JP2005159835A - Surface acoustic wave filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cascaded multiplex mode surface acoustic wave filter with small insertion loss in a passband and with a wide passband. <P>SOLUTION: Three interdigital electrodes (IDT) 202, 203 and 204 or 208, 209 and 210 are closely provided along the propagation direction of exciting surface acoustic waves on a piezoelectric substrate, and reflectors 205 and 206 or 211 and 212 are provided on both sides of the series of IDTs, thereby providing 3IDT cascaded duplex mode filters 201 and 207 utilizing at least a primary or third-order oscillation mode generated by acoustic coupling among the three IDTs. A dummy electrode finger 224 is provided between electrode fingers 202a, 202b, 208a and 208b of the IDTs and a bus bar to suppress slant radiation of the surface acoustic waves. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a surface acoustic wave filter used for a mobile radio terminal or the like, and more particularly to a surface acoustic wave filter having a wide pass band and a small insertion loss in the pass band.

  In recent years, with an increase in data communication speed of mobile phones, communication systems are being shifted to ones having a wide transmission / reception band. In order to effectively use the frequency, the interval between the transmission and reception bands has been narrowed, and a high-angle filter having a wide band and a steep cut-off characteristic is required.

  Patent Document 1 below describes a method for realizing a surface acoustic wave device having a wide pass band and low loss while maintaining a large attenuation outside the pass band in the vicinity of the high frequency side of the pass band.

  The surface acoustic wave device described in Patent Document 1 includes a surface acoustic wave filter having at least two comb-shaped electrodes on a piezoelectric substrate, and a surface acoustic wave resonator connected in series to the surface acoustic wave filter. The surface acoustic wave resonator is characterized in that spurious due to SSBW is shifted to the resonance frequency.

  An example of the surface acoustic wave device described in Patent Document 1 is shown in FIG. The surface acoustic wave device shown in FIG. 11 is in series with two three comb electrode (IDT) type longitudinally coupled double mode surface acoustic wave filters 101 and 107 and 3IDT type longitudinally coupled double mode surface acoustic wave filters 101 and 107. Four surface acoustic wave resonators 113, 117, 121, and 125 connected to each other are provided.

  At this time, at least one of the surface acoustic wave resonators 113, 117, 121, and 125 has a resonance frequency within the passband of the 3IDT type longitudinally coupled double mode surface acoustic wave filters 101 and 107, and an antiresonance frequency of 3IDT type. The configuration is such that the dummy electrode finger 132 is provided between the electrode finger 122b of the IDT and the bus bar, and is set to be located outside the pass band of the longitudinally coupled double mode surface acoustic wave filters 101 and 107.

In the above configuration, by providing the dummy electrode finger 132, the spurious attributed to the SSBW of the surface acoustic wave resonator can be shifted to the resonance frequency side, and while maintaining a large attenuation outside the passband, A surface acoustic wave device having a small insertion loss on the inner high frequency side and a wide pass band is obtained.
JP 2003-309448 A

  However, it is difficult to improve the insertion loss in the entire pass band, particularly on the low frequency side, by inserting dummy electrode fingers into the surface acoustic wave resonator as in the conventional surface acoustic wave filter. Further, in a surface acoustic wave filter having no surface acoustic wave resonator, it is not an effective means.

  In the multimode surface acoustic wave filter, the present invention improves the insertion loss on the high-frequency side in the passband, and has a wide passband capable of reducing the loss on the low-frequency side in the passband and good shoulder characteristics. It is an object of the present invention to provide a simple surface acoustic wave filter.

  In order to achieve the above object, the first means of the present invention is arranged on the piezoelectric substrate in close proximity along the propagation direction of the surface acoustic wave exciting the plurality of comb electrodes, and reflectors on both sides of the comb electrode rows. In a longitudinally coupled multimode surface acoustic wave filter that uses a plurality of vibration modes generated by acoustic coupling between the plurality of comb electrodes, the main waveguide is configured to suppress oblique radiation of the surface acoustic waves. It has a configuration in which the outer sound velocity is made slower than the surface acoustic wave velocity at the crossing portion of the comb-shaped electrode as a waveguide.

  According to a second means of the present invention, in the first means, a dummy electrode finger is provided between the electrode finger of the comb-shaped electrode and the bus bar in order to suppress the oblique emission of the surface acoustic wave. The acoustic velocity is made slower than the surface acoustic wave velocity at the crossing portion of the comb-shaped electrode as a waveguide.

  The third means of the present invention is characterized in that, in the second means, the width of the electrode fingers of the comb-shaped electrode is different from the width of the dummy electrode fingers.

  A fourth means of the present invention is characterized in that, in the third means, the width of the dummy electrode finger is wider than the width of the electrode finger of the comb-shaped electrode.

  According to a fifth means of the present invention, on the piezoelectric substrate, three comb-shaped electrodes are arranged close to each other along the propagation direction of the surface acoustic wave that excites the three comb-shaped electrodes, and reflectors are arranged on both sides of these comb-shaped electrode rows, In the three-comb electrode type longitudinally coupled double-mode surface acoustic wave filter using the first and third vibration modes generated by acoustic coupling between the three comb-shaped electrodes, the oblique emission of the surface acoustic wave is suppressed. As described above, the sound velocity outside the main waveguide is configured to be slower than the surface acoustic wave velocity at the crossing portion of the comb-shaped electrode as the waveguide.

  According to a sixth means of the present invention, in the fifth means, a dummy electrode finger is provided between the electrode finger of the comb-shaped electrode and the bus bar in order to suppress oblique emission of the surface acoustic wave, and The acoustic velocity is made slower than the surface acoustic wave velocity at the crossing portion of the comb-shaped electrode as a waveguide.

  The seventh means of the present invention is characterized in that, in the sixth means, the width of the electrode fingers of the comb-shaped electrode and the width of the dummy electrode fingers are different.

  An eighth means of the present invention is characterized in that, in the seventh means, the width of the dummy electrode finger is wider than the width of the electrode finger of the comb-shaped electrode.

  According to a ninth means of the present invention, in the sixth means, the length of the dummy electrode is set to 0.2λ or more, where λ is a wavelength defined by the pitch of the comb electrodes. Is.

  According to a tenth means of the present invention, in any one of the first to ninth means, a gap between the electrode finger of the comb-shaped electrode and the bus bar is narrowed so as to suppress oblique radiation of the surface acoustic wave. It is characterized by this.

  The present invention is configured as described above, and by providing dummy electrode fingers, it is possible to suppress oblique emission of surface acoustic waves. As a result, the loss in the passband is significantly reduced and the bandwidth is increased. Can be realized.

  As described above, the present invention is characterized in that in the longitudinally coupled multimode surface acoustic wave filter, dummy electrode fingers are provided between the electrode fingers of the comb-shaped electrode (IDT) and the bus bar. According to this configuration, the oblique emission of the surface acoustic wave is suppressed, and it can be confined in the lateral direction (cross width direction).

  In order to confine the surface acoustic wave in the lateral direction (cross width direction), as shown in FIG. 22, the velocity Vb in the bus bar portion is higher than the surface acoustic wave velocity Vi in the intersection portion of the comb-shaped electrode (IDT) 603 that is a waveguide. Need to be delayed (Vb <Vi). The provision of the dummy electrode finger 601 between the electrode finger of the comb-shaped electrode (IDT) 603 and the bus bar is intended to reduce the speed Vb of the bus bar portion.

  In addition, the gap (GAP) portion 602 between the electrode fingers of the comb-shaped electrode and the bus bar can also be considered to radiate surface acoustic waves. As a means for suppressing the oblique emission of the surface acoustic waves, the gap portion (GAP) ) 602 needs to be narrowed to reduce the speed of sound in the gap.

  Such a means is an effective means that can realize a low loss and a wide band even in a surface acoustic wave filter having no surface acoustic wave resonator.

  Next, specific examples of the present invention will be described together with comparative examples.

  As a first embodiment of the present invention, a reception filter for a 1.9 GHz band PCS (Personal Communications System) having a balanced-unbalanced conversion function (impedance on the unbalanced side is 50Ω and impedance on the balanced side is 150Ω) is provided. Let's take an example.

  First, a comparative example 1 is shown in FIG. On the piezoelectric substrate, three comb electrodes (IDT) are arranged close to each other along the propagation direction of the surface acoustic wave that excites them, and reflectors 205 and 206 are arranged on both sides of these IDT rows 202, 203, and 204. In addition, there is an elastic surface between the unbalanced signal terminal 221 and the 3IDT type longitudinally coupled double mode surface acoustic wave filter 201 that uses the first and third vibration modes generated by acoustic coupling between the three IDTs. Wave resonators 213 are connected in series.

  Similarly, a surface acoustic wave resonator 217 is connected in series between the 3IDT type longitudinally coupled double mode surface acoustic wave filter 207 and the unbalanced signal terminal 221. The 3IDT type longitudinally coupled double mode surface acoustic wave filter 207 includes an IDT 208 in which the direction of the center IDT 202 in the 3IDT type longitudinally coupled double mode surface acoustic wave filter 201 is inverted to invert the phase.

  In the first embodiment, as shown in FIG. 1, in the 3IDT type longitudinally coupled double mode surface acoustic wave filters 201 and 207 of the surface acoustic wave filter of Comparative Example 1 (FIG. 2), electrode fingers 202a and 202b of IDT are used. The dummy electrode fingers 224 are provided between the bus bars.

The detailed design of the surface acoustic wave filter in the first embodiment is as follows.
3IDT type longitudinally coupled double mode surface acoustic wave filters 201, 207
IDT wavelength λ _I1 : 2.03 μm
Reflector wavelength λ _R1 : 2.04 μm
Cross width W: 31.5λ _I1
Dummy electrode length: 1.2λ _I1
GAP length: 0.6μm
Surface acoustic wave resonators 213 and 217
IDT wavelength λ _I2 : 2.006 μm
Reflector wavelength λ _R2 : 2.006 μm
Cross width W: 37.5λ _I2
IDT logarithm: 100 pairs GAP length: 1.0 μm
Next, the effect of the first embodiment will be described. The characteristic results of Comparative Example 1 and the first embodiment are shown in FIGS. FIG. 3 shows the transmission characteristics, and FIG. 4 shows the squareness ratio.

  3 and 4, the insertion loss of Comparative Example 1 is 2.26 dB, whereas the insertion loss of 2.19 dB and 0.07 dB is improved in the first embodiment.

  Further, the loss at the end of the pass band is 1.30 dB in Comparative Example 1 at 1930 MHz, whereas it is improved by 1.59 dB and 0.16 dB in the first embodiment, and in 1990 MHz, While the comparative example 1 is 2.24 dB, the improvement effect of 1.98 dB and 0.26 dB was obtained in the first embodiment.

  It can also be seen that the squareness ratio (−10 dB bandwidth / −3 dB bandwidth) is steep as 1.207 in Comparative Example 1 and 1.163 in the first embodiment.

  Next, FIGS. 5 and 6 show changes in insertion loss (FIG. 5) and squareness ratio (FIG. 6) when the length of the dummy electrode finger is changed to 3.0λ. When the length of the dummy electrode finger is between 0.2λ and 1.2λ in both insertion loss and squareness ratio, good results of low loss and high squareness are obtained.

  As a second embodiment of the present invention, a reception filter for 900 MHz band EGSM (Extended Global System Mobile Communication) having a balanced-unbalanced conversion function (impedance on the unbalanced side is 50Ω and impedance on the balanced side is 150Ω). An example will be described.

  As Comparative Example 2, as shown in FIG. 7, four 3IDT type longitudinally coupled double mode surface acoustic wave filters 301, 307, 313, 319 are formed on a piezoelectric substrate so as to have a balance-unbalance conversion function. .

  First, the 3IDT type longitudinally coupled double mode surface acoustic wave filter 301 will be described. On the piezoelectric substrate, three comb electrodes (IDTs) are arranged close to each other along the propagation direction of the surface acoustic wave that excites them, and reflectors 305 and 306 are arranged on both sides of these IDT rows 302, 303, and 304. The first and third vibration modes generated by acoustic coupling between the three IDTs are used.

  Next, the 3IDT type longitudinally coupled double mode surface acoustic wave filter 307 will be described. The 3IDT type longitudinally coupled double mode surface acoustic wave filter 307 has the same characteristics as the 3IDT type longitudinally coupled double mode surface acoustic wave filter 301.

  The arrangement of the 3IDT type longitudinally coupled double mode surface acoustic wave filter 307 is such that the propagation direction of the surface acoustic wave of the 3IDT type longitudinally coupled double mode surface acoustic wave filter 307 is elastic in the 3IDT type longitudinally coupled double mode surface acoustic wave filter 301. It is parallel to the propagation direction of the surface wave, and is symmetrical with respect to the 3IDT type longitudinally coupled double-mode surface acoustic wave filter 301 with the propagation direction as the symmetry line.

  Next, the 3IDT type longitudinally coupled double mode surface acoustic wave filter 313 will be described. The 3IDT type longitudinally coupled double mode surface acoustic wave filter 313 has the same characteristics as the 3IDT type longitudinally coupled double mode surface acoustic wave filter 301.

  The arrangement of the 3IDT type longitudinally coupled double mode surface acoustic wave filter 313 is such that the propagation direction of the surface acoustic wave of the 3IDT type longitudinally coupled double mode surface acoustic wave filter 313 is elastic in the 3IDT type longitudinally coupled double mode surface acoustic wave filter 301. It becomes parallel to the propagation direction of the surface wave, and is symmetrical with respect to the 3IDT type longitudinally coupled double mode surface acoustic wave filter 301 with the direction perpendicular to the propagation direction as the symmetry line. Yes.

  Next, the 3IDT type longitudinally coupled double mode surface acoustic wave filter 319 will be described. The arrangement of the 3IDT type longitudinally coupled double mode surface acoustic wave filter 319 is such that the propagation direction of the surface acoustic wave of the 3IDT type longitudinally coupled double mode surface acoustic wave filter 319 is elastic in the 3IDT type longitudinally coupled double mode surface acoustic wave filter 313. It is parallel to the propagation direction of the surface wave, and is symmetrical with respect to the 3IDT type longitudinally coupled double mode surface acoustic wave filter 313 with the propagation direction as a symmetric line. The IDT 320 at the center of the longitudinally coupled double mode surface acoustic wave filter 319 is inverted, and the phase is inverted.

  In the second embodiment, as shown in FIG. 8, in the surface acoustic wave filter of Comparative Example 2 (FIG. 7), a dummy electrode finger 328 is provided between the electrode finger of the IDT and the bus bar. To do.

The detailed design of the surface acoustic wave filter in the second embodiment is as follows.
3IDT type longitudinally coupled double mode surface acoustic wave filters 301, 307, 313, 319
IDT wavelength λ _I : 4.188 μm
Reflector wavelength λ _R : 4.236 μm
Cross width W: 29.5λ _I
Dummy electrode length: 1.0λ _I
GAP length: 0.944 μm
Next, the effect of the second embodiment will be described. FIG. 9 and FIG. 10 show the characteristic results of Comparative Example 2 and the second embodiment. FIG. 9 shows the transmission characteristics, and FIG. 10 shows the squareness ratio.

  9 and 10, the insertion loss of Comparative Example 2 is 2.47 dB, whereas in the second embodiment, the insertion loss of 2.11 dB and 0.36 dB is improved.

  Further, the loss at the end of the passband is 925 MHz and the comparative example 2 is 2.47 dB, whereas in the second embodiment, the improvement is 2.11 dB and 0.36 dB, and at 960 MHz, While the comparative example 2 was 2.08 dB, the improvement effect of 2.05 dB and 0.03 dB was obtained in the second embodiment.

  It can also be seen that the squareness ratio (−10 dB bandwidth / −3 dB bandwidth) is steep as 1.200 in Comparative Example 2 and 1.176 in the second embodiment.

  A modification of the first embodiment is shown in FIG. In the surface acoustic wave filter shown in FIG. 12, the unbalanced input terminal 221 in the first embodiment shown in FIG. 1 is connected to the 3IDT type longitudinally coupled double mode surface acoustic wave filter 201 side and the 3IDT type longitudinally coupled double mode elastic surface. 12 are used as the two balanced output terminals 422 and 423 in FIG. 12, and the balanced output terminal 222 and the balanced output terminal 223 in the first embodiment shown in FIG. This is a configuration used as one unbalanced input terminal 421. Even in this configuration, the effect of the present invention can be obtained.

  Further, as another embodiment of the surface acoustic wave filter of the present invention, as shown in FIG. 13, the present invention also has a configuration in which the width of the dummy electrode finger 424 is different from the width of the electrode fingers 402a and 402b of the IDT. The effect is obtained. This configuration aims at a surface acoustic wave confinement effect in which the surface acoustic wave velocity Vb in the bus bar portion is further reduced by increasing the width of the dummy electrode.

  In addition, in the configurations of FIGS. 14, 15, 16, and 17 in which the ground electrode is changed in the 3IDT type longitudinally coupled double mode surface acoustic wave filter with respect to the first and second embodiments of the present invention. The effects of the present invention can be obtained.

  The surface acoustic wave filter of FIG. 14 has a configuration in which the unbalanced input terminal 421 in FIG. 12 is used as the ground, and the ground-side electrode fingers 402b and 408b of the IDTs 402 and 408 in FIG. 12 are used as the unbalanced input terminal.

  15 is configured to connect the ground side electrode fingers 302b and 314b of the IDTs 302 and 314 of the 3IDT type longitudinally coupled double mode surface acoustic wave filters 301 and 313 connected to the unbalanced input terminal 325 in FIG. In this configuration, only the electrode fingers 330 are taken.

  Further, in the configuration of FIG. 16, the ground side electrode fingers 308b and 320b of the IDTs 308 and 320 of the 3IDT type longitudinally coupled double mode surface acoustic wave filters 307 and 319 connected to the balanced input terminals 326 and 327 in FIG. This is a configuration taken only with the finger 330.

  17 is a combination of FIG. 15 and FIG. 16, and the IDTs 302, 308, 314, 320 of the IDTs 302, 308, 314, 320 of the 3IDT type longitudinally coupled double mode surface acoustic wave filters 301, 307, 313, 319 are shown. In this configuration, 302b, 308b, 314b, and 320b are taken only by the electrode fingers 330.

  Both the first and second embodiments are surface acoustic wave filters having a balance-unbalance conversion function, but the effects of the present invention can be obtained even in a configuration without a balance-unbalance conversion function. .

  For example, as shown in FIG. 18, there is no balanced-unbalanced conversion function, but a 3IDT configuration dual-coupled dual mode having a dummy electrode finger 224 between the surface acoustic wave resonator filter 213 and the unbalanced output terminal 222. Even in a configuration in which the surface acoustic wave filters 201 are connected in series, or in a configuration in which two 3IDT type longitudinally coupled dual mode surface acoustic wave filters 301 and 307 having dummy electrode fingers 328 are connected in cascade as shown in FIG. The effect is obtained.

  In addition, the surface acoustic wave filter configured as shown in FIGS. 20 and 21 slows the speed of sound in the gap (GAP) portion between the electrode finger of the comb electrode (IDT) and the dummy electrode without providing the dummy electrode finger. It aims at the confinement effect of the surface acoustic wave.

It is an electrode pattern schematic for demonstrating the surface acoustic wave filter of the 1st form of this invention. It is the electrode pattern schematic for demonstrating the surface acoustic wave filter which is not provided with the dummy electrode finger as the comparative example 1 for comparing with the said 1st Embodiment. It is a figure for demonstrating the frequency characteristic (passage characteristic) of the surface acoustic wave filter of the said 1st Embodiment and the comparative example 1. FIG. It is a figure for demonstrating the squareness ratio of the surface acoustic wave filter of the said 1st Embodiment and the comparative example 1. FIG. It is a figure for demonstrating the change of insertion loss when the length of a dummy electrode finger is changed in the surface acoustic wave filter of the said 1st Embodiment. In the surface acoustic wave filter of the first embodiment, it is a diagram for explaining a change in squareness ratio when the length of a dummy electrode finger is changed. It is an electrode pattern schematic for demonstrating the surface acoustic wave filter which is not provided with the dummy electrode as the comparative example 2 for comparing with the said 2nd Embodiment. It is an electrode pattern schematic for demonstrating the surface acoustic wave filter of the 2nd form of this invention. It is a figure for demonstrating the frequency characteristic (passage characteristic) of the surface acoustic wave filter of the said 2nd Embodiment and the comparative example 2. FIG. It is a figure for demonstrating the squareness ratio of the surface acoustic wave filter of the said 2nd Embodiment and the comparative example 2. FIG. It is the electrode pattern schematic for demonstrating the surface acoustic wave filter by a prior art. It is an electrode composition schematic diagram showing the surface acoustic wave filter of other modifications in the 1st embodiment of the above. It is the electrode pattern schematic which shows the surface acoustic wave filter of the other modification in the said 1st Embodiment. It is an electrode composition schematic diagram showing the surface acoustic wave filter of other modifications in the 1st embodiment of the above. It is the electrode block schematic diagram which shows the surface acoustic wave filter of the other modification in the said 2nd Embodiment. It is the electrode block schematic diagram which shows the surface acoustic wave filter of the other modification in the said 2nd Embodiment. It is the electrode block schematic diagram which shows the surface acoustic wave filter of the other modification in the said 2nd Embodiment. It is an electrode composition schematic diagram showing the surface acoustic wave filter of other modifications in the 1st embodiment of the above. It is the electrode block schematic diagram which shows the surface acoustic wave filter of the other modification in the said 2nd Embodiment. It is an electrode pattern enlarged view which shows the surface acoustic wave filter based on other embodiment of this invention. It is an electrode pattern enlarged view which shows the surface acoustic wave filter based on other embodiment of this invention. It is an electrode pattern enlarged view for demonstrating the effect | action in the surface acoustic wave filter of the 1st form of this invention.

Explanation of symbols

201, 207: Three comb electrode (IDT) type longitudinally coupled double mode surface acoustic wave filters 202, 203, 204, 208, 209, 210, 214, 218: Comb electrode (IDT)
205, 206, 211, 212, 215, 216, 219, 220: reflector 213, 217: surface acoustic wave resonator 221: unbalanced signal terminal 222, 223: balanced signal terminal 224: dummy electrode finger 225: gap (GAP) ) Part

Claims (10)

On the piezoelectric substrate, a plurality of comb electrodes are arranged close to each other along the propagation direction of the surface acoustic wave, and reflectors are arranged on both sides of the comb electrode rows, and acoustic coupling between the plurality of comb electrodes is performed. In a longitudinally coupled multimode surface acoustic wave filter using a plurality of generated vibration modes,
A longitudinally coupled multimode characterized in that the acoustic velocity outside the main waveguide is made slower than the surface acoustic wave velocity at the crossing portion of the comb-shaped electrode as the waveguide so as to suppress the oblique emission of the surface acoustic wave. Surface acoustic wave filter. The longitudinally coupled multimode surface acoustic wave filter according to claim 1,
In order to suppress the oblique emission of the surface acoustic wave, a dummy electrode finger is provided between the electrode finger of the comb electrode and the bus bar, and the sound velocity outside the main waveguide is changed to the elastic surface at the intersection of the comb electrode as the waveguide. A longitudinally coupled multimode surface acoustic wave filter characterized by being slower than the wave velocity. The longitudinally coupled multimode surface acoustic wave filter according to claim 2,
A longitudinally coupled multimode surface acoustic wave filter characterized in that the width of the electrode fingers of the comb-shaped electrode and the width of the dummy electrode fingers are different. The longitudinally coupled multimode surface acoustic wave filter according to claim 3,
A longitudinally coupled multimode surface acoustic wave filter characterized in that the width of the dummy electrode fingers is wider than the width of the electrode fingers of the comb electrodes. On the piezoelectric substrate, three comb-shaped electrodes are arranged close to each other along the propagation direction of the surface acoustic wave, and reflectors are arranged on both sides of the comb-shaped electrode rows so that the three comb-shaped electrodes are arranged between the three comb-shaped electrodes. In a three-comb electrode type longitudinally coupled double mode surface acoustic wave filter that uses first and third vibration modes generated by acoustic coupling,
A three-comb electrode type characterized in that the acoustic velocity outside the main waveguide is made slower than the surface acoustic wave velocity at the crossing portion of the comb-shaped electrode which is a waveguide so as to suppress the oblique emission of the surface acoustic wave. Longitudinal coupled dual mode surface acoustic wave filter. The three-comb electrode type longitudinally coupled double mode surface acoustic wave filter according to claim 5,
In order to suppress the oblique emission of the surface acoustic wave, a dummy electrode finger is provided between the electrode finger of the comb electrode and the bus bar, and the sound velocity outside the main waveguide is changed to the elastic surface at the intersection of the comb electrode as the waveguide. A three-comb electrode type longitudinally coupled double mode surface acoustic wave filter characterized by being slower than a wave velocity. The three-comb electrode type longitudinally coupled double mode surface acoustic wave filter according to claim 6,
3. A three-comb electrode type longitudinally coupled dual mode surface acoustic wave filter characterized in that the width of the electrode fingers of the comb-shaped electrode is different from the width of the dummy electrode fingers. The three-comb electrode type longitudinally coupled double mode surface acoustic wave filter according to claim 7,
3. A three-comb electrode type longitudinally coupled dual-mode surface acoustic wave filter, wherein the width of the dummy electrode fingers is wider than the width of the electrode fingers of the comb-shaped electrodes. The three-comb electrode type longitudinally coupled double mode surface acoustic wave filter according to claim 6,
3. A three-comb electrode type longitudinally coupled double-mode surface acoustic wave filter, wherein the length of the dummy electrode is set to 0.2λ or more, where λ is a wavelength defined by the pitch of the comb-shaped electrodes. The surface acoustic wave filter according to any one of claims 1 to 9,
A surface acoustic wave filter characterized in that a gap between an electrode finger of the comb-shaped electrode and a bus bar is narrowed so as to suppress oblique radiation of the surface acoustic wave.

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