AU2007100379A4 - A filter - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Applicant: 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 or central filter, for coupling voice frequency equipment to a DSL data and telephone transmission line.

Background of the Invention Almost every telephone in a typical 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 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 filter, such as filter 10 shown in Figure 1, may be positioned between the VFE equipment and the pair of twisted copper wires. Such a conventional filter is designed to provide a high level of DSL frequency attenuation, such as that required by the European ETSI TS101-952-1-5 and AS/ACIF S041, and usually -3comprises a four-stage balanced circuit including two switched bridging impedances (in-line filter) or unswitched bridging impedances (single first in-line or central filter), which commonly include high voltage capacitors.

Figure 1 shows a circuit diagram 10 of a conventional filter. The filter is arranged for connection to a pair of twisted cooper wires, which carries telephone signals and high frequency DSL data, on the left side of the circuit diagram allowing only telephone signals to pass to a VFE on the right side. Inductors 11, 11A and capacitors 12, 14 are series-connected when switch 16 is in an open position. When the switch 16 is in a closed position (or not included), the inductors 11, 11A and capacitor 12 are series connected, which provides initial 2 nd order attenuation to the high frequency DSL signal in "off-line" state. Tank circuits 18 and 18A provide impedances for voice frequency impedance matching and therefore increase return loss. Tank circuits 20, 20A, 22, 22A in combination with capacitor 24, 26 (when switch 28 is in an open position) or in combination with capacitor 24 (when the switch 28 is not incorporated or in a closed position) provide further low-pass attenuation and a sharp cut-off at the lower frequency end of the DSL frequency band.

However, often VFE equipment, such as alarm systems or other systems that use two way in-band VF signalling, requires a very low noise floor to function in a reliable manner. Low-pass filters compliant with recent Australian or European standards, such as the European ETSI TS101- 952-1-5 and AS/ACIF S041, may still not provide attenuation of high frequency signals that is sufficient 4 to control unwanted effects on the overall noise floor.

Other telecommunications applications, such as ISDN services, also have a similar requirement for isolation of DSL signals from existing services.

There is a need for technological advancement.

Summary of the Invention The present invention provides a filter for coupling a voice frequency equipment (VFE) to a transmission line that is suitable for transmission of DSL data and telephone signals, the filter comprising: first and second conductive paths, each conductive path comprising a series of tank circuits including at least one first tank circuit at least two second tank circuits (T2) and at least one impedance means, 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 path coupling the first and the second conductive paths; at least one capacitor positioned along the at least one connecting path; and an inductance means; wherein the at least one capacitor, together with the inductance means, the tank circuits T2 and the at least one impedance means, form an attenuating filter component arranged so that a high frequency signal will experience an attenuation before reaching the VFE.

As the filter includes at least two tank circuits T2 and the at least one impedance means in each series, the high frequency signal experiences an attenuation, typically a significant attenuation, before reaching the VFE, and the likelihood for noise floor at the VFE to be sufficient to cause a mal-function of the VFE is reduced.

Such a filter is particularly suitable for usage as single first in-line or central filter.

The capacitor may be a first capacitor and the filter may include a first and a second connecting path that couple the first and second conductive paths.

Further, the filter may include a further capacitor and at least one switch. The filter typically is arranged so that the in use the switch switches the further capacitor in or out of one of the connection paths. In this case the filter is particularly suitable for application as an inline filter, such as a balanced in-line filter.

The impedance means may comprise an inductor and/or resistor. Typically, the impedance means is provided in form of a tank circuit T2.

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 at least one inductance means before passing through any one of the tank circuits.

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 at least one damping impedance typically is arranged to improve the longitudinal balance performance of the in-line filter and 6 O typically 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.

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_Each second tank circuit T2, typically has a cut-off frequency at or around the lower frequency end of the DSL frequency band.

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

Alternatively, each series may be formed by T2, T1 and T2, T2 in that order. In this case the second connecting path typically is coupled to the series at positions between the tank circuits T1 and T2 of each series.

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

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

7- In each of the above variations one of the second tank circuits T2 may be replaced by the impedance means.

In one specific embodiment of the present invention the filter comprises at least two connecting conductive paths coupling the first and the second conductive paths at positions adjacent to tank circuits and at least one connecting impedance is positioned along each connecting path.

The filter may also comprise a third connecting path which couples the first and the second conductive path and a third capacitor may be positioned along the third connecting path.

Further, the filter may comprise a fourth connecting path which couples the first and the second conductive path and a fourth capacitor may be positioned along the fourth connecting 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 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.

8 Brief Description of the Drawings Figure 1 shows a diagram of an in-line or central 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, Figure 4 shows a diagram of a single first in line or central filter according to a third embodiment of the present invention, Figure 5 shows a diagram of a single first in line or central according to a fourth embodiment of the present invention, Figure 6 shows a diagram of a single first in line or central filter according to a fifth embodiment of the present invention, Figure 7 shows a diagram for an in-line filter according to a sixth embodiment of the present invention, and Figure 8 shows a diagram for an in-line filter according to a seventh 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 9 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 216 and switches 226, 228 and 230 which are used to switch connections between lines 220 and 222 via impedances 220, 222 and 224. 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 226, 228 and 230 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.

Three 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 204, 206, 208 and 212, 214 and 216 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 204, 206, 208 and 212, 214 and 216. 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 10 advantage for higher speed DSL services over longer distances.

Figure 3 shows circuit diagram for 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, provide initial attenuation to the high frequency DSL signal. Tank circuits 310 and 310A provide voice frequency impedance matching and tank circuits 308, 308A, 324 324A and 326 326A in combination with capacitor 318 (when switch 322 is closed) or with capacitor 318 in series with capacitor 316, inductor 317 and resistor 319 (when the switch 322 is open) provide substantial attenuation of the high frequency.

In this embodiment each second tank circuit 308, 308A, 324 and 324A, 326 and 326A comprises a damping resistor connected in series with a capacitor of the tank circuit T2.

The switch 322 typically is a solid state switching circuit 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 11 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 and 326 and 326A, as well as attenuation in combination with capacitor 306 and inductors 302 and 302A to reduce peak signal levels across an open switch 322 typically to levels below any breakdown levels for the switch 322.

In this embodiment, the tank circuits 310 and 310A are arranged for VFE impedance matching and the tank circuits 308, 308A, 324, 324A, 326 and 326A 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.

Figure 4 shows a diagram for a single first in line or central filter 400 according to a further embodiment of the present invention. In this embodiment the filter does not include any switches in the connecting paths (compared with the filter 300 the switch 322 and the LCR comprising components 317, 316, 319 are removed). As in the abovedescribed filter 300 described above, the capacitor 306 together with the inductances 302 and 302A provide initial 12 attenuation of a high frequency signal. Tank circuits 310 and 310A provides voice frequency impedance matching and tank circuits 308, 308A, 324, 324A 326 and 326A in combination with capacitor 318 provide substantial attenuation of the high frequency.

Figure 5 shows the circuit diagram for a single first in line or central 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, an additional connecting capacitor 328 is included. Unbalance to earth characteristics can be optimised in this configuration if necessary.

Figure 5A shows the circuit diagram for a single first in line or central filter 500 according to a further embodiment of the present invention. The filter 510 is related to filter 500 described above. In the filter 510, however, the second tank circuit 308/308A is replaced by an impedance element comprised of inductors.

Figure 6 shows the a circuit diagram for a single first in line or central filter 550 that includes the components of the filter 500 and further includes a connecting capacitor 330 for additional high frequency characteristic flexibility.

Figure 7 shows a circuit diagram for a filter 550 that includes the components of the filter 500. In this embodiment the filter 550 is a balanced in-line filter and includes switches 320. Further, the filter 550 includes capacitors 329, 316 and 331 in each connection path.

13 Figure 8 shows a circuit diagram for an in-line filter 700. The filter 700 is related to the filter 300 shown in Figure 3. Compared with the filter 300 the tank circuits 308, 308A, 310 and 310A have been interchanged and an additional connecting path with capacitors 312, 314 and switch 320 has been included.

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, filters of the types shown in Figures 4 to 8 can be configured with tank circuit 310 and 310A being positioned as the l t 2 nd 3 rd or 4th tank circuit. In addition, in filter 500 capacitor 306 and/or 318 may not be present and capacitors 330 and/or 328 may then have respective adjusted capacitances.

Claims (4)

1. A filter for coupling a voice frequency equipment (VFE) to a transmission line that is suitable for transmission of DSL data and telephone signals, the filter comprising: first and second conductive paths, each conductive path comprising a series of tank circuits including at least one first tank circuit at least two second tank circuits (T2) and at least one impedance means, 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 path coupling the first and the second conductive paths; at least one capacitor positioned along the at least one connecting path; and an inductance means; wherein the at least one capacitor, together with the at least one inductance means, the tank circuits T2 and the impedance means, form an attenuating filter component arranged so that a high frequency signal will experience an attenuation before reaching the VFE.

2. The filter as claimed in claim 1 wherein the at least one impedance means is a second tank circuit T2

3. The filter as claimed in claim 1 or 2 wherein the capacitor is a first capacitor and the filter includes first and second connecting paths that couple the first and second conductive paths; the filter further including a further capacitor and at least one switch, the filter 15 being arranged so that in use the at least one switch switches the further capacitor in or out of at least one connection path.

4. The filter as claimed in any one of the preceding claims comprising at least two connecting conductive paths coupling the first and the second conductive paths at positions adjacent to tank circuits and at least one connecting impedance is positioned along each connecting path. A filter substantially as herein described with reference to Figure 2 8.