Dielectric filter

Abstract: The present invention provides a small dielectric filter suitable for use in a high frequency band equal to or higher than 3 GHz. an input/output electrode made up of island type of conductive film is formed on one surface of said dielectric located on each end portion; in each of said dielectrics located on each end respectively, an earth electrode is formed on almost of all remaining area of said surface so as to be isolated from said input/output electrode and is also formed on all of the other surfaces with an exception of connecting surfaces; in an intermediate dielectric, an earth electrode is formed on all surfaces other than the connecting surface; and a conductive film connected to the earth electrode is formed on a part of at least one of the connecting surfaces of the dielectrics to be connected.Three or more elements of resonators may be integrally formed on a dielectric block, and, in that case, a through-hole is formed between the resonators.

Description: FIELD OF THE INVENTION The present invention relates to a dielectric filter and in particular to a small dielectric filter suitable for use in a high frequency band equal to or higher than 3 GHz. PRIOR ART With the spread of mobile communication device, a frequency band higher than that in current operation is considered to be made use of. In the conventional mobile communication, the frequency band up to about 2 GHz is used, and a combination of dielectric coaxial resonators has been mainly employed as a filter used in the mobile station. When the dielectric coaxial resonator is used, however, in the frequency band equal to or higher than 3 GHz, an axial dimension thereof has to be made shorter due to the frequency, which makes it extremely thinner and also makes it difficult to form an input and output coupling. In addition, to secure high Q, an outer diameter of the dielectric shall be made larger. For example, in order to secure a Q required at a frequency of 5 GHz, 10-odd mm of outer diameter is necessary. This goes against a requirement for making an electronic unit smaller and is not practical. Instead of coaxial TEM mode resonator, TE mode resonator may be considered to be used, which results in larger size of structure and requires a complex structure of input and output coupling. SUMMARY OF THE INVENTION The object of the present invention is to provide a dielectric filter, which provides sufficient filtering characteristic at high frequency band, for example, within the range of 3 GHz to 30 GHz, and meets the requirement for high Q, downsizing and thinner thickness. The object is solved by a dielectric filter having the features disclosed in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an exploded perspective view of a dielectric filter not covered by the claims; Fig. 2 is an explanatory diagram illustrating a characteristic of a dielectric filter of Fig. 1; Fig. 3 is a perspective view of an embodiment according to the present invention; Fig. 4 is an explanatory diagram illustrating a characteristic of another dielectric filter according to the present invention; wherein, each of reference numerals 11, 12, 13 designates a dielectric; 31 designates a dielectric (block); each of 14, 15, 34, 35 designates an input/output electrode; each of 16, 17, 18, 36 designates an earth conductor; each of 19, 20 designates a conductive strip; each of 39 and 40 designates a through hole. DESCRIPTION OF THE PREFERRED EMBODIMENT Though a resonance mode of a dielectric filter according to the present -invention has not been completely analyzed, it is supposed that said dielectric filter operates just like a waveguide. It is supposed that an island type of electrode film formed on one surface of the dielectric is used as an input/output coupling structure and a coupling between the resonators is generated on a connecting surface or inside of the dielectric to make a filtering characteristic. There will now be described a preferred embodiment of the present invention with reference to the attached drawings. Fig. 1 is an exploded perspective view of a dielectric filter not covered by the claims prior to being assembled. In this filter, three dielectric resonators are connected to make a unit. A rectangular parallelepiped dielectric 11, 13 with a dimension of 6.41 x 6.0 x 2.5 mm 3 and a dielectric constant of 37 is disposed on each end side respectively, and an island type of conductive film 14, 15 with a dimension of 1.4 x 1.4 mm 2 is formed on a central portion of said 6.41 x 6.0 mm 2 surfaces respectively. A conductive film 16, 17 is formed surrounding said conductive film 14, 15 placing a distance of 0.5 mm therefrom, and a conductive film is also formed on all of other surfaces excepting a connecting surface to form an earth electrode by being connected to said conductive film 16, 17. An intermediate dielectric resonator 12 has a dimension of 5.75 x 6.0 x 2.5 mm 3 and a conductive film 18 is formed on all the surfaces thereof excepting connecting surfaces to form an earth electrode. In the connecting portions of the dielectrics 11, 12, 13, though the dielectrics are exposed, conductive strips 19, 20 are formed thereon extending from the surface on which the input/output electrode being formed to the opposite surface thereof to adjust a coupling between the resonators. In this embodiment, 2 mm width of conductive strip is formed on a central portion of the connecting surface. In each of the connecting portions between the dielectric resonators 11, 13 each being located on each end respectively and the intermediate dielectric resonator 12 connected thereto, said conductive strip may be formed on either of the connecting surfaces. In this embodiment, for example, said conductive strip may not be formed on the resonator 11, and may not be formed also on an invisible connecting surface of the resonator 12. Thus, the conductive film may be formed on at least one of the connecting surfaces. Fig. 2 is an explanatory diagram illustrating a characteristic of the dielectric filter made up by connecting the dielectrics shown in Fig. 1. It is shown that the center frequency is in 5.81 GHz, 3 dB bandwidth is 184 MHz, and an insertion loss at a peak point is 0.77 dB. Fig. 3 is a perspective view of an embodiment of the present invention, in which three dielectric resonators are integrally formed on one dielectric block. In this embodiment, the dielectric block 31 has a dimension of 19.22 x 6.00 x 2.50 mm 3 and a dielectric constant of 37, and each of input/output electrodes 34, 35 is formed on each end portion on a surface of 19.22 x 6.00 mm 2 respectively, and a dielectric resonator having no input/output electrode is disposed in a central portion, and each of through holes 39, 40 is formed between said input/output electrodes and said central dielectric resonator for adjusting the coupling between the resonators.. Each of the through holes 39 and 40 is formed by a size of 1.6 x 0.5 mm 2 at a location of 6.37 mm apart from a longitudinal end surface of the dielectric block 31 respectively. Thereby, the dimension of the central dielectric resonator is defined to be 5.48 x 6.00 mm 2 . An input/output electrode 34, 35 having a dimension of 1.4 x 1.4 mm 2 is formed on the surface of the dielectric on each end portion, and a conductive film 36 is formed on almost of all remaining area of said surface surrounding said input/output electrodes 34, 35 placing 0.5 mm of distance therefrom and also on all of other surfaces to form an earth electrode. Fig. 4 is an explanatory diagram illustrating a characteristic of the dielectric filter obtained from the dielectric block shown in Fig. 3. It is shown that the center frequency is in 5.80 GHz, 3 dB bandwidth is 163 MHz, and an insertion loss at a peak point is 0.82 dB. Though, in the embodiment shown in Fig. 3, the coupling is adjusted by the through hole formed between the resonators, a groove formed on a side surface of the dielectric block may be also employed for adjusting the coupling. Additionally, in case of connection shown in Fig. 1, a conductive film may be formed on both sides instead of conductive strip to expose the dielectric on the central portions. As shown in above embodiments, a dimension of the dielectric forming the resonator located on each end portion shall be different from that of the dielectric forming the resonator located on the central portion. This comes from the difference therebetween in an effective dielectric constant, and thereby the dimension of the dielectric located on each end portion shall be larger than that on the central portion. An arrangement of the dielectric resonators is not limited to the example shown above, but another structure including a bend therein may be also employed. As shown in above embodiments, a dimension of the dielectric forming the resonator located on each end portion shall be different from that of the dielectric forming the resonator located on the central portion. This comes from the difference therebetween in an effective dielectric constant, and thereby the size of the dielectric located on each end portion shall be larger than that on the central portion. In above embodiment, the dielectric constant of each dielectric is 37. According to the present invention, a small and thin dielectric filter capable of being used in a frequency band width equal to or more than 3 GHz may be provided. In addition, an easily producible and inexpensive dielectric filter may be provided since it can be made by merely forming a conductive film on a surface of the rectangular parallelepiped dielectric. Further, the frequency of extreme may be arbitrarily set since the dielectric resonators located on the input/output end portions can be brought into capacitive coupling depending on the arrangement thereof and, in addition, the coupling condition thereof can be easily adjusted.

Claims: A dielectric filter in which three or more resonators are integrally formed in a rectangular parallelepiped dielectric block (31), said dielectric filter characterized in that : in each of the dielectric resonators respectively located on each end portion of said dielectric block (31) with respect to a longitudinal direction thereof, an input/output electrode (34, 35) made up of island type of conductive film (34, 35) is formed respectively on the same surface of said dielectric block (31), and an earth electrode (36) is formed on almost of all remaining area of said same surface so as to be isolated from said input/output electrode (34, 35) and is also formed on all of the other surfaces; in each of the other dielectric resonators, an earth electrode (36) made up of conductive film is formed on all surfaces thereof; and between the dielectric resonators, a through hole (39, 40) extending from the surface on which the input/output electrode (34, 35) is formed to the surface opposite thereto is formed.

Inventors: MIYASHITA MEIJI, SANO KAZUHISA, MIYASHITA, MEIJI, SANO, KAZUHISA
Applicants: TOKO INC, TOKO, INC.
Application year: 2000
Main classification: H01P1
IPC codes: H01P1