci\retype\P60058 .doc 13 Figure 3 shows a diagram of an in-line filter according to a second embodiment of the present invention, Figure 4 shows a diagram of an in-line filter according to a third embodiment of the present invention, Figure 5 and show diagrams of an in-line filter according to a forth embodiment of the present invention, Figure 6 shows a portion of a diagram of a filter according to the an embodiment of the present invention Figure 7 and show portions of diagrams of a filter according to an 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 and Figure 10 shows a portion of a diagram of a filter according to another 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 H:\lisaf\keep\Speci\retype\P60058 .doc 14 222 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, H:\lisaf\keep\Speci\retype\P60058 doe 15 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 in-line 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. 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 H:\lisaf\keep\Speci\retype\P60058 .doc 16 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.

Figure 4 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 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 318.

Figure 5 shows a balanced in-line 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 shows the same circuit as Figure 5 (a) except that tank circuits 308, 308A, 324 and 324A do not include damping resistors.

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 H:\lisaf\keep\Speci\retype\P60058 .doc 17 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 shows a portion of a diagram of a filter 700. The filter 700 is related to the filter 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.

H:\lisaf\keep\Speci\retype\P60058 .doc 18 Figure 7 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 Although the invention 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.

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 H:\lisaf \keep\Speci\retype\P60058 .doc 19 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-toback diodes in parallel with the damping resistor, baseto-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.

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Claims (4)

1. 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 (TI and T2), each first tank circuit T1 being arranged for VFE frequency impedance matching and each second tank circuit being arranged for band-stop/ low pass filtering, first and a second connecting path coupling the first and the second conductive paths, first and second capacitors positioned along the first and the second connecting paths respectively, one switching means, the switching means being positioned in the second connecting path for switching the second capacitor in or out of the second connecting path, wherein the or each first capacitor, together with an inductance means, form an attenuating filter component arranged so that a high frequency signal will experience an attenuation before reaching the switching means.

2. 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 (TI and T2), each first tank circuit T1 being arranged for VFE frequency impedance matching and each second tank circuit T2 being arranged for band stop and/or low pass H:\lisaf\keep\Speci\retype\P60058 .doc 21 filtering, at least one connecting conductive path coupling the first and the second conductive paths at positions between adjacent tank circuits, at least one connecting impedance means positioned along the connecting path, at least one switching means, the or each switching means being arranged for switching the, or at least one of the, impedance means 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.

3. Afilter 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 (TI and T2), each first tank circuit being arranged for VFE frequency impedance matching and each second tank circuit being arranged for band-stop/ low pass filtering, at least one connecting path coupling the first and the second conductive paths, impedance means positioned along the or each connecting path, 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. H:\lisaf\keep\Speci\retype\P60058 .doc 22

4. The filter of claim 3 comprising a current dependent impedance which is incorporated into the filter so that the damping impedance is controlled by an amount of DC current flowing through at least one conducting path. A filter substantially as herein described with reference to drawings 2 to Dated this 12th day of July 2006 ADVANCED CIRCUITS AND SYSTEMS PTY LTD By Its Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia. H:\lisaf\keep\Speci\retype\P60058 .doc