Dielectric resonator filter

Abstract: A dielectric resonator filter comprises dielectric resonators, an enclosure having a main body, a lid, and partition walls, interstage-coupling tuning windows, interstage-coupling tuning bolts, input/output terminals, and input/output coupling probes. Resonance-frequency tuning members each composed of a conductor plate and a bolt coupled integrally thereto are attached to the enclosure lid. Spurious-mode suppressing means such as rings (71,72) attached to the bolts of the resonance-frequency tuning members or bolts attached to the conductor plates or to the enclosure lid are disposed in a spurious-mode excitation space, whereby the occurrence of a disturbed characteristic in the pass band (or stop band) is suppressed. In another embodiment, the spurious-mode suppressing means comprise tuning plates (61y,61b) of different sizes.

Description: The present invention relates to a multi-purpose dielectric resonator filter for use at a mobile communication base station to serve as each of a receiving filter, a transmitting filter, a duplexer, and the like. Conventionally, band pass filters for allowing the passage of only signals in a specified frequency band have been used at base stations for mobile communication such as a mobile phone. For example, a receiving system uses a receiving filter to remove signals for communication systems using the other frequency bands and a transmitting system uses a transmitting filter not to send undesired electric waves to the systems using the other frequency bands. Such filters for use at the base stations are required to have a sufficiently low loss to provide the base stations with an adequate receiving sensitivity and power efficiency, a sharp filter characteristic provided for a reduced interval in frequency band between the adjacent base stations, and reduced size and weight for easier mounting on the overheads of the base stations. As an example of a filter satisfying such requirements, a dielectric resonator filter composed of a plurality of dielectric resonators coupled to each other has been proposed, which comes in various configurations. FIG. 4 is a perspective view schematically showing an example of a conventional six-stage dielectric resonator filter. As shown in FIG. 4 , the conventional dielectric resonator filter comprises six cylindrical dielectric resonators 511A to 511F formed by sintering a dielectric powder material. The resonance frequency of each of the dielectric resonators 511A to 511F is determined by the height and diameter of the cylindrical configuration thereof. In this example, the six dielectric resonators 511A to 511F operate as a six-stage band pass filter. An enclosure 520 of the dielectric resonator filter comprises a main body 521 composed of a bottom wall and side walls, a lid 522, partition walls 523A to 523G connected to each other to partition, into chambers, a space enclosed by the enclosure main body 521 . The dielectric resonators 511A to 511F are disposed on a one-by-one basis in the respective chambers defined by the partition walls 523A to 523G of the enclosure 520 . Interstage-coupling tuning windows 524A to 524E for providing electromagnetic field couplings between the resonators are provided between the five partition walls 523A to 523E of the seven partition walls 523A to 523G and the side walls of the enclosure main body 521. The interstage-coupling tuning windows 524A to 524E are provided with respective interstage-coupling tuning bolts 531A to 531E each for tuning the strength of an electromagnetic field coupling between the resonators. The enclosure main body 521 is provided with input/output terminals 541 and 542 each composed of a coaxial connector to input and output a high-frequency signal to and from the outside. Input/output coupling probes 551 and 552 are connected to the respective core conductors of the input/output terminals 541 and 542 . Resonance-frequency tuning members 561A to 561F each composed of a disk and a bolt formed integrally to tune the resonance frequency of the corresponding one of the dielectric resonators 511A to 511F are attached to the enclosure lid 521. The resonance-frequency tuning members 561A to 561F are disposed to have their respective center axes at the same plan positions as the respective center axes of the dielectric resonators 511A to 511F (i.e., at the concentric positions). Since the frequency characteristics including passband width and attenuation characteristic of a dielectric resonator filter are generally determined by the resonance frequency and Q factor of each of the resonators and an amount of coupling between the individual dielectric resonators, the configuration and the like of each of the dielectric resonators are calculated from the specifications of the frequency characteristics of the filter at the design stage. In practice, however, filter characteristics as designed cannot be obtained due to an error in the configurations of the dielectric resonators and enclosure and to a mounting error. To provide filter characteristics as designed, the resonance-frequency tuning members 561A to 561F are provided in the conventional dielectric resonator filter to render the respective resonance frequencies of the dielectric resonators 511A to 511F variable. In addition, the interstage-coupling tuning bolts 531A to 531E are provided to render the strengths of interstage couplings variable. Through the tuning using the tuning mechanism, desired filter characteristics are provided. For the resonance-frequency tuning members 561A to 561F , a structure as shown in FIG. 4 has been used widely in which the frequency characteristics of the dielectric resonators 511A to 511F are made variable by tuning the distance between conductor plates opposed to the dielectric resonators 511A to 511F and the dielectric resonators 511A to 511F by using the bolts. The dielectric resonator filter having such a structure operates as follows. If a high-frequency signal transmitted from, e.g., a signal source or an antenna and inputted into the enclosure 520 via the input/output terminal 541 has a frequency within the pass band of the filter, the signal couples to an electromagnetic field mode in the input-stage dielectric resonator 511A by the effect of the input/output coupling probe 551 so that TE01δ as a basic resonance mode is excited. The resonance mode couples to respective electromagnetic field modes in the subsequent dielectric resonators 511B, 511C, ... in succession through the interstage-coupling tuning windows 524A, 524B, ... so that the electromagnetic field mode excited in the dielectric resonator 511F couples to the output-side input/output probe 552 and the high-frequency signal is outputted from the input/output terminal 542 . On the other hand, the high-frequency signal having a frequency outside the pass band of the filter is reflected without coupling to the resonance mode in the dielectric resonator and sent back from the input/output terminal 541 . However, the foregoing conventional dielectric resonator filters has following drawbacks. FIG. 6 shows an example of the frequency characteristic of the dielectric resonator filter shown in FIG. 4 . In FIG. 6 , the horizontal axis represents the frequency (GHz) and the vertical axis represents the transmission characteristic (dB). As can be seen from the drawing, an attenuation pole P1 (valley) with an enhanced transmission characteristic exists in the pass band, which indicates that the filter characteristic has been degraded. The present inventors have assumed the cause of such a degraded filter characteristic as follows. FIG. 5 shows an electromagnetic field mode in the vicinity of the conductor plate of each of the resonance-frequency tuning members 561 of the dielectric resonator filter shown in FIG. 4 . In the drawing is shown the result of analyzing the distribution of an electric field in a cross section passing through the axis of the resonance-frequency tuning member by an electromagnetic field simulation using a FDTD method. As shown in FIG. 5 , a spurious electromagnetic field mode is produced in a space defined by the conductor plate of the resonance-frequency tuning member 561 and the enclosure lid 522 . As a result, the spurious electromagnetic field mode couples to a high-frequency signal to cause the state of resonance so that the spurious attenuation pole P1 (valley portion) is assumed to appear in the frequency characteristic. The spurious mode reacts more sensitively to the movement of the resonance-frequency tuning member than the resonance frequency in a basic mode required to provide the filter characteristic and changes greatly. Consequently, the attenuation pole resulting from the spurious mode frequently passes through a near-passband region when the vertical position of the resonance-frequency tuning member is changed to tune the filter characteristic and disturb the waveform of the filter characteristics, which presents a large obstacle to the tuning operation. In the worst case, the spurious mode enters the pass band of the filter even after the resonance-frequency tuning operation is completed to degrade the filter characteristic, as shown in FIG. 6 . US-A-4 335 365 describes a dielectric resonator within a cavity, comprising a tuning disk and a sleeve of absorbing material for damping the spurious resonances within the cavity. It is the object of the present invention to facilitate the operation of tuning a dielectric resonator filter and providing a dielectric resonator filter with an excellent frequency characteristic by focusing attention on the fact that the cause of the degraded characteristic in the conventional dielectric resonator filters is the spurious mode produced between the resonance-frequency tuning member as a mechanism for tuning the filter characteristic and the wall surface of the enclosure and providing means for eliminating the spurious mode. This object is solved by the subject matter of the independent claim. Preferred embodiments are defined in the dependent claims. A dielectric resonator filter according to the present invention comprises: a plurality of dielectric resonators; an enclosure enclosing the plurality of dielectric resonators to function as a shield against an electromagnetic field; and a plurality of resonance-frequency tuning means provided on a one-by-one basis for the plurality of dielectric resonators, each of the plurality of resonance-frequency tuning means including a conductor plate disposed in a space enclosed by the enclosure to have a first surface opposed to a surface of the corresponding one of the dielectric resonators and a second surface opposed to an inner surface of the enclosure, the resonance-frequency tuning means being capable of changing distances between the conductor plates and the dielectric resonators, the conductor plate of at least one of the plurality of resonance-frequency tuning means having a size different from sizes of the conductor plates of the other resonance-frequency tuning means. If a tuning is made by increasing the diameter or thickness of the conductor plate of each of the resonance-frequency tuning means provided additionally on some of the dielectric resonators, the frequency in the spurious mode changes with the size of the conductor plate. By using this, the disturbed characteristic resulting from the spurious mode can be moved from the pass band (or stop band) to another frequency region, so that the occurrence of a disturbed characteristic in the pass band (or stop band) is suppressed. Preferably, the conductor plate of each of the resonance-frequency tuning means has a disk-shaped configuration. FIG. 1 is a perspective view schematically showing a structure of a dielectric resonator filter; FIG. 2 is a perspective view schematically showing a structure of a dielectric resonator filter according to an third embodiment of the present invention; FIG. 3 is a graph showing the relationship between the position of a resonance-frequency tuning member and respective frequencies in a basic mode and a spurious mode, which have been measured to examine the effect of a resonance-frequency tuning member with a spurious-mode suppressing function; FIG. 4 is a perspective view schematically showing an example of the conventional six-stage dielectric resonator filter; FIG. 5 shows an electromagnetic field mode in the vicinity of the conductor plate of the resonance-frequency tuning member of the dielectric resonator filter shown in FIG. 21 ; FIG. 6 shows an example of the frequency characteristic of the dielectric resonator filter shown in FIG. 4 ; FIG. 1 is a perspective view schematically showing a structure of a dielectric resonator filter useful for understanding the present invention. As shown in FIG. 1 , the dielectric resonator filter comprises six cylindrical dielectric resonators 11A to 11F formed by sintering a dielectric powder material. The resonance frequency of each of the dielectric resonators 11A to 11F is determined by the height and diameter of the cylindrical configuration thereof. In this example, the six dielectric resonators 11A to 11F operate as a six-stage band pass filter. An enclosure 20 of the dielectric resonator filter comprises a main body 21 composed of a bottom wall and side walls, a lid 22 , partition walls 23A to 23G connected to each other to partition, into chambers, a space enclosed by the enclosure main body 21 . The dielectric resonators 11A to 11F are disposed on a one-by-one basis in the respective chambers defined by the partition walls 23A to 23G of the enclosure 20 . Interstage-coupling tuning windows 24A to 24E for providing electromagnetic field couplings between the resonators are provided between the five partition walls 23A to 23E of the seven partition walls 23A to 23G and the side walls of the enclosure main body 21 . The interstage-coupling tuning windows 24A to 24E are provided with respective interstage-coupling tuning bolts 31A to 31E each for tuning the strength of an electromagnetic field coupling between the resonators. The enclosure main body 21 is provided with input/output terminals 41 and 42 each composed of a coaxial connector to input and output a high-frequency signal to and from the outside. An input coupling probe 51 and an output coupling probe 52 are connected to the respective core conductors of the input/output terminals 41 and 42. Resonance-frequency tuning members 61A to 61F (resonance-frequency tuning means) each composed of a disk-shaped conductor plate and a bolt coupled integrally thereto to tune the resonance frequency of the corresponding one of the dielectric resonators 11A to 11F are attached to the enclosure lid 22. The resonance-frequency tuning members 61A to 61F are disposed to have their respective center axes at the same plan positions as the respective center axes of the dielectric resonators 11A to 11F (i.e., at the concentric positions). Specifically, the enclosure lid 22 is provided with screw holes which are at nearly concentric positions to the cylindrical dielectric resonators 11A to 11F such that the respective bolts of the resonance-frequency tuning members 61A to 61F are engaged with the screw holes of the enclosure lid 22. The resonance frequencies can be tuned by rotating the resonance-frequency tuning members 61A to 61F around the axes and thereby changing the distances between the conductor plates and the dielectric resonators 11A to 11F. Since the frequency characteristics including passband width and attenuation characteristic of a dielectric resonator filter are generally determined by the resonance frequency and Q factor of each of the resonators and an amount of coupling between the individual dielectric resonators, the configuration and the like of each of the dielectric resonators are calculated from the specifications of the frequency characteristics of the filter at the design stage. In practice, however, filter characteristics as designed cannot be obtained due to an error in the configurations of the dielectric resonators and enclosure and to a mounting error. To provide filter characteristics as designed, the resonance-frequency tuning members 61A to 61F are provided in the conventional dielectric resonator filter to render the respective resonance frequencies of the dielectric resonators 11A to 11F variable. In addition, the interstage-coupling tuning bolts 31A to 31E are also provided to render the strengths of interstage couplings variable. Through the tuning using the tuning mechanism, desired filter characteristics are provided. This filter has spurious-mode suppressing rings 71 and 72 (spurious-mode suppressing means) which are composed of a conductor and have screw holes for engagement with the bolts, of the input- and output-stage resonance-frequency tuning members 61A and 61F are attached to the bolts. To illustrate the effects achieved by the provision of the spurious-mode suppressing rings 71 and 72 , a description will be given first to the operation of the dielectric resonator filter. If a high-frequency signal transmitted from, e.g., a signal source or an antenna (not shown in FIG. 1 ) and inputted into the enclosure 20 via the input/output terminal 41 has a frequency within the pass band of the filter, the signal couples to an electromagnetic field mode in the input-stage dielectric resonator 11A by the effect of the input coupling probe 51 so that TE01δ as a basic resonance mode is excited. The basic resonance mode couples to respective electromagnetic field modes in the subsequent dielectric resonators 11B, 11C, ... in succession through the interstage-coupling tuning windows 24A, 24B, ... so that the electromagnetic field mode excited in the dielectric resonator 11F couples to the output coupling probe 52 and the high-frequency signal is outputted from the input/output terminal 42 . On the other hand, the high-frequency signal having a frequency outside the pass band of the filter should be reflected without coupling to the basic resonance mode in the dielectric resonator and sent back from the input terminal 41 . For the foregoing filter to operate precisely, each of the dielectric resonators 11A to 11F should have a precise resonance frequency and each of the interstage-coupling tuning windows 24A, 24B, ... should provide an interstage coupling having a precise strength. However, filter characteristics as designed cannot be provided due to an error in the configurations of the dielectric resonators 11A to 11F and enclosure 20 and to a mounting error. To provide filter characteristics as designed, the resonance-frequency tuning members 61A to 61F are provided and the conductor plates are moved upwardly or downwardly by rotating the bolts of the resonance-frequency tuning member 61A to 61F. As a result, the distances between the conductor plates of the resonance-frequency tuning members 61A to 61F and the dielectric resonators 11A to 11F located therebelow change to change the resonance frequencies of the dielectric resonators 11A to 11F. In addition, the interstage coupling bolts 31A to 31E are provided to render the strengths of interstage couplings variable. Through the tuning using the tuning mechanism, desired filter characteristics are provided. If the amounts of insertion of the interstage-coupling tuning bolts 31A to 31E are increased to reduce the distances between the tip portions thereof and the side walls opposed thereto, e.g., the electromagnetic field coupling between the adjacent dielectric resonators (e.g., 11B and 11C ) via the interstage-coupling tuning window (e.g., 24B ) is intensified. If the resonance-frequency tuning members 61A to 61F are lowered in position to reduce the distances between the dielectric resonators and the conductor plates, the resonance frequencies of the dielectric resonators are increased. The functions described above are common to the conventional dielectric resonator filters. However, the present embodiment features the spurious-mode suppressing rings 71 and 72 as spurious-mode suppressing means which are provided in a spurious-mode excitation space (the space R1 shown in FIG. 22 ) in the region between the resonance-frequency tuning members 61A and 61F and the enclosure lid 22. If the surfaces (lower surfaces) of the respective conductor plates of the resonance-frequency tuning members 61A and 61F opposed to the dielectric resonators 11A and 11F are assumed to be first surfaces and the surfaces (upper surfaces) of the conductor plates opposed to the inner surface of the enclosure lid 22 are assumed to be second surfaces, it follows that the spurious-mode suppressing rings 71 and 72 are disposed in the space R1 between the second surfaces of the conductor plates and the inner surface of the enclosure. The arrangement functions to suppress the production of the spurious mode shown in FIG. 5 . From the viewpoint of electromagnetic fields, the provision of the spurious-mode suppressing rings 71 and 72 reduces the vertical size of the spurious-mode excitation space R1 and thereby reduces the guide wavelength of the excited spurious mode, so that the filter characteristic shifts toward higher frequencies. Moreover, the length of the narrow portion R3 (see FIG. 5 ) connecting from the spurious-mode excitation space R1 (see FIG. 5 ) to the space R2 (see FIG. 5 ) in which the dielectric resonators 11A and 11F are disposed is increased, which makes the passage of an electromagnetic wave through the narrow portion R3 difficult and weakens the coupling between the spurious mode and respective modes in the dielectric resonators 11A and 11F. As a result, the occurrence of a disturbed characteristic such as an undesired attenuation pole P1 (see FIG. 6 ) in the pass band of the dielectric resonator filter composed of the six dielectric resonators 11A to 11F can be suppressed. FIG. 2 is a perspective view schematically showing a structure of a dielectric resonator filter according to an embodiment of the present invention. As shown in the drawing, the dielectric resonator filter according to the present embodiment comprises, as the spurious-mode suppressing means, resonance-frequency tuning members 61X and 61Y with a spurious-mode suppressing function each having a larger-diameter conductor plate in place of the spurious-mode suppressing rings 71 and 72 . Since the structure of the dielectric resonator filter according to the present embodiment is the same as the structure of the dielectric resonator filter described above and shown in FIG. 1 except for the structures of the resonance-frequency tuning members 61X and 61Y with the spurious-mode suppressing function, the description of the components shown in FIG. 2 which have the same function as in that filter is omitted by retaining the same reference numerals as in FIG. 1 . The basic operation of the dielectric resonator filter according to the present embodiment is the same as that of the foregoing dielectric resonator filter. In the dielectric resonator filter according to this embodiment, each of the conductor plates of the resonance-frequency tuning members 61X and 61Y with the spurious-mode suppressing function has a larger diameter so that the guide wavelength of an electromagnetic wave in a direction parallel to the conductor plates is increased and the spurious mode shifts accordingly to lower frequencies. This suppresses the occurrence of the disturbed characteristic such as the undesired attenuation pole P1 (see FIG. 6 ) in the pass band. FIG. 3 is a graph showing, when a single-stage filter (discrete resonator) is used, the relationship between the position of the resonance-frequency tuning member with the spurious-mode suppressing function and respective frequencies in the basic mode and in the spurious mode, which have been measured to examine the effect of the resonance-frequency tuning member with the spurious-mode suppressing function. The single-stage filter used to obtain the data shown in FIG. 3 comprises a cylindrical dielectric resonator composed of a dielectric material with a relative dielectric constant of 41 and having a diameter of 27 mm and a height of 12 mm, a cubic enclosure having inner sides of 40 mm, and a resonance-frequency tuning member with a conductor plate having a diameter of 15 mm, 25 mm, or 35 mm and with a bolt having a thickness of 1 mm and compliant with the standard M6. As shown in FIG. 3 , the frequency in the spurious mode differs depending on the diameter of the conductor plate. If the spurious mode enters the pass band to disturb the filter characteristic in a multi-stage dielectric resonator filter having a plurality of dielectric resonators disposed therein, the spurious mode can be expelled from the pass band by changing the diameter of the conductor plate of each of the resonance-frequency tuning members causing the spurious mode. If the effect is to be described in terms of electromagnetic fields, an increase in the diameter of the conductor plate of each of the resonance-frequency tuning members 61X and 61Y with the spurious-mode suppressing function increases the guide wavelength of an electromagnetic wave in a direction parallel to the conductor plates so that the spurious mode shifts toward lower frequencies. Although the present embodiment has provided the first-and six-stage dielectric resonators 11A and 11F with the additional resonance-frequency tuning members 61X and 61Y with the spurious-mode suppressing function having conductor plates with diameters larger than those of the conductor plates of the other frequency tuning members, the structure of the dielectric resonator filter according to the present invention is not limited to the present embodiment. It is also possible to provide the other-stage dielectric resonators 11 , such as the second- and third-stage dielectric resonators, with the additional resonance-frequency tuning members with the spurious-mode suppressing function. The stages of the dielectric resonators in which the resonance-frequency tuning members with larger-diameter conductor plates should be provided can be determined selectively and appropriately depending on the structures of the dielectric resonators, the enclosure, and the like. Although this embodiment has described the multi-stage filter having the six dielectric resonators as an example of the dielectric resonance filter to which the present invention is applied, the structure of the dielectric resonator filter according to the present invention is not limited to the foregoing embodiment. The effects of the present invention are achievable if the dielectric resonator filter has stages other than six stages. Although this embodiment has described the band pass filter as an example of the dielectric resonator filter to which the present invention is applied, the structure of the dielectric resonator filter according to the present invention is not limited to the foregoing embodiment. The effects of the present invention are achievable with another type of filter such as a band stop filter. Although FIG. 3 shows the result of measurement obtained by using the discrete resonator to define the effects by experiment, it will be appreciated that another multi-stage filter can also achieve the same effects irrespective of the number of stages by adopting the structure of each of the embodiments. Although this embodiment has disposed the dielectric resonators in a lower part of the space enclosed by the enclosure main body and disposed the conductor plates of the resonance-frequency tuning members above the dielectric resonators, it is also possible to dispose the dielectric resonators in the upper part of the space enclosed by the enclosure main body and dispose the conductor plates of the resonance-frequency tuning members below the dielectric resonators. In that case, the effects of the present invention can be achieved by disposing the spurious-mode suppressing members between the conductor plates of the resonance-frequency tuning members and the bottom surface of the enclosure main body.

Claims: A dielectric resonator filter comprising: a plurality of dielectric resonators (11A to 11F); an enclosure (20) enclosing said plurality of dielectric resonators to function as a shield against an electromagnetic field; and a plurality of resonance-frequency tuning means (61b to 61e, 61X, 61Y) provided on a one-by-one basis for said plurality of dielectric resonators (11A to 11F), each of said plurality of resonance-frequency tuning means (61 b to 61e, 61X, 61Y) including a conductor plate disposed in a space enclosed by said enclosure (20) to have a first surface opposed to a surface of the corresponding one of said dielectric resonators (11A to 11F) and a second surface opposed to an inner surface of said enclosure (20), said resonance-frequency tuning means (61b to 61e, 61X, 61Y) being capable of changing distances between the conductor plates and said dielectric resonators (11A to 11F), characterised in that the conductor plate of at least one (61X, 61Y) of the plurality of resonance-frequency tuning means (61b to 61e, 61X, 61Y) having a size different from sizes of the conductor plates of the other resonance-frequency tuning means (61b to 61e). The dielectric resonator filter of claim 1, wherein the conductor plate of each of said resonance-frequency tuning means (61b to 61e, 61X, 61Y) has a disk-shaped configuration.

Inventors: ENOKIHARA AKIRA, MINAMI KUNIHIKO, NAKAMURA TOSHIAKI, OKAJIMA MICHIO, OKAZAKI YASUNAO, TACHIBANA MINORU, ENOKIHARA, AKIRA, MINAMI, KUNIHIKO, NAKAMURA, TOSHIAKI, OKAJIMA, MICHIO, OKAZAKI, YASUNAO, TACHIBANA, MINORU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 2001
Main classification: H01P1
IPC codes: H01P1