Abstract:
A filter with an equivalent circuit that functions as well as physically larger filters without substantial drop off in performance.

Description:
FIELD OF THE INVENTION 
     This invention enables development and production of high electrical performance filters in sizes much smaller than what is capable with existing technologies, using an improved equivalent circuit. 
     BACKGROUND OF THE INVENTION 
     A ceramic body with a coaxial hole bored through its length forms a resonator that resonates at a specific frequency determined by the length of the hole and the effective dielectric constant of the ceramic material. The holes are typically circular, or elliptical. A dielectric ceramic filter is formed by combining multiple resonators. The holes in a filter must pass through the entire block, from the top surface to the bottom surface. This means that the depth of hole is the exact same length as the axial length of a filter. The axial length of a filter is set based on the desired frequency and available dielectric constant of the ceramic. 
     The ceramic block functions as a filter because the resonators are coupled inductively and/or capacitively between every two adjacent resonators. These components are formed by the electrode pattern which is designed on the top surface of the ceramic block couplings and plated with a conductive material such as silver or copper. 
     Ceramic filters are well known in the art and are generally described for example in U.S. Pat. Nos. 4,692,726, 4,823,098, 4,879,533, 5,250,916 and 5,488,335, all of which are hereby incorporated by reference as if fully set forth herein. 
     With respect to its performance, it is known in the art that the band pass characteristics of a dielectric ceramic filter are sharpened as the number of holes bored in the ceramic block are increased. The number of holes required depends on the desirable attenuation properties of the filter. Typically a simplex filter requires at least two holes and a duplexer needs more than three holes. This is illustrated in FIG. 9 where graph  10  represents the filter response with fewer holes than graphs  12  and  14 . It is apparent that graph  14  which is the response of the filter with the most holes, is the sharpest of the three responses shown. Referring to FIG. 10, it can be seen that the band pass characteristic of a particular dielectric ceramic filter is also sharpened with the use of trap holes bored into the ceramic block. Solid line graph  21  represents the response of a filter without a high end trap. Dashed line graph  23  represents the response of the same filter with a high end trap. 
     Trap holes, or traps as they are commonly referred to, are resonators which resonate at a frequency different from the primary filter holes, commonly referred to simply as holes. They are designed to resonate at undesirable frequencies. Thus, the holes transmit signals at desirable frequencies while the traps remove signals at the undesirable frequencies, whether low end or high end. In this manner the characteristic of the filter is defined, i.e. high pass, low pass, or band pass. The traps are spaced from holes a distance greater than the spacing between holes so as to avoid mutual interference between the holes and traps. As shown in FIG. 11, whereas holes  31  are separated from each other a distance equal to D, a distance of 2D is placed between trap  33  and the transmission hole nearest to trap  33 . The precise distance between trap and transmission pole is one of design choice for achieving a specified performance, but it is preferably 1 to 10 mm. Traditionally, the traps will be spaced from 1.5D to 2D from the holes. 
     Conventionally the holes  41  and traps  43  in a ceramic filter are positioned along a straight line. This design together with the spacing requirements addressed above limits the extent to which a filter may be reduced in size. Specifically, the performance characteristics of a given filter are a function of its width, length, number of holes and diameter of holes. The usual axial length L is 2 to 20 mm. The width w is determined by the number of holes. The usual width of the block filter is 2 to 70 mm. Reducing the number of holes, the diameter of the holes, or the spacing between holes, will effect the performance. Accordingly, it is desirable to have a design for a dielectric ceramic filter which can effectively reduce the size of a given filter while maintaining its given performance characteristics. 
     Equivalent circuits are generally those circuits with the same overall current, impedance, phase, and voltage relationships as a more-complicated counter part that it usually replaces. 
     There is a need for dielectric ceramic filters used in advanced communication applications such as CDMA and TDMA cellular phones with higher electrical performances and a smaller physical size. However the existing methods to develop a filter with higher electrical performance is to add additional transmission poles and/or trap resonators in a filter, which causes an increase in the size of the new filter. 
     SUMMARY OF THE INVENTION 
     This invention describes a new design for increasing the electrical performance without increasing the size of a high performance ceramic filter. To achieve this purpose, this invention describes a new equivalent circuit of dielectric ceramic filter with a new printed pattern on the filter to realize the new equivalent circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a typical equivalent circuit of a prior art filter. 
     FIG. 2 illustrates the typical printed pattern of a prior art filter designed in accordance with the equivalent circuit of FIG.  1 . 
     FIG. 3 illustrates the equivalent circuit for a filter designed in accordance with the present invention. This new equivalent circuit design has a similar electronic performance as the prior art filter of FIG. 1, but is physically smaller. 
     FIGS. 4A-4B illustrate one preferred embodiment of a printed pattern for a filter designed to perform as the equivalent circuit of FIG.  3 . 
     FIG. 5 compares the similarity in electrical performance between the filter designed in accordance with the present invention shown in FIG. 3 and a prior art filter, as shown in FIG.  1 . 
     FIG. 6 illustrates the equivalent circuit for a duplexer designed in accordance with another embodiment of the present invention. 
     FIGS. 7A-7B illustrates one preferred embodiment of a printed pattern for a duplexer designed to perform as the equivalent circuit of FIG.  6 . FIGS. 7C-7D,  7 E- 7 G,  7 G- 7 H and FIGS. 7J-7K and additional preferred embodiments and their equivalent circuits. 
     FIG. 8 illustrates another preferred embodiment of a printed pattern for a filter designed to perform as the equivalent circuit of FIG.  3 . This filter has two (2) transmission poles and one (1) trap resonator, but it can work as a filter with three (3) transmission poles and one (1) trap resonator. 
     FIG. 9 illustrates the increased sharpness of the band pass response of a dielectric ceramic filter as the number of holes in the filter increase. 
     FIG. 10 illustrates the effectiveness of traps in removing high end frequencies. 
     FIG. 11 is representative of the spacing between holes and hole and trap on a conventional ceramic block filter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In prior art filters, as shown in FIGS. 1 and 2, transmission holes  10  are separated from each other a distance equal to D (FIG. 2) and a distance of 2D (FIG. 2) is placed between trap holes  12  and the nearest transmission hole  10 . The precise distance is a design choice for acheiving a specific performance. However, the need for trap holes with their requisite spacing requirements in a filter adds a significant constraint to the degree to which the filter can be made smaller. 
     One embodiment of the invention is a filter with 4 transmission poles and 2 trap resonators (total 6 holes), shown in FIGS. 4A-4B. Capacitances C 1 , C 2  and C 3  are shown in FIG.  4 B. 
     C 1  is the capacitance of coupling between input/output electrode and resonator θ 1 ; C 2  is the capacitance of coupling between θ 1  and θ 2 ; and C 3  is the capacitance of coupling between input/output electrode and resonator θ 2 . Z is the inductance of coupling between θ 1  and η 2 . The shaded portion of the electric pattern, weakens C 2 . As a result of the weakened C 2 , Z is relatively strengthened. 
     Resonator θ 1  functions as a transmission pole by the coupling of Z and C 2 , so that θ 1  can compose 5 transmission poles by cooperation with the other 4 transmission poles of θ 2 , θ 3 , θ 4  and θ 5 . (See FIG.  3 ). 
     Furthermore, θ 1  also functions as a trap resonator by adjusting the coupling of C 1 , C 2  and C 3  as to be C 1 &gt;C 3 &gt;C 2 . Thus, θ 1  can work as both a transmission pole and a trap resonator. Due to the unique pattern of the filter, θ 1  can act as both a trap resonator and transmission pole, thus reducing filter size by eliminating one transmission pole. (See FIGS.  3  and  4 A). 
     This means higher electrical performance can be achieved while having a smaller filter size by using this new design of equivalent circuit. 
     A new electrode pattern of conductive material was developed, as shown in FIGS. 4A and 4B to realize the effect of the new equivalent circuit. Each value of W, L, X 1  and Y 1  in FIG. 4A are the following ranges. 
     W: 0.5 mm≧W≧0.1 mm 
     L: 3.0 mm≧L≧0.5 mm 
     X 1 : 4.0 mm≧X 1 &gt;1.0 mm 
     Y 1 : 2.0 mm≧Y 1 ≧0 mm 
     FIG. 4B shows parameters C 1 , C 2  and C 3 . C 1  is controlled by the distance between pattern  1  of conductive material for input/output electrode and pattern  3  of conductive electrode connected to conductive material on the inner surface of hole of θ 1  resonator (FIG.  3 ), and C 3  is controlled by the distance between pattern  1  and pattern  3  of conductive material connected to conductive material on the inner surface of hole θ 2  resonator (FIG.  3 ). C 1 , C 2  and C 3  are capacitances of coupling as described above in FIG. 4B. Z is an inductive coupling and is controlled by the pattern  2  of conductive material that is opposed to the pattern  1  and is connected to the conductive material on the side wall. The relationship of C 1 , C 2  and C 3 , to each other is as follows, C 1 &gt;C 3 &gt;C 2 . 
     FIG. 5 shows the electrical data of the filters developed by the existing technology and by our new technology along with the requested specification. Although the present invention&#39;s filter is smaller, due to the less amount of holes, than currently available filters, its performance matches the electrical performance of larger filters using presently available technology. The electrical performance of the present invention (the filter of FIG. 3) is represented by the rigid lines as is shown in FIG.  5 . The electrical performance of a prior art filter (the filter of FIG. 2) is represented by the broken line as shown in FIG.  5 . 
     We can also apply the concepts of this new filter technology to a duplexer. FIGS. 7A-7B is an embodiment of a printed pattern duplexer of the present invention. FIG. 6 is its equivalent circuit for a duplexer designated in accordance with another embodiment of the present invention. FIG.  6  and FIGS. 7A-7K show examples of new equivalent circuits and printed patterns, as applied to a duplexer. The duplexer of FIG.  6  and FIGS. 7A-7B has eight (8) transmission poles including four (4) transmission poles  20 , four (4) transmission poles, θ 2 , θ 3 , θ 4  and θ 5 , and three (3) trap resonators, including trap resonators  40  on each end of the duplexer and trap resonator θ 1 , but it can work as a filter with nine (9) transmission poles, and three (3) trap resonators, in which θ 1  serves as both a transmission pole and a trap resonator. In most cases, the higher band is the receiver band and the lower band is the transmitter band at the mobile phone terminal sides. These designations become reversed at the base station sides. However, it is noted that the relationship of the receiver band and the transmitter band, on the one hand, and the higher/lower bands on the other hand are not always consistent. 
     Each value of W, L, X 1  and Y 1  for the duplexer filter are the following ranges. 
     W: 0.5 mm≧W≧0.1 mm 
     L: 3.0 mm≧L&gt;0.5 mm 
     X 1 : 4.0 mm≧X 1 ≧1.0 mm 
     Y 1 : 2.0 mm≧Y 1 ≧0 mm 
     The relationship of C 1 , C 2  and C 3 , to each other is as follows, C 1 &gt;C 3 &gt;C 2 . C 1 , C 2  and C 3  are shown on FIG.  7 B. 
     In particular, FIGS. 7C,  7 D,  7 E,  7 F,  7 G,  7 H,  7 J, and  7 K allow for the concept of a resonator θ 1  working as both a transmission pole and as a trap resonator. Such a resonator θ 1  allows for a duplexer that requires minimal space. The resonator θ 1  acts as a transmission pole and as a trap resonator because of the unique relationship between the capacitances of capacitance couplings C 1 , C 2  and C 3 , in the manner as is described for FIGS. 4B and 7B above. The unique pattern of the duplexers allows for the resonator θ 1  to act as both a trap resonator and a transmission pole. In particular, FIGS. 7C,  7 D,  7 E,  7 F,  7 G,  7 H,  7 J, and  7 K show that using the inventive patterns taught in the present application, one may vary the number of transmission poles and trap holes as desired and still obtain a duplexer that is smaller in size than traditional duplexers because of a resonator acting as a trap hole and trap resonator. 
     FIG.  7 C and corresponding equivalent circuit in FIG. 7D show 8 transmission poles  20  and a resonator θ 1 , which acts as both a transmission pole and a trap resonator due to the relationship of capacitance couplings C 1 , C 2  and C 3  and inductance Z. FIG.  7 E and corresponding equivalent circuit in FIG. 7D show 7 transmission poles  20 , a trap resonator  40  and a resonator θ 1 , which acts as both a transmission pole and a trap resonator due to the relationship of capacitance couplings C 1 , C 2  and C 3  and inductance Z. FIG.  7 G and corresponding equivalent circuit in FIG. 7H show 5 transmission poles  20 , 2 trap resonators  40  and resonator θ 1 , which acts as both a transmission pole and a trap resonator due to the relationship of capacitance couplings C 1 , C 2  and C 3  and inductance Z. FIG.  7 J and corresponding equivalent circuit in FIG. 7K show 5 transmission poles  20 , a trap resonator  40  and a resonator θ 1 , which acts as both a transmission pole and a trap resonator due to the relationship of capacitance couplings C 1 , C 2  and C 3  and inductance Z. 
     It should be noted that capacitance couplings C 1 , C 2  and C 3  work in a manner similar to that described for FIG. 4B above to allow for a resonator θ 1  to wok as both a transmission pole and a trap resonator to allow for a reduced-size duplexer. 
     FIG. 8A illustrates another embodiment of the present invention, with FIG. 8B showing the equivalent circuit. This figure has two (2) transmission poles and one (1) trap resonator, but it can work as a filter with three (3) transmission poles and one (1) trap resonator. 
     In particular, FIG. 8A shows resonators θ 1 , θ 2  and θ 3 , with θ 1  acting as both a transmission pole and a trap resonator because of the relationship between C 1 , C 2  and C 3  as described above. 
     According to the above results, this new filter technology can be applied to many filters and duplexers which are of a smaller size with higher electrical performance than currently available filters. The foregoing merely illustrates the principles of the present invention. Those skilled in the art will be able to devise various modifications, which although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope.