Abstract:
A duplexing communication signal filter has a prismoid dielectric core having three sets of paired opposed sides. The dielectric core defines at least one through-hole passageway between one set of the pair opposed side and a void in one of the paired sides other than the apertured opposed sides. Present on the core of dielectric material is a surface-layer pattern of metallized and unmetallized areas including a relatively expansive metallized region to provide a reference potential, an unmetallized region surrounding one or more of the apertures, a transmitter pad, a receiver pad spaced apart from the transmitter pad, an antenna pad positioned between the transmitter pad and the receiver pad, and a second unmetallized region on the void to provide an unmetallized void.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to dielectric block filters for radio-frequency signals, and in particular, to monoblock single pass-band and duplexing filters.  
         BACKGROUND  
         [0002]    Ceramic block filters offer several advantages over lumped component filters. The blocks are relatively easy to manufacture, rugged, and relatively compact. In the basic ceramic block filter design, the resonators are formed by typically cylindrical passages, called through-holes, extending through the block from the long narrow side to the opposite long narrow side. The block is substantially plated with a conductive material (i.e. metallized) on all but one of its six (outer) sides and on the inside walls formed by the resonator holes.  
           [0003]    One of the two opposing sides containing through-hole openings is not fully metallized, but instead bears a metallization pattern designed to couple input and output signals through the series of resonators. This patterned side is conventionally labeled the top of the block, though the “top” designation may also be applied to the side opposite the surface mount contacts when referring to a filter in the board mounted orientation. In some designs, the pattern may extend to sides of the block, where input/output electrodes are formed.  
           [0004]    The reactive coupling between adjacent resonators is affected, at least to some extent, by the physical dimensions of each resonator, by the orientation of each resonator with respect to the other resonators, and by aspects of the top surface metallization pattern. Interactions of the electromagnetic fields within and around the block are complex and difficult to predict.  
           [0005]    These filters may also be equipped with an external metallic shield attached to and positioned across the open-circuited end of the block in order to cancel parasitic coupling between non-adjacent resonators and other components of the RF application device.  
           [0006]    Although such RF signal filters have received widespread commercial acceptance since the 1980s, efforts at improvement on this basic design continued.  
           [0007]    In the interest of allowing wireless communication providers to provide additional service, governments worldwide have allocated new higher RF frequencies for commercial use. To better exploit these newly allocated frequencies, standard setting organizations have adopted bandwidth specifications with compressed transmit and receive bands as well as individual channels. These trends are pushing the limits of filter technology to provide sufficient frequency selectivity and band isolation.  
           [0008]    Coupled with the higher frequencies and crowded channels are the consumer market trends towards ever smaller wireless communication devices (e.g. handsets) and longer battery life. Combined, these trends place difficult constraints on the design of wireless components such as filters. Filter designers may not simply add more space-taking resonators or allow greater insertion loss in order to provide improved signal rejection.  
           [0009]    A specific challenge in RF filter design is providing sufficient attenuation (or suppression) of signals that are outside the target passband at frequencies which are integer multiples of the frequencies within the passband. The label applied to such integer-multiple frequencies of the passband is “a harmonic.” Providing sufficient signal attenuation at the third (3 rd ) harmonic has been a persistent challenge.  
           [0010]    Therefore, it would be desirable to provide an RF filter that better attenuates 3 rd  harmonic frequencies without sacrificing other performance parameters such as size, passband insertion loss and material costs.  
         SUMMARY  
         [0011]    This invention overcomes problems of the prior art by providing a ceramic block RF filter having improved 3 rd  harmonic rejection in a small size.  
           [0012]    An embodiment of this invention is a duplexing communication signal filter suitable for use in a mobile communication device and connection to an antenna, a transmitter and a receiver for filtering an incoming signal from the antenna to the receiver and for filtering an outgoing signal from the transmitter to the antenna. The filter comprises a prismoid dielectric core having three sets of paired opposed sides. The dielectric core defines at least one through-hole passageway between one set of the pair opposed sides. The through-hole passageways terminate in opposing apertures to provide a set of apertured opposed sides. The core also defines a void in one of the paired sides other than the apertured opposed sides.  
           [0013]    Present on the core is a surface-layer pattern of metallized and unmetallized regions including a relatively expansive metallized region to provide a reference potential, an unmetallized region surrounding at least one of the apertures, a transmitter pad metallized region, a receiver pad metallized region spaced apart from the transmitter pad metallized region, an antenna pad metallized region positioned between the transmitter pad metallized region and the receiver pad metallized region and a second unmetallized region on the void to provide an unmetallized void.  
           [0014]    The void is preferably elongate having a slot-like shape. When present on a parallelepiped-shaped core, the slot has an orientation such that the slot is perpendicular to the pair of opposed apertured surfaces and parallel to the edges at the interface of the side surfaces.  
           [0015]    In an alternate embodiment of the present invention a signal filter having input and output electrodes is provided. Specifically the filter comprises a rigid core of dielectric material, preferably with a rectangular parallelepiped shape, and a surface-layer pattern of metallized and unmetallized regions supported by the core. The core has a top surface, bottom surface and at least four side surfaces. The core defines a series of through-holes, each extending from an opening on the top surface to an opening on the bottom surface. The core also defines a void on one of the four side surfaces. The surface-layer pattern of metallized and unmetallized regions includes an expansive region of metallization to absorb off-band signals, an unmetallized region substantially surrounding at least one opening, an unmetallized region on the void, an input connection region of metallization and an output connection region of metallization spaced apart from the input connection region.  
           [0016]    There are other advantages and features of this invention which will be more readily apparent from the following detailed description of the preferred embodiment of the invention, the drawings, and the appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0017]    In the Figures,  
         [0018]    [0018]FIG. 1 is an enlarged perspective (or more precisely an isometric) view of a duplexing filter according to the invention;  
         [0019]    [0019]FIG. 2 is an enlarged perspective view of the filter of FIG. 1 showing details of an opposing side surface; and  
         [0020]    [0020]FIG. 3 is an enlarged top side view of FIG. 1.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    While this invention is susceptible to embodiment in many different forms, this specification and the accompanying drawings disclose only preferred forms as examples of the invention. The invention is not intended to be limited to the embodiments so described, however. The scope of the invention is identified in the appended claims.  
         [0022]    Referring to FIGS. 1, 2 and  3 , an antenna duplexer or RF filter  10  includes an elongate, parallelepiped (or “box-shaped”) core of ceramic dielectric material  12 . Core  12  has three sets of opposing side surfaces, a top  14  and a bottom  16 , opposing long sides  18  and  20 , and opposing narrow sides  22  and  24 . The interface between sides  18 ,  20 ,  22  and  24  define parallel edges  26  and a bevel  28 . Bevel  28  facilitates automated part placement during filter fabrication and subsequent application device assembly.  
         [0023]    Core  12  defines a series of six through-hole passageways  30 A through  30 F, which extend from an aperture  34  on top side  14  to a bottom side  16  aperture (not separately shown). Core  12  also defines an elongated void  80  located in side surface  18 . Void  80  is oriented parallel to edges  26  and perpendicular to top and bottom surfaces  14  and  16 . Similarly, an elongated unmetallized void  82  is located in side surface  20 . Slot  82  is aligned parallel to vertical edges  26  and perpendicular to top and bottom surfaces  14  and  16 . When having the preferred elongate, substantially rectangular shape, voids  80  and  82  are conveniently labeled slots.  
         [0024]    Core  12  is rigid and is preferably made of a ceramic material selected for mechanical strength, dielectric properties, plating compatibility, and cost. The ceramic material is preferably a fired, rigid barium-containing ceramic with a dielectric constant in the range of about 25 to about 87, but most preferably 37.5. The preparation of suitable dielectric ceramics is described in U.S. Pat. No. 6,107,227 to Jacquin et al. and U.S. Pat. No. 6,242,376, the disclosures of which are hereby incorporated by reference to the extent they are not inconsistent with the present teachings. Core  12  is preferably prepared by mixing separate constituents in particulate form (e.g., Al 2 O 3 , TiO 2 , Zr 2 O 3 ) with heating steps followed by press molding and then a firing step to react and inter-bond the separate constituents.  
         [0025]    Filter  10  includes a pattern of metallized and unmetallized regions (or areas)  40 . Pattern  40  includes an expansive, wide region of metallization  42 , six unmetallized regions  44 ,  46 ,  48 ,  50 ,  92  and  94 , a transmitter metallized connection pad  52 , a receiver metallized connection pad  56 , and an antenna metallized connection pad  54 .  
         [0026]    Expansive metallized region  42  covers portions of top surface  14  and side surface  18 , and substantially all of bottom surface  16 , side surfaces  20 ,  22 ,  24  and the sidewalls  32  of through holes  30 . Expansive metallized region  42  extends contiguously from within the resonator holes  30  towards both top surface  14  and bottom surface  16 . Region  42  serves as a local ground.  
         [0027]    Core  12  and pattern  40  together form a series of through-hole resonators  25 A, B, C, D, E and F. The portions of expansive metallized region  42  extending around openings  34  of through-holes  30  are can be labeled “resonator pads.” Filter  10  has six through-holes  30  and six corresponding resonator pads  60 A, B, C, D, E and F.  
         [0028]    Pattern  40  includes six unmetallized regions  44 ,  46 ,  48 ,  50 ,  92  and  94  present on portions of top surface  14  and side surface  18 . Unmetallized region  44  substantially surrounds (or circumscribes) resonator pad  60 A and transmitter connection region  52 . Unmetallized region  46  substantially surrounds antenna connection region  54  and resonator pad  60 C. Unmetallized area  48  substantially surrounds receiver electrode  56  and resonator pad  60 F. Unmetallized area  50  substantially surrounds resonator pad  60 B.  
         [0029]    Pattern  40  also includes unmetallized region  92  at void  80  and unmetallized region  94  at void  82 .  
         [0030]    Duplex filter  10  can be divided at an antenna electrode  54  into two branches of resonators  25 , a transmitter branch  72  and a receiver branch  74 . Transmitter branch  72  extends between antenna electrode  54  and end  24 , while receiver branch  74  extends in the opposite direction between antenna electrode  54  and end  22 . Each branch includes a plurality of resonators  25  and a respective input/output electrode. More specifically, transmitter branch  72  includes a transmitter electrode  52 , and receiver branch  74  includes a receiver electrode  56 . Transmitter electrode  52  and receiver electrode  56  are spaced apart from antenna electrode in opposite directions along the length of core  12 .  
         [0031]    Antenna, transmit and receive metallized regions  54 ,  52  and  56  are defined by metallization pattern  40  and extend over portions of both top surface  14  and side surface  18 . These electrodes extend onto side surface  18  where they serve as surface mounting connection points.  
         [0032]    Pattern  40  includes metallized areas and unmetallized areas. The metallized areas are spaced apart from one another and, when filter  10  is in use, are capacitively coupled. The amount of capacitive coupling is roughly related to the size of the metallized regions, the separation distance between adjacent metallized regions, the overall core configuration, and the dielectric constant of the dielectric material. Similarly, pattern  40  also creates inductive coupling between the metallized areas. Interactions of the electromagnetic fields within and around core  12  are complex and difficult to predict.  
         [0033]    The metallized areas of pattern  40  preferably comprise a coating of one or more layers of a conductive metal. A silver-bearing conductive layer is presently preferred. Suitable thick film silver-bearing conductive pastes are commercially available from The Dupont Company&#39;s Microcircuit Materials Division.  
         [0034]    The surface-layer pattern of metallized and unmetallized areas  40  on core  12  may be prepared by providing a rigid core of dielectric material including through-holes to predetermined dimensions. The outer surfaces and through-hole sidewalls are coated with one or more metallic film layers by dipping, spraying or plating.  
         [0035]    The pattern of metallized and unmetallized areas is then preferably completed by computer-automated laser ablation of designated areas on core  12 . This laser ablation approach results in unmetallized areas which are not only free of metallization but also recessed into the surfaces of core  12  because laser ablation removes both the metal layer and a slight portion of the dielectric material.  
         [0036]    Alternatively, selected surfaces of the fully metallized core precursor are removed by abrasive forces such as particle blasting resulting in one or more unmetallized surfaces. The pattern of metallized and unmetallized areas is then completed by pattern printing with thick film metallic paste.  
         [0037]    Filters according to the present invention are optionally equipped with a metallic shield positioned across top surface  14 . For a discussion of metal shield configurations, see U.S. Pat. No. 5,745,018 to Vangala. An important feature of the present invention is the side surface voids  80  and  82 . Void  80  is preferably taken from long side surface  18  in transmitter branch  72 . Most preferably, void  80  is taken from side surface  18  and aligned to a position between through-holes  30 B and  30 C. Void  82  is also preferably located in transmitter branch  72 , and more preferably aligned between through-holes  30 B and  30 C. Specified by reference to the location of the surface mount pads, voids  80  and  82  are both preferably aligned to positions between the antenna connection pad  54  and the transmitter connection pad  52 . The depth, width, and length of voids  80  and  82  can vary.  
         [0038]    Voids  80  and  82  can be formed by grinding, laser ablation, or machining the core  12  to remove a portion of expansive metallized region  42  and a portion of core  12 . Voids  80  or  82  can also be formed as a molded-in feature during the molding of ceramic material making up core  12 . For a molded-in void, a mask is placed over the void space during the metal coating process.  
         [0039]    Filter voids  80  and  82  preferably have a depth in the range of about 3percent to about 10 percent, and more preferably about 4 percent to about 7 percent, based on the thickness of the filter in the direction of the void. Referring to voids  80  and  82 , the thickness of filter  10  is a measure of the distance between side  18  and side  20 .  
         [0040]    Preferred filters according to the present invention exhibit a passband for the outgoing (i.e. transmit) signal from about 1920 MHz to about 1980 MHz with a maximum insertion loss of at most 1.5 decibels (dB) and a 5760 MHz S 21  attenuation of at least 10 decibels (dB), more preferably a 5760 MHz S 21  attenuation of 14 decibels (dB). Preferred filters according to the invention also preferably exhibit a passband for the incoming (i.e. receive) signal from about 2110 MHz to about 2170 MHz with a maximum insertion loss of at most about 2.0 decibels (dB).  
       STUDY EXAMPLES  
       [0041]    A group of ten filters were prepared according to the embodiment shown in FIGS. 1 through 3, and as specified in Table I, below.  
                           TABLE I                                       Filter length (side 24 to side 22)   9.80 mm           Filter board height (side 18 to 20)   1.85 mm           Filter width (side 14 to side 16)   5.30 mm           Through-hole 30    762 microns (μm)           diameter (uniform)           Core dielectric constant   37.5           Outgoing (transmit)   1920 to 1980 MHz           signal passband           Incoming (receive)   2110 to 2170 MHz           signal passband           Side 20 void distance   0.40 mm           from bottom surface 16           SM side 18 void distance   0.25 mm           from bottom surface 16                      
 
         [0042]    These example filters featured one or more elongate voids of varying position, depth, length and width. Presented in Table II, below, are the filter fabrication parameters that were varied for the comparison study.  
                                                                                                                                       TABLE II                           Exam-       Second           ple   First Void Position (μm)   Void Position (μm)   Third Void Position (μm)            Number   P   D   W   L   P   D   W   L   P   D   W   L                    1   A   46   432   4445                                       2   B   83   584   4470       3   C   67   787   4521       4   D   95   610   4470       5   E   61   457   4445       6   C IO     69   610   3302       7   B   43   432   4318   C   43   559   4318       8   C   91   635   4587   D   58   559   4470       9   C   132   864   4496   C IO     71   584   3327       10   C   76   737   4420   D   46   597   4394   C IO     30   571   3556                  
 
         [0043]    In Table II, column label P is a reference to the relative position of the void along the length of the filter  10 . Position markers A, B, C, D and E showing the possible length-wise alignment of the voids are provided in FIG. 3. The position marker subscript IO indicates that the void space was taken from the surface mount side of the filter. Column labels D, L and W are a reference to the void depth, length and width, respectively.  
         [0044]    Example filters  1  through  5  included a single unmetallized void on side  20 , i.e. the side opposite the surface mount regions  54 ,  52  and receiver  56 . Example filter  6  included a single void on the surface mount side  18 , i.e. the I/O side. Example filters  7  and  8  each included two separate voids on side  20 . Example filter  9  included a void on side  20  and a void on surface mount side  18 . Example filter  10  included three voids, two on side  20  and one on surface mount side  18 . The example filters were evaluated by measuring the type  21  Scattering Parameter using a network analyzer. Scattering Parameters were defined and related testing methods were developed to address the complexity of measuring and comparing electric devices for high frequency applications. S-parameters are ratios of reflected and transmitted traveling waves measured at specified component connection points. An S 21  data point or plot is a measure of insertion loss, a ratio of an output signal at an output connection to an input signal at an input connection, at one or a range of input signal frequencies.  
         [0045]    For a discussion of Scattering Parameters and associated test standards and equipment, please consult the following references: Anderson, Richard W. “S-Parameter Techniques for Faster, More Accurate Network Design,”  Hewlett - Packard Journal , vol. 18, no. 6, Feb. 1967; Weinert, “Scattering Parameters Speed Design of High Frequency Transistor Circuits,”  Electronics , vol. 39, no. 18, Sep. 5, 1986; or Bodway, “Twoport Power Flow Analysis Using Generalized Scattering Parameters,” Microwave Joumal , vol. 10, no. 6, May 1967.  
         [0046]    More specifically, each example filter was evaluated by first fabricating a duplexing filter without voids having the passbands specified in Table I. The filter without voids was then tested to obtain an S 21  plot. Selected S 21  data points were recorded and are presented in TABLE III, below, under the row heading “ctrl.” After control testing, one or more unmetallized voids were added by laser ablation to the tested filter as specified in TABLE II.  
         [0047]    The void-added example filters were then retested to obtain a second S 21  plot. S 21  data were recorded and are presented in TABLE III, below, next to the corresponding control measurements.  
                                                             TABLE III                           Transmit   Receive                       Maximum   Maximum   Attenuation   Attenuation   Attenuation       Example   Insert. Loss   Insert. Loss   @ 3960 MHz   @ 5760 MHz   @ 5940 MHz       No.   (dB)   (dB)   (dB)   (dB)   (dB)                                1 ctrl   1.2   1.6   34.9   5.9   7.2           1.2   1.6   32.2   5.3   8.3       2 ctrl   1.3   1.67   34.6   6.5   8.2           1.3   1.67   35.5   7.8   7.6       3 ctrl   1.13   1.74   34.8   5.6   6.1           1.4   1.75   37.5   8.6   3.2       4 ctrl   1.25   1.89   34.2   6.7   7.7           1.23   1.99   37.5   16.9   7.9       5 ctrl   1.1   1.76   34.5   6.8   7.6           1.1   2.0   33.6   9.6   8.7       6 ctrl   1.16   1.68   35.3   6.9   7           1.27   1.69   35.2   10.8   5       7 ctrl   1.2   1.64   35.6   6   6.1           1.3   1.6   37.1   6.9   3.6       8 ctrl   1.3   1.64   34.3   5.0   7.5           1.37   1.65   37   19   4.6       9 ctrl   1.1   1.66   34.8   6.4   4.9           1.24   1.67   37   14.4   11       10 ctrl    1.14   1.68   35.4   6.8   7.8           1.34   1.69   38.3   22.5   9.4                  
 
         [0048]    S 21  data were recorded for the maximum insertion loss over the transmit passband (1920-2980 MHz), the maximum insertion loss over the receive passband (2110-2170 MHz), two times the high end of the transmit passband (3960 MHz), three times the low end of the transmit passband (5760 MHz) and three times the high end of the transmit passband (5940 MHz).  
         [0049]    Example 9 was identified as the preferred embodiment. Example 9 exhibited a significant improvement in attenuation at the harmonic target frequencies and only minor additional signal losses in the transmit and receive passbands. A specification for Example 9 is presented below in TABLE IV.  
                           TABLE IV                                       Filter length   9.80 mm           Filter height   1.85 mm           Filter width   5.30 mm           Through-hole 30    762 microns (μm)           diameter (uniform)           Core dielectric constant   37.5           Outgoing (transmit)   1920 to 1980 MHz           signal passband           Incoming (receive)   2110 to 2170 MHz           signal passband           Void 82 distance   0.40 mm           from bottom surface 16           Void 82 length   4.50 mm           Void 82 width   0.86 mm           Void 82 depth   0.13 mm           Void 80 distance   0.25 mm           from bottom surface 16           Void 80 width   0.58 mm           Void 80 length   3.33 mm           Void 80 depth   0.07 mm                      
 
         [0050]    Numerous variations and modifications of the embodiments described above may be effected without departing from the spirit and scope of the novel features of the invention. It is to be understood that no limitations with respect to the specific system illustrated herein are intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.