Dual-Mode Filters Using Dielectric Resonators With Apertures

1. Field of the Invention

This invention relates to dual-mode filters and particularly to dual-mode filters having dielectric resonators containing apertures.

2. Description of the Prior Art

Dual-mode dielectric resonator filters have been widely used in cellular radios and satellite multiplexers. Although, the use of dielectric resonator technology offers a significant reduction in weight and size in comparison with the waveguide resonator technology, it is known that the spurious performance of dual-mode dielectric resonator filters is not satisfactory for many satellite applications. In satellite multiplexers, improving the spurious performance of such filters will readily translate to higher communication capacity, or cost saving, or further reduction in weight and size or a combination of these factors.

Implementation of dual-mode dielectric resonator filters has been conventionally accomplished by using the resonator configuration shown in FIG. 1, where a solid cylindrical dielectric resonator R, housed within a metallic enclosure M, operates in either the dual HEH.sub.11 mode or the dual HEE.sub.11 mode. It is also known that the proximity of the resonant frequency of the HEE.sub.11 mode to that of the HEH.sub.11 mode interferes with the filter performance causing undesirable spurious response.

The resonant characteristics of the conventional resonator shown in FIG. 1 have been described by K. A. Zaki and C. Chen (IEEE, MTT-34, No. 7, pp. 815-824). A typical mode chart for this resonator is illustrated in FIG. 2 in which the abscissa and ordinate represent the diameter to height ratio and the resonant frequency of the first four modes. Although the location of the spurious response can be controlled by adjusting the resonator dimensions, even with the choice of the optimum dimensions the attainable spurious separation is not adequate to meet the stringent requirements of recent satellite systems. A need has therefore arisen for a dual-mode dielectric resonator with improved spurious performance.

U.S. Pat. No. 4,028,652 issued June, 1977 to K. Wakino, et al. describes a single mode filter having a dielectric resonator containing one or more apertures. Undesirable spurious responses are said to be reduced. The patent does not however suggest the use of dual-mode operation of any of the described resonant structures.

U.S. Pat. No. 4,706,052 issued November, 1987 to Jun Hiattori, et al. describes a single-mode filter design in which a variety of differently shaped, layered and dimensioned dielectric resonators are disclosed and described. While the resonators do not contain apertures, the stated purpose of the invention is to improve the spurious performance of single-mode dielectric resonators operating in the TEH.sub.01 mode. There is no suggestion to use dual-mode operation.

An object of the present invention is the provision of a dual-mode filter having dielectric resonator structure operating either in the dual HEH.sub.11 mode or the dual HEE.sub.11 mode, said filter having a remarkable improved spurious performance as compared to prior art.

Another object of the present invention is the provision of a dual-mode filter having a dielectric resonator structure in which the improvement of the spurious performance can be achieved with a simple and reduced weight construction.

A dual-mode filter has at least one cavity resonating in a dual-mode. The at least one cavity contains a dielectric resonator. The resonator contains at least one aperture and the at least one aperture extends partially through said resonator and is sized and located to shift a resonance frequency of a spurious mode to a higher frequency range distance from a principal mode.

The foregoing and other objects and advantages of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention.

In the drawings:

FIG. 1 is a side elevation view of a prior art dielectric resonator;

FIG. 2 is a graph illustrating a typical mode chart for the prior art resonator shown in FIG. 1;

FIG. 3 is a partial sectional side view of one embodiment of a dielectric resonator according to the present invention;

FIG. 4 is a partial sectional side view of another embodiment of a resonator according to the present invention;

FIG. 5 is a graph illustrating the resonant characteristics of the dielectric resonator configurations shown;

FIG. 6 is a partial sectional side view illustrating a support for a dielectric resonator inside a metallic enclosure;

FIG. 7 is a partial sectional side view of a dielectric resonator having three discs with an aperture in a centre disc;

FIG. 8 is a partial sectional side view of a dielectric resonator having two discs with a centrally located aperture on an inner surface of each disc;

FIG. 9 is a graph illustrating the spurious performance of the dielectric resonator configurations shown; and

FIG. 10 is a perspective view illustrating the use of one of the disclosed dielectric resonator configurations in a dual-mode filter.

In FIG. 1, there is shown a prior art dielectric resonator R supported on a support N and enclosed in a metal casing M. The resonator R has a diameter D and a length L.

In FIG. 2, there is shown a graph of the resonance frequency of a cavity of a dual-mode filter containing the resonator R from FIG. 1 when measured against the ratio of diameter divided by length for various different modes.

FIGS. 3 and 4 show two embodiments of the present invention employing a dielectric resonator structure operating in the HEH.sub.11 mode, whereby the resonant frequency of the spurious HEE.sub.11 mode is shifted into a higher frequency zone. In FIG. 3, there is shown a solid dielectric disc R.sub.2 sandwiched between two other discs R.sub.1 and R.sub.3 having through apertures H.sub.1 and H.sub.2 in a centre. The three discs R.sub.1, R.sub.2, R.sub.3 have the same diameter and are attached together by a bonding material, for example, TRANSBOND (a trade mark). Since bonding layers L.sub.1 and L.sub.2 are located away from a center (z=0) where the electric field intensity of the HEH.sub.11 mode is high, the unloaded Q of the resonator is little affected by the loss tangent of the bonding material.

In FIG. 4, there is shown a dielectric resonator similar to that shown in FIG. 3 where two blind apertures A.sub.1 and A.sub.2 are machined into a solid cylindrical resonator R. It is to be noted that the apertures A.sub.1 and A.sub.2 may have cylindrical or any desired shape. The said apertures may be partially or totally filled with another dielectric material with a dielectric constant lower than that of the dielectric resonator. Each of the dielectric resonators shown in FIGS. 3 and 4 is mounted on a support N inside a metallic enclosure M. The supports can be made of low loss dielectric constant material, for example, REXOLITE (a trade mark), quartz or MURATA Z (a trade mark). The metallic enclosure can have cylindrical, square or any other desired shape, as long as it provides shielding around the described resonator.

By way of example, the resonant characteristics of a resonator of the type shown in FIG. 4 with a diameter D=17.8 mm, height L=5.8 mm and aperture diameter Ds=4.0 mm, is measured for different values of aperture depth Hs. For the given D/L ratio, the first three consecutive resonant modes are TEH.sub.01, HEH.sub.11 and HEE.sub.11. FIG. 5 shows the percentage frequency separation (f.sub.HEE11 -f.sub.HEH11)/f.sub.HEH11 between the operating mode HEH.sub.11 and the spurious mode HEE.sub.11 versus the ratio Hs/L. The values given at Hs/L=0.0 and Hs/L=0.5 represent respectively the percentage spurious separation exhibited by the conventional solid dielectric resonator and by a dielectric resonator in a coaxial cylindrical form. It can be seen that the resonator configuration described in FIG. 4 offers a 30% improvement in the percentage frequency separation over that exhibited by the prior art solid resonator shown in FIG. 1. Since the electric field intensity of TEH.sub.01 and HEH.sub.11 modes is minimum at z=+1/2, the provision of shallow apertures A.sub.1 and A.sub.3 at the top and bottom faces has a negligible effect on the resonance frequencies of these two modes, and consequently on the frequency separation between them. It is to be also noted that for a given diameter D, height L and aperture depth Hs, the frequency separation between the HEH.sub.11 mode and the HEE.sub.11 mode is controlled by the aperture diameter Ds. An improvement in the frequency separation of more than 30% can be achieved by the choice of the optimum values of Ds and Hs.

FIG. 6 illustrates a support for the dielectric resonators inside the metallic enclosure M. A support in cup-form N is fitted into the aperture A.sub.2, and is bonded to the dielectric resonator by an adhesive material. The support is screwed to the metallic enclosure using a plastic screw S.sub.1 and a blind nut S.sub.2. There is a layer L.sub.4 of pliable adhesive, for example, scotechweld between the base of the support and the enclosure body which acts as vibration damping material and adds extra strength. This support configuration provides mechanical integrity, minimizes Q degradation and guarantees design repeatability with accurate placement of the dielectric resonator.

In FIG. 7, there is shown a further embodiment of the present invention whereby the basic mode of operation is the HEE.sub.11 mode. The resonator described in FIG. 6 has three dielectric discs R.sub.1, R.sub.2, R.sub.3, all having the same diameter and being attached together by a bonding material. The middle disc R.sub.2 has a through aperture H.sub.3 in a center. The aperture H.sub.3 may have a cylindrical shape or any other desired shape. This disc deforms the fields of the HEH.sub.11 mode causing its resonance frequency to be shifted into a higher frequency range while negligibly affecting that of the operating HEE.sub.11 mode. Since the discs are bonded close to the resonator center (z=0), where the electric field of the HEE.sub.11 mode is minimum, the loss tangent of the adhesive layers L.sub.1 and L.sub.2, which holds the three discs together, has little effect on the loss performance of the resonator.

FIG. 8 illustrates a dielectric resonator which functions in a similar manner as the dielectric resonator disclosed in FIG. 7. The resonator has two identical dielectric discs R.sub.4, R.sub.5 having blind apertures A.sub.3, A.sub.4 attached together by a bonding material. The aperture may be of cylindrical shape or any other desired shape. It may be filled partially or totally with dielectric material of lower dielectric constant. In both of FIGS. 7 and 8, the dielectric resonator is mounted on a support N and is accommodated in a metallic enclosure M. Since the electric field of the TEH.sub.01 mode is zero at r=0, the apertures in the disclosed resonator given in FIGS. 7 and 8 have a negligible effect on the separation between the resonant frequency of the TEH.sub.01 mode and the operating HEE.sub.11 mode.

FIG. 9 illustrates the measured percentage frequency separation between the HEE.sub.11 and HEH.sub.11 modes for the two-disc resonator configuration given in FIG. 8, wherein D=17.8 mm, L=10.9 mm and Ds=5.0 mm. In this example, the D/L ratio is chosen such that the first three consecutive resonant modes are TEH.sub.01, HEE.sub.11 and HEH.sub.11. From FIG. 9, it can be seen that a larger percentage frequency separation between the operating HEE.sub.11 and the spurious HEH.sub.11 is achieved by the proposed two-disc resonator. With the choice of the optimum values of Hs and Ds, more than 50% improvement can be achieved in the percentage frequency separation between these two modes.

By way of example, FIG. 10 shows a 4-pole dual-mode filter employing the dielectric resonator configuration disclosed in FIG. 3. The filter comprises of two cavities M.sub.1, M.sub.2 and an iris I. The dimensions of the cavities M.sub.1 and M.sub.2 are arranged to be below cutoff for waveguide modes over the frequency range of interest. The cavity M.sub.1 contains a dielectric resonator R.sub.1, two tuning screws T.sub.1, T.sub.2, a coupling screw T.sub.3 and a coaxial probe P.sub.1. The dielectric resonator is operating in the dual HEH.sub.11 mode and is mounted inside the cavity by a support N.sub.1. The coupling between the two orthogonal HEH.sub.11 modes is achieved by the screw T.sub.3, which is located at 45.degree. and 135.degree. with respect to the tuning screws T.sub.2 and T.sub.1. The function of the coaxial probe is to couple electromagnetic energy into the filter or out of the filter. The cavity M.sub.2 is nearly identical to the cavity M.sub.1. It contains a dielectric resonator R.sub.2, two tuning screws T.sub.4 and T.sub.5, a coupling screw T.sub.6 and a coaxial probe P.sub.2. The iris I provides intercavity coupling through the aperture O. The two cavities M.sub.1 and M.sub.2 and the iris I are bolted together by screws (not shown) to construct the filter. While the filter has two physical cavities, due to the dual-mode operation of the dielectric resonator, there are four electrical cavities whose resonance frequencies are controlled by the tuning screws T.sub.1, T.sub.2, T.sub.4 and T.sub.5.

FIG. 10 is included to illustrate the use of one of the resonators described in FIGS. 3, 4, 6, 7 and 8 in dual-mode filters and is not meant to limit the scope of the invention. It will be readily apparent to those skilled in the art that it will be possible to design a dual-mode filter, with any reasonable number of cavities, using any of the dielectric resonators included within the scope of the claims. Such a filter will have an improved spurious performance as compared to prior art.

It is to be noted that the dielectric discs illustrated in FIGS. 2 and 6 can be attached together by a bonding material or can be laminated in the axial direction. Although the present invention has been fully described by way of example in connection with a preferred embodiment thereof, it should be noted that various changes and modifications will be apparent to those skilled in the art. By way of example, the support structure is not restricted to the planar configurations described above. Other configurations, for example, mounting on microstrip substrates or mounting the resonators axially in cylindrical cavities could be utilized.