WO2006031158A1

WO2006031158A1 - Over-protection circuit for indoor digital signal communication

Info

Publication number: WO2006031158A1
Application number: PCT/SE2004/001344
Authority: WO
Inventors: Håkan SÖDERBERG
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2004-09-16
Filing date: 2004-09-16
Publication date: 2006-03-23

Abstract

The present invention relates to over-voltage protection devices and solves the problem of providing a flexible and easy-installed over-protection circuit which supports a desired single point grounding requirement and which protects an indoor installed electronic equipment from an over-voltage on a shield or conductor of a cable directly or in-directly connected to external equipment irrespective if coaxial or twisted pair cable is pre-installed in a construction or building. This is achieved by an gas discharge tube arrangement (20) and a transformer device set (30). The gas discharge arrangement (20) separates the transformer device set (30) and each of said cable conductor and cable shield from said single point ground (50) as long as a surge voltage of the gas discharge tube arrangement (20) is not exceeded.

Description

Over-protection circuit for indoor digital signal communication

TECHNICAL FIELD

The present invention relates to protection circuits .

More specifically, the present invention relates to an over-voltage protection circuit.

BACKGROUND OF THE INVENTION

Today there exists over-voltage protection (OV) connected to signal cables for analogue communications for indoor installations in different Distribution frames. This over-voltage protection is earthed/ grounded directly towards building earth/ ground and separated from other hardware earth /ground. For signal cables for analogue communications there is no shield, so there is OV protection only connected to the wire, i.e. conductor.

There is also OV protection in connection with digital signal communications cables for outdoor installations in the input to containers and cabinets, such as an antenna that is connected to a base station installation via a coaxial cable. The wire is here in open contact with the antenna and OV protection is therefore needed for protecting the indoor equipment installations. The shield can in this situation be grounded/ earthed directly towards building earth and needs no separate OV protection.

It is also well-known to connect the OV protection to the shield in the digital signal communications cables. In this situation the cable shield is not opened. It is therefore no need to have OV protection connected to the cable wire, since the cable wire is protecting the shield.

Today many indoor equipment is installed having single-point grounding. The indoor installations are connected to the building earth through one or a number of Main Earth Terminals (MET), which is a tree- structure of the ground connection conductor, instead of a ring structure, which is multi- point connection. One exception is the Digital Distribution Frame (DDF), which is not allowed to be connected directly to MET due to the occurrence of Electro Magnetic Currents (EMC).

An DDF is a contact interface, e.g. a rack entity or unit comprising contact devices, for providing cross-connections and a coupling point for defining the interface between different equipment installation providers. An electric potential difference higher than 2500 V, usually higher than 500 V, is defined as an over-voltage according to a desirable over-voltage requirement for equipments in central office environment.

For digital communications over signal cables connected to indoor installations, such as central office environment, it is desirable to have OV protection connected both to the shield and the wire. It is desirable to provide the shield of each cable with an OV protection as there quite often is a problem to connect the shield to the same grounding as the installed in-door equipment, because the external/ out-door cables, which is connected to external/ out-door equipment, may be situated in a location having another grounding point or MET. Another problem is that the shield must be cut and opened due to cross-connections or due to change from outdoor to indoor cables.

The OV protection principle for analogue signal communication cannot be used for digital installations since only the cable wire is connected to earth.

The digital over-voltage protection for outdoor cabinets and containers should not be used for indoor installation, since multipoint grounding is used in outdoor cabinets and containers. Multipoint grounding implements risk for introducing disturbing noise, high voltages, high currents, etc. Therefore, multipoint grounding is not desirable to install in indoor sites due to the distance to the building earth. BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to solve the problem of providing a flexible and easy-installed over-protection circuit which supports the desired single point grounding requirement and which protects an indoor installed electronic equipment, especially in central office environment, from an over-voltage on a shield or conductor of a cable directly or in-directly connected to external equipment irrespective if coaxial or twisted pair cable is pre-installed in a construction.

These objects are achieved by the claimed invention according to claims 1-

9.

One advantage with the present invention is that the invention provides lightning and power induction protection.

Yet another advantage is that the present invention blocks ground currents to enter connected equipment via the external cable.

Further, the present invention enables connection of both twisted pairs and coaxial cable.

Another advantage is that the invention can be used on cables enabling high digital signalling bit rates of at least 34 Mb/s.

Additionally, one advantage is an optional supervision function indicates if internal protection components starts to wear out.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic illustration of a over-voltage protection circuit according to the invention.

Fig. 2 is an exploded view of an over-protection module according to the present invention. Fig. 3 shows a module according to figure 2 mounted in a frame structure, i.e. a rack.

Fig. 4 is a schematic illustration of an in-door installation comprising the present invention. Fig. 5 is a wiring scheme for an integrated connector solution which is possible to use in combination with a OVP circuit according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to over-voltage protection devices.

Fig.1 shows an over-voltage protection (OVP) circuit 10 according to a first embodiment of the invention. The circuit 10 has one set of external cable connectors 48A and one set of internal cable connectors 48B. The number of connectors in the external cable connector set is the same as in the internal cable connector set. Each one of the connectors comprises at least one conductor connection terminal 1-6, preferably one for each of the conductors or wires to be connected to the circuit 10. The number of connection terminals in a connector, such as an integrated connector, is limited.

The preferred embodiment of the invention is designed for providing alternative cable connection options regarding coaxial and twisted pair cables, i.e. connecting coaxial cable to coaxial cable, twisted pair cable to twisted pair cable and coaxial cable to twisted pair cable or vice versa.

Coaxial cable has one central wire 7 (see figure 4) or conductor 7 surrounded by a woven metal thread shield 8 (see fig. 4) for eliminating electromagnetic radiation. The twisted pair cable has also a woven metal thread shield 8, but contain a twisted pair of wires or conductors 7 instead of one central conductor. One of the wires in the twisted pair is a return path. The shield of the coaxial cable has said return path function. The preferred embodiment is designed for indoor/in-house installed digital signal communication systems, e.g. communication equipment, such as Radio Base Stations (RBS) etc, comprising at least one transceiver and at least one receiver for radio communication. Therefore, one cable is selected for transmitting signals and one cable for receiving signals via external antennas. The option to chose between alternative cable types, i.e. coaxial or twisted pair cable, requires that each of the two sets described above comprises four cable plug connectors, two for coaxial cables and two for twisted pairs, and four shield terminals for connecting the shield of each cable.

Each cable will be provided with a cable plug, which fits into a cable plug connector 48 (see fig. 2) attached to the OVP circuit 10. It's possible to use one kind of cable plug and cable plug connector for twisted pair cable and a different kind of cable plug and corresponding cable plug connector for coaxial cables. However, if a multiple-pole/multi-terminal connector is used, the same kind of multiple-pole connector may be used independently of cable type, i.e. integrating possibility to use different cable types . In the present embodiment, such an integrated cable plug with a corresponding integrated cable plug connector with at least six poles or connection terminals is necessary. A wiring scheme has to be decided for avoiding unwanted cross-connections. Such a wiring scheme is illustrated in fig. 5. Poles no.1 and no. 6 may be reserved for twisted pair cable tpl, poles no. 3 and no. 4 for twisted pair cable tp2, pole no. 2 for the central conductor of coaxial cable coaxl and pole no.5 for the central wire of coaxial cable coax2. If cables tpl and coaxl always are used as transmitter cables, cables tp2 and coax2 are used as receiver cables.

Pole no. 1 of the cable plug connector of the external cable connector set is connected via a circuit board wiring or printed circuit conductor 9 (see fig. 1) to pole no. 1 in the cable plug connector of the internal cable connector set. In the same way, poles no. 2-6 in the cable plug connector of the external cable connector set are connected to it's corresponding pole in the cable plug of the internal cable connector set.

Returning to Fig. 1, the over-voltage protection of the circuit 10 will now be described in more detail. As mentioned above, it is desirable to provide the shield of each cable with an OV protection as there quite often is a problem to connect the shield to the same grounding as the installed in-door equipment, because the external/ out- door cables, which is connected to external/ out-door equipment, may be situated in a location having another grounding point or MET. Another problem to be addressed is that the shield must be opened due to cross-connections or due to change from outdoor to indoor cables, i.e. external and internal cables, respectively.

As described above, poles no. 1-6 and the corresponding terminals in the cable plug connector of the external cable connector set are connected via a signal conductor 9, implemented as a circuit board wiring or printed circuit conductor etc, to it's corresponding terminals and poles 1-6, respectively, in the cable plug of the internal cable connector set. Each signal conductor is OV protected through a Gas Discharge Tube 11 (LV(Low-Voltage)-GDT) in a Gas Discharge Tube arrangement 20. A Gas Discharge Tube 11 is a well- known electrical component having three connection terminals, isolated from each other by a gas having a characteristic surge voltage. Each GDT 11 has a surge voltage and the GDT is not conducting as long as said surge voltage is not exceeded. Any GDT having a preferred surge voltage may be chosen depending on purpose and protection requirements.

The conductors 7 of a twisted pair cable can share one GDT 11, but connected to different terminals of the GDT. For coaxial cables, the central conductor 7 and the shield 8, which has the function of a return path or return conductor in a coaxial cable, can share one GDT 11. The twisted pair cable shields 8 must also be connected to the protection arrengement and further one separate GDT may be added to the GDT arrangement 20, i.e. one separate GDT for each pair of twisted pair cables. However, by connecting all shields of the external cables to a common shield connection, it's possible to use the GDTs already having shield connections to the coaxial cable shields even though the shields of the coaxial cables function as return paths. The number of GDTs is optimized to a minimum of GDTs. Shields and conductors is not cross-connected over the GDT:s as the terminals of a GDT are isolated from each other by the gas in the GDT.

The shields of the external cables are connected to shield terminals J1-J4, respectively, connected to the common shield connection 16. The common cable shield connection or the external cable wires 7 are not directly connected to the local equipment installation 46 (in fig. 4) and/ or to the protection ground/ earth. The external cable shields 8 and the wires 7 are connected to a local protection ground 49 via the GDT arrangement 20 of the OVP circuit 10.

The OV protection circuit 10 according to the invention provides also the feature of alternative cable connection option to be able to alternate between coaxial cables and twisted pair cables. Said feature is accomplished by one transformer device set 30 for each cross-connection between coaxial and twisted pair cables. A transformer device makes it possible to connect a twisted pair cable having one impedance value and a coaxial cable having another impedance value to each other. For instance, coaxial cables have 75 ohm impedance and the twisted pair cables 120 ohm. In this embodiment, the transformer device 30 is a balun, but other transformer devices is possible to use.

One embodiment of the invention comprises an over-current device 24 which is coupled in parallel with the GDT arrangement 20 between the wires /conductors 7 and shields 8 of the external cables 40,54 and the local grounding 49. The over-current device 24 is here exemplified as a Direct Current-barrier, DC-barrier 24, comprising a resistor (24a) and a capacitor (24b), which are connected in parallel between the wires/ conductors 7 and shields 8 of the external cables 40,54 and the local grounding 49. The shields of the internal cables are connected to shield terminals of the internal cable connector set, which are connected to said DC-barrier 24. In this way, both the shields of the external cables and the internal cables may be connected for preventing any ground currents to harm and damage the equipment installations 46.

Further, slow acting high current fuses 22 are connected to the circuit board wires 9 or printed circuit conductors 9.

In another embodiment of the invention, each circuit board wiring 9 or printed circuit conductor 9 is provided with a Positive Temperature Coefficient (PTC) resistor 18 for protection against over-currents. The resistance of a PTC resistor 18 is increasing with increasing voltage over the resistor.

Yet another embodiment of the invention may comprise a supervision function circuit which checks the condition of the internal protection components, e.g. indicates if any of the GDTs 11 is not functioning properly due to age.

In figure 2, an OVP circuit 10 according to the invention is illustrated. The OVP circuit 10 according to the invention comprises a number of components: cable plug connectors 48 , baluns, GDTs and DC-barrier components. All said components are mounted on a carrying structure, such as a circuit board 26. Optional components, like PTC resistors, slow acting high current fuses and components for the supervision circuit, may be arranged on the same circuit board as the first mentioned components. The circuit board has one top surface on which the cable plug connectors are mounted, and one opposite surface, i.e. underside. All connectors 48 (48A and 48B) are arranged along one side of the circuit board. The cable plug connector's outer plug receiving part is extending outside the edge of the circuit board side. Some components may be arranged and attached on the underside of a circuit board 26. A favourable arrangement of components is to attach them on one side of the centre line between the centres of the long sides of the underside surface and leave the other half empty. Said arrangement offer the possibility to use two circuit boards 26 instead of one and mount two circuit boards with their undersides turned towards each other in a kind of sandwich design. This is a compact design. The components for two pairs of cables, where one cable of each pair is connected to an output plug connector and the other to an input plug connector, are arranged on each circuit board. Distance organs attached to the undersides will hold the circuit boards together on a determined distance. The cable plug connectors for in- and output cables, both external and internal cables, will be directed in the same direction. A common earth connector is arranged and attached on the opposite long sides of the circuit boards, directed in the opposite direction. This circuit board sandwich 26 is mounted to a front panel 44 with the cable plug connector's plug receiving part extending out through corresponding holes in the front panel. The circuit board sandwich 26 may be fastened to the front panel by screws. The circuit board sandwich 26 is inserted in an isolating housing 46 through an open side, and said panel is fastened by means of fastening organs to the isolating housing 46. When the panel 44 has been fastened to the housing 46, the common earth connector is extending through a hole in the wall of the housing that is opposite to the panel. The housing with the attached circuit board carrying panel form an OVP module. Any electromagnetic radiation will not be able to pass the through the walls of the module. A grounding wire 62 (see fig. 3) is attachable to the common earth connector.

In figure 3, an OVP module 60 according to the invention is attached in a distribution frame 70, e.g. a rack. Said rack is able to receive and carry a number of OVP modules according to the invention. Figure 4 is a schematic illustration of an in-door installation of the present invention connected between internal Radio Base Station equipment and external Radio Base Station equipment, e.g. an antenna 42. Even though the shields 8 is not fully covering the cable conductors 7 in the illustration, it is understood that the shields in a implemented embodiment of the invention is covering and protecting the cable conductors so that no leakage of electromagnetic radiation or other noise is possible to leave or influence the cable conductors 8. A digital distribution frame DDF 52 has been installed by e.g. the building constructor. The antenna 42 is connected to the DDF 52 via a shielded cable 54. Signals are carried via an external cable 40 between the DDF 52 and the over- voltage protection circuit 10 according the present invention. The external cable 40 may be a coaxial or twisted pair cable for high bit rates of at least 34 Mb/s. The OV circuit 10 comprises the gas discharge tube arrangement 20, here illustrated as switches, which will start conducting and connect the over-voltage and over-current on the shield and/ or the conductor or conductors of the external cable via a ground wire 56 or earth cable to a grounding point 49, here a Main Earth Terminal (MET), which is connected to the ground/ earth 50, here the building earth, via a ground bar 80. It is important to realise, that in normal operation, the GDTs of the GDT arrangement 20 is not conducting and the shield of the external cable is connected to the shield of an internal cable 44 and the conductor/ conductors of the external cable 40 is connected to the conductor/ conductors of the internal cable. The internal cable may be selected to be either coaxial cable or twisted pair cable irrespective of the external cable, which either is coaxial or twisted pair cable. This is possible due to the transformer devices (not shown), e.g. baluns, in the OV circuit. The internal cable 44, constituting a cable pair with the external cable 40, is connected between the OV circuit 10 and in- door installation equipment 46 of a telecommunication node, e.g. comprising a Radio Node Controller and Media Gateway. A power source 58 is feeding the node. The DDF 52 is protected via a ground wire 62 to the node, i.e. the in-door installation equipment. Said power source and the in¬ door installation equipment 46 are coupled via a Main Earth Terminal 49 (MET) to the same earth/ ground, which therefore is a Single-point.

If an over- voltage that is exceeding the surge voltage of the GDT: s is introduced, either on the shield or the conductor(s) of a cable, the GDT 11, which the shield or the conductor(s) is connected to, will start to conduct and the over-voltage and over-current will be conducted to earth, which may be other than the single-point ground.

The above-described invention solves the problem of providing a flexible and easy-installed over-protection circuit which supports the desired single point grounding requirement and which protects an indoor installed electronic equipment from an over-voltage on a shield or conductor of a cable directly or in-directly connected to external equipment irrespective if coaxial or twisted pair cable is pre-installed in a construction or building.

The present invention is not limited to the above-described preferred embodiments. Various alternatives, modifications and equivalents may be used. Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appended claims.

Claims

PATENT CLAIMS

1. An over- voltage protection circuit (10) arranged for serial connection between at least one pair of cables (tpl-tpljtpl-coaxljcoaxl-tpljcoaxl- coaxl;tp2-tp2; tp2-coax2;coax2-tp2;coax2-coax2), said pair comprising an external cable (40) connected to a device of an external equipment (42) and to a corresponding internal cable (44) connected to indoor equipment installations (46) protected by a single point ground (50), said circuit (10) having separate cable connectors (48) for coaxial cables and twisted pair cables, said cables comprising at least one conductor (1,2,3,4,5,6,7,8) and one shield (J1,J2,J3,J4), characterized in that the over-voltage circuit (10) comprises a gas discharge tube arrangement (20) and a transformer device set (30), each cable conductor and cable shield of the at least one external cable (40) is connected to the gas discharge tube arrangement (20) and to a dedicated transformer device (30) of said set, wherein the gas discharge arrangement (20) separates the transformer device set (30) and each of said external cable conductor and cable shield from a local ground (49), which may be other than said single point ground (50), as long as a surge voltage of the gas discharge tube arrangement (20) is not exceeded, and each dedicated transformer device (30) is connected between a cable connector (48) for a coaxial cable and a cable connector (48) for a twisted pair cable.

2. An over- voltage protection circuit according to claim 1, characterized in that each shield of an external cable is connected to a common external shield connection (16) of said circuit (10).

3. An over- voltage protection circuit according to claim 1 or 2, characterized in that said circuit comprises one or more Positive Temprature Coefficient (PTC) resistors (18) connected between at least one cable pair.

4. An over- voltage protection circuit according to any of claims 1-3, characterized in that said circuit comprises one or more slow acting high current fuses (22) connected between at least one cable pair.

5. An over-voltage protection circuit according to any of claims 1-4, characterized in that the circuit comprises an over-current protection device (24), which is arranged in parallel with the gas discharge tube arrangement (20) between the external cables (40,54) and a local ground (49, MET).

6. An over-voltage protection circuit according to any of claims 1-5, characterized in that said circuit is arranged on a circuit board (26).

7. An over- voltage protection circuit according to any of claims 1-6, characterized in that the circuit is enclosed in a isolating housing (46) having a panel (44) through which cable plug connection devices (48) is extending.

8. An over- voltage protection circuit according to any of claims 1-7, characterized in that gas discharge tube arrangement (20) comprises at least one 3-pole/3-terminal gas discharge tube (11) having input terminals which is isolated from each other.

9. An over-voltage protection circuit according to any of claims 1-8, characterized in that the transformer device set (30) comprises at least one balun having four connection terminals.