Patent Publication Number: US-6714103-B2

Title: TEM band pass filter having an evanescent waveguide

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a band pass filter, and particularly, to a highly compact and easily fabricated band pass filter. 
     DESCRIPTION OF THE PRIOR ART 
     In recent years, marked advances in miniaturization of communication terminals, typically mobile phones, has been achieved thanks to miniaturization of the various components incorporated therein. One of the most important components incorporated in a communication terminal is a band pass filter. 
     As shown in “A Novel TE 10δ  Rectangular Waveguide Resonator and Its Bandpass Filter Applications (Proceedings of the Korea-Japan Microwave Workshop 2000, September 2000)”, p. 88, FIG. 8, such a band pass filter is known wherein a plurality of TE mode half-wave (λ/2) dielectric resonators are disposed on a printed circuit board at predetermined spacing. In the band pass filter described in this paper, the distances between the resonators (air gaps) work as so-called “evanescent waveguides” to couple the adjacent resonators at a predetermined coupling constant. 
     As a need continues to be felt for still further miniaturization of the various communication terminals, further miniaturization of the band pass filter incorporated therein is also required. 
     In the band pass filter described above, however, the resonators must be mounted on the printed circuit board because they are coupled by the air gaps. The overall size of the band pass filter tends to be large because it is constituted of a plurality of independent components. 
     Further, in the band pass filter described above, the air gaps must be exactly adjusted to obtain desired characteristics. Even slight errors in the adjustment of the air gaps change the characteristics of the band pass filter markedly. Therefore, this makes the band pass filter described above very difficult to fabricate. The cost of the band pass filter is therefore high. 
     Thus, a compact and easily fabricated band pass filter is desired. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a compact and easily fabricated band pass filter. 
     The above and other objects of the present invention can be accomplished by a band pass filter comprising: a first half-wave (λ/2) resonator having a first open end on which an input terminal is formed and a second open end opposite to the first open end, a second half-wave (λ/2) resonator having a third open end on which an output terminal is formed and a fourth open end opposite to the third open end, and an evanescent waveguide interposed between the second open end of the first resonator and the fourth open end of the second resonator, the first half-wave (λ/2) resonator, the second half-wave (λ/2) resonator, and the evanescent waveguide being a single unit. 
     According to this aspect of the present invention, because the first half-wave (λ/2) resonator, the second half-wave (λ/2) resonator, and the evanescent waveguide are a single unit, they do not have to be mounted on a printed circuit board to form an air gap. Therefore, the overall size of the band pass filter can be reduced and fabrication of the band pass filter is simplified. 
     In a preferred aspect of the present invention, the first half-wave (λ/2) resonator, the second half-wave (λ/2) resonator, and the evanescent waveguide are made of a single dielectric unit. 
     In a further preferred aspect of the present invention, an overall dimension of the band pass filter is a substantially rectangular prismatic shape. 
     In a further preferred aspect of the present invention, a passing band of the band pass filter is not less than 5 GHz. 
     The above and other objects of the present invention can be also accomplished by a band pass filter comprising: 
     first and second dielectric blocks each of which has a top surface, a bottom surface, first and second side surfaces opposite to each other, and third and fourth side surfaces opposite to each other; 
     a third dielectric block in contact with the first side surface of the first dielectric block and the first side surface of the second dielectric block; 
     metal plates formed on the top surfaces, the bottom surfaces, the third side surfaces, and the fourth side surfaces of the first and second dielectric blocks; 
     a first electrode formed on the second side surface of the first dielectric block; and 
     a second electrode formed on the second side surface of the second dielectric block. 
     Also according to this aspect of the present invention, an air gap does not have to be formed by mounting components on a printed circuit board. Therefore, the overall size of the band pass filter can be miniaturized and fabrication of the band pass filter is simplified. 
     In a preferred aspect of the present invention, the first dielectric block and the second dielectric block have the same dimensions. 
     In a further preferred aspect of the present invention, the third dielectric block has a first side surface in contact with the first side surface of the first dielectric block, a second side surface in contact with the first side surface of the second dielectric block, a third side surface parallel to the third side surface of the first dielectric block, a fourth side surface parallel to the fourth side surface of the first dielectric block, a top surface parallel to the top surface of the first dielectric block, and a bottom surface parallel to the bottom surface of the first dielectric block on which a metal plate is formed. 
     In a further preferred aspect of the present invention, the bottom surfaces of the first to third dielectric blocks are coplanar. 
     In a further preferred aspect of the present invention, the top surfaces of the first to third dielectric blocks are coplanar. 
     In a further preferred aspect of the present invention, the members of at least one pair of surfaces among a first pair consisting of the top surfaces of the first and third dielectric blocks, a second pair consisting of the third surfaces of the first and third dielectric blocks, and a third pair consisting of the fourth surfaces of the first and third dielectric blocks fall in different planes. 
     In a further preferred aspect of the present invention, the first dielectric block and the metal plates formed on the top surface, bottom surface, second side surface, and third side surface thereof constitute a first half-wave (λ/2) dielectric resonator, the second dielectric block and the metal plates formed on the top surface, bottom surface, second side surface, and third side surface thereof constitute a second half-wave (λ/2) dielectric resonator, and the third dielectric block constitutes an evanescent waveguide. 
     The above and other objects of the present invention can be also accomplished by a band pass filter comprising: a plurality of half-wave (λ/2) dielectric resonators and at least one evanescent waveguide interposed between adjacent half-wave (λ/2) dielectric resonators, the half-wave (λ/2) dielectric resonators and the evanescent waveguide being made of a single dielectric unit. 
     Also, according to this aspect of the present invention, an air gap does not have to be formed by mounting components on a printed circuit board. Therefore, the overall size of the band pass filter can be miniaturized and fabrication of the band pass filter is simplified. 
     In a preferred aspect of the present invention, an overall dimension of the band pass filter is a substantially rectangular prismatic shape. 
     In another preferred aspect of the present invention, at least one slit is formed in the dielectric block at a portion thereof acting as the evanescent waveguide. 
     The above and other objects of the present invention can be also accomplished by a band pass filter comprising: a dielectric block of substantially rectangular prismatic shape constituted of a first portion lying between a first cross-section of the dielectric block and a second cross-section of the dielectric block substantially parallel to first cross-section and second and third portions divided by the first portion and metal plates formed on the surfaces of the dielectric block, thereby enabling the first portion of the dielectric block and the metal plates formed thereon to act as an evanescent waveguide, the second portion of the dielectric block and the metal plates formed thereon to act as a first resonator, and the third portion of the dielectric block and the metal plates formed thereon to act as a second resonator, the metal plates being formed on, among the surfaces of the second and third portions of the dielectric block, each surface which is substantially perpendicular to the cross-sections. 
     According to this aspect of the present invention, since the band pass filter is constituted of the dielectric block of rectangular prismatic shape, the mechanical strength is extremely high and low in cost. 
     In a preferred aspect of the present invention, the metal plates further include a first exciting electrode formed on, among the surfaces of the second portion of the dielectric block, a surface which is substantially parallel to the cross-sections and a second exciting electrode formed on, among the surfaces of the third portion of the dielectric block, a surface which is substantially parallel to the cross-sections. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view from one side showing a band pass filter  1  that is a preferred embodiment of the present invention. 
     FIG. 2 is a schematic perspective view from the opposite side showing the band pass filter  1  of FIG.  1 . 
     FIG. 3 is an exploded schematic perspective view showing the band pass filter  1  of FIG.  1 . 
     FIG. 4 is a schematic diagram showing the strength of an electric field generated by a half-wave (λ/2) dielectric resonator. 
     FIG.  5 ( a ) is a schematic diagram showing current flow in a half-wave (λ/2) dielectric resonator. FIG.  5 ( b ) is a schematic diagram showing a parallel metal plate waveguide mode electric field at the reference plane of FIG.  5 ( a ). 
     FIG. 6 is an equivalent circuit diagram of the band pass filter  1  shown in FIGS. 1 to  3 . 
     FIG. 7 is a graph showing the frequency characteristic curve of the band pass filter  1  shown in FIGS. 1 to  3 . 
     FIG. 8 is a graph showing the relationship between the thickness h of an evanescent waveguide  4  and an odd mode resonant frequency f odd  and an even mode resonant frequency f even . 
     FIG. 9 is a graph showing the relationship between the thickness h of an evanescent waveguide  4  and a coupling constant k. 
     FIG. 10 is a schematic perspective view from one side showing a band pass filter  1 ′ in which the thickness h of the evanescent waveguide  4  is set to smaller than 0.965 mm. 
     FIG. 11 is a schematic perspective view from the opposite side showing the band pass filter  1 ′ of FIG.  10 . 
     FIG. 12 is a graph showing the frequency characteristic curve of the band pass filter  1 ′ shown in FIGS. 10 and 11. 
     FIG. 13 is a schematic perspective view from one side showing a band pass filter  20  that is another preferred embodiment of the present invention. 
     FIG. 14 is a schematic perspective view from the opposite side showing the band pass filter  20  of FIG.  13 . 
     FIG. 15 is a schematic perspective view from one side showing a band pass filter  40  that is a further preferred embodiment of the present invention. 
     FIG. 16 is a schematic perspective view from the opposite side showing the band pass filter  40  of FIG.  15 . 
     FIG. 17 is a schematic perspective view from one side showing a band pass filter  60  that is a further preferred embodiment of the present invention. 
     FIG. 18 is a schematic perspective view from the opposite side showing the band pass filter  60  of FIG.  17 . 
     FIG. 19 is a schematic perspective view from one side showing a band pass filter  90  that is a further preferred embodiment of the present invention. 
     FIG. 20 is a schematic perspective view from the opposite side showing the band pass filter  90  of FIG.  19 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be explained with reference to the drawings. 
     As shown in FIGS. 1 to  3 , a band pass filter  1  that is a preferred embodiment of the present invention is constituted of a first resonator  2 , a second resonator  3 , and an evanescent waveguide  4  interposed between the first and second resonators  2  and  3 . 
     The first resonator  2  and the second resonator  3  are symmetrical. Each is composed of a dielectric block whose length, width, and thickness are 1.3 mm, 5.1 mm, and 1.0 mm. These dielectric blocks are made of dielectric material whose dielectric constant ∈r=37. The evanescent waveguide  4  is composed of a dielectric block whose length, width, and thickness are 0.2 mm, 5.1 mm, and 1.0 mm. It is made of the same dielectric material as the dielectric blocks composing the first and second resonators  2  and  3 . Thus, the band pass filter  1  measures 2.8 mm, 5.1 mm, and 1.0 mm in length, width, and thickness. 
     The first resonator  2 , the second resonator  3 , and the evanescent waveguide  4  are combined such that their bottom surfaces are coplanar. It is worth noting that this does not mean that they are physically different components. That is, the band pass filter  1  of this preferred embodiment is constituted of the single dielectric unit of substantially rectangular prismatic shape. 
     In this specification, the surfaces opposite to the associated bottom surfaces of the dielectric blocks composing the first resonator  2 , the second resonator  3 , and the evanescent waveguide  4  are each defined as a “top surface.” Among the surfaces of the dielectric blocks composing the first and the second resonators  2  and  3 , each surface in contact with the evanescent waveguide  4  is defined as a “first side surface.” Among the surfaces of the dielectric blocks composing the first and the second resonators  2  and  3 , each surface opposite to the first side surface is defined as a “second side surface.” The remaining surfaces of the dielectric blocks composing the first and second resonators  2  and  3  are defined as a “third side surface” and a “fourth side surface” with respect to each block. Among the surfaces of the dielectric block composing the evanescent waveguide  4 , the surface in contact with the first side surface of the first resonator  2  is defined as a “first side surface.” Among the surfaces of the dielectric block composing the evanescent waveguide  4 , the surface in contact with the first side surface of the second resonator  3  is defined as a “second side surface.” The remaining surfaces of the dielectric block composing the evanescent waveguide  4  are defined as a “third side surface” and a “fourth side surface.” Therefore, “length,” “width,” and “thickness” of the first resonator  2 , the second resonator  3 , and the evanescent waveguide  4  are defined by the distance between the first and second side surfaces, the distance between the third and fourth side surfaces, and the distance between the top and bottom surfaces, respectively. The third side surfaces of the first resonator  2 , second resonator  3 , and evanescent waveguide  4  are coplanar, and the fourth side surfaces of the first resonator  2 , second resonator  3 , and evanescent waveguide  4  are also coplanar. 
     As shown in FIGS. 1 to  3 , metal plates  5 ,  6 , and  7  are formed on the entire top surface, the entire third side surface, and entire fourth side surface of the first resonator  2  and a metal plate  9  is formed on the bottom surface of the first resonator  2  except at a clearance portion  8 . These metal plates  5 ,  6 ,  7 , and  9  are short-circuited with one another. Similarly, metal plates  10 ,  11 , and  12  are formed on the entire top surface, the entire third side surface, and entire fourth side surface of the second resonator  3  and a metal plate  14  is formed on the bottom surface of the second resonator  3  except at a clearance portion  13 . These metal plates  10 ,  11   12 , and  14  are short-circuited with one another. A metal plate  15  is formed on the entire bottom surface of the evanescent waveguide  4 . These metal plates  5 ,  6 ,  7 ,  9 ,  10 ,  11 ,  12 ,  14 , and  15  are thus short-circuited with one another and grounded. 
     As shown in FIGS. 1 and 3, an exciting electrode  16  whose height and width are 0.8 mm and 3.1 mm is formed on the second side surface of the first resonator  2  where the clearance portion  8  prevents the exciting electrode  16  from being in contact with the metal plate  9  formed on the bottom surface. Similarly, as shown in FIG. 2, an exciting electrode  17  whose height and width are 0.8 mm and 3.1 mm is formed on the second side surface of the second resonator  3  where the clearance portion  13  prevents the exciting electrode  17  from being in contact with the metal plate  14  formed on the bottom surface. One of the exciting electrodes  16  and  17  is used as an input electrode, and the other is used as an output electrode. 
     The metal plates  5 ,  6 ,  7 ,  9 ,  10 ,  11 ,  12 ,  14 , and  15  and the exciting electrodes  16  and  17  are made of silver. However, the present invention is not limited to using silver and other kinds of metal can be used instead. 
     No electrode is formed on the remaining surfaces of the first resonator  2 , second resonator  3 , and evanescent waveguide  4 , which therefore constitute open ends. 
     Each of the first resonator  2  and the second resonator  3  having the above described structure acts as a half-wave (λ/2) dielectric resonator. The evanescent waveguide  4  having the above-described structure acts as an E-mode waveguide. 
     The characteristics of the half-wave (λ/2) dielectric resonators constituted by the first resonator  2  and the second resonator  3  will now be explained. 
     FIG. 4 is a schematic diagram showing the strength of an electric field generated by the half-wave (λ/2) dielectric resonator. 
     As shown in FIG. 4, in this type of the half-wave (λ/2) dielectric resonator, the electric field is minimum at the side surfaces (the third and fourth side surfaces), on which the metal plates short-circuiting the metal plates formed on the top and bottom surfaces are formed, and the electric field is maximum at a symmetry plane, which is not exposed to the air. Therefore, in this type of the half-wave (λ/2) dielectric resonator, the radiation loss is much smaller than that of a quarter-wave (λ/4) dielectric resonator. The overall size of the half-wave (λ/2) dielectric resonator is almost double that of a quarter-wave (λ/4) dielectric resonator having the same characteristics. However, in this type of half-wave (λ/2) dielectric resonator, the resonant frequency is inversely proportional to the width of the dielectric block. Therefore, in the case where the desired resonant frequency is relatively high, such as 5.25 GHz, the overall size of the half-wave (λ/2) dielectric resonator should be small. 
     As shown in FIG.  5 ( a ), in this type of the half-wave (λ/2) dielectric resonator, current flows along the x-axis, which is the direction of mode propagation. The location of the exciting electrode is not along the direction of the mode propagation. For this type of excitation, the TE-mode electric field of the parallel metal waveguide mode is also excited in addition to the expected TEM-mode. 
     FIG.  5 ( b ) is a schematic diagram showing the TE-mode electric field of the parallel metal plate waveguide mode at the reference plane of FIG.  5 ( a ). 
     In a band pass filter constituted of two TEM-mode half-wave (λ/2) dielectric resonators, the TE-mode electric fields of the parallel metal waveguide mode are opposite in direction and capacitive coupling occurs between them which is the direct coupling between I/O ports. 
     FIG. 6 is an equivalent circuit diagram of the band pass filter  1  shown in FIGS. 1 to  3 . 
     In this figure, the first resonator  2  and the second resonator  3  are represented by two L-C parallel circuits  18 - 1  and  18 - 2 , respectively. The evanescent waveguide  4  is represented by an L-C parallel circuit  19  consisting of an inductor Lm and a capacitor Cm. The L-C parallel circuit  19  gives an internal coupling between the first resonator  2  and the second resonator  3 . The exciting electrodes  16  and  17  are represented by two capacitances Ce. The capacitance Cd represents the direct coupling capacitance between the exciting electrodes  16  and  17 . 
     FIG. 7 is a graph showing the frequency characteristic curve of the band pass filter  1  shown in FIGS. 1 to  3 . 
     In this Figure, S 11  represents the reflection coefficient, and S 21  represents the transmission coefficient. As shown in FIG. 7, the resonant frequency of the band pass filter  1  is approximately 5.25 GHz and its 3-dB bandwidth is approximately 410 MHz. Further, attenuation poles appear at approximately 4.8 GHz and 7.2 GHz because the dominant coupling between the two resonators by the evanescent waveguide  4  is inductive. No attenuation pole appears in the case where the dominant coupling between the two resonators by the evanescent waveguide  4  is capacitive. As is apparent from FIG. 7, the lower edge of the passing band of the frequency characteristics is sharpened compared with the higher edge of the passing band. 
     FIG. 8 is a graph showing the relationship between the thickness h of the evanescent waveguide  4  and the odd mode resonant frequency f odd  and even mode resonant frequency f even . 
     As shown in FIG. 8, although the even mode resonant frequency f even  has very little dependence upon the thickness h of the evanescent waveguide  4 , the odd mode resonant frequency f odd  markedly decreases with increasing thickness h. In the region where the thickness h of the evanescent waveguide  4  is smaller than 0.965 mm (first region), the odd mode resonant frequency f odd  is higher than the even mode resonant frequency f even . In the region where the thickness h of the evanescent waveguide  4  is higher than 0.965 mm (second region), the even mode resonant frequency f even  is higher than the odd mode resonant frequency f odd . In the region where the thickness h of the evanescent waveguide  4  is 0.965 mm, the odd mode resonant frequency f odd  and the even mode resonant frequency f even  are equal to each other. This implies that the dominant coupling between the two resonators by the evanescent waveguide  4  is capacitive in the first region, and the dominant coupling between the two resonators by the evanescent waveguide  4  is inductive in the second region. 
     The coupling constant k can be represented by the following equation.              k   =         f   even   2     -     f   odd   2           f   even   2     +     f   odd   2                 (   1   )                         
     The relationship between the thickness h of the evanescent waveguide  4  and the coupling constant k can be obtained by referring to the equation (1). 
     FIG. 9 is a graph showing the relationship between the thickness h of the evanescent waveguide  4  and the coupling constant k obtained from the equation (1). 
     The coupling constant k can be considered as a combination of the capacitive coupling constant k c  and the inductive coupling constant k i . 
     As shown in FIG. 9, the coupling constant k total  exponentially increases with increasing thickness h of the evanescent waveguide  4  and becomes zero at a thickness h of 0.965 mm. This means that the capacitive coupling constant k c  and the inductive coupling constant k i  are equal to each other when the thickness h of the evanescent waveguide  4  is 0.965 mm. In the region where the thickness h of the evanescent waveguide  4  is smaller than 0.965 mm (first region), the capacitive coupling constant k c  becomes greater than the inductive coupling constant k i . In the region where the thickness h of the evanescent waveguide  4  is greater than 0.965 mm (second region), the capacitive coupling constant k c  becomes smaller than the inductive coupling constant k i . 
     As is apparent from FIG. 9, in the case where the thickness h of the evanescent waveguide  4  is set to 1.0 mm as in the band pass filter  1  according to this embodiment, the dominant coupling of the first resonator  2  and the second resonator  3  becomes inductive, and k is approximately 0.055. In this case, the external quality factor becomes approximately 17.6. 
     Because, as described above, the band pass filter  1  according to this embodiment is constituted of the first resonator  2 , the second resonator  3 , and the evanescent waveguide  4  as a single unit, an air gap does not have to be formed by mounting components on a printed circuit board. Therefore, the overall size of the band pass filter  1  can be reduced and fabrication of the band pass filter  1  is simplified. 
     Further, according to the band pass filter  1 , owing to the fact that half-wave (λ/2) dielectric resonators are used for the first resonator  2  and the second resonator  3 , the radiation loss occurring at the open ends is very small compared with the case of using quarter-wave (λ/4) dielectric resonators. The overall size of a half-wave (λ/2) dielectric resonator is almost double that of a quarter-wave (λ/4) dielectric resonator. However, in the TEM-mode dielectric resonator, the radiation loss is proportional to the square of the resonant frequency, whereas the size of the resonator is inversely proportional to the resonant frequency. Therefore, in the case where the desired resonant frequency is relatively high, such as over 5 GHz, the band pass filter  1  of this embodiment is particularly effective. 
     According to the band pass filter  1 , the dominant coupling between the first resonator  2  and the second resonator  3  becomes inductive by setting the thickness h of the evanescent waveguide  4  being 1.0 mm (&gt;0.965 mm). However, capacitive dominant coupling between the first resonator  2  and the second resonator  3  can be obtained by setting the thickness h of the evanescent waveguide  4  being smaller than 0.965 mm. Next, another band pass filter whose dominant coupling between the first resonator  2  and the second resonator  3  is inductive by setting the thickness h of the evanescent waveguide  4  being smaller than 0.965 mm will be explained. 
     FIG. 10 is a schematic perspective view from one side showing a band pass filter  1 ′ in which the thickness h of the evanescent waveguide  4  is set to smaller than 0.965 mm. FIG. 11 is a schematic perspective view from the opposite side showing the band pass filter  1 ′ of FIG.  10 . 
     As shown in FIGS. 10 and 11, the band pass filter  1 ′ has the same structure and the same dimension as the band pass filter  1  except that the thickness h of the evanescent waveguide  4  is set to 0.93 mm. Therefore, a dielectric unit of such a shape can be fabricated by forming a slit on a single dielectric unit at a portion corresponding to the top surface of the evanescent waveguide  4 . As is apparent from FIG. 9, in the case where the thickness h of the evanescent waveguide  4  is set to 0.93 mm as in the band pass filter  1 ′, the dominant coupling of the first resonator  2  and the second resonator  3  becomes capacitive, and k is approximately −0.055. 
     FIG. 12 is a graph showing the frequency characteristic curve of the band pass filter  1 ′ shown in FIGS. 10 and 11. 
     In this Figure, S 11  represents the reflection coefficient, and S 21  represents the transmission coefficient. As shown in FIG. 12, the resonant frequency of the band pass filter  1 ′ is approximately 5.5 GHz and its 3-dB bandwidth is approximately 410 MHz. No attenuation poles appear in contrast to the band pass filter  1 . This is because that the dominant coupling between the two resonators by the evanescent waveguide  4  is capacitive. As is apparent from FIG. 12, the higher edge of the passing band of the frequency characteristics is sharpened compared with the lower edge of the passing band. 
     As described above, according to the band pass filter of this embodiment, the desired coupling constant k can be obtained by controlling the thickness h of the evanescent waveguide  4  so that the desired frequency characteristic can be obtained. 
     It is worth noting that the coupling constant k between the first resonator  2  and the second resonator  3  can be controlled based on not only the thickness h of the evanescent waveguide  4  but also the width of the evanescent waveguide  4 . Another preferred embodiment where the coupling constant k is controlled based on the width of the evanescent waveguide will be explained. 
     FIG. 13 is a schematic perspective view from one side showing a band pass filter  20  that is another preferred embodiment of the present invention. FIG. 14 is a schematic perspective view from the opposite side showing the band pass filter  20  of FIG.  13 . 
     As shown in FIGS. 13 and 14, the band pass filter  20  that is another preferred embodiment of the present invention is constituted of a first resonator  21 , a second resonator  22 , and an evanescent waveguide  23  interposed between the first and second resonators  21  and  22 . The top surfaces, bottom surfaces, first side surfaces, second side surfaces, third side surfaces, and fourth side surfaces of the dielectric blocks composing the first and second resonators  21  and  22  and the evanescent waveguide  23  are defined the same as the corresponding surfaces of the band pass filter  1  explained earlier. 
     In the band pass filter  20  of this embodiment, the width of the evanescent waveguide  23  is set narrower than the widths of the first resonator  21  and the second resonator  22 , whereas the thickness of the evanescent waveguide  23  are set to equal to thicknesses of the first resonator  21  and the second resonator  22 . The top surfaces, bottom surfaces, and fourth side surfaces of the first resonator  21 , second resonator  22 , and evanescent waveguide  23  are thus coplanar. A dielectric unit of such a shape can be fabricated by forming a slit on a single dielectric unit at a portion corresponding to the third side surface of the evanescent waveguide  23 . 
     As shown in FIGS. 13 and 14, metal plates  24 ,  25 , and  26  are formed on the entire top surface, entire third side surface, and entire fourth side surface of the first resonator  21 ; and a metal plate  28  is formed on the bottom surface of the first resonator  21  except at a clearance portion  27 . These metal plates  24 ,  25 ,  26 , and  28  are short-circuited with one another. Similarly, metal plates  29 ,  30 , and  31  are formed on the entire top surface, entire third side surface, and entire fourth side surface of the second resonator  22 ; and a metal plate  33  is formed on the bottom surface of the second resonator  22  except at a clearance portion  32 . These metal plates  29 ,  30 ,  31 , and  33  are short-circuited with one another. A metal plate  34  is formed on the entire bottom surface of the evanescent waveguide  23 . These metal plates  24 ,  25 ,  26 ,  28 ,  29 ,  30 ,  31 ,  33 , and  34  are thus short-circuited with one another and grounded. 
     As shown in FIG. 13, an exciting electrode  35  is formed on the second side surface of the first resonator  21  where the clearance portion  27  prevents the exciting electrode  35  from being in contact with the metal plate  28  formed on the bottom surface. Similarly, as shown in FIG. 14, an exciting electrode  36  is formed on the second side surface of the second resonator  22  where the clearance portion  32  prevents the exciting electrode  36  from being in contact with the metal plate  33  formed on the bottom surface. One of the exciting electrodes  35  and  36  is used as an input electrode, and the other is used as an output electrode. 
     Each of the first resonator  21  and the second resonator  22  having the above described structure acts as a half-wave (λ/2) dielectric resonator. The evanescent waveguide  23  having the above-described structure acts as an E-mode waveguide. 
     In the band pass filter  20 , the coupling constant k total  can be controlled based on the width of the evanescent waveguide  23 . 
     Because, as described above, the band pass filter  20  according to this embodiment is constituted of the first resonator  21 , the second resonator  22 , and the evanescent waveguide  23  as a single unit, the overall size thereof can be reduced and fabrication of the band pass filter is simplified. 
     A further preferred embodiment of the present invention will now be explained. 
     FIG. 15 is a schematic perspective view from one side showing a band pass filter  40  that is a further preferred embodiment of the present invention. FIG. 16 is a schematic perspective view from the opposite side showing the band pass filter  40  of FIG.  15 . 
     As shown in FIGS. 15 and 16, the band pass filter  40  that is a further preferred embodiment of the present invention is constituted of a first resonator  41 , a second resonator  42 , and an evanescent waveguide  43  interposed between the first and second resonators  41  and  42 . The top surfaces, bottom surfaces, first side surfaces, second side surfaces, third side surfaces, and fourth side surfaces of the dielectric blocks composing the first and second resonators  41  and  42  and the evanescent waveguide  43  are defined the same as the corresponding surfaces of the band pass filters  1  and  20  explained earlier. 
     In the band pass filter  40  of this embodiment, the width of the evanescent waveguide  43  is set narrower than the widths of the first resonator  41  and the second resonator  42 , whereas the thickness of the evanescent waveguide  43  is set equal to thicknesses of the first resonator  41  and the second resonator  42 . The top surfaces and bottom surfaces of the first resonator  41 , second resonator  42 , and evanescent waveguide  43  are thus coplanar. A dielectric unit of such a shape can be fabricated by forming slits in a single dielectric unit at portions corresponding to the third and fourth side surfaces of the evanescent waveguide  43 . 
     As shown in FIGS. 15 and 16, metal plates  44 ,  45 , and  46  are formed on the entire top surface, entire third side surface, and entire fourth side surface of the first resonator  41 ; and a metal plate  48  is formed on the bottom surface of the first resonator  41  except at a clearance portion  47 . These metal plates  44 ,  45 ,  46 , and  48  are short-circuited with one another. Similarly, metal plates  49 ,  50 , and  51  are formed on the entire top surface, entire third side surface, and entire fourth side surface of the second resonator  42 ; and a metal plate  53  is formed on the bottom surface of the second resonator  42  except at a clearance portion  52 . These metal plates  49 ,  50 ,  51 , and  53  are short-circuited with one another. A metal plate (not shown) is formed on the entire bottom surface of the evanescent waveguide  43 . These metal plates  44 ,  45 ,  46 ,  48 ,  49 ,  50 ,  51 , and  53  and the metal plate formed on the bottom surface of the evanescent waveguide  43  are thus short-circuited with one another and grounded. 
     As shown in FIG. 15, an exciting electrode  55  is formed on the second side surface of the first resonator  41  where the clearance portion  47  prevents the exciting electrode  55  from being in contact with the metal plate  48  formed on the bottom surface. Similarly, as shown in FIG. 16, an exciting electrode  56  is formed on the second side surface of the second resonator  42  where the clearance portion  52  prevents the exciting electrode  56  from being in contact with the metal plate  53  formed on the bottom surface. One of the exciting electrodes  55  and  56  is used as an input electrode, and the other is used as an output electrode. 
     Each of the first resonator  41  and the second resonator  42  having the above described structure acts as a half-wave (λ/2) dielectric resonator. The evanescent waveguide  43  having the above-described structure acts as an E-mode waveguide. 
     In the band pass filter  40 , as in the band pass filter  20  of the preceding embodiment, the coupling constant k total  can be controlled based on the width of the evanescent waveguide  43 . 
     Because, as described above, the band pass filter  40  according to this embodiment is constituted of the first resonator  41 , second resonator  42 , and evanescent waveguide  43  as a single unit, the overall size thereof can be miniaturized and fabrication of the band pass filter is simplified. 
     A further preferred embodiment of the present invention will now be explained. 
     FIG. 17 is a schematic perspective view from one side showing a band pass filter  60  that is a further preferred embodiment of the present invention. FIG. 18 is a schematic perspective view from the opposite side showing the band pass filter  60  of FIG.  17 . 
     As shown in FIGS. 17 and 18, the band pass filter  60  that is a further preferred embodiment of the present invention is constituted of a first resonator  61 , a second resonator  62 , a third resonator  63 , a first evanescent waveguide  64  interposed between the first and second resonators  61  and  62 , and a second evanescent waveguide  65  interposed between the second and third resonators  62  and  63 . That is, the band pass filter  60  of this embodiment is a kind of 3-stage band pass filter. 
     The first resonator  61 , second resonator  62 , third resonator  63 , first evanescent waveguide  64 , and second evanescent waveguide  65  are combined such that their bottom surfaces are coplanar. It is worth noting that this does not mean that they are physically different components, but they constitute a single dielectric unit having slits in the top surface thereof at portions acting as the first evanescent waveguide  64  and second evanescent waveguide  65 . That is, the band pass filter  60  of this preferred embodiment is also constituted of a single dielectric unit. 
     In this specification, the surfaces opposite to the associated bottom surfaces of the dielectric blocks composing the first resonator  61 , second resonator  62 , third resonator  63 , first evanescent waveguide  64 , and second evanescent waveguide  65  are each defined as a “top surface.” Among the surfaces of the dielectric blocks composing the first and second resonators  61  and  62 , each surface in contact with the first evanescent waveguide  64  is defined as a “first side surface.” Among the surfaces of the dielectric blocks composing the first and second resonators  61  and  62 , each surface opposite to the first side surface is defined as a “second side surface.” The remaining surfaces of the dielectric blocks composing the first and second resonators  61  and  62  are defined as a “third side surface” and a “fourth side surface” with respect to each block. Among the surfaces of the dielectric block composing the third resonator  63 , the surface in contact with the second evanescent waveguide  65  is defined as a “first side surface.” Among the surfaces of the dielectric block composing the third resonator  63 , the surface opposite to the first side surface is defined as a “second side surface.” The remaining surfaces of the dielectric block composing the third resonator  63  are defined as a “third side surface” and a “fourth side surface.” Among the surfaces of the dielectric block composing the first evanescent waveguide  64 , the surface in contact with the first side surface of the first resonator  61  is defined as a “first side surface.” Among the surfaces of the dielectric block composing the first evanescent waveguide  64 , the surface in contact with the first side surface of the second resonator  62  is defined as a “second side surface.” The remaining surfaces of the dielectric block composing the first evanescent waveguide  64  are defined as a “third side surface” and a “fourth side surface.” Among the surfaces of the dielectric block composing the second evanescent waveguide  65 , the surface in contact with the first side surface of the third resonator  63  is defined as a “first side surface.” Among the surfaces of the dielectric block composing the second evanescent waveguide  65 , the surface in contact with the second side surface of the second resonator  62  is defined as a “second side surface.” The remaining surfaces of the dielectric block composing the second evanescent waveguide  65  are defined as a “third side surface” and a “fourth side surface.” 
     The third side surfaces of the first resonator  61 , second resonator  62 , third resonator  63 , first evanescent waveguide  64 , and second evanescent waveguide  65  are coplanar, and the fourth side surfaces thereof are also coplanar. 
     As shown in FIGS. 17 and 18, metal plates  66 ,  67 , and  68  are formed on the entire top surface, entire third side surface, and entire fourth side surface of the first resonator  61 ; and a metal plate  70  is formed on the bottom surface of the first resonator  61  except at a clearance portion  69 . These metal plates  66 ,  67 ,  68 , and  70  are short-circuited with one another. Metal plates  71 ,  72 ,  73 , and  74  are formed on the entire top surface, entire third side surface, entire fourth side surface, and entire bottom surface of the second resonator  62 . These metal plates  71 ,  72 ,  73 , and  74  are short-circuited with one another. Metal plates  75 ,  76 , and  77  are formed on the entire top surface, entire third side surface, and entire fourth side surface of the third resonator  63 ; and a metal plate  79  is formed on the bottom surface of the third resonator  63  except at a clearance portion  78 . These metal plates  75 ,  76 ,  77 , and  79  are short-circuited with one another. Further, metal plates  80  and  81  are formed on the entire bottom surfaces of the first and second evanescent waveguides  64  and  65 , respectively. These metal plates  66 ,  67 ,  68 ,  70 ,  71 ,  72 ,  73 ,  74 ,  75 ,  76 ,  77 ,  79 ,  80 , and  81  are thus short-circuited with one another and grounded. 
     As shown in FIG. 17, an exciting electrode  82  is formed on the second side surface of the first resonator  61  where the clearance portion  69  prevents the exciting electrode  82  from being in contact with the metal plate  70  formed on the bottom surface. Similarly, as shown in FIG. 18, an exciting electrode  83  is formed on the second side surface of the third resonator  63  where the clearance portion  78  prevents the exciting electrode  83  from being in contact with the metal plate  79  formed on the bottom surface. One of the exciting electrodes  82  and  83  is used as an input electrode, and the other is used as an output electrode. 
     Each of the first to third resonators  61  to  63  having the above-described structure acts as a half-wave (λ/2) dielectric resonator. Each of the first and second evanescent waveguides  64  and  65  having the above-described structure acts as an E-mode waveguide. 
     In the band pass filter  60 , frequency characteristics having sharp edges compared with above described band pass filters  1 ,  20 , and  40  can be obtained by setting the coupling constant k1 total  between the first resonator  61  and the second resonator  62  and the coupling constant k2 total  between the second resonator  62  and the third resonator  63  to substantially the same value. The coupling constant k1 total  between the first resonator  61  and the second resonator  62  can be controlled based on the thickness of the first evanescent waveguide  64 . The coupling constant k2 total  between the second resonator  62  and the third resonator  63  can be controlled based on the thickness of the second evanescent waveguide  65 . In a three state band pass filter, |k1 total |=|k2 total |. 
     Because, as described above, the band pass filter  60  according to this embodiment is constituted of the first resonator  61 , second resonator  62 , third resonator  63 , first evanescent waveguide  64 , and second evanescent waveguide  65  as a single unit, the overall size thereof can be reduced and fabrication of the band pass filter is simplified. 
     A further preferred embodiment of the present invention will now be explained. 
     FIG. 19 is a schematic perspective view from one side showing a band pass filter  90  that is a further preferred embodiment of the present invention. FIG. 20 is a schematic perspective view from the opposite side showing the band pass filter  90  of FIG.  19 . 
     As shown in FIGS. 19 and 20, the band pass filter  90  that is a further preferred embodiment of the present invention is constituted of a first resonator  91 , a second resonator  92 , and an evanescent waveguide  93  interposed between the first and second resonators  91  and  92 . The top surfaces, bottom surfaces, first side surfaces, second side surfaces, third side surfaces, and fourth side surfaces of the dielectric blocks composing the first and second resonators  91  and  92  and the evanescent waveguide  93  are defined the same as the corresponding surfaces of the band pass filters  1 ,  20 , and  40  explained earlier. 
     In the band pass filter  90  of this embodiment, like in the band pass filter  1  described above, the thickness of the evanescent waveguide  93  is set smaller than that of the first resonator  91  and the second resonator  92 , whereas the width of the evanescent waveguide  93  is set equal to that of the first resonator  91  and the second resonator  92 . The bottom surfaces, third side surfaces, and fourth side surfaces of the first resonator  91 , second resonator  92 , and evanescent waveguide  93  are thus coplanar. A dielectric unit of such a shape can be fabricated by forming a slit in a single dielectric unit at a portion corresponding to the top surface of the evanescent waveguide  93 . 
     As shown in FIGS. 19 and 20, metal plates  94 ,  95 , and  96  are formed on the entire top surface, entire third side surface, and entire fourth side surface of the first resonator  91 ; and a metal plate  98  is formed on the bottom surface of the first resonator  91  except at a clearance portion  97 . These metal plates  94 ,  95 ,  96 , and  98  are short-circuited with one another. Similarly, metal plates  99 ,  100 , and  101  are formed on the entire top surface, entire third side surface, and entire fourth side surface of the second resonator  92 ; and a metal plate  103  is formed on the bottom surface of the second resonator  92  except at a clearance portion  102 . These metal plates  99 ,  100 ,  101 , and  103  are short-circuited with one another. A metal plate  104  is formed on the entire bottom surface of the evanescent waveguide  93 . These metal plates  94 ,  95 ,  96 ,  98 ,  99 ,  100 ,  101 ,  103 , and  104  are thus short-circuited with one another and grounded. 
     As shown in FIG. 19, an exciting electrode  105  is formed on the second side surface of the first resonator  91 . The exciting electrode  105  is in contact with the metal plate  94  formed on the top surface whereas the clearance portion  97  prevents the exciting electrode  105  from being in contact with the metal plate  98  formed on the bottom surface. Similarly, as shown in FIG. 20, an exciting electrode  106  is formed on the second side surface of the second resonator  92 . The exciting electrode  106  is in contact with the metal plate  99  formed on the top surface whereas the clearance portion  102  prevents the exciting electrode  105  from being in contact with the metal plate  103  formed on the bottom surface. One of the exciting electrodes  105  and  106  is used as an input electrode, and the other is used as an output electrode. The exciting electrodes  105  and  106  are inductive exciting electrodes whereas the exciting electrodes used in the above described embodiments are capacitive exciting electrodes. 
     Each of the first resonator  91  and the second resonator  92  having the above described structure acts as a half-wave (λ/2) dielectric resonator. The evanescent waveguide  93  having the above-described structure acts as an E-mode waveguide. 
     In the band pass filter  90 , like in the band pass filter  1 , the coupling constant K total  can be controlled based on the thickness of the evanescent waveguide  93 . 
     Because, as described above, the band pass filter  90  according to this embodiment is constituted of the first resonator  91 , second resonator  92 , and evanescent waveguide  93  as a single unit, the overall size thereof can be reduced and fabrication of the band pass filter is simplified. 
     The present invention has thus been shown and described with reference to specific embodiments. However, it should be noted that the present invention is in no way limited to the details of the described arrangements but changes and modifications may be made without departing from the scope of the appended claims. 
     For example, in the above described embodiments, the dielectric blocks for the resonators and the evanescent waveguide are made of dielectric material whose dielectric constant ∈ r is 37. However, a material having a different dielectric constant can be used according to purpose. 
     Further, the dimensions of the resonators and the evanescent waveguide specified in the above described embodiments are only examples. Resonators and an evanescent waveguide having different dimensions can be used according to purpose. 
     Furthermore, in the band pass filters  1 ,  60 , and  90 , the coupling constant is controlled based on the thickness of the evanescent waveguide, and in the band pass filters  20  and  40 , the coupling constant is controlled based on the width of the evanescent waveguide. However, the coupling constant can be controlled based on both thickness and width of the evanescent waveguide. 
     Further, the band pass filter  60  is configured to have three stages by using three resonators, but a band pass filter can also be configured to have four or more stages by using four or more resonators. 
     Because, as described above, the band pass filter according to the present invention is constituted of the resonators and the evanescent waveguide interposed between the resonators as a single unit, an air gap does not have to be formed by mounting components on a printed circuit board. Therefore, the overall size of the band pass filter can be miniaturized and fabrication of the band pass filter is simplified. Further, in the band pass filter according to the present invention, because the half-wave (λ/2) dielectric resonators are used, the radiation loss occurring at the open ends is very small. 
     Therefore, the present invention provides a band pass filter that can be preferably utilized in communication terminals such as mobile phones and the like, LANs (Local Area Networks), ITS (Intelligent Transport Systems) and various communication systems, where filtering is needed.