JP7801372B2 - Acoustic wave filter and communication device - Google Patents

Description

One aspect of the present disclosure relates to an acoustic wave filter.

Patent document 1 listed below discloses a ladder-type filter having multiple series resonators and multiple parallel resonators as an example of an elastic wave filter.

Japanese Patent Application Publication No. 2012-156741

An acoustic wave filter according to one aspect of the present disclosure has a plurality of parallel resonators including a plurality of series resonators, a first parallel resonator having a first resonant ripple, and other parallel resonators, wherein the first resonant ripple is located between the main resonant frequency and anti-resonant frequency of the first parallel resonator, and the main resonant frequency of the first parallel resonator is higher than the main resonant frequency of at least one of the other parallel resonators.

1 is a diagram illustrating a configuration of a main part of an acoustic wave filter according to a first embodiment. FIG. 1 is a diagram illustrating a configuration of a main part of an acoustic wave filter as a comparative example. 4A and 4B are diagrams illustrating the attenuation characteristics of the acoustic wave filter of the first preferred embodiment and the acoustic wave filter of a comparative example. FIG. 4 is a diagram illustrating frequency characteristics of a first parallel resonator and a normal parallel resonator. FIG. 2 is a plan view showing an example of the configuration of a resonator. FIG. 6 is a cross-sectional view taken along line Ic-Ic in FIG. 5. FIG. 6 is an enlarged plan view of a portion of the IDT electrode in FIG. 5 . FIG. 6 is an enlarged plan view of a portion of the reflector in FIG. 5 . 10A and 10B are diagrams illustrating a configuration of a main part of an acoustic wave filter according to a second embodiment of the present invention. FIG. 10 is a diagram illustrating the attenuation characteristics of the acoustic wave filter according to the second embodiment and the acoustic wave filter according to the first embodiment. FIG. 4 is a diagram illustrating frequency characteristics of a second parallel resonator and a first parallel resonator. FIG. 10 is a diagram illustrating a schematic configuration of a communication device according to a third embodiment.

[Embodiment 1]
The acoustic wave filter 100A according to the first embodiment will be described below. For ease of explanation, components having the same functions as those described in the first embodiment will be denoted by the same reference numerals in the following embodiments, and their descriptions will not be repeated. For brevity, descriptions of well-known technical matters will also be omitted as appropriate. Unless otherwise specified, the configurations and values described in this specification are merely examples. Therefore, unless otherwise specified, the positional relationships of the components are not limited to the examples in the figures. Furthermore, the components are not necessarily drawn to scale.

(Configuration of Acoustic Wave Filter 100A)
FIG. 1 is a diagram illustrating the configuration of a main part of an acoustic wave filter 100A. The acoustic wave filter 100A includes a plurality of (e.g., five) series resonators 1S and a plurality of (e.g., four) parallel resonators 1P. As shown in FIG. 1, the acoustic wave filter 100A may be, for example, a ladder-type filter. In this specification, the series resonators 1S and the parallel resonators 1P are collectively referred to as resonators 1. A configuration example of the resonator 1 as an acoustic wave element will be described later. In this specification, when distinguishing between the five series resonators 1S, they are referred to as series resonators 1S-1 to 1S-5, respectively. When distinguishing between the four parallel resonators 1P, they are referred to as parallel resonators 1P-1 to 1P-5, respectively.

The acoustic wave filter 100A includes multiple parallel resonators 1P, including a first parallel resonator 1PA having a first resonance ripple and other parallel resonators. As described below, the first parallel resonator 1PA has frequency characteristics that differ from the other parallel resonators. In the example shown in Figure 1, the first parallel resonator 1PA is parallel resonator 1P-2.

In contrast, in the example of embodiment 1, the three parallel resonators other than the first parallel resonator 1PA, i.e., parallel resonators 1P-1, 1P-3, and 1P-4, each have equivalent frequency characteristics. For this reason, in embodiment 1, parallel resonators 1P-1, 1P-3, and 1P-4 are also collectively referred to as normal parallel resonators 1PN.

As shown in FIG. 1, the acoustic wave filter 100A may have an input terminal Tin, an output terminal Tout, a series wiring SL connecting multiple series resonators 1S, and multiple parallel wiring PL connecting multiple parallel resonators 1P. Hereinafter, for example, the input terminal Tin will be abbreviated simply as Tin where appropriate. The acoustic wave filter 100A may also have multiple ground terminals TGND corresponding to the multiple PLs. In the example of FIG. 1, the acoustic wave filter 100A has four PLs and four TGNDs. When distinguishing between the four PLs, they are referred to as PL-1 to PL-4. When distinguishing between the four TGNDs, they are referred to as TGND-1 to TGND-4.

As shown in FIG. 1, the acoustic wave filter 100A may be configured as a filter that filters an electrical signal input to Tin and outputs the filtered electrical signal to Tout. An SL may be connected to Tin and Tout. Therefore, each of the series resonators 1S-1 to 1S-5 may be connected to Tin and Tout via an SL. In the example of FIG. 1, the series resonator 1S-1 is the series resonator closest to Tin. In contrast, the series resonator 1S-5 is the series resonator closest to Tout (in other words, the farthest from Tin).

In the example shown in Figure 1, PL-1 branches off from SL between series resonators 1S-1 and 1S-2 and is connected to TGND-1. PL-2 branches off from SL between series resonators 1S-2 and 1S-3 and is connected to TGND-2. PL-3 branches off from SL between series resonators 1S-3 and 1S-4 and is connected to TGND-3. PL-4 branches off from SL between series resonators 1S-4 and 1S-5 and is connected to TGND-4.

As shown in Figure 1, parallel resonators 1P-1 to 1P-4 may be connected to TGND-1 to TGND-4 via PL-1 to PL-4, respectively. With this configuration, unwanted components contained in the electrical signal can be filtered by diverting them to TGND via parallel resonator 1P. In the example of Figure 1, parallel resonator 1P-1 is the parallel resonator closest to Tin. In contrast, parallel resonator 1P-4 is the series resonator closest to Tout.

(Configuration of Acoustic Wave Filter 100R as Comparative Example)
Before describing the frequency characteristics (more specifically, the attenuation characteristics) of the acoustic wave filter 100A, the configuration of an acoustic wave filter 100R as a comparative example will be described. FIG. 2 is a diagram showing the configuration of a main part of the acoustic wave filter 100R. The acoustic wave filter 100R includes only normal parallel resonators 1PN as the multiple parallel resonators 1P. That is, unlike the acoustic wave filter 100A, the parallel resonator 1P-2 of the acoustic wave filter 100R is the normal parallel resonator 1PN.

(Comparison of Attenuation Characteristics Between Acoustic Wave Filter 100A and Acoustic Wave Filter 100R)
Fig. 3 illustrates the attenuation characteristics of the acoustic wave filter 100A and the acoustic wave filter 100R. In the graph of Fig. 3, the horizontal axis represents frequency (unit: MHz), and the vertical axis represents attenuation (unit: dB). In the graph of Fig. 3, the solid line represents the characteristics of the acoustic wave filter 100A (first embodiment), and the dotted line represents the characteristics of the acoustic wave filter 100R (comparative example). The attenuation can also be referred to as the transmission amount. Therefore, the attenuation characteristics can also be referred to as the transmission characteristics.

As described above, acoustic wave filter 100A, unlike acoustic wave filter 100R, includes a first parallel resonator 1PA as the parallel resonator 1P. Therefore, as shown in FIG. 3 , acoustic wave filter 100A generates a first resonance ripple due to first parallel resonator 1PA, unlike acoustic wave filter 100R.

Additionally, in the acoustic wave filter 100A, the first resonant ripple is located between the main resonant frequency f1a and the anti-resonant frequency f1b of the first parallel resonator 1PA. Examples of f1a and f1b will be described later with reference to FIG. 4. Furthermore, f1a is higher than the main resonant frequency of at least one of the other parallel resonators. Therefore, the first parallel resonator 1PA may be configured to satisfy these conditions.

As shown in FIG. 3 , in acoustic wave filter 100A, the first resonance ripple improves the steepness of the attenuation characteristics in the transition band located between the pass band and stop band of the acoustic wave filter compared to acoustic wave filter 100R. Additionally, in acoustic wave filter 100A, by setting f1a as described above, the risk of a protruding portion of the first resonance ripple occurring in the transition region on the low frequency side can be reduced. Therefore, acoustic wave filter 100A can improve the steepness of the attenuation characteristics in the transition region on the low frequency side compared to acoustic wave filter 100R. As described above, acoustic wave filter 100A can improve the steepness of the attenuation characteristics of an acoustic wave filter in a manner different from conventional techniques.

f1a may be higher than the main resonant frequency of all of the other parallel resonators. In this case, the steepness of the attenuation characteristics in the transition region on the low frequency side can be further improved. Therefore, in the example of embodiment 1, f1a may be higher than the main resonant frequency of the normal parallel resonator 1PN.

(Comparison of frequency characteristics between the first parallel resonator 1PA and the normal parallel resonator 1PN)
4 illustrates the frequency characteristics of the first parallel resonator 1PA and the normal parallel resonator 1PN. In each graph in FIG. 4, the solid line indicates the characteristic of the first parallel resonator 1PA, and the dotted line indicates the characteristic of the normal parallel resonator 1PN.

In Figure 4, the graph labeled 4000A shows the impedance characteristics of the first parallel resonator 1PA and the normal parallel resonator 1PN. In Figure 4, to illustrate the difference in waveform between the first parallel resonator 1PA and the normal parallel resonator 1PN, the frequency is shifted to align the main resonant frequency (phase 0°). In this graph, the horizontal axis represents frequency (unit: MHz), and the vertical axis represents the absolute value (magnitude) of the impedance (unit: Ohm). The graph labeled 4000B shows the impedance phase characteristics of the first parallel resonator 1PA and the normal parallel resonator 1PN. In this graph, the horizontal axis represents frequency (unit: MHz), and the vertical axis represents the impedance phase (unit: degree). In the following description, the impedance phase will be abbreviated as simply "phase."

As shown in Figure 4, f1a (the main resonant frequency of the first parallel resonator 1PA) is the frequency at which the absolute value of the impedance of the first parallel resonator 1PA is minimum. f1a is also the frequency at which the phase becomes 0° in a frequency band where the phase increases monotonically. In the example of embodiment 1, f1a = 1945.3 MHz. In contrast, the main resonant frequency of the normal parallel resonator 1PN is 1943.2 MHz.

f1b (the anti-resonance frequency of the first parallel resonator 1PA) is the frequency at which the absolute value of the impedance of the first parallel resonator 1PA is maximum. f1b is also the frequency at which the phase becomes 0° in a frequency band where the phase monotonically decreases. In the example of embodiment 1, f1b = 2016.0 MHz.

The frequency f1r of the first resonant ripple is the frequency at which the phase takes a minimum value in the passband of the acoustic wave filter 100A. In the example of embodiment 1, f1r = 1964.0 MHz. As shown in Figure 4, in the acoustic wave filter 100A, the first resonant ripple is located between f1a and f1b. In this specification, "the first resonant ripple is located between f1a and f1b" means "f1r is located between f1a and f1b."

At f1r, the phase (impedance phase) of the first parallel resonator 1PA may be 50° or less. In this case, a larger first resonance ripple can be formed, further improving the steepness of the attenuation characteristics in the low-frequency transition region of the acoustic wave filter 100A. In the example shown in FIG. 4, the phase of the first parallel resonator 1PA at f1r is approximately 20°.

(Example of the Position of the First Parallel Resonator 1PA)
1, in the acoustic wave filter 100A, the first parallel resonator 1PA may be connected to a certain PL among the multiple parallel wirings PL-1 to PL-4, excluding the parallel wiring PL-1 closest to Tin and the parallel wiring PL-4 closest to Tout. In the example of FIG. 1, the first parallel resonator 1PA is connected to PL-2.

If a resonator 1 having a ripple is located near Tin or Tout, the ripple may adversely affect the impedance matching between the acoustic wave filter 100A and an external device. Therefore, by positioning the first parallel resonator 1PA as described above, the risk of the first ripple adversely affecting the impedance matching with an external device can be reduced. This allows for an acoustic wave filter 100A that can easily be impedance matched with an external device.

(Example of the configuration of the resonator 1)
FIG. 5 is a plan view showing an example configuration of a resonator 1 (acoustic wave element) according to one aspect of the present disclosure. In FIG. 5, a SAW (Surface Acoustic Wave) element is shown as an example of an acoustic wave element. In the following description, for convenience, a Cartesian coordinate system (xyz coordinate system) shown in FIG. 5 is introduced. In the example of the first embodiment, the x direction is the propagation direction of acoustic waves. In contrast, the y direction is an example of a direction intersecting the x direction in the plan view.

Figure 6 is a cross-sectional view taken along line Ic-Ic in Figure 1. As shown in Figure 6, the z direction in the example of embodiment 1 is the thickness direction of each component of the resonator 1. The thickness s in Figure 6 is the thickness of the electrode fingers 32, which will be described below. In the following, the positive side of the z direction is considered to be the upper side, and the term "top surface" will be used.

As will be apparent to those skilled in the art, the acoustic wave according to one embodiment of the present disclosure is not limited to a SAW. The acoustic wave may be any wave that can be conceptualized as propagating along the piezoelectric substrate of the acoustic wave element. The acoustic wave may be, for example, a BAW (Bulk Acoustic Wave). Therefore, a BAW element can be cited as another example of an acoustic wave element according to one embodiment of the present disclosure. Therefore, the acoustic wave filter according to one embodiment of the present disclosure may be a SAW filter or a BAW filter.

The resonator 1 may have (i) a piezoelectric substrate 2, (ii) an IDT (Interdigital Transducer) electrode 3 provided on the upper surface 2A of the piezoelectric substrate 2, and (iii) a pair of reflectors 4A and 4B corresponding to the IDT electrode. The IDT electrode is also referred to as an excitation electrode. In this specification, the reflectors 4A and 4B are also collectively referred to as reflectors 4. The reflectors 4 may be positioned so as to sandwich the IDT electrode 3 in the x-direction.

The piezoelectric substrate 2 may be made of a single crystal substrate having piezoelectric properties. For example, the piezoelectric substrate 2 may be made of a single crystal of lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ).

The IDT electrode 3 may have a first comb-tooth electrode 30a and a second comb-tooth electrode 30b. In this specification, the first comb-tooth electrode 30a and the second comb-tooth electrode 30b are also collectively referred to as comb-tooth electrodes 30. In the following description, the subscript "a" is appropriately assigned to each component corresponding to the first comb-tooth electrode, and the subscript "b" is appropriately assigned to each component corresponding to the second comb-tooth electrode. For these components as well, the generic name corresponding to the comb-tooth electrode 30 is used as appropriate.

The comb-tooth electrode 30 may have (i) two bus bars 31 facing each other in the y direction, and (ii) a plurality of electrode fingers 32 extending in the y direction from one bus bar 31 (e.g., the first bus bar 31a) to the other bus bar 31 (e.g., the second bus bar 31b). As shown in Figure 6, the first electrode fingers 32a and the second electrode fingers 32b may be positioned alternately and repeatedly on the upper surface 2A of the piezoelectric substrate 2 with a generally constant spacing in the x direction (see also Figure 7). Figure 7 is an enlarged plan view of a portion of the IDT electrode 3 in Figure 5 (e.g., the central portion 3a of the IDT electrode 3).

The comb-tooth electrode 30 may have dummy electrode fingers 33 that face each electrode finger 32. As shown in FIG. 7 , the first dummy electrode finger 33a may extend from the first bus bar 31a toward the second electrode finger 32b. The second dummy electrode finger 33b may extend from the second bus bar 31b toward the first electrode finger 32a. However, the comb-tooth electrode 30 does not necessarily have to have dummy electrode fingers 33.

The busbar 31 may be formed, for example, as an elongated piece having a generally constant width in the x direction and extending linearly in the x direction. Therefore, the edges of the busbar 31 on opposing sides may be linear. In contrast, the multiple electrode fingers 32 may be formed, for example, as an elongated piece having a generally constant width in the x direction and extending linearly in the y direction.

As shown in Figures 6 and 7, the multiple electrode fingers 32 in the IDT electrode 3 may have a pitch Pt1 in the x-direction. Pt1 is, for example, the pitch (repetition interval) between the centers of the multiple electrode fingers 32. Therefore, Pt1 can also be expressed as the pitch from the center of a first electrode finger 32a to the center of a second electrode finger 32b adjacent to the first electrode finger 32a. As an example, Pt1 may be set equal to half the wavelength λ (λ/2) of the acoustic wave excited by the IDT electrode 3. In this case, λ can be expressed as 2 x Pt1.

The multiple electrode fingers 32 may have a width w1 in the x-direction. w1 may be set appropriately depending on the electrical characteristics required of the resonator 1. For example, w1 may be set so that the ratio of w1 to Pt1 (w1/Pt) is a value within a predetermined range.

As shown in Figure 6, the IDT electrode 3 may be composed of, for example, a conductive layer 15 made of metal. In addition, a protective layer 5 may be provided on the piezoelectric substrate 2 so as to cover the IDT electrode 3 and the reflector 4 (not shown in Figure 6).

Next, we will describe the reflector 4. The reflector 4 may have (i) two reflector bus bars 41 facing each other in the y direction, and (ii) a plurality of reflector electrode fingers 42 extending in the y direction between the reflector bus bars 41. The reflector bus bar 41 may be formed, for example, as an elongated member having a generally constant width in the x direction and extending linearly in the x direction. In contrast, the plurality of reflector electrode fingers 42 may be formed, for example, as an elongated member having a generally constant width in the x direction and extending linearly in the y direction.

Figure 8 is an enlarged plan view of a portion of the reflector 4 in Figure 5. As shown in Figure 8, the multiple reflector electrode fingers 42 may have a pitch Pt2 in the x-direction. Pt2 may be, for example, the pitch between the centers of the multiple reflector electrode fingers 42. Therefore, Pt1 can also be expressed as the distance from the center of one reflector electrode finger 42 to the center of an adjacent reflector electrode finger 42. The multiple electrode fingers 32 may have a width w2 in the x-direction. w2 may be set to a value approximately equal to w1. As an example, w2 may be set equal to w1.

The reflector 4 may be configured to reflect the acoustic waves excited by the IDT electrode 3. Therefore, Pt2 may be set to a value similar to Pt1. As an example, Pt2 may be set equal to Pt1. In addition, as shown in FIG. 5 above, the reflector 4 may be positioned at a distance G from the IDT electrode 3. The distance G may be, for example, from (i) the center of the electrode finger 32 located at the end of the IDT electrode 3 on the reflector 4 (e.g., reflector 4A) side to (ii) the center of the reflector electrode finger 42 located at the end of the reflector 4 on the IDT electrode 3 side.

In the example of embodiment 1, in the normal parallel resonator 1PN, Pt1 is set to the first pitch a1. In addition, in the normal parallel resonator 1PN, Pt2 and G are also set equal to a1. That is, embodiment 1 illustrates a case in which the normal parallel resonator 1PN has only the first pitch as the electrode finger pitch. In this specification, the electrode finger pitch collectively refers to Pt, Pt2, and the spacing G. With this configuration, a normal parallel resonator 1PN without resonant ripple is realized.

(Example of Configuration of First Parallel Resonator 1PA)
The first parallel resonator 1PA may include (i) a first region having a first pitch a1 as the electrode finger pitch of the first parallel resonator 1PA, and (ii) a second region having a second pitch b1 as the electrode finger pitch, the second pitch being smaller than the first pitch. The region REGa1 shown in FIG. 7 is an example of the first region, and the region REG1b shown in FIG. 8 is an example of the second region. This configuration achieves the first parallel resonator 1PA having a first resonant ripple. In the example of the first embodiment, a1 = 0.9882 μm and b1 = 0.7313 μm. That is, in the example of the first embodiment, the second pitch is 0.74 times the length of the first pitch. By making the ratio of the second pitch to the first pitch smaller than 0.8, higher steepness can be achieved in the acoustic wave filter.

The first parallel resonator 1PA may have only two electrode finger pitches. In other words, the electrode finger pitch of the first parallel resonator 1PA may have only two pitches: a first pitch or a second pitch. This configuration makes it possible to realize a first parallel resonator 1PA that has only the first resonant ripple as a single resonant ripple. The first pitch and second pitch may be referred to as the main pitch and sub-pitch, respectively, of the first parallel resonator 1PA.

The first region and the second region can be located anywhere in the first parallel resonator 1PA. As shown in FIG. 7, the first region may be located within the IDT electrode 3. In contrast, as an example, as shown in FIG. 8, the second region may be located within the reflector 4. For example, if the spacing G is set equal to a1, the second region can be located within the reflector 4 by setting Pt2 equal to b1.

As another example, the second region may be located between the IDT electrode 3 and the reflector 4. For example, if Pt2 is set equal to a1, the second region can be located between the IDT electrode 3 and the reflector 4 by setting the spacing G equal to b1. As is clear from the above explanation, the second region may be formed by two or more electrode fingers (collectively referring to the electrode fingers of the IDT electrode and the reflector electrode fingers).

If the second region were located within the IDT electrode 3, there is a concern that this could adversely affect the performance of the first parallel resonator 1PA, such as its power handling capability. Therefore, by locating the second region within the reflector 4 or between the IDT electrode 3 and the reflector 4, the risk of such adverse effects occurring can be reduced.

[Embodiment 2]
(Configuration of Acoustic Wave Filter 100B)
FIG. 9 is a diagram illustrating a configuration of a main part of an acoustic wave filter 100B according to a second embodiment. As illustrated in FIG. 9 , the acoustic wave filter 100B may include a second parallel resonator 1PB having a second resonance ripple as one of the other parallel resonators. As will be described later, the second parallel resonator 1PB may have frequency characteristics different from those of the first parallel resonator 1PA and the normal parallel resonator 1PN. In the example illustrated in FIG. 9 , the second parallel resonator 1PB is the parallel resonator 1P-3. Therefore, the normal parallel resonator 1PN in the example illustrated in FIG. 9 is the parallel resonators 1P-1 and 1P-4.

(Comparison of Attenuation Characteristics Between Acoustic Wave Filter 100B and Acoustic Wave Filter 100A)
Fig. 10 illustrates the attenuation characteristics of the acoustic wave filter 100B and the acoustic wave filter 100A. In the graph of Fig. 10, the horizontal axis represents frequency and the vertical axis represents attenuation. In the graph of Fig. 10, the solid line represents the characteristics of the acoustic wave filter 100B (Embodiment 2), and the dotted line represents the characteristics of the acoustic wave filter 100A (Embodiment 1). The characteristics of the acoustic wave filter 100A shown in Fig. 10 are the same as those shown in Fig. 3.

As described above, the acoustic wave filter 100B according to the second embodiment differs from the acoustic wave filter 100A in that it further includes a second parallel resonator 1PB as the parallel resonator 1P. In this case, as shown in FIG. 10 , the acoustic wave filter 100B generates a second resonant ripple due to the second parallel resonator 1PB, unlike the acoustic wave filter 100A.

Additionally, in the acoustic wave filter 100B, the second resonance ripple may be located between the main resonance frequency f2a and the anti-resonance frequency f2b of the second parallel resonator 1PB. Examples of f2a and f2b will be described later with reference to FIG. 11. As shown in FIG. 10, in the acoustic wave filter 100B, the second resonance ripple further improves the steepness of the attenuation characteristics in the transition band.

The second resonant ripple may be located on the lower frequency side than the first resonant ripple. In other words, the frequency f2r of the second resonant ripple may be set lower than f1r. Therefore, the second parallel resonator 1PB may be configured to satisfy these conditions. As shown in Figure 10, in the acoustic wave filter 100B, the second resonant ripple can further improve the steepness of the attenuation characteristics in the transition region on the low frequency side.

(Comparison of frequency characteristics between the first parallel resonator 1PA and the normal parallel resonator 1PN)
Fig. 11 illustrates the frequency characteristics of the second parallel resonator 1PB and the first parallel resonator 1PA. In each graph in Fig. 11, the solid line indicates the characteristics of the second parallel resonator 1PB, and the dotted line indicates the characteristics of the first parallel resonator 1PA. The characteristics of the first parallel resonator 1PA shown in Fig. 11 are the same as those in Fig. 4. In Fig. 11, each plot showing the characteristics of the second parallel resonator 1PB is shifted by +5.7 MHz on the horizontal axis for comparison with the characteristics of the first parallel resonator 1PA.

In Figure 11, the graph labeled 11000A shows the impedance characteristics of the second parallel resonator 1PB and the first parallel resonator 1PA. The graph labeled 11000B shows the phase characteristics of the second parallel resonator 1PB and the first parallel resonator 1PA. In the example of embodiment 2, f2a = 1939.6 MHz, f2b = 2010.0 MHz, and f2r = 1956.0 MHz.

At f2r, the phase (impedance phase) of the second parallel resonator 1PB may be 50° or less. In this case, a larger second resonance ripple can be formed, further improving the steepness of the attenuation characteristics in the low-frequency transition region of the acoustic wave filter 100B. In the example shown in FIG. 11, the phase of the second parallel resonator 1PB at f2r is approximately 20°.

In this specification, the difference between f1r and f1a is represented as Δf1, and the difference between f2r and f2a is represented as Δf2.
Δf1=f1r−f1a
Δf2=f2r-f2a
As shown in FIG. 11 , Δf1 may be greater than Δf2. With this configuration, the steepness of the attenuation characteristics can be improved in two stages in the transition region on the low frequency side. In the example of the second embodiment, Δf1=20.8 MHz and Δf2=16.4 MHz.

(Example of the Position of the Second Parallel Resonator 1PB)
In the acoustic wave filter 100B, the second parallel resonator 1PB may be connected to a certain PL among the parallel wirings PL-1 to PL-4, excluding the parallel wiring PL-1 closest to Tin and the parallel wiring PL-4 closest to Tout. In the example shown in Fig. 9, the second parallel resonator 1PB is connected to PL-3. This configuration enables the acoustic wave filter 100B to be easily impedance-matched with an external device.

(Example of the configuration of the second parallel resonator 1PB)
The second parallel resonator 1PB may include (i) a third region having a third pitch a2 as the electrode finger pitch of the second parallel resonator 1PB, and (ii) a fourth region having a fourth pitch b2 as the electrode finger pitch that is smaller than the third pitch. The region REGa2 shown in FIG. 7 is an example of the third region, and the region REGb2 shown in FIG. 8 is an example of the fourth region. This configuration makes it possible to realize the second parallel resonator 1PB having a second resonant ripple. In the example of embodiment 2, a2 = 0.9914 μm and b2 = 0.7535 μm.

The second parallel resonator 1PB may have only two electrode finger pitches. In other words, the electrode finger pitch of the second parallel resonator 1PB may have only two pitches: a third pitch and a fourth pitch. The third pitch and the fourth pitch may be referred to as the main pitch and the sub-pitch, respectively, of the second parallel resonator 1PB. This configuration makes it possible to realize a second parallel resonator 1PB having only the second resonant ripple as a single resonant ripple. Therefore, it is possible to realize an acoustic wave filter 100B having only the first resonant ripple and the second resonant ripple.

The third and fourth regions in the second parallel resonator 1PB can be located anywhere in the second parallel resonator 1PB. The third region may be located within the IDT electrode 3. In contrast, as an example, the fourth region may be located within the reflector 4 or between the IDT electrode 3 and the reflector 4. By locating the fourth region within the reflector 4 or between the IDT electrode 3 and the reflector 4, the risk of adverse effects on performance aspects such as power handling capability of the second parallel resonator 1PB can be reduced.

[Embodiment 3]
FIG. 12 is a diagram illustrating a schematic configuration of a communication device 151 according to a third embodiment. The communication device 151 is an application example of an acoustic wave filter according to an aspect of the present disclosure, and performs wireless communication using radio waves. The communication device 151 may include an acoustic wave filter according to an aspect of the present disclosure (e.g., the acoustic wave filter 100A). For example, the communication device 151 may include a duplexer 101 configured using the acoustic wave filter. The communication device 151 in the example of FIG. 12 may include one duplexer 101 as a transmit filter 109 and another duplexer 101 as a receive filter 111.

In communication device 151, a transmission information signal TIS containing information to be transmitted may be modulated and frequency-increased (converted to a high-frequency signal having a carrier frequency) by RF-IC (Radio Frequency-Integrated Circuit) 153, and converted into a transmission signal TS. Bandpass filter 155 may remove unwanted components from TS outside the transmission passband. The TS after removing the unwanted components may then be amplified by amplifier 157 and input to transmission filter 109.

The transmit filter 109 may remove unnecessary components outside the transmission passband from the input transmit signal TS. The transmit filter 109 may output the TS after removing the unnecessary components to the antenna 159. The antenna 159 may convert the TS, which is an electrical signal input to it, into radio waves as a wireless signal and transmit the radio waves outside the communication device 151.

The antenna 159 may also convert received external radio waves into a received signal RS, which is an electrical signal, and input the RS to the receiving filter 111. The receiving filter 111 may remove unwanted components outside the receiving passband from the input RS. The receiving filter 111 may output the received signal RS after the unwanted components have been removed to the amplifier 161. The output RS may be amplified by the amplifier 161. The bandpass filter 163 may remove unwanted components outside the receiving passband from the amplified RS. The RS after the unwanted components have been removed may be frequency-downshifted and demodulated by the RF-IC 153, and converted into a received information signal RIS.

The TIS and RIS may be low-frequency signals (baseband signals) containing appropriate information. For example, the TIS and RIS may be analog audio signals or digitized audio signals. The passband of the radio signals may be set as appropriate and may conform to various known standards.

[Additional Notes]
The invention according to the present disclosure has been described above based on the drawings and examples. However, the invention according to the present disclosure is not limited to the above-described embodiments. In other words, the invention according to the present disclosure can be modified in various ways within the scope of the present disclosure, and embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the invention according to the present disclosure. In other words, it should be noted that a person skilled in the art can easily make various modifications or corrections based on the present disclosure. It should also be noted that these modifications or corrections are included in the scope of the present disclosure.

1 Resonator 1P, 1P-1 to 1P-4 Parallel resonators 1PA First parallel resonator 1PB Second parallel resonator (other parallel resonators)
1PN Normal parallel resonator (other parallel resonators)
1S, 1S-1 to 1S-5: Series resonator 3: IDT electrode 4, 4A, 4B: Reflector 100A, 100B: Acoustic wave filter 151: Communication device PL: Parallel wiring PL-1: Parallel wiring (parallel wiring closest to input terminal)
PL-2 Parallel wiring (parallel wiring to which the first parallel resonator is connected)
PL-3 Parallel wiring (parallel wiring connected to the second parallel resonator)
PL-4 Parallel wiring (parallel wiring closest to the output terminal)
SL Series wiring Tin Input terminal Tout Output terminal a1 First pitch b1 Second pitch a2 Third pitch b2 Fourth pitch REGa1 First region REGb1 Second region REGa2 Third region REGb2 Fourth region f1a Main resonance frequency of first parallel resonator f1b Anti-resonance frequency of first parallel resonator f1r Frequency of first resonance ripple f2a Main resonance frequency of second parallel resonator f2b Anti-resonance frequency of second parallel resonator f2r Frequency of second resonance ripple Δf1 Difference between f1r and f1a Δf2 Difference between f2r and f2a

Claims (14)

a plurality of series resonators;
a plurality of parallel resonators including a first parallel resonator having a first resonant ripple and other parallel resonators;
and
The first parallel resonator is
a first region having a first pitch as the electrode finger pitch of the first parallel resonator;
a second region having an electrode finger pitch of the first parallel resonator that is smaller than the first pitch;
and
a ratio of the second pitch to the first pitch is less than 0.8, so that the first parallel resonator has the first resonant ripple;
the first resonant ripple is located between a main resonant frequency and an anti-resonant frequency of the first parallel resonator,
The acoustic wave filter, wherein a main resonant frequency of the first parallel resonator is higher than a main resonant frequency of at least one of the other parallel resonators. The acoustic wave filter of claim 1, wherein the main resonant frequency of the first parallel resonator is higher than the main resonant frequencies of all of the other parallel resonators. The acoustic wave filter of claim 1, wherein the phase of the impedance of the first parallel resonator is 50° or less at the frequency of the first resonant ripple. An input terminal,
An output terminal;
a series wiring that connects the plurality of series resonators;
a plurality of parallel wirings connecting the plurality of parallel resonators,
2. The acoustic wave filter according to claim 1, wherein the first parallel resonator is connected to a parallel wiring of the plurality of parallel wirings, excluding a parallel wiring closest to the input terminal and a parallel wiring closest to the output terminal. The acoustic wave filter according to claim 1, wherein the electrode finger pitch of the first parallel resonator has only two options: the first pitch and the second pitch. The first parallel resonator is
an IDT electrode;
a pair of reflectors sandwiching the IDT electrode in the propagation direction of the acoustic wave,
The acoustic wave filter according to claim 1 , wherein the second region is located within the reflector. the first parallel resonator has an IDT electrode;
The acoustic wave filter according to claim 1 , wherein the second region is located within the IDT electrode. The first parallel resonator is
an IDT electrode;
a pair of reflectors sandwiching the IDT electrode in the propagation direction of the acoustic wave,
The acoustic wave filter according to claim 1 , wherein the second region is located between the IDT electrode and the reflector. a second parallel resonator among the other parallel resonators has a second resonant ripple;
The acoustic wave filter according to claim 1 , wherein the second resonant ripple is located between a main resonant frequency and an anti-resonant frequency of the second parallel resonator. The acoustic wave filter of claim 9, wherein the second resonant ripple is located on the lower frequency side than the first resonant ripple. The difference between the frequency of the first resonant ripple and the main resonant frequency of the first parallel resonator is Δf1,
When the difference between the frequency of the second resonant ripple and the main resonant frequency of the second parallel resonator is represented as Δf2,
The acoustic wave filter according to claim 9 , wherein Δf1>Δf2. An input terminal,
An output terminal;
a series wiring that connects the plurality of series resonators;
a plurality of parallel wirings connecting the plurality of parallel resonators,
The acoustic wave filter according to claim 9 , wherein the second parallel resonator is connected to a parallel wiring other than the parallel wiring closest to the input terminal and the parallel wiring closest to the output terminal. The second parallel resonator is
a third region having a third pitch as the electrode finger pitch of the second parallel resonator;
a fourth region in which the electrode finger pitch of the second parallel resonator is a fourth pitch that is smaller than the third pitch;
The acoustic wave filter according to claim 9 , wherein the electrode finger pitch of the second parallel resonator has only two pitches, the third pitch and the fourth pitch. A communication device having the acoustic wave filter described in claim 1.