Dielectric filter device

Abstract: A dielectric filter device including at least two dielectric resonator bodies juxtaposed to each other each provided with a through bore which has an inner conductive layer, a first dielectric substrate provided with electrodes on its surfaces for forming an input and output capacities and a second dielectric substrate provided with capacitvely coupled electrodes on its inner surfaces for forming a coupling capacity and an electrode which cooperates with the capacitvely coupled electrodes to form stray capacities, the first and second dielectric substrates being superimposed and mounted on the front surfaces of the juxtaposed dielectric resonator bodies. Each dielectric resonator body comprises two longitudinally divided halves, and the innser surface of one of the halves which form the outermost resonator body are provided with conductive connecting lines each of which has one end connected with the inner conductive layer in the through bore and the other end connected with an input or output terminal provided on the lateral surface of the associated resonator body.

Description: BACKGROUND OF THE INVENTION The present invention relates to a dielectric filter device having a plurality of juxtaposed coaxial dielectric resonators. An example of a conventional dielectric filter device of this kind is disclosed in Japanese Patent Kokai No. 3-136502 in which the dielectric filter device comprises a plurality of juxtaposed coaxial dielectric resonators, each of the dielectric resonators includes an outer conductive layer provided on its outer surface except its front surface and an axially extended bore whose inner surface is coated with an inner conductive layer. Each dielectric resonator is covered with a printed-circuit board with which a plurality of conductors having a predetermined pattern are connected so as to form a capacitor circuit which generates a coupling capacitance. Each conductor is connected with the inner conductive layer of the associated coaxial resonator. An input and output terminals are connected with the inner conductive layers of the outermost positioned coaxial resonators with which capacitors may be externally connected to maintain an input and output capacities. Also, In order to adjust a frequency response of the filter device and reduce the length of each resonator stray capacities may be provided. In that case a capacitor is connected between the inner conductive layer of each coaxial resonator and a ground terminal. However, such a conventional filter device has disadvantages that the arrangement becomes complicated and is bulky because the input and output coupling capacities and the couping between the resonators are made by the same dielectric substrate and it is necessary to provide elements to be externally mounted other than the printed-circuit board for ensuring the required capacities and the stray capacities. Also it is difficult or substantially impossible to set and adjust the capacities after the filter is assembled. Japanese Patent Kokai No. 61-156903 discloses another conventional filter device which comprises a plurality of juxtaposed coaxial dielectric resonators each including an axially extended bore whose inner surface is provided with an inner conductive layer, a connecting terminal having one end inserted into the bore and connected with the inner conductive layer and other end extended from the front surface, and an outer conductive layer provided on the intended portion of the outer surface. Each resonator is constructed by superimposing two dielectric block halves divided longitudinally in such a manner that bore halves provided in them are opposited so as to complete the bore. With the arrangement disclosed in this reference, it is essential that lead wires should be connected with the inner conductive layers positioned at both the outermost coaxial dielectric resonators of the filter by soldering or the like. Therefore, lead wire guiding holes should be provided on both side portions of the dielectric block halves in a direction orthogonal to the axis of the bore halves. Each lead wire is guided outwardly through the corresponding hole. The guiding holes are given an enough inner dimension to prevent the thickness of the lead wires or any solder from obstructing the jointing of the dielectric block halves. However, to form the block halves with such lead wire guiding holes a complicated mold must be prepared, which results in that not only the forming of the block halves is costly, but also the block halves is weakened because of the present of lead wire guiding holes. SUMMARY OF THE INVENTION According to the present invention, there is provided a dielectric filter device having the features set out in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an exploded perspective view of a dielectric filter device, which does not embody the present invention but is shown for the better understanding thereof; Fig. 2 is a plan view of a first dielectric substrate used in the dielectric filter device of Fig. 1, wherein (A) and (B) show an inner and outer surfaces thereof, respectively; Fig. 3 is a plan view of a second dielectric substrate used in the dielectric filter device of Fig. 1, wherein (A) and (B) show an inner and outer surfaces thereof, respectively; Fig. 4 is a schematic longitudinal section of the dielectric filter device of Fig. 1 taken along the center axis of one of dielectric resonators; Fig. 5 is an equivalent circuit diagram of the dielectric filter device shown in Fig. 1; Fig. 6 is an exploded perspective view of a dielectric filter device embodying the present invention; Fig. 7 is a plan view of a dielectric substrate used in the dielectric filter device of Fig. 6, wherein (A) and (B) show an inner and outer surfaces thereof, respectively; Fig. 8 is a schematic cross section of the dielectric filter device of Fig. 6 taken along the axis of an input and output terminals; Fig. 9 is a schematic longitudinal section of the dielectric filter device of Fig. 6 taken along the center axis of one of dielectric resonators; and Fig. 10 is an equivalent circuit diagram of the dielectric filter device shown in Fig. 6. DETAILED DESCRIPTION Referring to Figs. 1 to 4, there is shown a dielectric filter device. Although this device does not embody the present invention, its operation will be described to allow a better understanding thereof. The illustrated filter device comprises two juxtaposed dielectric coaxial resonator bodies 1a and 1b each of which is made of titanium oxide dielectric ceramic material and is shaped as a rectangular parallelepiped. The resonator bodies 1a and 1b have through bores 2a and 2b, respectively, each of which extends from the front end to rear end along the center axes thereof. Each of the through bores 2a and 2b has an inner surface provided with an inner conductive layer 3a or 3b. Each of the resonator bodies 1a and 1b is provided with an outer conductive layer 4a or 4b on the outer surface except the front surface thereof. Reference numerals 5a and 5b represent connecting plug members of metal each of which has one end securely fitted into the front end portion of the bore 2a or 2b to ensure the electric connection thereof with inner conductive layer 3a or 3b and the other end or front end 6a or 6b of reduced diameter extended outwardly from the front surface of the associated resonator body 1a or 1b. A first and second dielectric substrates 7 and 8 are superimposed to each other and are disposed over the front surfaces of the resonator bodies 1a and 1b. These dielectric substrates are made of dielectric ceramic material. The first dielectric substrate 7 has an inside surface on which as shown in Fig. 2-(A) an input and output conductors 9a and 9b (or 9b and 9a) are formed opposite to the coaxial resonator bodies 1a and 1b, respectively and an outside surface on which as shown in Fig. 2-(B) connecting conductors 10a and 10b are formed opposite to the input and output conductors. Fitting holes 13a and 13b are provided to be extended through the input or output conductor, the first dielectric substrate 7 and the connecting conductor for inserting the front ends 6a and 6b of the connecting plug members 5a and 5b, respectively. The input and output conductors 9a and 9b (or 9b and 9a) are partially removed at the edge portions of the fitting holes 13a and 13b so that they are not connected with the inserted connecting plug members 5a and 5b as shown in Figs. 2-(A) and 4. The connecting conductors 10a and 10b are spread to the edge portion of the fitting holes 13a and 13b so that they are connected with the front ends 6a and 6b of the inserted connecting plug members 5a and 5b as shown in Figs. 2-(B) and 4. The first dielectric substrate 7 is also provided with two slots 14a and 14b at the positions where they do not come into contact with the connecting conductors 10a and 10b. The second dielectric substrate 8 has an inside surface on which as shown in Fig. 3-(A) conductors 15a and 15b having an interdigitated pattern are so formed that they come into contact with the connecting conductors 10a and 10b, respectively, when the first and second dielectric substrates 7 and 8 are superimposed to each other. The interdigitated portions of both the conductors 15a and 15b form a capacitive coupling. On the outside surface of the second dielectric substrate 8, as shown in Fig. 3-(B), a ground conductor 16 is formed opposite to the conductors 15a and 15b. Fitting holes 17a and 17b are provided to be extended through the conductors 15a and 15b, the second dielectric substrate 8 and the ground conductor 16 for inserting the front ends 6a and 6b of the connecting pulg members 5a and 5b, respectively. The ground conductor 16 is partially removed at the edge portions of the fitting holes 17a and 17b so that it is not connected with the inserted connecting plug members 5a and 5b as shown in Fig. 4. The conductors 15a and 15b are connected through the connecting conductors 10a and 10b on the first dielectric substrate 7 with the respective connecting pulg members 5a and 5b and thus the inner conductive layers 3a and 3b in the resonator bodies 1a and 1b. Alternatively, the conductors 15a and 15b may be arranged so that they are directly connected with the respective connecting pulg members 5a and 5b. Also, the second dielectric substrate 8 is provided with slots 18a and 18b at the positions where they do not come into contact with the conductors 15a and 15b but are aligned with the slots 14a and 14b in the first dielectric substrate 7. Terminal plates 19a and 19b are brought into contact with the input and output conductors 9a and 9b (or 9b and 9a) on the first substrate 7. The terminal plates 19a and 19b have connecting legs 20a and 20b which are inserted into the slots 14a and 18a; 14b and 18b in the first and second substrates 7 and 8, and are connected with an external input and output conductor lines on a printed-circuit board (not shown), thereby connecting the input and output conductors 9a and 9b with the external input and output conductor lines, respectively. Also, the ground conductor 16 is grounded via a casing or the like not shown. When the first and second dielectric substrates 7 and 8 are superimposed to each other and mounted on the front portion of the juxtaposed resonator bodies 1a and 1b, the inner conductive layers 3a and 3b in the resonator bodies 1a and 1b are electrically connected via the connecting plug members 5a and 5b with the connecting conductors 10a and 10b on the outside surface of the first dielectric substrate 7. Each of the input and output conductors 9a and 9b and the corresponding connecting conductor 10a or 10b between which the first dielectric substrate 7 is sandwiched form an input and output capacities C1 and C2, respectively as shown in the equivalent circuit of Fig. 5. The input and output capacities C1 and C2 are connected via the terminal plates 19a and 19b with an external input and output terminals 21 and 22, respectively. The connecting conductors 10a and 10b are connected with the interdigitated conductors 15a and 15b whose capacitive coupling forms a coupling capacity C3 (Fig. 5). Each of the interdigitated conductors 15a and 15b and the ground conductor 16 between which the second dielectric substrate 8 is sandwiched form stray capacities C4 and C5, respectively as shown in the equivalent circuit of Fig. 5. These stray capacities C4 and C5 are grounded via the ground conductor 16. In this embodiment the filter device comprises two coaxial resonator bodies. However the filter device may be constructed by using three or more coaxial resonator bodies juxtaposed. In that case, the input and output conductors on the inside surface of the first dielectric substrate should be positioned so that they are correspondent to both the outermost coaxial resonator bodies. Opposite to the thus positioned the input and output conductors the connecting conductors should also be arranged on the outer surface of the first dielectric substrate so as to form the input and output capacities C1 and C2. On the inside surface of the second dielectric substrate there should be arranged the conductors of the same number as the resonator bodies used for forming coupling capacities. Figs. 6 to 9 illustrate an embodiment of the present invention, in which the illustrated filter device comprises two juxtaposed coaxial dielectric resonator bodies 23a and 23b. Each resonator body is made of titanium oxide dielectric ceramic material, is shaped as a rectangular parallelepiped and is constructed by superimposing two sections longitudinally divided as halves 23a-1 and 23a-2; 23b-1 and 23b-2 to each other. The halves have inner surfaces which are to be superimposed to each other. On each of the inner surfaces a longitudinally extended groove 24a; 24b of semi-circular cross section is formed. Coated on each groove 24a; 24b is an inner conductive layer 25a; 25b. These inner conductive layers 25a and 25b may be formed by using a screen printing or other suitable thin film forming procedures. An outer conductive layer 26a; 26b is formed on the outer surface of each of the halves except the inner and front surfaces thereof. Also, as shown in Fig. 6, formed on the inner surface of one 23a-1; 23b-1 of the sections is a conductive connecting line 27a; 27b which is extending from the inner conductive layer 25a; 25b in the groove 24a; 24b to a rectangular input or output terminal 28a; 28b on the lateral surface of the section. These input and output terminals 28a and 28b (or 28b and 28a) may be provided by partially removing the outer conductive layer portions on the lateral surfaces of the respective sections so as to form rectangular portions electrically separated from the outer conductive layer. When the halves 23a-1 and 23a-2; 23b-1 and 23b-2 are assemblied to form the respective resonator body 23a; 23b, the semi-circular grooves 24a and 24a; 24b and 24b form a through bore. Fitted into the thus formed through bores of the respective resonator bodies 23a and 23b are connecting plug members 29a and 29b of metal each of which has an outer end or front end 30a; 30b of reduced diameter extended outwardly from the front surface of the associated resonator body 23a; 23b. As shown in Fig. 8, the resonator bodies 23a and 23b are mounted on a printed circuit board P, and the input and output terminals 28a and 28b may be connected with intended conducting wires, not shown, on the board P by soldering generally designated by the reference numeral 31 without using any lead wire. Disposed on the front surfaces of the juxtaposed resonator bodies 23a and 23b is a dielectric substrates 32 which is made of dielectric ceramic material. The dielectric substrate 32 has an inside surface on which as shown in Figs. 6 and 7-(A) conductors 33a and 33b having an interdigitated pattern are so formed that they come into contact with the connecting plug members 29a and 29b, respectively, when dielectric substrate 32 is mounted on the front surfaces of the juxtaposed resonator bodies 23a and 23b. The interdigitated portions of both the conductors 33a and 33b form a capacitive coupling. On the outside surface of the dielectric substrate 32, as shown in Fig. 7-(B), a ground conductor 34 is formed opposite to the conductors 33a and 33b. Fitting holes 35a and 35b are provided to be extended through the conductors 33a and 33b, the dielectric substrate 32 and the ground conductor 34 for inserting the front ends 30a and 30b of the connecting plug members 29a and 29b, respectively. The ground conductor 34 is partially removed at the edge portions of the fitting holes 35a and 35b so that it is not connected with the inserted connecting plug members 29a and 29b as shown in Fig. 9. The input and output terminals 28a and 28b are connected with an external input and output conducting wires not shown, respectively, and the ground conductor 34 is grounded via a casing (not shown) or the like. The inner conductive layers 25a and 25b are connected with the conductors 33a and 33b, respectively. The capacitive coupling between the conductors 33a and 33b forms a coupling capacity C1 as shown in Fig. 10. The respective conductors 33a and 33b are opposite to the ground conductor 34 via the dielectric substrate 32 so as to form stray capacities C2 and C3 as shown in Fig. 10. As in the case of the previous embodiment illustrated in Figs. 1 to 4, the embodiment illustrated in Figs. 6 to 9 may be modified as follows. The filter device may include three or more coaxial resonator bodies juxtaposed, in which the input and output terminals are provided only on both the outermost resonator bodies. On the inside surface of the dielectric substrate there may be arranged the conductors of the same number as the resonator bodies used for forming coupling capacities. As illustrated and described above, according to the present embodiment, by superimposing of two dielectric substrates it is possible to suitably set an input and output capacities, a coupling capacity and stray capacities and thus no external capacitor is required, which results in a simplified arrangement. By the provision of the stray capacities each resonator body can be reduced in its length. By partially removing or adding the ground conductor on the second dielectric substrate after assembling of the resonator bodies, the coupling capacity and stray capacities can be easily adjusted to obtain a desired frequency response characteristic. Therefore, the present embodiment can provide a dielectric filter device of reduced size having an excellent characteristic. Furthermore, since both the outermost resonator bodies are provided with conductive connecting lines each of which has one end connected with the inner conductive layer in the through bore and the other end connected with the input or output terminal provided on the lateral surface of the associated resonator body, it is not necessary to use any complicated molds for producing the halves of the resonator body and to provide any lead wire guiding hole on the resonator body which reduces the mechanical strength thereof. Therefore, the filter device of the present embodiment has advantages that it can be easily prepared with lower cost and that it can be surface-mounted on the printed-circuit board.

Claims: A dielectric filter device comprising at least two juxtaposed coaxial dielectric resonator bodies (23a, 23b), each having an outer surface including a first end surface, characterized in that each of the resonator bodies (23a, 23b) is constructed by stacking two longitudinally divided dielectric block halves (23a-1, 23a-2; 23b-1, 23b-2) to each other so that the confronting surfaces thereof are joined to each other, each of the halves has an outer conductive layer (26a, 26b) provided on the outer surface except the first end surface thereof and a longitudinally extended groove (24a, 24b) on the confronting surface thereof, each of the longitudinally extended grooves (24a, 24b) is provided with an inner conductive layer (25a, 25b) on its confronting surface and is put together to form an axially extended through bore into which a connecting member (29a, 29b) is fitted, and the inner conductive layers of both the outermost resonator bodies (23a, 23b) are outwardly extended along the joined confronting surfaces of the dielectric block halves while being kept insulated from the outer conductive layer (26a, 26b) to form an input and output terminals (28a, 28b).

Inventors: ITO KENJI, SHIMIZU HIROYUKI, ITO, KENJI, SHIMIZU, HIROYUKI
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD.
Application year: 1993
Main classification: H01P1
IPC codes: H01P1