Dielectric filter

Abstract: A dielectric filter comprising a dielectric ceramic block provided with three or more than three linearly arranged resonators, coupling through bores arranged between adjacent ones of the resonators and input/output pads arranged vis-a-vis the respective outermost resonators for capacitive coupling wherein there are provied a pair of outer coupling electric paths extending from the respective input/output pads onto the open-circuit end surface and arranged in juxtaposition with the respective outermost resonators with a coupling gap provided therebetween and a pair of inner coupling electric paths extending from the respective input/output pads onto the open-circuit end surface and arranged between the outermost resonators in juxtaposition with the inner resonator or the respective inner resonators adjacent to the outermost resonators with a coupling gap provided therebetween so as to remarkably reduce or eliminate the influence of the fly back x and consequently improve the attenuation performance of the dielectric filter.

Description: BACKGROUND OF THE INVENTION [Field of the Invention) The present invention relates to a dielectric filter comprising a plurality of juxtaposed resonators, and a duplexer using such resonators. [Prior Art] FIG. 1 of the accompanying drawings illustrates a known typical dielectric filter comprising a dielectric ceramic block A, three or more than three resonators B provided in the block A respectively by coating the peripheral walls of three or more than three through bores with a conducting material, an external conductor C covering the outer surfaces of the dielectric ceramic block A but not open-circuit end surface having the openings of the through bores, a pair of coupling through bores D arranged between adjacent ones of the resonators B in parallel with the latter and having no coating on the peripheral walls thereof and a pair of input/output pads E arranged on a lateral surface of the dielectric ceramic block A at respective positions abutting the open-circuit end surface of the block A and facing the respective outermost resonators in such a way that they are electrically insulated from the external conductor C. With such arrangement the magnetic field coupling intensity of the adjacent resonators may be intensified because dielectrics are cleared off between adjacent resonators B by a coupling through bore D. A variety of such dielectric filters are currently known. With such a known arrangement, however, a fly back x is produced at an attenuation pole in a higher frequency zone of a resonance frequency band under the influence of the coupling through holes D to hold the output level above the threshold value of -50dB as shown in FIG. 4B and reduce the attenuation w to such an extent that the filter does not operate satisfactorily in terms of attenuation and is apt to generate noise. EP-A-0,367,061 shows a dielectric filter according to the preamble of claim 1. It would therefore be desirable to provide a dielectric filter that is free from any degradation in the attenuation effect of the filter due to a fly back x. It would also be desirable to provide a duplexer using such dielectric filter. SUMMARY OF THE INVENTION According to one aspect of the invention, there is provided a dielectric filter as set out in claim 1. The dielectric filter further may comprise a rectangularly extended conductor arranged on the open-circuit end surface of said dielectric ceramic block to surround the internal conductor of said inner resonator or each of said respective inner resonators and connected to the internal conductor of said inner resonator or each of said respective inner resonators. Preferably, the free end of each of said inner coupling electric paths may be arranged to be adjacent to said respective rectangularly extended conductor on the open-circuit end surface of said dielectric ceramic block. Also, the free end of each of said outer coupling electric paths may be provided with an arcuate coupling tip to surround the opening of the associated through bore with the coupling gap therebetween. Futhermore, rectangularly extended conductors may be arranged on the open-circuit end surface of said dielectric ceramic block to surround the internal conductors of said respective outermost resonators and connected to the internal conductors of said respective outermost resonators. Preferably, the input/output pads may be disposed on an outer surface portion of the dielectric ceramic block to be brought into contact with a printed circuit board when the filter is mounted on the circuit board. Alternatively, the input/output pads may be disposed on other outer surface portion than that to be brought into contact with a printed circuit board when the filter is mounted on the circuit board. Each of said rectangularly extended conductors arranged to surround the internal conductors of said respective outermost resonators and connected thereto may be provided with at least one notch, and each of said outer coupling electric paths may be provided with a corresponding coupling projection which is engaged with said notch. With such an arrangement, it has been proved that the input/output pads are capacitively coupled with the inner resonator or the respective resonators by means of the inner coupling electric paths to effectively reduce or eliminate the influence of the above described fly back x. The invention also extends to such a dielectric filter in which the three or more resonators and the outer coupling electric path together form a transmitter section of a duplexer. The duplexer may further comprise a receiver section. The receiver section comprises a plurality of resonators provided in a row, one of the input/output pads is associated with the innermost resonators of the transmitter and receiver sections and is arranged to be operated as an input pad for said receiver section, and an output pad is capacitively coupled to the outermost resonator of the receiver section. With this arrangement again, the influence of the fly back x will be effectively reduced or eliminated. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective view of a comparable known dielectric filter; FIG. 2 is a schematic perspective view of a first embodiment of a dielectric filter according to the invention.; FIG. 3 is a schematic plan view of the dielectric filter of FIG. 2; FIG. 4A is a graph showing an attenuation waveform of the dielectric filter shown in FIGS. 2 and 3; FIG. 4B is a graph showing the attenuation waveform of the comparable dielectric filter shown in FIG. 1 which is equivalent to one obtained by using the dielectric filter illustrated in FIGS. 2 and 3 from which internal coupling electric paths are removed as will be described hereinafter; FIG. 5 is a schematic perspective view of a second embodiment of a dielectric filter according to the invention; FIG. 6 is a schematic perspective view of a third embodiment of a dielectric filter according to the invention; FIG. 7 is a circuit diagram of an equivalent circuit of the dielectric filter according to the invention; FIG. 8 is a schematic perspective view of a fourth embodiment of a dielectric filter according to the invention to be used as a duplexer; FIG. 9 is another schematic perspective view of the fourth embodiment of dielectric filter placed upside down relative to FIG. 8; FIG. 10A is a graph showing the attenuation waveform of the dielectric filter illustrated in FIGS. 8 and 9; and FIG. 10B is a graph showing the attenuation waveform of a comparable dielectric filter. DETAILED DESCRIPTION OF THE INVENTION Now the present invention will be described by referring to the accompanying drawings that illustrate preferred embodiments of the invention. The components that are same or similar throughout the embodiments will be denoted respectively by same reference symbols and will be described without redundancy. FIGS. 2 and 3 shows a first embodiment of dielectric filter 1a according to the invention, which is a three pole type dielectric filter having three resonators. The illustrated dielectric filter 1a comprises a substantially rectangularly parallelepipedic dielectric ceramic block 2 made of a titanium oxide type material and provided with three resonators 3A, 3B and 3C that are arranged in parallel with each other. The resonators 3A, 3B and 3C are realized by forming through bores 4A, 4B and 4C running through the dielectric ceramic block 2 and coating the peripheral inner walls of the through bores 4A, 4B and 4C with a conductive material to produce internal conductors 5. A pair of coupling through bores 6 are arranged respectively between the resonators 3A and 3B and between the resonators 3B and 3C and in parallel with the through bores 4A, 4B and 4C to couple the adjacent resonators. No internal conductor is arranged on the coupling through bores 6 and 6. The coupling intensity of the adjacently located resonators 3A and 3B and that of the resonators 3B and 3C are boosted by the respective coupling through bores 6 and 6. An external conductor 7 is formed by covering the outer surfaces 8B-8F of the dielectric ceramic block 2 also with a conductive material but not an open-circuit end surface 8A having the openings of the through bores 4A, 4B and 4C. The external conductor 7 operates as a shield or ground electrode. Note that the resonators 3A, 3B and 3C have a resonance length substantially equal to a quarter of the resonance frequency λ or λ/4. A rectangularly extended conductor 9 is arranged on the open-circuit end surface 8A of dielectric ceramic block 2, surrounding the open end of the resonator 3B, and connected to the internal conductor 5 thereof on the peripheral wall of the through bore 4B. The principal components of the illustrated dielectric filter embodying ― the invention will now be described. A pair of input/output pads 10 are arranged on a lateral surface 8E (which is to be directly mounted on a printed circuit board not shown) of the dielectric ceramic block 2 vis-a-vis the respective outermost resonators 3A and 3C for capacitive coupling and insulated from the external conductor 7. Therefore, the input/output pads 10 are to be electrically connected to conductors arranged, for example, on the printed circuit board. On the open-circuit end surface 8A of dielectric ceramic block 2 are provided outer coupling electric paths 11a which are respectively extending from the input/output pads 10 onto the open-circuit end surface 8A and arranged in juxtaposition with the respective openings of the outermost resonators 3A and 3C with a coupling gap provided therebetween. On the open-circuit end surface 8A of dielectric ceramic block 2 are also provided inner coupling electric paths 12a which are respectively extending from the respective input/output pads 10 onto the open-circuit end surface 8A and arranged between the outermost resonators 3A and 3C in the areas adjacent to the conductor 9 on the inner resonator 3B with a coupling gap provided therebetween. The free ends of the outer coupling electric paths 11a are provided with respective arcuate coupling tips lie to surround the respective openings of the through bores 4A and 4C with a coupling gap g1 disposed therebetween. The inner coupling electric paths 12a are L-shaped and provided with respective straight coupling tips 12e separated from the conductor 9 by a coupling gap g2. The input/output pads 10 are capacitively coupled to the respective resonators 3A and 3C by way of the respective outer coupling electric paths 11a and also to the resonator 3B by way of the respective inner coupling electric paths 12a. FIG. 4A shows the attenuation waveform of the illustrated dielectric filter. FIG. 4B shows the attenuation waveform of a comparable dielectric filter realized with no inner coupling electric paths. By comparing the two graphs, it will be appreciated that the waveform of FIG. 4B has a fly back x produced in a higher frequency zone of the resonance frequency band under the influence of the coupling through bores 6 to hold the output level above the threshold value of -50dB. Contrary to this, the fly back x is reduced to less than -50dB in this illustrated embodiment to make the filter perform remarkably in terms of attenuation. FIG. 5 illustrates a second embodiment of dielectric filter 1b according to the invention having outer coupling electric paths 11b having a profile different from that of their counterparts of the first embodiment illustrated in FIGS. 2 and 3. In this second embodiment, rectangularly extended conductors 13 are arranged on the open-circuit end surface 8A of dielectric ceramic block 2, surrounding the respective open ends of the resonators 3A and 3C, and connected to the internal conductors 5 thereof on the peripheral walls of the through bores 4A and 4C. Thus, the outer coupling electric paths 11b are linearly formed and arranged in juxtaposition with the respective front edges of the rectangular conductors 13 with a coupling gap g1 provided therebetween. Meanwhile, inner coupling electric paths 12b similar to their counterparts of the first embodiment are extending from the respective input/output pads 10 and arranged in juxtaposition with the respective lateral edges of the rectangular conductor 9 of the resonator 3B with a coupling gap g2 provided therebetween. In each of the above embodiments, the input/output pads 10 are capacitively coupled to the respective resonators 3A and 3C by way of the respective outer coupling electric paths 11a and 11b and also to the resonator 3B by way of the respective inner coupling electric paths 12a and 12b. Unlike known dielectric filters where the input/output pads are juxtaposed with along a lateral side of the dielectric block and capacitively coupled to the respective resonators by way of dielectrics, the input/output pads 10 of each of the above embodiments can be displaced appropriately in the direction of the arrows in FIGS. 2 and 5 along the lateral surface 8E of the dielectric ceramic block 2 to respective positions corresponding to the related conductors on a printed circuit board so long as the electric connections between the pads and the outer coupling electric paths and between the pads and the respective inner coupling electric paths remain. FIG. 6 illustrates a third embodiment of dielectric filter 1c according to the invention, where the input/output pads 10 are arranged respectively on oppositely disposed lateral surfaces 8C and 8D of the dielectric ceramic block 2. With this arrangement, the input/output pads 10 may be exposed on the printed circuit board on which the dielectric filter is arranged so that the related leads may be arranged easily to connect the dielectric filter and the printed circuit board by way of the input/output pads 10. In this embodiment again, outer coupling electric paths 11c and inner coupling electric paths 12c are extending respectively from the input/output pads 10 and juxtaposed with the respective resonators 3A, 3B and 3C with coupling gaps g1 and g2 provided therebetween for capacitive coupling in a manner as described above by referring to the first and second embodiments. As will be understood from the above description, a variety of different profiles and arrangements may be conceivable for the outer and inner coupling electric paths corresponding to the positions of the input/output pads 10 and the contours of the conductors 9 and 13. FIG. 7 shows a circuit diagram of an equivalent circuit of the illustrated filter 1a. FIGS. 8 and 9 schematically illustrate a fourth embodiment of dielectric filter 1d according to the invention which is to be used as a duplexer comprising a transmitter section T and a receiver section R. In this embodiment, the transmitter section T includes a pair of input/output pads 10a and 10b, outer and inner coupling electric paths 11d and 12d extending respectively from the input/output pads 10a and 10b and arranged respectively in juxtaposition with the substantially rectangular conductors 9 and 14 of resonators 3A, 3B and 3C for capacitive coupling in a manner as described earlier. In this embodiment, the rectangular conductors 14 of the outermost resonators 3A and 3C may be provided with notches 15 and the outer coupling electric paths 11d may be provided with corresponding coupling projections 16 projecting into the respective notches 15, whereas the inner coupling electric paths 12d have a L-shaped profile as in the case of the preceding embodiments. Note that, in this embodiment, the outer input/output pad 10a actually operates as an input pad whereas the inner input/output pad 10b operates as an output pad. The receiver section R of this embodiment has a four-poled structure in which four resonators 17A, 17B, 17C and 17D are provided in a row. An output pad 10c is capacitively coupled to the outermost resonator 17D of the receiver section R whereas the inner input/ output pad 10b of the transmitter section T is shared by the receiver section R and operating as an input pad of the latter. With the embodiment illustrated in FIGS. 8 and 9, alternatively the rectangular conductors 14 of the outermost resonators 3A and 3C may be formed without any notches as in the arrangement of the second embodiment illustrated in FIG. 5. FIG. 10A shows the waveform of the transmission wave Tx and that of the reception wave Rx of dielectric filter 1d in the fourth embodiment. FIG. 10B a comparable graph obtained by a dielectric filter realized by removing only the inner coupling electric paths 12d and 12d from the fourth embodiment. By comparing the two graphs, it will be appreciated that the fly back y is remarkably suppressed in the transmission wave Tx of FIG. 10A. With the respective embodiments, the resonators 3A, 3B and 3C are realized by using through bores having a circular cross section, the through bores may be replaced by through holes having a square, oval or rectangular cross section. Thus, resonators having a variety of different cross sectional views may be conceivable. Thus, in the dielectric filter embodying the invention, with the provision of the outer coupling electric paths extending from the respective input/output pads onto the open-circuit end surface of the dielectric ceramic block and arranged in juxtaposition with the respective outermost resonators with a coupling gap provided therebetween and the provision of the inner coupling electric paths also extending from the respective input/output pads onto the open-circuit end surface of the dielectric ceramic block and arranged between the outermost resonators in juxtaposition with the inner resonator or the respective inner resonators adjacent to the outermost resonators with a coupling gap provided therebetween, the input/output pads and the inner resonator or resonators are capacitively coupled with the respective input/output pads to remarkably reduce or eliminate the influence of the fly back x or y and consequently improve the attenuation performance of the dielectric filter and reduce the generation of noise.

Claims: A dielectric filter comprising a dielectric ceramic block (2), three or more than three resonators (3A, 3B, 3C) provided in a row in said dielectric ceramic block (2) respectively, each resonator (3A, 3B, 3C) including a through bore (4A, 4B, 4C) and an internal conductor (5) on a peripheral wall of the through bore (4A, 4B, 4C), an external conductor (7) covering the outer surfaces (8B, 8C, 8D, 8E, 8F) of said dielectric ceramic block excepting an open-circuit end surface (8A) having openings of the through bores (4A, 4B, 4C) and a pair of input/output pads (10) arranged on a lateral surface (8E; 8C, 8D) of said dielectric ceramic block (2) at respective positions abutting the open-circuit end surface (8A) of the dielectric ceramic block (2) and facing the respective outermost resonators (3A, 3C) in such a way that they are electrically insulated from said external conductor (7), the filter comprises outer coupling electric paths (11a, 11b, 11c) extending from said respective input/output pads (10) onto the open-circuit end surface (8A) and arranged in juxtaposition with the respective outermost resonators (3A, 3C) with a first coupling gap (g1) provided therebetween and inner coupling electric paths (12a, 12b, 12c) extending from the respective input/output pads (10) onto the open-circuit end surface (8A) and arranged between the outermost resonators (3A, 3C) in juxtaposition with the inner resonator or the respective inner resonators (3B) adjacent to the outermost resonators (3A, 3C) with a second coupling gap (g2) provided therebetween, characterized in that coupling through bores (6) are arranged between adjacent ones of said resonators (3A, 3B, 3C) in parallel with the latter. A dielectric filter as claimed in claim 1, wherein it further comprises a rectangularly extended conductor (9) arranged on the open-circuit end surface (8A) of said dielectric ceramic block (2) to surround the internal conductor (5) of said inner resonator or each of said respective inner resonators (3B) and connected to the internal conductor (5) of said inner resonator or each of said respective inner resonators (3B). A dielectric filter as claimed in claim 2, wherein each of said inner coupling electric paths (12a, 12b, 12c) has a free end arranged to be adjacent to said respective rectangularly extended conductor (9) on the open-circuit end surface (8A) of said dielectric ceramic block (2). A dielectric filter as claimed in claim 1, wherein each of said outer coupling electric paths (11a, 11b, 11c) has a free end provided with an arcuate coupling tip (11e) to surround the opening of the associated through bore (4A, 4C) with the first coupling gap (g1) therebetween. A dielectric filter as claimed in claim 1, wherein it further comprises rectangularly extended conductors (13) arranged on the open-circuit end surface (8A) of said dielectric ceramic block (2) to surround the internal conductors (5) of said respective outermost resonators (3A, 3C) and connected to the internal conductors (5) of said respective outermost resonators (3A, 3C). A dielectric filter as claimed in claim 1, wherein the lateral surface of said dielectric ceramic block (2) where said input/output pads are disposed is an outer surface portion (8E) to be brought into contact with a printed circuit board when the filter is mounted on the circuit board. A dielectric filter as claimed in claim 1, wherein said input/output pads (10) are provided on other outer surface portions (8C, 8D) than that to be brought into contact with a printed circuit board when the filter is mounted on the circuit board. A dielectric filter as claimed in claim 1, wherein each of said rectangularly extended conductors (13) arranged to surround the internal conductors (5) of said respective outermost resonators (3A,3C) and connected thereto is provided with at least one notch, and each of said outer coupling electric paths (11a, 11b, 11c) is provided with a corresponding coupling projection which is engaged with said notch. The dielectric filter of any preceding claim, in which the three or more resonators (3A, 3B, 3C) and the outer coupling electric paths together form a transmitter section (T) of a duplexer, the duplexer further comprising a receiver section (R). The dielectric filter of claim 9, wherein the receiver section (R) comprises a plurality of resonators (17A, 17B, 17C, 17D) provided in a row, one of said input/output pads (10b) is associated with the innermost resonators (3c) of the transmitter (T) and receiver (R) sections and is arranged to be operated as an input pad for said receiver section (R), and an output pad (10c) is capacitively coupled to the outermost resonator (17D) of the receiver section (R).

Inventors: HINO SEIGO, HINO, SEIGO
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD.
Application year: 1997
Main classification: H01P1
IPC codes: H01P1