Abstract:
A cavity filter which provides for fine tuning of the bandwidth of the filter. The filter provides for both capacitive cross-coupling and inductive coupling between physically adjacent but electrically non-adjacent resonators in the filter. The isolation of the filter can be fine tuned by adjusting the inductive coupling between these resonators, which has the effect of attenuating the cross-coupling effect between these resonators.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to microwave frequency filters. More specifically, this invention relates to a microwave frequency cavity filter whose bandwidth can be precisely fine-tuned with a minimum of effort, expense, and service interruptions.  
           [0003]    2. Discussion of the Related Art  
           [0004]    The rapid growth in cellular telephony and wireless communications has created enormous demand for bandwidth in the microwave radio frequency spectrum. As wireless technologies that depend on the microwave spectrum have become more popular, the microwave portion of the radio spectrum has become more crowded. Unused microwave frequencies are occupied by wireless service providers as soon as they become available, forcing wireless communication firms operating in the same location to provide their services on adjacent frequencies, without the benefit of any “empty” bandwidth between them. Because of this congestion, wireless providers need a way to isolate the transmission and reception of their frequencies from neighboring frequencies that are used for other services or by other providers.  
           [0005]    To accomplish this frequency isolation, resonator filters have been developed. These filters are built to permit only the frequencies in a certain range to pass through. This frequency range is called the pass band, and the frequencies inside this range are called bandpass frequencies. The frequencies outside of the pass band fall into the stop bands, and are blocked by the filter.  
           [0006]    While a number of resonator filter designs have been developed, one of the most common filters for use in microwave communications is the cavity filter. This type of filter consists of a number of resonators placed inside physically adjacent hollow metal cavities, thereby forming cavity resonators. By inductively coupling two or more adjacent resonators, the bandpass frequencies of these resonators are combined, forming a resonator filter with a bandwidth encompassing a range of frequencies.  
           [0007]    But in order to properly block the undesired frequencies in the stop band of the filter, some physically adjacent resonators in the filter are capacitively cross-coupled, which effectively cancels out certain frequencies in the filter. Capacitive cross-coupling attenuates the slope of the frequency response curve of the filter between the edge of the pass band and the edge of the stop band, allowing the filter to more precisely match the desired pass band without also erroneously passing frequencies outside of the pass band that may be used for other signals or which may be owned by other service providers. In essence, adjusting the capacitive cross-coupling within the filter fine tunes the isolation of the filter.  
           [0008]    In this regard, capacitive cross-coupling and inductive coupling have the opposite effect on the signals passed between adjacent resonators. For this reason, conventional cavity filters do not employ both capacitive cross-coupling and inductive coupling between a given pair of resonators.  
           [0009]    In conventional cavity filters, the inductive coupling between adjacent resonators is accomplished by placing a gap in the wall separating the two cavities. The size of the gap determines the amount of coupling. A common method of providing the capacitive cross-coupling in these conventional filters is to extend a metal bar across the wall separating two electrically non-adjacent resonators. The length of the bar determines the capacitive cross-coupling. In order to precisely select the frequency cutoff of the filter between the pass band and the stop band, the cross-coupling bar must have very precise physical dimensions.  
           [0010]    Furthermore, in order to fine tune the filter for tolerance purposes, the physical length of the bar must be changed, either by means of a fine tuning screw at one end of the bar, or more commonly by replacing the bar with another one of different length.  
           [0011]    But adjustment of the capacitive cross-coupling by either means is cumbersome and impractical. First, conventional cavity filters used for microwave signals are quite large and are made entirely of metal with covers or lids made of lead that cover the resonator cavities as well as the cross-coupling bars. Replacing or adjusting the cross-coupling bar requires physically removing this lead cover, which is difficult and labor intensive.  
           [0012]    Furthermore, manufacturing the cross-coupling bars to the precise physical dimensions and tolerances required in conventional filters makes them expensive, which adds further to the overall cost of the filter.  
           [0013]    Given these problems with conventional filters as well as the increased need for precise tuning of filter bandwidth at low cost, what is needed is a cavity filter that can be manufactured at a reduced cost but whose bandwidth can be very precisely tuned and adjusted with a minimum of effort and without interruption of service.  
         SUMMARY OF THE INVENTION  
         [0014]    The invention is directed to a cavity filter. According to a first aspect of the invention, the resonator comprises a filter housing having at least two cavities separated by a cavity wall; a filter cover for covering said filter housing; and a plurality of resonators respectively disposed in said cavities, wherein at least two of the resonators are coupled to each other by both an inductive coupler and a capacitive cross-coupler.  
           [0015]    Specifically, the capacitive cross-coupler includes a bar that extends from the cavity wall into each of the cavities and the inductive coupler is an opening in the cavity wall between the cavities. The inductive coupler also includes an adjustable fine tuner comprising a screw threaded through either the filter cover or the filter housing, such that the screw extends into the opening in the cavity wall.  
           [0016]    The invention is also directed to a method of fine tuning the slope of the frequency response curve of the cavity filter described above by attenuating the capacitive cross-coupling effect indirectly by adjusting the fine tuner of the inductive coupler. Specifically, the fine tuner is adjusted from the exterior of the filter by turning the screw further into the opening in the cavity wall, thereby increasing the inductance of the inductive coupler, reducing the capacitance between the two resonators. Similarly, by unscrewing the screw, it is retracted from the opening, reducing the inductance of the coupler and increasing the capacitance between the two resonators. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The objects and advantages of the present invention will be made more clear with reference to the following drawings, in which like elements have been given like reference characters. In particular:  
         [0018]    [0018]FIG. 1 is a top view of a cavity filter of the present invention;  
         [0019]    [0019]FIG. 2 is a front view of a cavity wall of the cavity filter of the present invention which includes both capacitive cross-coupler and inductive coupler between the electrically non-adjacent resonators of FIG. 1;  
         [0020]    [0020]FIG. 3 is a sample frequency response curve of a cavity filter of the present invention;  
         [0021]    [0021]FIG. 4 is a front view of another alternate embodiment of the present invention showing the same cavity wall as FIG. 2 but with a different inductive coupler., 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    The preferred embodiment of the invention is described with reference to FIGS. 1 and 2, showing a four-cavity filter  100 . According to a preferred embodiment, the resonator filter  100  includes a filter housing  102  and a filter cover  104 . Provided in the housing  102  are a plurality of resonators  106 ,  108 ,  110 , and  112 . The resonators are inductively coupled in series such that resonator  106  is coupled to resonator  108 , resonator  108  is coupled to resonator  110 , and resonator  10  is coupled to resonator  112 . These resonators are separated from each other by cavity walls  114 ,  116 ,  118 , and  123  that form a cross-shaped arrangement. As shown in FIG. 1, walls  114 ,  116 , and  118  extend only partially to the perimeter walls  120  of the filter housing  102  leaving a gap  122  therebetween. Hence, the walls permit inductive coupling between resonators  106 - 108 ;  108 - 110 ; and  110 - 112 .  
         [0023]    On the other hand, it is preferable that cavity wall  123  extends all the way to the perimeter wall  120 . This cavity wall  123  electrically separates the first resonator  106  in the series from the last resonator  112  in the series. Hence resonators  106  and  112  are not inductively coupled in the way that the other resonators are, and are therefore are not electrically adjacent in the series despite being physically adjacent.  
         [0024]    Because they are physically adjacent, resonators  106  and  112  can be capacitively cross-coupled using the cross-coupling bar  124 . Referring to FIG. 3, the purpose of the cross-coupling bar  124  is to attenuate the slope  126  of the cutoff in the frequency response curve  128  between the pass band  130  and the stop bands  132  in FIG. 3. In order to fine tune this capacitive cross-coupling effect, the invention includes an inductive coupler in cavity wall  123  in the form of a notch  134  provided in cavity wall  123  and an associated fine tuning screw  136 , shown in FIG. 2. The fine tuning screw  136  extends through the filter cover  104  into the notch  134 . The capacitance cross-coupling effect can be changed by turning the screw from the exterior of the filter  100 . More specifically, when the screw is turned so that it extends further into the notch  134  the inductance provided by the notch is raised thereby reducing the effective length of the cross-coupling bar  124  and, attendantly, the capacitive cross-coupling between resonators  106  and  112 .  
         [0025]    Conversely, when the screw is turned in the opposite direction (i.e., to shorten the distance that the fine tuning screw  136  extends into the notch  134 ), the inductance provided by the notch is reduced thereby increasing the effective length of the cross-coupling bar  124 , and, attendantly, the capacitive cross-coupling between resonators  106  and  112 .  
         [0026]    Referring to FIG. 23, the filter cover  104  encloses the resonator cavity. According to the preferred embodiment, the filter cover  104  is made of lead, while the housing  102  is made of iron. Of course, the invention is not limited in this respect. The cross-coupling bar  124  is held in the cavity wall  123  by a collar  138 , made of an electrically insulating material such as plastic. As noted above, the tuning screw  1136  extends through the filter cover  104  into the notch  134 . While notch  134  can be of any height equal to or less than the height of wall  123 , in the preferred embodiment the notch provides only fine adjustment of the capacitive effect of the cross-coupling bar  124 . Therefore, the height of the notch is only between twenty and fifty percent of the height of the wall  123 . Again, however, it should be understood that the invention is not limited to any particular height.  
         [0027]    [0027]FIG. 4 illustrates additional embodiment of the invention. In particular, in the embodiment of FIG. 4, both the bar  124  and the notch  134  are set in the middle of cavity wall  123 . The tuning notch  134  is provided above the bar  124 . This embodiment shows a filter which can be easily changed from one capacitive cross-coupling level to another by easily replacing the bar, but which also retains the ability to fine tune the bar  124  once it is set in place by adjusting the tuning screw  136  that extends through the top of the filter cover  104 . More specifically, with this arrangement, the insulating collar  138  that holds the bar  124  in place can be easily removed by sliding it out through the slot  134 .  
         [0028]    Having described the invention with particular reference to the preferred embodiments, it will be obvious to those skilled in the art to which the invention pertains after understanding the invention, that various modification s and changes may be made therein without departing from the spirit and scope of the invention as defined by the claims appended hereto.