Strip dual mode ring resonator and band-pass filter composed of the resonators

Abstract: A strip loop resonator, comprising: a loop-shaped strip line (153, 162) for resonating a microwave in a dual mode, the electric line length ( L =360 degrees) of the loop-shaped strip line being equivalent to a wavelength of the microwave to resonate the microwave circulated in the loop-shaped strip line according to an average line impedance of the loop-shaped strip line; an input strip line (152) for inputting the microwave to an input point of the loop-shaped strip line in electromagnetic coupling; and an output strip line (154) for outputting the microwave resonated in the loop-shaped strip line from an output point of the loop-shaped strip line in electromagnetic coupling, characterized in that the loop-shaped strip line comprises: a pair of coupling partial lines (153a; 153b, 162a; 162b) arranged in parallel to each other to couple the coupling partial lines of the loop-shaped strip line to each other in electromagnetic coupling, each of the coupling partial lines having an electric length 1 (1<90 degrees) and a first line impedance Z1; a first side strip line of which both ends are connected to first side ends of the coupling partial lines, the first side strip line having an electric length 2 (2>90 degrees) and a second line impedance Z2 different from the first line impedance Z1, the input and output points of the loop-shaped strip line being placed at the first side strip line, and the output point of the loop-shaped strip line being spaced a quarter of the wavelength of the microwave apart from the input point of the loop-shaped strip line; and a second side strip line of which both ends are connected to second side ends of the coupling partial lines, the second side strip line having an electric length 3 ( 3 = 360 - 2*1 - 2 ) and a third line impedance Z3 different from the first line impedance Z1.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION: The present invention relates to a strip dual mode ring resonator utilized to resonate waves in frequency bands ranging from an ultra high frequency (UHF) band to a super high frequency (SHF) band, and relates to a band-pass filter composed of a series of resonators which is utilized as a communication equipment or measuring equipment. 2. DESCRIPTION OF THE RELATED ART: A half-wave length open end type of strip ring resonator has been generally utilized to resonate microwaves ranging from the UHF band to the SHF band. Also, a one-wave length strip ring resonator has been recently known. In the one-wave length strip ring resonator, no open end to reflect the microwaves is required because an electric length of the strip ring resonator is equivalent to one-wave length of the microwaves. Therefore, the microwaves are efficiently resonated because electric energy of the microwaves resonated is not lost in the open end. In addition, in cases where a band-pass filter is composed of a plurality of strip ring resonators arranged in series, a strip dual mode ring resonator functioning as a two-stage filter is required to efficiently filter the microwave in the band-pass filter. 2-1 PREVIOUSLY PROPOSED ART: A first conventional resonator is described. Fig. 1A is a plan view of a one-wave length strip ring resonator in which no open end is provided. Fig. 1B is a sectional view taken generally along the line I-I of Fig. 1A. Each of constitutional elements of the ring resonator shown in Fig. 1A is illustrated in Fig. 1B. As shown in Fig. 1A, a one-wave length strip ring resonator 11 conventionally utilized is provided with an input strip line 12 in which microwaves are transmitted, a closed ring-shaped strip line 13 in which the microwaves transferred from the input strip line 12 are resonated, and an output strip line 14 to which the microwaves resonated in the strip ring 13 are transferred. As shown in Fig. 1B, the input and output strip lines 12, 14 and the ring-shaped strip line 13 respectively consist of a strip conductive plate 15, a dielectric substrate 16 surrounding the strip conductive plate 15, and a pair of conductive substrates 17a, 17b sandwiching the dielectric substrate 16. The ring-shaped strip line 13 has an electric length equivalent to a wavelength of the microwave. The electric length of the ring-shaped strip line 13 is determined by correcting a physical line length of the ring-shaped strip line 13 with a relative dielectric constant ε r of the dielectric substrate 16. The input strip line 12 is arranged at one side of the strip ring 13 and is coupled to the ring-shaped strip line 13 in capacitive coupling. That is, when the microwaves transmit through the input strip line 12, electric field is induced in a gap space between the input strip line 12 and the ring-shaped strip line 13. Therefore, the intensity of electric field in the ring-shaped strip line 13 is also increased at a coupling point P1 adjacent to the input strip line 12 to a maximum value. The output strip line 14 is arranged at an opposite side of the strip ring 13. In other words, the output strip line 14 is spaced 180 degrees (a half-wave length of the microwaves) in the electric length apart from the input strip line 12. In this case, the intensity of the electric field in the ring-shaped strip line 13 is maximized at a coupling point P2 adjacent to the output strip line 14 because the output strip line 14 is spaced 180 degrees in the electric length apart from the input strip line 12. Therefore, the output strip line 14 is electrically coupled to the ring-shaped strip line 13 in capacitive coupling. In the above configuration, when microwaves are transmitted in the input strip line 12, electric field is induced at a gap portion between the input strip line 12 and the ring-shaped strip line 13 by the microwaves. Therefore, the intensity of the electric field in the ring-shaped strip line 13 is maximized at the coupling point P1 adjacent to the input strip line 12. Thereafter, the electric field induced at the coupling point P1 is diffused into the ring-shaped strip line 13 as traveling waves. In other words, the microwaves are transferred from the input strip line 12 to the ring-shaped strip line 13. In this case, a part of the travelling waves are transmitted in a clockwise direction, and a remaining part of the travelling waves are transmitted in a counterclockwise direction. In cases where the wavelength of the microwaves is equivalent to the electric length of the ring-shaped strip line 13, the microwaves are resonated in the ring-shaped strip line 13. Therefore, the intensity of the microwaves in the ring-shaped strip line 13 is amplified. Thereafter, the intensity of the electric field in the ring-shaped strip line 13 is maximized at the coupling point P2 adjacent to the output strip line 14 because the output strip line 14 is spaced 180 degrees in the electric length apart from the input strip line 12. Therefore, the electric field is induced at a gap space between the ring-shaped strip line 13 and the output strip line 14. As a result, the microwave resonated in the ring-shaped strip line 13 is transferred to the output strip line 14. Accordingly, the strip ring resonator 11 functions as a resonator of the microwaves. In this case, the microwaves can be resonated in the strip ring 13 even though the electric length of the ring-shaped strip line 13 is an integral multiple of the wavelength of the microwaves. The strip ring resonator 11 is often utilized to estimate the dielectric substrate 16 because a resonance frequency (or a central frequency) of the microwaves is shifted according to a physical shape of the dielectric substrate 16 and the relative dielectric constant ε r of the dielectric substrate 16. The strip ring resonator 11 is described in detail in the literature "Resonant Microstrip Ring Aid Dielectric Material Testing", Microwaves & RF, page 95-102, April, 1991. 2-2 ANOTHER PREVIOUSLY PROPOSED ART: A second conventional resonator is described. Fig. 2 is a plan view of a strip dual mode ring resonator functioning as a two-stage filter. As shown in Fig. 2, a strip dual mode ring resonator 21 conventionally utilized is provided with an input strip line 22 in which microwaves are transmitted, a one-wave length strip ring 23 electrically coupled to the input strip line 22 in capacitive coupling, and an output strip line 24 electrically coupled to the strip ring 23 in capacitive coupling. The input strip line 22 is coupled to the strip ring 23 through a gap capacitor 25, and the output strip line 24 is coupled to the strip ring 23 through a gap capacitor 26. Also, the output strip line 24 is spaced 90 degrees (or a quarter-wave length of the microwaves) in the electric length apart from the input strip line 22. The strip ring 23 has an open end stub 27 in which the microwaves are reflected. The open end stub 27 is spaced 135 degrees (or 3/8-wave length of the microwaves) in the electric length apart from the input and output strip lines 22, 24. In the above configuration, the action of the strip dual mode ring resonator 21 is qualitatively described in a concept of travelling waves. When travelling waves are transmitted in the input strip line 22, electric field is induced in the gap capacitor 25. Therefore, the input strip line 22 is coupled to the strip ring 23 in the capacitive coupling, so that a strong intensity of electric field is induced at a point P3 of the strip ring 23 adjacent to the input strip line 22. That is, the travelling waves are transferred to the coupling point P3 of the strip ring 23. Thereafter, the travelling waves are circulated in the strip ring 23 to diffuse the electric field strongly induced in the strip ring 23. In this case, a part of the travelling waves are transmitted in a clockwise direction and a remaining part of the travelling waves are transmitted in a counterclockwise direction. An action of the travelling waves transmitted in the counterclockwise direction is initially described. When the travelling waves transmitted in the counterclockwise direction reach a coupling point P4 of the strip ring 23 adjacent to the output line 24, the phase of the travelling wave shifts by 90 degrees. Therefore, the intensity of the electric field at the coupling point P4 is minimized. Accordingly, the output strip line 24 is not coupled to the strip ring 23 so that the travelling waves are not transferred to the output strip line 24. Thereafter, when the travelling waves reach the open end stub 27, the phase of the travelling wave further shifts by 135 degrees as compared with the phase of the travelling wave reaching the coupling point P4. Because the open end stub 27 is equivalent to a discontinuous portion of the strip ring 23, a part of the travelling waves are reflected at the open end stub 27 to produce reflected waves, and a remaining part of the travelling waves are not reflected at the open end stub 27 to produce non-reflected waves. The non-reflected waves are transmitted to the coupling point P3. In this case, because the phase of the non-reflected waves transmitted to the coupling point P3 totally shifts by 360 degrees as compared with that of the travelling waves transferred from the input strip line 22 to the coupling point P3, the intensity of the electric field at the coupling point P3 is maximized. Therefore, the input strip line 22 is coupled to the strip ring 23 so that a part of the non-reflected waves are returned to the input strip line 22. A remaining part of the non-reflected waves are again circulated in the counterclockwise direction so that the microwaves transferred to the strip ring 23 are resonated. In contrast, the reflected waves are returned to the coupling point P4. In this case, the phase of the reflected waves at the point P4 further shifts by 135 degrees as compared with that of the reflected wave at the open end stub 27. This is, the phase of the reflected wave at the point P4 totally shifts by 360 degrees as compared with that of the travelling waves transferred from the input strip line 22 to the coupling point P3. Therefore, the intensity of the electric field at the coupling point P4 is maximized, so that the output strip line 24 is coupled to the strip ring 23. As a result, a part of the reflected wave is transferred to the output strip line 24. A remaining part of the reflected wave is again circulated in the clockwise direction so that the microwave transferred to the strip ring 23 is resonated. Next, the travelling waves transmitted in the clockwise direction is described. A part of the travelling waves transmitted in the clockwise direction are reflected at the open end stub 27 to produce reflected waves when the phase of the travelling waves shifts by 135 degrees. Non-reflected waves formed of a remaining part of the travelling waves reach the coupling point P4. The phase of the non-reflected waves totally shifts by 270 degrees so that the intensity of the electric field induced by the non-reflected waves is minimized. Therefore, the non-reflected waves are not transferred to the output strip line 24. That is, a part of the non-reflected waves are transferred from the coupling point P3 to the input strip line 22 in the same manner, and a remaining part of the non-reflected waves are again circulated in the clockwise direction so that the microwave transferred to the strip ring 23 is resonated. In contrast, the reflected waves are returned to the coupling point P3. In this case, because the phase of the reflected waves at the coupling point P3 totally shifts by 270 degrees, the intensity of the electric field induced by the reflected waves are minimized so that the reflected waves are not transferred to the input strip line 22. Thereafter, the reflected waves reach the coupling point P4. In this case, because the phase of the reflected waves at the coupling point P4 totally shifts by 360 degrees, the intensity of the electric field induced by the reflected waves is maximized. Therefore, a part of the reflected waves are transferred to the output strip line 24, and a remaining part of the reflected waves are again circulated in the counterclockwise direction so that the microwaves transferred to the strip ring 23 are resonated. Accordingly, because the microwaves can be resonated in the strip ring 23 on condition that a wavelength of the microwaves equals the electric length of the strip ring 23, the strip dual mode ring resonator 21 functions as a resonator and a filter. Also, the microwaves transferred from the input strip line 22 are initially transmitted in the strip ring resonator 23 as the non-reflected waves, and the microwaves are again transmitted in the strip ring resonator 23 as the reflected waves shifting by 90 degrees as compared with the non-reflected waves. In other words, two orthogonal modes formed of the non-reflected waves and the reflected waves independently coexist in the strip ring resonator 23. Therefore, the strip dual mode filter 21 functions as a dual mode filter. That is, the function of the strip dual mode filter 21 is equivalent to a pair of a single mode filters arranged in series. In addition, a ratio in the intensity of the reflected waves to the non-reflected waves is changed in proportional to the length of the open end stub 27 projected in a radial direction of the strip ring resonator 23. Therefore, the intensity of the reflected microwave transferred to the output strip line 24 can be adjusted by trimming the open end stub 27. The strip dual mode ring resonator 21 is proposed by J.A. Curtis "International Microwave Symposium Digest", IEEE, page 443-446(N-1), 1991. A further strip dual mode ring resonator is known from the article by M. Guglielmi et. al. entitled "Experimental Investigation of Dual-Mode Microstrip Ring Resonators" in Proceedings of the 20 th European Microwave Conference 1990, pages 901-906, wherein a strip dual mode ring resonator in which a microwave is resonated, comprises: a loop-shaped strip line having an electric length L=360 degrees equivalent to a wavelength of the microwave to resonate the microwave circulated therein in two different directions according to a line impedance thereof. Further, the strip dual mode ring resonator comprises an input strip line in which the microwave is transmitted, and an output strip line in which the microwave resonated in the loop-shaped strip line is transmitted. Additionally, a dual-mode behavior is obtained by exciting a specific resonant mode twice, with a rotation of 90° degrees in the respective field patterns to ensure the orthogonality of the two modes. 2-3 PROBLEMS TO BE SOLVED BY THE INVENTION: However, there are many drawbacks in the strip ring resonator 11. That is, it is difficult to manufacture a small-sized strip ring resonator 11 because a central portion surrounded by the ring-shaped strip line 13 is a dead space. Also, the electric length of the ring-shaped strip line 13 cannot be minutely adjusted after the ring-shaped strip line 13 is manufactured according to a photo-etching process or the like. In this case, the resonance frequency of the microwaves depends on the electric length of the ring-shaped strip line 13. Therefore, the resonance frequency of the microwaves cannot be minutely adjusted. In addition, in cases where a plurality of strip ring resonators 11 are arranged in series to compose a band-pass filter, it is difficult to couple the ring-shaped strip lines 13 to each other because the ring-shaped strip lines 13 are curved. Also, there are many drawbacks in the strip ring resonator 21. That is, a central frequency of the microwaves filtered in the strip ring resonator 21 cannot be minutely adjusted because the central frequency of the microwaves depends on the width of the open end stub 27 extending in a circumferential direction of the strip ring 23. Therefore, the central frequency of the microwaves manufactured does not often agree with a designed central frequency. As a result, a yield rate of the strip ring resonator 21 is lowered. Also, because a resonance width (or a full width at half maximum) can be adjusted only by trimming the length of the open end stub 27, the resonance width cannot be enlarged. In other words, in cases where the width of the open end stub 27 in the circumferential direction is widened to enlarge the resonance width, the phase of the reflected waves reaching the output strip line 24 undesirably shifts. As a result, the intensity of the microwaves transferred to the output strip line 24 is lowered at the central frequency of the microwaves resonated. Accordingly, in cases where a plurality of strip ring resonators 21 are arranged in series to compose a band-pass filter, the filter is limited to a narrow passband type of filter. SUMMARY OF THE INVENTION The object is to provide a small-sized strip dual mode ring resonator in which the resonance frequency is easily and minutely adjusted and the resonance width is narrow, and to provide a band-pass filter composed of the resonators. The object is achieved by the provision of a strip dual mode ring resonator according to claim 1 and a bandpass filter in which microwave is resonated, as specified in claim 12. BRIEF DESCRIPTION OF THE DRAWINGS The objects, features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which: Fig. 1A is a plan view of a conventional one-wave length type of strip ring resonator in which no open end is provided; Fig. 1B is a sectional view taken generally along the line I-I of Fig. 1A; Fig. 2 is a plan view of a conventional strip dual mode ring resonator functioning as a two-stage filter; Fig. 3 is a plan view of a strip dual mode ring resonator according to a first embodiment; Fig. 4 is a plan view of a strip dual mode ring resonator according to a second embodiment; Fig. 5 is a plan view of a band-pass filter in which four strip dual mode ring resonators shown in Fig. 16 are arranged in series according to a third embodiment; DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a strip dual mode loop resonator and a band-pass filter composed of the resonators according to the present invention are described with reference to drawings. Fig. 3 is a plan view of a strip dual mode ring resonator according to a first embodiment. As shown in Fig. 3, a strip dual mode ring resonator 151 comprises an input strip line 152 in which microwaves are transmitted, a loop-shaped strip line 153 in which the microwaves transferred from the input strip line 152 are resonated, an output strip line 154 in which the microwaves resonated in the loop-shaped strip line 153 are transmitted, an input coupling capacitor 155 having a lumped capacitance Cc for coupling the input strip line 152 to the loop-shaped strip line 153 in capacitive coupling, an output coupling capacitor 156 having the lumped capacitance Cc for coupling the loop-shaped strip line 153 to the output strip line 154 in capacitive coupling, and an open end stub 157 for changing the characteristic impedance of the loop-shaped strip line 153. An electric length of the loop-shaped strip line 153 agrees with a resonance wavelength λ o , and the loop-shaped strip line 153 is divided into three sections. A pair of widened strip lines 153a, 153b are provided in a first section of the loop-shaped strip line 153. The widened strip line sections 153a, 153b are arranged in parallel to each other. The widened strip lines 153a, 153b respectively have an electric length 1 (1<90°), a widened width W1, and a line impedance Z1. A second section of the loop-shaped strip line 153 is positioned at a first side (or a left side in Fig. 15) of the first block, and a U-shaped narrow strip line 153c having an electric length 2 (2>90°) is provided in the second section. One end of the U-shaped narrow strip line 153c is connected to a first side end of the widened strip line 153a, and the other end of the U-shaped narrow strip line 153c is connected to a first side end of the widened strip line 153b. A width of the narrow strip line 153c is W2 narrower than the widths W1 of the widened strip lines 153a, 153b, and a line impedance of the narrow strip line 153c is Z2. Because both straight portions of the U-shaped narrow strip line 153c are approached each other, the straight portions of the U-shaped narrow strip line 153c are coupled to each other in the electromagnetic coupling. A third section of the loop-shaped strip line 153 is positioned at a second side (or a right side in Fig. 15) of the first section, and a U-shaped narrow strip line 153d is provided in the third section. One end of the narrow strip line 153d is connected to a second end of the widened strip line 153a, and the other end of the narrow strip line 153d is connected to a second end of the widened strip line 153b. The narrow strip line 153d has an electric length 3, the width W2, and a line impedance Z3. In this case, a relational equation 2*1 + 2 + 3 = 360 degrees is satisfied. Also, the line impedance Z1 differs from the line impedance Z2 and the line impedance Z3 to produce four line impedance difference points at boundaries of the blocks in the loop-shaped strip line 153. Also, a flat surface is formed of an inside surface of the widened strip line 153a, an inside surface of the narrow strip line 153c, and an inside surface of the narrow strip line 153d. Also, another flat surface is formed of an inside surface of the widened strip line 153b, another inside surface of the narrow strip line 153c, and another inside surface of the narrow strip line 153d. That is, the widened strip lines 153a, 153b are manufactured by outwardly widening strip lines as compared with the narrow strip line 153c. Therefore, electromagnetic coupling between the widened strip lines 153a, 153b, electromagnetic coupling between both ends of the narrow strip line 153c, and electromagnetic coupling between both ends of the narrow strip line 153d are the same. The input and output strip lines 152, 154 are respectively formed of a plate capacitor, and are coupled to the narrow strip line 153c through the input and output coupling capacitors 155, 156. One end of the input coupling capacitor 155 is connected to an input point A of the narrow strip line 153c, and one end of the output coupling capacitor 156 is connected to an output point B of the narrow strip line 153c. The input and output points A, B are symmetrically positioned with respect to the narrow strip line 153c, and the output point B is spaced 90 degrees (or a quarter-wave length of the microwaves) in the electric length apart from the input point A. The open end stub 157 is connected to the middle of the narrow strip line 153d, and the open end stub 157 is arranged at equal intervals (or 135 degrees in the electric length) from the input and output points A, B. In the above configuration, microwaves having various wavelengths around the resonance wavelength λ o is transferred from the input strip line 152 to the loop-shaped strip line 153 because the input strip line 152 is coupled to the strip line 153 by the action of the input coupling capacitor 155. In the strip line 153, the line impedance of the strip line 153 is changed by the line impedance difference points in the strip line 153. Therefore, the microwaves are reflected in each of the blocks to produce reflected waves. Also, the microwaves are reflected in the open end stub 158. This is, the characteristic impedance of the strip line 153 is determined according to the electromagnetic coupling between the widened lines 153a, 153b, the line impedances Z1, Z2, and Z3 of the blocks, the electric lengths 1, 2, and 3, and the open end stub 157. Thereafter, the reflected waves are circulated in the strip line 153 in clockwise and counterclockwise directions. Thereafter, in cases where the wavelength of the microwaves agrees with the electric length of the strip line 153, the microwaves are resonated in the strip line 153. In this case, the intensity of the microwaves reflected in the open end stub 157 is varied by trimming the open end stub 158. Thereafter, the intensity of the electric field at the output point B is maximized by the microwaves resonated in the strip line 153. Therefore, the microwaves resonated are transferred to the output strip line 154 by the action of the output coupling capacitor 156. Accordingly, because the microwaves are resonated in the strip line 153 on condition that the wavelength of the microwaves agrees with the resonance wavelength λ o , the strip dual mode ring resonator 151 functions as a resonator and filter. Also, the microwaves transferred from the input strip line 152 are initially transmitted in the strip line 153 as non-reflected waves, and the microwaves are again transmitted in the strip line 153 as the reflected waves shifting by 90 degrees as compared with the non-reflected waves. In other words, two orthogonal modes formed of the non-reflected waves and the reflected waves independently coexist in the strip dual mode ring resonator 151. Therefore, the strip dual mode ring resonator 151 functions as a two-stage filter in the same manner as the conventional strip dual mode ring resonator 21. Also, because the characteristic impedance of the strip line 153 is determined according to the electromagnetic coupling between the widened lines 153a, 153b, the line impedances Z1, Z2, and Z3 of the blocks, the electric lengths 1, 2, and 3, and the open end stub 157, the characteristic impedance can be suitably adjusted in a wide range. Therefore, a resonance width of the resonated microwaves can be suitably adjusted by changing the characteristic impedance. That is, the strip dual mode ring resonator 151 having a widened resonance width can be manufactured. Also, a central frequency of the resonated microwaves can be adjusted by changing the characteristic impedance. Specifically, the central frequency of the resonated microwaves can be minutely adjusted by trimming the open end stub 157 after the strip dual mode ring resonator 151 is manufactured. Also, because the central frequency of the resonated microwaves can be adjusted after the strip dual mode ring resonator 151 is manufactured, a yield rate of the resonator 151 can be increased. Also, because the characteristic impedance can be suitably adjusted in a wide range, the resonator 151 having a superior performance can be stably manufactured. Next, a second embodiment of the third concept according to the present invention is described. Fig. 16 is a plan view of a strip dual mode ring resonator according to a second embodiment of the third concept. As shown in Fig. 4, a strip dual mode ring resonator 161 comprises the input strip line 152, a loop-shaped strip line 162 in which the microwaves transferred from the input strip line 152 are resonated, the output strip line 154, the input coupling capacitor 155, the output coupling capacitor 156, and the open end stub 157. An electric length of the loop-shaped strip line 162 agrees with a resonance wavelength λ o , and the loop-shaped strip line 162 is divided into three sections. A pair of straight strip lines 162a, 162b are provided in a first section of the loop-shaped strip line 162. The straight strip lines 162a, 162b are arranged in parallel to each other. The straight strip lines 162a, 162b respectively have an electric length 1 (1<90°), a width W1, and a line impedance Z1. A second section of the loop-shaped strip line 162 is positioned at a first side (or a left side in Fig. 16) of the first block, and a U-shaped narrow strip line 162c having an electric length 2 (2>90°) is provided in the second block. One end of the U-shaped narrow strip line 162c is connected to a first side end of the straight strip line 162a, and the other end of the U-shaped narrow strip line 162c is connected to a first side end of the straight strip line 162b. A width of the narrow strip line 162c is W2 narrower than the widths W1 of the straight strip lines 162a, 162b, and a line impedance of the narrow strip line 162c is Z2. Because both straight portions of the U-shaped narrow strip line 162c are approached each other, the straight portions of the U-shaped narrow strip line 162c are coupled to each other in the electromagnetic coupling. A third section of the loop-shaped strip line 162 is positioned at a second side (or a right side in Fig. 16) of the first block, and a U-shaped widened strip line 162d is provided in the third section. One end of the widened strip line 162d is connected to a second end of the straight strip line 162a, and the other end of the widened strip line 162d is connected to a second end of the straight strip line 162b. The widened strip line 162d has an electric length 3, a width W3 wider than W2, and a line impedance Z3. In this case, a relational equation 2*1 + 2 + 3 = 360 degrees is satisfied. Also, the line impedances Z1, Z2, and Z3 differ from each other. Therefore, there are four line impedance difference points at boundaries of the blocks in the loop-shaped strip line 162. Also, a flat surface is formed of an outside surface of the straight strip line 162a, an outside surface of the narrow strip line 162c, and an outside surface of the widened strip line 162d. Also, another flat surface is formed of an outside surface of the straight strip line 162b, an outside surface of the narrow strip line 162c, and an outside surface of the widened strip line 162d. That is, the straight and widened strip lines 162a, 162b, 162d are manufactured by inwardly widening strip lines as compared with the narrow strip line 162c. Therefore, a distance between the straight strip lines 162a, 162b is narrower than that between both ends of the narrow strip line 162c. Also, a distance between both ends of the widened strip line 162d is narrower than that between the straight strip lines 162a, 162b. As a result, electromagnetic coupling between the straight strip lines 162a, 162b is stronger than that between both ends of the narrow strip line 162c. Also, electromagnetic coupling between both ends of the widened strip line 162d is stronger than that between the straight strip lines 162a, 162b. The input and output strip lines 152, 154 are coupled to the narrow strip line 162c through the input and output coupling capacitors 155, 156. One end of the input coupling capacitor 155 is connected to an input point A of the narrow strip line 162c, and one end of the output coupling capacitor 156 is connected to an output point B of the narrow strip line 162c. The input and output points A, B are symmetrically positioned with respect to the narrow strip line 162c, and the output point B is spaced 90 degrees (or a quarter-wave length of the microwaves) in the electric length apart from the input point A. The open end stub 157 is connected to the middle of the widened strip line 162d, and the open end stub 157 is arranged at equal intervals (or 135 degrees in the electric length) from the input and output points A, B. In the above configuration, microwaves having various wavelengths around the resonance wavelength λ o are transferred from the input strip line 152 to the strip line 162 because the input strip line 152 is coupled to the strip line 162 by the action of the input coupling capacitor 155. In the strip line 162, the line impedance of the strip line 162 is changed by the line impedance difference points. Therefore, the microwaves are reflected in each of the blocks to produce reflected waves. Also, the microwaves are reflected in the open end stub 157. This is, the characteristic impedance of the strip line 162 is determined according to the electromagnetic coupling between both ends of the narrow strip line 162c, the electromagnetic coupling between the straight strip lines 162a, 162b, the electromagnetic coupling between both ends of the widened strip line 162d, the line impedances Z1, Z2, and Z3 of the blocks, the electric lengths 1, 2, and 3, and the open end stub 157. Thereafter, the reflected waves are circulated in the strip line 162 in clockwise and counterclockwise directions. Thereafter, in cases where the wavelength of the microwaves agrees with the electric length of the strip line 162, the reflected waves are resonated in the strip line 162. In this case, the intensity of the microwaves reflected in the open end stub 157 is varied by trimming the open end stub 168. Thereafter, intensity of electric field at the output point B is maximized by the microwaves resonated in the strip line 162. Therefore, the microwaves resonated are transferred to the output strip line 154 by the action of the output coupling capacitor 156. Accordingly, because the straight strip lines 162a, 162b and the widened strip line 162d are inwardly widened each other, an occupied area required to manufacture the strip dual mode ring resonator 161 can be minimized as compared with the resonator 151 shown in Fig. 3. Also, the strip dual mode ring resonator 161 functions as a dual mode resonator and filter in the same manner as the resonator 151 shown in Fig. 3. Also, a resonance width and a central frequency can be adjusted in the same manner as the resonator 151 shown in Fig. 15. Also, because the central frequency of the resonated microwaves is adjusted by changing the characteristic impedance of the strip line 162 and the length of the open end stub 157, a yield rate of the resonator 161 can be increased in the same manner as the resonator 151 shown in Fig. 15. In the second embodiment , all of the narrow and widened strip lines 162a, 162b, 162c, 162d are coupled to each other in the electromagnetic coupling. However, it is not necessary to couple all of the narrow and widened strip lines 162a, 162b, 162c, 162d to each other. In the first and second embodiments, the open end stub 157 is attached to the narrow strip line 153d and the widened strip line 162d. Also, the input and output coupling capacitors 155, 156 are arranged to couple the input and output strip lines 152, 154 to the narrow strip lines 153c, 162c. However, it is preferred that the input and output gap capacitors 52, 54 shown in Fig. 5 be arranged to couple the input and output strip lines 152, 154 to the narrow strip lines 153c, 162c. Next, a third embodiment according to the present invention is described. Fig. 5 is a plan view of a band-pass filter in which four strip dual mode ring resonators 161 shown in Fig. 4 are arranged in series according to a third embodiment of the third concept. As shown in Fig. 5, a band-pass filter 171 according to the third embodiment comprises a series of four strip dual mode ring resonators 161. That is, the strip dual mode ring resonator 161 in a first stage is connected with the strip dual mode ring resonator 161 in a second stage through an inter-stage coupling capacitor 172, the strip dual mode ring resonator 161 in the second stage is connected with the strip dual mode ring resonator 161 in a third stage through an inter-stage coupling capacitor 173, and the strip dual mode ring resonator 161 in the third stage is connected with the strip dual mode ring resonator 161 in a fourth stage through an inter-stage coupling capacitor 174. In the above configuration, each of the strip lines 162 in the strip dual mode ring resonators 161 functions as a dual mode resonator and filter. Therefore, the band-pass filter 171 functions as an eight-stage filter. Accordingly, because central hollow portions of the resonators 161 are minimized, and because the central hollow portions are efficiently utilized to couple the strip lines 162a to 162d, an area occupied by the filter 171 can be minimized. In the third embodiment, three resonators 161 according to the second embodiment is utilized to manufacture the filter 171. However, the number of the resonators 161 is not limited to four. Also, it is preferred that a plurality of resonators 151 shown in Fig. 15 be arranged in series to manufacture a band-pass filter. Also, it is preferred that the resonators 151, 161 be combined. Also, it is preferred that the filter 171 comprise a multilayer type of resonators in which a plurality of resonators 151 or 161 are arranged in a tri-plate structure. In the first and third embodiment, the strip lines (or balanced strip lines) are utilized to manufacture the resonators 151, 161 and the filter 171. However, it is preferred that microstrip lines be utilized to manufacture the resonators 151, 161 and the filter 171.

Claims: A strip dual mode ring resonator (151) in which a microwave is resonated, comprising: a loop-shaped strip line having an electric length L=360 degrees equivalent to a wavelength of the microwave to resonate the microwave circulated therein in two different directions according to a line impedance thereof, the loop-shaped strip line comprising a pair of parallel line sections (153a,b) and which are electromagnetically coupled to each other, the parallel line sections respectively having an electric length 1 degrees (1<90 degrees) and a line impedance Z1, a first side strip line (153c) through which first side ends of the parallel line sections (153a,b) are connected, the first side strip line having an electric length 2 degrees whereby 2>90 degrees and a line impedance Z2 differing from the line impedance Z1, and a second side strip line (153d) through which second side ends of the parallel line sections are connected, the second side strip line having an electric length 3 degrees whereby 3 = 360 - 2 * 1 - 2 and a line impedance Z3 differing from the line impedance Z1, an input strip line in which the microwave is transmitted, an input impedance element (155) for coupling the input strip line to the first side strip line of the loop-shaped strip line in electromagnetic coupling to transfer the microwave from the input strip line to an input point (A) of the first side strip line, an output strip line in which the microwave resonated in the loop-shaped strip line is transmitted, and an output impedance element (156) for coupling the output strip line to the first side strip line of the loop-shaped strip line in electromagnetic coupling to transfer the microwave from an output point (B) of the first side strip line to the output strip line, the output point (B) of the first side strip line being spaced 90 degrees in the electric length apart from the input point (A) of the first side strip line. A resonator according to claim 1, the strip dual mode ring resonator additionally includes an open end stub (157) for reflecting the microwave to change the line impedance of the loop-shaped strip line, the open end stub being arranged at a middle point of the second side strip line to be spaced a three-eighth of the wavelength of the microwave apart from the input and output points of the first side strip line, and intensity of the microwave reflected by the open end stub being changed by trimming the open end stub. A resonator according to claim 1, the strip dual mode ring resonator additionally includes a capacitor having a variable capacitance for changing the line impedance of the loop-shaped strip line, one end of the capacitor being connected to a middle point of the second side strip line to be spaced a three-eighth of the wavelength of the microwave apart from the input and output points of the loop-shaped strip line, and another end of the capacitor being grounded. A resonator according to claim 1 in which the widths of the parallel lines are wider than the widths of the first and second side strip lines. A resonator according to claim 1 in which the distance between the parallel line sections is narrower than the distance between both ends of the first side strip line which is bent in U shape. A resonator according to claim 5 in which the distance between both ends of the second side strip line which is bent in U shape is narrower than the distance between the parallel line sections. A resonator according to claim 1 in which the input impedance element is an input coupling capacitor for coupling the input strip line to the first side strip line of the loop-shaped strip line in capacitive coupling, and the output impedance element is an output coupling capacitor for coupling the output strip line to the first side strip line of the loop-shaped strip line in capacitive coupling. A resonator according to claim 7 in which the input coupling capacitor has a lumped capacitance. A resonator according to claim 7 in which the input coupling capacitor has a distributed capacitance. A resonator according to claim 7 in which the output coupling capacitor has a lumped capacitance. A resonator according to claim 7 in which the output coupling capacitor has a distributed capacitance. A band-pass filter (171) for filtering a microwave, comprising: a plurality of loop-shaped strip lines (161) arranged in series, each of the loop-shaped strip lines having an electric length  L =360 degrees equivalent to a wavelength of the microwave to resonate the microwave circulated therein in two different directions according to a line impedance thereof, each of the loop-shaped strip lines comprising a pair of parallel line sections and which are electromagnetically coupled to each other the parallel line sections respectively having an electric length 1 degrees (1<90 degrees) and a line impedance Z1, a first side strip line through which first side ends of the parallel line sections are connected, the first side strip line having an electric length 2 degrees (2>90 degrees) and a line impedance Z2 differing from the line impedance Z1, and a second side strip line through which second side ends of the parallel line sections are connected, the second side strip line having an electric length 3 degrees (3 = 360 - 2*1 - 2) and a line impedance Z3 differing from the line impedance Z1; an input strip line (152) in which the microwave is transmitted; an input impedance element (153) for coupling the input strip line to the first side strip line of the loop-shaped strip line arranged in a first stage in electromagnetic coupling to transfer the microwave from the input strip line to an input point of the first side strip line; a plurality of inter-stage impedance elements (172,173,174) which each are arranged between a pair of loop-shaped strip lines; an output strip line (154) in which the microwave resonated in the loop-shaped strip line is transmitted; and an output impedance element (156) for coupling the output strip line to the first side strip line of the loop-shaped strip line arranged in a final stage in electromagnetic coupling to transfer the microwave from an output point of the first side strip line to the output strip line, the output point of the first side strip line being spaced 90 degrees in the electric length apart from the input point of the first side strip line in each of the loop-shaped strip lines.

Inventors: FUJIMURA MUNENORI, HASEGAWA MAKOTO, MAKIMOTO MITSUO, TAKAHASHI KAZUAKI, FUJIMURA, MUNENORI, HASEGAWA, MAKOTO, MAKIMOTO, MITSUO, TAKAHASHI, KAZUAKI, Fujimura, Munenori, Mijamatsu-ryo
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H01P1
IPC codes: H01P1, H01P7