A dielectric filter

Abstract: A dielectric filter comprises a plurality of coaxial resonators each having an outer peripheral conductor and an inner peripheral conductor formed by an outer peripheral surface and an inner peripheral surface of a dielectric member with a conductive member and having one end face short-circuited, wherein at least one of said plurality of coaxial resonators has a length different than that of the other coaxial resonators, and has the same basic resonance frequency as that of said other coaxial resonators by a resonance frequency correcting capacitance connected to the inner peripheral conductor.

Description: The present invention relates to a dielectric filter. BACKGROUND OF THE INVENTION Conventionally, a dielectric filter using a dielectric coaxial resonator has been constructed by respectively connecting one-end face short-circuited type coaxial resonators each formed by providing a dielectric member with a through hole and coating an outer peripheral surface of the dielectric member and an inner peripheral surface of the through hole with a conductive member (for example, silver) and capacitive-coupling electrodes formed on the dielectric substrate to the respective coaxial resonators. In recent years, communication apparatuses have been decreased in weight and bulk in the field of mobile communication. With decreasing weight and bulk, smaller-sized dielectric filters have been requested. Meanwhile, the ratio of an inner coaxial diameter to an outer coaxial diameter must be 3.6 so as to obtain a high Qu value (unloaded Q value). In manufacturing a small-sized dielectric filter, therefore, if the outer coaxial diameter is not more than 4 mm, the inner coaxial diameter is not more than 1.2 mm. It is thus difficult to insert a member for external connection into the through holes of the above described coaxial resonators to connect the same with an external circuit as described above. The present applicant has proposed U. S. Patent Application S. N. 671, 615 as a dielectric filter for solving the problem. Furthermore, when a filter is constituted by coaxial resonators comprising not less than three stages, the length of the coaxial resonator in the middle stage is made longer than those of the coaxial resonators in the first stage and the final stage. Therefore, there occurs the problem that coaxial resonators having various lengths are required. On the other hand, in mobile communication apparatus, an antenna duplexer for separating and combining signals having different frequencies depending on the frequency is used. Such an antenna duplexer comprises a dielectric filter for transmission and a dielectric filter for reception which differ in center frequency. In such a dielectric filter, the interval between the center frequencies of a receiving band and a transmitting band becomes shorter with higher frequency in mobile communication, so that it is difficult to obtain out-of-passband attenuation required. Therefore, the dielectric filter used in the antenna duplexer must have an attenuation pole in its characteristics. Examples of a method of forming a pole in an attenuation region include one for directly connecting resonators by a reactance element with at least one resonator being skipped to form such a pole in a dielectric filter disclosed in Japanese Patent Laid-Open Gazette No. 77703/1987. EP-A-0208424 describes a dielectric filter with a quarter wavelength coaxial resonator. The filter is formed by coupling a plurality of resonators via an electromagnetic field. JP-A-60165102 describes a dielectric filter having five resonators provided in cascade, wherein adjacent resonators are coupled by coupling holes and coaxial capacitors are used for the coupling between input/output resonators and an external circuit. It is desirable to provide a small-sized dielectric filter in which an antiresonant capacitance for obtaining filter characteristics having an attenuation pole and a coupling between coaxial resonators can be achieved with good workability. It is also desirable to provide a dielectric filter for preventing an input-output direct coupling which is achieved through a ground electrode due to the fact that the grounding of the ground electrode is in an incomplete state. It is also desirable to provide a small-sized dielectric filter in which undesired filter characteristics in a resonance frequency in higher TEM (Transverse electromagnetic) mode can be solved without complicating a circuit. It is also desirable to provide an antenna duplexer using the dielectric filter as filters for transmission and reception. It is also desirable to make a matching circuit for matching input impedances smaller in size in providing an antenna duplexer using the dielectric filter as filters for transmission and reception, to achieve smaller size and higher performance of the antenna duplexer. SUMMARY OF THE INVENTION According to the invention there is provided a dielectric filter comprising a plurality of coaxial resonators each having an outer peripheral conductor and an inner peripheral conductor and having one end face short-circuited, wherein at least one of said plurality of coaxial resonators has a length different than that of the other coaxial resonators, characterised in that said at least one coaxial resonator has the same basic resonance frequency as that of said other coaxial resonators by a resonance frequency correcting capacitance connected to the inner peripheral conductor. The filter may comprise at least one resonator and a substrate having a conductive track thereon, the resonator having a recessed surface portion and being mounted to the substrate at said recessed portion such that there is capacitance between the resonator and a portion of the conductive track. The filter may comprise a plurality of resonators and a substrate having a conductive track thereon, the resonators being mounted to the substrate so as to form a capacitor with respective portion of the track and wherein at least one of the resonators is shorter in length than the other resonators. In view of the above described points, a dielectric filter may comprise a plurality of coaxial resonators, each of the above described coaxial resonators comprising a dielectric block having an outer peripheral surface and an inner peripheral surface parallel to a common axis and having a first and second end faces crossing the common axis, first and second conductive layers coating the outer peripheral surface and the inner peripheral surface, a third conductive layer formed on the second end face for short circuiting the first and second conductive layers, and a depressed part formed by removing a portion of the first conductive layer on the side of the first end face or a portion of the first conductive layer including the dielectric block; a dialectic substrate having a plurality of capacitance forming electrodes for forming capacitances between the electrodes and the second conductive layers of the coaxial resonators and external connection means formed thereon and mounted on the depressed part; and reactance means provided on the dielectric substrate for coupling the capacitance forming electrodes to each other. Furthermore, a dielectric filter may comprise a dielectric block having an outer peripheral surface and a plurality of inner peripheral surfaces parallel to a common axis and having first and second end faces crossing the common axis; a first conductor layer formed on the outer peripheral surface; a plurality of second conductive layers formed on the plurality of inner peripheral surfaces; a third conductive layer formed on the second end face for short-circuiting the first and second conductive layers; a depressed part formed by removing a portion of the first conductive layer on the side of the first end face or a portion of the first conductive layer including the dielectric block; a dielectric substrate having a plurality of capacitances forming electrodes for forming capacitance between the electrodes and the second conductive layers and external connection means formed thereon and mounted on the depressed part; and reactance means provided on the dielectric substrate for coupling the capacitance forming electrodes to each other. The capacitance forming electrodes provided on the dielectric substrate form antiresonance capacitances between the electrodes and the second conductive layers, and the reactance element for coupling the capacitance forming electrodes is provided on the dielectric substrate, thereby to obtain a small-sized dielectric filter which can be easily manufactured. Furthermore, a dielectric filter may comprise a plurality of coaxial resonators, each of the above described coaxial resonators comprising a dielectric block having an outer peripheral surface and an inner peripheral surface parallel to a common axis and having first and second end faces crossing the common axis, first and second conductive layers coating the outer peripheral surface and the inner peripheral surface, a third conductive layer formed on the second end face for short-circuiting the first and second conductive layer, and a depressed part formed by removing a portion of the first conductive layer on the side of the first end face or a portion of the first conductive layer including a dielectric block; and a dielectric substrate having a plurality of electrodes for external connection formed on its one major surface and having a plurality of ground electrodes corresponding to the external connection electrodes and electrically insulated from one another formed on the other major surface and mounted on the depressed part with the one major surface being abutted thereon. Furthermore, a dielectric filter may comprise a dielectric block having an outer peripheral surface and a plurality of inner peripheral surfaces parallel to a common axis and having first and second end faces crossing the common axis; a first conductive layer formed on the outer peripheral surface; a plurality of second conductive layers formed on the plurality of inner peripheral surfaces; a third conductive layer formed on the second end face for short-circuiting the first and second conductive layers; a depressed part formed by removing a portion of the first conductive layer on the side of the first end face or a portion of the first conductive layer including the dielectric block; and a dielectric substrate having a plurality of electrodes for external connection formed on its one major surface and having a plurality of ground electrodes corresponding to the external connection electrodes and electrically insulated from one another formed on the other major surface and mounted on the depressed part with the one major surface being abutted thereon. The plurality of ground electrodes formed on the dielectric substrate are electrically insulated from one another. Accordingly, all parts, excluding a reflected part of power which is not coupled to the coaxial resonator on the input side flow into the ground, resulting in no input-output direct coupling. Furthermore, in a dielectric filter which comprises a plurality of coaxial resonators each having an outer peripheral conductor and an inner peripheral conductor formed by coating an outer peripheral surface and an inner peripheral surface of a dielectric member with a conductive member and having one end face short-circuited, at least one of the plurality of coaxial resonators has a length different from those of the other coaxial resonators, and has the same basic resonance frequency as those of the other coaxial resonators by a resonance frequency correcting capacitance connected to the inner peripheral conductor. In the above described dielectric filter, even if the basic resonance frequencies of the plurality of coaxial resonators are the same by the resonance frequency correcting capacitance, the coaxial resonators differ in length and thus, differ in higher resonance frequency component. Consequently, it is possible to achieve a small-sized dielectric filter in which pass characteristics in a resonance frequency in a higher TEM mode are unrestrained and spurious filter characteristics are improved without complicating a circuit. Furthermore, an antenna duplexer may comprise a filter for reception and a filter for transmission each comprising a plurality of coaxial resonators each having an outer peripheral conductor and an inner peripheral conductor formed by coating an outer peripheral surface and an inner peripheral surface of a dielectric member with a conductive member and having one end face short-circuited, and having a depressed part formed by removing a portion of the outer peripheral conductor on the open face side or a portion of the outer peripheral conductor including the dielectric member; and a dielectric substrate having a plurality of capacitance forming electrodes for forming capacitances between the electrodes and the inner peripheral conductors of the coaxial resonators, a reactance element for coupling the capacitance forming electrode, and a matching circuit on the reception side and a matching circuit on the transmission side for connecting the filter for reception and the filter for transmission to one antenna formed thereon, each of the filter for reception and the filter for transmission being constructed by mounting the depressed part of the plurality of coaxial resonators on positions where the capacitance forming electrodes corresponding to the inner peripheral conductors of the coaxial resonators are formed on the dielectric substrate. The matching circuits can be simultaneously provided on the dielectric substrate comprising the capacitance forming electrodes of both the filters, thereby to simplify the construction and make the manufacture easy. Furthermore, the antenna duplexer may comprise a filter for reception and a filter for transmission each including coaxial resonators each having one end face short-circuited, which has an outer peripheral conductor and inner peripheral conductors formed by providing a dielectric member with at least two holes and coating an outer peripheral surface of the dielectric member and inner peripheral surfaces of the through holes with a conductive member and has a depressed part formed by removing a portion of the outer peripheral conductor on the open face side or a portion of the outer peripheral conductor including the dielectric member; and a dielectric substrate having a plurality of capacitance forming electrodes for forming capacitances between the electrodes and the inner peripheral conductors, a reactance element for coupling the capacitance forming electrodes, and a matching circuit on the reception side and a matching circuit on the transmission side for connecting the filter for reception and the filter for transmission to one antenna, each of the filter for reception and the filter for transmission being constructed by mounting the depressed part on positions where the capacitance forming electrodes corresponding to the inner peripheral conductors are formed on the dielectric substrate. Additionally, an antenna duplexer may have a dielectric filter for reception and a dielectric filter for transmission, one end of both the dielectric filters being connected to an antenna shared terminal, the antenna shared terminal, the one end of the filter for reception, and the one end of the filter for transmission being respectively grounded through capacitive elements or inductive elements, and the above described antenna shared terminal being connected to the one end of the filter for reception through the capacitive element or the inductive element, while being connected to the one end of the filter for transmission through the capacitive element or the inductive element. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view showing the appearance of a first dielectric filter; Fig. 2 is a diagram showing an equivalent circuit of the first dielectric filter; Fig. 3 is a diagram showing the frequency characteristics of the first dielectric filter; Fig. 4 is a diagram showing the appearance of a second dielectric filter; Figs. 5A to 5F are schematic diagrams showing a capacitance portion formed on a dielectric substrate; Fig. 6 is a perspective view showing the appearance of a third dielectric filter; Fig. 7 is a diagram showing an equivalent circuit of the third dielectric filter; Fig. 8 is a diagram showing the frequency characteristics of the third dielectric filter; Figs. 9A and 9B are schematic diagrams showing an inductance formed on a dielectric substrate; Fig. 10A is a perspective view showing the appearance of a fourth dielectric filter; Fig. 10B is a transverse sectional view showing the fourth dielectric filter; Fig. 10C is a bottom view showing the fourth dielectric filter; Fig. 10D is a plan view showing the dielectric substrate; Fig. 11 is a diagram showing an equivalent circuit of the fourth dielectric filter; Fig. 12 is a diagram showing the frequency characteristics of the fourth dielectric filter; Fig. 13A is a perspective view showing the appearance of a fifth dielectric filter; Fig. 13B is a bottom view showing the fifth dielectric filter; Fig. 14 is a diagram showing an equivalent circuit of the fifth dielectric filter; Fig. 15A is a perspective view showing the appearance of a sixth dielectric filter; Fig. 15B is a bottom view showing the sixth dielectric filter; Fig. 16 is a diagram showing the frequency characteristics of the fourth dielectric filter; Fig. 17 is a perspective view showing the appearance of a dielectric filter according to the first embodiment of the present invention; Fig. 18A is an exploded perspective view showing a resonator comprising a coaxial resonator and a capacitance forming portion in the first embodiment of the present invention; Fig. 18B is a transverse sectional view showing the resonator comprising the coaxial resonator and the capacitance forming portion in the first embodiment of the present invention; Fig. 19 is a diagram showing an equivalent circuit of the first dielectric filter according to the first embodiment of the present invention; Fig. 20 is a diagram showing the frequency characteristics of the dielectric filter according to the first embodiment of the present invention; Figs. 21 A to 21C are cross sectional views showing a dielectric filter according to a second embodiment of the present invention; Fig. 22 is a perspective view showing the appearance of a first antenna duplexer; Fig. 23 is a schematic diagram showing an equivalent circuit of the antenna duplexer shown in Fig. 22; Fig. 24 is a perspective view showing the appearance of a second antenna duplexer; Fig. 25 is a schematic diagram showing an equivalent circuit of the antenna duplexer shown in Fig. 24; Fig. 26 is a perspective view showing the appearance of a third antenna duplexer; Fig. 27 is a schematic diagram showing an equivalent circuit of the antenna duplexer shown in Fig. 26; Fig. 28 is a schematic diagram showing an equivalent circuit of a fourth antenna duplexer; Fig. 29 is a diagram showing the frequency characteristics of the fourth antenna duplexer; Fig. 30 is a diagram showing the frequency characteristics of the first antenna duplexer; Fig. 31A is a perspective view showing the appearance of a conventional dielectric filter; Fig. 31B is a transverse sectional view showing the conventional dielectric filter; Fig. 32A is a perspective view showing a dielectric substrate in the conventional dielectric filter; Fig. 32B is a bottom view showing the dielectric substrate in the conventional dielectric filter; Fig. 32C is a transverse sectional view showing the dielectric substrate in the conventional dielectric filter; Fig. 33 is a diagram showing an equivalent circuit of the conventional dielectric filter; and Fig. 34 is a diagram showing the frequency characteristics of the conventional dielectric filter. DESCRIPTION OF THE EMBODIMENTS Fig. 1 is a diagram showing a first dielectric filter, showing an example in which three coaxial resonators 25 to 27 constitute a band-pass filter having a pole in the rejection range. Each of the coaxial resonators 25 to 27 is a one-end face short-circuited type coaxial resonator in which an outer peripheral surface of a dielectric member (dielectric constant of 40) of a TiO 2 - SnO 2 - ZrO 2 system provided with a through hole and an inner peripheral surface of the through hole are coated with a conductive member such as silver. Sides of the resonator in cross section perpendicular to the longitudinal direction are respectively 3 mm and the length of the resonator in the longitudinal direction is 4.8 mm. The coaxial resonators 25 to 27 are provided with a depressed part 28 formed by removing a portion of outer peripheral conductors 25c to 27c including dielectric members 25a to 27a to a depth of approximately 0.5 mm on the open end side. A dielectric substrate 29 made of alumina is mounted on the depressed part 28. Capacitance forming electrodes 29a to 29c, input-output electrodes 29d and 29e, and an interstage coupling electrode 29f are formed on the upper surface of the dielectric substrate 29. The capacitance forming electrodes 29a to 29c are opposed to inner peripheral conductors 25d to 27d through the dielectric members when the dielectric substrate 29 is mounted on the depressed part 28 of the coaxial resonators 25 to 27, to form antiresonance capacitances C10 to C12 as shown in Fig. 2. The values of the antiresonance capacitances C10 to C12 are determined by, for example, the areas of the capacitance forming electrodes 29a to 29c, and the distance between the inner peripheral conductors 25d to 27d and the capacitance forming electrodes 29a to 29c. The larger the values of the antiresonance capacitances C10 to C12 are, the deeper an attenuation pole in filter characteristics is formed and the larger the difference between the resonance frequency and the antiresonance frequency is. Chip capacitors 30 to 33 are provided in predetermined places between the above described electrodes 29a to 29e formed on the upper surface of the dielectric substrate 29. The chip capacitors 30 and 31 are used as interstage coupling capacitors respectively connected to the interstage coupling electrode 29f so as to couple the coaxial resonators 25 and 26 and 26 and 27. The chip capacitors 32 and 33 are used as input coupling capacitors respectively connected between the input-output electrodes 29d and 29e and the capacitance forming electrodes 29a and 29c. The input-output electrodes 29d and 29e are subjected to through-hole plating, and are connected to input-output stab electrodes (not shown) provided on the reverse surface of the dielectric substrate 29 by penetration. The dielectric filter thus constructed is shown in Fig. 2 as represented by an equivalent circuit, and filter characteristics having an attenuation pole on the low frequency side of a passband as shown in Fig. 3 are obtained. In the dielectric filter, the antiresonance capacitances are respectively formed between the inner peripheral conductors of the coaxial resonators and the capacitance forming electrodes formed on the dielectric substrate, and the capacitance forming electrodes are coupled to each other by a reactance element constituted by the interstage coupling capacitors 30 and 31, so that the respective resonators are coupled to each other to have frequency characteristics having an attenuation pole. Moreover, since the capacitance forming electrodes are coupled to each other by the reactance element constituted by the interstage coupling capacitors 30 and 31, an input-output direct coupling which is achieved through a ground electrode due to the fact that the grounding of the ground electrode is in an incomplete state is prevented. Consequently, the degradation of out-of-band attenuation can be restrained in the frequency characteristics of the filter. Fig. 4 is a diagram showing a second dielectric filter, showing an example of a dielectric filter including a plurality of coaxial resonators each having one end face short-circuited, which has an outer peripheral conductor and inner peripheral conductors formed by providing a dielectric member 34 with a plurality of through holes 34a to 34c and coating an outer peripheral surface of the dielectric member 34 and inner peripheral surfaces of the through holes with a conductive member. A depressed part 35 is formed by removing a portion of the outer peripheral conductor on the open face side of the coaxial resonators or a portion of the outer peripheral conductor including the dielectric member, and a dielectric substrate 29a which is the same as that in the above described first dielectric filter is mounted on the depressed part 35. Although in the above described first and second dielectric filters, the chip capacitors are used as a capacitance element provided on the dielectric substrate, the structures shown in Figs. 5A to 5F can be also used. More specifically, in Figs. 5A and 5B, a pair of electrodes 29g and 29h having ends opposed to each other with predetermined spacing which is formed on a dielectric substrate 29 constitute a capacitance. In Fig. 5B, each of electrodes 29g and 29h is formed in an interdigital shape, so that it is possible to form a larger capacitance than that shown in Fig. 5A. In Figs. 5C and 5D a dielectric piece 36 is disposed in an end of one of electrodes 29g and the other electrode 29h extends to the upper surface of the dielectric piece 36, to form a capacitance. Fig. 5C is a perspective view, and Fig. 5D is a cross sectional view. In Figs. 5E and 5F, a dielectric piece 38 having an electrode 37 provided on its upper surface is disposed on the upper surface of the electrodes shown in Fig. 5A, to form a capacitance. Fig. 5E is a perspective view, and Fig. 5F is a cross sectional view. Fig. 6 is a diagram showing a third dielectric filter, showing an example in which three coaxial resonators 39 to 41 are used to construct a band-rejection filter. Pattern inductors 42d to 42g connected to capacitance forming electrodes 42a to 42c are formed on a dielectric substrate 42. This dielectric filter is shown in Fig. 7 as represented by an equivalent circuit, and the filter characteristics thereof are band preventing characteristics having an attenuation pole as shown in Fig. 8. Although in the third dielectric filter, the pattern inductors 42d to 42g formed on the dielectric substrate 42 are used as an inductance element, the structures shown in Figs. 9A and 9B can be used. As shown in a perspective view of Fig. 9A, and a cross sectional view of Fig. 9B, an electrode 43a is formed on the upper surface of a pair of electrodes 42h and 42i opposed to each other with predetermined spacing which is provided on the dielectric substrate 42, and a dielectric piece 43 having a pair of plated-through holes 43b and 43c connected to each other is connected to the electrode 43a. In the above described first to third dielectric filters, the capacitance forming electrodes are coupled to each other by the reactance element constituted by the interstage coupling capacitors 30 and 31. Accordingly, an input-output direct coupling which is achieved through a ground electrode due to the fact that the grounding of the ground electrode is in an incomplete state is prevented. Consequently, it is possible to restrain the degradation of out-of-band attenuation in the frequency characteristics of the filter. In a dielectric filter used for forming a pole P in an attenuation region of filter characteristics by jump capacitances C3, C'3 and C"3 as shown in Fig. 32, however, when the grounding of a ground electrode 3B on a dielectric substrate 3' is incomplete, a capacitive coupling is achieved through the ground electrode 3b between connection electrodes 3d' and 3d", to significantly degrade the filter characteristics. Particularly in a microwave band of not less than 10 GHz, it is difficult to obtain complete grounding, so that out-of-band attenuation is significantly degraded in the frequency characteristics of the filter. In fourth to sixth dielectric filters, an input-output direct coupling which is achieved through a ground electrode due to the fact that the grounding of the ground electrode is in an incomplete state is prevented in a dielectric filter used for forming a pole P in an attenuation region of filter characteristics by a jump coupling. Fig. 10 is a diagram showing a fourth dielectric filter, showing an example in which a first coaxial resonator 50 and a second coaxial resonator 51 constitute a dielectric filter. The first coaxial resonator 50 is one on the input side, and the second coaxial resonator 51 is one on the output side. The first and second coaxial resonators 50 and 51 are respectively provided with depressed parts 52 and 52', and a dielectric substrate 53 having an input connection electrode 53a and an output connection electrode 53a' shown in Fig. 10D is mounted on the depressed parts 52 and 52', so that the connection electrodes 53a and 53a' are respectively coupled to inner peripheral conductors 50d and 51d' of the coaxial resonators 50 and 51. Furthermore, a coupling (interstage coupling) between the coaxial resonators 50 and 51 is achieved by joining windows (not shown) formed by removing portions of respective outer peripheral conductors 50c and 51c of the coaxial resonators 50 and 51 to each other. As shown in a bottom view of Fig. 10C, first and second ground electrodes 53b and 53b' are respectively formed on the reverse surface of the dielectric substrate 53 corresponding to the input-output connection electrodes 53a and 53a'. The spacing between the first and second ground electrodes 53b and 53b' is substantially a distance at which the electrodes are not coupled to each other and the ingress of noise is prevented, for example, 0.2 mm. Fig. 11 is a diagram showing an equivalent circuit of the fourth dielectric filter, where C1 and C1' indicate coupling capacitances respectively formed between the inner peripheral conductors 50d and 51d' of the coaxial resonators 50 and 51 and the connection electrodes 53a and 53a' on the dielectric substrate 53, C2 and C2' indicate capacitances respectively formed between the connection electrodes 53a and 53a' and the first and second ground electrodes 53b and 53b', C3 indicates an interstage coupling capacitance, and Z and Z' indicate impedance components which occur when the grounding of the ground electrodes 53b and 53b' is incomplete. Fig. 12 is a diagram showing the frequency characteristics of the fourth dielectric filter, where a broken line represents the frequency characteristics of a dielectric filter having a structure shown in Fig. 31. As can be seen from the characteristic diagram, an input-output direct coupling is prevented by dividing a ground electrode formed on the reverse surface of the dielectric substrate 53, to improve the out-of-band attenuation characteristics of the filter. Fig. 13 is a diagram showing a fifth dielectric filter, showing an example in which four coaxial resonators 61 to 64 constitute a filter. Fig. 13A is a perspective view, and Fig. 13B is a bottom view. The resonators 61 to 64 are provided with a depressed part 65, and a dielectric substrate 70 is mounted on the depressed part 65. The resonators 61 to 64 are coupled to each other through coupling windows (not shown). Input-output connection electrodes 70a and 70a' and capacitance forming electrodes 70c and 70c' are respectively formed on the upper surface of the dielectric substrate 70, and stab electrodes for external connection 70d and 70d' formed by extending the input-output connection electrodes 70a and 70a' and ground electrodes 70b 1 to 70b 4 in positions corresponding to the connection electrodes 70a to 70a' and the capacitance forming electrodes 70c and 70c' are formed on the lower surface thereof. Input-output coupling capacitances C21 and C21' are respectively formed between the input-output connection electrodes 70a and 70a' and inner peripheral conductors 61d and 64d of the coaxial resonators 61 and 64, and resonator length correcting capacitances C24 and C24' are respectively formed between the capacitance forming electrodes 70c and 70c' and the inner peripheral conductors 62d and 63d of the coaxial resonators 62 and 63, as shown in Fig. 14. Fig. 14 shows an equivalent circuit of the fifth dielectric filter. In the circuit diagram of Fig. 14, C23, C23' and C23" indicate interstage coupling capacitances, and C25 and C25' indicate capacitances respectively formed between the capacitance forming electrodes 70C and 70C' and the ground electrodes 70b 2 and 70b 3 . In the fifth dielectric filter, when a pole P is formed in an attenuation region of filter characteristics, the resonators are connected through a transmission line with at least one resonator being skipped. For example, the input-output connection electrode 70 and the capacitance forming electrode 70c' and/or the input-output connection electrode 70a' and the capacitance forming electrode 70c may be connected to each other through a transmission line in a skipped manner. Fig. 15 is a diagram showing a sixth dielectric filter, showing an example of a dielectric filter including a plurality of coaxial resonators each having one end face short-circuited, which has an outer peripheral conductor 81c and inner peripheral conductors formed by providing a dielectric member with a plurality of through holes 81b, 81b' and 81b" and coating an outer peripheral surface of the dielectric member and inner peripheral surfaces of the through holes with a conductive member. In the sixth dielectric filter, a depressed part 82 is formed by removing a portion of the outer peripheral conductor on the open face side of the coaxial resonators or a portion of the outer peripheral conductor including the dielectric member, and a dielectric substrate 83 having external connection electrodes 83a and 83a' and ground electrodes 83b and 83b' separated in a substantially central part is mounted on the depressed part 82, to construct a dielectric filter. When capacitance forming electrodes are formed on the dielectric substrate in the sixth dielectric filter, as in the fifth dielectric filter, a ground electrode provided on the reverse surface is divided corresponding to the capacitance forming electrodes. When a pole P is formed in an attenuation region of filter characteristics in the sixth dielectric filter, the external connection electrodes 83a and 83a' are respectively connected to each other through a transmission line, so that the resonators are connected to each other with one resonator being skipped. According to the fourth to sixth dielectric filters, the plurality of ground electrodes formed on the reverse surface of the dielectric substrate having at least the external connection electrodes which is mounted on the depressed part of the coaxial resonators are provided separated from each other corresponding to the external connection electrodes and the capacitance forming electrodes. Accordingly, an input-output direct coupling is prevented, thereby to make it possible to improve out-of-band attenuation in the frequency characteristics of the filter without degrading the noise shielding effect. Meanwhile, as the above described dielectric filter using the coaxial resonators, a filter using quarter wavelength type resonators which resonate at a wavelength which is one-fourth the wavelength in the resonance frequency by short-circuiting the one sides of the resonators. For example, it is assumed in the dielectric filter shown in Fig. 10 that ceramics of a TiO 2 - ZrO 2 - SnO 2 system is used as a material for the coaxial resonators, and the dielectric constant ε r thereof is 40. If the length of one side of each of the coaxial resonators in cross section perpendicular to the longitudinal direction is 3 mm, the diameter of the through hole is 0.8 mm, and the length 1 of the resonator is 4.6 mm, a characteristic impedance Z 0 is 12 Ω. In such a dielectric filter, impedances are matched as viewed from the input side in a particular frequency band, to exhibit the characteristics of a band-pass filter. The resonance conditions of the quarter wavelength type resonator are given by the following equation (1): Z 0 • tan (β · 1) = ∞ (where β is a propagation constant in the resonator) Furthermore, the length 1 of the resonator is given by the following equation (2) when c is taken as the speed of light: 1 = c / (4f 0 ε r 1/2 ) From the equation (2), the following equation is obtained: f 0 = c / (41 0 ε r 1/2 ) Consequently, the resonance frequencies of the coaxial resonators 50 and 51 shown in Fig. 10 are 2.6 GHz. Meanwhile, a resonator constituting a filter may, in some cases, generally exhibit undesired filter characteristics because it has not less than two resonance modes. For example, in order to make the length of the quarter wavelength type resonator the smallest, a basic TEM mode is generally utilized. However, there exists a higher TEM mode having a resonance frequency which is an odd order of the frequency in the basic TEM mode. In the dielectric filter shown in Fig. 10, the resonance frequency in the basic TEM mode is 2.6 GHz. At this time, the resonance frequency in the higher TEM mode exists in the vicinity of 7.8, 13.0 and 18.2 GHz. Fig. 16 shows the frequency characteristics of the dielectric filter shown in Fig. 10. The reason why large resonance characteristics appear in the resonance frequency in the higher TEM mode is that resonance frequencies of the coaxial resonators 50 and 51 in the higher TEM mode are equal, so that they are mutually reinforcing. Thus, spurious filter characteristics having a center frequency in the vicinity of a frequency which is an odd multiple of the center frequency of desired filter characteristics appear. Such spurious filter characteristics exert various adverse effects on microwave equipments. For example, when such unnecessary filter characteristics exist in a filter on the reception side, a microwave in another band enters a circuit. In addition, when such unnecessary filter characteristics exist in a filter on the transmission side, a microwave having a frequency other than the intrinsic center frequency is released in a space. In a first embodiment and a second embodiment of the invention, there is provided a small-sized dielectric filter in which spurious filter characteristics in a resonance frequency in a higher TEM mode can be solved without complicating a circuit. Fig. 17 and Fig. 18 are diagrams showing a dielectric filter according to the first embodiment of the present invention. In one-end face short-circuited type coaxial resonators 90 and 100, an outer peripheral surface of a dielectric member provided with through holes and inner peripheral surfaces of the through holes, excluding their open faces 90a and 100a are coated with a conductive member such as silver. A material for the resonators is ceramics of a TiO 2 - ZrO 2 - SnO 2 system, and the dielectric constant thereof is 40. The coaxial resonator 90 is the same as that shown in Fig. 10, and the resonance frequency thereof is 2.6 GHz. A capacitance forming portion 101 is mounted on the open face 100a of the coaxial resonator 100. Fig. 18A is an exploded perspective view of a coaxial resonator 100 and a capacitance forming portion 101, and Fig. 18B shows a transverse sectional view thereof. The length of one side of the coaxial resonator 100 in cross section perpendicular to the longitudinal direction is 3 mm, the diameter of the through hole is 0.8 mm, and the length of the resonator is 2.3 mm. A first conductor 100d electrically connected to an outer peripheral conductor 100b and a second conductor 100e electrically connected to an inner peripheral conductor 100c are formed on the open face 100a of the coaxial resonator 100. The capacitance forming portion 101 comprises a dielectric member 102 in the shape of a square pole having a bottom surface which is almost the same as the open face 100a of the coaxial resonator 100. The dielectric material thereof can be the same as that of the coaxial resonator or another dielectric material. The dielectric member 102 is provided with a through hole 102a, and is provided with a capacitance forming conductor 102b on its one surface. A surface, on which the capacitance forming conductor 102b is not formed, of the capacitance forming portion 101 is mounted on the open face 100a of the coaxial resonator 100 by an adhesive or a dielectric paste 96. The capacitance forming conductor 102b on the capacitance forming portion 101 respectively forms resonance frequency correcting capacitances C31 and C32 shown in Fig. 19 between the capacitance forming conductor 102b and the first conductor 100d and the second conductor 100e on the open face 100a of the coaxial resonator 100. The resonance frequency of the resonator comprising the coaxial resonator 100 and the capacitance forming portion 101 is set to 2.6 GHz which is the same as that of the coaxial resonator 90. Therefore, a combined capacitance C of the resonance frequency correcting capacitances C31 and C32 is set to 5 pF. The combined capacitance C is found from the following equation (4) showing the resonance conditions of the resonator: Zo • tan (β 1) = 1 /(2πf 0 C) The resonance frequency of the resonator in a higher TEM mode appears in the vicinity of 11.1, 21.0 and 31.2 GHz. On the other hand, the coaxial resonators 90 and 100 differ in length, so that the resonance frequency of the coaxial resonator 90 in a higher TEM mode appear in the vicinity of 7.8, 13.0 and 18.2 GHz, which differs from the above described one. Consequently, their respective higher harmonic components are not added to each other, so that undesired filter characteristics are restrained. Meanwhile, the coaxial resonators 90 and 100 are provided with a depressed part 95, similarly to the dielectric filter shown in Fig. 10, and a dielectric substrate 110 having external connection electrodes 110a and 110b formed thereon is mounted on the depressed part 95. Input-output coupling capacitances C33 and C34 are respectively formed between the external connection electrodes 110a and 110b and the inner peripheral conductors 90C and 100C of the coaxial resonators 90 and 100. In addition, the coaxial resonators 90 and 100 are coupled to each other through coupling windows (not shown) formed on the outer peripheral conductors. Fig. 19 shows an equivalent circuit of the dielectric filter shown in Fig. 18. L indicates a coupling inductance between the resonators 90 and 100. Fig. 20 is a diagram showing the frequency characteristics of the dielectric filter shown in Fig. 18. As can be seen from the characteristic diagram, pass characteristics in the resonance frequency in the higher TEM mode are restrained, as compared with the frequency characteristics shown in Fig. 16. Figs. 21A to 21C show a second embodiment, which are transverse sectional views showing examples in which the structures of a capacitance forming portion 101 mounted on a coaxial resonator 100 are different. A resonator shown in Fig. 21A is constructed by forming a first conductor 100e electrically connected to an inner peripheral conductor 100c on an open face 100a of the coaxial resonator 100 and mounting a dielectric member 102 having a capacitance forming conductor 102b electrically connected to an outer peripheral conductor 100b formed therein on the open face 100a of the coaxial resonator 100. A resonator shown in Fig. 21B is constructed by forming a first conductor 100d electrically connected to an outer peripheral conductor 100b on an open face 100a of a coaxial resonator 100 and mounting a dielectric member 102 having a capacitance forming conductor 102b electrically connected to an inner peripheral conductor 100c formed therein on the open face 100a of the coaxial resonator 100. In a resonator shown in Fig. 21C, a plurality of capacitance forming conductors 102b are disposed with them being alternately opposed to each other in a capacitance forming portion 101, thereby to make it possible to form a large capacitance. Although in the above described embodiment, resonance frequency correcting capacitances are connected to an inner peripheral conductor of only one of a pair of coaxial resonators, different capacitances may be connected to both the coaxial resonators so that the basic resonance frequencies of the coaxial resonators are equal. In such a manner, the lengths of both the coaxial resonators can be reduced, thereby to make it possible to make the filter small in size. As described in the foregoing, according to the first and second embodiments, it is possible to achieve a small-sized dielectric filter in which pass characteristics in a resonance frequency in a higher TEM mode are restrained and undesired filter characteristics are improved without complicating a circuit. Fig. 22 is a perspective view showing a first antenna duplexer using as a filter for reception (Rx) a band-pass filter having a pole in the rejection range 44 according to the first dielectric filter and using as a filter for transmission (Tx) a band-rejection filter 45 according to the third dielectric filter. Electrodes 46a to 46c for the above described band-pass filter having a pole in the rejection range 44, electrodes 46d to 46f for the band-rejection filter 45, and pattern inductors 46g to 46j are formed on the surface of a dielectric substrate 46 made of alumina having a ground electrode formed on its reverse surface, and chip capacitors 47 to 50 are disposed thereon. In addition, a matching circuit on the reception side 51 and a matching circuit on the transmission side 52 for connecting the filter for reception and the filter for transmission to one antenna. A Tx input electrode 46k connected to a transmitter 53, an Rx output electrode 46m connected to a receiver 54, and an antenna electrode 46n connected to an antenna 55 are formed, as shown in Fig. 23. Fig. 23 is a schematic diagram showing the antenna duplexer. The filter for reception (Rx) has properties of passing a received wave band and preventing a transmitted wave band, and the filter for transmission (Tx) has properties of passing a transmitted wave band and preventing a received wave band. Meanwhile, since an input impedance of the filter for reception (Rx) relative to a transmitted wave or an input impedance of the filter for transmission (Tx) relative to a received wave take a finite value, the impedances may, in some cases, be mismatched between both the filters and the antenna. Accordingly, the matching circuit on the reception side 51 and the matching circuit on the transmission side 52 for connecting the filters to one antenna are formed. The matching circuit on the reception side 51 and the matching circuit on the transmission side 52 are constructed using the change in phase due to a transmission line. More specifically, the phase of a signal wave varies depending on the length of the transmission line which is the length of the propagated distance. By selecting the length of the transmission line, the input impedance of the filter for transmission (Tx) in the received wave band is regarded as infinity, and the input impedance of the filter for reception (Rx) in the transmitted wave band is regarded as infinity. As a result, the antenna duplexer characteristics of a shared antenna are obtained. In such an antenna duplexer, the matching circuits 51 and 52 for adjusting the characteristics of the filters 44 and 45 are formed on the same dielectric substrate 46, to decrease the number of components and make the manufacture easy. Also in the first antenna duplexer, a dielectric filter using coaxial resonators, which is formed by coating an outer peripheral surface of a dielectric member provided with a plurality of through holes and inner peripheral surfaces of the through holes with a conductive member as in the second dielectric filter, may be used as filters for transmission and reception. Fig. 24 is a perspective view showing a second antenna duplexer using as a filter for reception (Rx) a band-pass filter having a pole in the rejection range 44 according to the first dielectric filter and using as a filter for transmission (Tx) a band-rejection filter 45 according to the third dielectric filter, as in the first antenna duplexer. The second antenna duplexer is the same as the first antenna duplexer except for the structures of a matching circuit on the reception side 51' and a matching circuit on the transmission side 52' for connecting the filter for reception and the filter for transmission to one antenna. The matching circuit in the first antenna duplexer is formed using the transmission line. In the matching circuit formed using the transmission line, however, the length of the line may be in the vicinity of a quarter wavelength in many cases, so that the matching circuit itself requires a large area, to increase the entire antenna duplexer in size. In the second antenna duplexer, therefore, the matching circuit is constructed using an inductance element and a capacitance element, to decrease the matching circuit in size. Pattern capacitors 51a and 52a are used as the capacitance portions, and line inductors 51b and 52b are used as the inductance elements. The line inductor 51 is grounded to a ground electrode on the reverse surface through a plated-through hole 51c. Fig. 25 is a schematic diagram showing the second antenna duplexer. In such an antenna duplexer, circuits 56 and 57 for adjusting the characteristics of the filters 44 and 45 and matching circuits 51 and 52 can be formed on the same dielectric substrate 46, thereby to reduce the number of components and make the manufacture easy. Also, in the second antenna duplexer, a dielectric filter using coaxial resonators, which is formed by coating an outer peripheral surface of a dielectric member provided with a plurality of through holes and inner peripheral surfaces of the through holes with a conductive member as in the second dielectric filter, may be used as filters for transmission and reception. Fig. 26 is a perspective view showing a third antenna duplexer using as a filter for reception (Rx) a band-pass filter having a pole in the rejection range 44 according to the first dielectric filter and using as a filter for transmission (Tx) a band-rejection filter 45 according to the third dielectric filter. In the eleventh embodiment, the whole of a matching circuit 52 is constituted by two inductances and one capacitance. Pattern inductors 4 and 6 are used as the inductances, and a chip capacitor 5 is used as the capacitance. Fig. 27 shows an equivalent circuit of the antenna duplexer. Inductance elements or capacitors 7 are formed between the filter for reception (Rx) 44 and the filter for transmission (Tx) 45 and a Tx input electrode 46k connected to a transmitter 53, an Rx output electrode 46m connected to a receiver 54, and an antenna electrode 46n connected to an antenna 55, as shown in Fig. 26. However, there is no strict solution for finding the values of the respective elements. Therefore, the following procedure is used so as to determine the values of the inductance elements or capacitors. The characteristics of the filter for reception (Rx) and the filter for transmission (Tx) are found by simulation or measurement. The characteristics are represented by data of a frequency having a sufficiently small width and an S parameter corresponding to the frequency, and are inputted to an electronic computer. A suitable objective function is determined to perform an optimization calculation. Examples of a method of optimization include the Monte Carlo method. An example of the objective function is represented by the following equation (5): G = ∫ f2 f1 (-100 S 46 n ·46 k +10 S 46 n ·46 n +10 S 46 k ·46 k + S 46 n ·46 k ) df + ∫ f4 f3 (-100 S 46 n ·46 k + 10 S 46 n ·46 n + 10 S 46 m ·46 m + S 46 m ·46 k ) df where f1 to f2 indicate frequencies in a received wave band, f3 to f4 indicate frequencies in a transmitted wave band, and Snm is an S parameter represented in decibel notation of the antenna duplexer, and terminal numbers correspond to those shown in Fig. 27. 100 and 10 which are coefficients of the S parameter are weights. As a result of the optimization, connection is switched to an opened state when the absolute value of the value of an element represented by an impedance is extremely large, while being switched to a short-circuited state when it is extremely small. The optimization is further performed to determine a final circuit. As a measure of the switching at this time, the connection is switched to an opened state when the absolute value is not less than 200 Ω, while being switched to a short-circuited state when it is not more than 10 Ω. Fig. 28 is a diagram showing an equivalent circuit of a fourth antenna; showing one example in which optimization is performed in the third antenna duplexer. The connection from an antenna electrode 46n to a ground conductor and the connection from a filter for transmission 45 to the ground conductor are switched to an opened state. An inductor 4 of approximately 6 nH is positioned between the antenna electrode 46n and the filter for transmission 45, and a capacitor 5 of approximately 1.5 pF is positioned between the antenna electrode 46n and a filter for reception 44. Further, an inductor 6 of approximately 10 nH is positioned between the filter for reception 44 and the ground conductor. Fig. 29 shows the characteristics of the fourth antenna duplexer. Terminal numbers of an S parameter correspond to those shown in Fig. 28. It is assumed that a transmitted wave band is 1.453 to 1.465 GHz, and a received wave band is 1.501 to 1.513 GHz. The insertion loss of a received wave is 2.7 dB and the return loss thereof is 9.2 dB, the insertion loss of a received wave is 1.7 dB and the return loss thereof is 16 dB, and the degree of separation between transmission and reception is 38 dB. Fig. 30 shows the characteristics of the first antenna duplexer. A transmitted wave band and a received wave band are the same as the above described ones. The insertion loss of a received wave is 2.9 dB and the return loss thereof is 11.4 dB, the insertion loss of a transmitted wave is 2.6 dB and the return loss thereof is 5.8 dB, and the degree of separation between transmission and reception is 38 dB. It can be confirmed from comparison between Fig. 29 and Fig. 30 that in the fourth antenna duplexer, the insertion loss is smaller and the return loss is larger, that is, the characteristics are higher. Capacitance forming electrodes provided on a dielectric substrate form antiresonance capacitances between the electrodes and inner peripheral conductors, and a reactance element for coupling the capacitance forming electrodes is formed on the dielectric substrate, thereby to obtain a dielectric filter which is easily manufactured and is small in size. Furthermore, in an antenna duplexer using the dielectric filter as filters for transmission and reception, matching circuits can be simultaneously provided on the dielectric substrate comprising capacitance forming electrodes of both the filters, thereby to simplify the construction and make the manufacture easy. It should be noted that the present invention is not limited to the embodiments as described above. It is envisaged that various modifications and variations to the above described embodiments could be made without falling outside the scope of the present invention as defined in the claims.

Claims: A dielectric filter comprising a plurality of coaxial resonators (90,100) each having an outer peripheral conductor (90b,100b) and an inner peripheral conductor (90c, 100c) and having one end face short-circuited, wherein at least one of said plurality of coaxial resonators (90,100) has a length different than that of the other coaxial resonators (90), characterised in that said at least one coaxial resonator (100) has the same basic resonance frequency as that of said other coaxial resonators (90) by a resonance frequency correcting capacitance (101) connected to the inner peripheral conductor (100c). The dielectric filter according to claim 1, wherein the outer peripheral conductor (90b,100b) and the inner peripheral conductor (90c,100c) are formed by coating an outer peripheral surface and an inner peripheral surface of a dielectric member with a conductive member, respectively. The dielectric filter according to claim 1 or 2, wherein said resonance frequency correcting capacitance is formed by a dielectric member (102) having a capacitance forming conductor (102b) formed thereon on an open end face (100a) of the coaxial resonator (100). The dielectric filter according to any preceding claim, wherein the dielectric member (102) is in the shape of a substantially square pole having a bottom surface almost the same in shape as the open end face (100a) of the coaxial resonator (100). The dielectric filter according to any preceding claim, wherein each of the coaxial resonators (90, 100) further comprises a depressed part (95) formed by removing a portion of the conductive member coating the outer peripheral surface of the dielectric member on one end face of the dielectric member or a portion of the conductive member coating the outer peripheral surface of the dielectric member together with a corresponding portion of the dielectric member; and wherein the filter further comprises a dielectric substrate (110) having a plurality of external connection electrodes (110a, 110b) formed thereon for forming input-output coupling capacitances between the external connection electrodes (110a, 110b) and the inner peripheral conductors (90c, 100c) of the coaxial resonators, each of the coaxial resonators (90, 100) being mounted to the dielectric substrate (110) at the depressed part (95). The dielectric filter according to any of claims 2 to 5, wherein the dielectric member comprises a through hole having an axis which is aligned with that of the resonator. The dielectric filter according to any of claims 2 to 6, wherein the dielectric member comprises a first major surface which is mounted to the open surface of the coaxial resonator and a second major surface opposite the first major surface and supporting the capacitance forming conductor (102b).

Inventors: IKEDA KACHIO, KOBAYASHI YASUMI, KUROKI KAZUHIKO, SHIBATA KENICHI, TAKEUCHI KOUSUKE, IKEDA, KACHIO, KOBAYASHI, YASUMI, KUROKI, KAZUHIKO, SHIBATA, KENICHI, TAKEUCHI, KOUSUKE
Applicants: SANYO ELECTRIC CO, SANYO ELECTRIC CO., LTD.
Application year: 1992
Main classification: H01P1
IPC codes: H01P1