ACOUSTIC WAVE FILTER

An acoustic wave filter includes a first filter circuit with a predetermined frequency band as a pass band and provided on a first path connecting first and second signal terminals, and an additional resonant circuit connected in parallel with at least a portion of the first filter circuit. The additional resonant circuit includes an IDT electrode group including IDT electrodes positioned along an acoustic wave propagation direction. At least one resonant frequency of one or more resonant frequencies of the additional resonant circuit is included within the pass band of the first filter circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acoustic wave filter including an acoustic wave resonator.

2. Description of the Related Art

In the related art, there is an acoustic wave filter including an acoustic wave resonator. As an example of this type of acoustic wave filter, Japanese Unexamined Patent Application Publication No. 2018-74539 discloses an acoustic wave filter including a filter circuit having a predetermined frequency band as a pass band, and a cancel circuit connected in parallel with the filter circuit. This acoustic wave filter enables improvements in attenuation characteristics and isolation characteristics of the filter circuit.

SUMMARY OF THE INVENTION

In the acoustic wave filter disclosed in Japanese Unexamined Patent Application Publication No. 2018-74539, however, as illustrated in FIG. 5 in Japanese Unexamined Patent Application Publication No. 2018-74539, a resonant frequency of the cancel circuit is located on a high frequency side far away from the pass band of the filter circuit. For this reason, in this acoustic wave filter, it is difficult to reduce insertion loss within the pass band.

Preferred embodiments of the present invention provide acoustic wave filters that each reduces or prevents insertion loss within a pass band of a filter circuit.

An acoustic wave filter according to an aspect of a preferred embodiment of the present invention includes a filter circuit with a predetermined frequency band as a pass band and provided on a first path connecting a first signal terminal and a second signal terminal, and an additional resonant circuit connected in parallel with at least a portion of the filter circuit. The additional resonant circuit includes an IDT electrode group including a plurality of IDT electrodes positioned along an acoustic wave propagation direction. At least one resonant frequency of one or more resonant frequencies of the additional resonant circuit is included within the pass band of the filter circuit.

An acoustic wave filter according to a preferred embodiment of the present invention enables a reduction in insertion loss within the pass band of the filter circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described in detail below with reference to the drawings. Incidentally, all of the preferred embodiments described below describe comprehensive or specific examples. Numerical values, shapes, materials, components, the arrangement and connection configuration of the components, and so forth that are described in the following preferred embodiments are merely examples and are not intended to limit the present invention. Of components in the following preferred embodiments, a component not described in an independent claim is described as an optional component. Furthermore, the sizes or size ratio of components illustrated in drawings are or is not necessarily exact. Additionally, in figures, components that are substantially the same are denoted by the same reference signs, and a repeated description is omitted or simplified in some cases. Furthermore, in the following preferred embodiments, when an element is referred to as being “connected” to another element, the element can not only be directly connected to the other element, but also electrically via another element or the like.

PREFERRED EMBODIMENTS

1. Configuration of Multiplexer

A configuration of a multiplexer including an acoustic wave filter according to a preferred embodiment of the present invention will be described with reference toFIG.1.

FIG.1is a circuit configuration diagram of a multiplexer5including an acoustic wave filter1according to the present preferred embodiment. Incidentally,FIG.1also illustrates an antenna element9.

The multiplexer5is a splitter or combiner including a plurality of filters. The multiplexer5includes the acoustic wave filter1including a first filter circuit10and an additional resonant circuit20, and a second filter circuit50. The first filter circuit10is a filter circuit having a predetermined frequency band as a pass band.

Furthermore, the multiplexer5includes a first signal terminal T1, a second signal terminal T2, and a third signal terminal T3.

The first signal terminal T1is connected to the acoustic wave filter1. Furthermore, the first signal terminal T1is connected to an RF signal processing circuit (not illustrated), for example, via an amplifier circuit (not illustrated) outside the multiplexer5.

The second signal terminal T2is a common terminal connected to each of the acoustic wave filter1and the second filter circuit50. Specifically, the second signal terminal T2is connected to the acoustic wave filter1via a node n0between the acoustic wave filter1and the second signal terminal T2and is further connected to the second filter circuit50via the node n0. Furthermore, the second signal terminal T2is connected to the antenna element9outside the multiplexer5. The second signal terminal T2is also an antenna terminal of the multiplexer5.

The third signal terminal T3is connected to the second filter circuit50. Furthermore, the third signal terminal T3is connected to an RF signal processing circuit (not illustrated), for example, via an amplifier circuit (not illustrated) outside the multiplexer5.

The acoustic wave filter1is located on a first path r1connecting the first signal terminal T1and the second signal terminal T2. The acoustic wave filter1is a transmission filter having, for example, an uplink frequency band (transmission band) as a pass band and is configured so that its pass band is lower than that of the second filter circuit50. The pass band of the acoustic wave filter1is the same as a pass band of the first filter circuit10.

The acoustic wave filter1includes the first filter circuit10, and the additional resonant circuit20additionally connected to the first filter circuit10. The additional resonant circuit20is a circuit to cancel an unwanted wave outside the pass band of the first filter circuit10. The additional resonant circuit20has a plurality of resonant frequencies (resonance points). At least one resonant frequency of one or more resonant frequencies of the additional resonant circuit20is included within the pass band of the first filter circuit10. A resonant frequency of the additional resonant circuit20will be described later.

The second filter circuit50is located on a third path r3connecting the second signal terminal T2and the third signal terminal T3. The second filter circuit50has, as a pass band, a frequency band different from the pass band of the first filter circuit10. The second filter circuit50is a reception filter having, for example, a downlink frequency band (reception band) as a pass band. The second filter circuit50includes, for example, by a plurality of series-arm resonators, a plurality of parallel-arm resonators, and a longitudinally coupled acoustic wave resonator.

Each of the acoustic wave filter1and the second filter circuit50has to have characteristics in which frequencies in its own band are passed and frequencies in another band located outside its own band are attenuated.

2. Configuration of Acoustic Wave Filter

Next, a configuration of the acoustic wave filter1will be described with reference toFIGS.1and2.

As illustrated inFIG.1, the acoustic wave filter1includes the first filter circuit10and the additional resonant circuit20.

The series-arm resonators S1to S3are disposed on the first path r1(series arm) connecting the first signal terminal T1and the second signal terminal T2. The series-arm resonators S1to S3are connected in series in this order from the first signal terminal T1toward the second signal terminal T2.

The parallel-arm resonators P1to P4are connected in parallel with one another on paths (parallel arms) connecting respective nodes n1, n2a, n2c, and n3between the first signal terminal T1and the series-arm resonators S1to S3arranged on the first path r1and reference terminals (grounds). Specifically, of the parallel-arm resonators P1to P4, one end of the parallel-arm resonator P1closest to the first signal terminal T1is connected to the node n1between the first signal terminal T1and the series-arm resonator S1, and the other end is connected to a reference terminal. One end of the parallel-arm resonator P2is connected to the node n2abetween the series-arm resonators S1and S2, and the other end is connected to a reference terminal. One end of the parallel-arm resonator P3is connected to the node n2cbetween the series-arm resonators S1and S2, and the other end is connected to a reference terminal. One end of the parallel-arm resonator P4is connected to the node n3between the series-arm resonators S2and S3, and the other end is connected to a reference terminal. Incidentally, the reference terminals on other end sides of the parallel-arm resonators P1and P2are connected in common, and the reference terminals on other end sides of the parallel-arm resonators P3and P4are connected in common. The nodes n2aand n2c, and a node n2bto be described may be the same node.

Thus, the first filter circuit10has a n-type ladder filter structure including three series-arm resonators S1to S3on the first path r1and four parallel-arm resonators P1to P4on the paths connecting the first path r1and the reference terminals.

Incidentally, for the series-arm resonators and parallel-arm resonators of the first filter circuit10, the number of series-arm resonators and the number of parallel-arm resonators are not limited to three or four. One or more series-arm resonators and two or more parallel-arm resonators may only be provided. Furthermore, an inductor may be provided between a parallel-arm resonator and a reference terminal. InFIG.1, some of the reference terminals to which the parallel-arm resonators are connected are connected in common. However, a decision on whether reference terminals are to be connected in common or individually can be made appropriately, for example, in accordance with a constraint of an installation layout of the first filter circuit10.

Next, the additional resonant circuit20of the acoustic wave filter1will be described. The additional resonant circuit20is a circuit that applies an opposite phase to an unwanted wave outside the pass band of the first filter circuit10to thus keep the unwanted wave from being output from the acoustic wave filter1.

The additional resonant circuit20is connected to a plurality of different nodes on the first path r1so as to be connected in parallel with at least a portion of the first filter circuit10. The additional resonant circuit20includes a first terminal21, which is a connection node on one end side of the additional resonant circuit20, a second terminal22, which is a connection node on the other end side, and an IDT (InterDigital Transducer) electrode group25disposed on a second path r2connecting the first terminal21and the second terminal22. Incidentally, a terminal herein refers to a point where radio frequency signals enter or leave the additional resonant circuit20.

Each of the first terminal21and the second terminal22are electrically connected to the IDT electrode group25. That is, a plurality of IDT electrodes of the IDT electrode group25are each connected directly to the first path r1via the second path r2, for example, without any capacitance element.

Furthermore, the first terminal21and the second terminal22are connected to respective different nodes on the first path r1. InFIG.1, the first terminal21is connected to the node n2bbetween the series-arm resonators S1and S2, and the second terminal22is connected to a node n4between the series-arm resonator S3and the second signal terminal T2.

FIG.2is a schematic diagram illustrating the IDT electrode group25included in the additional resonant circuit20of the acoustic wave filter1. Incidentally, inFIG.2, an electrode and a line are represented by solid lines.

As described above, the additional resonant circuit20includes the IDT electrode group25. The IDT electrode group25is an acoustic wave resonator group including a plurality of IDT electrodes31and32. The IDT electrode group25includes, for example, at least one longitudinally coupled resonator. The plurality of IDT electrodes31and32are adjacent to each other along an acoustic wave propagation direction D1. Electrode parameters of the plurality of IDT electrodes31and32are different from each other.

Furthermore, the additional resonant circuit20includes a plurality of reflectors41and42. The plurality of reflectors41and42are located, in the acoustic wave propagation direction D1, on both outer sides of the IDT electrode group25so that the IDT electrode group25is interposed therebetween. InFIG.2, the additional resonant circuit20including two reflectors41and42is exemplified.

The plurality of IDT electrodes31and32include a plurality of first comb-shaped electrodes31aand32aand a plurality of second comb-shaped electrodes31band32b. Of the plurality of IDT electrodes31and32, the IDT electrode31includes a pair of the first comb-shaped electrode31aand the second comb-shaped electrode31b. On the other hand, the IDT electrode32includes a pair of the first comb-shaped electrode32aand the second comb-shaped electrode32b.

The first comb-shaped electrode31aand the second comb-shaped electrode31bface each other. The first comb-shaped electrode32aand the second comb-shaped electrode32bface each other.

The plurality of first comb-shaped electrodes31aand32aare electrically connected to a plurality of different nodes on the first path r1. Specifically, the first comb-shaped electrode31ais connected to the node n2bvia the first terminal21, and the first comb-shaped electrode32ais connected to the node n4via the second terminal22. On the other hand, each of the second comb-shaped electrodes31band32bis connected to a ground.

Incidentally, although an example has been described above where the first terminal21of the additional resonant circuit20is connected to the node n2band the second terminal22is connected to the node n4, connections of the terminals are not limited to this. The first terminal21and the second terminal22may only be connected to nodes on both outer sides of two or more adjacent series-arm resonators on the first path r1. For example, the first terminal21may be connected to the node n1on the first path r1connecting the first signal terminal T1and the series-arm resonator S1or may be connected to the node n3. For example, the second terminal22may be connected to the node n3.

3. Structure of IDT Electrode Group of Additional Resonant Circuit

Next, the structure of the IDT electrode group25included in the additional resonant circuit20will be described. The IDT electrode group25includes, for example, a plurality of surface acoustic wave (SAW) resonators.

FIG.3includes a plan view and a cross-sectional view schematically illustrating the structure of the IDT electrode group25. Incidentally, the IDT electrode group25illustrated inFIG.3is intended to describe a typical structure of a resonator, and the number of electrode fingers included in an IDT electrode and a reflector, the length of each electrode finger, and so forth are not limited to this.

The IDT electrode group25includes a substrate320having piezoelectricity, and the plurality of IDT electrodes31and32on the substrate320. On both outer sides of the IDT electrode group25in the acoustic wave propagation direction D1, the plurality of reflectors41and42are provided.

As illustrated in the cross-sectional view inFIG.3, the IDT electrode group25and electrodes of the plurality of reflectors41and42are defined by the substrate320, an electrode layer325, and a dielectric layer326. The electrode layer325defines each of the IDT electrodes31and32and the electrodes of the plurality of reflectors41and42. The dielectric layer326is provided on the substrate320so as to cover each of the IDT electrodes31and32and each of the reflectors41and42.

The substrate320is a LiNbO3substrate (lithium niobate substrate), for example, with a cut-angle of about 127.5°. When a Rayleigh wave is used as an acoustic wave that propagates through the substrate320, it is desirable that a cut-angle of the substrate320be about 120°±20° or about 300°±20°, for example.

The electrode layer325has a structure in which a plurality of metal layers are laminated. The electrode layer325is formed by laminating, for example, a Ti layer, an Al layer, a Ti layer, a Pt layer, and an NiCr layer in this order from top to bottom.

The dielectric layer326is a film mainly including, for example, silicon dioxide (SiO2). The dielectric layer326is provided, for example, to adjust frequency-temperature characteristics of the IDT electrode group25, to protect the electrode layer325from an external environment, or to increase resistance to moisture.

As illustrated in the plan view inFIG.3, the IDT electrode31includes a pair of the first comb-shaped electrode31aand the second comb-shaped electrode31bthat face each other. The IDT electrode32includes a pair of the first comb-shaped electrode32aand the second comb-shaped electrode32bthat face each other.

Each of the first comb-shaped electrodes31aand32ahas a comb shape and includes a plurality of electrode fingers36athat are parallel or substantially parallel to one another and a busbar electrode37athat connects one ends of the plurality of respective electrode fingers36a. Each of the second comb-shaped electrodes31band32bhas a comb shape and includes a plurality of electrode fingers36bthat are parallel or substantially parallel to one another and a busbar electrode37bthat connects one ends of the plurality of respective electrode fingers36b. Each of the busbar electrodes37aand37bextends along the acoustic wave propagation direction D1. The plurality of electrode fingers36aand36bextend along a direction D2perpendicular or substantially perpendicular to the acoustic wave propagation direction D1. The plurality of electrode fingers36aand36binterdigitate with each other in the orthogonal direction D2and face each other in the acoustic wave propagation direction D1.

The first comb-shaped electrode31ais connected to the first terminal21via a line, and the first comb-shaped electrode32ais connected to the second terminal22via a line. The second comb-shaped electrode31bis connected to the ground via a line, and the second comb-shaped electrode32bis connected to the ground via a line. Incidentally, the ground may be a ground connection electrode (not illustrated) provided, for example, in or on a board of the multiplexer5.

A resonant frequency of the additional resonant circuit20can be adjusted, for example, by changing a pitch (λ/2) between a plurality of electrode fingers36aand36b, or an intersecting width of the plurality of electrode fingers36aand36b.

4. Bandpass Characteristics and Others of Acoustic Wave Filter

Bandpass characteristics and others of the acoustic wave filter1according to the present preferred embodiment will be described.

FIG.4is a graph illustrating bandpass characteristics of the acoustic wave filter1.FIG.5is a graph illustrating phase characteristics of the first filter circuit10and the additional resonant circuit20that are included in the acoustic wave filter1.

InFIGS.4and5, an example is illustrated where the acoustic wave filter1is a transmission filter and the second filter circuit50is a reception filter. Furthermore, an example is illustrated where the pass band of the acoustic wave filter1ranges from about 698 MHz to about 716 MHz and the pass band of the second filter circuit50ranges from about 728 MHz to about 746 MHz.

As illustrated inFIG.4, the additional resonant circuit20has a plurality of resonant frequencies. At least one resonant frequency of one or more resonant frequencies that the additional resonant circuit20has is included within the pass band of the first filter circuit10. InFIG.4, for example, of two resonant frequencies f1and f2of the additional resonant circuit20, the resonant frequency f1exists within the pass band of the first filter circuit10. Furthermore, of a plurality of resonant frequencies f1and f2, the other resonant frequency f2different from the resonant frequency f1is outside the pass band of the first filter circuit10. Specifically, the other resonant frequency f2is outside the pass band of the second filter circuit50and exists in a band of frequencies higher than the pass band of the second filter circuit50.

Hereinafter, the pass band of the first filter circuit10included in the acoustic wave filter1may be referred to as its own band, and the pass band of the second filter circuit50may be referred to as another band or the other band.

As indicated by phase characteristics inFIG.5, signal phases that pass through the first filter circuit10and the additional resonant circuit20are the same in its own band. On the other hand, signal phases that pass through the above-described respective circuits are the same in a certain frequency band in the other band but are opposite in most of a frequency band other than the certain frequency band.

Specifically, as for signal phases that pass through the first filter circuit10and the additional resonant circuit20, when a signal phase of the first filter circuit10exhibits a capacitive property in its own band, a signal phase of the additional resonant circuit20also exhibits the capacitive property. When a signal phase of the first filter circuit10exhibits an inductive property, a signal phase of the additional resonant circuit20also exhibits the inductive property. Thus, when a signal that passes through the additional resonant circuit20has the same characteristics as a signal that passes through the first filter circuit10in terms of the capacitive and inductive properties, the signal that passes through the first filter circuit10can be strengthened. That is, signal phases of the first filter circuit10and the additional resonant circuit20in its own band are made to coincide with each other, thereby enabling a reduction in insertion loss in its own band of the acoustic wave filter1.

On the other hand, signal phases of the first filter circuit10and the additional resonant circuit20exhibit the inductive property in a certain frequency band in the other band. However, in about 80% or more of a frequency band other than the certain frequency band, a signal phase of the first filter circuit10exhibits the capacitive property, and a signal phase of the additional resonant circuit20exhibits the inductive property. In other words, signal phases of the first filter circuit10and the additional resonant circuit20are opposite rather than the same in most portions (frequency regions) in the other band. Thus, when most of signals that pass through the additional resonant circuit20have characteristics opposite to signals that pass through the first filter circuit10in terms of the capacitive and inductive properties, a signal that passes through the first filter circuit10can be weakened. That is, signal phases of the first filter circuit10and the additional resonant circuit20in the other band are reversed, thus making it possible to keep the acoustic wave filter1from adversely affecting bandpass characteristics in the other band.

Furthermore, when signal phases of the first filter circuit10and the additional resonant circuit20are compared by using a difference between phase angles, a difference between phase angles of the above-described signal phases in its own band is smaller than a difference between phase angles of the above-described signal phases in the other band. For example, a difference between phase angles of the above-described signal phases in about 80% or more of its own band is about 10° or less, and a difference between phase angles of the above-described signal phases in about 80% or more of the other band is about 90° or more. When there is such a difference between differences between phase angles, a signal that passes through the first filter circuit10in its own band can be strengthened, and a signal that passes through the first filter circuit10in the other band can be weakened. This enables a reduction in insertion loss in its own band and an improvement in attenuation characteristics in the other band.

Next, bandpass characteristics of the acoustic wave filter1that exhibits the above-described characteristics will be described by comparison with bandpass characteristics of an acoustic wave filter in a comparative example.

FIG.6is a graph illustrating bandpass characteristics of acoustic wave filters in the present preferred embodiment and a comparative example. Incidentally, in the comparative example, there is provided an acoustic wave filter including no additional resonant circuit20, that is, an acoustic wave filter including only the first filter circuit10.

As illustrated inFIG.6, in the acoustic wave filter1according to the present preferred embodiment, an attenuation slope located at frequencies higher than its own band is steep in comparison with the acoustic wave filter in the comparative example. Furthermore, in the acoustic wave filter1according to the present preferred embodiment, attenuation in the other band is great in comparison with the acoustic wave filter in the comparative example. The reason why these characteristics are exhibited is because, as indicated by phase characteristics inFIG.5, some signal phases of the first filter circuit10and the additional resonant circuit20are opposite in a transition band (about 716 MHz to about 728 MHz) between its own band and the other band and the other band (about 728 MHz to about 746 MHz). Thus, the acoustic wave filter1according to the present preferred embodiment enables an improvement in attenuation characteristics outside the pass band in comparison with the acoustic wave filter in the comparative example.

Next, the amount of improvement in insertion loss in the pass band of the acoustic wave filter1will be described.

FIG.7Ais a graph illustrating the amount of improvement in insertion loss in the pass band of the acoustic wave filter1.FIG.7Bis a graph illustrating insertion losses of the first filter circuit10and the additional resonant circuit20in the acoustic wave filter1.

InFIG.7A, the vertical axis represents the amount of improvement (dB) in insertion loss of the acoustic wave filter1according to the present preferred embodiment with respect to insertion loss of the acoustic wave filter in the comparative example. Referring toFIG.6as an example, the amount of improvement in insertion loss is obtained by subtracting, at a frequency (for example, 716 MHz) at which a maximum insertion loss is reached in the pass band of the acoustic wave filter1, an insertion loss in the comparative example from an insertion loss in the present preferred embodiment. As a difference between the insertion loss in the present preferred embodiment and the insertion loss in the comparative example increases, the degree of improvement in insertion loss increases.

InFIG.7A, the horizontal axis represents a difference (dB) between insertion losses of the first filter circuit10and the additional resonant circuit20. Referring toFIG.7Bas an example, a difference between insertion losses of the first filter circuit10and the additional resonant circuit20is obtained by subtracting, at a frequency (for example, about 716 MHz) at which a maximum insertion loss is reached in the pass band, an insertion loss of a signal that passes through the additional resonant circuit20from an insertion loss of a signal that passes through the first filter circuit10. As a difference between the insertion loss of the signal that passes through the first filter circuit10and the insertion loss of the signal that passes through the additional resonant circuit20decreases (toward the right-hand side ofFIG.7A), the strength of the signal that passes through the additional resonant circuit20increases.

As illustrated inFIG.7A, when a difference between the insertion losses of the first filter circuit10and the additional resonant circuit20falls below about 25 dB, the amount of improvement in insertion loss represented by the vertical axis increases. That is, it is desirable that a difference between insertion losses of signals that pass through the first filter circuit10and the additional resonant circuit20be about 25 dB or less within the pass band of the acoustic wave filter1.

Next, a relationship between a resonant frequency F of series-arm resonators included in the first filter circuit10and the resonant frequency f1of the additional resonant circuit20will be described. The resonant frequency F of the series-arm resonators is, for example, a resonant frequency obtained when characteristics of one or more series-arm resonators are averaged. The resonant frequency F in this example is a resonant frequency obtained when characteristics of the series-arm resonators S2and S3with which the additional resonant circuit20is connected in parallel are averaged.

FIGS.8A and8Binclude graphs illustrating an example of insertion losses of series-arm resonators of the first filter circuit10and the additional resonant circuit20.FIGS.9A and9Bincludes diagrams illustrating an example of the series-arm resonators of the first filter circuit10and the additional resonant circuit20.FIGS.8A and9Aillustrate an example where the resonant frequency F of the series-arm resonators and the resonant frequency f1of the additional resonant circuit20do not coincide with each other.FIGS.8B and9Billustrate an example where two resonant frequencies F and f1coincide with each other.

For example, as illustrated inFIG.8A, when the resonant frequency f1of the additional resonant circuit20deviates from the resonant frequency F of the series-arm resonators, mismatching is likely to occur between a signal that passes through the first filter circuit10and a signal that passes through the additional resonant circuit20within the pass band. This is because the series-arm resonators have the property of having high impedance at a frequency different from their own resonant frequency. As illustrated inFIG.9A, when a signal passes through the series-arm resonators having high impedance, a potential difference V occurs across the additional resonant circuit20due to a voltage drop, and the additional resonant circuit20is excited by this potential difference V. For this reason, mismatching occurs between the signal that passes through the first filter circuit10and the signal that passes through the additional resonant circuit20.

For example, as illustrated inFIG.8B, when the resonant frequency f1of the additional resonant circuit20coincides with the resonant frequency F of the series-arm resonators, mismatching is less likely to occur between a signal that passes through the first filter circuit10and a signal that passes through the additional resonant circuit20within the pass band. This is because the impedance of the series-arm resonators reaches substantially 0 at their own resonant frequency, that is, it is equivalent to the occurrence of a short circuit. As illustrated inFIG.9B, even when a signal passes through the series-arm resonators whose impedance is substantially 0, a potential difference V is not very likely to occur in the additional resonant circuit20, and the additional resonant circuit20is almost not excited. For this reason, mismatching is less likely to occur between the signal that passes through the first filter circuit10and the signal that passes through the additional resonant circuit20.

Thus, it is desirable that the resonant frequency f1of the additional resonant circuit20coincide with the resonant frequency F of one or more series-arm resonators within the pass band of the first filter circuit10. Incidentally, the above description in which the resonant frequencies F and f1coincide with each other means, for example, that the resonant frequency f1of the additional resonant circuit20is located, of resonant frequencies of the one or more series-arm resonators, within a range from a minimum resonant frequency to a maximum resonant frequency.

Furthermore, if the additional resonant circuit20has a plurality of resonant frequencies (a plurality of local maxima of insertion loss) within the pass band of the first filter circuit10, it is desirable that, of the plurality of resonant frequencies, a value of a resonant frequency f1at which the lowest impedance (or insertion loss) is exhibited be closer to a value of the resonant frequency F of the series-arm resonators than a resonant frequency different from the resonant frequency f1at which the lowest impedance (or insertion loss) is exhibited. That is, the additional resonant circuit20has a plurality of resonant frequencies included within the pass band, and, of the plurality of resonant frequencies, a value of the resonant frequency f1at which the lowest impedance is exhibited may be closest to a value of the resonant frequency of one or more series-arm resonators.

SUMMARY

As described above, the acoustic wave filter1according to the present preferred embodiment includes the first filter circuit (filter circuit)10having a predetermined frequency band as a pass band and provided on the first path r1connecting the first signal terminal T1and the second signal terminal T2, and the additional resonant circuit20connected in parallel with at least a portion of the first filter circuit10. The additional resonant circuit20includes the IDT electrode group25including the plurality of IDT electrodes31and32positioned along the acoustic wave propagation direction D1. At least one resonant frequency f1of one or more resonant frequencies of the additional resonant circuit20is included within the pass band of the first filter circuit10.

For example, in the existing acoustic wave filter, since the resonant frequency of the cancel circuit is located far away from the pass band, a signal that passes through the cancel circuit is small in the pass band, and it is difficult to strengthen a signal that passes through the first filter circuit10. In contrast, in the acoustic wave filter1according to the present preferred embodiment, since the above-described at least one resonant frequency f1of the additional resonant circuit20is included within the pass band of the first filter circuit10, a signal that passes through the first filter circuit10within the pass band can be strengthened. This enables a reduction in insertion loss within the pass band of the first filter circuit10.

Furthermore, signal phases of the first filter circuit10and the additional resonant circuit20may be the same within the pass band.

Thus, when the signal phases are the same, a signal that passes through the first filter circuit10can be strengthened. This enables a reduction in insertion loss within the pass band of the first filter circuit10with certainty.

Furthermore, the additional resonant circuit20may have a plurality of resonant frequencies f1and f2, and the other resonant frequency f2different from the at least one resonant frequency f1may be outside the pass band of the first filter circuit10.

This acoustic wave filter1enables an improvement in attenuation characteristics outside the pass band.

Furthermore, a difference between phase angles of signal phases of the first filter circuit10and the additional resonant circuit20within the pass band may be smaller than a difference between phase angles of signal phases of the first filter circuit10and the additional resonant circuit20outside the pass band.

This enables, by using the additional resonant circuit20, a signal that passes through the first filter circuit10within the pass band to be strengthened and enables a signal that passes through the first filter circuit10outside the pass band to be weakened. Thus, insertion loss within the pass band can be reduced, and attenuation characteristics outside the pass band can be improved.

Furthermore, a difference between insertion losses of signals that pass through the first filter circuit10and the additional resonant circuit20may be about 25 dB or less within the pass band.

This enables an increase in the amount of improvement in insertion loss of the acoustic wave filter1according to the present preferred embodiment with respect to the acoustic wave filter in the comparative example.

Furthermore, the first filter circuit10may include one or more series-arm resonators, and the at least one resonant frequency f1of the additional resonant circuit20may coincide with a resonant frequency F of the one or more series-arm resonators within the pass band.

This causes mismatching between a signal that passes through the first filter circuit10and a signal that passes through the additional resonant circuit20to be less likely to occur within the pass band and enables a reduction in insertion loss of the first filter circuit10.

Furthermore, the additional resonant circuit20may have a plurality of resonant frequencies included within the pass band, and, of the plurality of resonant frequencies, a value of a resonant frequency f1at which the lowest impedance is exhibited may be closest to a value of the resonant frequency of the one or more series-arm resonators.

This enables a signal that passes through the first filter circuit10to be effectively strengthened. This enables a reduction in insertion loss within the pass band of the first filter circuit10.

Furthermore, the plurality of IDT electrodes31and32may be each connected directly to the first path r1via the second path r2different from the first path r1.

This enables a signal that passes through the additional resonant circuit20to be strengthened and enables a signal that passes through the first filter circuit10to be further strengthened. This enables a reduction in insertion loss within the pass band of the first filter circuit10.

Furthermore, the multiplexer5may include the above-described acoustic wave filter1, the first signal terminal T1, the second signal terminal T2, the third signal terminal T3, and the second filter circuit50having, as a pass band, a frequency band different from that of the first filter circuit10and disposed on the third path r3connecting the second signal terminal T2and the third signal terminal T3. The other resonant frequency f2different from the resonant frequency f1may be outside the pass band of the second filter circuit50. In most portions within the other band of the first filter circuit10, that is, in most portions within the pass band of the second filter circuit50, signal phases of the first filter circuit10and the additional resonant circuit20may be opposite rather than the same.

Other Preferred Embodiments

Acoustic wave filters according to preferred embodiments of the present invention has been described above. As for the present invention, the present invention also encompasses other preferred embodiments achieved by combining any components in the above-described preferred embodiments, modifications obtained by making various modifications to the above-described preferred embodiments within the scope of the present invention, and radio frequency front-end circuits and communication devices that each include an acoustic wave filter or multiplexer according to other preferred embodiments of the present invention.

In the above description, although an example has been described where the IDT electrode group25includes two IDT electrodes, the number of IDT electrodes is not limited to this and may be three or more.

In the above description, although an example has been described where the pass band of the acoustic wave filter1is lower than the pass band of the second filter circuit50, the pass band of the acoustic wave filter1is not limited to this and may be higher than the pass band of the second filter circuit50.

In the above description, although an example has been described where the acoustic wave filter1is a transmission filter, the acoustic wave filter1is not limited to this and may be a reception filter. Furthermore, the multiplexer5is not limited to a configuration in which both a transmission filter and a reception filter are included and may include only a transmission filter, or only a reception filter.

Furthermore, in the above description, although, as an example, the multiplexer including two filters has been described, preferred embodiments of the present invention can also be applied, for example, to a triplexer in which a common antenna terminal is provided for three filters, or a hexaplexer in which a common antenna terminal is provided for six filters. That is, the multiplexer may only include two or more filters.

Furthermore, each of the first signal terminal T1and the second signal terminal T2may be either an input terminal or an output terminal. For example, if the first signal terminal T1is an input terminal, the second signal terminal T2is an output terminal. If the second signal terminal T2is an input terminal, the first signal terminal T1is an output terminal.

Furthermore, the second filter circuit50is not limited to the above-described filter configuration and can be appropriately designed, for example, in accordance with filter characteristics that are desired. Specifically, the second filter circuit50may have a longitudinally coupled filter structure or may have a ladder filter structure. Furthermore, each resonator of the second filter circuit50is not limited to a SAW resonator and may be, for example, a BAW (Bulk Acoustic Wave) resonator. Additionally, the second filter circuit50may be constructed without any resonators and may be, for example, an LC resonant filter or dielectric filter.

Furthermore, materials of the electrode layer325and dielectric layer326of the IDT electrodes31and32and the reflectors41and42are not limited to the above-described materials. Furthermore, the IDT electrodes31and32do not have to have the above-described laminated structure. The IDT electrodes31and32may be made of a metal or alloy, such as Ti, Al, Cu, Pt, Au, Ag, or Pd, or may include a plurality of multilayer bodies made of the above-described metal or alloy.

Furthermore, although a substrate having piezoelectricity has been described as the substrate320in the preferred embodiments, this substrate may be a piezoelectric substrate including a single piezoelectric layer. The piezoelectric substrate in this case is made, for example, of a piezoelectric single crystal of LiTaO3, or another piezoelectric single crystal, such as LiNbO3. Furthermore, as for the substrate320on which the IDT electrodes31and32are formed, as long as the substrate320has piezoelectricity, a structure in which piezoelectric layers are laminated on a supporting substrate may be used in addition to a structure in which the entire substrate320is including a piezoelectric layer. Additionally, a cut-angle of the substrate320according to the above-described preferred embodiments is not limited. That is, a laminated structure, materials, and a thickness may be appropriately changed, for example, in accordance with a desired bandpass characteristics of the acoustic wave filter. Even a surface acoustic wave filter in which, for example, a LiTaO3piezoelectric substrate or LiNbO3piezoelectric substrate with an angle other than the cut-angle described in the above-described preferred embodiments is used can achieve a similar effect.

Preferred embodiments of the present invention can be widely used, as a multiplexer, a front-end circuit, and a communication device that each include an acoustic wave filter, in communication equipment, such as mobile phones.