Dielectric filter, dielectric duplexer, and communication apparatus

Abstract: A stepped portion (21) is formed inside the inner-conductor holes (2a, 2c) arranged between both side surfaces of a dielectric block (1). The axis at an open end of the holes (2a, 2c) is made to be different from the axis at a short-circuited surface. The open ends of the inner-conductor holes (2a, 2c) are formed so as to be close to corresponding input-output electrodes (5a, 5b).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dielectric filter and dielectric duplexer which have a plurality of inner-conductors inside a dielectric block, an external conductor disposed on the outer surface of the dielectric block and input-output electrodes disposed on the outer surface of the dielectric block and capacitance-coupled to inner-conductors, and to a communication apparatus including the same. 2. Description of the Related Art A dielectric filter having, for example, the construction shown in Fig. 7 has been disclosed (Japanese Utility Model Application No. 6-2802). Fig. 7 is a sectional view passing through two input-output electrodes. In this dielectric filter, three inner-conductor holes 2a, 2b, and 2c passing through between a pair of opposing end surfaces of a dielectric block 1 in the form of a nearly rectangular solid are provided, and inner-conductors 3a, 3b, and 3c are formed on respective internal surfaces thereof. At fixed locations on the external surface of the dielectric block 1 a pair of input-output electrodes 5a and 5b are formed, and on nearly the whole of the external surface, except the areas where the input-output electrodes 5a and 5b are formed, an external electrode 4 is formed. This dielectric filter is externally coupled via external coupling capacitances C1 and C2 generated between the input-output electrodes 5a and 5b and the inner-conductors 3a and 3c, respectively, which oppose the electrodes. Each of the inner-conductor holes 2a, 2b, and 2c is formed as a straight hole of the same axis having a constant inner diameter, and, of the three inner-conductor holes 2a, 2b, and 2c, the inner-conductor holes 2a and 2c acting as an input-output stage are arranged dose to the sides where the input-output electrodes 5a and 5b are formed. In this way, without allowing the Q 0 value of a resonator to fall by arranging an inner-conductor hole opposed to an input-output electrode to be close to the input-output electrode, a large external coupling capacitance can be obtained through an input-output electrode with a relatively small area. However, in the above conventional dielectric filter, as the whole inner-conductor holes acting as input-output stages are arranged close to the side surfaces where the input-output electrodes are formed, there is a problem of the Q 0 valve of the resonator being degraded. This is because the best Q 0 value of the resonator is obtained when the inner-conductor hole is arranged in the middle of the dielectric block and the Q 0 value deteriorates further when the hole is set further from the middle. EP A 0 664 572 discloses a dielectric filter, comprising a dielectric block, resonator holes having steps consisting of a portion having a larger inner diameter and a portion having a smaller inner diameter, wherein the smaller inner diameter portion of each resonator hole is formed on the side of a short-circuited end surface. By forming the smaller inner diameter portions of the resonator holes, close to each other, the coupling between the two resonators becomes inductive coupling. EP A 0 853 349 discloses a dielectric filter in which a stronger electromagnetic coupling than in conventional dielectric filter can be provided between adjacent resonator holes without changing the external shape and dimensions of a dielectric block. The resonator holes pass through opposing surfaces of a dielectric block, each including a large diameter hole section and a the small diameter hole section. The small diameter hole sections may be formed near a short-circuited end face of the dielectric block. The large diameter hole sections in the small diameter hole sections are connected to each other with their axes shifted from each other. EP A 0 863 566 discloses a dielectric filter. The dielectric filter has a dielectric block with resonator holes formed therein. Each of the resonator holes has a large-sectional-area portion and a small-sectional area portion so that the resonator hole has different respective inner diameters at an open-circuited end and the short-circuited end. It is the object of the present invention to provide a dielectric filter having an increased external coupling capacitance yet maintaining a high Q 0 value. This object is achieved by a dielectric filter according to claim 1. According to a further aspect, the invention provides for a dielectric duplexer and a communication apparatus including the inventive filter. One preferred embodiment of the present invention provides a dielectric filter comprising all features as set out in claim 1. Another preferred embodiment of the present invention provides a dielectric duplexer comprising at least two filter portions formed in a dielectric block, wherein at least one of the filter portions is composed of the above described dielectric filter. Yet another preferred embodiment of the present invention provides a communication apparatus comprising at least one of the above described dielectric filters and the dielectric duplexer. In a dielectric filter or dielectric duplexer having the above construction, as the open end of the inner-conductor hole opposed to an input-output electrode (first inner-conductor hole portion) is arranged so as to be near the input-output electrode, that is, as only part of the inner-conductor hole is made to be eccentric so as to be close to the input-output electrode, a large external coupling capacitance can be obtained, and the Q 0 value is less reduced. That is, when compared with conventional examples where the whole inner-conductor hole is made near an input-output-electrode, the Q 0 value of the resonator can be less reduced. Furthermore, when the first inner-conductor hole portion is formed so as to have a larger diameter than the other portion (second inner-conductor hole portion), the external coupling capacitance can be further increased. Therefore, a dielectric filter and dielectric duplexer having a small insertion loss and suitable characteristics can be obtained. Further, a communication apparatus according to the present invention is composed of a dielectric filter or dielectric duplexer having the above features and accordingly exhibits suitable characteristics. Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a perspective schematic illustration of a dielectric filter of a first embodiment; Fig. 2 is a sectional view taken along line X - X of Fig. 1; Fig. 3 is a sectional view taken on line Y - Y of Fig. 1; Fig. 4 is a sectional view passing through two input-output electrodes of a second embodiment; Fig. 5 is a perspective schematic illustration of a dielectric duplexer of a third embodiment; Fig. 6 is a block diagram of a communication apparatus of a fourth embodiment; and Fig. 7 is a sectional view passing through two input-output electrodes of a conventional dielectric filter. DESCRIPTION OF THE PREFERRED EMBODIMENTS The construction of a dielectric filter according to a first embodiment of the present invention is explained with reference to Figs. 1 through 3. Fig. 1 is a perspective illustration of the dielectric filter, Fig. 2 a sectional view taken along line X - X of Fig. 1, and Fig. 3 a sectional view taken along line Y - Y of Fig. 1. More, in the following perspective illustrations, the dotted shaded areas show the ground of dielectric blocks. The dielectric filter of the present embodiment is composed of a dielectric block 1 in the form of a nearly rectangular solid (hexahedron). Three inner-conductor holes 2a, 2b, and 2c passing through the dielectric block 1 between a pair of opposing end surfaces are formed, and inner-conductors 3a, 3b, and 3c, respectively, are formed on the inside surfaces of the holes. At fixed locations of the outer surface of the dielectric block 1 a pair of input-output electrodes 5a and 5b are formed, and on nearly the whole of the outer surface, except the areas where the input-output electrodes 5a and 5b are formed, an external conductor 4 is formed. In the vicinity of one opening surface of the inner-conductor holes 2a through 2c a portion having no inner-conductor (nonconductive portion) "g" is given, and this portion is made an open end of resonators composed of the inner-conductors 3a through 3c. That is, at one end surface (end surface at the front side of Fig. 1) each of the inner-conductors 3a through 3c is separated from the external conductor 4 by the nonconductive portion "g", and at the other end surface (end surface on the other side of Fig. 1) each of the inner-conductors is connected (short-circuited) to the external conductor 4, and this end surface is made a short-circuited surface. Each of the input-output electrodes 5a and 5b is formed over one main surface close to the open end and the neighboring side surface of the dielectric block 1. This dielectric filter is externally coupled through external coupling capacitances C1 and C2 generated between the input-output electrodes 5a and 5b and the respective inner-conductors 3a and 3c opposed to the electrodes. Further, the dielectric filter is mounted on a mounting board so that the surface where the input-output electrodes 5a and 5b are formed (upper surface of Fig. 1) becomes a mounting surface. Inside the inner-conductor holes 2a and 2c arranged at both side surfaces of the dielectric block a stepped portion 21 is formed to define a first inner-conductor hole portion and a second inner-hole portion divided thereby. The first inner-conductor hole portion has the open end and the second inner-hole portion extends to the short-circuited surface. The axis of the first inner-conductor hole portion is located at a different portion from that of the second inner-hole portion. Moreover, the inner diameters of the first inner-conductor hole portion and the second inner-hole portion are formed to be the same. The first inner-hole portion of the inner-conductor hole 2a is provided to be close to the input-output electrode 5a, and the first inner-hole portion of the inner-conductor hole 2c is provided near the input-output electrode 5b. The second inner-hole portion of the inner-conductor holes 2a and 2c and the inner-conductor hole 2b are arranged in the middle of the dielectric block 1 in the thickness direction. Thus, because in the dielectric filter of the present embodiment the input-output electrodes 5a and 5b are constructed to be closer to the inner-conductors 3a and 3c, respectively, which act as input-output stages and which are opposed to the electrodes, large external coupling capacitances C1 and C2 can be realized. Furthermore, as only the first inner-hole portion of the inner-conductor holes 2a and 2c are made eccentric so as to be dose to the input-output electrodes 5a and 5b, respectively, the Q 0 value is less reduced. It is desirable that the above stepped portion 21 be set at a location in which the desired external coupling capacitance can be obtained and be set as close as possible to the end surface at the open end so that the Q 0 value is less reduced. Further, the input-output electrodes are also generally provided to be as close as possible to the end surface at the open end so that the external coupling capacitance is increased. Next, the construction of a dielectric filter according to a second embodiment of the present invention is shown in Fig. 4. The appearance of the dielectric filter is nearly the same as that of the first embodiment shown in Fig. 1, but, as shown in the sectional view of Fig. 4, the inner diameter of the first inner-hole portion of the inner-conductor holes 2a and 2c opposed to the input-output electrodes 5a and 5b, respectively, are made larger than the inner diameter of the second inner-hole portion thereof. The first inner-hole portions of the inner-conductor holes 2a and 2c are arranged so as to be close to the input-output electrodes 5a and 5b, respectively. Because of this construction, larger external coupling capacitances can be obtained compared with the case of the first embodiment. In each of the above embodiments, although the inner-conductor holes having a substantially circular cross-section were shown, the cross-section of the inner-conductor holes is not limited to only a circular shape, and it may be polygonal, such as a square, etc., or an oval shape. The inner-conductor holes may be of mixed shapes. Further, the input-output electrodes may be formed only in a main surface acting as a mounting surface. Next, the construction of a dielectric duplexer according to a third embodiment of the present invention is shown in Fig. 5. In the dielectric duplexer, a transmission side composed of a bandpass filter of a three-stage resonator and a reception side composed of a bandpass filter of a four-stage resonator are formed in a dielectric block 1 in the form of a rectangular solid. Inner-conductor holes 2a through 2c constituting the resonators on the side of the transmission filter, inner-conductor holes 2d through 2g constituting the resonators on the side of the reception filter, and an inner-conductor hole 2h to obtain an external coupling common to both filters all pass through the dielectric block 1 between a pair of opposing end surfaces. On the inside surface of each of the inner-conductor holes 2a through 2h acting as resonator holes, inner-conductors 3a through 3h are formed, respectively. In the vicinity of one opening surface of each of the inner-conductors 3a through 3g, a portion having no conductor "g" is provided, and this portion is made an open end of each of the resonators. On the outer surface of the dielectric block 1, input-output electrodes 5a, 5b, and 5c are formed, and an external conductor 4 is formed on nearly the whole surface except the areas where the input-output electrodes 5a through 5c are formed. The input-output electrode 5a acting as a transmission terminal and the input-output electrode 5c acting as a reception terminal are formed over one main surface close to the end surface at the open end and over side surfaces of the dielectric block 1, and the input-output electrode 5b acting as an antenna terminal common to both filters is formed over the main surface and a short-circuited surface of the dielectric block. The inner-conductor 3h of the inner-conductor hole 2h is connected to the external conductor on the end surface at the open end and to the input-output electrode 5b on the short-circuited surface. Inside each of the inner-conductor hole 2a opposed to the input-output electrode 5a and the inner-conductor hole 2g opposed to the input-output electrode 5c, a stepped portion 21 is formed to define a first inner-conductor portion and a second inner-conductor portion. The first inner-hole portion has the open end and the second inner-hole portion extends to the short-circuited surface. The location of the axis of the first inner-conductor portion and that of the second inner-hole portion are formed so as to be different from each other. Further, each of the axis of the first inner-conductor portions are formed so as to be dose to the corresponding input-output electrodes 5a and 5c. In the dielectric duplexer, the input-output electrodes 5a and 5c are capacitance-coupled to the respective inner-conductors 3a and 3g opposed to the electrodes, and the inner-conductors are externally coupled through the external coupling capacitance. The inner-conductor 3h is electromagnetically coupled (meaning interdigital coupling in the present embodiment) to the neighboring inner-conductors 3c and 3d, and is externally coupled via this coupling. Also in the dielectric duplexer, as the first inner-hole portion of each of the inner-conductors 3a and 3g is constructed so as to be close to the input-output electrodes 5a and 5c, respectively, a large external coupling capacitance can be obtained and the Q 0 value is less reduced. In the present embodiment, each of the first and second inner-hole portions of the inner-conductor holes is shown to have the same inner diameter over its entire length. However, the holes are not limited to this, and the inner-conductor holes may be constructed to have the first and second inner-hole portions which has different sectional areas. Further, in the present embodiment, the common input-output portion (input-output electrode 5b) is constructed so as to be externally coupled via an electromagnetic coupling, but by providing the common input-output electrode on one main surface, the common input-output electrode may be externally coupled through the capacitance-coupling between the common input-output electrode and the inner-conductor opposed to the electrode. In this case, the inner-conductor holes opposed to the input-output electrodes of the reception-side filter and transmission-side fitter are formed as holes having a stepped portion, and the first inner-hole portion of each hole may be formed so as to be close to the input-output electrode. Moreover, in each of the above embodiments, an open end formed by providing a portion having no inner-conductor at one end portion of each of the inner-conductors was described, however, without forming any external conductor in one opening surface of the inner-conductor holes, the opening surface may be formed as an open surface of each of the resonators.. Furthermore, a coupling electrode connected to the inner-conductor may be formed in the open surface. Further, a comb-like type in which the open end of each of the resonators is arranged at one end surface of the dielectric block was described. However, the invention is not limited thereto, and the open end of each of the resonators may be arranged on any end surface. Next, the construction of a communication apparatus according to a fourth embodiment of the present invention is shown in Fig. 6. In Fig. 6, reference numeral 122 represents an antenna, 123 a duplexer, 124 a transmission filter, 125 a reception filter, 126 a transmission circuit, and 127 a reception circuit. The communication apparatus is constructed by connecting an antenna terminal ANT of the duplexer 124 to the antenna 122, a transmission terminal Tx to the transmission circuit 126, and a reception terminal Rx to the reception circuit 127. Here, a dielectric filter of the first or second embodiment can be used for the transmission filter 124 or the reception filter 125, and a dielectric duplexer described in the third embodiment can be used for the duplexer. By using a dielectric filter or duplexer according to the present invention a communication apparatus with suitable characteristics can be realized. As explained above, because a dielectric filter or duplexer according to the present invention is formed so that the open end of the inner-conductor hole opposed to the input-output electrode is close to the input-output electrode, a large external coupling capacitance can be obtained and the Q 0 value is less reduced, and accordingly high a Q 0 value can be realized. Further, by making the diameter at the open end of the inner-conductor hole opposed to the input-output electrode larger than that of the other portion, the external coupling capacitance can be further increased. Therefore, according to the present invention, a dielectric filter and dielectric duplexer having small insertion loss and suitable characteristics can be obtained. Further, by mounting a dielectric filter or duplexer according to the present invention a communication apparatus with suitable characteristics can be obtained.

Claims: A dielectric filter comprising: a dielectric block (1); an external conductor (4) disposed on an outer surface of the dielectric block (1); input-output electrodes (5a, 5b) disposed on the outer surface of the dielectric block (1); and a first inner-conductor hole (2a), a second inner-conductor hole (2b), and a third inner-conductor hole (2c), wherein the first inner-conductor hole (2a) and the third inner-conductor hole (2c) are capacitance-coupled to the input-output electrodes (5a, 5b), the second inner-conductor hole (2b) is disposed between the first inner-conductor hole (2a) and the third inner-conductor hole (2c), each of the first, second and third inner conductor hole (2a, 2b, 2c) has a first inner-conductor hole portion, a second inner-conductor hole portion, and an inner-conductor (3a, 3b, 3c) disposed on an inner surface thereof provided in the dielectric block (1), each of the inner-conductors (3a, 3b, 3c) has an open end in at least one opening surface of the inner-conductor hole (2a, 2, 2c) or in the vicinity of the opening surface; the first and third inner-conductor holes (2a, 2c) are adjacent to the input-output electrodes (5a, 5b) and have a stepped portion (21) between the first inner-conductor hole portion and the second inner-conductor hole portion, the first inner-conductor hole portion of the first and third inner-conductor holes (2a, 2c) has the open end and is located closer to the input-output electrode (5a, 5b) than the second inner-conductor hole portion of the first and third inner-conductors holes (2a, 2c), and the second inner-hole portions of the first, second and third inner-conductor holes (2a, 2b, 2c) are arranged in the middle of the dielectric brock (1) in the thickness direction; characterized in that the first inner-conductor hole portions of the first and third inner-conductor holes (2a, 2c) are offset from a plane in the dielectric block (1) in which the second inner-conductor hole portions of the first, second and third inner-conductor holes (2a, 2b, 2c) are disposed, whereby the first inner-conductor hole portions of the first and third inner-conductor holes (2a, 2c) are close to the input-output electrodes (5a, 5b), the sectional area of the first inner-conductor hole portion of the first and third inner-conductor holes (2a, 2c) is larger than that of the second inner-conductor hole portion of the first and third inner-conductor holes (2a, 2c), and the sectional area of the first inner-conductor hole portion of the second inner-conductor hole (2b) is the same as that of the second inner-conductor hole portion of the second inner-conductor hole (2b). The dielectric filter according to claim 1, wherein the sectional area of the first inner-conductor hole portion of the first, second and third inner-conductor holes (2a, 2b, 2c) is larger than that of the second inner-conductor hole portion of the first, second and third inner-conductor holes (2a, 2b, 2c). A dielectric duplexer comprising at least two filter portions formed in a dielectric block (1), wherein at least one of the filter portions is composed of the dielectric filter of one of claims 1 or 2. A communication apparatus comprising at least one of the dielectric filters of one of claims 1 or 2 and the dielectric duplexer of claim 3.

Inventors: ISHIHARA JINSEI, KATO HIDEYUKI, OKADA TAKAHIRO, ISHIHARA, JINSEI, KATO, HIDEYUKI, OKADA, TAKAHIRO, Ishihara, Jinsei, c/o(A170) Intell.Prop. Dept., Kato, Hideyuki, c/o(A170) Intell.Prop. Dept., Okada, Takahiro, c/o(A170) Intell.Prop. Dept.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 2000
Main classification: H01P1
IPC codes: H01P1, H01P7