Abstract:
Dielectric tube loaded metal cavity resonators and filters having a dielectric tube resonator extending substantially the full height of the metallic cavity are disclosed herein. The resonators and filters achieve low insertion loss in a size substantially smaller than conventional dielectric loaded resonators for equivalent quality factors. The dielectric tube resonators may be used with coaxial resonators to provide mixed resonator filter constructions.

Description:
FIELD OF THE INVENTION 
     This invention relates to TM01 cavity resonators and to filters achieving a low insertion loss and high Q in a small size. 
     BACKGROUND OF THE INVENTION 
     Coaxial cavity resonator filters and dielectric loaded single TE01 mode cavity resonators filters are two types of filter structures that have been widely used, especially in cellular-type telecommunications base stations, to provide high performance and high power handling. The typical quality factor (Q) of coaxial cavity resonators is from 2,000 to 8,000, while the Q of dielectric loaded TE01 mode cavity resonators varies from 12,000 to 40,000 when low loss, high dielectric constant ceramic materials are used. Usually, the cavity size of dielectric loaded TE01 mode cavity resonators is much greater than the size of the coaxial cavity resonators. To find a technology to fill the gap between these two technologies namely to produce a filter which has a Q greater than that of a coaxial cavity resonator filter, but which is of a size smaller than that of a TE01 coaxial cavity resonator has been a long time goal. It would be desirable to provide a dielectric loaded TE01 mode cavity resonator filter with a Q of 8000 to 12,000 without increasing the cavity size relative to coaxial cavity resonator technology, or to provide a similar Q with smaller size. 
     It would also be desirable to produce filters using both ceramic or metal disc loaded cavity resonators to achieve Qs in the ranges of 8,000 to 12,000 in a size smaller than is possible today when employing either coaxial cavity resonator and TE01 mode cavity resonator technologies. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an improved dielectric loaded cavity resonator filter is provided. The filter has at least one elongate dielectric tube resonator defining a clear through axial opening. The tube resonator is positioned in a conductive cavity such as a metallic cavity. The elongate dielectric tube resonator extends at least 70% of the height of the cavity and preferably extends substantially from the top to the bottom of the conductive cavity and has a length which is equal to or greater than its diameter. Means for securing the dielectric tube resonator in the cavity at each end of the tube resonator are provided. The securing means may comprise a mounting post at one end of the dielectric tube resonator. Desirably, the dielectric tube resonator defines centering formations in the clear-through axial opening and the centering formations engage the securing means at each end of the dielectric tube resonator. In a preferred form, the filter comprises a plurality of dielectric tube resonator/conductive cavities. The filter may also comprise a plurality of resonators, including at least one of the dielectric tube resonators and at least one coaxial resonator. The filter may also comprise tuning screws projecting into the dielectric tube resonators coaxial with the clear-through axial openings for adjusting the resonant frequency of the filter. 
     Also in accordance with the present invention, an improved dielectric loaded cavity resonator is provided comprising an enclosed housing defining a conductive cavity and an elongate cylindrical dielectric tube resonator defining a clear-through axial opening therein, the resonator being centrally located in the cavity and extending preferably substantially the full height of the cavity. In a most preferred form, the height of the dielectric tube resonator is equal to or greater than its diameter. 
    
    
     Further objects, features and advantages of the present invention will become apparent from the following description and drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of a dielectric tube resonator and cavity of the present invention. 
     FIG. 2 is a view like that of FIG. 1 showing a mounting assembly for the dielectric tube resonator. 
     FIG. 3 is a view like that of FIG. 2 showing a modified mounting assembly for the dielectric tube resonator. 
     FIG. 4 is a view like that of FIG. 2 showing a further modified mounting assembly for the dielectric tube resonator. 
     FIG. 5 is a plan view of a typical six resonator bandpass filter employing dielectric tube resonators and cavities of the type illustrated by FIGS. 1-4. 
     FIG. 6 is cross-sectional view of the filter of FIG. 5 taken substantially along line  5 — 5  of FIG.  5 . 
     FIG. 7 is a frequency response plot of the six resonator bandpass filter of FIG.  5 . 
     FIG. 8 is a plot showing the spurious performance of the six resonator bandpass filter of FIG.  5 . 
     FIG. 9 is a view like FIG. 6 but showing a mixed resonator filter employing both a tube resonator/cavity of the present invention and coaxial resonators/cavities. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a dielectric tube resonator/cavity  100  of the present invention comprises a housing  102  and a cover  104  defining a conductive cavity such as a metallic cavity  106 . Housing  102  is formed of a cast or machined metallic material, such as aluminum, or may be molded from a suitable nonconductive material, such as a plastic material, coated internally with a metallic conductive layer in a known manner. Cover  104  may be a conductive plate, or may be a plastic plate coated internally with a conductive material. Cover  104  is secured to housing  102  by screws (not shown) to define the cavity  106 . 
     A high dielectric constant dielectric tube which functions as a dielectric tube resonator  110  is centrally positioned in the conductive cavity and extends substantially from the bottom of the cavity to the inside surface of the cover. It is spaced sufficiently at one or both ends so that it is not mechanically stressed by the housing thereby to avoid undesired distortions. The TM01 mode is the primary resonant mode. Because there is no discontinuity of the tube resonator  110  in the axial direction, the cavity resonant frequency is independent of the cavity height, a feature which makes miniaturization of filters employing such tube resonator/cavity structures possible. 
     In a preferred embodiment of the present invention, a dielectric tube resonator  110  may be 2.28 inches in length. It defines an internal, clear-through cylindrical axial opening having an internal diameter of 0.38 inch and an external diameter of 1.68 inches. The dielectric tube resonator material may be ceramic and has a dielectric constant of about 45. The conductive housing  102  may be generally rectangular and defines internal cavity dimensions of 3.5 by 3.5 by 2.5 inches. Cover  104  is secured to the housing by a series of screws (not shown). 
     Referring now to FIG. 2, a typical arrangement for mounting a tube resonator  110 A having a high dielectric constant of about  20  to  50  with low loss in the cavity  106  is seen to comprise a centering or mounting post  120 A having a diameter substantially equal to that of the cylindrical opening in the resonator  110 A. Resonator  110 A defines top and bottom frustoconical internal formations  122 A and  124 A which may be chamfers of 45° and which are concentric with the cylindrical opening  126 A of the resonator  110 A. 
     Post  120 A is secured to, and projects upwardly from, the floor of the cavity  106  and into seating engagement within the central opening  126 A to center and locate the resonator  110 A. A rubber O-ring  128 A surrounds the post  120 A and engages the frustoconical lower regions  124 A of the tube resonator thereby to assist in seating and fixing the tube resonator  110 A and its lower region closely adjacent to the base of the cavity. At the top of the tube resonator  110 A a generally cone-shaped funnel  130 A having a chamfer to match the frustoconical formation is seated in the top end formation  122 A to center and locate the tube resonator  110 A at its top in the cavity  106 . Funnel  130 A is desirably threaded centrally so that a tuning screw  132 A may rotate relative thereto and may move coaxially within the central opening  126 A. Tuning screw  132 A defines a tool engaging formation of the outer end thereof. A locknut  134 A is provided to set and maintain an adjusted position of tuning screw  132 A. 
     A suitable dielectric tube resonator  110 A is made of ceramic, is 2.28 inches in height and 1.68 inches in diameter and defines a 0.38 inch central cylindrical opening. The post  120 A is of aluminum, and the funnel  130 A is of aluminum. The tuning screw  132 A is a threaded rod 0.20 inch in diameter and is of brass, but could be of plastic or other materials, as well. The dimensions of the conductive cavity are 3.5, by 3.5 by 2.5 inches (although the cavity may be cylindrical as well), and the frustoconical sections are at 45° to the vertical. 
     In the embodiment of FIG. 3, all of the parts, elements, and relationships may be the same as those of FIG. 2 except that the O-ring  128 A is omitted and a wave-washer  140 B is mounted in a shallow cylindrical slot  142 B formed in the base of the cavity  106  in a location which is aligned with the lower end of the dielectric tube resonator  110 A. The wave-washer  140 B provides biased engagement and seating of the tube resonator  110 A in the cavity  106 . The wave-washer may be of metal, but can be of non-metallic material as well. 
     Referring now to FIG. 4, a further dielectric tube resonator/coating  100 C is shown. The housing and cover may be the same as that of FIG.  1 . The dielectric tube resonator  110 C may typically be of a ceramic having a dielectric constant of 45. The resonator  110 C, extends from the base of the housing almost to the cover and occupies about 98% of the height of the cavity. Because the end gap is very small, the field distribution in the cavity has minor charge and the dielectric tube resonator cavity  100 C therefore performs very much like the other embodiment. 
     The internal diameter terminates at the base of the resonator in a frustoconical configuration with the head of a threaded fastener or screw  150 C which secures the resonator at the base of the housing so that it is tightly mounted against the cavity bottom wall and properly aligned with the mounting hole. There is no pressure exerted against the top of the resonator by the cover. A tuning screw  132 C which is located to function as described regarding the embodiments of FIGS. 2 and 3 is provided as well. 
     It will be clear from the foregoing that the means for securely mounting a tube resonator in a conductive cavity which extends substantially between the top and bottom of the cavity may be provided to form a resonator/cavity assembly useful for microwave applications. The resonant frequency can be adjusted by a judiciously positioned tuning screw mounted on the cover. If, for some reason, the housing and cover dictate it, the tuning screw could enter the housing from its bottom, as through the post of FIGS. 2,  3  and  4 , with like effect. Other tuning arrangements may be used as well. 
     The tube resonator/cavity assemblies described are gainfully deployed in bandpass filters employing a plurality of such dielectric tube resonators, such as the six dielectric tube resonator bandpass filter of FIGS. 5 and 6. 
     Referring now to FIGS. 5 and 6, a six tube resonator bandpass filter  190  of the present invention comprises six dielectric tube resonator/cavities  200 ,  300 ;  202 ,  302 ;  204 ,  304 ;  206 ,  306 ;  208 ,  308 ; and  210 ,  310 . Adjacent pairs of dielectric tube resonator/cavities are respectively coupled through adjacent irises or windows  220 ,  222 ,  224 ,  226 , and  228  for known purposes. A variety of iris configurations may be used. Resonator/cavities  200 ,  300  and  202 ,  302  are coupled by a coupling bar  240  mounted in an electrically insulating holder  242 . Isolation walls such as isolation wall  260  may be provided, consistent with filter design necessities and characteristics. The filter  190  also comprises a connector such as a threaded connector  250  having an input/output coupling loop  252  and a further threaded connector  254  also having an input/output coupling loop  256 . Typically, connectors  250 ,  254  are coaxial connectors. 
     As best shown by FIG. 6, tube resonators  200 ,  202 ,  204 ,  206 ,  208  and  210  are seen to be elongated dielectric tube resonators which extend substantially from the inside bottoms of the associated conductive cavities defined by the housing  280  to the inside tops of the cavities as defined by the cover  282 . The resonators may be mounted and located at their tops and bottoms as described in connection with FIGS. 1-4. Adjustable threaded tuning screws, such as tuning screws  207 ,  209  and  211 , may be supplied for each of the respective tube resonators, and a tuning screw  241  may be provided for the coupling bar  240 , as well. 
     In the filter of FIG. 5, the dielectric tube resonators may be 1.68 inches in outside diameter and 0.38 inch in inside diameter, and 2.38 inches in length, namely having a length which is about 1.5 times the diameter. 
     FIGS. 7 and 8 show the frequency response and spurious resonant frequencies  700 ,  702  of a bandpass filter constructed according to the embodiment of FIG.  5 . As can be seen, the filter passes frequencies in the band between 463.5 MHz and 465 MHz. In the embodiment from which the plots of FIGS. 7 and 8 were recorded, a resonator Q of approximately 10,000 was achieved at a resonant frequency of 464 MHz. As can be seen in FIG. 8, the first spurious resonant frequency  700  occurs at 896 MHz, a ratio of 1.93 between the first spurious resonant frequency and the primary resonant frequency. 
     Although an exemplary filter in accordance with the present invention has been designed for use in the 450 MHz range, filters for frequencies of from 400 MHz to 3 GHz may be made as well, with advantages comparable to those of the present embodiment. 
     Because the general filter cavity design employing coaxial resonators is similar to that employing tube resonators of the present invention, it has been determined that a mixed resonator filter may be employed with advantageous results. Such a filter is shown in FIG.  9 . 
     As there seen, a mixed, three cavity filter  290 , which comprises resonators disposed in three cavities, may include two metallic coaxial resonator/cavities  406 ,  506  and  410 ,  510 , and a dielectric tube resonator/conductive cavity  408 ,  508 . Coaxial connectors  450 ,  454  having coupling loops  452 ,  456 , respectively may be provided, as may be irises such as irises  426  and  428 . Tuning screws  407 ,  441 ,  409 ,  443  and  411 , like those in the embodiment of FIGS. 5 and 6, may similarly be provided for similar purposes, namely for tuning the resonators and coupling bars. 
     Thus, filters taking advantage of the dielectric tube resonators of the present invention and known coaxial resonators may be produced having Qs in the ranges of 8000 to 12000, but in sizes smaller than is otherwise possible currently. The adjacent and non-adjacent coupling mechanisms and frequency and coupling tuning screws are also applicable to both types of resonators, and therefore may be used in a mixed filter employing dielectric tube resonator/cavities of the present invention. The dielectric tube resonators preferably extend substantially the full heights of the cavities in which they are positioned, and minimally extend at least 70% of the height of the cavity. 
     Not only may the dielectric tube resonators of the present invention be used in bandpass filters of the types illustrated and described so far, and in filters used for microwave frequencies, they may be also used in a variety of other frequencies, in bandstop (notch) filters, and, among other things, in oscillator designs, as well. 
     Use of the dielectric tube resonator/cavity arrays of the present invention makes it possible to provide dielectric loaded resonator/cavity structures and dielectric loaded cavity resonator filters having reduced dimensions or having increased quality factors as compared to presently available dielectric loaded cavity structures and filters, all while making it possible to utilize conventional means for frequency tuning, for providing mutual and cross couplings between the resonators, and for providing input/output couplings to the resonators. Use of the dielectric tube resonator arrangements of the present invention also permit the use of mixed filters employing dielectric tube resonators and coaxial resonators with couplings among them to realize a variety of complex filter functions within a compact unit with high performance. 
     It will be apparent to those skilled in the art that modifications may be made in the foregoing embodiments without departing from the spirit and scope of the invention. Accordingly, it is intended that the present invention not be limited except as may be necessary in view of the appended claims.