Patent Publication Number: US-2012032760-A1

Title: Compact planar resonators with z-axis folding

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of United States Provisional Application No. 61/371,419 filed Aug. 6, 2010, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to filters for electronic circuits. In particular, this invention relates to an improved structure for a planar filter that is compact in size. 
     Filters are commonly used in electronic circuits for removing undesired frequency components from an electronic signal, enhancing desired frequency components in the electronic signal, or both. Such filters for electronic circuits are typically classified as being either (1) high-pass filters, wherein frequency components below a predetermined level are blocked and frequency components below a predetermined level are passed; (2) low-pass filters, wherein frequency components below a predetermined level are passed and frequency components below a predetermined level are blocked; (3) band-pass filters, wherein frequency components within a predetermined range are passed and frequency components outside of the predetermined range are blocked; and (4) band-reject filters, wherein frequency components within a predetermined range are blocked and frequency components outside of the predetermined range are passed. A wide variety of electronic circuit filter structures of these general types are known in the art. 
     One well known type of electronic circuit filter is commonly referred to as a planar filter. A typical planar filter is characterized by a relatively narrow strip of an electrically conductive material that functions as a resonator and one or more relatively wide strips of an electrically conductive material that function as a ground plane(s). As is well known, the size, shape, and other characteristics of the resonator define frequency or frequencies at which the planar filter is desired to operate. In a microstrip type of planar filter, the resonator extends parallel to the ground plane, but is separated therefrom by an intervening substrate formed from a dielectric material (i.e., a material that is a poor conductor of electricity). In a stripline type of planar filter, the resonator extends parallel between two ground planes and is separated from each by an intervening dielectric substrate. 
     Planar filters of this general type find widespread, but not exclusive, use in printed circuit board, with the planar filter designed as part of the printed circuit board because the same techniques and processes used to design and manufacture the printed circuit board can be used to design and manufacture the planar filter. However, it has been found that in some instances, the planar filter can occupy an undesirably large amount of physical space on the printed circuit board. Thus, it would be desirable to provide an improved structure for a planar filter that is compact in size in comparison to equivalent structures. 
     SUMMARY OF THE INVENTION 
     This invention relates to an improved structure for a planar filter that is compact in size in comparison to equivalent structures. The invention provides a technique to reduce the width and length dimensions of a planar resonator by folding the resonator in the Z-axis (i.e., height). This invention permits printed circuit board filters made with planar resonators to have a reduced form factor due to folding the planar resonator. Folding in the plane of the planar filter (defined here as the x-axis and y-axis) has been done for years and abundant prior art exists. This invention folds the planar filter in the z-axis, which may increase the height dimension, but drastically reduces width and length dimensions. 
     Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a prior art structure for a stripline type of planar filter. 
         FIG. 2  is an enlarged sectional elevational view of a portion of the prior art planar filter illustrated in  FIG. 1 . 
         FIG. 3  is an exploded perspective view of an improved structure for a stripline type of planar filter in accordance with this invention. 
         FIG. 4  is an enlarged sectional elevational view of a portion of the inventive planar filter illustrated in  FIG. 13   
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in  FIGS. 1 and 2  a prior art structure for a stripline type of planar filter, indicated generally at  10 . The prior art planar filter  10  includes a plurality of resonators  11 . Three of such resonators  11  are shown in  FIG. 1 , although it is known to provide a greater or lesser number thereof. Each of the resonators  11  is flat and elongated in shape and is formed from a material that is a good conductor of electricity, such as a metallic material. In the prior art planar filter  10 , each of the resonators  11  extends in a single plane, and all of the resonators  11  are co-planar. As is well known, the size, shape, and other characteristics of the resonator define frequency or frequencies at which the prior art planar filter  10  is desired to operate. For example, the prior art planar filter may be designed to function at ultra high frequencies. The resonators  11  may, for example, be approximately one-quarter of a wavelength at the operating frequency of the planar filter  10 , and the length thereof may be approximately 7.9 inches. 
     The resonators  11  are disposed between a first ground plate  12  and a second ground plate  13 . Each of the first and second ground plates  12  and  13  is planar in shape and is formed from a material that is a good conductor of electricity, such as a metallic material. The first ground plate  12  is spaced apart from the resonators  11  by a first substrate  14  that is formed from a dielectric material (i.e., a material that is a poor conductor of electricity). Similarly, the second ground plate  13  is spaced apart from the resonators  11  by a second substrate  15  that is also formed from a dielectric material. When assembled as shown in  FIG. 2 , the resonators  11 , the first and second ground plates  12  and  13 , and the first and second ground plates  14  and  15  form the prior art planar filter  10 . 
       FIGS. 3 and 4  illustrate an improved structure for a planar filter, indicated generally at  20 , in accordance with this invention. The planar filter  20  includes a plurality of resonators  21 . Three of such resonators  21  are shown in  FIG. 1 , although this invention may be practiced with a greater or lesser number thereof. Each of the illustrated resonators  21  is elongated in shape and is formed from a material that is a good conductor of electricity, such as a metallic material. As is well known, the size, shape, and other characteristics of the resonator define frequency or frequencies at which the planar filter  20  is desired to operate. 
     In this invention, none of the resonators  21  extends in a single plane, as do the prior art resonators  11  described above. Rather, each of the resonators  21  extends in two or more planes. As best shown in  FIG. 4 , each of the resonators  21  includes a first portion  21   a  that extends in a first plane, a second portion  21   b  that extends from the first portion  21   a  and extends in a second plane, and a third portion  21   c  that extends from the second portion  21   b  and extends in a third plane. In the illustrated embodiment, the first portions  21   a  and the third portions  21   c  of each of the resonators extend in respective planes that are spaced apart and parallel to one another, while the second portions  21   b  of each of the resonators  21  extend generally perpendicular to the planes defined by the first and third resonators  21   a  and  21   c , respectively. However, each of the resonators  21  may be formed having any other desired shape or combination of shapes. 
     The resonators  21  are disposed between a first ground plate  22  and a second ground plate  23 . Each of the illustrated first and second ground plates  22  and  23  is planar in shape and is formed from a material that is a good conductor of electricity, such as a metallic material. The first ground plate  22  is spaced apart from the first portions  21   a  of the resonators  21  by a first substrate  24  that is formed from a dielectric material (i.e., a material that is a poor conductor of electricity). Similarly, the second ground plate  23  is spaced apart from the third portions  21   c  of the resonators  21  by a second substrate  25  that is also formed from a dielectric material. 
     A third ground plate  26  is disposed between the first portions  21   a  of the resonators  21  and the third portions  21   c  of the resonators  21 . The illustrated third ground plate  26  is planar in shape and is formed from a material that is a good conductor of electricity, such as a metallic material. The third ground plate  26  is spaced apart from the first portions  21   a  of the resonators  21  by a third substrate  27  that is formed from a dielectric material. Similarly, the third ground plate  26  is spaced apart from the third portions  21   c  of the resonators  21  by a fourth substrate  28  that is also formed from a dielectric material. 
     As shown in  FIGS. 3 and 4 , the third substrate  27  has a plurality of openings  27   a  formed therethrough. In the illustrated embodiment, the third substrate  27  has three of such openings  27   a  formed therethrough, one for each of the three illustrated resonators  21 . However, the third substrate  27  may have any desired number of such openings  27   a  formed therethrough. Similarly, the fourth substrate  28  has a plurality of openings  28   a  (one of which is illustrated in  FIG. 4 ) formed therethrough. In the illustrated embodiment, the fourth substrate  28  has three of such openings  28   a  formed therethrough, one for each of the three illustrated resonators  21 . However, the fourth substrate  28  may have any desired number of such openings  28   a  formed therethrough. Lastly, the third ground plate  26  has a plurality of openings  26   a  (one of which is illustrated in  FIG. 4 ) formed therethrough. In the illustrated embodiment, the third ground plate  26  has three of such openings  26   a  formed therethrough, one for each of the three illustrated resonators  21 . However, the third ground plate  26  may have any desired number of such openings  26   a  formed therethrough. 
     As shown in  FIG. 4 , the openings  26   a ,  27   a , and  28   a  are provided to allow respective portions of the resonators  21  to extend therethrough. Specifically, the second portions  21   b  of the resonators  21  extend through the openings  26   a ,  27   a , and  28   a  to provide electrical continuity between the first and third portions  21   a  and  21   c  of the resonators  21 . Thus, it can be seen that the first portions  21   a  of the resonators  21  are disposed between the first and third substrates  24  and  27 , respectively, while the third portions  21   c  of the resonators  21  are disposed between the second and fourth substrates  25  and  28 . At the same time, the third ground plate  26  is disposed between third and fourth substrates  27  and  28 . When assembled as shown in  FIG. 4 , the resonators  21 , the ground plates  22 ,  23 , and  26 , and the substrates  24 ,  25 ,  27 , and  28  form the planar filter  20  of this invention. 
     Thus, it can be seen that the resonators  21  of this invention are folded in the lengthwise or elongated direction such that the first and third portions  21   a  and  21   c  thereof are aligned in a direction that is perpendicular to the respective planes in which they extend (the vertical direction when viewing  FIG. 4 ). This configuration would not qualify as a stripline type of planar filter  20  without the third ground plane  26  extending between the first and third portions  21   a  and  21   c  of the resonators  21 . Using the sample dimensions discussed above in connection with the prior art planar filter  10  illustrated in  FIGS. 1 and 2 , the orientation of the first and third portions  21   a  and  21   c  reduces the overall length of the resonators  21  from about 7.9 inches to about 4 inches, which is nearly a 50% reduction in length. This is a significant reduction in size, which would allow the planar filter  20  to be used physical spaces where the prior art planar filter  10  would not be appropriate. 
     The illustrated planar filter  20  is a stripline type, which is characterized by the resonators extending parallel between the two ground planes and being separated from each by the intervening dielectric substrates. However, it will be appreciated that this invention may be practiced in connection with other types of planar filters  20 . For example, this invention could be embodied as a microstrip type of planar filter, wherein the resonators extends parallel to the ground plane, but are separated therefrom by an intervening dielectric substrate. 
     The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.