Band-pass filter comprising series coupled split gap resonators arranged along a circular position line

Herein disclosed is a band-pass filter for a radio wave having a wavelength range of a high frequency, such as a microwave and a milliwave. The band-pass filter comprises: a dielectric substrate; input and output terminals; and a plurality of conductive strip line resonators being capable of resonating with a predetermined wavelength. Each of the strip line resonators has two ends and bent line extending from one end to the other end with a predetermined length corresponding to the wavelength. The one end and the other end are placed face to face with each other to provide a gap therebetween. In the band-pass filter, the plurality as arranged on the dielectric substrate in series and spaced apart from each other at predetermined intervals along a predetermined position line and coupled with each other through the inductive and capacitive coupling to transfer the signal between the resonators one after another. Each of the adjoining resonators has a predetermined intensity of the coupling between them in accordance with a relationship between the positions of the gaps of the adjoining resonators. As a result, the band-pass filter can be miniaturized as regulating a desired intensity of coupling between resonators.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a band-pass filter for filtering a radio 
wave having a wavelength range of a high frequency, such as a microwave 
and a millimeter wave. 
2. Description of the Related Art 
There have so far been proposed wide varieties of band-pass filters for 
filtering a radio wave having a wavelength range of a high frequency, such 
as a microwave and a millimeter wave. This kind of band-pass filter 
comprises a plurality of resonators, for instance, a wave guide resonator, 
a cavity resonator or a strip line resonator, being capable of resonating 
with a desired frequency. 
The band-pass filter is utilized for a wide variety of communication 
equipment which has needed to be miniaturized in recent years. The 
band-pass filter, therefore, also needs miniaturizing. The use of the 
strip line resonators can make the band-pass filter to be substantially 
miniaturized in comparison with the other resonators, i.e., a wave guide 
resonator or a cavity resonator. For this reason, the band-pass filter 
including the strip line resonators is useful for the miniaturized 
communication equipment. 
Referring to FIG. 14 of the drawings, there is shown a conventional 
band-pass filter 1 comprising a plurality of micro strip line resonators 
represented by the reference numerals 2, 3, 4, 5 and 6 each having a 
predetermined wavelength for resonating such as a half wavelength 
.lambda./2 or a quarter wavelength .lambda./4. The resonators 2-6 are 
arranged on a dielectric substrate 9 in longitudinally parallel 
relationship and apart from each other at predetermined intervals 
represented by the reference characters "La, Lb, Lc and Ld" in FIG. 14. 
The dielectric substrate 9 has a length represented by the reference 
character "L" as shown in FIG. 14. The length L of the dielectric 
substrate 9 should be more than the sum of all of intervals La, Lb, Lc, 
and Ld. 
The radio wave signal is inputted to the first resonator 2 through an input 
terminal 7. The first resonator 2 resonates with the predetermined 
wavelength. The resonating signal is then transferred from the first 
resonator 2 to the second resonator 3 by way of the inductive and 
capacitive coupling. The signal is transferred from the second resonator 3 
through the fifth resonator 6 one after another while each of the 
resonators resonates with its resonating wavelength. The resonating signal 
is thus outputted from the fifth resonator 6 through an output terminal 8. 
The band-pass filter 1 can thus obtain the filtered signal having the 
desired wavelength. 
However, a drawback encountered in the conventional band-pass filter of the 
above-described nature is that the band-pass filter 1 needs a large amount 
of strip line resonators, so as to obtain a signal having superior 
characteristics, for instance, a sharp skirt form of a band-edge and a 
narrow passing band. Furthermore, the band-pass filter 1 needs to extend 
the space at the interval La, Lb, Lc and Ld in order to reduce the 
intensity of the coupling between the resonators 2-6. As a result, not 
only the length L of the dielectric substrate 9 but also the size of the 
band-pass filter 1 increases. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a band-pass filter for 
a radio wave having a wavelength range of a high frequency, such as a 
microwave and a millimeter wave. The band-pass filter can be miniaturized 
under the condition that the intensity of the coupling between resonators 
is reduced and the filtered signal has superior characteristics. 
In accordance with an aspect of the present invention, there is provided a 
band-pass filter comprising: a dielectric substrate; input and output 
terminals; and a plurality of conductive strip line resonators being 
capable of resonating with a predetermined wavelength. Each of the strip 
line resonators has two ends and bent line extending from one end to the 
other end with a predetermined length corresponding to the wavelength. The 
one end and the other end are placed face to face with each other to 
provide a gap therebetween. The plurality of resonators have a first 
resonator coupled with the input terminal being capable of resonating with 
the predetermined wavelength and a second resonator arranged apart from 
the first resonator at a predetermined interval and coupled with the first 
resonator through an inductive and capacitive coupling therebetween, being 
capable of resonating with a predetermined wavelength, and further coupled 
with the output terminal to output the resonating signal. 
In the band-pass filter, the plurality of resonators further have at least 
a third resonator intervening between the first and second resonators. The 
plurality of resonators are on the dielectric substrate in series and 
space apart from each other at predetermined intervals along a loop shape 
position line extending from the first resonator to the second resonator. 
The third resonator is coupled with the first and second resonators 
through the inductive and capacitive coupling so that the signal is 
transferred from the first resonator to the second resonator through the 
intervening resonators. Each of the adjoining resonators has a 
predetermined intensity of the coupling between them in accordance with a 
relationship between the positions of the gaps of the adjoining 
resonators. 
Each of the strip line resonators may be shaped into a circular form having 
an opening portion interposed between the one end and the other end. 
Alternatively, each of the strip line resonators may be shaped into a 
U-shaped form having an opening portion interposed between the one end and 
the other end. 
The loop shaped position line may be substantially a circular line 
encircled around a center of the substrate with a predetermined radius. 
The interval between the first and second resonators may be larger than 
that between of any other adjoining resonators. 
Alternatively, the band-pass filter further comprises shielding means 
interposed between the first and second resonators for shielding the 
electromagnetic to prevent the coupling between the first and second 
resonators. The band-pass filter further comprises shielding means placed 
on a center of the loop shaped position line for shielding against 
electromagnetic energy to prevent the coupling between the resonators 
except for the adjoining resonators with each other. 
In the band-pass filter according to the present invention, the signal may 
be microwave or millimeter wave.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Throughout the following detailed description, similar reference characters 
and numbers refer to similar elements in all figures of the drawings and 
they may not be described in detail for all drawing figures. 
Referring now to FIGS. 1 through 9 of the drawings, a preferred embodiment 
of the band-pass filter according to the present invention will be 
explained hereinafter. 
As shown in FIG. 1, the band-pass filter 100 comprises a dielectric 
substrate 110 and a plurality of conductive strip line resonators 10 
serially arranged on the dielectric substrate 110. The dielectric 
substrate 110 has a ground plate made out of a metal forming a disk shape 
having a diameter of 50 mm and thickness of 0.3 .mu.m and a dielectric 
material layer made out of a dielectric material, such as MgO, LaAlO.sub.3 
or Al.sub.2 O.sub.3, deposited on the ground plate to form the dielectric 
material layer having a thickness of 0.5 mm. In the ground plate of the 
dielectric substrate 110, the diameter may be 10 mm-100 mm, while the 
thickness may be 0.1 .mu.m-10 .mu.m. In the dielectric material layer of 
the dielectric substrate 110, the thickness may be 0.1 mm-10 mm. 
Each of the strip line resonators 10 is made out of a conductive material, 
such as a metal, e.g., Au or Cu, or a superconductive material, e.g., 
YBa.sub.2 Cu.sub.3 O.sub.7, TlBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 or Nb, 
deposited on the dielectric material layer of the dielectric substrate 110 
by the conventional pattern formation manner. In this embodiment, each of 
the strip line resonators 10 is made out of a YBa.sub.2 Cu.sub.3 O.sub.7 
having a thickness of 0.3 .mu.m and a width of 0.5 mm. In each of the 
strip line resonators 10, the thickness may be 0.1 .mu.m-10 .mu.m, while 
the width may be 0.1 mm-10 mm. The length of each of the resonators 10 
will be described in the following description. 
Each of the strip line resonators 10 is designed to resonate with a 
predetermined resonating wavelength of .lambda./2. In this embodiment, 
each of the strip line resonators 10 has two ends and curved line 
extending from one end to the other end with a predetermined length 
corresponding to the resonating wavelength to be placed face to face with 
each other to form a circular form having an opening portion interposed 
between the one end and the other end. Each of the strip line resonators 
10 has a diameter of 10 mm. The length of the opening portion between the 
one end and the other end may be, but not limited to, 0.5 mm. 
The strip line resonators 10 are arranged on the dielectric substrate 110 
at predetermined first to eleventh positions represented by the reference 
numerals "11, 12, 13, 14, 15, 16, 17, 18, 19, 20 and 21" in FIG. 1. In 
FIG. 1, the resonator placed at the first position 11, hereinafter 
referred to as "the first resonator 11", is coupled with an input terminal 
(IN) 22, while the resonator placed at the eleventh position 21, 
hereinafter referred to as "the eleventh resonator 21", is coupled with an 
output terminal (OUT) 23. The other resonators placed at the second to 
tenth positions 12, 13, 14, 15, 16, 17, 18, 19 and 20 are hereinlater 
referred to as "the second to tenth resonators 12 to 20", respectively. 
All of the strip line resonators 11 to 21 are arranged on the dielectric 
substrate 110 in series and apart from with each other at predetermined 
intervals of 1-10 mm along a predetermined position line 112 on which the 
center of each of the circular resonators 11 to 21 is put. The position 
line 112 represented by the broken line in FIG. 1 extends from the first 
resonator 11 to the twelfth resonator 21 and is formed into a loop shape 
having a center "O" and a radius "M". In this embodiment, the radius M may 
be several mm to 100 mm. 
It will be explained hereinafter the operation of the above band-pass 
filter 100. 
The signal is inputted to the band-pass filter 100 through the input 
terminal (IN) 22. The inputted signal is transferred to the first 
resonator 11 while the first resonator 11 resonates with its resonating 
wavelength. The resonating signal is transferred from the first resonator 
11 to the second resonator 12 through the inductive and capacitive 
coupling. The signal is transferred from the second resonator 12 to the 
adjoining resonator, i.e., the third resonator 13 while the second 
resonator 12 resonates with its resonating wavelength. Then, the signal is 
serially transferred between the adjoining resonators through the 
inductive and capacitive coupling, finally, transferred from the tenth 
resonator 20 to the eleventh resonator 21 through the inductive and 
capacitive coupling while each of the resonators resonate with its 
resonating wavelength. The filtered signal is thus outputted from the 
band-pass filter 100 through the output terminal (OUT) 23. 
It will be understood from the above description of the operation of the 
band-pass filter 100 that each of resonators is coupled with another 
resonator through the inductive and capacitive coupling. The intensity of 
this coupling is determined on the basis of a relationship between the 
positions of the opening portion of these resonators. As a result, the 
intensity of the coupling can vary in accordance with variation of the 
above position relationship. 
As shown in FIG. 1, the directions from centers of the resonators 11, 12, 
13, 14, 15, 16, 17, 18, 19, 20 and 21 toward the opening portions of the 
resonators 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 and 21 are represented 
by the arrows 11a, 12a, 13a, 14a, 15a, 16a, 17a, 18a, 19a, 20a and 21a 
respectively. In this embodiment, all of the arrows 11a to 21a direct 
toward the center O of the circular position line 112, i.e., the opening 
portions of the resonators 11 to 21 face the center O of the circular 
position line 112. 
The distance between the center of the first resonator 11 and the center of 
the eleventh resonator 21 is larger than the distances between the centers 
of the other resonators, e.g., the first and second resonators 11 and 12 
or the second and third resonators 12 and 13, in order to particularly 
reduce the intensity of the coupling between the input terminal (IN) 22 
and the output terminal (OUT) 23 as small as possible. The band-pass 
filter 100 thus constructed can prevent crosstalk from occurring between 
the first and eleventh resonators 11 and 21. 
In the first embodiment, the first and eleventh resonators 11 and 21 are 
coupled with the input terminal (IN) 22 and the output terminal (OUT) 23, 
respectively, with an electrical tapped connection. Alternatively, the 
resonator 10 arranged at the first or eleventh positions 11 or 21 in FIG. 
1 may be spaced apart from the output terminal 25 as shown in FIG. 2. In 
this case, the resonator 10 is coupled with the terminal 25 by way of the 
inductive and capacitive coupling. 
FIG. 3 shows a pair of resonators represented by the reference characters 
"A and B" in explanation for the position relationship between the 
resonators. The position relationship between the resonators can be 
determined based on various parameters including a length of the 
resonator, a distance D between the center of the resonator A and the 
center the resonator B, and a relationship between the positions of the 
opening portions A' and B' of the adjoining resonators A and B. 
Here, the length of the resonator corresponds to the desired resonating 
wavelength. The distance D is determined based on the radius M of the 
position line 112 and the number of the resonators formed on the 
dielectric substrate 110. As a result, the parameter which can influence 
the intensity of the coupling between the adjoining resonators is only the 
position relationship. 
The above position relationship is referred to an angle formed by the line 
between a center of the resonator and an intermediate point between the 
one end and the other end of the resonator with respect to a vertical axis 
line vertically extending from the center of the resonator. The angles of 
the resonators A and B are represented by the reference characters 
.theta..sub.A and .theta..sub.B in FIG. 3, respectively. In the resonators 
A and B, the angles .theta..sub.A and .theta..sub.B can independently vary 
between 0 and 360 degree. Consequently, the intensity of the coupling 
between the resonators A and B can vary in accordance with the angles 
.theta..sub.A and .theta..sub.B without varying the distance D. 
The first embodiment of the band-pass filter 100 has, therefore, an 
advantage over the prior art in miniaturizing the band-pass filter and 
varying the intensity of coupling between the adjoining resonators in 
accordance with the relationship between the opening portions of the 
adjoining resonators. 
Referring to FIG. 4 of the drawings, there is illustrated a variation of 
intensity of the coupling in accordance with variety of position 
relationships between the adjoining resonators. In this case, the length 
of the resonator is .lambda./2. There are shown six examples of the 
position relationship in FIG. 4. In the first example, in which the 
opening portion of a pair of resonators 101 are opposite to each other, 
the intensity of the inductive coupling between them is the largest among 
all of these examples. Since the length of the resonator is .lambda./2, 
the peak of the wavelength in the resonator, in which the electric current 
density is the largest, just appears at an adjoining point opposite to the 
opening portion of the resonator, thereby causing the strong inductive 
coupling between the resonators. 
This means that the intensity of the inductive coupling between the 
adjoining resonators varies in accordance with the relationship between 
the positions at which the peaks of the electric current density in the 
adjoining resonators appear. It will be clearly understood from the above 
description that the pair of resonators 101 of the first example, a pair 
of resonators 103 of the third example, a pair of resonators 104 of the 
fourth example and a pair of resonators 105 of the fifth example are 
arranged in order of the intensity of the inductive coupling as shown in 
FIG. 4. 
In actual fact the intensity of the coupling between the pair of resonators 
should be obtained by integrating the intensity of the inductive and 
capacitive coupling over all of microscopic area in the resonator on the 
basis of variety of parameters, such as a thickness of the dielectric 
substrate, a dielectric constant of the dielectric substrate or width and 
length of the resonator, utilized for designing the band-pass filter. The 
exact intensity of the coupling should be calculated based on the 
determined parameters by performing the numerical analysis, for instance, 
simulation of the electromagnetic filed. 
Therefore, in a pair of resonators 102 of the second example and a pair of 
resonators 106 of the sixth example, the intensity of the coupling between 
the pair of resonators, in which the arrows indicating the central current 
density points of them cross at right angle with each other, is indicated 
as illustrated in FIG. 4, but not limited to these examples. 
Referring to FIG. 5 of the drawings, a comparative diagram in the size of 
the substrate 110 of the band-pass filter 100 according to the present 
invention compared with that of the substrate 910 of the conventional 
filter. In FIG. 5, the diameter of the substrate 110 is represented by the 
reference character "L1". On the other hand, the reference character "L2" 
represents a diameter of the disk shaped substrate indicated by a broken 
line in FIG. 5. This disk shaped substrate is necessary for the substrate 
910 of the conventional filter to be made when the present invention of 
the filter and the conventional filter have the same number of the 
resonators of 11 and the same characteristics in the coupling. The disk 
shaped substrate is then cut-off and shaped into a rectangular form. 
As shown in FIG. 5, the diameter L1 of the substrate 110 of 2 inches can be 
reduced in comparison with the diameter L2 in the substrate 910 of the 
conventional filter of 4 inches. When the substrate has a small dielectric 
loss, i.e., a single crystal, as well as a large area, it is difficult and 
expensive to make this substrate. Therefore, the diameter of the substrate 
may be preferably small in substrate manufacturing process. 
Furthermore, the area of the substrate 110 is approximately 2025 mm.sup.2, 
while the area of the substrate 910 of the conventional filter is 
approximately 4500 mm.sup.2. As a result, the area of the substrate 110 
can also reduced to less than half of the area of the substrate 910 of the 
conventional filter. Therefore, the filter according to the present 
invention can be miniaturized in comparison with the conventional filter. 
The band-pass filter according to the present invention is not limited to 
that shown in FIG. 1. FIG. 6 shows another layout diagram of the band-pass 
filter according to the present invention. As shown in FIG. 6, the 
band-pass filter 120 has eleven circular strip line resonators same as 
those of the band-pass filter 100 shown in FIG. 1. The resonators may be 
arranged in series at the points 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 
and 21 on the dielectric substrate 110 so as to have the arrows indicating 
the directions of the opening portions of the resonators direct toward the 
outside against the center O. 
Furthermore, the layout of the resonators in the band-pass filter may be 
alternated in the manner as shown in FIG. 7. The resonators of the 
band-pass filter 130 may be arranged in series at the points 11, 12, 13, 
14, 15, 16, 17, 18, 19, 20, and 21 on the dielectric substrate 110 so as 
to have the arrows direct toward the same directions along the position 
line 112 as shown in FIG. 7. 
Alternatively, the band-pass filter 140 may further comprise, if necessary, 
another suitable peripheral device circuit, such as a low noise amplifier 
(LNA) located in a center space of the dielectric substrate 110 as shown 
in FIG. 8. In this case, the band-pass filter 140 thus constructed can 
reduce the area of the communication equipment including the another 
peripheral device circuit. 
The resonator 10 of the band-pass filter according to the present invention 
may have the other forms as shown in FIG. 9. In FIG. 9, the resonator 10a 
has the same form, i.e., a circular form, as the resonator 10 in the above 
description. The resonator 10b has an elliptic shape. The resonator 10c 
has a polygonal shape. The resonator 10d has a rectangular shape. As shown 
in FIG. 9, the variation of the shapes of the resonators result in the 
variation of the intensity of the coupling, for instance, but not limited 
to, the circular shape of the resonator 10a indicates the smallest 
intensity, while the rectangular shape of the resonator 10d indicates the 
largest intensity. It will be understood from the above description that 
the resonator may be formed into any desired shapes so as to obtain the 
desired intensity of the coupling. The variation of the intensity of the 
coupling is shown in FIG. 9 as an example under the specific condition in 
which all of the resonators have the same condition except for the shape 
of the resonator. 
Referring to FIGS. 10 and 11, there is shown a second preferred embodiment 
of the band-pass filter according to the present invention. As shown in 
FIG. 10, the band-pass filter 200 comprises a dielectric substrate 210 
having a rectangular shape and a plurality of strip line resonators. Each 
of the resonators is the same as that of the first embodiment. The 
resonators are arranged on the rectangular shaped dielectric substrate 210 
in series along a straight line and spaced apart from each other. As shown 
in FIG. 10, all of the opening portions of the resonators direct toward 
the same direction vertical with the straight line. 
Alternatively, the opening portions of the resonators may direct different 
directions from each other as shown in FIG. 11. The band-pass filter 220 
thus constructed has different characteristic in the coupling from that of 
the band-pass filter 200 shown in FIG. 10. This results in the fact that 
the second embodiment of the band-pass filter can also obtain a desired 
characteristic in the coupling by varying the position relationship 
between the opening portions of the adjoining resonators without varying 
the distance between the adjoining resonators. 
Referring to FIG. 12, there is shown a third preferred embodiment of the 
band-pass filter according to the present invention. As shown in FIG. 12, 
the band-pass filter 300 comprises a dielectric substrate 310 having a 
circular shape and a plurality of strip line resonators 30. In this 
embodiment, each of the strip line resonators 30 has a U-shaped form or a 
hairpin curved form. Each of the U-shaped resonators 30 has two straight 
lines radially and outwardly extending from the inside of the dielectric 
substrate 310 and an arc portion interposed between outside ends of the 
straight lines to form an opening portion at inside ends of the straight 
lines. The opening portion of each of the U-shaped resonators 30 faces the 
center of the dielectric substrate 310. The resonators 30 are arranged at 
the positions 31, 32, 33, 34, 35, 36, 37, 38, 39, 40 and 41 in series and 
spaced apart from each other at predetermined intervals along a circular 
line indicated by the broken line. 
In the band-pass filter 300 thus constructed, the length of the resonator 
30 can be increased by extending the length of the straight line without 
extending the area of the dielectric substrate 310. This is essential for 
the band-pass filter utilized for the low frequency. 
Referring to FIG. 13 of the drawings, a fourth preferred embodiment of the 
band-pass filter according to the present invention will be described 
hereinafter. The fourth embodiment of the band-pass filter 400 exemplifies 
the positive intention of reducing the undesired coupling in the band-pass 
filter 100 shown in FIG. 1. 
As shown in FIG. 13, the band-pass filter 400 comprises a filter case 53 in 
which the band-pass filter 100 and first and second shields 51 and 52 
assemble. The first and second shields 51 and 52 are made of a conductive 
material, such as a metal, e.g., Au or Cu, or a superconductive material, 
e.g., YBa.sub.2 Cu.sub.3 O.sub.7, TlBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 or 
Nb, and electrically connected to the filter case 53. 
The first shield 51 has, but not limited to, a rectangular board shape 
having a predetermined width. The first shield 51 stands on the dielectric 
substrate 110 of the band-pass filter 100 and interposed between the first 
resonator 11 and the eleventh resonator 21, so that the undesired coupling 
between the first resonator 11 and the eleventh resonator 21 can be 
prevented by shielding against electromagnetic energy. The second shield 
52 has, but not limited to, a cylindrical shape having a predetermined 
width. The second shield 52 stands on a center of the dielectric substrate 
110 of the band-pass filter 100 to shield against electromagnetic energy 
to prevent the coupling among the resonators. 
The filter case 53 has a bottom plate on which the band-pass filter 100 is 
located, and side plates each standing along the edge of the bottom plate 
to encircle the band-pass filter 100 to form a cavity accommodating the 
band-pass filter 100. The filter case 53 has a top plate to put the lid on 
the cavity. 
The fourth embodiment of the band-pass filter 400 thus constructed has an 
advantage over the prior art in shielding the undesired coupling between 
the resonators to lead to the fact that the band-pass filter 400 has an 
improved quality of the filtering characteristic. Furthermore, the first 
resonator 11 through the eleventh resonator 21 can be arranged on the 
dielectric substrate 110 of the band-pass filter 100 as close as possible. 
This results in the fact that the fourth embodiment has an advantage in 
miniaturizing the band-pass filter. 
The many features and advantages of the invention are apparent from the 
detailed specification, and thus it is intended by the appended claims to 
cover all such features and advantages of the invention which fall within 
the true spirit and scope thereof. Further, since numerous modifications 
and changes will readily occur to those skilled in the art, it is not 
desired to limit the invention to the exact construction and operation 
illustrated and described herein, and accordingly, all suitable 
modifications and equivalents may be construed as being encompassed within 
the scope of the invention.