Patent Publication Number: US-2023155267-A1

Title: Metallized dielectric waveguide filters having irregular shaped resonant cavities, slanted metallized openings and/or spurious coupling windows

Description:
FIELD 
     The present invention relates generally to communications systems and, more particularly, to filters that are suitable for use in cellular communications systems. 
     BACKGROUND 
     Filters are electronic devices that selectively pass signals based on the frequency of the signal. Various different types of filters are used in cellular communications systems. Moreover, as new generations of cellular communications services have been introduced—typically without phasing out existing cellular communications services—both the number and types of filters that are used has expanded significantly. Filters may be used, for example, to allow radio frequency (“RF”) signals in different frequency bands to share selected components of a cellular communications system and/or to separate RF data signals from power and/or control signals. Conventionally, metal resonant cavity filters have been used to implement many of the filters used in cellular communications systems. However, metal resonant cavity filters tend to be heavy and expensive to manufacture. As the number of filters used in a typical cellular communications system has proliferated, the need for smaller, lighter and/or less expensive filters has increased. 
     The “response” of a filter refers to the amount of energy that passes from a first port (e.g., an input port) of the filter to a second port (e.g., an output port) of the filter as a function of frequency. A filter response will typically include one or more pass-bands, which are frequency ranges where the filter passes signals with relatively small amounts of attenuation. A filter response also typically includes one or more stop-bands. A stop-band refers to a frequency range where the filter will substantially not pass signals, usually because the filter is designed to reflect backwards any signals that are incident on the filter in this frequency range. In some applications, it may be desirable that the filter response exhibit a high degree of “local selectivity,” meaning that the transition from a pass-band to an adjacent stop-band occurs over a narrow frequency range. One technique for enhancing local selectivity is to add transmission zeros in the filter response. A “transmission zero” refers to a portion of a filter frequency response where the amount of signal energy that passes is very low. Transmission zeros are most typically achieved using cross-couplings. 
     Resonant cavity filters include a plurality of resonant cavities. Cross-coupling, which is the most common technique used to increase local selectivity in a resonant cavity filter, refers to intentional coupling between non-adjacent resonating cavities. Depending on the relative location of the transmission zero with respect to the pass-band, the sign of the required cross-coupling may vary. When cross-couplings are used to create transmission zeros, the resonant cavities are often arranged in some form of a planar grid as opposed to a single row of resonant cavities. Such a two-dimensional distribution of resonant cavities facilitates coupling between resonant cavities that are not adjacent each other along a main transmission path through the filter (i.e., cross-couplings). U.S. Pat. No. 5,812,036 (“the &#39;036 patent”), the contents of which are incorporated herein by reference, discloses various resonant cavity filters that have such two-dimensional cavity arrangements that include cross-coupling. 
       FIG.  1    is a schematic top sectional view of a conventional metal resonant cavity filter  10  (which is one of the filters discussed in the above-referenced &#39;036 patent). As shown in  FIG.  1   , the resonant cavity filter  10  includes a metallic housing  12  that has walls  14  formed therein that define resonant cavities  18 - 1  through  18 - 6 . The filter  10  also includes a floor  28  and a top cover (not shown) that encloses the cavities. While the example filter  10  illustrated in  FIG.  1    includes a total of six cavities  18 , it will be appreciated that any appropriate number of cavities  18  may be provided as necessary to provide a filter having desired filtering characteristics. Note that herein when multiple of the same elements or structures are provided, they may be referred to in some instances using two part reference numerals, where the two parts are separated by a dash. Herein, such elements may be referred to individually by their full reference numeral (e.g., cavity  18 - 2 ) and may be referred to collectively by the first part of the applicable reference numeral (e.g., the cavities  18 ). 
     Still referring to  FIG.  1   , a coaxial resonating element or “resonator”  20 - 1  through  20 - 6  is provided in each of the respective cavities  18 - 1  through  18 - 6 . The walls  14  may include openings or “windows”  16 - 1  through  16 - 5  that allow resonators  20  in adjacent ones of the cavities  18  to couple to each other along a U-shaped main transmission path that extends from an input port  22  to an output port  24  of the filter  10 . An RF signal input at input port  22  generally follows the main transmission path and hence passes through each of the resonant cavities  20  in numerical order (i.e., from cavity  20 - 1  to cavity  20 - 2 , . . . to cavity  20 - 6 ) and is output at output port  24  as a filtered signal. In addition, the filter  10  includes two cross-coupling windows  26 - 1 ,  26 - 2  that enable cross-coupling between two pairs of non-adjacent resonators  20  (namely, between the resonators  20 - 1  and  20 - 6  in cavities  18 - 1  and  18 - 6  and between the resonators  20 - 2  and  20 - 5  in cavities  18 - 2  and  18 - 5 ). The main couplings between the five sequential pairs of resonators  20  and the two cross-couplings between the two pairs of non-adjacent resonators  20  contribute to the overall transfer function of the filter  10 . As discussed above, the cross-couplings may be used to generate transmission zeros in the filter response. 
     Another known type of filter is the metallized dielectric waveguide filter. A waveguide is a metal conduit that may be used to confine and direct RF signals. A metallized dielectric waveguide filter is a waveguide filter that is formed using one or more blocks of dielectric material that have metallized exterior surfaces. Metallized dielectric waveguide filters are typically formed by metallizing the outside of one or more ceramic blocks using a metallization process such as screen printing, spray coating, dip coating or thin film metallization process. 
     The use of waveguides filled with a solid dielectric material allows a reduction in the overall size of the filter. Generally speaking, the higher the dielectric constant of the dielectric material, the greater the dimensions of the filter may be reduced. Metalized dielectric waveguide filters can exhibit a very high ratio of Q factor to volume, have low insertion losses, and can readily handle 10-20 Watts of power without generating unacceptable levels of passive intermodulation products. As such, metalized dielectric waveguide filters may be well-suited for many cellular applications. Metallized dielectric waveguide filters, however, can be relatively heavy, and hence they are generally only used at higher frequencies where the shorter wavelength of the RF signals reduces the overall size and weight of the filter. 
     A metallized dielectric waveguide filter includes a plurality of resonant cavities that are defined along a main transmission path that extends between an input port and an output port of the filter. Pairs of vertically-extending metallized openings which extend through the dielectric block are formed within the dielectric block. These pairs of metallized openings form metal walls within the dielectric block in order to define the individual resonant cavities within the metallized dielectric block. Openings between each pair of vertically-extending metal-plated holes form coupling windows. An electromagnetic wave that enters a resonant cavity is reflected back and forth between the two ends thereof, and will resonate at a characteristic frequency based on a given geometry of the resonant cavity. The resonance effect can be used to selectively pass certain frequencies through the coupling window into the next resonant cavity along the main transmission path. Additional openings in the form of “cross-coupling windows” may be provided between resonant cavities that are not adjacent each other along the main transmission path. These cross-coupling windows may be used to generate transmission zeros in the frequency response of the filter, as explained above. 
       FIG.  2 A  is a schematic top sectional view of a conventional 7-order Chebyshev metallized dielectric waveguide filter  100  that has a filter response with a pair of transmission zeros.  FIG.  2 B  is a schematic top view of the metallized dielectric waveguide filter  100  of  FIG.  2 A . As shown in  FIGS.  2 A- 2 B , the metallized dielectric waveguide filter  100  comprises a solid block of dielectric material  110  that has metallization plated on the outer sidewalls  120  and top surface  122  ( FIG.  2 B ) thereof. While not shown in the views of  FIGS.  2 A- 2 B , the bottom surface of the dielectric block  110  is also plated with metal. As shown in  FIG.  2 B , first and second input/output ports  112 - 1 ,  112 - 2  are provided on either side of the dielectric block  110 . The first and second input/output ports  112 - 1 ,  112 - 2  may comprise, for example, coaxial connectors that are mounted within respective vertical openings  114 - 1 ,  114 - 2  in the dielectric block  110  (here vertical refers to the direction of an axis that is perpendicular to the plane defined by the top surface  122  of filter  100 . The openings  114  may extend, for example, about halfway through the dielectric block  110 . 
     Additional metallized openings  130 - 1 ,  130 - 2 ,  130 - 3  extend vertically all of the way through the dielectric block  110 . Each metallized opening  130  may comprise one or more segments  132 , which may be linear segments. The metallized openings  130  divide the dielectric block  110  into seven resonant cavities  140 - 1  through  140 - 7 . Each metallized opening  130  is fairly long, and two of the three metallized openings  130  extend to a sidewall  120  of filter  100 . Coupling windows  150 ,  160  are provided between selected adjacent pairs of resonant cavities  140 . The coupling windows may include main coupling windows  150  and a cross-coupling window  160 . 
       FIG.  3    is a schematic diagram illustrating the resonant cavities  140  included in the conventional metallized dielectric waveguide filter  100  of  FIGS.  2 A- 2 B  and the different couplings between these resonant cavities  140 . As shown in  FIG.  3   , the resonant cavities are disposed in two rows R 1 , R 2 , with each extending generally parallel to a longitudinal axis L of the filter  100  (see  FIG.  2 B ). Each resonant cavity  140  is approximately the same size. Since the filter  100  is an odd order filter (i.e., it has an odd number of resonant cavities  140 , the first row R 1  has four resonant cavities  140  while the second row R 2  only has three resonant cavities  140 . As such, the dielectric block  110  has a “cut-out” section on the left side of bottom row R 2  where no dielectric material is provided. As such, sidewalls  120 - 2  and  120 - 3  are each divided into two sidewall segments  120 - 2 A,  120 - 2 B and  120 - 3 A,  120 - 3 B. The main transmission path through the filter  100  extends from the first input/output port  112 - 1  through the seven resonant cavities  140 - 1  through  140 - 7  in numerical order to the second input/output port  112 - 2 , as shown by the arrows between resonant cavities  140  in  FIG.  3   . The filter has six “main couplings” which refer to the couplings between adjacent resonant cavities  140  along the main transmission path. These main couplings are labelled k 1,2 , k 2,3 , k 3,4 , k 4,5 , k 5,6 , and k 6,7  in  FIG.  3   . There also is one cross-coupling between resonant cavities  140 - 2  and  140 - 5  (through cross-coupling window  160 ) which is labelled k 2,5  in  FIG.  3   . 
     SUMMARY 
     Pursuant to some embodiments of the present invention, metallized dielectric waveguide filters are provided that include first and second input/output ports and a dielectric block that has metallized top and bottom surfaces and metallized sidewalls. The dielectric block further includes a plurality of metallized openings that extend into the interior of the dielectric block, and these metallized openings divide the dielectric block into a plurality of resonator cavities. A first of the metallized openings extends at an oblique angle with respect to a first of the metallized outer sidewalls. 
     In some embodiments, the first and/or a second of the metallized openings may extend at an angle of between 15° and 75° with respect to the first of the metallized outer sidewalls. In some embodiments, the outer sidewalls may define a generally rectangular shape. 
     In some embodiments, the dielectric block may be divided into first and second rows of resonator cavities, where each of these rows extends parallel to a longitudinal axis of the metallized dielectric waveguide filter. 
     In some embodiments, a plurality of main coupling windows may be provided within the dielectric block that define a main transmission path through the dielectric block. The main transmission path may extend from the first input/output port sequentially through each of the resonator cavities to the second input/output port. The main transmission path may cross from the first row of resonator cavities to the second row of resonator cavities at least once, at least twice, or at least three times in example embodiments. In some embodiments, the first of the metallized openings may be positioned between one of the resonator cavities in the first row and one of the resonator cavities in the second row. 
     In some embodiments, at least one cross-coupling window may be provided within the dielectric block that is configured to allow a pair of resonator cavities that are not adjacent each other along the main transmission path to cross-couple. 
     In some embodiments, the first of the metallized opening may form a first wall between the first of the resonator cavities and a third of the resonator cavities, a second of the metallized opening may form a second wall between the first of the resonator cavities and a second of the resonator cavities that is between the first of the resonator cavities and the third of the resonator cavities along the main transmission path. The first and second walls may define a coupling window between the first of the resonator cavities and the third of the resonator cavities. 
     In some embodiments, the dielectric block may be divided into first and second rows of resonator cavities that extend parallel to a longitudinal axis of the metallized dielectric waveguide filter, and the first and third of the resonator cavities may be in different ones of the first and second rows. 
     In some embodiments, a second of the metallized opening forms a first wall between a second of the resonator cavities and a third of the resonator cavities that is adjacent the second of the resonator cavities along the main transmission path, and a third of the metallized opening forms a second wall, the second wall and the third wall defining a coupling window between the second of the resonator cavities and a fourth of the resonator cavities. 
     In some embodiments, the dielectric block may be divided into first and second rows of resonator cavities, where each of these rows extends parallel to a longitudinal axis of the metallized dielectric waveguide filter, and the fourth of the resonator cavities and the second of the resonator cavities are in different ones of the first and second rows. 
     In some embodiments, a second of the metallized openings may extend substantially into a central region between a pair of resonator cavities that are adjacent each other along the main transmission path so as to define first and second coupling main windows between the pair of resonator cavities. 
     In some embodiments, at least one of the resonator cavities may have an angled inner sidewall. 
     Pursuant to further embodiments of the present invention, metallized dielectric waveguide filters are provided that include first and second input/output ports and a dielectric block that has metallized top and bottom surfaces and metallized outer sidewalls, the dielectric block further including a plurality of metallized openings that extend into the interior of the dielectric block, the metallized openings dividing the dielectric block into a plurality of resonator cavities. A first of the metallized opening forms a first wall between a first of the resonator cavities and a second of the resonator cavities, the second resonator cavity being adjacent the first of the resonator cavities along a main transmission path through the dielectric block that extends between the first input/output port and the second input/output port sequentially through each of the resonator cavities, and a second of the metallized opening forms a second wall. The first wall and the second wall define a coupling window between the first of the resonator cavities and a third of the resonator cavities. 
     In some embodiments, the third of the resonator cavities may be adjacent the second of the resonator cavities along the main transmission path. 
     In some embodiments, the resonator cavities are arranged in first and second rows that extend parallel to a longitudinal axis of the metallized dielectric waveguide filter, and the first of the resonator cavities and the third of the resonator cavities are in different ones of the first and second rows. 
     In some embodiments, the first of the metallized openings may extend at an oblique angle with respect to a first of the metallized outer sidewalls. 
     In some embodiments, the first of the metallized openings may extend at an angle of between 15° and 75° with respect to the first of the metallized outer sidewalls. 
     In some embodiments, the main transmission path may cross from the first row of resonator cavities to the second row of resonator cavities at least once. 
     In some embodiments, a third of the metallized openings may extend substantially into a central region between a pair of resonator cavities that are adjacent each other along the main transmission path so as to define first and second coupling main windows between the pair of resonator cavities. 
     Pursuant to further embodiments of the present invention, metallized dielectric waveguide filters are provided that include first and second input/output ports and a dielectric block that has metallized top and bottom surfaces and metallized outer sidewalls, the dielectric block further including a plurality of metallized openings that extend into the interior of the dielectric block, the metallized openings dividing the dielectric block into a plurality of resonator cavities. A main transmission path is defined through the dielectric block that extends between the first input/output port and the second input/output port sequentially through each of the resonator cavities. A first of the metallized openings extends substantially into a central region between a first of resonator cavities and a second of the resonator cavities that are adjacent each other along the main transmission path so as to define first and second coupling main windows between the first and second of the resonator cavities. 
     In some embodiments, the dielectric block may be divided into a first row of resonator cavities and a second row of resonator cavities, where each of the first row of resonator cavities and the second row of resonator cavities extends parallel to a longitudinal axis of the metallized dielectric waveguide filter, and wherein the first of resonator cavities is in the first row and the second of resonator cavities is in the second row. 
     In some embodiments, the first of the metallized openings may extend at an angle of between 15° and 75° with respect to both a first of the metallized outer sidewalls and with respect to a second of the metallized outer sidewalls that is substantially perpendicular to the first of the metallized outer sidewalls. 
     In some embodiments, the first of the metallized openings may be positioned between one of the resonator cavities in the first row and one of the resonator cavities in the second row. 
     In some embodiments, a second of the metallized openings may form a first wall between a third of the resonator cavities and a fifth of the resonator cavities, a third of the metallized opening forms a second wall between the third of the resonator cavities and a fourth of the resonator cavities that is between the third of the resonator cavities and the third fifth the resonator cavities along the main transmission path, the first wall and the second wall defining a coupling window between the third of the resonator cavities and the fifth of the resonator cavities. 
     In some embodiments, the dielectric block may be divided into first and second rows of resonator cavities that each extend parallel to a longitudinal axis of the metallized dielectric waveguide filter, and the third of the resonator cavities and the fifth of the resonator cavities are in different ones of the first and second rows. 
     Pursuant to further embodiments of the present invention, metallized dielectric waveguide filters are provided that include first and second input/output ports and a dielectric block that has metallized top and bottom surfaces and metallized outer sidewalls, the dielectric block further including a plurality of metallized openings that extend into the interior of the dielectric block, the metallized openings forming metallized inner walls within the dielectric block. The metallized outer walls and the metallized inner walls define a plurality of main coupling windows that form a main transmission path through the dielectric block that extends between the first input/output port and the second input/output port sequentially through each of the resonator cavities, a first cross-coupling window between two of the resonant cavities that are not adjacent each other along the main transmission path, and a first spurious coupling window. The first spurious coupling window is configured to generate a cross-coupling between first and second of the resonant cavities that are not adjacent each other along the main transmission path that substantially cancels coupling between the first and second of the resonant cavities that occurs along the main transmission path. 
     In some embodiments, the dielectric block may be divided into first and second rows of resonator cavities, that each extend parallel to a longitudinal axis of the metallized dielectric waveguide filter. The first and second of the resonator cavities are in different ones of the first and second rows. 
     In some embodiments, the spurious coupling window may be defined between a first of the metallized inner walls and a second of the metallized inner walls. 
     In some embodiments, the first metallized inner wall may be between the first of the resonator cavities and the second of the resonator cavities and the second metallized inner wall is between the first of the resonator cavities and a third of the resonator cavities that is in between the first and second of the resonator cavities along the main transmission path. 
     In some embodiments, the first metallized inner wall may be between the first of the resonator cavities and a third of the second of the resonator cavities and the second metallized inner wall is between the first of the resonator cavities and a fourth of the resonator cavities that is in between the first and second of the resonator cavities along the main transmission path. 
     In some embodiments, the first metallized inner wall may be between the first of the resonator cavities and a third of the resonator cavities and the second metallized inner wall is between the second of the resonator cavities and the third of the resonator cavities, and the third of the resonator cavities is in between the first and second of the resonator cavities along the main transmission path. 
     In some embodiments, a first of the metallized inner walls may extend at an oblique angle with respect to a first of the metallized outer sidewalls. The oblique angle may be an angle of between 15° and 75°. 
     In some embodiments, the dielectric block may be divided into first and second rows of resonator cavities that each extend parallel to a longitudinal axis of the metallized dielectric waveguide filter. The main transmission path may cross from the first row of resonator cavities to the second row of resonator cavities at least twice. In some embodiments, the first and second of the resonant cavities may be in different ones of the first and second rows. 
     In some embodiments, a first of the metallized openings may extend substantially into a central region between a pair of resonator cavities that are adjacent each other along the main transmission path so as to define first and second coupling main windows between the pair of resonator cavities. In some embodiments, the spurious coupling window may be defined by the first and second of the metallized openings. 
     In some embodiments, at least one of the resonator cavities may have an angled inner sidewall. 
     Pursuant to still further embodiments of the present invention, metallized dielectric waveguide filters are provided that include first and second input/output ports and a dielectric block that has metallized top and bottom surfaces and metallized outer sidewalls, the dielectric block further including a plurality of metallized openings that extend into the interior of the dielectric block, the metallized openings dividing the dielectric block into a plurality of resonator cavities. The resonator cavities are arranged in a first row and a second row within the dielectric block, where each of the first row of resonator cavities and the second row of resonator cavities extends parallel to a longitudinal axis of the metallized dielectric waveguide filter. A first of the metallized openings forms an inner wall that separates one of the resonator cavities in the first row from one of the resonator cavities in the second row. A longitudinal axis of the first of the metallized openings forms an oblique angle with the longitudinal axis of the metallized dielectric waveguide filter. 
     In some embodiments, the oblique angle is between 15° and 75°. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic top sectional view of a conventional resonant cavity filter that has cross-coupling between selected cavities. 
         FIG.  2 A  is a schematic top sectional view of a conventional 7-order Chebyshev filter metallized dielectric waveguide filer. 
         FIG.  2 B  is a schematic top view of the metallized dielectric waveguide filter of  FIG.  2 A . 
         FIG.  3    is a schematic diagram illustrating the resonant cavities included in the metallized dielectric waveguide filter of  FIGS.  2 A- 2 B  and the different couplings between these resonant cavities. 
         FIG.  4 A  is a graph illustrating the simulated return loss and insertion loss for an ideal 7-order Chebyshev filter having a response with a pair of symmetric transmission zeros. 
         FIG.  4 B  is a graph illustrating the simulated return loss and insertion loss for the filter of  FIGS.  2 A- 2 B . 
         FIG.  5 A  is a perspective view of a metallized dielectric waveguide filter according to embodiments of the present invention with the input/output connectors thereof omitted. 
         FIG.  5 B  is a top view of the metallized dielectric waveguide filter of  FIG.  5 A . 
         FIG.  5 C  is a schematic perspective view of one of the resonant cavities of the metallized dielectric waveguide filter of  FIGS.  5 A- 5 B . 
         FIG.  6    is a schematic diagram illustrating the resonant cavities included in the metallized dielectric waveguide filter of  FIGS.  5 A- 5 B  and the different couplings between these resonant cavities. 
         FIG.  7    is a graph illustrating the simulated return loss and insertion loss for the filter of  FIGS.  5 A- 5 B . 
     
    
    
     DETAILED DESCRIPTION 
     While the conventional metallized dielectric waveguide filter  100  described above with reference to  FIGS.  2 A- 3    may be suitable for various applications, it has a number of potential disadvantages. First, conventional metallized dielectric waveguide filters may be relatively heavy and expensive to manufacture. As the number of filters used in a typical cellular communications system has proliferated, weight and expense concerns become heightened. Second, conventional metallized dielectric waveguide filters, and particularly odd-order filter designs, may be more fragile than most conventional filters, due to the above-described long slots that extend through the dielectric block of these filters. If the filter is subjected to twisting forces, there is a danger that the filter may crack or even break. Third, while careful design of the coupling windows may reduce unintended coupling between non-adjacent resonant cavities, it may be very difficult to completely eliminate such unwanted couplings, and these unwanted couplings may reduce the RF performance of the filter. 
     For example,  FIG.  4 A  is a graph illustrating the simulated return loss (curve  190 ) and insertion loss (curve  192 ) performance of an ideal metallized dielectric waveguide 7-order Chebyshev bandpass filter that is designed to have a passband of 3.6-3.8 GHz (the Q-value used in the simulation was assumed to be 1500). As shown in  FIG.  4 A , a return loss of less than −19 dB is achieved across the entire pass band, and the return loss rises very sharply just outside of the pass band (to less than −1 dB within about 25 MHz on either side of the pass band). The insertion loss curve  192  also exhibits excellent performance, with the insertion loss being less than 1 dB across the full pass band and with two symmetric transmission zeros (nearly 60 dB rejection) at about 50 MHz on either side of the pass band. In contrast,  FIG.  4 B  illustrates the simulated response of the conventional metallized dielectric waveguide 7-order Chebyshev bandpass filter  100  of  FIGS.  2 A- 2 B . In  FIG.  4 B , curve  194  shows the simulated return loss performance and curve  196  shows the simulated insertion loss performance. While the performance is somewhat similar to the ideal performance shown in  FIG.  4 A , it can be seen that the transmission zeros are not very symmetric, in both location and depth. Thus, improved metallized dielectric waveguide filter designs are desirable that address the above-described issues with conventional designs. 
     Pursuant to embodiments of the present invention, metallized dielectric waveguide filters are provided that are smaller, lighter and more mechanically robust than conventional metallized dielectric waveguide filters, and which also exhibit improved RF performance. The metallized dielectric waveguide filters according to embodiments of the present invention may include new coupling window structures that are formed using smaller metallized openings through the dielectric block. The use of such smaller metallized openings may improve the mechanical integrity of the filter and the new coupling windows may also allow the filter to be designed to cancel unwanted couplings between resonant cavities that are not adjacent each other along the main transmission path through the filter. By at least partially cancelling these unwanted couplings, the RF performance of the filter may be improved. Additionally, at least some of the resonant cavities may have irregular shapes, which may allow the overall dimensions of the filter to be reduced, particularly in odd-order filter designs. For example, one or more metallized openings may be formed through the dielectric block that extend through the dielectric block at an oblique angle with respect to the sidewalls of the filter and/or with respect to a longitudinal axis of the filter. 
     The metallized dielectric waveguide filters according to embodiments of the present invention may be small, low loss and exhibit a very high Q factor. They may also have high power handling capabilities, and be reasonably lightweight. These filters may also be cheaper and easier to manufacture than conventional die-cast coaxial cavity filters. 
     The metallized dielectric waveguide filters according to embodiments of the present invention may include a dielectric block that has metallized top and bottom surfaces and metallized sidewalls. A plurality of metallized openings that extend into the interior of the dielectric block, and these metallized openings may divide the dielectric block into a plurality of resonator cavities. In some embodiments, one of the metallized openings may extend at an oblique angle with respect to a first of the metallized outer sidewalls. 
     In other embodiments, a first of the metallized openings may form a first wall between a first of the resonator cavities and a second of the resonator cavities that is adjacent the first of the resonator cavities along a main transmission path through the dielectric block. A second of the metallized opening may form a second wall. The first and second walls may define a coupling window between the first of the resonator cavities and a third of the resonator cavities. 
     In still other embodiments, one of the metallized openings may extend substantially into a central region between a first of resonator cavities and a second of the resonator cavities that are adjacent each other along the main transmission path so as to define first and second coupling main windows between the first and second of the resonator cavities. 
     In yet additional embodiments, the metallized openings may form a plurality of metallized openings forming metallized inner walls within the dielectric block. The metallized inner and outer walls may define (1) a plurality of main coupling windows that form the main transmission path through the dielectric block, (2) a first cross-coupling window between two of the resonant cavities that are not adjacent each other along the main transmission path, and (3) at least one spurious coupling window. The spurious coupling window may be configured to generate a cross-coupling between first and second of the resonant cavities that are not adjacent each other along the main transmission path that substantially cancels coupling between the first and second of the resonant cavities that occurs along the main transmission path. 
     In further embodiments, the resonator cavities may be arranged in first and second rows within the dielectric block, where each of these rows extend parallel to a longitudinal axis of the metallized dielectric waveguide filter. One of the metallized openings may form an inner wall that separates one of the resonator cavities in the first row from one of the resonator cavities in the second row. A longitudinal axis of this metallized opening may form an oblique angle with the longitudinal axis of the metallized dielectric waveguide filter. 
     Metallized dielectric waveguide filters according to embodiments of the present invention will now be discussed in greater detail with respect to  FIGS.  5 A- 7   . 
       FIGS.  5 A and  5 B  are a perspective view and top view, respectively, of a metallized dielectric waveguide filter  200  according to certain embodiments of the present invention. The filter  200  may be, for example, a band pass filter that is designed to operate in the 3.6-3.8 GHz band, with a pass band return loss of less than −17 dB, a pass band insertion loss of less than 1.2 dB, out-of-band rejection of at least 40 dB in the 3400-3540 MHz and 3860-4000 MHz frequency bands, and pass band ripple of less than 0.9 dB. The filter  200  has a 7-order Chebyshev design with a pair of symmetric transmission zeros. The filter has a “single layer” design with two rows of resonant cavities formed in a metallized dielectric block. 
     As shown in  FIGS.  5 A- 5 B , the metallized dielectric waveguide filter  200  is a generally rectangular device that extends along a longitudinal axis L. The filter also extends along a “transverse” axis T and a vertical axis V, with each axis being perpendicular to the other two axes. The filter  200  has sidewalls  220 - 1  through  220 - 4  a top surface  222  and a bottom surface  224  (not visible in the figures). The filter comprises a solid block of dielectric material  210  that has metallization plated or otherwise formed on the exterior thereof. A dielectric constant of the dielectric material of the dielectric block  210  may be, for example, between 15 and 40. In an example embodiment, the dielectric material may comprise a ceramic. First and second input/output ports  212 - 1 ,  212 - 2  are provided on either side of the filter  200 . The first and second input/output ports  212 - 1 ,  212 - 2  may comprise, for example, coaxial connectors (not shown) that are mounted within respective vertical openings  214 - 1 ,  214 - 2  in the dielectric block  210 . The openings  214  may extend, for example, about halfway through the dielectric block  210 . 
     A plurality of metallized openings  230 - 1  through  230 - 7  extend vertically all of the way through the dielectric block  210 . Each metallized opening  230  may be formed, for example, by drilling, cutting or punching a hole all the way through the dielectric block  210  prior to the metallization operation that is used to plate or otherwise deposit a metal layer on the sidewalls  220  and top and bottom surfaces  222 ,  224  of the dielectric block  210 . The metallization operation may likewise coat the interior of the openings through the dielectric block  210  to form the metallized openings  230 . The metallized openings  230  tend to be much shorter (in their length direction) as compared to the metallized openings  130  in the conventional metallized dielectric waveguide filter  100  discussed above. 
     The metallized openings  230  divide the dielectric block  210  into seven resonant cavities  240 - 1  through  240 - 7 .  FIG.  5 C  is a schematic view of one of the resonant cavities  240  of metallized dielectric waveguide filter  200 . As shown in  FIG.  5 C , the resonant cavity  240  has a length l, a width w and a height h. The length dimension is the longitudinal direction of the dielectric block  210  in which the resonant cavity  240  is formed. The width and height dimensions are transverse to the length dimension and perpendicular to each other. All three dimensions are also shown in  FIG.  5 C  with respect to resonant cavity  240 . 
     Each resonant cavity  240  has a resonant frequency. For a resonant cavity  240  that has a rectangular shape, the resonant frequency f res  may be determined based on the dimensions of the cavity and the dielectric constant (ε) of the dielectric material as follows: 
     
       
         
           
             
               
                 
                   
                     fres 
                     - 
                     
                       c 
                       ⁢ 
                       a 
                       ⁢ 
                       v 
                     
                   
                   = 
                   
                     
                       1 
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                         
                           
                             μ 
                             ⁢ 
                             ε 
                           
                         
                       
                     
                     · 
                     
                       
                         
                           
                             ( 
                             
                               π 
                               w 
                             
                             ) 
                           
                           2 
                         
                         + 
                         
                           
                             ( 
                             
                               π 
                               l 
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where μ is the magnetic permeability of the dielectric material. Typically each resonant cavity  240  is designed to have approximately the same resonant frequency, which may be the center frequency of the pass band of the filter  200 . 
     As is apparent from Equation (1), a desired resonant frequency for a resonant cavity  240  can be obtained by manipulating the length (l) and width (w) of the resonant cavity  240  and the dielectric constant of the dielectric block  210 . However, the length l and width w (as well as the height h) heavily impact the electric and magnetic field distributions within the resonant cavity  240 . Consequently, the length l, width w and height h must also be selected to take into account the couplings that are required between adjacent and non-adjacent resonant cavities  240  in order to obtain a desired filter response. 
     Referring again to  FIGS.  5 A- 5 B , he resonant cavities  240 - 1  through  240 - 7  are located in similar positions to the resonant cavities  140 - 1  through  140 - 7  in filter  100 , but resonant cavities  240 - 1  and  240 - 3  are redesigned to extend into the lower left hand portion of dielectric block  210  in the view of  FIG.  5 B , which allows the rectangular footprint of filter  200  to be reduced as compared to the rectangular footprint of filter  100  (the “rectangular footprint” refers to the smallest rectangle that completely encloses the filter when the filter is viewed from above). Coupling windows  250 ,  260  are provided between selected adjacent pairs of resonant cavities  240 . The coupling windows may include main coupling windows  250 - 1  through  250 - 6 , a cross-coupling window  260 , and spurious coupling windows  270 - 1  through  270 - 3 . The coupling windows  250 ,  260  are formed either (1) between two or more of the metallized openings  230  or (2) between a metallized opening  230  and one of the sidewalls  220  of filter  200 . 
     The main coupling windows  250  are similar to the main coupling windows  150  of filter  100 , and the cross-coupling window  260  is similar to the cross-coupling window  150  of filter  100 . In each case, these “windows”  250 ,  260  represent a region in the dielectric block  210  that is between two adjacent resonant cavities  240  where no metallization is present so that RF energy may pass through the window between the two adjacent cavities  240 . The spurious coupling windows  270  are formed by implementing the metallized openings  130  of filter  100  as a plurality of smaller metallized openings  230  (e.g., one small metallized opening for each segment of the metallized openings  130 ), where small regions where no metallization is present are left between the metallized openings  230  in order to form the additional spurious coupling windows  270 . The spurious coupling windows  270  may be smaller than the main coupling windows  250  and/or may be located in positions where the electromagnetic fields are lower in the resonant cavities  240 , and hence the amount of RF energy that will pass through the spurious coupling windows  270  is generally less than the amount of RF energy that will pass through the main coupling windows  250 . 
     A challenge with using metallized dielectric waveguide filters in cellular systems is that these filters tend to generate undesired or “spurious” modes at frequencies that are close to the pass band. Waveguide filters may be designed to transmit an electromagnetic wave in either a transverse electric (TE) mode or a transverse magnetic (TM) mode, as is well understood by those of ordinary skill in the art. In waveguide transmission systems, including waveguide filters, other undesired transmission modes may arise that may negatively affect the response of the filter. These undesired modes are referred to as “spurious modes.” Spurious modes may result in the amount of rejection being reduced in a frequency range that is above the pass band frequency range. In many cases, cellular operators may require that the filters used in base station antennas have extremely high degrees of rejection at frequencies that are close to the pass band. If spurious modes fall within frequency ranges where such high degree of rejection is required, it may be difficult to meet the attenuation specifications. 
     The filter  200  further includes a metallized circular hole  280  is formed in the top surfaces of each of resonant cavities  240 - 2 ,  240 - 3 ,  240 - 4 ,  240 - 5  and  240 - 6 . These metallized circular holes  280  are referred to herein as “blind holes” and may be used to shape the electromagnetic field to increase the coupling through a coupling window  250  and/or to increase the center frequency of the first spurious mode, which may help extend the pass band of the filter response. Each blind hole  280  may comprise an opening that is formed in the top portion or ceiling of the resonant cavity  240 . The sidewalls and floor of this hole are metallized in the metallization process applied to the dielectric block  210  to form the blind hole  280 . While the blind holes  280  are shown as having a circular cross-section, it will be appreciated that the blind holes  280  may have any appropriate shape. 
       FIG.  6    is a schematic diagram illustrating the resonant cavities  240  included in the metallized dielectric waveguide filter  200  of  FIGS.  5 A- 5 B  and the different couplings between these resonant cavities  240 . As shown in  FIG.  6   , the resonant cavities are disposed in two rows R 1 , R 2 , with each row extending generally parallel to a longitudinal axis L of the filter  200 . The main transmission path through the filter  200  extends from the first input/output port  212 - 1  through the seven resonant cavities  240 - 1  through  240 - 7  in numerical order to the second input/output port  212 - 2 , as shown by the solid arrows in  FIG.  6   . The filter has six “main couplings” which refer to the six segments of the solid arrows that extend between pairs of adjacent resonant cavities  240  along the main transmission path. The magnitudes of these main couplings are labelled k 1,2 , k 2,3 , k 3,4 , k 4,5 , k 5,6 , and k 6,7  in  FIG.  6   . There is one cross-coupling between resonant cavities  240 - 2  and  240 - 5  (through cross-coupling window  260 ) which is labelled k 2,5  in  FIG.  6   . The value of the percent coupling k i,j  between two resonant cavities is defined by Equation (2): 
     
       
         
           
             
               
                 
                   ki 
                   , 
                   
                     j 
                     = 
                     
                       
                         
                           
                             f 
                             odd 
                           
                           - 
                           
                             f 
                             even 
                           
                         
                         
                           
                             
                               f 
                               odd 
                             
                             · 
                             
                               f 
                               even 
                             
                           
                         
                       
                       · 
                       100 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where f odd  is the center frequency of the first transmission mode, and f even  is the center frequency of the second transmission mode. 
     The couplings can be characterized by their polarity (positive or negative). Positive couplings (e.g., couplings k 1,2 , k 2,3 , k 3,4 , k 4,5 , k 5,6 , k 6,7 ) may be readily generated by having the magnetic field distributions in the resonant cavities  240  overlap in the vicinity of the main coupling windows  250  that connect adjacent resonant cavities  240 . The magnitude of the coupling may be controlled by the size of the coupling window  250 . Unfortunately, as the size of a main coupling window  250  is increased, the center frequency of the first spurious mode shifts toward the pass band. This may be problematic when it is necessary to have a high level of rejection at frequencies that are close to the pass band. The blind holes  280  may be used to push the center frequency of the first spurious mode higher in frequency in order to at least partially counteract the reduction on the center frequency that occurs when the size of the main coupling window  250  is increased. The use of blind holes  280  also changes the resonant frequency of a resonant cavity  240 , but this may be compensated for by changing the length of the resonant cavity  240 . The provision of blind holes  280  may also increase the magnitude of the negative cross-coupling k 2,5 . Unfortunately, the Q factor of the filter  200  is reduced by the provision of the blind holes  280 , and the decrease in Q factor increases with increasing depths for the blind holes  280 . Thus, the depths of the blind holes  280  may need to be limited to maintain a minimum required Q factor for the filter  200 . 
     As is further shown in  FIG.  6   , three spurious coupling windows  270  are provided that allow for coupling between three additional pairs of resonant cavities  240  that are not adjacent each other along the main transmission path. Applicants have recognized that unintended (and unwanted) coupling may occur between certain pairs of non-adjacent resonant cavities  240  including between resonant cavities  240 - 1  and  240 - 3 , between resonant cavities  240 - 2  and  240 - 4 , and between resonant cavities  240 - 5  and  240 - 7 . This coupling may occur, for example, because some RF energy may enter a particular resonant cavity  240  through a first main coupling window  250  and then leave the resonant cavity  240  through a second main coupling window without resonating (or with reduced resonation) so that this RF energy effectively appears as a small main coupling between two non-adjacent resonant cavities  240 . Applicants believe that these unintended couplings are at least partially responsible for the non-symmetries seen in the insertion loss and return loss curves of  FIG.  4 B . Pursuant to embodiments of the present invention, the spurious coupling windows  270  may allow equal magnitude, oppositely signed couplings to pass between the three pairs of resonant cavities  240  identified above in order to cancel the unintended couplings. While complete or nearly complete cancellation typically optimizes performance, it will be appreciated that the magnitudes of the couplings through the spurious coupling windows  270  need not always be equal magnitude and/or exactly opposite in phase to the above-described unintended couplings. 
     Referring again to  FIGS.  5 A- 6   , it can be seen that metallized openings  230 - 1  and  230 - 3  through  230 - 8  all are generally line-shaped openings that have respective longitudinal axes that extend either generally parallel to or generally perpendicular to the longitudinal axis L of filter  200 . The orientation of these metallized openings  230  is consistent with the orientations of the metallized openings  130  included in the conventional metallized dielectric filter  100  discussed above. Notably, however, metallized opening  230 - 2  has a different, “slant” orientation so that the longitudinal axis of metallized opening  230 - 2  forms an oblique angle with the longitudinal axis L of filter  200 . Moreover, since the sidewalls  220  of filter  200  extend either parallel or perpendicular to the longitudinal axis L, the longitudinal axis of metallized opening  230 - 2  also forms an oblique angle with respect to each of the sidewalls  220 - 1  through  220 - 4 . In the depicted embodiment, the longitudinal axis of metallized opening  230 - 2  forms an angle of about 45° with the longitudinal axis L of filter  200  as well as with sidewalls  220 - 1  and  220 - 2 . In example embodiments, the longitudinal axis of metallized opening  230 - 2  may form an angle of between about 15° and about 75° with the longitudinal axis L of filter  200  as well as with sidewalls  220 - 1  and  220 - 2 . In other example embodiments, the longitudinal axis of metallized opening  230 - 2  may form an angle of between about 30° and about 60° with the longitudinal axis L of filter  200 . 
     Metallized opening  230 - 2  acts as a sidewall of resonant cavities  240 - 1  and  240 - 3 . Since metallized opening  230 - 2  is slanted, resonant cavities  240 - 1  and  240 - 3  each have substantially non-rectangular shapes, unlike the resonant cavities  140  of conventional metallized dielectric filter  100 . The use of a slanted resonant cavity sidewall, combined with configuring the filter  200  to have a rectangular shape, allows for an overall reduction in the rectangular footprint of filter  200  as compared to conventional filter  100 . In particular, since the lower left corner of the dielectric block  110  of filter  100  is “filled in” in the implementation of filter  200 , it becomes possible to extend the width of resonant cavity  240 - 1  as compared to resonant cavity  140 - 1 , which allows the length of resonant cavity  240 - 1  to be reduced as compared to the length of resonant cavity  140 - 1 . Similarly, the additional region of dielectric material added in filter  200  allows resonant cavity  240 - 3  to be shifted to the left (in the views of  FIGS.  5 B and  6   ) as compared to the position of resonant cavity  140 - 3  of filter  100 . Since resonant cavities  240 - 1  and  240 - 3  do not extend as far to the right from sidewall  220 - 2  as do the corresponding resonant cavities  140 - 1 ,  140 - 3  of filter  100 , the overall length of filter  200  may be reduced significantly as compared to the filter  100  (e.g., from about 57 mm to about 48 mm). In addition, a small reduction in the width of filter  200  as compared to the width of filter  100  may also be possible. As a result, the rectangular footprint of filter  200  may be on the order of 20% smaller than the rectangular footprint of filter  100 , which represents a significant savings in terms of space, cost and weight. The slanted metallized opening  230 - 2  facilitates these savings by allowing the additional volume of dielectric material that “fills in” the recess of filter  100  to be split between resonant cavities  240 - 1  and  240 - 3 , and by also directing the resonation of the RF energy within each cavity in a useful manner. 
     It should also be noted that metallized openings  230  are substantially shorter than metallized openings  130  that are included in the conventional filter  100 . Two of the three metallized openings  130  also extend to the edge of the filter  100 . These metallized openings  130  significantly degrade the structural integrity of filter  100  and, in particular, make filter  100  very susceptible to twisting forces that could crack the filter  100  or even break it in half. In contrast, filter  200  has much shorter metallized openings  230  and none of the metallized openings  230  (at least in the depicted embodiment) extend to a sidewall  220  of filter  200 . This provides a much more mechanically robust filter  200 . Additionally, long metallized openings such as the metallized openings  130  of filter  100  are more difficult to manufacture than shorter metallized openings, and hence the filter  200  may also be easier to manufacture than the filter  100 . 
     As shown in  FIGS.  5 A- 5 B , metallized opening  230 - 4  is positioned substantially in the middle of the region of dielectric block  210  that is between resonant cavities  240 - 4  and  240 - 5 . As shown in  FIG.  6   , this design creates a pair of main coupling windows  250 - 4 A,  250 - 4 B (see  FIG.  5 A ) between resonant cavities  240 - 4  and  240 - 5  as opposed to a single coupling window  250 . This design allows the spurious coupling window  270 - 2  to generate a desired amount of coupling between resonant cavities  240 - 2  and  240 - 4 , while also allowing the appropriate amount of coupling between resonant cavities  240 - 4  and  240 - 5 . 
       FIG.  7    is a graph illustrating the simulated return loss and insertion loss for the filter of  FIGS.  5 A- 5 B . As shown in  FIG.  7   , the filter  200  exhibits excellent return loss (curve  290 ) and insertion loss (curve  292 ) performance. As shown in  FIG.  7   , a return loss of less than −19 dB is achieved across the entire pass band, and the return loss rises very sharply just outside of the pass band (to less than −1 dB within about 25 MHz on either side of the pass band). The insertion loss curve also exhibits excellent performance, with the insertion loss being less than 0.6 dB across the full pass band, and two symmetric transmission zeros (60 dB rejection) are provided at about 50 MHz outside of either side of the pass band. Filter  200  also exhibits better out-of-band rejection as compared to filter  100 , which may be important in some applications. The spurious coupling windows are attributed as providing the improved symmetry in the locations and depths of the transmission zeros. 
     Ports  212 - 1  and  212 - 2  are referred to as input/output ports above. This is because in may embodiments the filters may process both transmit and receive signals, and hence RF signals may input to the filter through both input/output ports during normal operation. thus, it will be appreciated that RF energy can flow through the filters according to embodiments of the present invention along the main transmission paths thereof in either direction. 
     Embodiments of the present invention have been described above with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present invention. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (i.e., “between” versus “directly between”, “adjacent” versus “directly adjacent”, etc.). 
     Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer or region to another element, layer or region as illustrated in the figures. It will be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” “comprising,” “includes” and/or “including” when used herein, specify the presence of stated features, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, operations, elements, components, and/or groups thereof. 
     Aspects and elements of all of the embodiments disclosed above can be combined in any way and/or combination with aspects or elements of other embodiments to provide a plurality of additional embodiments.