Surface acoustic wave resonator filter

Abstract: A surface acoustic wave resonator filter includes an interdigital electrode on a piezoelectric substrate and has a two-dimensional mode coupling structure coupling a longitudinal resonation in the longitudinal direction consistent with a propagating direction of a surface acoustic wave and a transversal mode resonation in a transverse direction perpendicular to the propagating direction.

Description: The present invention relates to a surface acoustic wave resonator filter. More specifically, the invention relates to a band-pass filter of the type of a surface acoustic wave (SAW) resonator utilizing a longitudinal mode resonance of a surface acoustic wave and having a wide band characteristics. An equivalent circuit of the conventional two pole mode SAW resonator is illustrated in Fig. 7A. It has been known that such SAW resonator includes two series resonating circuits 11 and 12 having frequencies f 11 and f 12 . The phases of two resonating frequencies f 11 and f 12 are in opposite phase relationship. This opposite phase relationship is equivalently illustrated by a transformer 13 . When an impedance matching is established in the two pole mode SAW resonators under a certain condition, a filter characteristics as illustrated in Fig. 7B is obtained with a pass band between the resonance frequencies f 11 and f 12 . A center frequency in the pass band is f 1 . As a typical filter employing this principle, monolithic quartz filter has been well known. With such filter construction, each insertion loss at the resonance frequencies f 11 and f 12 becomes substantially consistent with 3 dB point attenuating 3 dB from a minimum insertion loss (generally 0 dB) of the pass band. As conventional means for realizing such two pole mode resonator, there are a longitudinal mode resonance type for generating two resonance modes in a propagating direction of the surface acoustic wave and a transversal mode type for generating two resonance modes in a direction transverse to the propagating direction. This two pole mode resonator has been disclosed in M. Tanaka et al., "NARROW BANDPASS FILTER USING DOUBLE-MODE SAW RESONATORS ON QUARTZ" in the 38th Annual Frequency Control Symposium of IEEE, 1984, and M. Tanaka et al., "Narrow Bandpass Double Mode SAW Filter", of the 15th EM Symposium of Electric Communication Committee, 1986. Figs. 8A and 8B show the concrete construction of this two pole mode resonator. Fig. 8A shows a transversal mode coupling type filter, in which two resonators a and b are proximately arranged longitudinally in perpendicular to the propagating direction of the surface acoustic wave. The resonator a has an IDT (interdigital) electrode 1a at a center and reflectors 3a and 4a at both sides of the IDT electrode 1a . Similarly, the resonator b has an IDT electrode 1b at the center and reflectors 3b and 4b at both sides of the IDT electrode 1b . In this transversal mode coupling type filter, the band-pass filter characteristics as shown in Fig. 7B is obtained by a coupling of two modes (even symmetric mode illustrated by f 1 and odd symmetric mode illustrated by f 2 ). Fig. 8B shows a longitudinal mode coupling type filter, in which two IDT electrodes 1 and 2 are proximately arranged in the longitudinal direction as the propagating direction of the surface acoustic wave. At both sides of these IDT electrodes 1 and 2 , reflectors 3 and 4 are arranged respectively. With this longitudinal mode coupling type filter, the band-pass filter characteristics as illustrated in Fig. 7B is obtained by the coupling of two modes generated between the input side IDT electrode 1 and the output side IDT electrode 2 . In such type of the surface acoustic wave filter, it is necessary to expand the interval between the resonance frequencies of two modes to be used in order to widen the filter characteristics. There is a restriction on the design parameter to cause various problems, such as large use impedance, degradation of the attenuation magnitude out of the band and so forth. Therefore, there is a limitation for widening the band. IEEE 1988 Ultrasonics Symposium Proceedings, 2-5 October 1988, pages 195-202, discloses SAW resonator filters with longitudinal and transverse mode coupling, respectively. The resonators comprise one resonance frequency only. Therefore, it is an object of the present invention to provide a surface acoustic wave resonator filter which permits widening of bands with simple construction. This object is solved with the features of the claim. The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken limitative to the present invention, but are for explanation and understanding only. In the drawings: Figs. 1A and 1B are illustration respectively showing filter characteristics of the present invention; Fig. 2A is an equivalent circuit diagram of a filter according to the present invention; Fig. 2B is a characteristic chart of the filter of the invention; Fig. 3A is an illustration showing a construction of one embodiment of the present invention; Fig. 3B is an illustration showing art not included in the invention; Figs. 4A and 4B show results of actual measurement of filter characteristics of one embodiment of the invention; Figs. 5A and 5B show results of actual measurement of the characteristics in case that another embodiment of the filter according to the invention is used for impedance matching; Figs. 6A and 6B show results of actual measurement of the characteristics in case that another embodiment of the filter according to the invention is used for impedance matching; Fig. 7A is an equivalent circuit diagram of the conventional SAW resonator; Fig. 7B is an illustration of a characteristics of the conventional SAW resonator; Fig. 8A is an illustration showing a construction of the conventional transversal mode coupling type filter; and Fig. 8B is an illustration showing a construction of the conventional longitudinal mode coupling type filter. Initially, consideration is given for the case where is a two pole mode SAW resonator A having resonance frequencies f 11 and f 12 and a two pole mode resonator B having resonance frequencies f 21 and f 22 . When an impedance matching is established for using as a filter, a filter characteristics becomes as illustrated in Fig. 1B , assuming the center frequencies in the pass-bands are respectively f 1 and f 2 . In order to establish a single filter characteristics as illustrated in Fig. 1A by composing these two filter characteristics A and B , it is essential conditions that, in the cross point of these two characteristics A and B , the power of each output signal of each filter is half of the power of the output signal of the pass band and two signals are in the same phase. The equivalent circuit of the resonators A and B satisfying these essential conditions is as illustrated in Fig. 2A . The equivalent circuit of the resonator A has a parallel connection 14 of the series resonating circuit having resonance frequencies f 11 and f 1 2 . Similarly, the equivalent circuit of the resonator B has a parallel connection of a series resonating circuit having resonance frequencies f 21 and f 22 . Then, the output signals, the high band side resonance frequency f 12 of the resonator A and the low band side resonance frequency f 21 of the resonator B are to be matched to each other in the vector sense (both in the power and phase). This relationship is illustrated in the transformer 16. At this time, the composite resonance characteristics becomes as illustrated in Fig. 2B . Then, the filter characteristics with established impedance matching becomes wide band as illustrated in Fig. 1A . The embodiment of the filter construction of the present invention satisfying the above-mentioned characteristics is illustrated in Fig. 3A. As shown in Fig. 3A , the shown embodiment is constructed by two sets of longitudinal mode coupling type resonators illustrated in Fig. 8B proximately arranged in the transverse direction and transversal mode coupling being acoustically established between these two resonators. Namely, the SAW filter according to the invention is designed in a two-dimensional mode coupling type structure by coupling the longitudinal mode resonance in the longitudinal direction as the propagating direction of the surface acoustic wave and the transversal mode resonation in the transverse direction perpendicular to the propagating direction. In the construction shown in Fig. 3A , two longitudinal mode coupling type resonators A and B are arranged on a piezoelectric substrate 20 in a cross proximity to each other for permitting acoustical transversal mode coupling. An Input IDT electrode 1A and a dummy electrode 2A are arranged in the longitudinal direction. Also, reflectors 3A and 4A are arranged at respective sides thereof. The IDT electrode 1A , the dummy electrode 2A and reflectors 3A and 4A form the first longitudinal mode resonator A . An output IDT electrode 1B and a dummy electrode 2B are also arranged in the longitudinal direction. Reflectors 3B and 4B are arranged at respective sides. The IDT electrode 1B , the dummy electrode 2B and the reflectors 3B and 4B form the second longitudinal mode resonator B . It should be noted that the dummy electrodes 2A and 2B correspond to a single IDT electrode of the longitudinal resonator illustrated in Fig. 8B and have acoustically equivalent function to the IDT electrode. Further discussion will be given for these dummy electrodes 2A and 2B . In the structure of Fig. 3A, the transversal mode coupling is the same as the transversal mode coupling between adjacent two grating waveguides, similarly to the transversal mode coupling type resonator of Fig. 8A . On the other hand, the longitudinal mode coupling utilizes resonance mode by a multiple reflection between the reflectors 3 and 4 , resonance mode by multiple reflection between the IDT electrodes, and further resonance mode within the IDT electrode, similarly to the longitudinal mode coupling type resonator illustrated in Fig. 8B. The dummy electrodes 2A and 2B in the structure of Fig. 3A have equivalent structure to the IDT electrodes except for the structure of electrically shorting the IDT electrodes. Therefore, in the structure where two resonators A and B are adjacent in the transverse direction, both resonators A and B become equivalent to a uniform grating wave guide for the presence of the dummy electrodes 2A and 2B . Therefore, the desired transversal mode coupling can be obtained. On the other hand, observing as the coupling in the longitudinal direction, it is necessary that the dummy electrodes 2A and 2B are acoustically equivalent to the IDT electrodes. Because of presence of the dummy electrodes, the multiple reflecting resonance between the reflectors, the multiple reflecting resonance between the IDT electrodes, and the resonance of the IDT electrode per se , as the longitudinal mode resonance can be utilized as in the longitudinal mode coupling type resonator. The electrodes are arranged to generate two longitudinal mode resonance in the longitudinal direction, SAW propagation in even symmetric mode ( f 1 ) and odd symmetric mode ( f 2 ) in the transverse direction so that two transversal mode characteristics can be obtained in the even symmetric mode and odd symmetric mode. This is illustrated in a form of the equivalent circuit as shown in Fig, 2A , in which the resonating circuit 14 represents the even symmetric mode and the resonating circuit 15 represents odd symmetric mode. Respective resonating circuits 14 and 15 have two series resonating circuits having two longitudinal modes. In the shown construction, it is necessary to establish matching as shown in Fig. 2B between the higher side longitudinal mode resonance f 12 of the even symmetric mode and the lower side longitudinal mode resonance f 21 of the odd symmetric mode. This can be achieved by appropriately selecting the number of IDT pairs N, the longitudinal mutual interval between the IDT electrodes, longitudinal distances between the IDT electrode and the reflectors. With the construction set forth above, the composite resonance characteristics shown in Fig. 2B can be obtained. The results of actual measurement are shown in Figs. 4A and 4B. Fig. 4B shows in enlarged scale the frequency range near 250 MHz. When the filter of Fig. 3A is used with matched impedance, a flat characteristics in the pass band can be obtained as illustrated in Figs. 5A and 5B . It should be noted that Fig. 5B shows in enlarged scale the frequency range near 250 MHz. The shown SAW filter employs a ST cut quartz substrate, which exhibits capability of increasing of band width to 0.15% of relative band (3 dB band/center frequency) whereas the conventional relative band has been approximately 0.1%. Furthermore, as can be appreciated from the equivalent circuit of Fig. 2A , since the phase is reversed between the even symmetric mode and the odd symmetric mode (by transformer 16 ), attenuation rate can be improved for mutual cancellations of the signals of respective modes in the frequency range out of the pass band. Fig. 3B shows art of the SAW not according to the present invention. In this case, a parallel connection is established between input and output of two longitudinal mode coupling type resonators A and B . The transversal mode is considered to be established by electrical coupling. The resonator A comprises a pair of IDT electrodes 1A and 2A, and reflectors 3A and 4A at both sides thereof. Also, the resonator B comprises a pair of IDT electrodes 1B and 2B , the reflectors 3B and 4B at both sides thereof. The IDT electrodes 1A and 2B are electrically connected, and the IDT electrodes 2A and 1B are electrically connected to each other. As a results, two resonators A and B are connected in parallel between the input IN and the output OUT . Even in this construction, the equivalent circuit the same as Fig. 2A can be established. Here, it becomes necessary that the signals from two resonators has to have opposite phase relationship. Therefore, in Fig. 3B , the relationship of the polarities of the IDT electrodes of the resonators A and B are selected to be opposite to each other. Namely, upper intersecting electrodes of the IDT electrodes 1A and 2A are grounded so that lower intersecting electrodes of the IDT electrodes 1B and 2B are grounded. On the other hand, as another method, an opposite phase relationship of signals of two resonators A and B can be established by shifting an interval between the IDT electrodes 1B and 2B of the resonator B for n/2 (n is odd integer) wavelength relative to the interval between the IDT electrodes 1A and 2A of the resonator A . Figs. 6A and 6B show results of actual measurement of the characteristics for the filter having the structure of Fig. 3B . It should be noted that Fig. 6A is illustrated in the expanded scale for the frequency range near 250 MHz. As set forth above, according to the present invention, it becomes possible to widen the band of the filter characteristics by acoustically coupling two pole modes SAW resonators and composing the characteristics of two resonators of the frequency axis.

Claims: A surface acoustic wave resonator filter including: a first longitudinal mode coupling type resonator (A) connected to an input terminal (IN), and a second longitudinal mode coupling type resonator (B) connected to an output terminal (OUT) and acoustically coupled to said first longitudinal mode coupling type resonator (A), characterized in that each of said first and second longitudinal mode coupling type resonators comprises a plurality of resonance frequencies, wherein said first and second longitudinal mode coupling type resonators are arranged mutually adjacent to each other on a piezo electric substrate (20) for establishing transversal mode coupling., wherein each of said first and second longitudinal mode coupling type resonators (A, B) comprises two resonance frequencies, wherein one of the resonance frequencies of the first resonator (A) is equal to one of the resonance frequencies of the second resonator (B), wherein the transversal mode coupling of the first and second longitudinal mode coupling type resonators is an acoustic coupling, and wherein each of the first and second longitudinal mode coupling type resonators has a dummy electrode (2A, 2B) arranged adjacent the own interdigital electrode (1A, 1B) in the longitudinal direction and adjacent to an interdigital electrode (1B, 1A) of the other resonator in the transverse direction for establishing the acoustic coupling.

Inventors: YAMAMOTO YASUSHI, YAMAMOTO, YASUSHI, Yamamoto, Yasushi, c/o NEC Corporation
Applicants: NRS TECHNOLOGIES INC, NRS TECHNOLOGIES INC.
Application year: 1993
Main classification: H03H9
IPC codes: H03H9