High frequency narrow-band multi-mode filter

Abstract

A high frequency narrow-band pass multi-mode filter in constructed in such manner that SAW of SSBW resonators are closely disposed to each other on a single piezoelectric substrate to generate different vibration modes of different resonance frequencies, conditions that the resonators are acoustically coupled to each other to cause these vibration modes, are experimentally found, and the above conditions are satisfied by the filter. In order to reduce the ohmic loss of a very narrow common bus bar, the number of strip pairs of electrode in both interdigital transducers (hereinafter referred to as IDT) is minimized and instead reflectors are provided on the both sides TDTs to compensate for the reduction of its Q value. A part of the bus bar for the IDT electrodes is formed integral with some or all of gratings of the reflectors, whereby the connecting positions of the grating electrodes with respect to lead terminals can be positioned with a high freedom.

We claim: 
     
       1. A high-frequency narrow-band pass .[.multi-mode.]. .Iadd.double-mode .Iaddend.filter comprising first and second resonators arranged adjacent to each other, each of said resonators having the same resonance frequency and comprising a pair of comb-type interdigital transducer electrode arrays, the electrodes of said arrays being interleaved with each other, and a common bus bar provided between .Iadd.and connected to .Iaddend.said first and second resonators, said first and second resonators being arranged in directions perpendicular to .[.the.]. .Iadd.a .Iaddend.propagation .[.directions.]. .Iadd.direction .Iaddend.of waves excited by said resonators and in close proximity to each other on a piezoelectric substrate so that acoustic coupling .Iadd.occurs .Iaddend.between said first and second resonators .[.is attained.]., thereby causing first and second resonance frequencies of different vibrating modes, the overlap of the electrodes of said respective pairs of electrode arrays of said first and second resonators defining respective first and second acoustic .[.aperatures.]. .Iadd.apertures .Iaddend.of width W and the distance between said first and second acoustic aperatures defining a gap g, said filter utilizing said first and second resonance frequencies of different vibration modes generated by the acoustic coupling, wherein said width w and gap g satisfies the relation w≦20λ and g≦6λ, where λ is the wavelength of .[.the wave excited by said resonators.]. .Iadd.a wave corresponding to said same resonance frequency of each of said resonators, wherein reflectors comprising a multiplicity of gratings made of metal strips are provided on both sides of said resonators to reflect waves corresponding to said first and second resonance frequencies of different vibrating modes resulting from said acoustic coupling, and   wherein said common bus bar is formed integrally with and connected to at least one of the gratings of the reflectors, and said common bus bar is grounded through at least one grating such that the ohmic loss of said bus bar and the loss of said filter is reduced.Iaddend..   
     
     
       2. A filter as set forth in claim 1, wherein a wave excited by said resonators is a surface skimming bulk wave (SSBW). 
     
     
       3. A high frequency narrow-band pass .[.multi-mode.]. .Iadd.double mode .Iaddend.filter of multi-section type, wherein said filter set forth in claim 1 forms a unit section and a plurality of said unit sections are connected in a tandem manner so as to provide filter characteristics of a desired shape factor and relative attenuation. .[. 
     
     
       4.  A filter as set forth in claim 3, wherein reflectors comprising a multiplicity of gratings made of grooves in said substrate are provided on both sides of said resonators to reflect a wave excited by said resonators..]. .[.5. A filter as set forth in claim 3, wherein reflectors comprising a multiplicity of gratings made of metal strips are provided on both sides of said resonators to reflect a wave excited by said 
     
     
        resonators..]. 6. A .[.filter as set forth in claim 1.]. .Iadd.high-frequency narrow-band pass double-mode filter comprising first and second resonators arranged adjacent to each other, each of said resonators having the same resonance frequency and comprising a pair of comb-type interdigital transducer electrode arrays, the electrodes of said arrays being interleaved with each other, and a common bus bar provided between and connected to said first and second resonators, said first and second resonators being arranged in directions perpendicular to a propagation direction of waves excited by said resonators and in close proximity to each other on a piezoelectric substrate so that acoustic coupling occurs between said first and second resonators, thereby causing first and second resonance frequencies of different vibrating modes, the overlap of the electrodes of said respective pairs of electrode arrays of said first and second resonators defining respective first and second acoustic apertures of width w and the distance between said first and second acoustic apertures defining a gap g, said filter utilizing said first and second resonance frequencies of different vibration modes generated by the acoustic coupling, wherein said width w and gap g satisfies the relation w≦20λ and g≦6λ, where λ is the wavelength of a wave corresponding to said same resonance frequency of each of said resonators.Iaddend., wherein reflectors comprising a multiplicity of gratings made of grooves in said substrate are provided on both sides of said resonators to reflect .[.a wave excited by said resonators.]. .Iadd.waves corresponding to said first and second resonance frequencies of different vibrating modes   
     
     
        resulting from said acoustic coupling.Iaddend.. .[.7.  A filter as set forth in claim 6, wherein said common bus bar is formed integrally with at least some of said gratings of the reflectors, whereby the connecting points of said interdigital transducer electrodes with lead terminals can 
     
     
        be positioned with a high degree of freedom..]. 8. A filter as set forth in claim .[.7.]. .Iadd.6.Iaddend., wherein waves excited by said resonators .[.are.]. .Iadd.include different types of .Iaddend.waves other than .[.SAW excitable by said resonators.]. .Iadd.a surface acoustic 
     
     
        wave.Iaddend.. .[.9.  A filter as set forth in claim 6, wherein said common bus bar is formed integrally with the gratings of said reflectors and is connected to the gratings, and said common bus bar is grounded, whereby connecting points of said common bus bar with the lead terminals are positioned with a high degree of freedom and the ohmic loss of said 
     
     
        common bus bar and the loss of the filter is reduced..]. 10. A filter as set forth in claim 6, wherein .Iadd.the reflectors each have an inner side adjacent the resonators and an outer side opposite the inner sides and wherein .Iaddend.said common bus bar is extended across the gratings of said reflectors and is not connected to the gratings, .[.and the width of.]. said .Iadd.common .Iaddend.bus bar .[.in the gratings.]. .Iadd.having a width which .Iaddend.is gradually increased .Iadd.from the 
     
     
        inner side of a reflector .Iaddend.toward the outer side thereof. 11. A filter as set forth in claim .[.6.]. .Iadd.1.Iaddend., wherein .Iadd.the reflectors each have an inner side adjacent the resonators and an outer side opposite the inner side and wherein .Iaddend.said common bus bar is extended across the gratings of said reflectors and is connected to the gratings, .[.and the width of.]. said common bus bar .[.in the gratings.]. .Iadd.having a width which .Iaddend.is gradually increased .Iadd.from the inner side of a reflector .Iaddend.toward the outer .[.sides.]. .Iadd.side 
     
     
        .Iaddend.thereof. .[.12.  A filter as set forth in claim 6, wherein said common bus bar is connected to lead patterns, said patterns are provided in place between said resonators and said reflectors, are extended from a point to which said common bus bar is connected to a peripheral edge of 
     
     
        said piezo-electric substrate..]. 13. A filter as set forth in claim 1, 
     
     
        wherein preferably g≦3λ. 14. A filter as set forth in claim 1, wherein a wave excited by said resonators is a surface acoustic wave (SAW). .[.15. A filter as set forth in claim 1, wherein reflectors comprising a multiplicity of gratings made of metal strips are provided on both sides of said resonators to reflect a wave excited by said 
     
     
        resonators..]. .Iadd.16.  A filter as set forth in claim 1 wherein said common bus bar has a width on the order of g/2, and is connected to 
     
     
        substantially all of said multiplicity of gratings. .Iaddend. .Iadd.17.  A filter as set forth in claim 1 wherein said common bus bar has a width, said width is slightly less than twice a width associated with an 
     
     
        individual interdigital transducer electrode. .Iaddend. .Iadd.18.  A filter as set forth in claim 1 wherein said common bus bar is connected to 
     
     
        substantially all of said gratings. .Iaddend. .Iadd.19.  A tandem connected multi-sectioned double-mode surface acoustic wave (SAW) filter formed on a single piezoelectric substrate, comprising: a pair of double mode filter sections formed in spaced parallel alignment on a surface of said single piezoelectric substrate, each of said double mode filter sections itself comprising: first and second SAW resonators, having the same resonant frequency, arranged in parallel, closely opposed relationship so as to be acoustically coupled to each other and thereby generate two different vibration modes of first and second resonance frequencies, each of said resonators comprising an interdigitated transducer (IDT) having first and second comb type electrode arrays that are interleaved with each other, there being a central common bus bar disposed between said first and second resonators and extending in a direction parallel to a propagation direction of surface acoustic waves excited by each of said resonators, the first electrode array of each resonator extending from and being electrically connected to said common bus bar,     a reflector at each end of said interdigitated transducer, each of said reflectors comprising a grating made of metal strips,   said common bus bar extending beyond both ends of said interdigitated transducers and being connected to substantially all of the metal strips of said reflectors, so that said transducers are grounded through said common bus bar and said reflectors,   said pair of double mode filter sections being separated by sufficient distance so that acoustic coupling therebetween does not take place,   an interconnection bus, having a width wider than widths of said common bus bars of the pair of filter sections, interconnecting the second electrode array of the first resonator of one of said filter sections to the second electrode array of the second resonator of the other filter section, thereby interconnecting an output of one filter section to an input of the other filter section, and   a connection bus for the second electrode array of said second resonator of said one filter section and a like connection bus for the second electrode array of said second resonator of the other filter section, the connection buses having a width wider than widths of said common bus bars and being disposed to facilitate input and output connection to said tandem   
     
     
        connected filter. .Iaddend. .Iadd.20.  A surface acoustic wave transducer according to claim 19 wherein in each SAW resonator, the first and second electrode arrays define respective first and second acoustic apertures of width w and the distance between said first and second acoustic apertures defines a gap g, said filter utilizing said first and second resonance frequencies of different vibration modes resulting from said acoustic coupling between said pair of resonators, wherein said width w and gap g satisfy the relation w≦20λ and g≦6λ, where λ is a wavelength of a wave corresponding to said same resonance 
     
     
        frequency of each of said resonators. .Iaddend. .Iadd.21.  A surface acoustic wave transducer according to claim 20 wherein said common bus bar 
     
     
        has a width on the order of g/2. .Iaddend. .Iadd.22.  A surface acoustic wave transducer according to claim 21 wherein each common bus bar has a width, said width is slightly less than twice a width associated with an 
     
     
        individual array electrode. .Iaddend. .Iadd.23.  A high-frequency narrow-band pass multi-mode filter comprising at least two resonators arranged adjacent to one another, each of said resonators having the same resonance frequency and comprising a pair of comb-type interdigital transducer electrode arrays, the electrodes of said arrays being interleaved with each other, and wherein a common bus bar is provided between and connected to two of said adjacent resonators, each of said resonators being arranged in directions perpendicular to a propagation direction of waves excited by said resonators on a piezoelectric substrate so that acoustic coupling occurs between said resonators, thereby causing multiple resonance frequencies of different vibrating modes, the overlap of the electrodes of respective pairs of electrode arrays of said resonators defining respective first and second acoustic apertures of width w and the distance between said first and second acoustic apertures defining a gap g, said filter utilizing said multiple resonance frequencies of different vibration modes generated by the acoustic coupling, wherein said width w and gap g satisfies the relation w≦20λ and g≦6λ, where λ is the wavelength of a wave corresponding to said same resonance frequency of each of said resonators, wherein reflectors comprising a multiplicity of gratings made of metal strips are provided on both sides of said resonators to reflect waves corresponding to said multi-mode vibrations resulting from said acoustic coupling, and   wherein said common bus bar is formed integrally with and connected to at least one of the gratings of the reflectors, and said common bus bar is grounded through at least one grating such that the ohmic loss of said bus   
     
     
        bar and the loss of said filter is reduced. .Iaddend. .Iadd.24.  A high frequency narrow-band multi-mode filter of multi-section type as set forth in claim 23, wherein the gratings have an outer side and the common bus bar has a width, wherein said common bus bar is extended across the gratings of said reflectors, and the width of said bus bar in the gratings 
     
     
        is gradually increased toward the outer side thereof. .Iaddend. .Iadd.25. A high frequency narrow-band multi-mode filter of multi-section type, wherein said filter set forth in claim 23 forms a unit section and a plurality of said unit sections are connected in a tandem manner so as to provide filter characteristics of a desired shape factor and relative 
     
     
        attenuation. .Iaddend. .Iadd.26.  A high-frequency narrow-band pass multi-mode filter comprising at least two resonators arranged adjacent to one another, each of said resonators having the same resonance frequency and comprising a pair of comb-type interdigital transducer electrode arrays, the electrodes of said arrays being interleaved with each other, and wherein a common bus bar is provided between and connected to two of said adjacent resonators, each of said resonators being arranged in directions perpendicular to a propagation direction of waves excited by said resonators on a piezoelectric substrate so that acoustic coupling occurs between said resonators, thereby causing multiple resonance frequencies of different vibrating modes, the overlap of the electrodes of respective pairs of electrode arrays of said resonators defining respective first and second acoustic apertures of width w and the distance between said first and second acoustic apertures defining a gap g, said filter utilizing said multiple resonance frequencies of different vibration modes generated by the acoustic coupling, wherein said width w and gap g satisfies the relation w≦20λ and g≦6λ, where λ is the wavelength of a wave corresponding to said same resonance frequency of each of said resonators, wherein reflectors comprising a multiplicity of gratings made of grooves in said substrate are provided on both sides of said resonators to reflect waves corresponding to said vibrations of different modes resulting from said acoustic coupling. .Iaddend.