Dielectric filter with attenuation pole

Abstract: There is disclosed a dielectric filter with attenuation pole having sharp attenuation characteristic. The dielectric block employed in the present dielectric filter is composed of a uniform integral dielectric single-block including TiO₂, BaO. The dielectric block of this type comprises a plurality of resonators extending from a top surface toward a bottom surface thereof, a plurality of adjusting patterns provided over the top surface thereof, an outer conductor formed by side surfaces and the bottom surface thereof and grounded, input and output electrodes, wherein the adjusting patterns, the outer conductor, input and output electrodes are plated, for example, by silver, and insulted cable having one end connected to the outer conductor and the other end connected to the output electrode though a plurality of resonators thereabove.

Description: BACKGROUND OF THE INVENTION Field of the Invention and Related Art: The present invention relates to a dielectric filter with attenuation pole that is particularly adapted for use in an antenna of a mobile phone system. There are many known techniques relating to a ceramic bandpass filter to be used in a mobile phone. Fig. 1 shows a typical example of these techniques which comprises an integral dielectric single-block 101 (hereinafter referred to as simply dielectric block), a plurality of resonators 102 extending from a top surface toward a bottom surface of the dielectric block 101 , a plurality of adjusting patterns 103 provided over the top surface of the dielectric block 101 , input and output electrodes 104 and metallized patterns 105 provided over side surfaces and the bottom surface of the dielectric block 101 . The inner conductors are formed within inner walls of the resonators 102. These inner conductors are connected to the adjusting patterns 103 at the top surface of the dielectric block 101 . The inner conductors are connected to the metallized patterns 105 at the bottom surface of the dielectric block 101 . The metallized patterns 105 are grounded. JP-A-62 157 402 discloses a dielectric filter in accordance with the first part of claim 1. Coupling holes 6 are arranged between the resonators in order to electromagnetically couple the resonators. An insulated semi-rigid cable 7 is provided having capacitor electrodes 9 at both ends. The cable 7 serves to skip two resonators in order to form two poles at both sides of the center frequency in the filter characteristic of the dielectric filter. A similar arrangement is disclosed in JP-A-62 77 703. JP-A-61 62 203 discloses a dielectric filter with a plurality of dielectric resonators and slot couplings formed between the resonators. Frequency adjusting patterns are provided on the top surface of the dielectric blocks. Two of these frequency adjusting patterns also serve as electrodes for applying voltage. SUMMARY OF THE INVENTION It is an object of the present invention to provide a dielectric filter with attenuation pole having less antenna return loss in a passband. It is another object of the present invention to provide a dielectric filter with attenuation pole enabling to obtain a sharp attenuation characteristic with fewer elements in constitution thereof. To achieve the above objects, the present invention provides a dielectric filter with attenuation pole comprising: a dielectric block having a top surface, a bottom surface, and side surfaces, the bottom surface and side surfaces being covered by an outer conductor; a plurality of resonators extending from the top surface toward the bottom surface, and having inner conductors therein; coupling means arranged to couple capacitance between a plurality of resonators; two electrodes arranged to apply voltage to the dielectric block; and an insulated cable connected with its one end to the outer conductor and with its other end to one of the two electrodes. The above and other objects, features and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a dielectric filter having no attenuation pole; Fig. 2 is a perspective view of a dielectric filter with attenuation pole according to an embodiment of the present invention; Fig. 3 is a cross sectional view taken along A-A of the dielectric filter with attenuation pole in Fig. 2; Fig. 4 is an equivalent circuit of the dielectic filter with attenuation pole in Fig. 2; and Fig. 5 is a view of assistance in explaining frequency-attenuation rate characteristic of a dielectric filter with attenuation pole. DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described with reference to Figs. 2 through 5. The dielectric filter of the present invention is composed of a uniform dielectric single-block 201 (hereinafter referred to as simply dielectric block) including TiO₂, BaO. The dielectric block 201 has dimensions of 9.4 mm in height X 6.0 mm in width X 28.1 mm in length. Hollow resonators 202, 203, 204 , 205, respectively extending from a top surface toward a bottom surface of the dielectric block 201 which are arranged in parallel with each other to form first to fourth stages of resonators. The resonators 202 through 205 respectively have inner conductors 212, 213, 214, 215 formed in the inner walls thereof. The inner conductors 212 through 215 are respectively connected at each one end thereof to frequency adjusting patterns 222, 223, 224, 225 formed over the top surface of the dielectric block 201 , and at each other end thereof to an outer conductor 206 formed and extended over side surfaces and the bottom surface. Formed between the frequency adjusting patterns 222 through 225 are coupling capacitance adjusting patterns 231, 232, 233. Formed at both ends of the top surfaces of the dielectric block 201 are electrodes 250, 251. Preferably, the inner conductors, frequency adjusting patterns, the outer conductor, the capacitance adjusting patterns and the electrodes are respectively plated with copper, silver, etc. An insulated cable 270 is connected at one end thereof to an electrode 250 and at the other end thereof to the outer conductor 206 through the first through fourth resonators 202 through 205 thereabove. The insulated cable 270 is disposed over the top surface of the dielectric block 201 . An electric signal applied to the electrode 250 generates electromagnetic field by the first stage resonator 202. The resultant electromagnetic field is transmitted to the second stage of resonator 203 via the coupling capacitance adjusting pattern 231 adjacent to the first resonator 202. Likewise, the electromagnetic field is successively transmitted to the third and fourth stages of resonators 204, 205 via the coupling capacitance adjusting patterns 232, 233. The fourth stage of the resonator 205 transmits the electromagnetic field transmitted from the third stage of the resonator 204 to the electrode 251. The electromagnetic field transmitted to the electrode 251 is transmitted to a load connected to the electrode 250 as an electric signal. The electric signal applied to the electrode 250 is also transmitted to the outer conductor 206 via the insulated cable 270. Since the dielectric filter according to this invention includes a quarter-wave length coaxial resonator, the electromagnetic field becomes maximum at the top surface of the dielectric block 201 . And the insulted cable 270 is disposed over the top surface of the dielectric block 201 , so there exists coupling capacitance between each resonator 202, 203, 204, 205 and insulated cable 270. Each element as illustrated in Fig. 2 corresponds to an equivalent circuit in Fig. 4 which have relations listed hereunder. As shown in Fig. 4, a parallel resonator circuit is composed of the coupling capacitors C1, C3, C5, C7, C9, the inductance L11, L22, L33, L44, L55, and the coupling capacitors Cc1, Cc2, Cc3, Cc4. Attenuation pole is appeared in the attenuation characteristic of the dielectric filter due to the parallel resonator circuit. That is, a resonant frequency of the parallel resonator circuit provides a transmission/zero characteristic, which cause attenuation infinite to generate the pole. When the dielectric filter is employed as an antenna for the mobile phone system, the attenuation characteristic relative to the frequency is illustrated in Fig. 5. An advance mobile phone system (AMPS) having transmission lines of 832 channels has an attenuation standard of 31 dB in 869 MHz. The attenuation according to the present invention becomes 34 dB which meet the attenuation standard of the AMPs.

Claims: A dielectric filter with attenuation pole comprising; a dielectric block (201) having a top surface, a bottom surface, and side surfaces, the bottom surface and side surfaces being covered by an outer conductor (206); a plurality of resonators (202, 203, 204, 205) extending from the top surface toward the bottom surface, and having inner conductors (212, 213, 214, 215) therein; characterized by coupling means (231, 232, 233) arranged to couple capacitance between a plurality of resonators (202-205); two electrodes (250, 251) arranged to apply voltage to the dielectric block (201); and an insulated cable (270) connected with its one end to the outer conductor (206) and with its other end to one of the two electrodes (250, 251). A dielectric filter with attenuation pole according to Claim 1, wherein the resonators (202-205) are disposed at predetermined distances from one another. A dielectric filter with attenuation pole according to Claim 1, wherein the coupling means (231-233) are metallized patterns formed on the top surface of the dielectric block (201). A dielectric filter with attenuation pole according to Claim 1, wherein the insulated cable (270) has one end connected to the outer conductor (206) and the other end connected to one of the electrodes (250, 251) through a plurality of resonators (202-205) thereabove. A dielectric filter with attenuation pole according to Claim 3, wherein the metallized patterns are formed between a plurality of resonators (202-205). A dielectric filter with attenuation pole according to one of the preceding claims, characterized by adjusting means (222, 223, 224, 225) arranged to adjust respective resonant frequencies of a plurality of resonators (202-205); A dielectric filter with attenuation pole according to Claim 6, wherein the adjusting means (222-225) are metallized patterns provided over the top surface of the dielectric block (201) and connected to the inner conductors (212-215). A dielectric filter with attenuation pole according to claim 6 or 7, characterized in that a plurality of resonators (202-205) are disposed in parallel with each other.

Inventors: GUNJI KATSUHIKO, HORII HIROYUKI, KOMAZAKI TOMOKAZU, MASHIMO AKIRA, ONISHI NORIO, SAKURAI YOSHIMITU, GUNJI, KATSUHIKO, HORII, HIROYUKI, KOMAZAKI, TOMOKAZU, MASHIMO, AKIRA, ONISHI, NORIO, SAKURAI, YOSHIMITU
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1988
Main classification: H01P1
IPC codes: H01P7, H01P1