Distributed-constant-line-type filter

Abstract: A line width (w1) on the grounded side of distributed-constant-line-type resonators (2, 3) which form a distributed-constant-line-type filter (1) is made wider than a line width (w2) on the open side. By increasing the line width (w1) on the grounded side, where a larger electric current flows, of a distributed-constant-line-type resonator (1), the line resistance of the distributed-constant line decreases, making it possible to reduce the loss of the distributed-constant-line-type resonator (2, 3) and further to reduce the insertion loss of the distributed-constant-line-type filter (1). Further, by decreasing the line width (w2) on the open side to less than the line width (w1) on the grounded side, an increase in the size of the filter (10) because the line width (w1) on the grounded side is increased can be prevented.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter device and, more particularly, to a distributed-constant-line-type filter. 2. Description of the Related Art Since distributed-constant-line-type filters are typically formed of a strip line, they are thinner and lighter than filters using a block resonator, and are used for signal processing of a portable telephone set requiring a strictly smaller size. Fig. 9 shows an example of a distributed-constant-line-type filter formed by combining conventional distributed-constant-line-type resonators. A distributed-constant-line-type filter 100 is a comb-line-type filter, namely, a comb-type filter formed of a plurality of distributed-constant-line-type resonators 101, 102, 103, 104 and 105, and a grounding electrode 106 connected to the resonators. The resonators are disposed at such positions as to be coupled to each other. One end of each of the distributed-constant-line-type resonators 101 to 105 is an open end. A distributed-constant line 107 for input is connected to the outermost resonator 101. A distributed-constant line 108 for output is connected to another outermost resonator 105. The distributed-constant-line-type resonators 101 to 105, the grounding electrode 106, the input/output distributed-constant lines 107 and 108 are strip lines or may be microstrip lines. Since the distributed-constant-line-type resonators 101, 102, 103, 104 and 105 form a resonance circuit, components having a resonance frequency of a resonance circuit from among signal components from the input 107 are sent to the output 108, and other signals are reflected by the resonance circuit and return to the input 107. That is, the distributed-constant-line-type filter 100 operates as a band-pass filter. As a main factor for a signal loss in the distributed-constant line, there is a loss due to resistance (line resistance). In order to reduce loss, it is common practice to widen the line width so as to lower the line resistance. However, in the conventional filter 100, if the line width of the resonator is widened, the spacing between the adjacent lines becomes narrow, and the coupling between the resonators becomes too strong, causing the characteristics of the filter to vary. In order to reduce the loss of the line and adjust the coupling between the resonators to a predetermined level, the spacing between the adjacent lines may be widened. However, the size of the filter in the right-to-left direction in the figure increases. EP-A-0688058 relates to a resonator comprising a dielectric substrate having a plurality of strip lines disposed on a main surface thereof. One end of the strip lines is fed around the edge of the dielectric substrate and connected to a ground electrode on the back side thereof. An input terminal and an output terminal are connected directly to the outermost of the strip lines on the back side of the dielectric substrate. The strip lines are arranged in an alternative fashion extending from both sides of the upper surface of the substrate such that narrowed top end portions thereof overlap each other in spaced apart relationship so as to be electromagnetically coupled. The Patent Abstracts of Japan, Vol. 96, No. 10, October 31, 1996 (see also JP 08167801 A) show a high frequency filter comprising two spirally formed strip lines. First ends of the strip lines are connected to input/output electrodes, whereas second ends of the strip lines are connected to a ground electrode. EP-A-0 641 035 relates to a filter device comprising two width-stepped microstrip line resonators, wherein a signal input terminal and a signal output terminal is capacatively coupled to one end of the resonators respectively and wherein the other ends are grounded. It is the object of the present invention to provide a small filter device having a small amount of signal loss. This object is achieved by a filter device according to claim 1. According to the present invention, in a filter device having distributed-constant-line-type resonators, since the line widths are different depending upon the section, a compact resonator is provided. Further, the line width is wider in a section within the resonator where the current amplitude is large and narrower in a section where the current amplitude is small; therefore, it is possible to effectively reduce the conductor loss of the resonator. That is, in comparison with a method of reducing conductor loss by widening the line width in all the resonators, it is possible to effectively reduce the conductor loss while keeping the resonator compact. Furthermore, since the distributed-constant line which forms the resonator is folded, the resonator becomes compact. Even if the section in the line where the current amplitude is large is widened, it is possible to keep the entire size of the folding compact by narrowing the line width in the section where the current amplitude, which is another line which forms the folding, is small. The above and further objects, aspects and novel features of the invention will become more apparent from the following detailed description when read in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the construction of an embodiment of a distributed-constant-line-type filter according to the present invention; Fig. 2 shows the construction of another embodiment of a distributed-constant-line-type filter according to the present invention; Fig. 3 shows the construction of yet another embodiment of a distributed-constant-line-type filter according to the present invention; Fig. 4 shows the construction of a further embodiment of a distributed-constant-line-type filter according to the present invention; Fig. 5 shows the construction of still a further embodiment of a distributed-constant-line-type filter according to the present invention; Fig. 6 shows the construction of still a further embodiment of a distributed-constant-line-type filter according to the present invention; Fig. 7 shows the construction of still a further embodiment of a distributed-constant-line-type filter according to the present invention; Fig. 8 shows the construction of still a further embodiment of a distributed-constant-line-type filter according to the present invention. Fig. 9 shows the construction of a conventional distributed-constant-line-type filter; and Fig. 10 shows the construction of a comb-shaped coupling capacitor. Fig. 11 shows the construction of an another embodiment of a distributed-constant-line-type filter according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a distributed-constant-line-type filter 1 of the present invention. The distributed-constant-line-type filter 1 includes distributed-constant-line-type resonators 2 and 3, and a grounding electrode 4 connected to one end of each of these resonators. The length of each resonator, namely, the lengths from 2a to 2b and from 3a to 3b are approximately one fourth of the wavelength of the signal of a frequency used. That is, each of the distributed-constant-line-type resonators 2 and 3 is a λ/4-type resonator. In order to reduce the mounting area, the distributed-constant-line-type resonators 2 and 3 are formed into a folded shape. This shape is called a meandering shape. In order to reduce leakage of the propagation signal, the corners of the bent sections of the distributed-constant-line-type resonators 2 and 3 should preferably be cut. A distributed-constant line 7 for input and a distributed-constant line 8 for output are connected to the open ends of the distributed-constant-line-type resonators 2 and 3 via comb-shaped coupling capacitors 5 and 6, respectively. A comb-shaped coupling capacitor is an independent strip line having a shape such as that shown in Fig. 10, having the advantage that λ/4 within the resonator can be shortened, namely, a shorter length of the resonators 4 and 5 is required. However, when there is enough mounting space, the coupling capacitors may not be used. In the distributed-constant-line-type resonators 2 and 3, the line width w2 of near the open end is different from the line width w1 near the grounding conductor, that is, w1 > w2. The resonators, the grounding conductor, and the input and output lines are formed of, for example, strip lines or microstrip lines. The signal input from the distributed-constant line 7 for input is input to a resonance circuit formed of the distributed-constant-line-type resonators 2 and 3 via a coupling capacitor 5. The signal components having the resonance frequency of the resonance circuit from among the input signal components are output from the distributed-constant line 8 for output, and the signals of the frequency other than the resonance frequency are reflected. That is, the distributed-constant-line-type filter 1 is a band-pass filter. Generally speaking, in the λ/4 resonator, in the distributed-constant-line-type filter, one end of which is open and the other end grounded, the amplitude of a high-frequency current which flows through the distributed-constant line is larger on the grounded-end side and decreases toward the open-end side. In order to reduce the insertion loss of the band-pass filter, the conductor loss of the distributed-constant-line-type resonator may be decreased. In order to effectively decrease the conductor loss of the distributed-constant-line-type resonator, it is effective to enlarge the area of the strip line in a section where the amplitude of a high-frequency current is large in order to decrease the line resistance of that section. From the point of view of decreasing the line resistance, it is ideal to increase the strip line width in all the sections of the distributed-constant-line-type filter. However, since there is a strict demand for the mounting area, the area of the strip line is increased only in the most effective section. For example, the mounting area of a band-pass filter for a portable telephone according to the present invention should be, for example, within 2 mm x 2 mm. A wider strip line of a part of the resonator increases the mounting area. In order to prevent this, the strip line width should preferably be narrow in a section where the amplitude of the high-frequency current is small within the resonator. In the λ/4 resonator, since the amplitude of the electric current near the open ends is very small, the strip line width near the open ends is narrowed. Although a decrease in the line width increases the line resistance, the contribution to the conductor loss of all the resonators is small. Fig. 2 shows another embodiment of a distributed-constant-line-type filter according to the present invention. The basic construction of this embodiment is the same as that of the filter shown in Fig. 1. The differences are as described below. Since the corners of the bent sections of distributed-constant-line-type resonators 11 and 12 are not cut, the electrode area of those sections increases, and the line resistance decreases, making it possible to reduce the loss of the distributed-constant-line-type resonators and the insertion loss of the filter. As a result, the confinement characteristic of the propagation signal in the bent sections decreases slightly; however, this is an effective construction when it is desirable to reduce the insertion loss of the filter by a larger amount. Fig. 3 shows yet another embodiment of a distributed-constant-line-type filter according to the present invention. A distributed-constant-line-type filter 20 includes spiral-shaped distributed-constant-line-type resonators 21 and 22. An insulating film 28 is provided at the intersection of the lines. Also in this filter, the line widths w5 > w6 are set so that w5 > w6. Fig. 4 shows a further embodiment of a distributed-constant-line-type filter according to the present invention. A distributed-constant-line-type filter 30 is an interdigital-type filter such that distributed-constant-line-type resonators 31 and 32 whose length is approximately one fourth of the wavelength of a desired frequency, one end of which is open and the other end connected to grounding electrodes 33 and 34, respectively, and grounded, are each formed in a meandering shape, and two of them are arrayed so as to be coupled to each other. A distributed-constant line 37 for input and a distributed-constant line 38 for output are connected to one end of each of the distributed-constant-line-type resonators 31 and 32 via comb-shaped coupling capacitors 35 and 36, respectively. In the distributed-constant-line-type resonators 31 and 32, the line width of one end is different from the line width of the other end. In each of them, the line width is w8 on the one end side, and the line width is w7 on the other end side, with w7 being wider than w8. Fig. 5 shows still a further embodiment of a distributed-constant-line-type filter according to the present invention. A distributed-constant-line-type filter 40 includes spiral-shaped distributed-constant-line-type resonators 41 and 42. An insulating film 49 is provided at the intersection of the lines. The line widths w9 > w10 are set so that w9 > w10. Fig. 6 shows still a further embodiment of a distributed-constant-line-type filter 50 according to the present invention. The distributed-constant-line-type filter 50 includes distributed-constant-line-type resonators 51 and 52 similarly to the above-described filters. The resonators 51 and 52 are folded to reduce the mounting area in the same manner as the other above-described filters. In order that the filter 50 functions as a band-pass filter, the resonators 51 and 52 must be magnetically coupled to each other. The magnetic coupling is established between a section 510 and a section 520. Therefore, the spacing between the sections 510 and 520 is at a distance at which desired magnetic coupling can be established. However, magnetic coupling between the other sections is possible, for example, between the section 521 and the section 510, and between a section 511 and the section 520, and others. However, this coupling between the other sections might cause spurious signals in the filter. Therefore, for example, the section 511 should preferably be situated away from the section 520. However, if this is done, the mounting area of the filter increases. Therefore, an outer section 512 having a small contribution to the coupling between the resonators is brought close to the section 511. That is, the section having a large contribution to the coupling between the resonators, namely, the section having a larger current amplitude, is made as far away as possible from the more adjacent resonator, thereby preventing an occurrence of a spurious signal. Fig. 7 shows still a further embodiment of a distributed-constant-line-type filter according to the present invention. In Fig. 7, a distributed-constant-line -type filter 60 is a comb-line-type filter such that distributed-constant-line-type resonators 61, 62, 63, and 64 formed of a distributed-constant line whose length is approximately one fourth of a desired frequency, one end of which is open and the other end connected to a grounding electrode 65 and grounded, are each formed in a meandering shape and four of them are arrayed so as to be coupled to each other. A distributed-constant line 68 for input and a distributed-constant line 69 for output are connected to one end of each of the distributed-constant-line-type resonators 61 and 64 via comb-shaped coupling capacitors 66 and 67, respectively. Fig. 8 shows still a further embodiment of a distributed-constant-line-type filter according to the present invention. In Fig. 8, a distributed-constant-line-type filter 70 is an interdigital-type filter such that distributed-constant-line-type resonators 71, 72, 73, and 74 formed of a distributed-constant line whose length is approximately one fourth of a desired frequency, one end of which is open and the other end connected to grounding electrodes 75 and 76 and grounded, are each formed in a meandering shape and four of them are arrayed so as to be coupled to each other. A distributed-constant line 79 for input and a distributed-constant line 80 for output are connected to one end of each of the distributed-constant-line-type resonators 71 and 74 via comb-shaped coupling capacitors 77 and 78, respectively. In Figs. 7 and 8, the line width and the spacing between the adjacent sections are set in the same manner as in Figs. 2, 4, and 6. By forming the distributed-constant-line-type resonator into multiple steps of three or more as described above, the attenuation level at both ends of the passing band of the distributed-constant-line-type filter can be increased. In each of the above-described embodiments, the distributed-constant line for input may be used as a distributed-constant line for output, and the distributed-constant line for output may be used as a distributed-constant line for input. Each of the above-described embodiments is a filter using a λ/4 resonator. However, the present invention can be applied to a filter using another type of resonator, for example, a 3λ/4 resonator. In a distributed-constant-line-type filter 200 of Fig. 11, in 3λ/4 resonators 201 and 202, sections where the amplitude of the electric current reaches a maximum are sections 201a and 201d. In the 3λ/4 resonator, since it can be considered that waves of 3λ/4 enter the resonator, there are two maximum points of the current amplitude within the resonator. That is, the current amplitude reaches a maximum near the sections 201a and 201d. Therefore, by widening the strip line widths of the sections 201a and 201d and narrowing the strip line widths of the other sections, for example, the sections 201b and 201c, it is possible to realize a reduction in the insertion loss of the filter while maintaining the filter at a small size.

Claims: A filter device (1; 10; 20; 30; 40; 50; 60; 70), comprising: at least two distributed-constant-line-type resonators (2, 3; 11, 12; 21, 22; 31, 32; 41, 42; 51, 52; 61, 62, 63, 64; 71, 72, 73, 74) each of which has a strip line constituting said resonator and having a first end (2a, 3a) and a second end (2b, 3b) ; a signal input terminal (7; 16; 26; 37; 47; 56; 68) capacitively coupled to the first end of the strip line of one of the resonators; a signal output terminal (8; 17; 27; 38; 48; 57; 69; 80) capacitively coupled to the first end of the strip line of another one of the resonators, wherein the second ends of the strip lines are grounded; and characterized in that the width (w1, w2; w3, w4; w5, w6; w7, w8; w9, w10; w11, w12) of at least one of the strip lines is wider in a section where a current amplitude of a signal at a resonance frequency of said strip line is large and narrower in a section where said current amplitude on said strip line is low, wherein the width of said at least one strip line is greater near said second end than near said first end. A filter device (1; 10; 20; 30; 40; 50; 60; 70) according to claim 1, wherein the length of the strip lines of the resonators is approximately one fourth of the wavelength of a signal having a resonance frequency of said resonators (2, 3; 11, 12; 21, 22; 31, 32; 41, 42; 51, 52; 61, 62, 63, 64; 71, 72, 73, 74). A filter device (1; 10; 20; 30; 40; 50; 60; 70) according to claim 1, wherein the line width near a section where the current amplitude of a standing wave which occurs within said resonator having said at least one strip line by a signal having a resonance frequency of said resonator reaches a maximum is greater than the line width in the other sections. A filter device (1; 10; 20; 30; 40; 50) according to one of claims 1 to 3, wherein either one of said distributed-constant-line-type resonators is folded. A filter device (1) according to claim 4, wherein said folded bent section is tapered. A filter device (20; 40) according to one of claims 1 to 3, wherein either one of said distributed-constant-line-type resonators is shaped in a spiral. A filter device (40) according to one of claims 1 to 6, further comprising a first grounding conductor (43) connected to one of said distributed-constant-line-type resonators (41) and a second grounding conductor (44) connected to another one of distributed-constant-line-type resonators (42). A filter device (40) according to claim 7, wherein said grounding conductors (43, 44) are disposed in such a manner as to face each other, and said resonators (41, 42) are disposed between said grounding conductors (43, 44) so as to be coupled to each other.

Inventors: SASAKI YUTAKA, TANAKA HIROAKI, SASAKI, YUTAKA, TANAKA, HIROAKI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H01P1
IPC codes: H01P1