Half-wavelength resonator type high frequency filter

Abstract: A half-wavelength resonator type high frequency filter has N half-wavelength resonators (where N is an integer not smaller than 2); an input terminal; an output terminal; first matching part for matching the first of the resonators to the input terminal; second matching part for matching the N-th of the resonators to the output terminal; and (N-1) interstage coupling part for coupling the resonators with one another, and wherein excitation positions of the first of the resonators and the N-th of the resonators are displaced from the center positions of the respective resonators toward an end thereof, and at least one of the interstage coupling part is electrically connected to its associated resonators at positions other than both ends thereof.

Description: The present invention relates to a half-wavelength resonator type high frequency filter, exemplified primarily by a dielectric filter used in wireless equipment such as a portable telephone. In recent years, there has been an increasing demand for half-wavelength resonator type high frequency filters as compact and high performance filters that have superior selectivity characteristics in order to achieve efficient utilization of frequencies in wireless communications. An example of a prior art half-wavelength resonator type high frequency filter will be described below with reference to the drawings. Figure 6 shows the configuration of a prior art half-wavelength resonator type high frequency filter constructed using strip lines. In Figure 6, reference numerals 41 and 42 are half-wavelength resonators. Reference numeral 43 is an input terminal, and 44 is an output terminal. Reference numeral 45 is an input matching circuit block Yt, 46 is an output matching circuit block Yt, and 47 is an interstage coupling capacitor Cg. The half-wavelength resonators 41 and 42 are each a strip line with both ends open, and the input matching circuit block 45 and output matching circuit block 46 are formed, for example, by input and output coupling capacitors. The operation of the thus constructed half-wavelength resonator type high frequency filter will be described below. First, the resonators are excited at the midpoints of the respective strip lines, i.e., the dividing points between L1 and L2 and between L3 and L4, via the input and output matching circuit blocks which are, for example, input and output coupling capacitors. The interstage coupling capacitor Cg is electrically connected to both of the resonators at the respective resonator side 48. The thus constructed filter exhibits a band pass characteristic with its pass band center frequency at the antiresonant frequency of the resonators and a transfer characteristic with attenuation poles formed at series-resonant frequencies of the L1 and L4 sections of the strip lines where they are equivalently grounded. In this case, if we consider only the fundamental mode, the number of attenuation poles is one per resonator. In the above configuration, however, since each resonator resonates only at one specific frequency in the fundamental mode, the number of filter attenuation poles is limited to the number of resonators used. Further, the magnitude of attenuation is not sufficient. Another problem is that since there is a significant limitation on input/output matching, freedom in attenuation pole frequency control is limited. JP-A-56116302 discloses a high frequency filter according the preamble portion of claim 1. In view of the above-outlined problems, it is an object of the present invention to provide a half-wavelength resonator type high frequency filter that permits to freely control the filter's attenuation pole frequencies. This object is solved by a high frequency filter as set forth in claim 1. Prefered embodiments are subject to various dependent claims. Figure 1 is a diagram showing the circuit configuration of a half-wavelength resonator type high frequency filter according to one embodiment of the present invention. Figure 2 is a diagram showing the structure of the half-wavelength resonator type high frequency filter according to one embodiment of the present invention. Figure 3 is a characteristic diagram of the half-wavelength resonator type high frequency filter of Figure 2. Figure 4 is a diagram showing the structure of a half-wavelength resonator type high frequency filter in an embodiment different from the embodiment shown in Figure 2. Figure 5 is a diagram showing the circuit configuration of a half-wavelength resonator type high frequency filter in an embodiment different from the embodiment shown in Figure 1. Figure 6 is a diagram showing the configuration of a half-wavelength resonator type high frequency filter according to the prior art. A half-wavelength resonator type high frequency filter according to one embodiment of the present invention will be described below with reference to the drawings. Figure 1 shows the configuration of the half-wavelength resonator type high frequency filter. In Figure 1, reference numerals 1 and 2 are half-wavelength resonators. Reference numeral 3 is an input terminal, and 4 is an output terminal. Reference numeral 5 is an input matching means Yt, 6 is an output matching means Yt, and 7 is an interstage coupling means which is formed, for example, an interstage coupling capacitor Cg. The input and output terminals are tap-fed to the resonators via the input and output matching means. The interstage coupling capacitor Cg is electrically connected to both of the half-wavelength resonators 1 and 2 at positions other than both ends thereof. L1 and L2 indicate the distances from the excitation point of the first half-wavelength resonator to the respective ends thereof, and L3 and L4 indicate the distances from the excitation point of the second half-wavelength resonator to the respective ends thereof. In this embodiment, the relations L1 ≠ L2, L3 ≠ L4, L2 ≠ L4, L1 ≠ L3, and L1 + L2 = L3 + L4 are satisfied. Figure 2 shows an example of a pattern diagram of the present embodiment constructed with coplanar waveguides (CPW). In this example, half-wavelength resonators 25 and 26 are TEM mode coplanar waveguide with both ends open, and are formed on a dielectric substrate 21 made of alumina or the like. Reference numeral 22 indicates a grounding pattern. Input and output matching circuit blocks are constructed, for example, from an input coupling capacitor 27, which is formed by a gap between an input transmission line 23 and the resonator 25, and an output coupling capacitor 28, which is formed by a gap between an output transmission line 24 and the resonator 26. Likewise, the interstage coupling capacitor Cg can be formed from an interstage coupling capacitor 29 formed by a gap between the waveguides. The interstage coupling capacitor Cg is electrically connected to the resonators 25 and 26 at intermediate points along the respective waveguides excluding both ends thereof, as earlier described. This example has the characteristic that the excitation point of each resonator is at the same position as the coupling point between the resonators. The operation of the thus constructed half-wavelength resonator type high frequency filter will be described below with reference to Figures 1 and 2. In the configuration of this embodiment, when the excitation point or the coupling point of the resonators is set slightly displaced from the center point, for example, each waveguide section of approximately one-quarter wavelength, extending from the excitation point to the end thereof, series resonates and generates an attenuation pole. Accordingly, two attenuation poles can be generated with each half-wavelength resonator. The attenuation pole frequency can be set as desired by adjusting the connection point between the input matching means 5 and output matching means 6 and the interstage coupling means 7Cg. Input/output impedance matching can be accomplished with relative ease by selecting the configuration of the matching means and the way the excitation point is taken. Figure 3 shows an example of the filter characteristic of the configuration of the present invention shown in Figure 2. As shown by the graph of the transfer amount, four attenuation poles #1 to #4 are formed using the two-stage filter configuration. In this way, excellent selectivity characteristics can be obtained despite the compact size of the filter. As described above, according to the present embodiment, by displacing the excitation point of each resonator from its center point toward one end thereof, and by connecting the interstage coupling means to the resonators at positions other than both ends thereof, a larger number of attenuation poles can be generated than the prior art configuration, and excellent selectivity characteristics can thus be obtained. Figure 2 has shown coplanar waveguides, but it will be appreciated that the present invention can also be carried out using microstrip lines as shown in Figure 4. In the figure, reference numeral 200 is a grounding electrode layer. Figure 5 shows an example in which three or more resonators are used; in this example, not all the interstage coupling means are connected to the ends of their associated resonators 100, but one interstage coupling means 71 is connected to the ends of its associated resonators. As described above, according to the present embodiment, a larger number of attenuation poles can be generated than the prior art configuration, and excellent selectivity characteristics can thus be obtained. Further, by forming the matching means by coupling capacitors, a high frequency filter having attenuation poles can be constructed with simple configuration. By configuring the excitation means as a tap feeding type, the configuration of the high frequency filter having attenuation poles can be further simplified. By constructing the resonators as TEM resonators with both ends open, the fabrication of the filter can be made easier. The magnitude of the attenuation poles can be made sufficiently large. The attenuation poles can be generated at desired frequencies, and excellent selectivity characteristics can be obtained with simple configuration.

Claims: A high frequency filter comprising: N resonators, where N is an integer not smaller than 2, including at least two half-wave resonators (25, 26) with both ends open; an input terminal (23); an output terminal (24); first matching means (27) for matching the first of said resonators (25) to said input terminal (23) at a first excitation point; second matching means (28) for matching the second of said resonators (26) to said output terminal (24) at a second excitation point; and inter-stage coupling means (29) for coupling said resonators (25, 26) with one another at each coupling point characterized in that sections of each of said half-wavelength resonators (25, 26) are extending from said excitation point or said coupling point to the respective open ends thereof with different lengths (L1, L2, L3, L4) such that said excitation point or said coupling point of said half-wavelength resonators (25, 26) is offset from the center position of said respective half-wavelength resonators (25, 26), said sections are series resonated at respective resonance frequencies, and in that said inter-stage coupling means (29) is substantially at the same position as said excitation point. The high frequency filter according to claim 1, characterized in that said half-wavelength resonators (25, 26) are each shaped in the form of the letter L, and the adjoining half-wavelength resonators (25, 26) are disposed close together at positions substantially mirror-symmetric to each other so that bent portions of said adjoining half-wavelength resonators (25, 26) are electromagnetically coupled to each other and that said inter-stage coupling means (29) is substantially at the same position as the bent portions of said adjoining half-wavelength resonators (25, 26). The high frequency filter according to claim 1 or 2, characterized in that said first matching means (27) and said second matching means (28) are each formed by a coupling capacitor. The high frequency filter according to claim 3, characterized in that said matching means (27, 28) is made in the form of a strip with one end thereof positioned in close proximity to the bent portion of its corresponding one of said half-wavelength resonators (25, 26). The high frequency filter according to one of claims 1 to 4, characterized in that at least one of said inter-stage coupling means (29) is electrically connected to its associated resonator at a position other than both ends thereof. The high frequency filter according to one of the claims 1 to 5 characterized in that said half-wavelength resonators (25, 26) are TEM-mode resonators. The high frequency filter according to one of claims 1 to 6 characterized in that any one of the lengths of the sections (L1, L2, L3, L4) is different from the lengths of the other sections thereof.

Inventors: AWAI IKUO, ISHIZAKI TOSHIO, WADA KOUJI, AWAI, IKUO, ISHIZAKI, TOSHIO, WADA, KOUJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1999
Main classification: H01P1
IPC codes: H01P3, H01P1