Patent Publication Number: US-8988171-B2

Title: Multi-resonator waveguide bandpass filter

Description:
TECHNICAL FIELD 
     The present invention relates to a bandpass type of filter that includes an E-plane parallel metal plate and, in particular, to a bandpass filter that includes a cross-coupling waveguide path within the waveguide. 
     BACKGROUND ART 
     When developing radio equipment, the aim is to achieve the maximum performance and the optimum functional characteristics in as small a space as possible. 
     The physical dimensions of passive circuits such as filters are determined by the design frequency. Consequently, there is only a small degree of freedom from the viewpoint of the flexible packaging of the respective components. 
     For example, in a bandpass filter which includes an E-plane parallel metal plate, the level of satisfaction with the standard in the pass bands (i.e., the abruptness of boundaries with stop bands) is decided solely by the number of stages. Because of this, there are cases when the overall filter length is too long for the space provided. 
     In bandpass filters in which a plurality of resonators are aligned in a row, technology is known in which, by coupling resonators together by crossing one or more resonators, an attenuation pole is created on the outside of the pass band thereby improving the attenuation characteristics. 
     “The combline bandpass filter” disclosed in Patent document 1 is known as a bandpass filter in which cross-coupling lines are formed. In the invention disclosed in Patent document 1, on the inside of one side surface of a metal case, a metal coupling loop is installed with one end thereof being bent in the direction of the open end of a resonance conductor, and the other end thereof being bent in the direction of the short-circuit end of the resonance conductor. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. H6-291512 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, the working of the metal coupling loop of the invention disclosed in Patent document 1, as well as the installation thereof inside a metal case are not simple tasks. 
     In some cases, a coaxial line is used for the cross-coupling line. However, in the same way as in the invention disclosed in Patent document 1, this is not straightforward from the viewpoint of creating a waveguide path, and a coaxial line also generates considerable loss. 
     The present invention has been conceived in view of the above described problems. One example of objects of the present invention is to provide a bandpass filter having simplified component shapes and also having simplified assembly, and which also has superior attenuation characteristics. 
     Means for Solving the Problem 
     A bandpass filter of the present invention includes: rectangular waveguides which are divided into two in a center of a broad plane; and a metal plate which has a substantially ladder shape, and is disposed between the rectangular waveguides in parallel with a narrow plane of the rectangular waveguides, and has a pair of beams and plurality of fins that connect the pair of beams. At least one other waveguide is formed by dividing a waveguide path within the rectangular waveguides vertically with respect to a direction which is parallel with the broad plane. At least three resonators is formed within the rectangular waveguides by the metal plate, and each of the other waveguides couples resonators together which crosses at least one of the plurality of resonators so as to form a pole outside a pass band. 
     Effect of the Invention 
     According to the present invention, it is possible to provide a bandpass filter having simplified component shapes and also having simplified assembly, and which also has superior attenuation characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing the structure of a bandpass waveguide filter according to a first exemplary embodiment of the present invention. 
         FIG. 2  is a view showing the structure of an E-plane parallel metal plate which is used in the bandpass waveguide filter according to the first exemplary embodiment of the present invention. 
         FIG. 3  is a cross-sectional view showing the bandpass waveguide filter according to the first exemplary embodiment of the present invention. 
         FIG. 4  is a view showing an example of the pass characteristics of a bandpass filter. 
         FIG. 5A  is a view showing the structure of a rectangular waveguide which is applied to a bandpass waveguide filter according to a second exemplary embodiment of the present invention. 
         FIG. 5B  is a cross-sectional view showing the structure of a rectangular waveguide taken along a line A-A shown in  FIG. 5A . 
         FIG. 5C  is a view showing the structure of an E-plane parallel metal plate which is applied to the bandpass waveguide filter according to the second exemplary embodiment of the present invention. 
         FIG. 6  is a cross-sectional view showing a bandpass waveguide filter according to the second exemplary embodiment of the present invention. 
         FIG. 7A  is a view showing the structure of a rectangular waveguide which is applied to a bandpass waveguide filter according to a third exemplary embodiment of the present invention. 
         FIG. 7B  is a cross-sectional view showing a rectangular waveguide taken along lines B-B and B′-B′ shown in  FIG. 7A . 
         FIG. 7C  is a cross-sectional view showing a rectangular waveguide taken along lines C-C and C′-C′ shown in  FIG. 7A . 
         FIG. 7D  is a view showing the structure of an E-plane parallel metal plate which is applied to the bandpass waveguide filter according to the third exemplary embodiment of the present invention. 
         FIG. 8A  is a cross-sectional view showing the bandpass waveguide filter taken along a line B-B in  FIG. 7A  according to the third exemplary embodiment of the present invention. 
         FIG. 8B  is a cross-sectional view showing the bandpass waveguide filter taken along a line B′-B′ in  FIG. 7A  according to the third exemplary embodiment of the present invention. 
         FIG. 8C  is a cross-sectional view showing the bandpass waveguide filter taken along a line C-C in  FIG. 7A  according to the third exemplary embodiment of the present invention. 
         FIG. 8D  is a cross-sectional view showing the bandpass waveguide filter taken along a line C′-C′ in  FIG. 7A  according to the third exemplary embodiment of the present invention. 
         FIG. 9A  is a view showing another structure of the rectangular waveguide which is applied to the bandpass waveguide filter according to the third exemplary embodiment of the present invention. 
         FIG. 9B  is a cross-sectional view showing a rectangular waveguide taken along lines B-B and B′-B′ shown in  FIG. 9A . 
         FIG. 9C  is a cross-sectional view showing a rectangular waveguide taken along lines C-C and C′-C′ shown in  FIG. 9A . 
         FIG. 9D  is a view showing another structure of an E-plane parallel metal plate which is applied to the bandpass waveguide filter according to the third exemplary embodiment of the present invention. 
         FIG. 10A  is a view showing another structure of a rectangular waveguide which is applied to a bandpass waveguide filter according to an exemplary embodiment of the present invention in which a waveguide path which is located in the center in the height direction of the rectangular waveguide, and a waveguide path which is located adjacent to an H-plane thereof are both employed. 
         FIG. 10B  is a view showing another structure of an E-plane parallel metal plate which is applied to the bandpass waveguide filter according to the exemplary embodiment of the present invention in which the waveguide path which is located in the center in the height direction of the rectangular waveguide, and the waveguide path which is located adjacent to the H-plane thereof are both employed. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     First Exemplary Embodiment 
     A first exemplary embodiment in which the present invention has been favorably implemented will now be described. 
       FIG. 1  shows the structure of a bandpass waveguide filter according to the present exemplary embodiment. A coordinate system is set in which the longitudinal direction of the waveguide is set parallel to the z-direction, an H-plane (broad plane, first plane) is set parallel to the xz-plane, and an E-plane (narrow plane, second plane) is set parallel to the yz-plane. The width of the E-plane is narrower than the width of the H-plane. The E-plane may also be perpendicular relative to the H-plane. 
     Rectangular waveguides  1   a  and  1   b  which are divided into two at the center of the H-plane sandwich an E-plane parallel metal plate  2  so as to form a single waveguide. A coupling coefficient which is required for the bandpass filter is set to a desired value via the shape (i.e., via the plate thickness and the fin width and interval) of fins  21  which are disposed in a ladder shape. 
     A protruding portion  11   a  which protrudes in parallel with the xz-plane is formed inside the rectangular waveguide  1   a . A groove  12   a  which extends in the z-direction and whose depth direction extends in the x-direction and whose width direction extends in the y-direction is formed between the protruding portion  11   a  and an inside wall of the H-plane of the waveguide. The depth of the groove  12   a  is set in accordance with the coupling amount of the cross-coupled waveguide. It is not essential for the groove  12   a  to reach as far as the inside wall of the rectangular waveguide  1   a  (i.e., the E-plane). 
     Slope portions  13   a  are formed on the inside of the H-plane of the rectangular waveguide  1   a  with gaps provided between themselves and the end portions of the groove  12   a  in order to adjust the coupling amount. The opening direction of coupling windows  10   a  is set at an optional angle relative to the longitudinal direction of the rectangular waveguide  1   a . The dimensions of the fins  21  located at the front and rear of the coupling windows  10   a  are adjusted to dimensions which are different from those of the other fins  21  in order to adjust the coupling amount. 
     Although it is obscured in  FIG. 1  by the waveguide wall, the other rectangular waveguide  1   b  has the same structure as the rectangular waveguide  1   a.    
     The E-plane parallel metal plate  2  forms a plurality of, namely, three or more resonators within the rectangular waveguides  1   a  and  1   b . The E-plane parallel metal plate  2  is formed such that there are no fins  21  in the portion thereof which corresponds to the grooves in the rectangular waveguides  1   a  and  1   b . In other words, the E-plane parallel metal plate  2  has an open portion which corresponds to the shape of the grooves (namely, an open portion which has the same shape as the shape of the grooves) in a portion thereof which corresponds to the grooves in the rectangular waveguides  1   a  and  1   b . Namely, as is shown in  FIG. 2 , in a portion of the E-plane parallel metal plate  2  which corresponds to the position where the grooves  12   a  and  12   b  are located, the fins  21   d  to  21   f  are supported in a cantilever fashion, and a beam portion  22  is provided at a distal end thereof. 
     As a result of the E-plane parallel metal plate  2  being sandwiched between the pair of rectangular waveguides  1   a  and  1   b , as is shown in  FIG. 3 , a separate quadrangular waveguide-shaped waveguide path is formed inside the waveguide. Namely, inside the rectangular waveguides  1   a  and  1   b , at least one other waveguide is formed as a result of the waveguide path being divided vertically in a direction which is parallel to the H-plane. This waveguide-shaped waveguide path forms a cross-coupling waveguide path  3  which couples together the resonators of the bandpass filter. Namely, the cross-coupling waveguide path  3  couples resonators together by crossing at least one of the plurality of resonators that are formed inside the rectangular waveguides  1   a  and  1   b  by the E-plane parallel metal plate  2 . The cross-coupling waveguide path  3  functions as a cross-coupling line. The cross-coupling waveguide path  3  has a flat shape in which the height (i.e., the y-direction) dimension is smaller than the width (i.e., the x-direction) dimension. However, the aspect ratio thereof is not limited to any specific value and is a value which corresponds to the coupling amount. 
       FIG. 4  shows the pass characteristics of a 38 GHz band model bandpass filter. In  FIG. 4 , the curved line shown by the arrow A shows the pass characteristics of a bandpass filter when there is no cross-coupling. In  FIG. 4 , the curved line shown by the arrow B shows the pass characteristics of a bandpass filter when there is cross-coupling. As is shown in  FIG. 4 , at least one pole is formed by coupling resonators together using a cross-coupling line. The number of poles and the positions where they are formed can be altered by selecting the resonators to be coupled together. 
     In  FIG. 4 , at a position R where the pole is formed (in the vicinity of 38.15 GHz) the amount of attenuation is improved by not less than 20 dB. 
     The present invention is achieved not only when the dimensions and shapes of the coupling windows, fins, and slope portions are limited to particular numerical values and shapes, and the dimensions and the like of the coupling windows, fins, and slope portions can be adjusted in order to adjust the number and positions of the poles in accordance with the desired characteristics. As a consequence, a detailed description of the adjustment of the coupling amount is omitted here. 
     In the above described structure, because a separate waveguide (i.e., a cross-coupling waveguide path) is formed inside the rectangular waveguides  1   a  and  1   b , there is no need for the external dimensions of the bandpass filter to be changed. As a consequence, restrictions on the packaging space are alleviated and it is simple for the respective components inside a device to be packaged with flexibility. 
     Here, a structure in which the bandpass waveguide filter is symmetrical in the longitudinal direction of the rectangular waveguides is described as an example. However, it is not necessary for the bandpass waveguide filter to be symmetrical in the longitudinal direction of the rectangular waveguides. 
     In this manner, a cross-coupling waveguide path is formed inside a rectangular waveguide, and a pole is generated by cross-coupling resonators together, and the coupling amount is adjusted such that this pole is formed outside a pass band in a transient area between the pass band and a stop band. By employing this type of structure, it is possible to improve the pass characteristics of a bandpass filter. 
     Second Exemplary Embodiment 
     A second exemplary embodiment in which the present invention has been favorably implemented will now be described. In the same way as in the first exemplary embodiment, the bandpass waveguide filter of the present exemplary embodiment has a structure in which an E-plane parallel metal plate is sandwiched between a pair of rectangular waveguides which are divided into two at the center of the H-plane. 
       FIG. 5A  shows the structure of the inside of a rectangular waveguide  1   a  which constitutes the bandpass waveguide filter of the present exemplary embodiment.  FIG. 5B  is a cross-sectional view showing the rectangular waveguide  1   a  taken along a line A-A shown in  FIG. 5A . In the present exemplary embodiment, a pair of protruding portions  14   a  are formed substantially parallel with the H-plane in the vicinity of the center in the height direction of the E-plane of the rectangular waveguide  1   a . The portion which is sandwiched between the pair of protruding portions  14   a  forms a groove  15   a.    
       FIG. 5C  shows the structure of an E-plane parallel metal plate  2  which constitutes the bandpass waveguide filter of the present exemplary embodiment. A pair of beam portions  23  which correspond to the pair of protruding portions  14   a  are each supported in a cantilever fashion by fins. 
     One end of each pair of protruding portions  14   a  and beam portions  23  is formed in a slope shape, and the coupling amount can be adjusted by altering the shape and dimensions of this portion. 
     The structure of the rectangular waveguide  1   b  is the same as that of the rectangular waveguide  1   a.    
     As is shown in  FIG. 6 , in the present exemplary embodiment, if the E-plane parallel metal plate  2  is sandwiched between the pair of rectangular waveguides  1   a  and  1   b , then a cross-coupling waveguide path  4  is formed in a center portion in the height direction of the waveguide. 
     In the bandpass waveguide filter of the present exemplary embodiment as well, in the same way as in the first exemplary embodiment, a pole is generated by cross-coupling resonators together, and the coupling amount is adjusted such that this pole is positioned outside a pass band in a transient area between the pass band and a stop band. By employing this type of structure, it is possible to improve the pass characteristics of a bandpass filter. 
     Third Exemplary Embodiment 
     A third exemplary embodiment in which the present invention has been favorably implemented will now be described. In the same way as in the bandpass waveguide filters of the first and second exemplary embodiments, the bandpass waveguide filter of the present exemplary embodiment has a structure in which an E-plane parallel metal plate is sandwiched between a pair of rectangular waveguides which are divided into two at the center of the H-plane. 
       FIG. 7A  shows the structure of the inside of a rectangular waveguide  1   a  which constitutes the bandpass waveguide filter of the present exemplary embodiment.  FIG. 7B  is a cross-sectional view showing the rectangular waveguide  1   a  taken along lines B-B and B′-B′ shown in  FIG. 7A .  FIG. 7C  is a cross-sectional view showing the rectangular waveguide  1   a  taken along lines C-C and C′-C′ shown in  FIG. 7A . In the present exemplary embodiment, two protruding portions  16   a  and  17   a  are formed at different positions in the longitudinal direction of the rectangular waveguide  1   a . One protruding portion  16   a  is provided substantially parallel with the H-plane in the vicinity of the H-plane on the underside of the rectangular waveguide  1   a . The other protruding portion  17   a  is provided substantially parallel with the H-plane in the vicinity of the H-plane on the topside of the rectangular waveguide  1   a . Grooves  18   a  and  19   a  are formed between the protruding portions  16   a  and  17   a  and the inner walls of the waveguide. 
     There are no fins in those portions of the E-plane parallel metal plate  2  which correspond respectively to the two grooves  18   a  and  19   a  in the rectangular waveguide  1   a . In other words, the E-plane parallel metal plate  2  has open portions which correspond to the shape of the grooves (namely, open portions which have the same shape as the shape of the grooves) in portions thereof which correspond to the grooves in the rectangular waveguides  5  and  6 . Namely, as is shown in  FIG. 7D , in portions which correspond to the respective positions where the grooves  18  and  19  are located, the fins are supported in a cantilever fashion. Beam portions  24  and  25  are provided at a distal end of these fins. 
     The structure of the rectangular waveguide  1   b  is the same as that of the rectangular waveguide  1   a.    
       FIG. 8A  is a cross-sectional view showing the bandpass waveguide filter taken along a line B-B in  FIG. 7A  according to the third exemplary embodiment.  FIG. 8B  is a cross-sectional view showing the bandpass waveguide filter taken along a line B′-B′ in  FIG. 7A  according to the third exemplary embodiment.  FIG. 8C  is a cross-sectional view showing the bandpass waveguide filter taken along a line C-C in  FIG. 7A  according to the third exemplary embodiment.  FIG. 8D  is a cross-sectional view showing the bandpass waveguide filter taken along a line C′-C′ in  FIG. 7A  according to the third exemplary embodiment. 
     As is shown in  FIGS. 8A through 8D , in the present exemplary embodiment, if the E-plane parallel metal plate  2  is sandwiched between the pair of rectangular waveguides  1   a  and  1   b , then two cross-coupling waveguide paths  5  and  6  are formed within the waveguide. Because each of these cross-coupling waveguide paths  5  and  6  forms a pole, by adjusting the coupling amounts such that each of these poles is positioned outside a pass band in a transient area between the pass band and a stop band, it is possible to further improve the pass characteristics. 
     Here, a structure in which cross-coupling waveguide paths are formed respectively in the vicinity of the H-plane on the top side of the rectangular waveguide and the vicinity of the H-plane on the underside thereof has been used as an example. However, as is shown in  FIGS. 9A through 9D , it is also possible to employ a structure in which a plurality of cross-coupling waveguide paths are formed in the vicinity of one of the H-planes of the rectangular waveguide  1   a .  FIG. 9A  is a view showing another structure of the rectangular waveguide which constitutes the bandpass waveguide filter according to the third exemplary embodiment.  FIG. 9B  is a cross-sectional view showing the rectangular waveguide  1   a  taken along lines B-B and B′-B′ shown in  FIG. 9A .  FIG. 9C  is a cross-sectional view showing a rectangular waveguide  1   a  taken along lines C-C and C′-C′ shown in  FIG. 9A .  FIG. 9D  is a view showing another structure of an E-plane parallel metal plate which forms part of the bandpass waveguide filter according to the third exemplary embodiment. 
     In the bandpass waveguide filter of the present exemplary embodiment as well, in the same way as in the bandpass waveguide filter of the first exemplary embodiment, a pole is generated by cross-coupling resonators together, and by adjusting the coupling amount such that this pole is positioned outside a pass band in a transient area between the pass band and a stop band, it is possible to improve the pass characteristics of a bandpass filter. 
     In a bandpass filter according to one exemplary embodiment of the present invention, a metal plate which has a substantially ladder shape and in which a pair of beams are connected by a plurality of fins is disposed in parallel with the narrow plane between rectangular waveguides which are divided into two in the center of the broad plane. At least one other waveguide is formed within the rectangular waveguides by dividing the waveguide path in the direction of the narrow plane. Each of the other waveguides couples resonators together by crossing at least one of the resonators that are formed inside the rectangular waveguides by the metal plate, and thereby forms a pole outside the pass band. 
     A bandpass filter according to another exemplary embodiment of the present invention includes: rectangular waveguides which are divided into two in a center of a broad plane; and a metal plate which has a substantially ladder shape, and is disposed between the rectangular waveguides in parallel with a narrow plane of the rectangular waveguides, and has a pair of beams and plurality of fins that connect the pair of beams. At least one other waveguide is formed by dividing a waveguide path within the rectangular waveguides vertically with respect to a direction which is parallel with the broad plane. At least three resonators are formed within the rectangular waveguides by the metal plate, and each of the other waveguides couples resonators together which crosses at least one of the plurality of resonators so as to form a pole outside a pass band. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the claims. 
     For example, using rectangular waveguides and an E-plane parallel metal plate such as those shown in  FIGS. 10A and 10B , it is also possible to use a combination of a waveguide path which is located in the center in the height direction of the rectangular waveguide, and a waveguide path which is located adjacent to the H-plane.  FIG. 10A  is a view showing the structure of a rectangular waveguide which is applied to a bandpass waveguide filter according to another exemplary embodiment of the present invention.  FIG. 10B  is a view showing the structure of an E-plane parallel metal plate which is applied to a bandpass waveguide filter according to another exemplary embodiment of the present invention. 
     In this manner, various transformations are possible in the present invention. 
     This application is the National Phase of PCT/JP2009/006966, filed Dec. 17, 2009, which is based upon and claims the benefit of priority from Japanese patent application No. 2008-332321, filed on Dec. 26, 2008, the disclosure of which is incorporated herein in its entirety by reference. 
     INDUSTRIAL APPLICABILITY 
     The bandpass waveguide filter according to the respective exemplary embodiments described above can be applied to RF transmission-reception separation circuits which are located in the input section of simple radio devices that are designed to firm up flexible base network systems at a low cost. 
     REFERENCE SYMBOLS 
     
         
           1   a ,  1   b  Rectangular waveguide 
           2  E-plane parallel metal plate 
           3 ,  4  Cross-coupling waveguide path 
           10   a  Coupling window 
           11   a ,  11   b ,  14   a ,  14   b ,  16   a ,  16   b ,  17   a ,  17   b  Protruding portion 
           12   a ,  12   b ,  15   a ,  15   b ,  18   a ,  18   b ,  19   a ,  19   b  Groove 
           13   a  Slope portion 
           21 ,  21   a  to  21   i  Fin 
           22 ,  23 ,  24 ,  25  Beam portion