PROTECTION DEVICE AGAINST PULSED CURRENTS

Abstract

A protection device against pulsed currents intended to transmit signals having frequencies lying in a transmission frequency band. The protection device has a signal conduction path and a shielding disposed around the signal conduction path. The signal conduction path has two spark gaps mounted in series and an inductor element linking a portion of the signal conduction path situated between the spark gaps. The inductor element is linked to the shielding. The protection device is configured as a high-pass filter allowing passage over the signal conduction path of the signals having frequencies lying within the transmission frequency band.

Claims

1. A protection device (1) against pulsed currents intended to transmit signals having frequencies lying within a transmission frequency band, the protection device (1) comprising a signal conduction path and a shielding disposed around the signal conduction path, the signal conduction path comprising: two spark gaps (4) mounted in series; and an inductor element (5) linking a portion of the signal conduction path situated between the two spark gaps (4) to said shielding; such that the protection device (1) is configured as a high-pass filter allowing passage over the signal conduction path of the signals having frequencies lying within the transmission frequency band.

2. The protection device (1) according to claim 1, further comprising at least one capacitive element (6) mounted in parallel with one said spark gap (4) on the signal conduction path.

3. The protection device (1) according to claim 2, wherein said at least one capacitive element (6) comprises a capacitor having plates separated by a dielectric insulator (13).

4. The protection device (1) according to claim 3, wherein the signal protection path comprises at least one pair of electrodes, each electrode of the pair of electrodes comprising a first surface and a second surface adjacent to the first surface, wherein the first surfaces of the pair of electrodes are positioned facing one another and said spark gap is mounted between the first surfaces of the pair of electrodes, wherein the second surfaces of the pair of electrodes are situated facing one another and said dielectric insulation being mounted between the second surfaces of the pair of electrodes, in such a way that the second portions of the pair of electrodes form the plates of the capacitor.

5. The protection device (1) according to claim 4, wherein each of the electrodes of the pair of electrodes comprises a blind bore, the first surface being positioned at the bottom of the blind bore, such that a meeting of said blind bores forms an inner space housing said spark gap (4), the second surface being positioned around the blind bore.

6. The protection device (1) according to claim 4, having an elongate form in a longitudinal direction, wherein each of the electrodes of the pair of electrodes has a form of revolution about an axis of revolution parallel to the longitudinal direction.

7. The protection device (1) according to claim 4, wherein the inductor element (5) has a central part (14a) and a peripheral part (14b), the central part (14a) being in electrical contact with one said electrode of the pair of electrodes, the peripheral part (14b) being in electrical contact with the shielding.

8. The protection device (1) according to claim 1, wherein the inductor element (5) comprises a coil (14) having a flat spiral form.

9. The protection device (1) according to claim 1, wherein at least one of the two spark gaps (4) comprises: an insulating jacket (15) delimiting an inner space and having two apertures respectively at two opposite ends of the inner space; two spark-gap electrodes (16) closing the two apertures of the inner space in a gastight manner, each spark-gap electrode comprising an inner part (16a) housed in the inner space of the insulating jacket (15) and an outer part (16b) accessible from the outside of the insulating jacket (15), the inner part (16a) having an end surface (17), the end surfaces (17) of said spark-gap electrodes (16) being positioned facing one another so as to delimit between them an air gap (18); and an inert gas captive in the inner space of the insulating jacket (15).

10. The protection device (1) according to claim 9, wherein the insulating jacket (15) is made of ceramic.

11. The protection device (1) according to claim 9, wherein the seal-tightness between the spark-gap electrodes (16) and the insulating jacket (15) is produced by brazing.

12. The protection device according to claim 10, wherein the ends of the insulating jacket (15) comprise a layer (19) of an alloy of iron and nickel, the seal-tightness between the spark-gap electrodes (16) and the insulating jacket (15) being produced by brazing.

13. The protection device according to claim 9, wherein the spark-gap electrodes (16) are made of a metal chosen from the group composed of copper and alloys thereof.

14. The protection device (1) according to claim 9, wherein the gas captive in the insulating jacket (15) is chosen from the group composed of argon, neon, hydrogen, nitrogen, rare gases and mixtures of these gases.

15. The protection device (1) according to claim 1, further comprising two terminal connectors (30) for coaxial cable (3), each terminal connector (30) comprising a peripheral conductive portion (30a) intended to be linked to the peripheral shielding of a coaxial cable (3) and a central conductive portion (30b) intended to be linked to the central core of a coaxial cable (3), wherein the signal conduction path is in electrical contact with the central conductive portion (30b) of each of the terminal connectors (30), and wherein the shielding is in electrical contact with the peripheral conductive portion (30a) of each of said terminal connectors (30).

Description

BRIEF DESCRIPTION OF THE FIGURES

[0041] The invention will be better understood and other aims, details, features and advantages thereof will become more clearly apparent from the following description of several particular embodiments of the invention, given purely in an illustrative and nonlimiting manner, with reference to the attached drawings.

[0042] FIG. 1 is a schematic representation of a coaxial cable radiofrequency signal transmission system comprising a protection device according to a first embodiment of the invention.

[0043] FIG. 2 is a schematic representation similar to FIG. 1 comprising a protection device according to a second embodiment of the invention.

[0044] FIG. 3 is a perspective schematic view of the protection device according to the second embodiment of the invention.

[0045] FIG. 4 is a perspective schematic view of the protection device represented in FIG. 3, the body being omitted.

[0046] FIG. 5 is a schematic view in cross-section on a plane at right angles to the longitudinal axis of the protection device represented in FIG. 3.

[0047] FIG. 6 is a schematic view of a spark gap that can be used in the protection device represented in FIG. 3, according to the first embodiment.

[0048] FIG. 7 is a schematic view similar to that of FIG. 6, according to a second embodiment.

[0049] FIG. 8 is a graphic representation of the return loss as a function of the frequency of the protection device according to the second embodiment of the invention.

[0050] FIG. 9 is a graphic representation of the insertion loss as a function of the frequency of the protection device according to the second embodiment of the invention.

DETAILED DESCRIPTION

[0051] The embodiments hereinbelow are described in relation to a protection device intended to limit the transient overvoltages and overcurrents in a coaxial cable radiofrequency signal transmission system.

[0052] Referring to FIG. 1, a protection device 1 is installed on a coaxial cable bidirectional transmission line 3, for example used for the reception or the transmission of radiofrequency signals lying within a given operating frequency band. In particular, the peripheral shielding of the coaxial cable can serve as earth potential. The protection device 1 is generally incorporated in a coaxial coupling comprising two terminal connectors 30 intended to be interposed on the coaxial cable transmission line 3. More details on such a coaxial coupling can be found in the patent application FR-A-3061813.

[0053] The coaxial cable transmission line 3 can belong to a telecommunication network incorporating equipment to be protected (not represented), for example radiocommunication equipment in CDMA, GSM/UMTS, WiMAX or TETRA base stations.

[0054] Some events can provoke the flow of high pulsed currents on the coaxial cable transmission line 3, which take the form of abrupt overvoltages and overcurrents over a brief instant. Now, such increases in the voltage and/or the intensity of the electrical current can cause transmission interruptions, even damage the equipment linked to the coaxial cable transmission line 3.

[0055] To limit the transient overvoltages and overcurrents, the protection device 1 diverts to the earth, via the peripheral shielding, the pulsed current discharge induced in the coaxial cable transmission line 3.

[0056] The protection device 1 comprises a signal conduction path arranged electrically on the central core of the coaxial cable transmission line 3 and a shielding in electrical contact with the peripheral shielding of the coaxial cable transmission line 3.

[0057] The signal conduction path comprises two spark gaps 4 mounted in series, and an inductor 5 linking a portion of the signal conduction path situated between the two spark gaps 4 to the shielding.

[0058] In normal operating conditions, i.e. in the absence of transient overvoltages or overcurrents, the radiofrequency signals are transmitted without loss of integrity in the coaxial cable transmission line 3.

[0059] On the one hand, the spark gaps 4 have very low capacitance values such that the protection device 1 operates as a high-pass filter which blocks the direct current flows and the low frequencies but allows the radiofrequency signals to pass. In dimensioning, for the typical characteristic impedance of 50 , spark gaps 4 exhibit capacitance values of the order of 5.3 pF and an inductance 5 exhibiting inductance values of the order of 6.6 nH, the protection device 1 allows a cut off frequency of the order of 600 MHz.

[0060] As a variant, if the capacitance values of the spark gaps 4 are too low to be compatible with the operating frequency band of the radiofrequency signal transmission system, capacitive elements 6 can be mounted in parallel with the coaxial cable transmission line 3, as represented in FIG. 2. In dimensioning, for the typical characteristic impedance of 50 , a pair of spark gaps 4 exhibiting capacitance values of the order of 0.7 pF and a pair of capacitive elements 6 exhibiting capacitance values of the order of 63 pF and an inductance 5 exhibiting inductance values of the order of 79.6 nH, the protection device 1 allows a cut off frequency of the order of 50 MHz.

[0061] On the other hand, the inductance 5 is configured to exhibit a very high impedance to the radiofrequency signals, in particular in the operating frequency band of the transmission system such that the protection device 1 insulates the central core of the coaxial cable transmission line 3 from the peripheral shielding serving as a ground potential.

[0062] Conversely, in the event of transient overvoltages or overcurrents induced in the central core of the coaxial cable transmission line 3, for example under the effect of lightning, the pulsed current generated is diverted to the peripheral shielding serving as a ground potential, which makes it possible to protect the equipment linked to the coaxial cable transmission line 3.

[0063] For example, when lightning strikes the coaxial cable transmission line 3, a strong pulsed current characterized by a direct current flow and low-frequency electromagnetic waves is propagated along the coaxial cable transmission line 3 to reach the signal conduction path of the protection device 1. One of the two spark gaps 4 is subjected to a transient overvoltage whose value exceeds a certain threshold corresponding to a spark-over voltage of the spark gap 4. Advantageously, the spark-over voltage is chosen to be a little greater than the nominal operating voltage of the coaxial cable transmission line 3. The spark gap 4 then sparks over suddenly, and becomes conductive with a very low resistance such that it behaves as a closed switch. Downstream of the spark gap 4, the inductor 5, which has a zero impedance in terms of direct current and very low impedance at low frequencies, provokes a short circuit diverting the pulsed current generated by the lightning to the shielding.

[0064] Referring to FIG. 3, the protection device 1 takes the form of a rectangular body 7, for example made of brass, developing along a longitudinal axis X between two ends 8. The body 7 forms the shielding of the protection device 1. At each end 8, the protection device 1 comprises a terminal connector 30 to couple the protection device 1 to the coaxial cable transmission line 3. The terminal connectors 30 are of generally cylindrical form about the longitudinal axis X. Each terminal connector 30 comprises a peripheral conductive portion 30a intended to be linked to the peripheral shielding of the coaxial cable 3 and a central conductive portion 30b intended to be linked to the central core of the coaxial cable 3. The body 7 forming the shielding of the protection device 1 is in electrical contact with the peripheral conductive portion 30a of each of the terminal connectors 30.

[0065] Referring to FIGS. 4 and 5, the body 7 of the protection device 1 is hollow and forms a cylindrical internal housing 10 for two pairs of electrodes 11a, 11b, two spark gaps 4 and an inductor 5. The pairs of electrodes 11a, 11b, the spark gaps 4, the inductor 5 and the terminal connectors 30 are coaxial.

[0066] Each electrode of a pair of electrodes 11a, 11b comprises a body with symmetry of revolution of flared form between two opposite ends. One end of the body of the electrode has a second electrode surface comprising a blind bore 12a, 12b with a flat bottom. The flat bottom forms the first electrode surface. In the internal housing 10, the first and second electrodes of a pair of electrodes 11a, 11b are arranged so as to position the first and the second electrode surface of the first electrode 11a facing, respectively, the first and the second electrode surface of the second electrode 11b. The meeting of the bores 12a, 12b of the first and second electrodes 11a, 11b forms an inner space dimensioned to accommodate a spark gap 4. Each pair of electrodes 11a, 11b thus grips a spark gap 4 in electrical contact with the first electrode surface at the bottom of the bores 12a, 12b.

[0067] A flat annular seal 13 made of polytetrafluoroethylene is inserted between the facing electrode surfaces of the first and second electrodes 11a, 11b. Each electrode surface forms the conductive plate of a capacitor mounted, by construction, in parallel with the spark gap 4 gripped in the pair of electrodes 11a, 11b. As a variant, materials other than polytetrafluoroethylene can be used to separate the conductive plates.

[0068] Each pair of electrodes 11a, 11b is housed in a respective part of the internal housing 10 representing half the internal housing 10. For each pair of electrodes 11a, 11b, the end without the bore 12a of the first electrode 11a is in electrical contact with the central conductive portion 30b of a terminal connector 30, and the end without a bore 12b of the second electrode 11b is in electrical contact with the inductor 5. The detail of the terminal connectors 30 is not represented in FIG. 5.

[0069] The inductor 5 comprises or consists of a circular flat spiral coil 14 having a central part 14a and a peripheral part 14b. As a variant, the spiral coil 14 can have a polygonal form (e.g. square, hexagonal, octagonal, etc.) or any other form. The spiral coil 14 is positioned in the middle of the internal housing 10 in the plane at right angles to the longitudinal axis X. The central part is in electrical contact with the second electrode of each pair of electrodes (as indicated above) while the peripheral part is in electrical contact with the body 7 of the protection device 1. The spiral coil 14 is thus mounted between the two spark gaps 4 and linked via the body 7 forming the shielding of the protection device 1 to the peripheral shielding of the coaxial cable 3.

[0070] Thus, the central conductive portion 30b of the terminal connectors 30, the pairs of electrodes 11a, 11b and the central portion 14a of the spiral coil 14 form the signal conduction path of the protection device 1.

[0071] Advantageously, to limit the phenomena of passive intermodulation of the protection device 1, the spiral coil 14 is made from a non-magnetic metal or from an alloy of non-magnetic metals, preferably an alloy of copper and of beryllium. Indeed, the ferromagnetic metals, such as iron or nickel, generally used in the known spark gap coaxial surge arrestors, exhibit nonlinear characteristics which generate distortions by intermodulation of the radiofrequency signals.

[0072] Referring to FIGS. 6 and 7, the spark gap 4 comprises an insulating jacket 15 of hollow cylindrical form developing between two ends along the longitudinal axis X. The insulating jacket 15 delimits an inner space of the spark gap emerging on two apertures situated respectively at the two opposite ends of the inner space of the spark gap 4. Advantageously, the insulating jacket 15 is made of ceramic.

[0073] The spark gap 4 also comprises two spark-gap electrodes 16. Advantageously, the spark-gap electrodes are made of copper or from an alloy of copper. Each spark-gap electrode 16 comprises an inner part 16a housed in the inner space of the insulating jacket and an outer part 16b protruding outside of the insulating jacket. As illustrated in FIG. 5, the outer part 16b is in electrical contact with the flat bottom of the bore 12a, 12b of an electrode 11a, 11b of the protection device 1. The inner part 16a has a flat end surface 17.

[0074] In the inner space of the spark gap 4, the end surfaces 17 of the two spark-gap electrodes 16 are positioned facing one another so as to delimit between them an air gap 18. The distance separating the end surfaces 17 of the spark-gap electrodes 16 makes it possible to define the spark-over voltage. When the voltage at a spark-gap electrode 16 reaches the spark-over voltage, an electrical current occurs between the spark-gap electrodes 16, forming an electrical arc. The spark gap 4 becomes conductive with a very low resistance which allows the passage of the pulsed current, then conveyed by the spiral coil 14 to the shielding of the protection device 1.

[0075] In order to limit the holding time or to stop the electrical arc between the spark-gap electrodes 16, an inert gas is captive in the insulating jacket 15, which includes the air gap 18. Such an inert gas is for example argon, neon, hydrogen, nitrogen, a rare gas, or a mixture of these gases. This inert gas is kept in the spark gap 4 at low pressure, for example at a pressure of 50 mbar. This low pressure affects the value of the spark-over voltage of the spark gap 4. The gas can be captive in the spark gap 4 at different pressures, depending on the spark-over voltage desired for the spark gap 4.

[0076] In order to ensure that the inert gas is captive in the spark gap 4, the inner space is sealed. As illustrated in FIG. 6, the seal-tightness of the inner space can be produced by hermetically sealing, through a rapid thermal cycle, the outer part 16b of the spark-gap electrode 16 on an open end of the insulating jacket 15. Advantageously, for a spark-gap electrode 16 made of copper and an insulating jacket 15 made of ceramic, the seal-tightness can be produced by a thermal cycle of 30 min at a maximum temperature of 1120 K.

[0077] As a variant, as illustrated in FIG. 7, a metal layer 19 can be used to cover the ends of the insulating jacket 15. The seal-tightness between the layer 19 and the outer part 16b of the spark-gap electrode 16 and between the layer 19 and the end of the insulating jacket 15 is for example produced by brazing. Advantageously, when the insulating jacket 15 is made of ceramic, the layer 19 is made of an alloy of iron and nickel, which exhibits an expansion coefficient very close to the expansion coefficient of ceramic. Thus, the layer 19 and the insulating jacket 15 expand and contract similarly such that the forces that they exert on one another in contraction or in expansion do not risk damaging the insulating jacket 15.

EXAMPLE

[0078] An example of the protection device 1 as described with reference to FIGS. 3 to 6 has been implemented with the following dimensioning: [0079] characteristic impedance of the order of 50 ; [0080] two spark gaps 4 each having a capacitance of the order of 0.7 pF; [0081] two capacitive elements 6 each having a capacitance of the order of 30.5 pF; [0082] an inductor 5 having an inductance value of the order of 28.5 nH.

[0083] The return loss, abbreviated RL, measured in decibelsquantifies the power loss of the signal reflected by a discontinuity in a transmission line. For a given frequency, the greater the return loss, the higher the performance levels of the transmission line. In particular, for a protection device intended to limit the transient overvoltages and overcurrents in a coaxial cable radiofrequency signal transmission line, losses of 20 dB or more are desirable in the transmission frequency band.

[0084] In FIG. 8, the graph 20 represents the return loss (dB) as a function of the frequency (MHz). The protection device according to the example exhibits a return loss varying between 20 dB and 50 dB in a transmission frequency band lying between 0.4 GHz and 2.7 GHz.

[0085] The insertion loss, abbreviated IL, measured in decibelsquantifies the power loss of the input signal with respect to that of the output signal resulting from the insertion of a device in a transmission line. For a given frequency, the lower the insertion loss, the higher the performance levels of the transmission line. In particular, for a protection device intended to limit the transient overvoltages and overcurrents in a coaxial cable radiofrequency signal transmission line 3, losses of 0.1 dB or less are desirable in the transmission frequency band.

[0086] In FIG. 9, the graph 21 represents the variation of the insertion loss (dB) as a function of the frequency (MHz). The protection device according to the example exhibits an insertion loss varying between 0 dB and 0.05 dB in a transmission frequency band lying between 0.4 GHz and 2.7 GHz.

[0087] Although the invention has been described in relation to several particular embodiments, it is perfectly clear that it is in no way limited thereto and that it encompasses all the technical equivalents of the means described and the combinations thereof if they fall within the context of the invention.

[0088] The use of the verb comprise or include and its conjugate forms does not preclude the presence of elements or steps other than those stated in a claim.

[0089] In the claims, any reference symbol between parentheses should not be interpreted as a limitation on the claim.