Abstract:
A high-frequency filter whose main structure is formed by stacking a plurality of patterned substrates. The high-frequency filter includes a metal grounding layer, a signal input port, a signal output port, and a plurality of resonator layers having resonators coupled to one another for transmitting signals. By utilizing coupling among resonators located on adjacent layers respectively instead of coupling among resonators on a single layer in the prior planar patterned filter, the structure of the high-frequency filter in the invention can be changed according to process limitations.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     a) Field of the invention  
         [0002]     The present invention relates to a high-frequency filter and, more particularly, to a high-frequency filter with a patterned multi-layer structure.  
         [0003]     b) Description of the Related Art  
         [0004]     As electronic equipments miniaturize, their internal elements develop toward having a compacted aggregation. Thus, in order to integrate with other elements, a method for forming filters on planar circuit boards has been made available.  FIG. 1A  shows a top view of a conventional interdigital planar filter.  FIG. 1B  shows a sectional view of the filter taken along the line A-A′ as shown in  FIG. 1A . As shown in  FIGS. 1A and 1B , the conventional interdigital planar filter is formed by patterning substrates with metal materials. The conventional interdigital planar filter includes a metal grounding surface  16 , a substrate (such as a printed circuit board)  15 , a plurality of resonators  11  formed on the substrate  15 , an input port  12 , and an output port  13 . The resonators  11  are electrically connected to the metal grounding surface  16  via through holes  14  and thereby shorted to ground; the through holes  14  of adjacent resonators are located at opposite ends (interdigitated). Furthermore, the adjacent resonators  11  can couple with each other.  
         [0005]     A signal is inputted into the input port  12  and transmitted to the resonator  11  connected thereof. The signal is then transmitted to the resonator  11  connected to the output port  13  through couplings between adjacent resonators  11  sequentially. At last, the signal is sent out from the output port  13 . Generally, the distance d between adjacent resonators  11  greatly affects the performance of the interdigital planar filter because it can determine the coupling strength between the two resonators. Common printed circuit boards have a lower dielectric constant (less than 5), which causes their electric field to be more spread out; therefore, if these printed circuit boards are used as substrates to make interdigital planar filters, couplings between adjacent resonators are easier to occur and the distance therebetween is often greater.  
         [0006]     Thus, a technology using ceramic materials as substrate has been developed for the purpose of minimizing filter sizes. However, ceramic materials have a very high dielectric constant (usually greater than 7.8). If interdigital planar filters are to be made on ceramic materials, adjacent resonators have to be in extreme proximity in order to accomplish the required coupling strength; the distance d has to be smaller than 100 μm, which is not feasible with the current processing technology. Hence, it is difficult to fabric interditial planar filters on ceramic substrates due to the essential spacing required.  
         [0007]     In view of the above problem, a filter that is able to change its structure to accommodate process limitations would solve the problems; at the same time, the filter can be made efficiently on ceramic substrates and the size of the filter can be minimized.  
       SUMMARY OF THE INVENTION  
       [0008]     An object of the invention is to provide a high-frequency filter, in which its structure can be adjusted in regards to the process limitations, and the high-frequency filter is easily made on ceramic substrates leading to minimization of the filter.  
         [0009]     The first embodiment of the invention provides a high-frequency filter composed of a plurality of patterned substrates stacked together, and each patterned substrate includes a top surface and a bottom surface. The high-frequency filter includes a first metal grounding layer having a first grounding surface, a plurality of resonator layers each having at least one resonator, and a signal input port and a signal output port, both located on one of the plurality of resonator layers. Every one of the resonators has a grounding end electrically connected to the first grounding surface, and the resonators are arranged in the same direction and disposed so that coupling occurs between each two resonators that are separately situated on adjacent resonator layers.  
         [0010]     Through the design of the invention, the structure of the high-frequency filter can be changed according to the process limitations, and thus the problem involving difficulties of making conventional interdigital planar filters on ceramic substrates is solved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1A  is a top view of a conventional interdigital planar filter;  
         [0012]      FIG. 1B  is a cross-sectional view of the planar filter taken along the line A-A′ shown in  FIG. 1A ;  
         [0013]      FIG. 2A  is an exploded view of a high-frequency filter, according to the first embodiment of the invention;  
         [0014]      FIG. 2B  is a bottom view of the high-frequency filter, according to the first embodiment of the invention (schematic diagram of the first metal grounding layer);  
         [0015]      FIG. 3A  is a cross-sectional view of the high-frequency filter taken along the line B-B′ in  FIG. 2A ;  
         [0016]      FIG. 3B  is a cross-sectional view of the high-frequency filter taken along the line C-C′ in  FIG. 2A ;  
         [0017]      FIG. 4A  is an exploded view of a high-frequency filter, according to the second embodiment of the invention;  
         [0018]      FIG. 4B  is a bottom view of the high frequency-filter, according to the second embodiment of the invention (schematic diagram of the first metal grounding layer);  
         [0019]      FIG. 5A  is an exploded view of a high-frequency filter, according to the third embodiment of the invention; and  
         [0020]      FIG. 5B  is a bottom view of the high-frequency filter, according to the third embodiment of the invention (schematic diagram of the first metal grounding layer).  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The invention may be described with greater clarity and particularity by reference to the accompanying drawings. The same reference numerals refer to the same parts throughout the various figures.  
         [0022]     A high-frequency filter, according to the invention, has a main structure formed by stacking a plurality of patterned substrates, the high-frequency filter includes at least one metal grounding layer, a signal input port, a signal output port, a plurality of resonator layers for transmitting signals. The plurality of resonator layers have resonators that short to ground and are able to couple with one another. The function and arrangement of each element is described in the following detailed description of the embodiments.  
         [0023]     Referring to  FIG. 2A , a high-frequency filter according to the first embodiment of the invention is formed by three patterned substrates  20 A,  20 B, and  20 C stacked together. The patterned substrates are made of a dielectric material, preferably a ceramic material so that the low temperature co-fired ceramic (LTCC) technology can be used, and the patterns on the patterned substrates are formed with metal materials. A first metal grounding layer  21  and a second metal grounding layer  25  are formed on the bottom surface of the lowest patterned substrate  20 A and the top surface of the highest patterned substrate  20 C, respectively. Furthermore, the second metal grounding layer  25  is a grounding surface formed on the entire top surface of the patterned substrate  20 C.  
         [0024]     Referring to  FIG. 2B , the first metal grounding layer  21  has contacting areas  26 ,  27  on its two ends for receiving signals by connecting to external devices, while the other area of the first metal grounding layer  21  is a grounding surface. The contacting areas  26 ,  27  are isolated from the grounding surface. It is to be understood that the second metal grounding layer  25  can be omitted to simplify the manufacture process, and moreover, an identification layer  30  for determining the direction of the filter may be added on the second metal grounding layer  25 .  
         [0025]     As shown in  FIG. 2A , resonator layers  22 ,  23  are formed separately on the top surfaces of the patterned substrates  20 A,  20 B, respectively. The resonator layers  22 ,  23  each has at least one linear resonator and all resonators are arranged in the same direction (in parallel). It needs to be noted that the simplified drawing only depicts resonators  22   a,    22   b  (on the resonator layer  22 ) and resonators  23   a,    23   b  (on the resonator layer  23 ) for description purpose. Each resonator has an grounding end that is able to connect to the first metal grounding layer  21  and the second grounding layer  25  concurrently via a through hole, and thus short to ground. For example, the resonator  23   a  is short to ground via a through hole c, and the resonator  22   a  is short to ground via a through hole d.  FIG. 3A  is used to describe the relative positions between the resonators. As shown in  FIG. 3A , when all resonators are viewed from the top of the filter, the sequence of resonators from left to right is:  23   a,    22   a,    23   b,    22   b.  In this embodiment, the resonators overlap partially with adjacent resonators, but in other embodiments, the adjacent resonators may have a distance thereinbetween. When the distance is appropriately controlled, any two resonators that are on adjacent layers individually and are adjacent (or partially overlapped) when all resonators viewed from the top of the filter are able to couple with each other. For instance, the resonator  22   a  can couple with the resonators  23   a,    23   b  separately, and the resonator  23   b  can couple with the resonators  22   a,    22   b  separately. In the invention, similar to the structure of conventional interdigital filters, the grounding ends (through holes) of any two resonators that lie on adjacent layers and couple with each other are on opposite sides of the two resonators.  
         [0026]     The resonator layer  22  further includes a signal input port  28  and a signal output port  29  as illustrated in  FIG. 2A . The signal input port  28  electrically connects to the contacting area  26  of the first metal grounding layer  21  via a through hole a, and the signal output port  29  electrically connects with the contacting area  27  of the first metal grounding layer  21  via a through hole g. On the other hand, the signal input port  28  electrically connects to the resonator  23   a  situated on the upper layer via a through hole b, and the signal output port  29  connects directly to the resonator  22   b  situated on the same layer.  FIG. 3B  illustrates the relative positions between the contacting area  26 , the signal input port  28 , and the resonator  23   a;  the left side of the signal input port  28  is below the resonator  23   a  and the right side is above the contacting area  26 .  
         [0027]     The path along which signals are transmitted in the high-frequency filter according to this embodiment is described below. A signal first enters the contacting area  26  of the first metal grounding layer  21  and then is transmitted to the signal input port  28  via the through hole a. The signal is then transmitted to the resonator  23   a  via the through hole b, and to the resonators  22   a,    23   b  and  22   b  sequentially by the couplings therebetween. Finally, the signal transmitted to the resonator  22   b  is sent to the signal output port  29 , and then to the contacting area  27  of the first metal grounding layer  21  via the through hole g, and is outputted therefrom.  
         [0028]     As described above, the invention uses coupling between resonators on adjacent layers to replace the coupling between resonators on a single layer in the conventional planar filter, and thus the structural design of the high-frequency filter of the invention has more flexibility. For instance, the distance between resonators on the same layer can be adjusted according to the process limitations, and the distance between resonators on different layers can also be adjusted according to the dielectric constant of the substrate and the required coupling strength. Thus, if the structure of the invention is applied to ceramic substrates, the problem encountered by the conventional interdigital filter is solved.  
         [0029]     Referring to  FIGS. 4A and 4B , the structure of the high-frequency filter in the second embodiment is similar to that in the first embodiment, except that the high-frequency filter in the second embodiment has three resonator layers  42 ,  43 ,  44 . In the high-frequency filter according to this embodiment, signals are transmitted through the following elements in sequence: the contacting area  46 , the signal input port  48 , the resonator  43   a,  the resonator  44   a,  the resonator  43   b,  the resonator  42   a,  the signal output port  49 , and the contacting area  47 .  
         [0030]     As shown in the second embodiment, the high-frequency filter according to the invention is not limited to have only two resonator layers but is able to add resonator layers in response to the requirement of the high-frequency filter. The amount and arrangement of resonators on each resonator layer can be adjusted according to specification needs.  
         [0031]     Referring to  FIGS. 5A and 5B , the structure of the high-frequency filter in the third embodiment is similar to that in the first embodiment, except that all of the resonators are in one zigzag shape. Moreover, a signal input port  58  connects directly with a resonator  52   a  of the same layer, and a signal output port  59  electrically connects to a resonator  53   b  of the upper layer via a through hole f″. Signals are transmitted in the high-frequency filter of this embodiment by passing through the following elements in sequence: the contacting area  56 , the signal input port  58 , the resonator  52   a,  the resonator  53   a,  the resonator  52   b,  the resonator  53   b,  the signal output port  59 , and the contacting area  57 .  
         [0032]     As shown in the third embodiment, the resonators of the high-frequency filter according to the invention can be made into zigzag or curviform shapes for further reducing the size of the filter, so long as the arrangement of the resonators fulfills the aforementioned requirements. Furthermore, the signal input port and signal output port can be placed on any resonator layer depending on the needs, but preferably on the lowest resonator layer for simple processing purpose (or on the resonator layer adjacent to a metal grounding layer with contacting areas). The connecting methods of the signal input port, signal output port, and the resonators can be determined based on the structure needed for the filter; for instance, connected by through holes or integration. Moreover, each through hole is filled with metal materials for transmitting signals or shorting to ground.  
         [0033]     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.