Abstract:
The present invention is a preferred monoblock ceramic bandpass duplexer filter. The preferred filter has at least three I/O pads. One of the pads is coupled to an antenna, another is connected to a transmission circuit and the last pad is connected to a receive circuit. The filter is comprised of two sections: a transmission section and a receive section. The transmission and receive sections include resonators disposed on respective sides of the antenna pad. A first alternative signal path is disposed adjacent the ends of the transmission resonators. A second alternative signal path is disposed adjacent to the ends of the receive resonators. Each alternative signal path couples adjacent and non-adjacent resonators. A further feature of the filter of the present invention includes a shunt zero resonator for the transmission section. To the contrary, the present invention allows the elimination of a shunt zero resonator for the receive section of the filter.

Description:
FIELD OF THE INVENTION 
     This invention relates to electrical filters and, in particular, to dielectric filters that provide increased attenuation proximate to the desired passband. 
     BACKGROUND OF THE INVENTION 
     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 cylindrical passages, called 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. 
     One of the two opposing sides containing holes 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. In some designs, the pattern may extend to sides of the block, where input/output electrodes are formed. 
     The reactive coupling between adjacent resonators is dictated, 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 are complex and difficult to predict. These 
     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 to achieve acceptable stopbands. 
     Although such RF signal filters have received wide-spread commercial acceptance since the 1970s, efforts at improvement on this basic design continued. 
     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. 
     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. 
     Therefore, the need continues for improved RF filters which can offer selectivity and other performance improvements, without increases in size or cost of manufacturing. This invention overcomes the size-to-selectivity compromise by providing a ceramic block RF filter having adaptable selectivity with a robust, relatively low cost control mechanism and relatively low insertion loss. 
     SUMMARY OF THE INVENTION 
     The present invention is a preferred duplexer filter that is a monolith (also referred to as a monoblock) of a dielectric ceramic that defines a plurality of resonators. The preferred filter has at least three input/output (I/O) pads. One of the pads is coupled to an antenna, another is connected to a transmission circuit and the last pad is connected to a receive circuit. The filter is comprised of two sections: a transmission section and a receive section. The transmission and receive sections include resonators disposed on respective sides of the antenna pad. 
     The filter of the invention also includes a first alternative signal path adjacent the ends of the transmission resonators. A second alternative signal path is disposed adjacent to the ends of the resonators. Each alternative signal path couples adjacent and non-adjacent resonators. A further feature of the filter of the present invention includes a shunt zero resonator for the transmission section. To the contrary, the present invention allows the elimination of a shunt zero resonator for the received section of the filter. 
     Specified more generally, a preferred RF signal filter according to the present invention includes a block of dielectric material having an input electrode and an output electrode spaced apart along the length of the block. The block defines an array of through-hole resonators extending between the input electrode and the output electrode. A resonator by-pass electrode extends from a position adjacent a first resonator of the array to a position adjacent a second resonator of the array. The first and second resonators are separated by at least one resonator of the array such that the by-pass electrode provides a parallel signal pathway between the first and second resonators. 
     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 
     In the FIGURES, 
     FIG. 1 is a perspective view of a filter incorporating the present invention; 
     FIG. 2 is a block schematic for the FIG. 2 filter; 
     FIG. 3 is a frequency response graph for RF signals around a U.S. PCS transmit band showing the performance of a ceramic duplexer filter according to the present invention and the performance of a conventional duplexer; 
     FIG. 4 is a frequency response graph for RF signals around a U.S. PCS receive band showing the performance of a ceramic duplexer filter according to the present invention and the performance of a conventional duplexer; 
     FIG. 5 is an enlarged fragmentary plan view of the transmitter section of the dielectric block filter of FIG. 2 with markings for specifying preferred dimensions; and 
     FIG. 6 is an enlarged fragmentary plan view of the transmitter section of a dielectric block filter according to an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     Referring to FIG. 1, the preferred embodiment of a filter  100  is shown. Filter  100  includes a block  110  which is comprised of a dielectric material that is selectively plated with a conductive material. Block  110  has a top surface  112 , a bottom (not separately shown) and sides, such as side  120 . The filter  100  can be constructed of a suitable dielectric material that has a low loss, a high dielectric constant and a low temperature coefficient of the dielectric constant. 
     The plating on block  110  is electrically conductive, preferably copper, silver or an alloy thereof. Such plating preferably covers all surfaces of the block  110  with the exception of a top surface  112 , the plating of which is described below. Of course, other conductive plating arrangements can be utilized. See, for example, those discussed in “Ceramic Bandpass Filter,” U.S. Pat. No. 4,431,977, Sokola et al., assigned to the present assignee and incorporated herein by reference to the extent it is not inconsistent. The plating is preferably coupled to a reference potential. 
     Block  110  includes nine holes  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108  and  109  ( 101 - 109 ), each extending from top surface  112  to a bottom surface (not shown) thereof. The surfaces defining holes  101 - 109  are likewise plated with an electrically conductive material. Each of the plated holes  101 - 109  is essentially a transmission line resonator comprised of a short-circuited coaxial transmission line having a length selected for desired filter response characteristics. For an additional description of the holes  101 - 109 , reference may be made to U.S. Pat. No. 4,431,977, Sokola et al., supra. Although block  110  is shown with nine plated holes  101 - 109 , the present invention is not limited to such. In fact, any number of plated holes greater than two can be utilized depending on the filter response characteristics desired. 
     According to the present invention, top surface  112  of block  110  is selectively plated with an electrically conductive material similar to the plating on block  110 . The selective plating includes input-output I/O pads, specifically transmit (Tx) electrode  114 , antenna (ANT) electrode  116  and receive (Rx) electrode  118 . Also included is plating  121 ,  122 ,  123 ,  124 ,  125 ,  126 ,  127 ,  128  and  129  ( 121 - 129 ) that surrounds holes  101 - 109  and ground plating  130 ,  132  and  134 . Finally, according to the present invention, alternative signal paths  136  and  138  are included in the selective plating on top surface  112 . 
     Plating  121 - 129  is used to capacitively couple the transmission line resonators, provided by the plated holes  101 - 109 , to ground plating  130 ,  132 ,  134  on top surface  112  of block  110 . Portions of plating  121 - 129  also couple the associated resonator of holes  101 - 109  to transmit electrode  114 , antenna electrode  116  and receive electrode  118 . Furthermore, alternative signal paths  136 ,  138  couple adjacent and non-adjacent proximate resonators of holes  101 - 109  through associated plating  121 - 129 . Plates  121 - 125 , holes  101 - 105 , ground plating  132 , alternative signal path  136  and transmit electrode  114  together make up a transmit section of duplexer filter  100 . Plates  126 - 129 , holes  106 - 109 , ground plating  134 , alternative signal path  138  and receiver electrode  118  together make up a receive section of filter  100 . 
     Coupling between the transmission line resonators, provided by the plated holes  101 - 109  in FIG. 1, is accomplished at least in part through the dielectric material of block  110  and is varied by varying the width of the dielectric material and the distance between adjacent transmission line resonators. The width of the dielectric material between adjacent holes  101 - 109  can be adjusted in any suitable regular or irregular manner, such as, for example, by the use of slots, cylindrical holes, square or rectangular holes, or irregular shaped holes. Furthermore, plated or unplated holes located between the transmission line resonators  101 - 109  can also be utilized for adjusting the coupling. 
     In addition, the plating  121 - 129  causes capacitive coupling between adjacent holes  101 - 109 . In light of that, the non-linear periphery of plates  121 - 129  is chosen to increase the capacitive coupling. Since capacitive coupling is also a function of distance, the periphery of plates  121 - 129  can be moved closer to the other plate of the capacitive coupling. As a result, if desired, the periphery can be made more linear. Such alteration of the periphery and distance is determined from the desired coupling. 
     This coupling between the transmission line resonators is shown diagrammatically in FIG.  2 . Circuit  200  represents a partial circuit model of filter  100  in FIG.  1 . Circuit (or filter)  200  includes a transmitter (Tx) section  210  and a receiver (Rx) section  205 . Both sections  205  and  210  include resonators (R)  215 , inter-resonator couplings (K)  220 , I/O couplings  225  and alternative signal paths  230 . Inter-resonator couplings  220  represent the capacitive coupling between plates  121 - 129  (of FIG.  1 ). I/O couplings  225  represent capacitive coupling between transmit electrode  114 , antenna electrode  116  and receive electrode  118 , and plating  121 - 129  (of FIG.  1 ). Transmitter section  210  additionally includes a shunt zero  235 , which includes a resonator  215  and an I/O coupling  225 . Sections  205  and  210  are coupled to a preferred antenna through I/O coupling  250 . 
     Alternative signal paths  230  each include, as shown, alternative path couplings  240  and transmission lines (TLINE)  245 . Alternative path couplings (KAPc)  240  represent the capacitive coupling between plating  121 - 129  and alternative signal paths  136 ,  138  (of FIG.  1 ). Couplings  240  and lines  245  electrically couple resonators  215  in parallel. To illustrate this parallel coupling, a resonator  215  is coupled through node  265  and a coupling  240  to node  255 . Node  255  is coupled in parallel through line  245 , coupling  240  and node  260  to a second resonator  215 , and through lines  245 , coupling  240  and node  270  to a third resonator  215 . 
     In a different perspective, nodes  260  and  265  are directly coupled as shown by a path line  275 . Path line  275  traverses couplings  240  and line  245 . In addition, nodes  265  and  270  are directly coupled as shown by path line  280 . Path line  275  traverses couplings  240  and lines  245 . Thus, according to the present invention, alternative signal paths  236 ,  238  provide additional coupling among resonators  215 . With the use of either alternative signal paths  230  ( 136  and  138  in FIG.  1 ), adjacent and non-adjacent resonators  215  that are proximate to said paths are coupled together. 
     Operationally, if node  285  provides a received signal as an output, lead  290  is coupled to an antenna and node  295  receives a transmit signal, then circuit  200  of FIG. 2 has transmitter section  210  exhibiting a four-pole passband generated by resonators  215 , three transmission zeroes generated by alternative signal path  230  proximate to resonators  215 , a shunt zero generated by shunt zero  235  and an alternative path zero generated by alternative signal path  235 . Receiver section  205  has a four-pole passband generated by resonators  215 , three transmission zeroes generated by alternative signal path  230  proximate to resonators  215  and an alternative path zero generated by alternative signal path  236 . 
     FIG. 5 is an enlarged fragmentary plan view of the transmitter section of the top of the dielectric block filter of FIG. 2 with markings W, G, and L for specifying preferred dimensions. The following corresponding list defines the preferred dimensions (in mils or 0.001″) of electrodes and spaces about the transmitter alternative signal path for an 1800 Mhz PCS duplexer: 
     3≦W 1 , W 2 , W 3 ≦12 
     3≦G 1 , G 2 , G 3 ≦15 
     3≦G 4 , G 5 , G 6 ≦15 
     50≦L 1 ≦500 
     10≦Block E R≦ 120 
     3≦W 4 , W 5 , W 6 ≦60 
     1≦W 7 , W 8 , W 9 ≦60 
     These dimensions are preferred for a US PCS duplexer (1800 Mhz) having an overall length of about 19.5 mm, an overall width of about 4 mm, and an overall height of 7.25 mm. 
     FIG. 6 shows a modification of transmitter alternative signal path  136  of FIG.  1 . Bar  636  is comprised of three portions  636   a ,  636   b  and  636   c  as shown. For this modification, each of those three portions is composed of a different composition. This in turn will provide a method(of varying the coupling between the portions of bar  636  and proximate plates  123 ,  124  and  125 . 
     Although the present invention is exemplified by a monoblock structure, duplexer ceramic bandpass filter described above, many variations exist that are contemplated to be within the present invention. To illustrate, a filter having only a receive or transmit section can utilize the present invention. Also, whether the filter is a duplexer or not, the number of holes should be at least three. If desired, a shunt zero resonator can be added to the receive section of the filter. 
     The present invention can be used with structures that separately formed resonators that are then used as a band pass or band stop filter. An alternative signal path can be formed by using discrete components between each separate resonator. However, if the resonators are connected, then the alternative signal path may be disposed as described for the preferred embodiment. 
     For both alternative signal paths, the geometry can be changed. To illustrate, each bar can be configured in a U-shape, an L-shape, a convex or concave arc, or with a nonlinear periphery like a zigzag, an undulation, a wave or a comb. Furthermore, the configuration can be changed for portions of the bar, while other portions have a different configuration. As stated above, the bar can include portions having different compositions. Any configuration may be considered to achieve the desired coupling. In addition, the alternative signal path can be comprised of metallization and discrete components. Such components can be wires, capacitors, resistors and inductors. 
     Moreover, the present invention can utilize more than one alternative signal path for the transmit or receive sections. To illuminate, another alternative signal path can be placed adjacent to plates  123 ,  124  and  125  on the opposite side of alternative signal path  136  in FIG.  1 . Or the other alternative signal path can be placed adjacent to plates  122 , 123  and  124  on the opposite side of alternative signal path  136 . A similar additional alternative signal path can be placed in the receive section of filter  100 . 
     Working Example 
     A ceramic duplexer filter for US PCS was fabricated as shown in in FIG. 1 for testing and comparison. The prepared FIG. 1 duplexer included a shield in accordance with the disclosure of U.S. Pat. No. 5,745,018 to Vangala, which is herein incorporated by reference to the extent it is not inconsistent. The frequency response of the improved duplexer about the US PCS transmit and receive bands was graphed together with a conventional duplexer designed for the same frequencies. 
     FIG. 3 is a frequency response graph for RF signals around a U.S. PCS transmit band showing the performance of a ceramic duplexer filter according to the present invention and the performance of a conventional duplexer A line  300  shows the transmit band performance of the conventional duplexer filter, i.e. without an alternative signal path  136 . The conventional transmitter section provides a passband  310 , a low-side zero  315  and a high-side zero  320 . Line  305  is the transmit band response of the improved duplexer which includes a alternative signal path  136 . Zeroes  315  and  320  are shifted to  315 ′ and  320 ′ to provide zeroes closer to passband  310 ′. Note that the associated passband  310 ′ extends over a greater range of frequency with a flatter attenuation curve than passband  310 . The advantages of the present invention can be seen from the graph in FIG.  3 . The use of the present invention provides better attenuation closer to the passband than a filter without the present invention. 
     FIG. 4 is a frequency response graph for RF signals around a U.S. PCS receive band showing the performance of a ceramic duplexer filter according to the present invention and the performance of a conventional duplexer. Line  400  is the receive bandfrequency response of the conventional duplexer, i.e. without a duplexer filter without alternative signal path. The represented receiver portion has a passband  410 , a low-side zero  415  and a high-side zero that extends off the graph. Line  405  shows the performance provided by the receiver section of the improved PCS duplexer filter according to the present invention. Zero  415  and the high-side zero are moved to  415 ′ and  420 ′ to provide zeroes closer to passband  410 ′. Note that the associated passband  410 ′ extends over a greater range of frequency than passband  410 . Thus, the use of the present invention provides better attenuation closer to the passband than a filter without the present invention. 
     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. 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.