Abstract:
The present invention relates to a radio frequency filter employing a notch structure, wherein the notch structure is a dual notch structure which comprises: a C notch structure formed in a predetermined region at a partition between two cavities to be cross-coupled; and an L notch structure formed together with the C notch structure in a predetermined region at a partition between two cavities.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation of International Application No. PCT/KR2012/006247 filed on Aug. 6, 2012, which claims priority to Korean Application No. 10-2011-0078275 filed on Aug. 5, 2011, which applications are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a radio frequency filter for use in a wireless communication system, and more particularly, to a radio frequency filter employing a notch structure. 
       BACKGROUND ART 
       [0003]    The term “radio frequency filter (hereinafter, referred to as ‘filter’) refers to an apparatus for performing a function of passing or filtering signals in any predetermined frequency band, which is classified into a low pass filter, a band pass filter, a high pass filter, a band stop filter and the like according to a frequency band to be filtered. 
         [0004]    The filter has an important characteristic such as an insertion loss and a skirt feature. The insertion loss means electric power to be lost when the signals pass through the filter, and the skirt feature means a steep extent of a passing band and a filtering band of the filter. 
         [0005]    The insertion loss and the skirt feature have a trade-off relationship with each other according to the number of stages (degrees) of the filter. As the stages of the filter increase, the skirt feature becomes better but the insertion loss becomes worse. 
         [0006]    In order to improve the skirt feature of the filter without the stages of the filter being increased, a method of forming a notch (reduction pole) is generally used. This is a method capable of enhancing the skirt feature of the filter, in which the notch is formed in a specific frequency band without the stages of the filter being increased, so as to enhance the skirt feature of the filter. 
         [0007]    There is a cross coupling method as a most general method forming the notch. A technology relating to formation of the notch using the cross coupling method is disclosed in U.S. Pat. No. 6,342,825, entitled “Band-pass filter having tri-section”, issued to Hershtig, Rafi and assigned to ‘K&amp;L Microwave Co.’ on Jan. 29, 2002. 
       SUMMARY 
       [0008]    The invention has been made to solve the above-mentioned problem in the prior art, and an aspect of the present invention is to provide a radio frequency filter employing a notch structure capable of providing a notch feature to the radio frequency filter. 
         [0009]    Another aspect of the present invention is to provide a radio frequency filter employing a notch structure capable of generating notch features at both ends of a three stage filter and a four stage filter. 
         [0010]    In accordance with an aspect of the present invention, a radio frequency filter employing a notch structure is provided. The radio frequency filter having a dual notch structure includes: a C type notch structure formed on a predetermined portion of a partition between two cavities to be cross coupled; and an L type notch structure formed along the C type notch structure. 
       Advantageous Effect 
       [0011]    As described above, the radio frequency filter employing the notch structure according to the present invention has a more efficient structure capable of providing multiple notch features to the radio frequency filter, particularly, allowing a generation of the notch feature in both the three stage filter and the four stage filter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a schematic view illustrating a structure of a general three stage filter; 
           [0013]      FIG. 2  is a graph illustrating a frequency filtering characteristic of the three stage filter shown in  FIG. 1 ; 
           [0014]      FIG. 3  is a schematic view illustrating a structure of the three stage filter employing a general notch structure; 
           [0015]      FIG. 4  is a circuit diagram illustrating an equivalent circuit of the three stage filter shown in  FIG. 3 ; 
           [0016]      FIG. 5  is a graph illustrating a frequency filtering characteristic of the three stage filter shown in  FIG. 3 ; 
           [0017]      FIG. 6  is a schematic view illustrating a structure of the three stage filter employing a notch structure according to a first embodiment of the present invention; 
           [0018]      FIG. 7  is a circuit diagram illustrating an equivalent circuit of the three stage filter shown in  FIG. 6 ; 
           [0019]      FIG. 8  is a graph illustrating a frequency filtering characteristic of the three stage filter shown in  FIG. 3 ; 
           [0020]      FIG. 9  is a schematic view illustrating a structure of four stage filter employing a notch structure according to a second embodiment of the present invention; 
           [0021]      FIG. 10  is a circuit diagram illustrating an equivalent circuit of the four stage filter shown in  FIG. 9 ; 
           [0022]      FIG. 11  is a graph illustrating a frequency filtering characteristic of the four stage filter shown in  FIG. 9 ; 
           [0023]      FIG. 12  is a schematic view illustrating a structure of the four stage filter employing a notch structure according to a third embodiment of the present invention; 
           [0024]      FIG. 13  is a graph illustrating a frequency filtering characteristic of the four stage filter shown in  FIG. 12 ; 
           [0025]      FIG. 14  is a schematic view illustrating a structure of a generally considered four stage filter; 
           [0026]      FIG. 15  is a circuit diagram illustrating an equivalent circuit of the four stage filter shown in  FIG. 14 ; 
           [0027]      FIG. 16  is a graph illustrating a frequency filtering characteristic of the four stage filter shown in  FIG. 14 ; 
           [0028]      FIG. 17  is a schematic view illustrating a structure of six stage filter employing a notch structure according to a fourth embodiment of the present invention; 
           [0029]      FIG. 18  is a circuit diagram illustrating an equivalent circuit of the six stage filter shown in  FIG. 17 ; 
           [0030]      FIG. 19  is a graph illustrating a frequency filtering characteristic of the six stage filter in  FIG. 17 ; 
           [0031]      FIG. 20  is a schematic view illustrating a structure of the six stage filter employing a notch structure according to a fifth embodiment of the present invention; 
           [0032]      FIG. 21  is a graph illustrating a frequency filtering characteristic of the six stage filter shown in  FIG. 20 ; 
           [0033]      FIG. 22  is a schematic view illustrating a structure of the six stage filter employing a notch structure according to a sixth embodiment of the present invention; and 
           [0034]      FIG. 23  is a graph illustrating a frequency filtering characteristic of the six stage filter shown in  FIG. 22 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Hereinafter, an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
         [0036]    Firstly, referring to  FIGS. 1 to 5 , formation of a notch using a cross-coupling will be described.  FIG. 1  is a schematic perspective view illustrating a structure of a general three stage filter, and  FIG. 2  is a graph illustrating a frequency filtering characteristic of the three stage filter shown in  FIG. 1 .  FIG. 3  is a schematic perspective view illustrating a structure of the three stage filter employing a notch structure,  FIG. 4  is a circuit diagram illustrating an equivalent circuit of the three stage filter shown in  FIG. 3 , and  FIG. 5  is a graph illustrating a frequency filtering characteristic of the three stage filter shown in  FIG. 3 . 
         [0037]    Typically, the filter has a structure in that a multiple dielectric substances or metallic resonators are connected by multiple stages in a plurality of cavities defined by a metal housing and a cover, and in  FIGS. 1 and 3 , the illustration of the structure of the metal housing will be omitted for convenience of the description. 
         [0038]    Referring to  FIGS. 1 and 3 , a three stage filter has a structure in which a first cavity  11 , a second cavity  12 , and a third cavity  13  are cross-coupled with one another, and have a first resonator  21 , a second resonator  22 , and a third resonator  23 , respectively. The first resonator  21  of the first cavity  11  is connected with an input connector  31  to receive an input signal, and the third resonator  13  of the third cavity  13  is connected with an output connector  32  to provide an output signal. 
         [0039]    Accordingly, a signal input into the input connector  31  is output to the output connector  32  after sequentially passing the first resonator  21 , the second resonator  22 , and the third resonator  23 , as indicated by an arrow in  FIG. 1 . 
         [0040]    In the filter having no the notch structure as shown in  FIG. 1 , only a sequential coupling between adjacent resonators is basically established, while in the filter employing a capacity (C) type notch structure  41  as shown in  FIG. 3 , a cross coupling between the resonators, which are not adjacent, is achieved. 
         [0041]    The C type notch structure  41  has a metal stick forming a capacitance coupling between the first resonator  21  and the third resonator  23  as an important structure. The metal stick penetrates an inner wall partitioning the first and third cavities  11  and  13 . At this time, the metal stick is covered with a supporting substance of a dielectric material (not shown) such as Teflon in order to electrically isolate the metal stick from the inner wall, and joined to the inner wall. At this time, the inner wall may have a through-hole structure formed therein in which the metal stick is installed, or may have a lower portion at which the metal stick is installed. However, since it is difficult to form the through-hole in the inner wall in an operating process, an upper end of the inner wall is partially cut off, and the metal stick covered with the supporting substance is generally installed on the corresponding cut off portion. Such a supporting substance functions as an insulation of the metal stick, and also is fixed to the corresponding portion at which the metal stick is installed because of an engagement with the cut off portion of the inner wall, resulting in a fixing of the metal stick. 
         [0042]    Referring to  FIGS. 2 and 4 , it is understood that the three stage filter having the C type notch structure  41  shown in  FIG. 3  creates a notch at a lower portion of a processing band (a low frequency band of the processing band), in comparison with the general three stage filter. 
         [0043]    A four or more stage filter may employ an inductor (L) type notch structure as well as the C type notch structure  41 , in which the L type notch structure is constituted in the form of a window formed in the partition between the receiving spaces of two resonators to be cross-coupled, and creates an inductance coupling between both resonators. 
         [0044]    The three stage filter shown in  FIG. 1  has no separate notch structure, but since a weak coupling is created between the first resonator  21  of the first stage filter and the third resonator  23  of the third stage filter through the third resonator  23  of the third stage filter, a weak L type notch may be formed without hardly having effect on the filtering characteristic. 
         [0045]    In the above-described structure, particularly, the three stage filter, the notch structure must have been employed only between the first stage and the third stage. It has been recognized that it has been difficult to implement multiple notches or to create notches at both ends of the processing bands, that is, a lower end and an upper end of the processing band, without a special structure being added inside or outside of the filter. 
         [0046]      FIG. 6  is a schematic perspective view illustrating a structure of the three stage filter employing a notch structure according to a first embodiment of the present invention,  FIG. 7  is a circuit diagram illustrating an equivalent circuit of the three stage filter shown in  FIG. 6 , and  FIG. 8  is a graph illustrating the frequency filtering characteristic of the three stage filter shown in  FIG. 3 . In  FIG. 6 , similarly to  FIGS. 1 and 3 , it is omitted to show the structure of the metal housing for the convenience of the description. 
         [0047]    Referring to  FIG. 6 , the three stage filter employing the notch structure according to the first embodiment of the present invention includes a first cavity  11 , a second cavity  12  and a third cavity  13 , which are arranged in the triangular form and sequentially coupled, and which include a first resonator  21 , a second resonator  22 , and a third resonator  23 , respectively. The first resonator  21  of the first cavity  11  is connected with an input connector  31  to receive an input signal, and the third resonator  13  of the third cavity  13  is connected with an output connector  32  to provide an output signal. 
         [0048]    At this time, according to the characteristic of the present invention, the C type notch structure  41  is installed to the partition between the first cavity  11  and the third cavity  13  to be cross-coupled, and in addition the L type notch structure  51  of the window structure extends to a lower end of the C type notch structure  41  and is formed on the partition. In  FIG. 6 , although it is shown that the L type notch structure  51  has an identical width to the cut-off portion of the partition in order to install the C type notch structure  41 , the L type notch structure  51  is slightly narrower than the width of the cut-off portion on which the C type notch structure  41 , i.e., a step portion, is installed, so that the supporting substance is rigidly fixed to the partition on which the C type notch structure  41  is installed. 
         [0049]    As shown in  FIG. 6 , the notch structure according to the first embodiment of the present invention may be regarded as a dual notch structure in which the C type notch structure  41  and the L type notch structure  51  are complex. 
         [0050]    As described above, if the three stage filter employs the dual notch structure of the present invention, it is understood that the notches are created at both ends of the processing band of the filter as indicated by an arrow in  FIG. 8 . That is, two notches are created at the lower end of the processed band, while one notch is created at an upper end of the processed band. 
         [0051]      FIG. 9  is a schematic perspective view illustrating the structure of the four stage filter employing the notch structure according to the second embodiment of the present invention,  FIG. 10  is a circuit diagram illustrating an equivalent circuit of the four stage filter, and  FIG. 11  is a graph illustrating a frequency filtering characteristic of the four stage filter in  FIG. 9 . 
         [0052]    Referring to  FIG. 9 , the four stage filter employing the notch structure according to the second embodiment of the present invention has a structure in that a first cavity  11 , a second cavity  12 , a third cavity  13  and a fourth cavity  14  are arranged in two columns and sequentially coupled with each other in two pairs. The first, second, third and fourth cavities  11 ,  12 ,  13  and  14  have a first, second, third and fourth resonators  21 ,  22 ,  23  and  24  respectively. The first resonator  21  of the first cavity  11  is connected with an input connector  31  to receive an input signal, and the fourth resonator  14  of the fourth cavity  14  is connected with an output connector  32  to provide an output signal. 
         [0053]    At this time, according to the characteristic of the present invention, the C type notch structure  41  is installed to the partition between the first cavity  11  and the fourth cavity  14 , and in addition the L type notch structure  51  of the window structure extends to a lower end of the C type notch structure  41  and is formed on the partition. That is, the notch structure according to the characteristic of the present invention shown in  FIG. 9  may have a similar structure to the dual notch structure shown in  FIG. 6 . 
         [0054]    As described above, if the four stage filter employs the dual notch structure of the present invention, it is understood that the notches are created at both ends of the processing band of the filter as shown in  FIG. 11 . That is, two notches are created at the lower and upper ends of the processing band, so as to implement the dual notches which are symmetrical to left and right. 
         [0055]      FIG. 12  is a schematic perspective view illustrating a structure of the four stage filter employing a notch structure according to the third embodiment of the present invention, and  FIG. 13  is a graph illustrating the frequency filtering characteristic of the four stage filter of  FIG. 12 . The equivalent circuit of the notch structure according to the third embodiment shown in  FIG. 12  is identical to the equivalent circuit of the second embodiment shown in  FIG. 10 . 
         [0056]    Referring to  FIG. 12 , the four stage filter employing the notch structure according to the third embodiment of the present invention has a structure in that a first cavity  11 , a second cavity  12 , a third cavity  13  and a fourth cavity  14  are sequentially coupled with one another, similarly to the third embodiment shown in  FIG. 9 . Each of the first resonator  12 , the second resonator  22 , the third resonator  23  and the fourth resonator  24  is prepared for each cavity. The first resonator  21  of the first cavity  11  is connected with an input connector  31  to receive an input signal, and the fourth resonator  14  of the fourth cavity  14  is connected with an output connector  32  to provide an output signal. 
         [0057]    At this time, according to the characteristic of the present invention, the C type notch structure  41  is installed to the partition between the first cavity  11  and the fourth cavity  14 , and in addition the L type notch structure  51  of the window structure is formed at another region on the partition, differently from the C type notch structure  41 . The C type notch structure  41  is formed at a center portion of the partition by reference of a plane surface, and the L type notch structure  51  is formed on any side of the partition to correspond to the type notch structure  41 . 
         [0058]    If the L type notch structure  51  and the C type notch structure  41  are separately installed on the partition, it is easy to separately tune the L type notch and the C type notch. 
         [0059]    In the case of employing the dual notch structure according to the third embodiment of the present invention as described above, as shown in  FIG. 13 , it is noted that the filter has the filter characteristic almost similar to the filter characteristic according to the second embodiment as shown in  FIG. 11 . 
         [0060]    On the other hand, in  FIG. 12 , it is shown that the L type notch structure  51  is formed at a position near the input connector  31  and the output connector  32 . However, as indicated by an arrow in  FIG. 12 , the L type notch structure  51  may be formed. In this case, the filter also may have a similar characteristic. In this case, however, the notch characteristic may be weak. 
         [0061]      FIG. 14  is a schematic perspective view illustrating a structure of a generally considered four stage filter,  FIG. 15  is a circuit diagram illustrating an equivalent circuit of the four stage filter of  FIG. 14 , and  FIG. 16  is a graph illustrating a frequency filtering characteristic of the four stage filter of FIG.  14 . 
         [0062]    Referring to  FIGS. 14 to 16 , the four stage filter employing the generally considered notch structure will be described. In comparison with the four stage filter according to the characteristic of the present invention, the four stage filter shown in  FIG. 14  has a C type notch structure  41  installed on a partition interposed between the first cavity  11  and the third cavity  13 , and the general L type notch structure  42  may be interposed between the first cavity  11  and the fourth cavity  14 . 
         [0063]    As described above, it is understood that only two notches are created in the four stage filter employing the notch structure as shown in  FIG. 14 . 
         [0064]      FIG. 17  is a schematic perspective view illustrating the structure of the six stage filter employing the notch structure according to the fourth embodiment of the present invention,  FIG. 18  is a circuit diagram illustrating an equivalent circuit of the six stage filter, and  FIG. 19  is a graph illustrating a frequency filtering characteristic of the six stage filter in  FIG. 17 . 
         [0065]    Referring to  FIG. 17 , the six stage filter employing the notch structure according to the fourth embodiment of the present invention has a structure in that a first cavity  11 , a second cavity  12 , a third cavity  13 , a fourth cavity  14 , a fifth cavity  15 , and a sixth cavity  16  are arranged in two columns, each of which has three cavities, and are sequentially coupled with each other. 
         [0066]    At this time, according to the characteristic of the present invention, the C type notch structure  41  is installed to the partition between the second cavity  12  and the fourth cavity  15 , and in addition the L type notch structure  51  of the window structure extends to a lower end of the C type notch structure  41  and is formed on the partition. That is, the notch structure according to the characteristic of the present invention shown in  FIG. 17  may have a similar structure to the dual notch structure shown in  FIG. 9 . 
         [0067]    In the case that the six stage filter employs the dual notch structure of the present invention as described above, it is understood that an additional notch is created at a lower end of the processing band of the filter as indicated by an arrow in  FIG. 19 . 
         [0068]      FIG. 20  is a schematic perspective view illustrating a structure of a six stage filter employing a notch structure according to the fifth embodiment of the present invention, and  FIG. 21  is a graph illustrating the frequency filtering characteristic of the six stage filter of  FIG. 20 . The equivalent circuit of the notch structure according to the fifth embodiment shown in  FIG. 20  may be identical to the equivalent circuit of the fourth embodiment shown in  FIG. 18 . 
         [0069]    Referring to  FIG. 20 , in the six stage filter employing the notch structure according to the fifth embodiment of the present invention, the C type notch structure  41  is installed on a partition between the second cavity  12  and the fourth cavity  15 , similarly to the filter in  FIG. 17 . The L type notch structure  51  of a window structure may be formed at another region on the partition, separately from the C type notch structure  41 . That is, the notch structure according to the characteristic of the present invention shown in  FIG. 20  may have a similar structure to the dual notch structure shown in  FIG. 12 . 
         [0070]    In the case of employing the dual notch structure according to the fifth embodiment of the present invention as described above, as shown in  FIG. 21 , it is noted that the filter has the filter characteristic almost similar to the filter characteristic according to the fourth embodiment as shown in  FIG. 19 . 
         [0071]      FIG. 22  is a schematic perspective view illustrating a structure of the six stage filter employing a notch structure according to the sixth embodiment of the present invention, and  FIG. 23  is a graph illustrating the frequency filtering characteristic of the six stage filter of  FIG. 22 . 
         [0072]    Referring to  FIG. 22 , the six stage filter employing the notch structure according to the sixth embodiment of the present invention has a similar structure to the structures of the embodiments shown in  FIGS. 17 and 20 , but has the difference in that the C type notch structure  41  and the L notch type structure  51  extending to a lower end of the C type notch structure  41  are formed at a first stage, i.e., a partition between a first cavity  11  and a sixth cavity  16 . 
         [0073]    In view of the filtering characteristic of the filter having the structure, as shown in  FIG. 23 , two notches are created at each of a lower end and an upper end of the processing band of the filter. 
         [0074]    As described above, the radio frequency filter employing the notch structure according to the embodiments of the present invention may be implemented, and also the notch structure according to the characteristic of the present invention may be applied to multiple stage filters having more than the six stages, beyond the above-described embodiments. Further, two or more dual notch structures having a combination of the L type notch and the C type notch according to the characteristic of the present invention may be employed to the multiple stage filters having more stage than the six stage filter. Furthermore, the general L type notch structure or the general C type notch structure is adjustably incorporated with the dual notch structure according to the characteristic of the present invention, and a combination of the L type notch or the C type notch with the dual notch structure may be employed to the filter.