Abstract:
A coaxial cavity resonator filter has a hollow cavity and a post having desired dimensions for achieving desired filter characteristics. A tuning element is supported within a metallic opening and is configured to electromagnetically interact with the post. The tuning element has a conductive core element where the orientation of the tuning element with the cavity is adjusted so as to achieve the desired filter characteristic. An insulator is configured to cover a portion of the conductive core element of the tuning element, at a location where the tuning element and the metallic opening interact. A portion of the insulator is threaded so as to allow the conductive core element vary its orientation within the cavity without contacting the metallic opening.

Description:
RELATED APPLICATION 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 13/675,327, filed on Nov. 13, 2012, the entirety of which is incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to Radio Frequency Communication transceivers and in particular to RF filters with reduced intermodulation distortion characteristics. 
         [0004]    2. Description of Related Art 
         [0005]    A typical wireless communication system, such as cellular transceiver, includes uplink and downlink channels separated in frequency. Such communication systems use filters to route, combine, and/or separate signals at different frequencies, to prevent interfering with other channels or systems, and/or to prevent being interfered with by other channels or systems. 
         [0006]    One type of filter used in such communication systems is constructed with coaxial cavity resonators, sometimes referred to as combline or interdigital resonators. These resonators typically consist of a metal outer conductor or cavity with a metal inner conductor. The inner conductor is electrically short circuited to the outer conductor at one end and open circuited at the other end. When an electromagnetic wave is coupled to this structure, the wave propagates along its length until it encounters the short circuit and is reflected back. This reflection causes a standing wave to be generated when the length of the inner conductor is approximately ¼ wave length long relative to the frequency of the coupled wave. Shorter lengths can also be used by capacitively loading the open circuit end. This standing wave can then be further coupled to adjacent resonators, allowing waves at specific frequencies to propagate while rejecting waves at other frequencies. 
         [0007]    However, coaxial cavity resonators can cause signal corruption. Signal corruption can occur when intermodulation Distortion (IMD) generated by the uplink or downlink signals fall unintentionally into the downlink or uplink frequency band, respectively. IMD in filters can create the very interference they are supposed to be preventing. 
         [0008]    As such there is a need to enhance the performance of such coaxial cavity resonators employed in wireless base stations and to specifically reduce or preferably eliminate intermodulation distortion. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0009]    As more spectrum is being allocated for wireless communications, the problem of intermodulation distortion has become more noticeable. A common construction of filters for wireless communication systems is machined metal housings using metal posts as combline or interdigital resonators. Current cost effective machining techniques are not accurate enough to produce these structures repeatedly so tuning elements are often employed to compensate for these inaccuracies. These tuning elements are often shaped as a threaded metal rod, with an arrangement for varying its length to achieve the desired filtering effect. Consequently, the contact area where the threads meet the housing is weak and/or intermittent. Current flows in these areas causing potential intermodulation distortion. 
         [0010]    Intermodulation distortion is generated when two or more signals encounter non-linear elements during transmission. One source of non-linearity is weak and/or intermittent metal to metal contact in areas where current flows. As such, the tuning elements intended to fine tune the resonator filter can cause the very distortion that they intended to overcome. In accordance with one embodiment of the invention a coaxial cavity resonator filter is provided having a cylindrical hollow post. The post is configured to receive a frequency tuning element. The post includes a first opening and an inner wall, such as a cylindrical wall having a diameter that is larger than the diameter of the tuning element. The post further includes a flange that forms a second opening having a specified height and a diameter that is smaller than the diameter of the inner wall of the first opening. 
         [0011]    An insulating support member is disposed within the post. The insulating support member is made of an insulating material such as Teflon® or a polyetherimide such as Ultem®, and it has a first head portion having a first diameter and a shoulder flange portion having a smaller diameter with a threaded internal wall. The shoulder flange portion of the insulating support is fitted within the second opening of the post. The insulating support is configured to receive a tuning element that can be screwed via its internal threaded portion. In an alternative embodiment, the tuning element includes an insulated threaded sleeve positioned at a desired portion along its length, and the second opening of the post is similarly threaded. As such, during operation the insulated threaded portion of the tuning element engages the threaded second opening and the length of the tuning element is adjusted to achieve a desired frequency response. 
         [0012]    In accordance with yet another embodiment the insulated sleeve is moveable along the length of the tuning element to provide an optimum location for the tuning element along the hollow tube of the post. 
         [0013]    In accordance with yet another embodiment the insulated sleeve is mounted in the cavity cover such that the tuning element is external to the resonator post. The length of the tuning element is adjusted to achieve the desired frequency response from the coaxial cavity resonator. In this configuration, the tuning element can also be used to adjust the coupling between adjacent resonators. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In accordance with various embodiments of the invention the following description and accompanying drawings describe the various features of the invention as claimed, wherein: 
           [0015]      FIG. 1  illustrates a coaxial cavity resonator filter according to one embodiment; 
           [0016]      FIGS. 2 a  and 2 b    illustrate a coaxial cavity resonator employed in the coaxial cavity resonator filter of  FIG. 1 ; 
           [0017]      FIGS. 3 a  and 3 b    illustrate an insulating support member according to one embodiment; 
           [0018]      FIG. 4  illustrates a tuning element according to one embodiment; 
           [0019]      FIG. 5  illustrates a tuning element and a locking nut within an insulating support member according to one embodiment of the invention; 
           [0020]      FIG. 6  illustrates another tuning element according to another embodiment; 
           [0021]      FIG. 7  illustrates a coaxial cavity resonator structure according to one embodiment; 
           [0022]      FIG. 8  illustrates a coaxial cavity resonator structure according to one embodiment; 
           [0023]      FIGS. 9 a -9 d    illustrate a coaxial cavity resonator filter having a solid post in accordance with one embodiment of the invention; 
           [0024]      FIGS. 10 a -10 b    illustrate another filter having a solid post in accordance with an embodiment of the invention; 
           [0025]      FIGS. 11 a -11 d    illustrate one embodiment of the invention with a hollow post; 
           [0026]      FIGS. 12 a  and 12 d    illustrate another embodiment of the invention with a hollow post; and 
           [0027]      FIG. 13  illustrates the test results employing a coaxial cavity resonator in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    The coaxial cavity resonator filters discussed in relation to various embodiments of the invention are typically employed in wireless base stations, such as cellular communication base stations. A desired characteristic of such filters is to have low insertion losses in the passband frequency range of the transmitted or received signals, along with high attenuation in the stopband frequency range close to the passband frequency range. 
         [0029]      FIG. 1  illustrates a coaxial cavity resonator filter structure of a transmitter/receiver filter  10 , in a housing  12 . Filter  10  includes a top plate (not shown), which is removed from the top portion of transmitter/receiver filter as illustrated in  FIG. 1 . A plurality of coaxial cavity resonators  14 ( a ),  14 ( b ),  14 ( c ) . . .  14 ( n ) are arranged to form desired filters. Such resonators in accordance with various embodiments of the invention are serially or sequentially coupled to obtain the desired filter characteristics. In one embodiment of the invention one set of filters  14  may be coupled to form the transmit filter of a base station. This filter receives the energy from the transmit section of the base station, and filters the energy according to a designated transmit-frequency passband. Another set of filters  14  corresponds to the receive filter of the base station. This filter receives energy from the radio antenna of the base station and filters the energy according to a designated receive-frequency passband. 
         [0030]      FIGS. 2 a  and 2 b    illustrate a coaxial cavity resonator  14  in accordance with one embodiment of the invention.  FIG. 2 b    is a section view of the resonator and  FIG. 2 a    is its top view. Coaxial cavity resonator  14  includes a hollow upper portion  16  and a hollow lower portion  18 , separated by an aperture  20 . The internal diameter of aperture  20  is smaller than the internal diameters of upper portion  16 , forming a flange  24 . The top portion includes an opening  22  with an internal diameter that is larger than the diameter of aperture  20 . In accordance with one embodiment of the invention, opening  22  has the same diameter as the internal diameter of upper portion  16 . In accordance with one embodiment of the present invention, the internal diameter of aperture  20  is about 8 millimeters with a tolerance of about +0.01 mm and −0.02 mm. The internal diameter of aperture  20  is about 6.25 mm, and the length of upper portion is about 10 mm. The internal diameter of lower portion  18  is about 12 mm and the length of lower portion  18  is about 51.77 mm. 
         [0031]      FIG. 3 a    illustrates an insulating support member  30  in accordance with one embodiment of the invention. Insulating support member  30  is made of an insulating material such as Ultem® or Teflon®, and is configured to fit within coaxial cavity resonator  14  illustrated in  FIGS. 2 a  and 2 b   . Insulating support member  30  includes a head portion  32  and a shoulder portion  34 . Head portion  32  has an outside diameter that is larger than the outside diameter of shoulder portion  34 . In accordance with one embodiment of the invention, the outside diameter of head portion  32  tapers towards the shoulder portion along taper  36 . Similarly, shoulder portion  34  tapers in via taper  38 . 
         [0032]    Furthermore, the inside diameter of shoulder portion  34  is threaded so as to accommodate the turning of a tuning element configured to pass through insulating support  30  as will be explained in more detail below. In accordance with one embodiment of the invention, the diameter of head portion  32  is about 8 mm. For this embodiment, the length of the shoulder portion is about 10 mm and the length of the head portion is about 2.5 mm providing an overall length of 12.5 mm for the insulated support member. 
         [0033]    The insulated support member is configured to fit within the coaxial cavity resonator, such as  14  illustrated in  FIG. 2 , such that the shoulder portion is fitted within aperture  20  and head portion  32  is disposed within the upper portion of the coaxial cavity resonator. Turning to  FIGS. 3 a  and 3 b   , insulating support member is illustrated, prior to placing it within coaxial cavity resonator  14   a.    
         [0034]    Once insulated support  30  is placed within the coaxial cavity resonator as described above, a tuning element  80  illustrated in  FIG. 4  can be threaded within the support member to adjust its length within the coaxial cavity resonator to achieve the desired frequency characteristics. To this end,  FIG. 4  illustrates tuning element  80  that has a specified length and diameter depending on the size of the coaxial cavity resonator is screwed into insulated support  30 . The outside diameter of tuning element  80  is threaded so that it engages the threaded inner diameter of insulating support  30 . Tuning element  80  has a head portion  82 , with a slot  84  for driving the element in and out of the insulating support and the coaxial cavity resonator. The pitch of the threads of tuning element  80  is designed to provide accurate control of the length of the tuning element within the coaxial cavity resonator. 
         [0035]      FIG. 5  illustrates the bottom side of filter  10  referred in  FIG. 1  above. One of the coaxial cavity resonators shown in  FIG. 1 , such as  14 ( d ) is fitted with an insulating support  30 . Thereafter, a tuning element  80  is inserted within the insulating support and threaded within the coaxial cavity resonator until the desired frequency response is achieved. A locking member, such as lock nut  89  assures that the tuning element remains at a specific length during the operation of filter  10 . In accordance with one embodiment of the invention, lock nut  89  is made of Ultem® or Teflon®. Once the proper length of tuning element  80  has been determined, a locking ember  89  is screwed on the tuning element to fix the effective length of the tuning element during the operation of the filter. 
         [0036]    In accordance with another embodiment of the invention, instead of using insulating support  30 , tuning element  80  is fitted with a threaded insulating sleeve. As such  FIG. 6  illustrates tuning element  80  having an insulating sleeve  92  that is threaded so as to allow the tuning of element  80  once it is within the coaxial cavity resonator. In accordance with this embodiment of the invention, tuning element  80  is inserted within the coaxial cavity resonator post, such that the sleeve portion of tuning element  80  engages the inner diameter of aperture  20  of the coaxial cavity resonator which is threaded. 
         [0037]      FIG. 7  illustrates a configuration where the tuning element  80  is external to a resonator post  106  and insulated from a cavity cover  120  by insulating support  110 . Resonator post  106  is enclosed in cavity  100 . A second cover  116  is employed to create an isolation cavity  118  such that any existing adjacent resonators do not couple through the portion of the tuning element that protrudes above cavity cover  120 . The same combinations of threaded support and tuning element described above are relevant here. The tuning element is moved in and out to achieve the desired frequency response. 
         [0038]    It is appreciated by those skilled in the art that, depending on frequency characteristics requirements, sometimes a single coaxial cavity resonator is employed and other times two or more coaxial cavity resonators are coupled together by employing an arrangement where a coupling tuning element is used to achieve the desired filter characteristics. In accordance with one embodiment of the present invention,  FIG. 8  illustrates a structure  102  having adjacent coaxial cavity resonators  104  and  106 . Resonators  104  and  106  are coupled through the magnetic field around the resonators. As described earlier, when an electromagnetic wave of the appropriate frequency is coupled to a resonator, a standing wave is generated. This standing wave has a magnetic field associated with it. The electromagnetic wave is sinusoidal so the resulting magnetic field is also sinusoidal. When this magnetic field is incident on the second resonator the electromagnetic wave will couple to the second resonator which in turn will generate a standing wave. This process can be repeated for any desired number of N resonators. The coupling tuning element between the resonators allows the magnetic field of the first resonator to couple to the element which in turn couples to the second resonator, creating a bridge that increases coupling. 
         [0039]    As such,  FIG. 8  illustrates the arrangement where coupling tuning element  108  is employed to provide coupling between the two coaxial cavity resonators. Regions  110 ,  112  and  114  identify the locations within the structure where insulating members are employed in accordance with the arrangements described above. Regions  110 ,  112  and  114  include insulating supports for receiving corresponding tuning elements, or in accordance with other embodiments of the invention, regions  110 ,  112  and  114  include tuning elements with corresponding insulated sleeves. 
         [0040]    In accordance with other embodiments, the intermodulation distortion effect can be substantially reduced in a variety of cavity resonator structures. For example.  FIGS. 9 a -9 d    illustrate an embodiment in connection with a solid resonator within a resonant cavity. As such,  FIG. 9 a    illustrates a filter  160  having a filter body  162  forming a cavity cube with five close sides and a top open side. A solid resonator  164  is disposed within the cavity. A cover  166  is placed over the open top of the cavity. Cover  166  includes an opening  168  for allowing tuning element  80  to engage within the cavity. A lock nut member  89  is screwed on the tuning element as illustrated in  FIG. 9 a   . Thereafter a shield plate  170  is placed over the filter to isolate the filter from its adjacent environment.  FIG. 9 b    is a side view of filter  160  illustrating the manner tuning element  80  engages the resonant cavity. In accordance with one embodiment of the invention,  FIG. 9 c    illustrates tuning element  80  that is a made of a conductor covered by a threaded insulator having external threads as discussed before.  FIG. 9 d    is a cross section of tuning element  80  showing the conductive element embedded within the insulator covering  92 . During operation, tuning element  80  is adjusted by accessing the opening within shield  170 . 
         [0041]      FIGS. 10 a  and 10 b    illustrate another embodiment of filter  160 . In this embodiment an insulator member such as a plug  180  is inserted within opening  160 . Insulator member  180  has a double flange configuration, such that when inserted into the opening, the upper flange engages against the upper surface of cover  166  and the lower flange engages against the bottom surface of the cover. In accordance with one embodiment, the inside wall portion of the plug is threaded so as to allow tuning element  80  to move along the inside surface of the plug until the desired frequency response is achieved. 
         [0042]      FIGS. 11 a  through 11 d    show another embodiment where the resonator post  190  is hollow, allowing the tuning element  80  to engage with the cavity resonator from its bottom side.  FIGS. 11 c  and 11 d    illustrate a tuning element  80  having the same construction as the one depicted in  FIGS. 9 c    and  9   d.    
         [0043]      FIGS. 12 a  and 12 b    show yet another embodiment where the resonator post is also hollow, allowing the tuning element  80  to engage with the cavity resonator from the bottom side. As illustrated in  FIG. 12 b   , insulating support member  30  is inserted within the resonator post, and screw  80  is inserted within the insulating support member. A cover  192  is disposed on the hollow cavity. Turning element  80  can be accessed and adjusted from the bottom side of the cavity filter. 
         [0044]    The intermodulation distortion effect is substantially eliminated by using the various embodiments of the present invention as described above. For example,  FIG. 13  illustrates the effects of intermodulation distortion, with and without the insulating support arrangement of the present invention. As illustrated, graph  210  represents the distortion level without the insulating support employed in accordance with the present invention, and line graph  220  represents the distortion level with the insulating support employed in accordance with the present invention. Line  230  illustrates an exemplary acceptable frequency response. While all points on graph  210  are above line  230 , all points on graph  220  are within the acceptable limits. 
         [0045]    As such, in accordance with various embodiments of the present invention, an arrangement for insulating the tuning element of a coaxial cavity resonator from the remaining portions of the structure provides a substantial reduction in intermodulation distortion. 
         [0046]    While only certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes or equivalents will now occur to those skilled in the art. It is therefore, to be understood that this application is intended to cover all such modifications and changes that fall within the true spirit of the invention.