GB2427110A - Current activated solid state switch - Google Patents

Abstract

A switch S for use in an in-line DSL filter 20. Switch S includes one PNP bipolar transistor Q1 connected in direct parallel to one NPN transistor Q2, with a current sensing element Z connected between the two emitter pins of transistors Q1 and Q2, forming Port 1, and two base pins of transistors Q1 and Q2, forming Port 2. DC current, in either direction, between Port 1 and Port 2 thus turns on either transistor Q1 or transistor Q2, thereby creating a low impedance between Port 1 and the two collector pins of transistors Q1 and Q2, forming Port 3.

Description

CURRENT ACTIVATED SOLID STATE SWITCH

The present invention generally relates to high frequency data communications or Digital Subscriber Line (DSL) systems, and more particularly to a solid state switch for allowing conventional Voice Frequency Customer Equipment (VF CE) or ISDN Equipment, for example a telephone, to operate simultaneously with a DSL system.

A portion of the disclosure of this patent specification contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent specification or parts thereof as it appears in the file or records of a Patent Office, but otherwise reserves all copyright rights whatsoever.

Presently, some users, particularly home users, of the Internet, or other digital-based services, do not have access to a dedicated high-speed data communications link and must rely on a standard analog or Plain Old Telephone Service (POTS) line. POTS is an analog service provided over copper wires usually referred to as twisted pair. Analog or POTS line modems are typically only able to reach data transfer rates of up to 56 Kbps.

Conventional analog modems transmit and receive information on POTS lines via a Public Switched Telephone Network (PSTN).

DSL systems were developed to transfer digital data over the same twisted pair lines which are used for POTS. DSL has significant advantages, most notably a much larger data transfer rate than a conventional analog modem. Both POTS and DSL can be provided over the same copper wires so that no additional expense of providing additional lines for DSL is required. POTS operates at frequencies below 4 KHz while DSL operates at frequencies above 4 KHz up to several MHz.

For modern high frequency data communications or DSL systems to operate simultaneously with conventional POTS Voice Frequency Customer Equipment (VF CE), such as a telephone, modem, facsimile, etc., on one pair of twisted copper wires between a telephone exchange and a subscriber, the VF CE should preferably: (1) not load the line with low impedance at high frequencies; (2) not receive any high frequency signals which may cause VF CE malfunction; and, (3) not pass any spurious high frequency signals back to the line that may cause malfunction of the DSL system.

Referring to Fig. 1, in the case of a well designed subscriber installation, isolation of VF CE and DSL systems is commonly achieved by the use of one passive multiple order balanced low pass filter 10 that allows DC and telephone related signals to pass (including ring voltage and out of band signalling). Filter 10 is normally installed at a building 12 entry point of telephone subscriber line 14, originating from telephone exchange 16, and is termed a central filter, (master splitter or splitter). One, or more commonly multiple, VF CE 17 are connected to the output of filter 10.

However, it is common for a single telephone line 14 entry point in a building 12 to be either inaccessible or inconveniently located, in addition to multiple units of VF CE being installed. In this case, the only readily accessible or convenient elements for the installation of a filter is the VF CE, such as telephones, computer modems, facsimiles, etc. .

Hence, each piece of VF CE 17 must be individually connected via a filter. The resultant installation of multiple filters of the central type 10 in parallel across the telephone line 14 would result in an unacceptable impedance on telephone line 14, resulting in VF CE and/or DSL equipment malfunction.

Therefore, a special filter 18, termed an in-line (distributed, splitterless or micro-) filter, is required to be installed between each piece of VF CE 17 and the line 14. For reasons of circuit simplicity and low cost, in-line filters commonly use the same passive balanced circuit designs as central filters, but with some circuit elements that would otherwise affect the total loading impedance, switched ON or OFF in sympathy with the existence or otherwise of DC line current, which in turn provides an indication of whether the respective VF CE is in use or not.

A switch of filter 1 8 should be responsive to an absolute level of DC current (polarity independent), should pass low level AC signals when ON, and should also block low level AC signals when OFF. Moreover, the switch should also be sensitive enough to activate for a few milli-amps of DC line current whilst at the same time present a low series impedance inline with the filter elements. The switch should also be able to withstand high AC ring voltage, line interruption and other high energy transient events.

It is known to use low-pass filters, being simple filters that block all signals above a certain frequency. Since voice information is transmitted at a frequency below 4 KHz in POTS, the low-pass filters are built to block signals above 4 KHz, preventing DSL data signals from interfering with standard telephone calls.

AU 368 13/99 discloses a filter using contacts of an electromagnetic relay whose coil is incorporated in the telephone line loop. The contacts of a line current sensing relay cause a capacitor to be inserted into a filter when an associated telephone is brought into an off- hook mode, to alter the characteristics of the filter. However, there are disadvantages associated with relays, including reed relays, when used for this purpose. A major disadvantage is that such relays are barely sensitive enough for reliable operation at low current values, typically 20 to 50 mA and as a result of hysteresis require a larger current to turn on. This is because the relay coil is serially connected in the line loop and its coil resistance therefore needs to be relatively low. Further, the fast rise time of current when the contacts of the relay close may cause errors in some forms of signal transmission such as DSL transmission. Electromagnetic switching components are also less reliable than solid state components. Also as a result of not being magnetically screened electromagnetic switching components tend to degrade the high frequency performance of a filter.

As discussed in US 6680641, it is also known to use JFET transistors for switching AC signals. While the drainlsource of a JFET transistor is capable of conducting current in both directions, a control voltage must be applied to the transistor's gate of a polarity that matches the JFET type (P-channel or N-channel). Generating a unipolar control voltage when the line feed polarity can be either positive or negative is difficult. Although two JFETs could be used, the relatively high turn-on voltage of JFETs make them less attractive for this application.

Another known AC switch is an optically coupled MOS switch, sometimes known as a MOS opto-coupler. While this type of switch can also handle low level AC signals, its operating LED is typically unipolar which requires a rectifier bridge if the device is to operate with a line-feed polarity of either polarity. Such a rectifier bridge is disadvantageous in that it introduces a relatively high voltage drop. Further, this type of switch is relatively expensive.

US 6680641 discloses a bidirectional bipolar transistor switch for switching low level AC signals. A pair of bipolar transistors are coimected in anti-parallel and are operated by a common current sensing resistor in the base/emitter circuit of both transistors. However, this arrangement is believed to suffer from the disadvantage that the significant portion of excess line current is passed through the baseemitter junction of one of the bipolar transistors. The base emitter junction maximum current specification for typical small signal bipolar transistors is well below the required surge current specification for a VF CE device. In the more commonly used form of this circuit this problem is addressed by the use of two pairs of two series diodes to be in-line as the bias element combined with individual base resistors for the bipolar transistors. This results in an undesirable voltage drop for each switch used in a filter and a total DC loop resistance that will not comply with many technical specifications, such as the European ETSI TS1O1 and TR1O1 series technical specifications and numerous derived national standards. A further disadvantage of this circuit is the need for separate bipolar transistor base current control resistors, increasing cost and complexity further.

It should be understood that reference to DSL includes all types and variations of DSL, including, but not limited to, Asymmetric DSL (ADSL), Very high bit-rate DSL (VDSL), Symmetric DSL (SDSL), Rate-adaptive DSL (RADSL), or the like. DSL technologies include discrete multitone (DMT), carrierless amplitude and phase modulation (CAP), high-speed DSL (VDSL), and other technologies.

This identifies a need for a switch which addresses or at least ameliorates at least some of

the problems inherent in the prior art.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge.

According to a first broad form, the present invention provides a current activated solid state switch, including: an impedance component connected between a first node and a second node; a first transistor, an emitter of the first transistor connected to the first node and a base of the first transistor connected to the second node; and, a second transistor, an emitter of the second transistor connected to the first node and a base of the second transistor connected to the second node, and a collector of the first transistor connected to a collector of the second transistor to form a third node.

In a preferred form, the first transistor is a PNP type and the second transistor is a NPN type, or, the first transistor is a NPN type and the second transistor is a PNP type.

In a further preferred form, the impedance component is a sensing element and includes a resistor in parallel with a diode.

According to particular non-limiting forms, various components can additionally be attached between the first node and the second node to improve operating characteristics, such as, for example, a pair of transistors connected in parallel andlor a pair of diodes connected in parallel.

In other particular, but non-limiting, forms there is further provided that: the first node and the second node connect to one wire of a telephone line and the third node connects to the other wire of the telephone line; the switch is a bidirectional switch; and/or, switching is between VF CE or ISDN and DSL devices using the telephone line.

Optionally, but not necessarily, the switch is part of an in-line filter, such as an in-line DSL filter.

In accordance with further specific optional embodiments, provided by way of example only: two switches are provided on alternate sides of the wires of a telephone line; the in- line filter includes one or more resistors in parallel with an impedance component, wherein an increase in longitudinal balance of the filter is achieved; and/or, the one or more resistors are 1% resistors and the impedance component is a resonant filter element.

An example embodiment of the present invention should become apparent from the following description, which is given by way of example only, of a preferred but non- limiting embodiment, described in connection with the accompanying figures.

Fig. 1 (prior art) illustrates a block diagram of DSL filter arrangements; Fig. 2 illustrates a block diagram of an example in-line filter; Fig. 3 illustrates a block diagram of an example bidirectional switch; Fig. 4 illustrates a block diagram of a further example bidirectional switch and balanced filter segment; Fig. 5 illustrates a block diagram of an alternate example bidirectional switch and balanced filter segment; and, Fig. 6 illustrates a block diagram of another alternate example bidirectional switch and balanced filter segment.

The following modes, given by way of example only, are described in order to provide a more precise understanding of the subject matter of preferred embodiments. In the figures, incorporated to illustrate features of example embodiments, like reference numerals are used to identify like parts throughout the figures.

Referring to Fig. 2, illustrating a block diagram of an in-line filter 20, the in-line filter 20 is positioned between a line side 22 and a telephone side 24. In-line filter 20 includes impedance components ZI, Z2 and Z3 on line 26 and impedance components Z1A, Z2A and Z3A on line 28. Impedance components Z4 and Z5 are switched into or out of in-line filter 20 by switches Si and S2 respectively. When there is a need to switch an even number of impedance components Z4 and Z5 in filter 20, switches Si and S2 are preferably placed on alternate sides of filter 20, as illustrated, to maintain the overall electrical balance.

Referring to Fig. 3, in one particular preferred form the present invention can be considered to be a bidirectional switch S. Switch S can be used in filter 20 as switch Si and/or switch S2. Switch S includes one PNP bi-polar transistor Qi connected in direct parallel to one NPN transistor Q2, with a current sensing element Z connected between the two emitter pins of transistors Ql and Q2, at node 31 and forming Port 1, and two base pins of transistors QI and Q2, at node 32 and forming Port 2. DC current, in either direction, between Port I and Port 2 thus turns on either transistor Qi or transistor Q2, thereby creating a low impedance between Port I and the two collector pins of transistors QI and Q2, connected at node 33 and forming Port 3.

Other embodiments of the present invention are possible. According to another embodiment, and referring to Fig. 4, a more detailed embodiment of switch S is illustrated.

For simplicity, operation of switch S for only one direction of DC current is discussed.

Filter architecture may also be different to that presented in Fig. 2. For example, switch SI and associated impedance component Z4 may themselves be connected in parallel with in- line filter impedance component Z2. Then switch S2 and associated impedance component Z5 would be connected in parallel with filter impedance component Z2A. The resultant filter would again have a change of characteristics and impedance as a result of the presence of DC line current.

Current sensing element Z (i.e. impedance component) of Fig. 3 includes resistor Ri in parallel with diode Dl. For DC quiescent line currents which are below a line current defined as representing a VF CE OFF state, i.e. a telephone is ON HOOK, all quiescent current flows through Ri, insignificant current flows through diode Dl and transistor QI remains in an OFF state, thereby presenting a high impedance to impedance component Z4.

When line current increases above a predefined VF CE OFF state threshold, that is VF CE is now ON, i.e. a telephone is OFF HOOK, diode Dl becomes the significant path for current flow and the voltage across diode Dl will saturate at one diode voltage drop.

Resistor Ri provides an accurate way of setting the VF CE ON/OFF line current switching threshold.

Diode Dl could typically be a general purpose silicon diode. Such diodes have an equivalent circuit that is not ideal and includes a small series resistance which, amongst other factors, is due to the finite bulk resistance of the semiconductor material used in the diode. The combination of the significant current flow through diode Dl and the small series resistance results in sufficient voltage across diode Dl to turn ON transistor Qi via transistor base resistor R2 presenting a low impedance to impedance component Z4. The current gain and small transistor collector load on transistor Ql ensure saturation is maintained.

Diode Dl therefore serves in three roles. Firstly, protecting transistor QI base from over- current by conducting the significant portion of line current. Secondly, setting a knee- point just above the switch-on threshold of transistor QI. And thirdly, protecting the reverse direction transistor Q2 base-emitter junction from over-voltage.

Resistor R3 is used to maintain a DC charge across impedance component Z4 in the event that impedance component Z4 is capacitive, then when transistor QI switches ON, in-rush current should be much reduced. In addition, resistor R3 provides a small amount of DC bias to transistor Qi collector.

If DC current is in the reverse direction in switch S, then the description presented hereinbefore equally applies by substitution of transistor Q2 for transistor Ql and substitution of diode D2 for diode Dl. It would then be obvious to the person skilled in the art how switch S operates for a reverse direction of DC current.

Diodes D3 and D4 provide OFF state over-voltage protection for transistor Qi and transistor Q2. Diodes D3 and D4 could be substituted for back to back zener diodes should the level of AC signal to be switched be higher than a diode voltage drop. Capacitor Cl can also be provided in parallel with diodes D3 and D4.

Capacitor Cl provides a path for undesirable high frequency AC signals such as transient or radio frequency signals to bypass the switch. The impedance of capacitor Cl would be high with respect to the impedance of impedance component Z4.

Suitable component values for the detailed embodiment of the switch S are: diodes Dl, D2 and D3, D4 pairs - dual series signal diode type BAV99; PNP transistor type BC857; NPN transistor type BC847; resistor RI - 56R; resistor R2 - 56R; resistor R3 - 100k ohm; and, capacitor Cl - lnF.

Resistors R4, R4A, R5 and R5A, provided across impedance components Zi, ZIA, Z2 and Z2A respectively, are used to improve the longitudinal balance of filter 20. Preferably, resistors R4, R4A, R5 and R5A are 1% resistors and present an accurate limiting impedance across a parallel resonant filter element Zi, Z1A, Z2 or Z2A, that substantially improves the longitudinal balance of filter 20. Obviously, more than one resistor could be provided to achieve a similar effect as resistor R4, R4A, R5 or R5A.

Switch S could be provided as a stand-alone device or incorporated into VF CE or ISDN equipment, such as a telephone, modem, facsimile, etc. Other embodiments are also possible, for example, referring to Fig. 5, additional transistors Q3 and Q4, with resistor R6, can be attached as illustrated. Transistors Q3 and Q4 are connected in parallel and are provided between transistors QI and Q2 and Port 2. An advantage of this arrangement is that transistors QI and Q2 cannot be turned on by low level AC signals, thereby maintaining a high OFF state impedance between Port 1 and Port 3 even when no DC bias is present.

A further particular embodiment is illustrated in Fig. 6, wherein diodes Dl and D2 of Fig. 4 are replaced by transistors Q3 and Q4. When the transistor base maximum current specification is not exceeded, transistors Q3 and Q4 can replace diodes Dl and D2 in the sensing element Z. An advantage of this arrangement is that transistors Ql and Q2 cannot -lo- be turned on by low level AC signals, thereby maintaining a high OFF state impedance between Port 1 and Port 3 even when no DC bias is present. This circuit also has higher sensitivity to low level DC loop currents and the component count is reduced.

Further simplification of the circuit can be made by utilizing the intrinsic voltage limitation of the BE PNP diode in series with the BC NPN diode or BE NPN diode in series with the BC PNP diode. When the transistor base maximum current specification is not exceeded, protection diodes D3 and D4 can be omitted.

Telecommunication companies worldwide are demanding tighter or stricter specifications for DSL filters, mainly to allow the introduction of faster DSL services. The circuit designs illustrated in the figures can provide several important advantages. For example: (1) A single forward diode drop in the line current path is a significant advantage since this reduces the total loop resistance compared with other solid state solutions, and facilitates compliance with major technical specifications; (2) This in turn makes longitudinal balance (earth unbalance) easier to control and results in less circuit cost and complexity; (3) A single diode has a much higher surge current capability than a bipolar transistor base-emitter junction; (4) A single resistor only is needed to control base current of both bipolar transistors, resulting in less circuit cost and complexity; (5) Back to back diodes in the line current path provide robust protection against ring voltage, over-voltages and over-currents; (6) Back to back diodes in the signal path provide robust protection against transients across the switch; (7) Noise associated with zener diodes is avoided; (8) The unreliability and insensitivity of electromechanical switches is avoided; and (9) Semiconductors used are common and relatively inexpensive.

Thus, there has been provided a bidirectional switch for allowing conventional VF CE or ISDN equipment, for example a telephone, to operate simultaneously with a DSL system.

Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Although a preferred embodiment has been described in detail, it should be understood that various changes, substitutions, and alterations can be made by one of ordinary skill in the art without departing from the scope of the present invention.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), andlor all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims (20)

1. CLAIMS: I. A current activated solid state switch, including: an impedance

   component connected between a first node and a second node; a first transistor, an emitter of the first transistor connected to the first node and a base of the first transistor connected to the second node; and, a second transistor, an emitter of the second transistor connected to the first node and a base of the second transistor connected to the second node, and a collector of the first transistor connected to a collector of the second transistor to form a third node.
2. 2. The switch as claimed in claim 1, wherein the impedance component includes a resistor connected in parallel with a diode.
3. 3. The switch as claimed in either claim 1 or claim 2, wherein at least one resistor is connected between the second node and the collector of the second transistor.
4. 4. The switch as claimed in any one of the claims 1 to 3, wherein the switch is a bidirectional switch.
5. 5. The switch as claimed in any one of the claims 1 to 4, wherein the first transistor is a PNP type and the second transistor is a NPN type.
6. 6. The switch as claimed in any one of the claims 1 to 4, wherein the first transistor is a NPN type and the second transistor is a PNP type.
7. 7. The switch as claimed in any one of the claims I to 6, wherein the impedance component includes a resistor connected in parallel with two opposing diodes.
8. 8. The switch as claimed in any one of the claims I to 6, wherein the impedance component includes a resistor connected in parallel with two opposing diodes and at least one transistor.
9. 9. The switch as claimed in claim 8, wherein the impedance component includes two transistors, a base of each transistor connected to the first node, a collector of each transistor connected to the second node, and an emitter of each transistor connected to one side of the two opposing diodes.
10. 10. The switch as claimed in any one of the claims 1 to 6, wherein the impedance component includes a resistor connected in parallel with two transistors, a collector of each of the transistors connected to the second node.
11. 11. The switch as claimed in any one of the claims 1 to 10, wherein in use the first node and the second node are connected to a first wire of a telephone line and the third node is connected to a second wire of the telephone line.
12. 12. The switch as claimed in any one of the claims 1 to 11, wherein the switch is adapted to allow switching between VF CE, ISDN and DSL devices which utilise a telephone line.
13. 13. The switch as claimed in any one of the claims I to 12, wherein the switch is part of an in-line DSL filter.
14. 14. An in-line filter including at least one bidirectional switch, the at least one bidirectional switch including: an impedance component connected between a first node and a second node; a first transistor, an emitter of the first transistor connected to the first node and a base of the first transistor connected to the second node; and, a second transistor, an emitter of the second transistor connected to the first node and a base of the second transistor connected to the second node, and a collector of the first transistor connected to a collector of the second transistor to form a third node.
15. 15. The in-line filter as claimed in claim 14, wherein the first node and the second node are adapted to be connected to a first wire of a telephone line and the third node is adapted to be connected to a second wire of the telephone line.

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16. 16. The in-line filter as claimed in claim 15, wherein a further switch is provided and is adapted to be connected between the first wire and the second wire of the telephone line.
17. 17. The in-line filter as claimed in claim 16, wherein the further switch is connected to alternate sides of the in-line filter relative to the at least one switch.
18. 18. The in-line filter as claimed in claim 14, wherein the in-line filter includes an impedance component in series with the at least one switch and connected to the third node.
19. 19. The in-line filter as claimed in claim 14, wherein the in-line filter includes one or more resistors in parallel with the impedance component whereby an increase in longitudinal balance of the in-line filter is achieved.
20. 20. The in-line filter as claimed in claim 14, wherein the impedance component is a resonant filter element.