Longitudinal coupling dual mode surface acoustic wave filter

Abstract

To provide satisfactory attenuation characteristics at a high-pass side of a pass band in a longitudinal coupling dual mode surface acoustic wave filter.SOLUTION: A longitudinal coupling dual mode surface acoustic wave filter comprises: a piezoelectric substrate; a plurality of IDT electrodes provided on the piezoelectric substrate while being disposed side by side in a lateral direction in which a surface acoustic wave moves forward; reflectors provided on the piezoelectric substrate while holding the plurality of IDT electrodes right and left therebetween; and an electrode finger group of at least one of the IDT electrodes which is formed in such a manner that a pitch that is a distance between right and left centers of adjacent electrode fingers provided in the IDT electrode becomes small and then large in a view of a propagation region of the surface acoustic wave from one of a left end and a right end toward the other in the IDT electrode.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a longitudinally coupled dual-mode surface acoustic wave filter.

Background Art

[0002] Various communication devices incorporate and use chips, which are electronic components including SAW (Surface Acoustic Wave) filters. As this SAW filter, a plurality of IDT (Inter Digital Transducer) electrodes and reflectors sandwiching these IDT electrodes from both sides are arranged side by side in the propagation direction (longitudinal direction) of SAWs in a piezoelectric substrate, and a longitudinally coupled dual-mode SAW filter that obtains filter characteristics by utilizing the coupling of two resonance modes resonating at different frequencies in the propagation direction is known.

[0003] Regarding the above longitudinally coupled dual-mode SAW filter, it will be described below as a DMS filter. This DMS (Double Mode SAW) filter is widely used because it has high attenuation, low loss, and can be designed to be small in obtaining a wide passband. For example, the DMS filter is shown in Patent Documents 1 and 2. However, regarding this DMS filter, it has been difficult to obtain good attenuation characteristics on the high-frequency side compared to the low-frequency side (low-frequency side) of the passband. More specifically, it has been difficult to relatively increase the attenuation gradient and the attenuation amount on the high-frequency side.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Disclosure of the Invention

Problems to be Solved by the Invention

[0005] In the case of a ladder-type filter in which a plurality of SAW resonators are combined, good attenuation characteristics can also be obtained on the high-frequency side of the passband. However, since a plurality of such SAW resonators are required, there is a problem that the chip including the filter becomes large. There are cases where it is required to design a filter so that the passband is less than 1 GHz. However, regarding the IDT electrodes included in the SAW resonator, it is necessary to increase the interval between the electrode fingers as the wavelength of the SAW propagating therethrough becomes larger. Therefore, when the passband of the filter is set to such a low band, it is particularly difficult to meet the requirement of chip miniaturization due to the increase in the size of each SAW resonator.

[0006] In addition, as specifications for mobile communication in which the frequency band used is 1 GHz or less, there are cases where the uplink frequency band is higher than the downlink frequency band, such as Band20, Band71, Band13, and Band14. As a filter for the downlink of these bands used in a mobile communication device, it is conceivable to use a DMS filter for the purpose of reducing the size of the chip including the filter and thus the size of the mobile communication device itself. However, as a filter for the downlink of such bands, it is required to obtain the largest attenuation amount in the uplink frequency band. Therefore, it is desirable to obtain good attenuation characteristics on the high-frequency side of the passband. As described above, it has been difficult to obtain good attenuation characteristics on the high-frequency side for conventional DMS filters.

[0007] Therefore, in order to improve the attenuation characteristics on the high-frequency side of the passband, a chip design in which a plurality of SAW resonators are combined with a DMS filter is also widely performed. However, due to the limitation of the chip size, the number of SAW resonators that can be formed is limited. Therefore, it has been difficult to make the chip a small-sized electronic component with good attenuation characteristics on the high-frequency side while keeping the chip size small. From the above-described circumstances, it is required to improve the attenuation characteristics on the high-frequency side of the passband of the DMS filter itself and to make the chip incorporated in the communication device small-sized.

[0008] The filter shown in Patent Document 1 has a configuration in which a DMS filter and a plurality of SAW resonators are formed on a common piezoelectric substrate, and a configuration in which a plurality of SAW resonators are combined with the above-described DMS filter. As described above, since the requirement of a plurality of SAW resonators is disadvantageous in reducing the chip size, there is a possibility that the filter may not sufficiently meet the further miniaturization requirement of recent electronic components. Further, Patent Document 2 shows that by adjusting the number of electrode fingers in the first to fifth IDT electrodes between these IDT electrodes, the attenuation gradient on the high-frequency side of the passband can be improved. However, the improvement amount shown in the document is small and insufficient as a countermeasure.

[0009] The present invention has been made in view of the above circumstances, and an object thereof is to obtain good attenuation characteristics on the high-frequency side of the passband in a vertically coupled dual-mode surface acoustic wave filter.

Means for Solving the Problems

[0010] The vertically coupled dual-mode surface acoustic wave filter of the present invention includes: a piezoelectric substrate, a plurality of IDT electrodes provided side by side in the left-right direction in which the surface acoustic wave propagates on the piezoelectric substrate, a reflector provided on the piezoelectric substrate so as to sandwich the plurality of IDT electrodes from left and right, in at least one of the IDT electrodes, an electrode finger group of the IDT electrode formed such that a pitch, which is a distance between the left and right centers of adjacent electrode fingers provided in the IDT electrode, becomes smaller and then larger when viewed from one of the left end and the right end toward the other end of the propagation region of the surface acoustic wave, and are provided.

Effects of the Invention

[0011] According to the present invention, good attenuation characteristics can be obtained on the high-frequency side of the passband in a vertically coupled dual-mode surface acoustic wave filter.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Mode for Carrying Out the Invention

[0013] 〈Comparative Form〉 To clearly show the configuration of the DMS filter 1A, which is a longitudinally coupled dual-mode surface acoustic wave filter as the first embodiment of the present invention, the DMS filter 1 in the comparative form will be described first with reference to the schematic plan view of FIG. 1. The DMS filter 1 includes IDT electrodes 11 to 13 each containing an electrode finger group, and two reflectors 14, which are formed on the surface of the piezoelectric substrate 15. The reflectors 14, IDT electrode 11, IDT electrode 12, IDT electrode 13, and reflector 14 are arranged in this order along the propagation direction of the SAW. Therefore, the two reflectors 14 are provided so as to sandwich the IDT electrodes 11 to 13 in the SAW propagation direction. Thus, the DMS filter 1 has a configuration with three IDT electrodes sandwiched between two reflectors, and the first-order resonance mode and the third-order resonance mode are excited, and these resonance modes are combined to obtain filter characteristics.

[0014] Each of the IDT electrodes 11 to 13 includes two bus bars 21 spaced apart from each other in a direction perpendicular to the SAW propagation direction, and electrode fingers 22 and dummy electrode fingers 23 extending in a direction perpendicular to the SAW propagation direction from the bus bars 21. In order to distinguish the two bus bars 21 in the same IDT electrode from each other, the bus bars formed on one side (the upper side in the figure) and the other side (the lower side in the figure) of the piezoelectric substrate 15 may be denoted as 21A and 21B, respectively. The bus bar 21A of the IDT electrode 12 is connected to the input terminal 16, the bus bars 21B of the IDT electrodes 11 and 13 are connected to the output terminal 17, and the other bus bars 21 are grounded.

[0015] Hereinafter, for convenience of explanation, the SAW propagation direction will be described as the left-right direction, and the sides where the IDT electrodes 11 and 13 are located will be described as the left side and the right side, respectively. And the direction perpendicular to the SAW propagation direction will be simply described as the perpendicular direction. For each of the IDT electrodes 11 to 13, the electrode fingers 22 extending from the bus bar 21A and the electrode fingers 22 extending from the bus bar 21B are alternately and repeatedly arranged when viewed from one side to the other side in the left-right direction.

[0016] Dummy electrode fingers 23 that are shorter than the electrode fingers 22 are formed so as to extend along the extension lines in the extending direction of each of the electrode fingers 22. The dummy electrode fingers 23 are provided between the bus bars 21A and 21B and are electrodes that do not contribute to vibration, for the purpose of preventing a decrease in the excitation of the electrode fingers 22 and suppressing spurious by adjusting the charge distribution in the region extending from the IDT electrode 11 to the IDT electrode 13. Note that a filter configuration may be adopted in which the dummy electrode fingers 23 are not provided.

[0017] Of the region extending from the IDT electrode 11 to the IDT electrode 13 sandwiched between the above-described bus bars 21A and 21B, the region where the electrode fingers 22 are arranged and the SAW propagates is indicated by a dotted line frame and designated as a track T. The distance between the centers on the left and right of two adjacent electrode fingers 22 in the track T is described as a pitch P. In the figure, one of the many pitches P formed by the groups of electrode fingers 22 is representatively illustrated.

[0018] Regarding two adjacent IDT electrodes among the IDT electrodes 11 to 13, assuming one IDT electrode and the other IDT electrode, a region formed by two or more electrode fingers 22 provided at the end on the other IDT electrode side in one IDT electrode and two or more electrode fingers 22 provided at the end on the one IDT electrode side in the other IDT electrode is defined as an inter-electrode end region 31. In the figure, this inter-electrode end region 31 is indicated by a dashed-dotted line frame. The pitch P in the inter-electrode end region 31 is smaller than the pitch P in other regions of the track T. By setting the pitch P of the inter-electrode end region 31 in this way, it has been confirmed that the passband of the DMS filter 1 becomes wider.

[0019] In addition, in the electrode-to-electrode end region 31, the regions of the IDT electrodes (IDT electrode 12 in this embodiment) where both the left and right ends are adjacent to other IDT electrodes are defined as end regions 32. That is, the end regions 32 are formed at both the left and right ends of the IDT electrode 12. In the IDT electrode 12, the pitch P of this end region 32 is set to be smaller than the pitch P of the regions other than the end region 32 in the region forming the track T. Note that the pitch P is the same size in each part within the electrode-to-electrode end region 31. Therefore, the pitch P of the end region 32 of the IDT electrode 12 is the same size as the pitch P of the regions included in the electrode-to-electrode end region 31 in the IDT electrodes 11 and 13.

[0020] 〈First Embodiment〉 Regarding the DMS filter 1A of the first embodiment, with reference to the schematic plan view of FIG. 2, the differences from the DMS filter 1 of the comparative form will be mainly described. In the DMS filter 1A, a narrow pitch region 33 with a small pitch P is provided in the left and right central portions of the IDT electrode 12 and in the regions away from the end region 32, and the narrow pitch region 33 is shown surrounded by a dashed-dotted line in the figure.

[0021] More specifically, when the regions adjacent to the left and right central portions of the IDT electrode 12 with respect to the end region 32 and located adjacent to both the left and right sides of the narrow pitch region 33 are defined as the first intermediate regions 34, the pitch P (second pitch) of the narrow pitch region 33 is smaller than the pitch P (third pitch) of the first intermediate region 34. Therefore, when looking at the part forming the track T of the IDT electrode 12 from one of the left end and the right end toward the other, the first intermediate region 34, the narrow pitch region 33, and the first intermediate region 34 are arranged in this order, so that the pitch P becomes small and then large. In other words, regions with a large pitch P are provided so as to sandwich the region with a small pitch P from the left and right.

[0022] Regarding the relationship of the pitch P in each region in more detail, the pitch P in the narrow pitch region 33 is smaller than the pitch P (the first pitch) in each end region 32, and the pitch of the first intermediate region 34 is larger than the pitch P in each end region 32. Therefore, in the IDT electrode 12 of the DMS filter 1A, the electrode fingers 22 are arranged such that the narrow pitch region 33 is formed as the region having the smallest pitch P at the left and right central portions.

[0023] The IDT electrodes 11 and 13 of the DMS filter 1A are configured in the same manner as the IDT electrodes 11 and 13 of the DMS filter 1. Therefore, for the DMS filter 1A, the end region 32, the narrow pitch region 33, and the first intermediate region 34 are provided in the group of electrode fingers 22 of the IDT electrode (IDT electrode 12) excluding the IDT electrodes at both the left and right ends among the odd-numbered IDT electrodes provided. Note that, similar to the DMS filter 1 in the comparative form, the DMS filter 1A has a bilaterally symmetric shape, and the IDT electrode 12 also has a bilaterally symmetric shape.

[0024] 〈Simulation 1〉 FIG. 3 is a graph showing the filter characteristics of the DMS filter 1 and the DMS filter 1A obtained by simulation (hereinafter referred to as simulation 1). As shown in the graph, the DMS filter 1 has a low attenuation gradient on the high-frequency side of the passband. On the other hand, for the DMS filter 1A, an attenuation pole (notch) is formed at around 660 MHz on the high-frequency side of the passband, which is not seen in the DMS filter 1. Due to this attenuation pole, both the attenuation gradient and the attenuation amount on the high-frequency side of the passband are larger for the DMS filter 1A than for the DMS filter 1.

[0025] 〈Measured Results〉 FIG. 4 shows the filter characteristics obtained by actual measurement of the DMS filter 1A. Similar to the result of Simulation 1 in FIG. 3, it was shown that an attenuation pole is formed near 660 MHz on the high-frequency side of the passband. It was shown that the DMS filter 1A having such a narrow pitch region 33 exhibits better attenuation characteristics on the high-frequency side. And for the DMS filter 1A, even without adopting a configuration in which a plurality of resonators are connected as in Prior Art Document 1, the DMS filter 1A itself exhibits such good attenuation characteristics on the high-frequency side. Therefore, by eliminating or reducing the number of resonators to be connected, it is possible to prevent the size of the piezoelectric substrate 15 from increasing and to reduce the size of the chip including the DMS filter 1A.

[0026] Incidentally, in FIG. 2 regarding the pitch P in the DMS filter 1A, it was shown that only three types are included, where the IDT electrode 12 includes the pitches P of the end region 32, the narrow pitch region 33, and the first intermediate region 34, and the IDT electrodes 11 and 13 include the same pitch P as the end region 32 or the first intermediate region 34. Even if there are only three types of pitch P in this way, the above-described attenuation pole is formed and good attenuation characteristics can be obtained. However, it is preferable to set regions where three or more pitches P are different so as to obtain desired filter characteristics.

[0027] Specifically, the first intermediate region 34 of the IDT electrode 12 is divided into a plurality of parts, and the regions other than the end region 32 of the IDT electrodes 11 and 13 are divided into a plurality of parts, and the pitch P is set for each of these divided regions. Also, in FIG. 2, the number of electrode fingers 22 forming the narrow pitch region 33 is three, the number of electrode fingers 22 forming each end region 32 is two, and the number of electrode fingers 22 forming the end region 31 between the electrodes in the IDT electrodes 11 and 13 is two. However, the number is not limited to such illustration and can be any number. The result of Simulation 1 shown in FIG. 3, the actual measurement result shown in FIG. 4, and the results of Simulations 2 and 3 described later are results obtained by setting a larger number of electrode fingers 22 than the number of electrode fingers 22 shown in FIG. 2 and setting more than three types of pitch P in this way. The specific setting of the pitch P will be described in the item of Simulation 3.

[0028] <Simulation 2> For the DMS filter 1A, a simulation (hereinafter referred to as Simulation 2) was conducted to examine the transition of filter characteristics when the pitch P of the narrow pitch region 33 was changed. The pitch P of the narrow pitch region 33 was set to 2.688 μm, 2.7264 μm, or 2.7252 μm. Although the pitch P of the narrow pitch region 33 is changed in this way, the pitch P in the end region 32 is larger for any of these values.

[0029] The graph in Fig. 5 shows the filter characteristics of the DMS filter 1A obtained by Simulation 2. As shown in the graph, as the pitch P of the narrow pitch region 33 increases, the attenuation pole moves to the low frequency side, so the attenuation gradient increases, and as the attenuation pole becomes smaller, the attenuation amount decreases. Therefore, it can be seen that if a large attenuation gradient is to be obtained, the pitch P of the narrow pitch region 33 may be made relatively large.

[0030] Even when the pitch P of the narrow pitch region 33 is set to the largest value among the set values (i.e., the state where the difference in pitch P from the end region 32 is the smallest), which is 2.7252 μm, a high attenuation gradient is obtained. Even if the pitch P of the narrow pitch region 33 is made larger than the pitch P of the end region 32, it is considered that such a relatively high attenuation gradient can be obtained. That is, in order to obtain both a high attenuation gradient and a high attenuation amount, it is preferable to make the pitch P of the narrow pitch region 33 smaller than the pitch P of the end region 32, but if only the attenuation gradient is considered, such a setting is not limited thereto. That is, in the IDT electrode 12, the pitch P of the narrow pitch region 33 is not necessarily made smaller than the pitch P of the end region 32.

[0031] <Simulation 3> A simulation was performed on a filter configured by connecting two resonators to a DMS filter 1A to examine the filter characteristics. Hereinafter, this simulation will be referred to as Simulation 3. Also, for the filter combined with these resonators, in order to avoid confusion with the DMS filter 1A alone, it will be hereinafter referred to as a filter component. The filter component in this Simulation 3 was designed as a filter component for the downlink of Band71 described in the items of the problems to be solved by the invention.

[0032] In Simulation 3, the conditions set for the DMS filter 1A are described below. The piezoelectric substrate 15 was made of 42° rotated Y-cut X-propagation LiTaO3, and the IDT electrodes of the two resonators connected to the DMS filter 1A were also formed on this piezoelectric substrate 15. The IDT electrodes 11 to 13 of the DMS filter 1A and the IDT electrodes of the resonators were made of an Al (aluminum) film, and the film thickness of the Al film was 576 nm. In addition, regarding the DMS filter set in other simulations and the DMS filter 1A used to obtain the actual measurement results in FIG. 4, the material of the piezoelectric substrate 15 and the IDT electrodes and the film thickness of the IDT electrodes are the same as the settings in this Simulation 3.

[0033] In this Simulation 3, the pitch P of each part of the DMS filter 1A was set to a value within the range of 2.7264 μm to 3.2832 μm. And, unlike that shown in FIG. 2, more types than three types were included in terms of size, but similar to that shown in FIG. 2, among the pitches P of each track T, the pitch P of the narrow pitch region 33 at the center left and right of the IDT electrode 12 was the smallest. The setting of this pitch P will be described in detail later.

[0034] The results of Simulation 3 are shown in the graph of FIG. 6. In this graph, the downlink frequency band of 617 MHz to 652 MHz in Band 71 is shown as L1, and the uplink frequency band of 663 MHz to 698 MHz is shown as L2. As is clear from the graph, the insertion loss in the downlink frequency band L1 is suppressed to 2 dB or less, and an attenuation of 40 dB or more is obtained in the uplink frequency band L2. Therefore, it has been shown that the DMS filter 1A is particularly useful as a filter for the downlink when the uplink frequency band is higher than the downlink frequency band.

[0035] FIG. 7 is a graph showing the setting of each pitch of the DMS filter 1A in Simulation 3. The position numbers on the horizontal axis of the graph correspond to the positions of the electrode fingers 22 that form the pitch P, and the smaller the number, the more the pitch P is formed to the left of the track T. When the electrode fingers 22 are repeatedly arranged at the same pitch P when viewed in the left-right direction, the repetition is shown as one position. That is, one position includes two or more electrode fingers, and only two adjacent electrode fingers 22 are not represented as one position.

[0036] For the IDT electrodes 11 and 13 at the left and right ends, four types of pitches are set, and are represented by the position numbers 1 to 4 and 16 to 19. In these IDT electrodes 11 and 13, the pitch P is set to decrease as it approaches the IDT electrode 12. For the central IDT electrode 12, six types (a total of 11 from the position numbers 5 to 15) of pitches P are set. The two electrode-end regions 31 described in FIGS. 1 and 2 are positions 4, 5, 15, and 16, and the end region 32 of the IDT electrode 12 is positions 5 and 15. The narrow pitch region 33 is position 10, and the first intermediate region 34 is positions 6 to 9 and 11 to 14.

[0037] <Second Embodiment> Regarding the DMS filter 1B of the second embodiment, differences from the DMS filter 1 will be mainly described with reference to FIG. 8, which is a graph showing the setting of the pitch P in the same manner as in FIG. 7. In the DMS filter 1B, the configuration of the IDT electrode 12 is different from that of the DMS filter 1A. Instead of setting the position 10 as the narrow pitch region 33, the positions 9 and 11 adjacent to both sides of the position 10 are set as the narrow pitch region 33.

[0038] More specifically, as shown in the graph of FIG. 8, the position 10 is configured as a second intermediate region having a pitch P larger than the pitches P of the end region 32 and the narrow pitch region 33. In this DMS filter 1B as well, the pitch P of the narrow pitch region 33 is smaller than the pitch P of the end region 32. Therefore, in the IDT electrode 12 of the DMS filter 1B, the electrode fingers 22 are arranged such that the narrow pitch region 33 is formed as a region having the smallest pitch P at positions deviated from the left and right central portions and the ends.

[0039] As described above, since the first intermediate region 34 (positions 5 to 8 or 12 to 15), the narrow pitch region 33 (position 9 or 11), and the second intermediate region (position 10) are arranged in this order, when looking at the IDT electrode 12 of this DMS filter 1B from one side to the other side, the pitch P becomes smaller and then larger.

[0040] 〈Simulation 4〉 FIG. 9 shows a graph of the passband characteristics of the DMS filter 1B obtained by simulation (hereinafter referred to as simulation 4). In the graph of FIG. 9, the passband characteristics of the DMS filter 1 of the first embodiment shown in the graph of FIG. 3 are also displayed for comparison. As is clear from the graph, it was confirmed that, similar to the DMS filter 1, an attenuation pole is formed on the high-frequency side of the passband for the DMS filter 1B, and good attenuation characteristics are obtained. From the results of this simulation 4, it can be seen that the narrow pitch region 33 is not limited to being set at the left and right central portions of the IDT electrode 12.

[0041] So far, a DMS filter with three IDT electrodes has been shown. However, a configuration with more IDT electrodes, such as 5, 7, 9, etc., may also be used. In practical applications, the number of IDT electrodes is often odd, but an even number may also be used.

[0042] <Third Embodiment> Hereinafter, the DMS filter 1C of the third embodiment will be described with reference to the schematic plan view of FIG. 10, focusing on the differences from the DMS filter 1A. This DMS filter 1C is provided with five IDT electrodes, which are shown as 41, 42, 43, 44, and 45 in order from the left side to the right side. The IDT electrodes 41 and 45 are configured in the same manner as the IDT electrodes 11 and 13 of the DMS filter 1A, respectively. One end of each of the left and right sides forms an electrode-to-electrode end region 31, and four positions with different pitches P from each other are set.

[0043] The IDT electrodes 42 to 44 have the same configuration as the IDT electrode 12 of the DMS filter 1A. Therefore, in the DMS filter 1C, among the IDT electrodes provided in an odd number, two end regions 32, two first intermediate regions 34, and a narrow pitch region 33 are provided in each of the electrode finger 22 groups of the IDT electrodes (42 to 44) excluding the IDT electrodes (41, 45) at both ends on the left and right, and eleven positions are set. In FIG. 10, only three types of pitches P are shown, similar to FIG. 2. The bus bars 21A of the IDT electrodes 41, 43, and 45 are connected to the input terminal 16, the bus bars 21B of the IDT electrodes 42 and 44 are connected to the output terminal 17, and the other bus bars 21 are grounded.

[0044] FIG. 11 is a graph showing the relationship of pitches at each position for the DMS filter 1C, similar to FIGS. 8 and 9. The positions in the IDT electrodes 41, 42, 43, 44, 45 are 1 to 4, 5 to 15, 16 to 26, 27 to 37, and 38 to 41, respectively. The positions 10, 21, and 32 set at the left and right central portions of each of the IDT electrodes 42, 43, and 44 are the narrow pitch regions 33, respectively. And the pitch P of position 21 is larger than the pitch P of positions 10 and 32. That is, in the DMS filter 1C, between the different IDT electrodes each having the narrow pitch region 33, the pitch P (second pitch) in the narrow pitch region 33 is different from each other. Specifically, the pitch P of positions 10 and 32 is set to 2.688 μm, and the pitch P of position 21 is set to 2.752 μm, respectively.

[0045] 〈Simulation 5〉 FIG. 12 shows a graph of the passband characteristics of the DMS filter 1C obtained by simulation (hereinafter referred to as simulation 5). In the graph of FIG. 12, the passband characteristics of the DMS filter 1A also shown in the graph of FIG. 3 are displayed for comparison. As shown in the graph, in the DMS filter 1C as well, an attenuation pole is formed on the high-frequency side of the passband, similar to the DMS filter 1A, but the number of these attenuation poles is different from each other. In the DMS filter 1A, there is one, while in the DMS filter 1C, there are two. This characteristic of having two attenuation poles appears because the pitch P of positions 10 and 32 is set to be different from the pitch P of position 21. The formation of a plurality of attenuation poles in this way is preferable from the viewpoint of increasing the degree of freedom in the design of the filter in realizing the desired filter characteristics.

[0046] In the DMS filter 1C, instead of having the same configuration as the IDT electrode 12 of the DMS filter 1A for each of the IDT electrodes 42 to 44, all or part of them may have the same configuration as the IDT electrode 12 of the DMS filter 1B described in FIG. 8.

[0047] Regarding the DMS filter 1C, assume that IDT electrodes 46 and 47 having the same configuration as the IDT electrodes 42 to 44 are arranged between the IDT electrodes 41 and 42 and between the IDT electrodes 44 and 45, respectively, and a total of seven IDT electrodes are provided. In that case, for example, the pitch P of the narrow pitch region 33 of the IDT electrodes 46 and 47, the pitch P of the narrow pitch region 33 of the IDT electrodes 44 and 45, and the pitch P of the narrow pitch region 33 of the IDT electrode 43 may be different in size. That is, the number of narrow pitch regions 33 with different sizes of the mutual pitch P can be appropriately set according to the number of IDT electrodes provided in the DMS filter, and it is not limited to two as in the configurations shown in FIGS. 10 and 11.

[0048] By the way, the IDT electrodes having the narrow pitch region 33 shown in each of the above-described embodiments have a symmetric shape about the left and right, but may have an asymmetric shape about the left and right in order to obtain desired filter characteristics. Also, the shape of the DMS filter described so far does not have to be symmetric about the left and right. However, from the viewpoint of more surely suppressing the generation of spurious in the passband, it is preferable that the IDT electrodes having the narrow pitch region 33 and the shape of the DMS filter are symmetric about the left and right.

[0049] Also, as described above, for the purpose of widening the passband, the electrode end region 31 with a relatively small pitch P is set. However, a configuration in which this electrode end region 31 is not provided may be used. Therefore, for example, in the IDT electrodes 12 of the DMS filters 1A and 1B of the first and second embodiments, the pitch P of the portions shown as the first intermediate region 34 and the end region 32 may be the same value.

[0050] As described above, in the present invention, by adjusting the pitch of the IDT electrodes, an attenuation pole is formed on the high-frequency side of the DMS filter to obtain good filter characteristics. For a DMS filter with a passband lower than 1 GHz, since it is likely to be enlarged, it is particularly effective to apply the present invention. Note that as the operating principle of the DMS filter does not depend on the materials of the piezoelectric substrate 15 and the electrodes used, materials other than the exemplified materials may be used. Specifically, for the piezoelectric substrate 15, piezoelectric materials other than LiTa03 such as LiNbO3, AlN, Sc-doped AlN, and ZnO may be used.

[0051] Further, by bonding the piezoelectric substrate 15 to another substrate such as SiO2 or Si, it may be configured as a multilayer substrate with improved temperature characteristics and losses. Alternatively, in order to improve the temperature characteristics and weather resistance, a functional inorganic film such as SiO2, SiON, or SiN may be formed to coat the IDT electrodes on the piezoelectric substrate 15. The IDT electrodes may be constituted by a metal film of a material other than Al, or may have a structure in which two or more metal films of different materials are laminated.

[0052] From the above, the DMS filter of the present invention may be configured as a so-called TC (Temperature Compensated)-SAW filter in which changes in the propagation speed of SAW due to the ambient temperature caused by the action of the film covering the IDT electrodes and changes in the linear expansion of the piezoelectric substrate due to the ambient temperature caused by the action of the substrate bonded to the piezoelectric substrate are suppressed.

[0053] Further, a DMS filter may be configured by forming the IDT electrodes and the reflector 14 on a laminated substrate in which the piezoelectric substrate 15 is formed relatively thinly in a film shape, a high-sound-speed substrate such as Si with a relatively high sound speed of bulk waves is provided below the piezoelectric substrate 15, and a low-sound-speed film such as SiO2 with a relatively low sound speed of bulk waves is interposed between the high-sound-speed substrate and the piezoelectric substrate 15. The DMS filter is a so-called TF (Thin Film)-SAW filter in which characteristics are improved by confining SAW in the laminated portion of the piezoelectric substrate 15 and the low-sound-speed film.

[0054] In addition, the present invention can form attenuation poles by adjusting the pitch at each location, and there is no limitation on the design of the electrode fingers 22 in the vertical direction. Therefore, in the example described above, the width of the electrode fingers 22 in the extending direction is uniform, but a non-uniform configuration may also be used. By making the width non-uniform in this way, the SAW velocity profile in the vertical direction of the track T can be adjusted, and a configuration may be adopted in which a piston mode in which the amplitude rapidly decreases at the end in the vertical direction of the track T is excited.

[0055] It should be considered that the embodiments disclosed this time are illustrative in all respects and not restrictive. Without departing from the scope of the appended claims and the gist thereof, the above embodiments may be omitted, substituted, changed, or combined in various forms.

Explanation of Reference Numerals

[0056] P pitch 1A DMS filter 11 - 13 IDT electrodes 14 Reflector 22 Electrode fingers

Claims

1. A piezoelectric substrate, a plurality of IDT electrodes provided side by side in the left - right direction in which an elastic surface wave propagates on the piezoelectric substrate, a reflector provided on the piezoelectric substrate so as to sandwich the plurality of IDT electrodes from left and right, in at least one of the IDT electrodes, an electrode finger group of the IDT electrode is formed such that a pitch, which is the distance between the left - right centers of adjacent electrode fingers provided in the IDT electrode, becomes smaller and then larger when looking from one of the left and right ends of the propagation region of the elastic surface wave toward the other, A longitudinally - coupled dual - mode elastic surface wave filter comprising the above.

2. The electrode finger group, is formed at each of the left and right ends of the IDT electrode by including the left and right end electrode fingers and the electrode fingers adjacent to the electrode fingers, and has an end region having a first pitch, a narrow - pitch region provided away from the end region and having a second pitch, a first intermediate region provided between the narrow - pitch region and the end region and having a third pitch different from the first pitch and larger than the second pitch, The longitudinally - coupled dual - mode elastic surface wave filter according to Claim 1, comprising the above.

3. The longitudinally - coupled dual - mode elastic surface wave filter according to Claim 2, wherein the second pitch is smaller than the first pitch.

4. The longitudinally - coupled dual - mode elastic surface wave filter according to Claim 2 or 3, wherein the narrow - pitch region is located at the left and right central portions of the IDT electrode.

5. A second intermediate region having a pitch larger than the second pitch is provided at the left and right central portions of the IDT electrode, The longitudinally - coupled dual - mode elastic surface wave filter according to Claim 2 or 3, wherein the narrow - pitch region is provided on each of the left and right sides of the second intermediate region.

6. Three or more odd - numbered IDT electrodes are provided, The longitudinally - coupled dual - mode elastic surface wave filter according to Claim 2, wherein the end region, the narrow - pitch region, and the first intermediate region are provided in the electrode finger group of the IDT electrodes excluding the left and right end IDT electrodes.

7. Five or more IDT electrodes are provided, The longitudinally - coupled dual - mode elastic surface wave filter according to Claim 6, wherein the second pitches are different from each other between two or more IDT electrodes having the narrow - pitch region.

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