Abstract:
A high-frequency, e.g., microwave, filter ( 100, 300, 400 ) is made, e.g., stamped or etched, from a single sheet ( 110, 310, 410 ) of electrically conductive material, e.g., a metal plate or a printed circuit board. The sheet defines a frame ( 112, 312, 412-413 ), one or more resonant filter elements ( 114, 311-315, 411-415 ) inside of the frame, one or more supports ( 116, 316-317, 416 ) connecting each resonant filter element to the frame, and a flange ( 118, 318, 418 ) on one of the resonant filter elements. The flange serves as an electrical contact to the filter; another flange ( 317, 417 ) on another element, or the frame itself, serves as a second contact. An electrically conductive housing ( 104, 304, 404 ) encapsulates both faces of the sheet.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application is a continuation-in-part of application of R. Barnett et al., entitled “Sheet-Metal Filter”, U.S. application Ser. No. 09/521,556, filed on Mar. 9, 2000, now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates to high-frequency, e.g., microwave, filters. 
     BACKGROUND OF THE INVENTION 
     The recent proliferation of, and resulting stiff competition among, wireless communications products have put price/performance demands on filter components that conventional technologies find difficult to deliver. This is primarily due to expensive manufacturing operations such as milling, hand-soldering, hand-tuning, and complex assembly. 
     SUMMARY OF THE INVENTION 
     This invention is directed to solving this and other problems and disadvantages of the prior art. According to the invention, a filter is made from a single sheet of electrically conductive material, e.g., metal, preferably by stamping. The sheet is preferably all metal, e.g., a metal plate or a stacked assembly of metal sheets, but it may also be a metal-laminated non-conductive substrate, e.g., a printed-circuit board. In the latter case, the filter may advantageously be made by etching. An electromagnetically conductive housing preferably encapsulates at least both faces of the sheet. The sheet of conductive material defines a frame, one or more resonator filter elements inside of the frame, and one or more supports attaching the resonators to the frame. At least one contact connected to the resonator filter element provides an electromagnetic contact thereto. Preferably, the contact is a flange on at least one of the resonators, also defined by the sheet of conductive material. Another flange or the frame itself serves as another contact to the filter. Illustratively, the flanged resonator is rectangular and the flange and the supports extend from a side of the rectangle, whereby the distance between the flange and an end of the rectangular resonator that lies on the same side of the supports as the flange primarily determines the input characteristics of the filter. The resonant frequency of the filter element is primarily determined by the length of the element (λ/2). Other factors, such as the width, the thickness, the tap point (L), and the resonators proximity to other metal also determine the resonant frequency. 
     Major benefits of the invention include low manufacturing costs, narrow (illustratively about 1%) bandwidth filters requiring no tuning, and high Q, relative to conventional technology. These and other features and advantages of the invention will become more evident from the following description of an illustrative embodiment of the invention considered with the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of a filter that includes a first illustrative embodiment of the invention; 
     FIG. 2 shows illustrative dimensions of the resonant element of the filter of FIG. 1; 
     FIG. 3 is a graph of first operational characteristics of the resonant element of FIG. 2; 
     FIG. 4 is a graph of second operational characteristics of the resonant element of FIG. 2; 
     FIG. 5 is a perspective view of a filter that includes a second illustrative embodiment of the invention; 
     FIG. 6 is a perspective view of a filter that includes a third illustrative embodiment of the invention; and 
     FIG. 7 is a perspective view of a filter that includes a fourth illustrative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a first bandpass filter  100 , which comprises an electrically conductive (e.g., metallic) filter layer  110  positioned inside a cavity formed by an electrically conductive housing  104 . The cavity is dimensioned to exhibit a waveguide cutoff frequency below the frequencies at which filter  100  is being used. Filter layer  110  is a single sheet of electrically conductive material, such as a sheet of aluminum, steel, kovar, copper, or molybdenum. All these metals should be plated with copper, gold, or silver to enhance their conductivity and corrosion resistance. Filter layer  110  also may be a metal-coated (laminated) insulating substrate, such as a printed-circuit board or plastic or ceramic. In the latter case, the printed-circuit may be metal-coated on both sides, with one of the sides forming a part of housing  104 . In the case of being a single sheet of metal, filter layer  110  is easily manufactured by stamping or etching. In the case of being a laminate, filter layer  110  is easily manufactured by etching or plating, including edge plating. Cutting or other manufacturing methods may also be used. Filter layer  110  need not be planar. Outer portions thereof may be bent substantially perpendicularly to the rest to form a part of the walls of housing  104 , or else are part of the interconnections between other filter layers or circuitry. Filter layer  110  comprises a frame  112 , a resonator (resonant filter element)  114  inside of frame  112 , supports  116  connecting resonator  114  to frame  112 , and a coupler; a second, ground, contact is formed by frame  112  and supports  116 . The coupler is shown in FIG. 1 as a contact flange  118  located at the  50  Ω tap point and extending from resonator  114 , and acts as an inductive coupler. The coupler can also be an out-of-side coupler, or a capacitive coupler, or any other desired coupler. Flange  118  forms a tap point between supports  116  and edges  122  of resonator  114 , so the closer flange  118  is to edge  122 , the more energy it couples in at a higher frequency. The inductive coupler formed by flange  118  may extend from resonator  114  in the plane of filter element  110 ′ through a gap  270  in frame  112 , as shown in FIG.  5 . This planar filter is enclosed in a closure formed by an electrically conductive housing  104 , which behaves as a waveguide with a cut-off frequency lower than the second harmonic frequency of the filter center frequency. This planar configuration comprising filter element  110 ′ as an input/output possesses up-down symmetry and nulls the coupling between the filter elements and the waveguide. Therefore it achieves automatic suppression of the waveguide modes which would otherwise be excited. As a consequence, the cut-off frequency of filter  100  is pushed up high, and the filter achieves very good suppression of second harmonics. However, flange  118  may be bent away from the plane of filter layer  110 , as shown in FIG. 1, to extend outside of housing  104  through an opening  120  therein to form a connectorless coupling to, e.g., an antenna. The bent-up flange  118  destroys the up-down symmetry of filter layer  110 ′ and hence destroys the suppression of the waveguide modes. In order to regain the high suppression of the waveguide modes at the second harmonic position, the bent-up flange  118  must be positioned at an integer multiple of waveguide half-wavelength of the second harmonic frequency of the filter&#39;s center frequency from the inside edge of frame  112 . It renders the flange  118  in a null of the electromagnetic fields of the waveguide modes at the second harmonic frequency. Preferably, both frame  112  and resonator  114  are rectangular in shape. 
     For a bandpass half-wavelength filter, the important parameters are the loaded Q of the end resonators (which forms the input/output coupling to the filter) the center frequency of each resonator, and the interresonator coupling coefficients. They can be calculated for the specific type of filter that is desired. Electromagnetic (EM) simulations are used to relate these parameters to the specific structures and physical dimensions of the resonators for realization of the filter, because it is usually very difficult if not impossible to solve the problems analytically due to the complexity of the studied structures. The dimensions of an illustrative endcoupling resonator  114  are shown in FIG.  2 . The dimension “L” between the edge of flange  118  that is closest to support  116  and an end  122  of resonator  114  that lies on the same side of support  116  as flange  118  is critical in that it is determinative of the input/output characteristics—the loaded Q and the center frequency f 0  of filter  100  and the loaded Q of the input and output resonators. It also de-tunes the center frequency f 0  of the input and output resonators from their natural, unloaded, half-wavelength resonance. The relationship of the loaded Q and center frequency ƒ o  to the parameter L is determined by simulations, whose results are shown in FIG. 3 as curves  210  and  220 . Simulations provide an invaluable means to study and optimize the overall structures through exploration of an enormous design space, which might be otherwise impossible. However, due to inaccuracy in EM modeling, several prototypes with dimensions close to those selected by simulations were built and measured to map out the exact dependence experimentally for fine adjustment to achieve a no-tuning design. Their results are also shown in FIG. 3 as curves  230  and  24 . It is clear from FIG. 3 that the desired loading Q and the center frequency may not coincide with each other. However, variation of the resonator&#39;s length, such as lengthening or shortening both ends by the same amount, will only affect the center frequency but not the Q. Hence, desired Q and center frequency can be achieved simultaneously. 
     FIG. 6 shows a third filter  300 , which comprises an electrically conductive filter layer  310  mounted inside an electrically conductive housing  304 . Filter layer  310  is also a single sheet of material, and comprises five resonators  311 - 315  to form a five-pole filter. Resonators  311 - 315  are capacitively coupled to each other at their adjacent edges across gap G. Resonators  311 - 315  are positioned inside a frame  312  and are connected thereto by supports  316  and  317 . Contact flanges  318  and  319  extend from sides  320  of the two outermost resonators  310  and  314 . Filter layer  310  is also easily manufactured by stamping or etching. Flange  318  is bent away from the plane of filter element  310  and extends outside of housing  304  via orifice  322  to form a first contact to filter  300 . Flange  319  extends outside of housing  304  through a gap  330  in frame  312  to form a second contact of filter  300 . Suppression of the low-frequency parasitic mode is achieved by designing the end resonators  311  and  314  properly such that the center frequency of the parasitic mode of the end resonators  311  and  314  are very different from that of the inner resonators  312 ,  313 , and  315 . 
     For the inner resonators, their center frequencies are mainly determined by their lengths, approximately inverse-proportionally. The coupling between the resonators is determined by the gap G between them. Usually the coupling will have a weak effect on the center frequency, which should be taken into consideration. In general, gap G is hard to describe by an analytical mathematical formula; fortunately it is not necessary because the coupling effects can generally be found by measurement. The measured relationship between gap width G and the coupling coefficient K and center frequency ƒ o  for filter  300  that uses the five resonators of FIG. 6 is shown in FIG.  4 . Coincidentally for this filter  300 , because of its specific geometry, the center frequency is independent of the coupling coefficient K. Therefore, the desired center frequency of the resonators can be achieved by adjusting their lengths without regard for the gaps between the resonators. This makes the filter easier to design. 
     With all the relevant dimensions mapped out, a desired frequency response can be achieved at any frequency. In addition to the desired frequency response in the desired bands, a filter will often display some parasitic modes at the undesired places. They can be reduced or eliminated on a case-to-case basis by manipulating the structures in a way that suppresses those undesired modes but not the desired one by properly engineering the width and the shape of tabs  316  so that they do not perturb the desired modes of propagation in the resonant elements. 
     FIG. 7 shows a fourth filter  400 , which also comprises an electromagnetically conductive filter layer  410  mounted inside an electromagnetically conductive housing  404 . This design is particularity suited for implementing a transceiver duplexer. Filter layer  410  defines dual side-by-side five-pole filters. Of course, any desired number of filters may be defined by a single filter layer  410 . The filters may be cascaded for better performance. Or, they may be used for different stages of a transmitter or a receiver. Or, one may be used for the transmitter and the other for the receiver of a wireless device. Filter layer  410  is a single sheet of material and defines two frames  412  and  413  each holding five resonators  424 - 428  that are connected thereto by supports  416 . Of course, each of the filters may have a different number of resonators, of different dimensions, to achieve different filter characteristics. Contact flanges  419  and  418  extend from sides  420  of the two outermost resonators  424  and  428  in each frame  412  and  413  and establish the input/output coupling to filter  400 . Alternately, this coupling can be obtained by coupling capacitively to the same elements  411  and  414 . Filter layer  410  is likewise easily manufactured by stamping or etching. Flanges  418  and  419  are bent away from the plane of filter layer  410  and extend through orifice  422  outside of housing  404  to form a pair of contacts to each of the two filters. 
     Of course, various changes and modifications to the illustrative embodiments described above will be apparent to those skilled in the art. For example, the resonators may be twisted to lie at an angle to the plane of the filter frame, e.g., at 90° thereto. Such changes and modifications can be made without departing from the spirit and the scope of the invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be covered by the following claims except insofar as limited by the prior art.