Filter, duplexer and communication device

Abstract: A filter (10) includes a dielectric resonator (20), a cavity (30) for holding the dielectric resonator (20) therein, an external connector (13) mounted on the cavity (30), and a coupling loop (12) connected to the external connector (13) so as to electromagnetically couple with the dielectric resonator (20). The coupling loop (12) is formed by bending a metal plate nearly into the shape of an L, and is provided with a rib (14) extending in a direction that is not parallel to the bending line.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter having a coupling loop, to a duplexer, and to a communication device. 2. Description of the Related Art As FIG. 10 shows, a conventional filter 110 comprises a dielectric resonant device 120, metal panels 111 having an external connector 113 that serves as an input-output connecting means and covers open portions of the dielectric resonant device 120, and a coupling loop 112. The dielectric resonant device 120 includes a frame 121 and a dielectric resonator 122 that are made of ceramic. The frame 121 is shaped like a parallelepiped with two opposing surfaces being open, and is provided with conductors 123 thereon. The dielectric resonator 122 is shaped like a rectangular parallelepiped, and is disposed inside the frame 121 so that its two opposing surfaces are integrated with the frame 121. The metal panels 111 are made of metal, such as iron or a nickel alloy, in order to achieve good electrical conductivity and to make the coefficient of linear expansion thereof the same as that of a dielectric. These metal panels 111 are connected to the conductors 123 of the dielectric resonant device 120, whereby a cavity 130 is formed as a whole. The coupling loop 112 is made of copper in view of electrical conductivity and rust prevention, and worked into the shape of an L. One end of the coupling loop 112 fits in a hole that is previously formed through the metal panel 111, and is fixed by soldering or the like. The other end of the coupling loop 112 is connected to the external connector 113. Since this other end of the coupling loop 112 is also worked into a corrugated shape, it can, for example, absorb impact that is applied from the side of the external connector 113. This has solved problems, for example, deformation of the coupling loop 112 due to stress from the outside, and separation of the coupling loop 112 from the metal panel 111. In the filter 110 mentioned above, current applied from the outside flows in the coupling loop 112 via the external connector 113. The current that flows through the coupling loop 112 generates a magnetic field, and this magnetic field couples with the dielectric resonator 122. In this case, the degree of coupling between the coupling loop 112 and the dielectric resonator 122 is adjusted based on the length, thickness, and width of the coupling loop 112 or the distance between the coupling loop 112 and the dielectric resonator 122. Such adjustment of the degree of coupling allows a filter having the required electrical characteristics. A coupling loop has its own natural frequency, and the natural frequency of a coupling loop in a conventional filter is about 260Hz. On the other hand, in normal use of the filter, a device itself, in which the filter is incorporated, vibrates with the vibrations applied from the outside. In this case, frequencies ranging from about 5Hz to 200Hz are a problem. There is a likelihood that a coupling loop will resonate with the vibrations from the outside. The coupling loop resonates because the frequency of the external vibrations is almost equivalent to the natural frequency of the coupling loop. Although the natural frequency of the conventional coupling loop does not coincide with the frequency of the external vibrations, if it remains about 260Hz, the attenuation amount is not sufficient near about 200Hz, which is an unnecessary signal, thereby affecting the filter characteristics to a degree that is not disregarded. As the coupling loop resonates with the external vibrations, the degree of coupling between the coupling loop and the dielectric resonator varies, and the electrical characteristics, such as return loss, are thereby disturbed. Moreover, reliability of the filter is deteriorated. In order to solve the above problems, it may be possible to further increase the natural frequency of the coupling loop so that the resonance with the external vibrations can be disregarded. Incidentally, the coupling loop can be regarded as having a beam structure. In general, the natural frequency of a beam is expressed by the following formula: Natural Frequency f = C l 2 EI pA where C is a constant, l is the length of the beam, E is the Young's modulus of the beam, I is the second moment of area of the beam, p is the density of the beam, and A is the sectional area of the beam. Referring to the above formula, it may be possible to reduce the length of the beam in order to increase the natural frequency of the coupling loop. Since the length of the beam has an influence on the degree of coupling with the dielectric resonator, however, it cannot be easily changed. Accordingly, it is good, in practice, to change the bending rigidity of the beam. The bending rigidity of the beam is given by the product of the Young's modulus and the second moment of area of the material. Therefore, the bending rigidity of the beam can be improved by increasing the Young's modulus or the second moment of area of the material. Although iron is available as a material having a high Young's modulus, the use of iron for the coupling loop causes a new problem, that is, thorough rust prevention is required. When the coupling loop is made of iron, in general, intermodulation (IM) is apt to occur, and therefore, the coupling loop is plated with silver. If the silver plate rusts, however, iron appears on the surface thereof, and IM is likely to occur. Although it may also be possible to increase the thickness of the coupling loop in order to increase the second moment of area, this results in an increase in the material cost. The coupling loop is formed by bending a metal plate into the shape of an L. Therefore, the strength of the bent portion is low, and this leads to a fear that the positional relationship between the coupling loop and the dielectric resonator may change. Furthermore, one end of the coupling loop on the side of the external connector has been heretofore corrugated so as to absorb impact from the external connector. It is, however, not so easy to corrugate an end of the coupling loop, and costs become high. FR 2118859 A concerns a cavity resonator having a coupling loop extending from an input/output connector to the inside of the cavity. Instead of forming the entire loop by a wire-shaped conductor, the loop is formed by a first portion of a wire-shaped conductor extending from the connector and by two thin conductive plates arranged substantially perpendicular to each other and extending from the end of the wire-shaped conductor to a wall of the cavity. US-A-5,625,330 describes a resonator coupling device having a rotatable ring for adjusting the loaded Q. The coupling device consists of a connector, a ring and a coupling loop extending into the interior of a resonator cavity. The loop is electrically and mechanically connected to a central conductor and is formed from a strip of sheet metal with one end connected to the central conductor and the other end being soldered in a slot of the coupling device. It is the object of the present invention to provide an improved filter hardly being effected by vibrations applied from the outside and having a high reliability. This object is achieved by a filter according to claim 1. According to a further aspect, the present invention provides for a duplexer using the inventive filter and according to yet a further aspect, the present invention concerns a communication device having such a duplexer using the inventive filter. According to an aspect of the present invention, there is provided a filter having a cavity, an input-output connecting means mounted in the cavity, and a coupling loop connected to the input-output connecting means so as to couple with a magnetic field inside the cavity, wherein a rib is formed on the coupling loop for increasing the natural frequency thereof. According to another aspect of the present invention, the coupling loop is formed by bending a metal plate along a bending line, and the rib extends in a direction that is not in parallel with the bending line. According to a further aspect of the present invention, the coupling loop includes a section having high rigidity and a curved section having low rigidity, one end of the high-rigidity section is connected to the cavity, the other end thereof is connected to one end of the low-rigidity curved section, and the other end of the low-rigidity curved section is connected to the input-output connecting means. Preferably, a dielectric resonator is disposed inside the cavity. Preferably, a rib is formed in a bent portion of the coupling loop. This structure makes it possible to increase the natural frequency of the coupling loop, and to thereby prevent the coupling loop from resonating with vibrations from the outside. Moreover, it is possible to mechanically reinforce the bent portion of the coupling loop, and to limit the change of degree of coupling, whereby a reliable filter can be provided. Accordingly, it is possible to easily manufacture at low cost a coupling loop that is connected to an external connector and that absorbs impact applied through the external connector. In addition, it is possible to provide a reliable filter in which change of degree of coupling is prevented. According to a further aspect of the present invention, there is provided a duplexer including at least two filters, input-output connecting means connected to the filters, and an antenna connecting means commonly connected to the filters, wherein at least one of the filters is a filter of the above-mentioned type. According to a still further aspect of the present invention, there is provided a communication device including the above-mentioned duplexer, a transmission circuit connected to at least one of the input-output connecting means in the duplexer, a receiving circuit connected to at least one of the input-output connecting means that differs from the input-output connecting means connected to the transmission circuit, and an antenna connected to the antenna connecting means in the duplexer. According to the above, it is possible to obtain a duplexer and a communication device having stable properties and high reliability. Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a filter according to the present invention; FIG. 2 is a perspective view of a coupling loop shown in FIG. 1; FIG. 3 is a perspective view showing another example of a coupling loop according to the present invention; FIG. 4 is a perspective view showing only a high-rigidity section of a further example of a coupling loop according to the present invention; FIG. 5 is a perspective view showing only a high-rigidity section of a further example of a coupling loop according to the present invention; FIG. 6 is a perspective view showing only a high-rigidity section of a further example of a coupling loop according to the present invention; FIG. 7 is a perspective view showing only a high-rigidity section of a further example of a coupling loop according to the present invention; FIG. 8 is an equivalent circuit diagram of a duplexer according to the present invention; FIG. 9 is a schematic view of a communication device according to the present invention; and FIG. 10 is a perspective view of a conventional filter. DESCRIPTION OF THE PREFERRED EMBODIMENTS A filter according to an embodiment of the present invention will be described below with reference to the attached drawings. A dielectric resonant device in this embodiment is of a type in which a dielectric resonator is disposed inside a frame. FIG. 1 is a schematic perspective view of the filter according to the present invention. In FIG. 1, the filter is cut along the plane perpendicular to the open surface of a dielectric resonant device 20, in order for the inside thereof to be seen well. Although coupling loops 12, and external connectors 13 that serve as input-output connecting means are provided for input and output, since they have the same structure, only one of the coupling loops 12 and one of the external connectors 13 are illustrated and described. The filter 10 generally comprises a TM mode dielectric resonant device 20, and metal panels 11 mounted to cover open portions of the dielectric resonant device 20. In the TM mode dielectric resonant device 20, a dielectric resonator 22 made of ceramic and shaped like a column is disposed inside a frame 21 that is also made of ceramic, and conductors 23 are formed by applying and baking silver paste on the frame 21. The metal panels 11, which are mounted at the open portions of the dielectric resonant device 20, are worked from a metal plate of iron, a nickel alloy, or the like, by pressing or stamping, and are provided with the coupling loop 12 and the external connector 13. The metal panels 11 are soldered so as to cover the open portions of the dielectric resonant device 20, and connected to the conductors 23 of the dielectric resonant device 20, whereby a cavity 30 is formed as a whole. A metal cover is further mounted on the filter 10, though it is not shown in FIG. 1, to maintain stability in installation, to prevent impact from being transmitted from the outside to the inside, and to mechanically reinforce an external connector section. The coupling loop 12 including a section having high rigidity 12a and a section having low rigidity 12b. The high-rigidity section 12a is formed by bending a metal plate of copper or the like into the shape of an L. In contrast, the low-rigidity section 12b is formed by curving a metal plate that is made of phosphor bronze or the like and that has a smaller thickness than that of the high-rigidity section 12a, so that it has flexibility. A rib 14 is formed in a part of the high-rigidity section 12a that is parallel to the metal panel 11 so as to increase the second moment of area. The high-rigidity section 12a also has a cutout portion 16 at one end thereof, as shown in FIG. 2, so that elastic force acts on the force exerted from the widthwise direction of the cutout portion 16. Moreover, two projecting portions 15 are formed to sandwich the cutout portion 16. The metal panel 11 has a hole whose bore is smaller than the width of the two projecting portions 15. By being inserted in this hole formed through the metal panel 11, the projecting portions 15 of the high-rigidity section 12a are retained from both sides. Then, the projecting portions 15 are temporarily attached by using the elastic force to expand, and are fixed by soldering. The high-rigidity section 12a has a hole at the other end. One end portion of the low-rigidity section 12b is inserted in the hole, bent, and fixed by soldering. The other end of the low-rigidity section 12b also has a hole, in which the leading end of a center conductor of the external connector 13 is fixedly inserted. One end of the high-rigidity section 12a that is connected to the metal panel 11 may be further bent into the shape of an L so that a surface having a hole 17 is formed to be bonded to the metal panel 11, as shown in FIG. 3. In this case, a projection is formed on the metal panel 11 by stamping or louvering the metal panel 11 inward. The projection of the metal panel 11 is inserted in the hole 17, bent, and fixed by soldering. This simplifies the operation and improves stability of the bonded portion. FIGS. 4 to 7 demonstrate variations of the rib 14, and each shows only the high-rigidity section 12a of the coupling loop 12 that is worked into the shape of an L and fixed to the metal panel 11. In the variation shown in FIG. 4, a rib 14 is also formed in a part of the high-rigidity section 12a in the coupling loop 12, which is nearly perpendicular to the metal panel 11, so that it extends in the direction nearly perpendicular to the metal panel 11, thereby further increasing the natural frequency. There may be provided a plurality of ribs 14, as shown in FIG. 5. In this case, the natural frequency of the coupling loop 12 can be further increased. When the rib 14 is shaped like a cross, as shown in FIG. 6, it is possible to increase the strength of the coupling loop 12 with respect to vibrations in other directions. As shown in FIG. 7, ribs 19 may be formed in bent portions of the high-rigidity section 12a of the coupling loop 12. This makes it possible to increase the mechanical strength of the bent portions, to prevent a change in the bending angle of the coupling loop 12, and to prevent a change in the degree of coupling between the coupling loop 12 and the dielectric resonator 22. As a result, a filter having high reliability can be provided. While the filter of this embodiment employs a dielectric resonant device in which a dielectric resonator shaped like a column is disposed inside a ceramic frame, the present invention is not limited to the illustrated embodiment. The present invention may also be applied to, for example, a filter using a double-mode or multiple-mode dielectric resonant device in which a cross-shaped dielectric resonator is placed. The present invention may also be applied to any device using a coupling loop, for example, a waveguide filter in which a coupling loop is mounted in a metal cavity. Next, a duplexer according to an embodiment of the present invention will be described with reference to FIG. 8. FIG. 8 is an equivalent circuit diagram of the duplexer of this embodiment. As shown in FIG. 8, a duplexer 35 of this embodiment comprises a four-stage transmission filter 40a composed of two double-mode dielectric resonant devices 20a and 20b, and a six-stage receiving filter 40b composed of three double-mode dielectric resonant devices 20c, 20d, and 20e. In each of the dielectric resonant devices 20a to 20e used in the transmission filter 40a and the receiving filter 40b, a cross-shaped dielectric resonator is placed in a frame having a conductor formed on the outer surface thereof, and two modes are coupled by forming a cutout for coupling at an intersection of the dielectric resonator, or other methods. A metal panel is mounted at an open portion of each of the dielectric resonant devices 20a to 20e so that it covers the open portion, and is provided with a coupling loop having the shape that has been described in the above embodiments. Coupling loops 12E and 12F, which are coupled to the first stage of the transmission filter 40a and the last stage of the receiving filter 40b, respectively, are connected to external connectors 13a and 13b mounted on the metal panel. Furthermore, coupling loops 12G and 12H coupled to the last stage of the transmission filter 40a and the first stage of the receiving filter 40b are commonly connected to a single external connector 13c. On the other hand, coupling loops are mounted between the second and third stages of the transmission filter 40a, between the second and third stages of the receiving filter 40b, and between the fourth and fifth stages of the receiving filter 40b in order to couple the dielectric resonant devices. A line, such as a coaxial line, connected to the external connector 13a, to which the coupling loop 12E coupled with the first stage of the transmission filter 40a is connected, is connected to an external transmission circuit, and a line connected to the external connector 13b, to which the coupling loop 12F coupled to the last stage of the receiving filter 40b is connected, is connected to an external receiving circuit. Furthermore, a line connected to the external connector 13c, to which the coupling loops 12G and 12H coupled to the last stage of the transmission filter 40a and the first stage of the receiving filter 40b are commonly connected, is connected to an antenna. In the duplexer 35 having such a configuration, a signal having a predetermined frequency, of signals input from the external transmission circuit to the transmission filter 40a, is output to the antenna, and a signal having a predetermined frequency, which is different from the aforesaid frequency, of signals input from the antenna to the receiving filter 40b, is output to the external receiving circuit. Furthermore, a communication device according to an embodiment of the present invention will be described with reference to FIG. 9. FIG. 9 is a schematic view of the communication device of this embodiment. As shown in FIG. 9, a communication device 50 of this embodiment comprises a duplexer 35, a transmission circuit 51, a receiving circuit 52, and an antenna 53. The duplexer 35 is the same as that described in the above embodiment. The coupling loop 12E coupled to the first stage of the transmission filter 40a is connected to the transmission circuit 51 via the external connector 13a and the line, and the coupling loop 12F coupled to the last stage of the receiving filter 40b is connected to the receiving circuit 52 via the external connector 13b and the line. Moreover, the coupling loop 12C coupled to the last stage of the transmission filter 40a and the coupling loop 12H coupled to the first stage of the receiving filter 40b are connected to the antenna 53 via the external connector 13c and the line. In the duplexer and the communication device mentioned in the above embodiments, since the natural frequency of the coupling loop is apart from the frequency that has been a problem, the properties are stabilized, thereby improving reliability. As mentioned above, according to the present invention, the natural frequency of the coupling loop is increased by forming a rib in the coupling loop, for example, increased to about 380Hz in the case in which a single rib is formed, as shown in FIG. 1. This allows a satisfactory attenuation amount near 200Hz. The resonation resulting from the external vibration, which has been a problem hitherto, can be reduced to a negligible degree, thereby preventing the electrical characteristics, such as return loss, from being disturbed. As a result, it is possible to provide a filter having high reliability. Furthermore, the bent portion of the coupling loop can be mechanically reinforced by a rib formed therein. As a result, it is possible to further improve stability of the coupling loop, and to prevent a change in the degree of coupling between the coupling loop and the electromagnetic field generated in the cavity. Still furthermore, the coupling loop including two sections, an L-shaped section having high rigidity, and a curved section having low rigidity. One end of the high-rigidity L-shaped section is connected to the cavity, the other end thereof is connected to one end of the low-rigidity curved section, and the other end of the low-rigidity curved section is connected to the external connector. This permits the low-rigidity section of the coupling loop to absorb impact from the external connector. Consequently, it is possible to provide a filter having stable characteristics.

Claims: A filter (10) having a cavity (30), input-output connecting means (13) mounted in said cavity (30), and a coupling loop (12) connected to said input-output connecting means (13) so as to couple with a magnetic field inside said cavity (30), characterized in that said coupling loop (12) comprises a rib (14) formed on said coupling loop (12) for increasing the natural frequency thereof. A filter (10) according to claim 1, wherein said coupling loop (12) is formed by bending a metal plate along a bending line, and said rib (14) extends in a direction that is not in parallel with said bending line. A filter (10) according to claim 1 or 2, wherein said coupling loop (12) includes a section (12a) having high rigidity and a curved section (12b) having low rigidity, one end of said high-rigidity section (12a) is connected to said cavity (30), the other end thereof is connected to one end of said low-rigidity curved section (12b), and the other end of said low-rigidity curved section (12b) is connected to said input-output connecting means (13), and said rib (14) is formed on said section (12a) having high rigidity. A filter (10) according to claim 2 or 3, wherein said rib (14) is formed on a bent portion of said coupling loop (12). A filter (10) according to one of claims 1 to 4, wherein a dielectric resonator (20) is disposed inside said cavity (30). A duplexer (35) comprising: at least two filters (40a, 40b); input-output connecting means (13a, 13b) connected to said filters (40a, 40b); and antenna connecting means (13c) commonly connected to said filters (40a, 40b), wherein at least one of said filters (40a, 40b) is a filter (10) according to any of claims 1 to 5. A communication device (50) comprising: a duplexer (35) according to claim 6; a transmission circuit (51) connected to at least one of input-output connecting means (13a) in said duplexer (35); a receiving circuit (52) connected to at least one of said input-output connecting means (13b) that differs from said input-output connecting means (13a) connected to said transmission circuit (51); and an antenna (53) connected to antenna connecting means (13c) in said duplexer (35).

Inventors: HIMI YOSHIHIRO, NAKATANI YUKIHIRO, NISHIYAMA TAIYO, WAKAMATSU HIROKI, HIMI, YOSHIHIRO, NAKATANI, YUKIHIRO, NISHIYAMA, TAIYO, WAKAMATSU, HIROKI, Himi, Yoshihiro Murata Manufacturing Co., Ltd., Nakatani, Yukihiro Murata Manufacturing Co., Ltd., Nishiyama, Taiyo Murata Manufacturing Co., Ltd., Wakamatsu, Hiroki Murata Manufacturing Co., Ltd.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1998
Main classification: H01P7
IPC codes: H01P7