Abstract:
The present invention relates generally to digital elliptic filters, and more particularly, but not exclusively to multi-layer digital elliptic filters and methods for their fabrication.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Application No. 61/757,102, filed on Jan. 26, 2013, the entire contents of which application are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to digital elliptic filters, and more particularly, but not exclusively to multi-layer digital elliptic filters and methods for their fabrication. 
     BACKGROUND OF THE INVENTION 
     While digital elliptic filters have been designed and fabricated, present manufacturable designs include a number of limitations that can inversely impact performance. For example, current digital elliptic filters may be inherently wideband (greater than 30%) and may not be suited to narrowband design due to physical limitations in the design and manufacture of such filters. In addition, the structure of current digital elliptical filters can present manufacturing challenges, because such filters can require a series of internal stubs that must be machined. Still further, the spacing of ground planes may result in junction effects which are difficult to compensate, especially at X-band (8-12 GHz) frequencies and above. Thus, it would be an advance in the art to provide digital elliptic filters having designs that are more readily manufactured at frequencies at or above X-band, as well as providing methods of their manufacture. 
     SUMMARY OF THE INVENTION 
     In one of its aspects the present invention may provide a multi-layer digital elliptic filter comprising a conductive enclosure having conductive walls defining a cavity therein. First and second conductive posts may be disposed within the cavity of the conductive enclosure, with conductive posts each having a respective first end connected to a selected conductive wall of the conductive enclosure. In addition, the second conductive post may have a post cavity disposed therein. A conductive stub may be disposed within the post cavity and electrically connected to the first conductive post such that the first and second conductive posts, the conductive stub, and the conductive enclosure have inductive and capacitive properties to provide a digital elliptic filter. The conductive stub may be either partially or fully contained within the post cavity. Moreover, the post cavity may include a longitudinal wall extending along a longitudinal axis of the second post, with a notch disposed in the longitudinal wall. A portion of the stub may be disposed within the notch to provide the electrical connection between the stub and the first conductive post. 
     In another of its aspects the present invention may provide a method of forming a multi-layer digital elliptic filter by a sequential build process. The method may include depositing a plurality of layers, where the layers comprise one or more of a conductive material and a sacrificial photoresist material, thereby forming a structure which comprises: a conductive enclosure, the enclosure having conductive walls defining a cavity therein; first and second conductive posts disposed within the cavity of the conductive enclosure, the conductive posts each having a respective first end connected to a selected conductive wall of the conductive enclosure, the second conductive post having a post cavity disposed therein; a conductive stub disposed within the post cavity and electrically connected to the first conductive post, wherein the first and second conductive posts, conductive stub, and conductive enclosure are configured to have inductive and capacitive properties to provide a digital elliptic filter. The method may also include removing the sacrificial photoresist. The method of forming a multi-layer digital elliptic filter may include forming a structure, wherein the conductive stub is partially or fully contained within the post cavity. In addition, the method of forming a multi-layer digital elliptic filter may include forming a structure, wherein the post cavity comprises a longitudinal wall extending along a longitudinal axis of the second post, the wall having a notch disposed therein. A portion of the stub may be disposed within the notch to provide the electrical connection between the stub and the first conductive post. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary and the following detailed description of exemplary embodiments of the present invention may be further understood when read in conjunction with the appended drawings, in which: 
         FIG. 1A  schematically illustrates an isometric view of an exemplary design of a physical realization of a digital elliptic filter in accordance with the present invention having a post structure (solid lines) enclosed within a metal box (dashed lines); 
         FIG. 1B  illustrates a lumped element diagram and high-pass frequency response corresponding to the design of  FIG. 1A ; 
         FIG. 1C  illustrates a lumped element diagram and frequency response of an alternative design having a band-stop frequency response; 
         FIG. 1D  illustrates the performance of the digital elliptic filter of  FIG. 1A , with the solid line showing Insertion Gain in dB (or |S21|) and the dashed line showing return loss in dB (or |S11|); 
         FIG. 2A  schematically illustrates a cross-sectional view of the digital elliptic filter and enclosing metal box of  FIG. 1A  taken along the sectioning line  2 A- 2 A; 
         FIG. 2B  schematically illustrates a cross-sectional view of the digital elliptic filter and enclosing metal box of  FIG. 1A  taken along the sectioning line  2 B- 2 B; 
         FIG. 3A  schematically illustrates the post structure of the digital elliptical filter of  FIG. 1A ; 
         FIG. 3B  schematically illustrates a cross-sectional view of the digital elliptical filter portion of  FIG. 3A  taken along the sectioning lines  3 B- 3 B; 
         FIG. 3C  schematically illustrates an enlarged fragmentary end view of the post structure illustrated in  FIG. 3A ; 
         FIG. 3D  schematically illustrates a cross-sectional view of the digital elliptical filter portion of  FIG. 3A  taken along the sectioning lines  3 D- 3 D; 
         FIG. 4A  schematically illustrates an isometric view of a further exemplary design of a physical realization of a digital elliptic filter in accordance with the present invention having a post structure (solid lines) enclosed within a metal box (dashed lines); 
         FIG. 4B  schematically illustrates a cross-sectional view of the digital elliptic filter of  FIG. 4A  taken along the sectioning line  4 B- 4 B; 
         FIG. 5  illustrates a lumped element diagram corresponding to the design of  FIGS. 4A-4B ; 
         FIG. 6A  schematically illustrates an isometric view of another exemplary design of a physical realization of a digital elliptic filter in accordance with the present invention having a post structure (solid lines) enclosed within a metal box (dashed lines) having connecting arms which project out beyond the ends of the posts of the digital elliptic filter; 
         FIG. 6B  schematically illustrates a cross-sectional view of the digital elliptical filter of  FIG. 6A  taken along the sectioning lines  6 B- 6 B; 
         FIG. 6C  schematically illustrates an enlarged fragmentary end view of the digital elliptical filter illustrated in  FIG. 6A ; 
         FIGS. 7A, 7B  schematically illustrate an isometric and end view, respectively, of yet a further exemplary design of a physical realization of a digital elliptic filter in accordance with the present invention having individual resonators of different height; and 
         FIGS. 8A-8D  schematically illustrate exemplary lumped element diagrams of digital elliptic filters of the present invention used in conjunction with low pass filters. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures, wherein like elements are numbered alike throughout,  FIG. 1A  schematically illustrates an isometric view of an exemplary design of a physical realization of a digital elliptic filter  100  of order n=3 in accordance with the present invention. The filter  100  is a distributed realization of the lumped element circuit having a high pass frequency response as shown in  FIG. 1B ; the insertion gain performance of the corresponding physical realization of the filter  100  is shown in  FIG. 1D . Turning to the specific exemplary physical structure of the filter  100  as illustrated in various views shown in  FIGS. 1A, 2A-3D , the filter  100  may include a post structure comprising first and second posts  110 ,  120  enclosed within and grounded to a hollow (air-filled) metal box  130  having an inner wall  132  and outer wall  131 . In addition, idealized 50 ohm ports  142 ,  144  may be modeled in the design as zero thickness “sheets” to represent where a signal is input/output to/from the filter  100 ,  FIGS. 1A, 2A . In a final physical implementation the idealized ports  142 ,  144  may be replaced with 50 ohm transmission lines, as illustrated and discussed below in connection with ports  642 ,  644  of  FIGS. 6A-6C , for example. 
     The first and second posts  110 ,  120  may have a length (LenRes) that is electrically equivalent to one quarter of a wavelength at which the filter  100  is designed to operate. The first and second posts  110 ,  120  may be configured to create an electrical response equivalent to an inductor to ground (e.g., L 1  and L 3 ,  FIG. 1B ) as well as an inductive coupling between the posts  110 ,  120  (e.g., L 2 ,  FIG. 1B ). The behavior of the first and second posts  110 ,  120  as inductors, and the values of the inductance of the first and second posts  110 ,  120 , may be determined by the specific configuration of the first and second posts  110 ,  120  and the metal box  130  relative to one another. 
     For example, in the exemplary configuration of  FIGS. 1A-3D , the first post  110  may be provided in the form of a rectangular solid, and the second post  120  may be provided in the form of a longitudinal post having a C-shaped cross-section taken perpendicular to the longitudinal axis,  FIG. 3D . In this regard, the second post  120  may include an upper portion  125  and a lower portion  123  joined by a vertical portion  124  defining a cavity  129  therebetween to provide the C-shape. (The C-shape is depicted with the opening to the right; however, the “C” could be reversed so that the opening in the C-shape of the second post  120  is to the left in  FIG. 3D .) An L-shaped stub  128  may be disposed within the cavity  129 , where the L-shape is defined by an arm portion  121  and longitudinal portion  122  of the stub  128 ,  FIGS. 1A, 2B-3D . The length of the longitudinal portion  122  may be foreshortened by an amount delS 2  to account for the length of the arm portion  121 ,  FIG. 3B . In addition, an opening  133  in the box  130  may optionally be provided to prevent electrical connection between the stub  128  and the box  130 . The vertical portion  124  may be foreshortened or notched by providing a notch  126  to permit the stub  128  to be fully enclosed within the second post  120  to deter electrical interaction between the stub  128  and metal box  130 . Specifically, the notch  126  may be configured such that the length of the arm portion  121  is minimized to minimize unwanted parasitic circuit elements, in so doing the range of impedances (and thus capacitances) may be increased. The stub  128  may be electrically connected to the first post  110  at the arm portion  121  of the stub  128 ,  FIG. 3B . In this particular exemplary configuration, the C-shaped second post  120  may create a physical element that provides the electrical equivalent of the series capacitor (C) of the equivalent lumped circuit illustrated in  FIG. 1B . Hence, the particular physical realization of the digital elliptical filter  100  of  FIGS. 1A, 2A-3D  provides the performance illustrated in  FIG. 1D . In addition, alternative designs in accordance with the present invention are contemplated which would provide physical realizations of a band-stop filter as illustrated in  FIG. 1C , which may be accomplished by modifying the configuration of the filter  100  such that the base of the posts  110 ,  120  are open circuited instead of short circuited, and connecting both ends of the stub  128  to the posts  110 ,  120 . 
     The design of the physical realization of the digital elliptical filter  100  may be facilitated through the use of suitable modeling software, such as ANSYS HFSS (ANSYS, Inc., Canonsburg, Pa. USA). In addition, a starting point for use with modeling software may be determined using the methodology disclosed in Horton et. al, The digital elliptic filter—a compact sharp cutoff design for wide bandstop or bandpass requirements, IEEE Transactions On Microwave Theory And Techniques, Vol. MTT-I5, No. 5, May 1967, the entire contents of which are incorporated herein by reference. 
     Design Example 
     A specific exemplary design of a physical realization of the digital elliptic filter  100  was performed using ANSYS HFSS, which design predicted the performance results illustrated in  FIG. 1D . With reference to the dimensioning lines illustrated in  FIGS. 1A, 2A-3D , the dimensions of the design are provided in Tables 1 and 2, where Table 1 includes the predefined values and Table 2 the values calculated by the design process. In the design, the thickness of the metal box  130  was not critical from a microwave design point of view, but was set at 0.25 mm on all sidewalls and 0.15 mm on top and bottom surfaces. The length of the posts  110 ,  120  (LenRes) was calculated to be electrically equal to one quarter of a wavelength at the mid-band frequency of the filter  100 . For the design, where the dielectric was essentially air, the mid band length (LenRes) was calculated by the equation 
               LenRes   =       ⁢       λ   4     =       ⁢       v   p       4   ·     f   0             ,         
where ν p  was the phase velocity of a wave propagating along the transmission line and f 0  was the center frequency of the filter&#39;s passband. For the present design having posts  110 ,  120  for a TEM (transverse electromagnetic) mode wave with an air dielectric, ν p  was equal to the speed of light in a vacuum or 2.998.10 8  m/s. The center frequency of the filter  100  was 25.0 GHz, making LenRes=2.998 mm. However, the length was then adjusted in simulation to correct for non-ideal effects to provide the value listed in Table 2.
 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Value (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 b 
                 0.7 
               
               
                   
                 t 
                 0.5 
               
               
                   
                 Ts 
                 0.1 
               
               
                   
                 Gs 
                 0.1 
               
               
                   
                 s01 
                 0.5 
               
               
                   
                 s23 
                 0.5 
               
               
                   
                 W3 
                 0.1 
               
               
                   
                 LenGap 
                 0.75 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Value (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 w1 
                 0.47 
               
               
                   
                 w2 
                 0.47 
               
               
                   
                 s12 
                 0.06 
               
               
                   
                 wInS2 
                 0.05 
               
               
                   
                 w4 
                 0.09 
               
               
                   
                 LenRes 
                 3.20 
               
               
                   
                 iA12 
                 0.39 
               
               
                   
                 delS2 
                 0.60 
               
               
                   
                 w5 
                 0.09 
               
               
                   
                 wNotch2 
                 0.215 
               
               
                   
                   
               
             
          
         
       
     
     Leaving the design example and turning to other exemplary configurations of the present invention,  FIGS. 4A, 4B  schematically illustrate an isometric and cross-sectional views, respectively, of a further exemplary design of a physical realization of a digital elliptic filter  400  where n is extended beyond 3. In particular, the digital elliptic filter  400  represents a specific example where n=7. For odd values of n, extending the digital elliptic filter  400  to include additional elements (of the unit type containing L 9 /L 8  and C 4 ) may be accomplished by adding additional circuit elements as shown in  FIG. 5 , which physically corresponds to adding additional posts. Thus, the n=7 digital elliptic filter  400  includes four posts  410 ,  420 ,  430 ,  440  with three interposed stubs  418 ,  428 ,  438 , where the posts  410 - 440  and stubs  418 - 438  may be configured and oriented relative to one another in a manner similar to that of the posts  110 ,  120  and stub  128  of the digital elliptic filter  100 . The stubs  418 ,  428 ,  438  may be fully or partially enclosed in corresponding posts  420 ,  430 ,  440 , respectively. 
     In yet another exemplary design of a physical realization of a digital elliptic filter in accordance with the present invention,  FIGS. 6A-6C  schematically illustrate isometric and cross-sectional views, respectively, of a digital elliptic filter  600 . The digital elliptic filter  600  may be similar to the digital elliptic filter  400  by containing four posts  610 ,  620 ,  630 ,  640  and three stubs  618 ,  628 ,  638 , which may be oriented relative to one another in a similar manner to the correspondingly named parts of the digital elliptic filter  400 . However, the digital elliptic filter  600  may differ from the digital elliptic filter  400  in that the stubs  618 ,  628 ,  638  may extend outward beyond the ends of the corresponding posts  620 ,  630 ,  640  in which the stubs  618 ,  628 ,  638  are otherwise enclosed,  FIGS. 6B, 6C . In addition, the digital elliptic filter  600  may include input and output ports  642 ,  644  electrically connected to posts  610 ,  640 , respectively, and grounded to the metal box  650 . The two ports  642 ,  644  may represent a 50 ohm physical transmission line. The ports  642 ,  644  may connect to posts  610 ,  640  in-plane with the posts  610 ,  640  as shown, or may connect to the posts  610 ,  640  from above or below, or by other suitable orientations, for example. 
     As yet a further exemplary design of a physical realization of a digital elliptic filter in accordance with the present invention,  FIGS. 7A, 7B  schematically illustrate isometric and end views, respectively, of an exemplary digital elliptic filter  700  in accordance with the present invention having individual resonators of different height. The digital elliptic filter  700  may be similar to the digital elliptic filter  600  as containing four posts  710 ,  720 ,  730 ,  740  and three stubs  718 ,  728 ,  738 , which may be oriented relative to one another in a similar manner to the correspondingly named parts in the digital elliptic filter  600 . However, the digital elliptic filter  700  may differ from the digital elliptic filter  600  in that one or more of the posts, e.g., post  740 , may have a height that differs from one or more of the remaining posts  710 ,  720 ,  730 ,  FIGS. 7B, 7C . In particular, the decreased height of post  740  permits the post  740  to have increased width, allowing the post  740  to more fully enclose the stub  738  associated therewith. 
     In another of its aspects, digital elliptic filters of the present invention (e.g., filters  100 ,  400 ,  600 ,  700 ) may be used in conjunction with one or more low pass filters to create a narrow bandwidth bandpass filter,  FIGS. 8A-8D . Such a combination can be advantageous in that the size of the digital elliptic filter can be reduced increasing its bandwidth. The low pass filter can then be one of several types, including lumped element, pseudo-lumped element, or stepped impedance. The low pass filter of the stepped impedance type may be particularly useful in that it can be used to route a signal in a manner similar to a transmission line. The digital elliptic filter and low pass filter combination is also well suited to diplexer and multiplexer designs,  FIGS. 8B-8D . For instance, the digital elliptic filter may be combined with a low pass filter to create a diplexer,  FIG. 8B , and the diplexer can then be cascaded to create a triplexer, quadplexer or higher order n-plexer,  FIGS. 8C-8D . In  FIGS. 8B-8D  the letters signify channels of increasing frequency, such that channel A is the lowest frequency, channel B is higher frequency than A, and so forth. 
     The exemplary designs of the present invention may be particularly amenable to fabrication by a sequential build process, such as the PolyStrata® process by Nuvotronics, LLC of Radford Va., USA. For instance the metal structures (e.g., posts  110 ,  120 ,  410 - 440 , metal boxes  150 ,  450 , and ports  642 ,  644 ) may be built up layer by layer by a sequential build process. (The PolyStrata® process is disclosed in U.S. Pat. Nos. 7,012,489, 7,148,772, 7,405,638, 7,948,335, 7,649,432, 7,656,256, 8,031,037, 7,755,174, and 7,898,356, 2008/0199656, 2011/0123783, 2010/0296252, 2011/0273241, 2011/0181376, 2011/0210807, the contents of which patents are incorporated herein by reference.) Thus, in another of its aspects the present invention provides a method of forming a multi-layer digital elliptic filter by a sequential build process. 
     These and other advantages of the present invention will be apparent to those skilled in the art from the foregoing specification. Accordingly, it will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the claims.