Abstract:
A dual mode ceramic filter has an enclosure with two cavities separated by a wall, and two TM dual-mode resonators, each TM dual-mode resonator positioned in a corresponding cavity. Each TM dual-mode resonator has first and second modes, and a body having a central portion with a plurality of arms extending outwardly from the central portion. The filter also has two input conductive members, each input conductive members positioned in a corresponding cavity. Each input conductive member is disposed proximate a corresponding TM dual-mode resonator for coupling between the input conductive member and the TM dual-mode resonator.

Description:
RELATED APPLICATION INFORMATION 
     The present application claims priority under 35 USC 119(e) of provisional patent application Ser. No. 60/651,182, filed on Feb. 9, 2005, incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to filters for wireless communications systems, and in particular, wireless base station filters. 
     BACKGROUND OF THE INVENTION 
     A wireless telecommunication system typically includes a plurality of base stations connected to a communication network. Each base station includes radio transceivers associated with a transmission tower. A typical base station includes one or more filters for processing IF signals. One such filter is known as a microwave cavity filter which includes resonators formed in cavities in order to provide a desired frequency response when signals are input to the filter. 
     One type of cavity filter design employs dual-mode resonators utilized in the cavity filters, providing desired filter functions while reducing the filter size compared to conventional cavity filters utilizing single mode resonators. However, many existing dual-mode resonators are difficult to manufacture due to the shape of the resonator structure. Other existing resonators that use hybrid modes, are too large and bulky for certain applications. 
     Accordingly, an objective of the present invention is to provide a structure for smaller base station cavity filters which avoids the above-noted problems. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the present invention provides a filter comprising; an enclosure having a cavity; a TM dual-mode resonator in the cavity, the TM dual-mode resonator having first and second modes and comprising a TM dielectric resonator body having a central portion with a plurality of arms extending outwardly from the central portion; and an input conductive member in the cavity. The input conductive member is disposed proximate the TM dual-mode resonator for coupling between the input conductive member and the TM dual-mode resonator. 
     Preferably, the dielectric resonator body comprises a cross shape, and the filter further comprises at least one tuning member that is adjacent one or more of the plurality of arms. The filter preferably further comprises a tuning member that is positioned adjacent or more of the plurality of arms, for tuning a magnetic field in the cavity. Preferably, all tuning is in the same direction. 
     In one example of the filter, at least two of the arms are offset relative to the central portion of the dielectric resonator body. In another example of the filter, the dielectric resonator body comprises an “X” shape, the cavity is essentially rectangular with essentially parallel top and bottom surfaces, and one arm of the dielectric resonator body is transverse relative to said cavity surfaces. Two or more arms of the dielectric resonator body can be transverse relative to said cavity surfaces, wherein a transmission zero is close to the bandbass of the filter. 
     In another example, the cavity can be essentially rectangular with essentially back and front surfaces, wherein the input conductive member is transverse relative to said cavity surfaces. 
     The filter can further comprise a first tuning member positioned proximate the dielectric resonator body in the cavity to preset the coupling, and a second tuning member positioned proximate the dielectric resonator body in the cavity for fine-tuning. The first tuning member comprises a step in a corner of the cavity, and the second tuning member comprises a tuning screw. The tuning members comprise metals covered with dielectric film. 
     In another example, the cavity is essentially rectangular with essentially parallel top and bottom surfaces, and essentially parallel front and back surfaces, and at least one arm of the dielectric resonator body is transverse relative to said cavity top and bottom surfaces, and the input conductive member is transverse relative to said top and bottom cavity surfaces. 
     In another aspect, the present invention provides another filter comprising; an enclosure having two cavities separated by a wall; two TM dual-mode resonators, each TM dual-mode resonator positioned in a corresponding cavity, each TM dual-mode resonator having first and second modes and comprising a TM dielectric resonator body having a central portion with a plurality of arms extending outwardly from the central portion; and two input conductive members, each input conductive members positioned in a corresponding cavity. Each input-conductive member is disposed proximate a corresponding TM dual-mode resonator for coupling between the input conductive member and the TM dual-mode resonator. 
     Preferably, the filter further comprises a tuning member for each cavity, positioned adjacent one or more of the plurality of arms of the dielectric resonator body in the cavity. A mode tuning member can be positioned adjacent two of the plurality of arms. The filter can include a tuning member for each cavity, positioned adjacent or more of the plurality of arms of the dielectric resonator body in the cavity, for tuning a magnetic field in the cavity. At least two of the arms of the dielectric resonator body in each cavity may be offset relative to the central portion of the dielectric resonator body. At least one dielectric resonator body preferably comprises an “X” shape. 
     In another example of the filter, for each cavity, a first tuning member is positioned proximate the dielectric resonator body in the cavity to preset the coupling, and a second tuning member positioned proximate the dielectric resonator body in the cavity for fine-tuning. For each cavity, the first tuning member comprises a step in a corner of the cavity, and the second tuning member comprises a tuning screw. 
     Further, each cavity is essentially rectangular with essentially parallel top and bottom surfaces, and essentially parallel front and back surfaces; at least one arm of the dielectric resonator body in each cavity is transverse relative to said cavity top and bottom surfaces; and the corresponding input conductive member is transverse relative to said top and bottom cavity surfaces. 
     In another aspect, the present invention provides another filter comprising; an enclosure having two cavities separated by a wall; two TM dual-mode resonators, each TM dual-mode resonator positioned in a corresponding cavity, each TM dual-mode resonator having first and second modes and comprising a TM dielectric resonator body having a central portion with a plurality of arms extending outwardly from the central portion; two input conductive members, each input conductive members positioned in a corresponding cavity, wherein each input conductive member is disposed proximate a corresponding TM dual-mode resonator for coupling between the input conductive member and the TM dual-mode resonator; and a cross-coupling member disposed in the two cavities via a opening in the wall for coupling between resonator modes. 
     The cross-coupling member is positioned adjacent the TM dielectric resonator bodies. In one example, the cross-coupling member providing coupling between resonator modes  1  and  4 . Preferably, the cross-coupling member comprises a closed loop which is not connected to cavity surfaces, and can comprise an “8” shape. The cross-coupling member is printed on a double-sided substrate. 
     In another aspect, the present invention provides another filter comprising; an enclosure having a cavity; a TM dual-mode resonator in the cavity, the TM dual-mode resonator having first and second modes and comprising a TM dielectric resonator body having a central portion with a plurality of arms extending outwardly from the central portion, the dielectric resonator body comprising an “X” shape defining a tilt angle γ between two arms of the dielectric resonator body, wherein the tilt angle γ ranges from about 66 degrees to about 83 degrees, such that the smaller the tilt angle γ, the higher the coupling factor K 12 ; and an input conductive member in the cavity, wherein the input conductive member is disposed proximate the TM dual-mode resonator for coupling between the input conductive member and the TM dual-mode resonator. 
     In one version, the dielectric resonator body is oriented in the cavity by an orientation angle relative to a centred point in the cavity whereby said arms of the dielectric resonator body are rotated around the centred point such that the arms are kept as far away as possible from the cavity surfaces to increase Q-value. A first arm of the dielectric resonator body is oriented in the cavity by an orientation angle φ relative to the input conductive member; and a second arm of the dielectric resonator body, adjacent to the first arm, is oriented in the cavity by an orientation angle θ relative to the input conductive member, such that the orientation angles θ and φ sets the total angle γ between the first and second arms. 
     The coupling Qe 2  between the second arm and the input conductive member depends on the angle θ, wherein the coupling Qe 2  is dependent on the angle θ. The coupling Qe 1  between the first arm and the input conductive member depends on the angle φ, wherein the coupling Qe 1  is dependent on the angle φ. The orientation angle θ can range from about 59 degrees to about 67 degrees. The orientation angle φ ranges from about 0 degrees to about 14 degrees. 
     In another version, the dielectric resonator body is oriented in the cavity by an orientation angle relative to the input conductive member, whereby said arms of the dielectric resonator body are rotated in the cavity such that the arms are kept as far away as possible from the cavity surfaces to increase Q-value. 
     The input conductive member can have a tilt angle relative to the cavity surfaces, such that orientation angles of the input conductive member relative to the arms of the dielectric resonator body are functions of the tilt angle of the input conductive member. Changing the tilt angle affects said orientation angles, resulting in changes in coupling between the arms and the input conductive member. 
     These, and other embodiments, features and advantages of the present invention will be apparent from the following specification taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a partially broken-away perspective view of a filter including two cavities, each cavity housing a transverse magnetic (TM) dual mode resonator, according to an embodiment of the present invention. 
         FIG. 1B  shows a perspective view of the interior filter of  FIG. 1A , which includes four resonators forming the two TM dual mode resonators. 
         FIG. 1C  shows details of a input pin spacing relative to a TM dual mode resonator in a cavity of the filter of  FIG. 1A , according to an embodiment of the present invention. 
         FIG. 2  shows a perspective view of interior of a cavity in another filter including a TM dual mode resonator with offset arms, according to an embodiment of the present invention. 
         FIG. 3A  shows a side view of the interior of a cavity in another filter including a TM dual mode resonator with tilted arms, according to an embodiment of the present invention. 
         FIG. 3B  is a graph showing an example frequency response of the filter of  FIG. 3A . 
         FIG. 4A  shows a side view of the interior of a cavity in another filter a including TM dual mode resonator, cross tilted up, according to an embodiment of the present invention. 
         FIG. 4B  is a graph showing an example frequency response of the filter of  FIG. 4A . 
         FIG. 5A  shows another filter including a TM dual mode resonator, cross tilted down, according to an embodiment of the present invention.  FIG. 5B  is a graph showing an example frequency response of the filter of  FIG. 5A . 
         FIG. 6  is a diagram shown an example coupling transmission zero for a filter including TM dual mode resonators, according to an embodiment of the present invention. 
         FIG. 7A  shows another filter including a TM dual mode resonator with tilted input, according to an embodiment of the present invention. 
         FIG. 7B  is a graph showing an example frequency response of the filter of  FIG. 7A . 
         FIG. 8  shows a perspective view of a filter including two cavities, each cavity housing a TM dual mode resonator with tilted input, according to an embodiment of the present invention. 
         FIG. 9  is a diagram shown an example coupling transmission zero for a filter including TM dual mode resonators, according to an embodiment of the present invention. 
         FIG. 10A  is a perspective view of a filter including two cavities, each cavity housing a TM dual mode resonator, with loop coupling between resonator modes  1  and  4 , according to an embodiment of the present invention. 
         FIG. 10B  is a graph showing an example frequency response of the filter of  FIG. 10A . 
         FIG. 11A  is a detail perspective view of the cross coupling in the filter of  FIG. 10A . 
         FIG. 11B  is a detail side view of the cross coupling in the filter of  FIG. 10A . 
         FIG. 12A  shows an example magnetic field diagram for tuning frequency, influenced by a metal along the side of a filter cavity which including TM dual mode resonators, according to an embodiment of the present invention. 
         FIG. 12B  shows an example magnetic field diagram for tuning frequency, influenced by a metal along a corner of a filter cavity which including TM dual mode resonators, according to an embodiment of the present invention. 
         FIG. 13  shows perspective view of a filter including two cavities, each cavity housing a TM dual mode resonator, and metals in the cavities for tuning frequency, according to an embodiment of the present invention. 
         FIG. 14  shows perspective view of a filter including two cavities, each cavity housing a TM dual mode resonator, and metals in the cavities for tuning couplings, according to an embodiment of the present invention. 
         FIG. 15A  shows example diagram of frequency tuning based on dimensions of a ceramic filter with a TM dual mode resonator having tilted arms, according to an embodiment of the present invention. 
         FIG. 15B  shows example diagram of frequency tuning based on dimensions of a ceramic filter having a TM dual mode resonator with tilted cross, according to an embodiment of the present invention. 
         FIG. 16A  is an example diagram showing effect of tilt angle between resonators forming a TM dual mode resonator, in a filter cavity, according to an embodiment of the present invention. 
         FIG. 16B  is an example diagram showing effect of angles between resonators forming a TM dual mode resonator and orientation of input coupling pin, in a filter cavity, according to an embodiment of the present invention. 
         FIG. 17  is an example diagram showing effect of angles between resonators forming a TM dual mode resonator, and tilt of input coupling pin, in a filter cavity, according to an embodiment of the present invention. 
         FIG. 18  is a graph showing an example frequency response of the filter of  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a structure for smaller base station filters. Specific embodiments and various features and aspects of the invention are described below. 
       FIG. 1A  is a partially broken-away perspective view of a filter  10  having a rectangular-shaped metal case  12 , according to an embodiment of the present invention.  FIG. 1B  shows a perspective view of the interior of the filter of  10   FIG. 1A , providing two cavities (i.e.,  14 A,  14 B) separated by a wall  15 , wherein each cavity houses a transverse magnetic (TM) dual mode resonator. A first TM dual mode resonator  16  is formed by resonator members  16 A,  16 B crossing each other at a mid-point to form a “cross” or “X” in cavity  14 A. A second TM dual mode resonator  18  is formed by resonator members  19 A,  19 B crossing each other at a mid-point to form as a “cross” or “X” in cavity  14 B. The filter case  12  further houses input pins (i.e.,  18 A,  18 B) coupled to coaxial connectors (i.e.,  20 A,  20 B). In this structure, there is coupling between the input pin  18 A and the resonators  16 A,  16 B. Similarly, there is coupling between the input pin  18 B and the resonators  19 A,  19 B. The resonators comprise low loss dielectric material such as e.g. ceramics. Other materials can also be used. 
     The example filter  10  operates in the frequency range 1920-1980 MHz with four resonators (two cavities). Further, Table 1 below provides additional specifics: 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 K12 = 0.0333 
                 f = 65.0 MHz 
               
               
                   
                 K23 = 0.0245 
                 f = 47.7 MHz 
               
               
                   
                 K12 = 0.0333 
                 f = 65.0 MHz 
               
               
                   
                   
               
             
          
         
       
     
     In Table 1, f represents frequency, and K 12 , K 23  represent resonance modes coupling coefficients for different frequencies. In addition, the coupling Qe from input pin to a resonator mode is about 23 at f=84.4 MHz. 
     In this example, the filter structure has a height and width of about 26 mm, represented by simulated performance data discussed further below. Smaller dimensions may also be provided, for example the size 22 mm may be preferred. All tuning is preferably from the same direction. 
       FIG. 1C  shows details of input pin  18 A spacing relative to a TM dual mode resonator formed by resonators  16 A,  16 B in the cavity  14 A of the filter  10  of  FIG. 1B . In this example, the input pin  18 A is a 5 mm input metal pin, and coupling to the input pin  18 A depends on the Gap distance between the ceramic resonators  16 A,  16 B and the input pin  18 A, as shown by example in Table 2 below. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Gap 
                 fA 
                 Qe 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 4 
                 1914 
                 21 
               
               
                 4.5 
                 1904 
                 24 
               
               
                 5 
                 1893 
                 27 
               
               
                 5.5 
                 1883 
                 31 
               
               
                   
               
             
          
         
       
     
     One approach is to use a screw or a step in the corner. However, this embodiment uses a step  26  to preset the coupling and a screw  28  for fine-tuning. 
       FIG. 2  shows a perspective view of interior of cavity  20  in another filter including a TM dual mode resonator formed by resonators  22  and  24 , wherein the resonator  22  has offset arms  22 A and  22 B, according to an embodiment of the present invention. The arms  22 A and  22 B are offset to contribute to the coupling between the two resonators by rotating the field to less orthogonality. In that case there is less metal in the cavity, which means better Q-value. This embodiment uses a step  26  to preset the coupling and a screw  28  for fine-tuning. 
       FIG. 3A  shows a side view of the interior of a cavity  31  in another filter  30  including a TM dual mode resonator formed by resonators  32 A,  32 B wherein the resonators  32 A,  32 B are tilted with respect to one another to form an “X”, according to an embodiment of the present invention. The input pin  34  is also shown in  FIG. 3A . With tilted resonators (arms) there is harder coupling between the pin  34  and resonator modes. As described further below, the coupling depends on the tilt angle between the two resonators  32 A,  32 B. A square step in the lower corner is no longer needed.  FIG. 3B  is a graph showing an example frequency response of the filter of  FIG. 3A . 
       FIG. 4A  shows a side view of the interior of a cavity  41  in another filter  40  including a TM dual mode resonator formed by resonators  42 A,  42 B wherein the resonators  42 A,  42 B form a “cross” or “X” that is tilted up in the cavity  41 , according to another embodiment of the present invention. The input pin  44  is also shown in  FIG. 4A .  FIG. 4B  is a graph showing an example frequency response of the filter of  FIG. 4A . 
     Referring back to  FIG. 4A , the entire TM dual mode resonator “cross” or “X” is turned (tilted) slightly, so that the input can couple to the second mode as well. This additional coupling creates a zero in transmission on the higher side of the spectrum, wherein as schematically shown by example in  FIG. 6 :
         Coupling k 02 →Transmission zero at high side   Harder coupling→Zero at lower frequency       

       FIG. 9  is a diagram shown an example coupling transmission zero for a filter including TM dual mode resonators, according to an embodiment of the present invention, wherein:
         Coupling a quadruple   with the right phase shift→Two zeroes       
       FIG. 5A  shows a side view of the interior of a cavity  51  in another filter  50  including a TM dual mode resonator formed by resonators  52 A,  52 B wherein the resonators  52 A,  52 B form a “cross” or “X” that is tilted down in the cavity  51 , according to an embodiment of the present invention. The input pin  54  is also shown in  FIG. 5A . The entire TM dual mode resonator cross (or X) is turned (tilted) slightly, so that the input can couple to the second mode as well. As in  FIG. 4A , the tilted cross (or X) results in a better attenuation on the higher side of the spectrum.  FIG. 5B  is a graph showing an example frequency response of the filter of  FIG. 5A . 
       FIG. 7A  shows a side view of the interior of a cavity  71  in another filter  70  including a TM dual mode resonator formed by resonators  72 A,  72 B which form a “cross” or “X” that is tilted down in the cavity  71 , according to an embodiment of the present invention. The input pin  74  is also shown in  FIG. 7A . The entire TM dual mode resonator cross (or X) is turned (tilted) slightly, and the input pin  74  is tilted relative to the resonators  72 A,  72 B. With a tilted input pin  74 , the coupling to mode  2  can be harder. In this way the transmission zero can be placed in the filter skirt very close to the passband.  FIG. 7B  is a graph showing an example frequency response of the filter of  FIG. 7A . In  FIG. 7B , the notch placed on the high side is very wide, and deep, with −60 dB as close as 2100 MHz. 
       FIG. 8  shows a perspective view of a filter  80  including a case  82  that forms two cavities  84 A,  84 B, which is a dual cavity implementation the example in  FIG. 7A , according to an embodiment of the present invention. In  FIG. 8 , the cavity  84 A houses the tilted input pin  88 A and a first TM dual mode resonator  85  formed by resonators  85 A,  85 B as a “cross” or “X”. The cavity  84 B houses the tilted input pin  88 B and a second TM dual mode resonator  87  formed by resonators  87 A,  87 B as a “cross” or “X”. 
       FIG. 10A  is a perspective view of a filter  100  having a case  102  that provides two cavities  104 A and  104 B, according to an embodiment of the present invention. The cavity  104 A houses input pin  108 A and a TM dual mode resonator formed by resonators  106 A,  106 B as a “cross” or “X”. The cavity  104 B houses input pin  108 B and a TM dual mode resonator formed by resonators  109 A,  109 B as a “cross” or “X”. Further, there is a loop  110  passing through an opening  17  in wall  15  between the two cavities  104 A and  104 B, providing coupling between resonator modes  1  and  4 . 
     Coupling is accomplished with a closed loop coupling  110 , which need not be connected to the cavity walls. The loop  110  is twisted in the form of a laying figure “8” for proper phase of the coupling. The loop  110  can for example be printed on a double-sided substrate card (e.g., Teflon substrate). Loops with different widths provide different position of the transmission zeroes.  FIG. 10B  is a graph showing an example frequency response of the filter of  FIG. 10A . 
     Coupling with a quadruple can make double transmission zeroes very close to the passband.  FIG. 11A  is a detail perspective view of the cross-coupling  110  in the filter of  FIG. 10A .  FIG. 11B  is a detail side view of the cross-coupling  110  in the filter of  FIG. 10A . Fine tuning can be performed with a screw  126  that blocks the loop  110 . 
       FIG. 12A  shows a top view of an example magnetic field  121  for tuning frequency, influenced by a metal  120  along the side of a filter cavity  122  which houses a TM dual mode resonator  124 , according to an embodiment of the present invention.  FIG. 12B  shows the magnetic field  121 , influenced by a metal  120  along a corner of the filter cavity  122 . The magnetic fields is influenced by a metal along the side, wherein the frequency is changed as a result. In the corner there is less influence i.e. less changes. A screw into the cavity will influence the field in the same way. Deeper penetration influences more of the field. 
       FIG. 13  shows perspective view of a filter  130  with a casing  127 , implementing two a cavity ( 132 A,  132 B) version of the examples in  FIGS. 12A and 12B . The cavity  132 A houses input pin  138 A and a TM dual mode resonator formed by resonators  136 A,  136 B as a “cross” or “X”. The cavity  132 B houses input pin  138 B and a TM dual mode resonator formed by resonators  139 A,  139 B as a “cross” or “X”. 
     To tune electric fields there have to be holes in the ceramic resonators  136 ,  139  in two orthogonal directions. This embodiment instead tunes the magnetic fields  121  ( FIGS. 12A-B ) close to the cavity walls  127 . In each cavity, resonance modes  1  and  4  are easily tuned with a screw (e.g.,  137 A or  137 B;  135 A or  135 B) from the top. Modes  2  and  3  are tuned with a metal bar  120  that is moved from the bottom and up. The bar can be moved with a screw from the top, placed in the corner. To prevent the moving bars from generating PIM the moving parts can be covered with a thin dielectric film. 
       FIG. 14  shows perspective view of a filter  140  with a case  142  providing two cavities  144 A and  144 B, according to an embodiment of the present invention. The cavity  144 A houses input pin  148 A and a TM dual mode resonator formed by resonators  146 A,  146 B as a “cross” or “X”. The cavity  144 B houses input pin  148 B and a TM dual mode resonator formed by resonators  149 A ,  149 B as a “cross” or “X”. Metals in the cavities  144 A and  144 B are for tuning couplings. Coupling between modes  1 - 2  and  3 - 4  can be done with screws  147 A from the top. Coupling of the modes  3 - 4  is done in the aperture opening  143  in the separating wall. Even this coupling can be done with a screw  147 B from the top, placed in the opening  143 . In the tilt up case ( FIG. 4A ) there may be a 4 mm long screw  43  with diameter of 6 mm, or a 10 mm long screw with 5 mm diameter, to get a shift of 10 MHz in k 23 . 
       FIG. 15A  shows example diagram of frequency tuning based on dimensions of a ceramic filter including a TM dual mode resonator  150  formed by resonators  150 A,  150 B as a “cross” or “X” having tilted arms, according to an embodiment of the present invention.  FIG. 15B  shows example diagram of frequency tuning based on dimensions of a ceramic filter having a TM dual mode resonator  152  formed by resonators  152 A,  152 B with tilted “cross” or “X”, according to an embodiment of the present invention. 
     Referring to the example in Table 3 below, the frequencies and the coupling between the modes are dependent on the dimensions of the ceramic resonators ( 150 A,  150 B in  FIG. 15A , and  152 A,  152 B in  FIG. 15B ). 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Dimensions 
                 Primary 
                 Secondary 
               
               
                   
                   
               
             
             
               
                   
                 M1A 
                 f1 
                 K12 
               
               
                   
                 M1B 
                 K12 
                 f1 
               
               
                   
                 M2A 
                 f2 
                 f1 
               
               
                   
                 M2B 
                 K12 
                 f2 
               
               
                   
                 Gap 
                 Qe 
                 f1 
               
               
                   
                 Wall 
                 K23 
                 f2 
               
               
                   
                   
               
             
          
         
       
     
     As is known by those skilled in the art, in Table 3 the terms f 1 , f 2 , etc., represent resonance frequencies of first, second, etc., resonance modes, and the terms K 0 , K 1 , K 12 , K 23 , etc., represent resonance modes coupling coefficients. 
     In Table 3 “Gap” is the distance between the ceramic resonators and the metal pins of the input, and “Wall” is the width of the separating wall  15  between the cavities. The dimensions M 1 A, M 1 B, M 2 A, M 2 B, are shown in  FIGS. 15A-B . The frequencies have the strongest dependence, wherein a change of 0.1 mm in dimensions can result in 10 MHz offset. 
     The filter may be first tuned with these dimensions to obtain a design centering. Then, when the filter is produced simpler tuning with the tuning screws can be performed. 
     The finished filter has only one small secondary effect in the tuning screws. Tuning of f 1  will make a shift in K 12  a few MHz. Table 4 below shows the difference in MHz when the screw for f 1  changes 12 mm, and the bar for f 2  changed 8 mm. K 12  is changed with a 3 mm screw on the right and on the left side of the resonators. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Tilted arms 
                 Tilt up 
                 Tilt down 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f1 
                 24 
                 20 
                 20 
               
               
                   
                 f2 
                 49 
                 33 
                 26 
               
               
                   
                 K12 
                 21 
                 15 
                 16 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 16A  is an example diagram showing effect of tilt angle γ between resonators  160 A,  160 B forming a TM dual mode resonator  160  as a “cross” or “X”, in a filter cavity, according to an embodiment of the present invention. As shown by example in Table 5 below, the angle γ between the two resonators  160 A,  160 B sets the coupling factor (coefficient) K 12  between the two modes. With orthogonal fields (γ=90°) there will be no coupling. With angle γ&lt;90° there will be a certain coupling. The more the two resonators  160 A,  160 B are aligned, the higher the coupling. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Angle γ 
                 K12 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 66.3 
                 0.0806 
               
               
                   
                 70.1 
                 0.0598 
               
               
                   
                 74.1 
                 0.0416 
               
               
                   
                 78.3 
                 0.0265 
               
               
                   
                 82.5 
                 0.0215 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 16B  is an example diagram showing effect of angles between resonators  162 A,  162 B forming a TM dual mode resonator  162  as a “cross” or “X”, and orientation of input coupling pin  164 , in a filter cavity, according to an embodiment of the present invention. When the arms, or the whole “cross” or “X” are tilted by an orientation angle φ, the resonators  162 A,  162 B are rotated around a centred point in the cavity. In this way the resonators  162 A,  162 B are kept as far away as possible from the walls of the cavity, and the Q-value will be high. Tables 6-7 below show effect of angle θ, and the orientation angle φ of the coupling pin  164  relative to the resonators  162 A,  162 B. Angle θ affects the coupling since it, together with angle φ, sets the total angle γ between resonators  162 A and  162 B. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Angle θ 
                 K12 
                 Qe2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 59.4 
                 0.0683 
                 224 
               
               
                 62.7 
                 0.0438 
                 282 
               
               
                 66.3 
                 0.0267 
                 387 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 Angle φ 
                 Qe1 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 25.7 
               
               
                   
                 4.4 
                 25.1 
               
               
                   
                 8.7 
                 26.0 
               
               
                   
                 13.0 
                 26.9 
               
               
                   
                   
               
             
          
         
       
     
     Qe 1  is the coupling from input pin  164  to resonator mode  1 , and Qe 2  is the coupling between input pin  164  and resonator mode  2 . The Qe 2  coupling depends on the angle θ. A smaller angle θ results in harder coupling from the input to mode  2 , which results in a transmission zero at the higher side of the spectrum. 
     The total angle γ=θ+φ is chosen for the proper K 12 , and the tilt angle φ of the “cross” or “X” is chosen to obtain the proper Qe 2 . Even Qe 1  is affected by the angle φ. However, this small change in Qe 1  can be corrected with the distance between input pin and the resonator cross (or X). The value of Qe 1  at φ=θ is a result of the metal screws in the cavity. 
       FIG. 17  is an example diagram showing effect of angles between resonators  170 A,  170 B forming a TM dual mode resonator  170  as a “cross” or “X”, and tilt of input coupling pin  172 , in a filter cavity, according to an embodiment of the present invention. Compared to  FIG. 16B , by further tilting the input pin  172 , the angle θ is decreased and the angle φ is increased. Angle φ at 26.7 degrees results in smaller coupling to mode  1  (higher Qe 1 ). This was compensated, by moving the input pin closer to the “cross” or “X” formed by the resonators  170 A,  170 B. For angle θ=51.7 results in a coupling to mode  2  (Qe 2 =177) so high that the transmission zero is very close to the filters pass band.  FIG. 18  is a graph showing an example frequency response of the filter of  FIG. 17 . 
     Simulations were performed in hfss. In one simulation, the filter structure included tuning screws and a coupling screw of 4 mm diameter with the length of 1 mm. The coupling screw is placed in the upper left corner of the cavity. All mechanical parts in the cavity will influence the fields and have to be included when performing design centring. The coupling can be set over a range wide enough to be used for base station filters. 
     The present invention has been described in considerable detail with reference to certain preferred versions thereof; however, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.