Filter device, multiplexer, and communication device and high-frequency front-end circuit equipped with filter device and multiplexer

Abstract

A filter device (100) comprises: an input terminal (Tin); an output terminal (Tout); a series arm circuit (SA) connected between the input terminal and the output terminal; and a parallel arm circuit (PA). The parallel arm circuit includes resonator circuits (RC1, RC2) connected in parallel between the series arm circuit and a ground potential (GND). The series arm circuit includes a series arm resonator (S1) and a capacitor (CS12) connected in series between the input terminal and the output terminal. The resonator circuit (RC1) includes a parallel arm resonator (P1) and an inductor (LP1), each of which is connected between one end of the capacitor (CS12) and the ground potential. The resonator circuit (RC2) includes a parallel arm resonator (P2) and an inductor (LP2), each of which is connected between another end of the capacitor (CS12) and the ground potential. Each of the resonators (S1, P1, P2) is an elastic wave resonator. The resonance frequency of the series arm resonator (S1) and the anti-resonance frequency of the parallel arm resonators (P1, P2) are lower than the lower limit frequency of a passband.

Description

Filter device, multiplexer, and high-frequency front-end circuit and communication device incorporating the same

[0001] The present disclosure relates to a filter device, a multiplexer, and a high-frequency front-end circuit and a communication device incorporating the same, and more particularly to a technique for improving the attenuation characteristics of a filter device using an acoustic wave resonator.

[0002] International Publication No. 2015 / 083415 (Patent Document 1) discloses a ladder-type filter device configured with series arm circuits and parallel arm circuits including acoustic wave resonators. In a typical ladder-type filter device, the resonant frequency of a resonator (series arm resonator) in the series arm circuit and the antiresonant frequency of a resonator (parallel arm resonator) in the parallel arm circuit are made to approximately match, thereby obtaining a bandpass filter in which the vicinity of the antiresonant frequency of the series arm circuit is the high-frequency stopband, the vicinity of the resonant frequency of the parallel arm circuit is the low-frequency stopband, and the frequency band between them is the passband.

[0003] International Publication No. WO 2015 / 083415

[0004] To broaden the bandwidth of a ladder-type filter device, it is necessary to set a wider interval between the antiresonant frequency of the series arm resonator and the resonant frequency of the parallel arm resonator. In this case, the fractional bandwidth of the filter device (= passband width / center frequency) may become wider than the fractional bandwidth of each resonator. In this case, the region between the antiresonant frequency of the parallel arm resonator and the resonant frequency of the series arm resonator becomes capacitive, and the passband characteristics in this region may deteriorate.

[0005] As a measure to suppress such degradation of passband characteristics, International Publication No. 2015 / 083415 (Patent Document 1) proposes a configuration in which an inductor is connected in parallel to a parallel arm resonator. The parallel arm resonator and the inductor connected in parallel to the resonator constitute an LC parallel resonator. By appropriately setting the resonant frequency of this LC parallel resonator, attenuation in the passband is suppressed and the passband characteristics are improved.

[0006] On the other hand, in the above configuration, the influence of the LC parallel resonator formed by the parallel arm resonator and the inductor may extend not only to the passband but also to the non-passband (stopband), and sufficient attenuation characteristics may not be obtained, particularly in the stopband, which is on the lower frequency side of the passband.

[0007] The present disclosure has been made to solve the above-mentioned problems, and its purpose is to improve the attenuation characteristics of a ladder-type filter device on the lower frequency side than the pass band.

[0008] A filter device according to a first aspect of the present disclosure is a filter device that passes signals in a predetermined frequency band, and includes an input terminal, an output terminal, a series arm circuit connected between the input terminal and the output terminal, and a parallel arm circuit. The parallel arm circuit includes a first resonator circuit and a second resonator circuit connected in parallel between the series arm circuit and ground potential. The series arm circuit includes a first series arm resonator and a first capacitor connected in series between the input terminal and the output terminal. The first resonator circuit includes a first parallel arm resonator and a first inductor, each connected between one end of the first capacitor and ground potential. The second resonator circuit includes a second parallel arm resonator and a second inductor, each connected between the other end of the first capacitor and ground potential. Each of the first series arm resonator, the first parallel arm resonator, and the second parallel arm resonator is an acoustic wave resonator. The antiresonant frequency of the first series arm resonator and the resonant frequency of each parallel arm resonator are lower than the lower limit frequency of the predetermined frequency band.

[0009] A multiplexer according to a second aspect of the present disclosure includes an input terminal, a first output terminal, a second output terminal, a first filter device, and a second filter device. The first filter device is connected between the input terminal and the first output terminal and configured to pass signals in a first frequency band. The second filter device is connected between the input terminal and the second output terminal and configured to pass signals in a second frequency band different from the first frequency band. Each of the first filter device and the second filter device includes a series arm circuit and a parallel arm circuit connected between the input terminal and the corresponding output terminal. The parallel arm circuit includes a parallel arm circuit including a first resonator circuit and a second resonator circuit connected in parallel between the series arm circuit and ground potential. The series arm circuit includes a first series arm resonator and a first capacitor connected in series between the input terminal and the output terminal. The first resonator circuit includes a first parallel arm resonator and a first inductor, each connected between one end of the first capacitor and ground potential. The second resonator circuit includes a second parallel arm resonator and a second inductor, each connected between the other end of the first capacitor and ground. Each of the first series arm resonator, the first parallel arm resonator, and the second parallel arm resonator is an acoustic wave resonator. The antiresonant frequency of the first series arm resonator and the resonant frequency of each parallel arm resonator are lower than the lower limit frequency of the corresponding frequency band.

[0010] In the ladder-type filter device according to the present disclosure, the antiresonant frequency of the series arm resonator and the resonant frequency of each parallel arm resonator are both set to be lower than the lower limit frequency of the pass band. As a result, in addition to the attenuation poles generated by each parallel arm resonator, an attenuation pole generated by the series arm resonator also occurs on the lower frequency side of the pass band. This improves the attenuation characteristics on the lower frequency side of the pass band.

[0011] 1 is a block diagram of a communication device equipped with a high-frequency front-end circuit to which a filter device according to a first embodiment is applied; FIG. 2 is an example of an equivalent circuit diagram of the filter device in FIG. 1; FIG. 3 is a schematic side view of the filter device in FIG. 1; FIG. 4 is a partial cross-sectional view of a first example of an acoustic wave device in FIG. 2; FIG. 5 is a partial cross-sectional view of a second example of an acoustic wave device in FIG. 2; FIG. 6 is a partial cross-sectional view of a third example of an acoustic wave device in FIG. 2; FIG. 7 is a diagram for explaining attenuation characteristics of a stopband on the low frequency side of the filter device according to the first embodiment; FIG. 8 is a diagram for explaining characteristics of each resonator according to a first comparative example; FIG. 9 is a diagram for explaining attenuation characteristics in the filter devices of the first embodiment and the first comparative example; FIG. 10 is an example of an equivalent circuit diagram of the filter device according to a second comparative example; FIG. 11 is a diagram for explaining attenuation characteristics in the filter devices of the first embodiment and the first comparative example; ３ 1 is a diagram for explaining the relative bandwidth when the piezoelectric body is LiTaO ３ 10 is a diagram for explaining the fractional bandwidth in the case of . FIG. 11 is an example of an equivalent circuit diagram of the filter device according to embodiment 2. FIG. 12 is a diagram for explaining the attenuation characteristics in the filter devices of embodiment 2 and comparative example 4. FIG. 13 is a diagram for explaining the attenuation characteristics in the filter devices of embodiment 2 and comparative example 5.

[0012] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated.

[0013] 1 is a block diagram of a communication device 10 equipped with a high-frequency front-end circuit 20 to which a filter device according to embodiment 1 is applied. The communication device 10 is, for example, a mobile terminal such as a mobile phone, a smartphone, or a tablet, or a personal computer with a communication function.

[0014] Referring to FIG. 1 , the communication device 10 includes, in addition to a high-frequency front-end circuit 20, an antenna ANT, an RF signal processing circuit (hereinafter also referred to as an "RFIC (Radio Frequency Integrated Circuit)") 30, and a baseband signal processing circuit (hereinafter also referred to as a "BBIC (Baseband Integrated Circuit)") 40. In outline, the communication device 10 upconverts an intermediate frequency (IF) signal transmitted from the BBIC 40 to a high-frequency signal using the RFIC 30, further amplifies the high-frequency signal using the high-frequency front-end circuit 20, and radiates the high-frequency signal from the antenna ANT. Furthermore, the communication device 10 extracts and amplifies a signal of a predetermined frequency band from a signal received by the antenna ANT using the high-frequency front-end circuit 20, downconverts the signal using the RFIC 30, and processes the signal using the BBIC 40.

[0015] The high frequency front-end circuit 20 includes a switch SW1, filter devices 100A to 100D, matching devices (MN) 110A to 110D and 120, power amplifiers PA1 and PA2, and low noise amplifiers LNA1 and LNA2.

[0016] The switch SW1 is a single-pole, triple-throw switch including a terminal TA connected to the antenna ANT and three switching terminals T1, T2, and T3. The terminals T1 and T2 are terminals connected to the transmitting circuit. The terminal T3 is a terminal connected to the receiving circuit. The switch SW1 is controlled by a control signal from, for example, an RFIC.

[0017] The high-frequency front-end circuit 20 of the first embodiment has, as a transmission-side circuit, two paths for transmitting high-frequency signals in different frequency bands. The first transmission path connected to the terminal T1 is provided with a filter device 100A, a matching device 110A, and a power amplifier PA1. The second transmission path connected to the terminal T2 is provided with a filter device 100B, a matching device 110B, and a power amplifier PA2.

[0018] The power amplifiers PA1 and PA2 amplify the high-frequency signals supplied from the RFIC 30 to a predetermined power level. The matching device 110A matches the impedance of the power amplifier PA1 with the impedance of the filter device 100A. The matching device 110B matches the impedance of the power amplifier PA2 with the impedance of the filter device 100B. The filter devices 100A and 100B are bandpass filters having different passbands. The filter devices 100A and 100B extract signals of the corresponding passbands from the high-frequency signals amplified by the power amplifiers PA1 and PA2, respectively, and output the extracted signals to the antenna ANT via the switch SW1.

[0019] When emitting a signal in a frequency band corresponding to the filter device 100A, the switch SW1 is switched to the terminal T1, and when emitting a signal in a frequency band corresponding to the filter device 100B, the switch SW1 is switched to the terminal T2. When receiving radio waves through the antenna ANT, the switch SW1 is switched to the terminal T3.

[0020] The receiving circuit includes a diplexer 105, matching circuits 110C and 110D, and low-noise amplifiers LNA1 and LNA2. The diplexer 105 also includes an input terminal T4, output terminals T5 and T6, a matching circuit 120, and filter devices 100C and 100D. The receiving circuit is a circuit that extracts signals of frequencies corresponding to the pass bands of the filter devices 100C and 100D from signals received by the antenna ANT, distributes the extracted signals to a first receiving path and a second receiving path, and transmits the extracted signals to the RFIC 30.

[0021] The matching circuit 120 matches the impedance between the antenna ANT and each reception path and distributes the reception signal to the first reception path and the second reception path. The filter devices 100C and 100D have different passbands and extract high-frequency signals of the corresponding passbands from the reception signal. The matching circuit 110C matches the impedance between the filter device 100C and the low-noise amplifier LNA1. The matching circuit 110D matches the impedance between the filter device 100D and the low-noise amplifier LNA2. The low-noise amplifiers LNA1 and LNA2 amplify the signals that have passed through the matching circuits 110C and 110D, respectively, with low noise and output the amplified signals to the RFIC 30.

[0022] Filter devices according to the present disclosure can be adopted as the filter devices 100A to 100D in the above-described communication device 10. In the following description, the filter devices 100A to 100D may also be collectively referred to as "filter device 100."

[0023] 2 to 6, the detailed configuration of the filter device 100 according to the first embodiment will be described. The filter device 100 is a filter that has, for example, the WiFi7 band (5150 MHz to 7125 MHz) as its passband and the n77 band (3300 MHz to 4200 MHz) and n79 band (4400 MHz to 5000 MHz), which communicate simultaneously with the WiFi7 band, as its stopband.

[0024] (1) Equivalent Circuit Fig. 2 is an equivalent circuit diagram of the filter device 100. Referring to Fig. 2, the filter device 100 includes an input terminal Tin, an output terminal Tout, a series arm circuit SA, a parallel arm circuit PA, and capacitors C13, C14, and C24.

[0025] The series arm circuit SA includes a series arm resonator S1 and capacitors CS12, CS23, and CS34 connected in series between the input terminal Tin and the output terminal Tout. One end of the capacitor CS12 is connected to the input terminal Tin, and the other end is connected to one end of the capacitor CS23. The other end of the capacitor CS12 is connected to one end of the capacitor CS34. The series arm resonator S1 is connected between the other end of the capacitor CS34 and the output terminal Tout. Instead of or in addition to the series arm resonator S1, a series arm resonator S2 may be provided between the input terminal Tin and the capacitor CS12.

[0026] The parallel arm circuit PA includes resonator circuits RC1 to RC4 connected in parallel between the series arm circuit SA and ground potential GND. More specifically, resonator circuit RC1 is connected between one end (connection node N1) of capacitor CS12 and ground potential GND, resonator circuit RC2 is connected between a connection node N2 between capacitor CS12 and capacitor CS23 and ground potential GND, resonator circuit RC3 is connected between a connection node N3 between capacitor CS23 and capacitor CS34 and ground potential GND, and resonator circuit RC4 is connected between a connection node N4 between capacitor CS34 and series arm resonator S1 and ground potential GND.

[0027] The capacitor C13 is connected between the connection nodes N1 and N3. The capacitor C24 is connected between the connection nodes N2 and N4. The capacitor C14 is connected between the connection nodes N1 and N4.

[0028] In other words, the resonator circuits RC1 and RC2 are capacitively coupled by the capacitor CS12, and the resonator circuits RC2 and RC3 are capacitively coupled by the capacitor CS23. The resonator circuits RC3 and RC4 are capacitively coupled by the capacitor CS34, and the resonator circuits RC1 and RC3 are capacitively coupled by the capacitor C13. Furthermore, the resonator circuits RC2 and RC4 are capacitively coupled by the capacitor C24, and the resonator circuits RC1 and RC4 are capacitively coupled by the capacitor C14.

[0029] The resonator circuit RC1 includes a parallel arm resonator P1, an inductor LP1, and a capacitor CP1. One end of the parallel arm resonator P1 is connected to the connection node N1, and the other end is connected to the ground potential GND via the capacitor CP1. That is, the parallel arm resonator P1 and the capacitor CP1 are connected in series between the connection node N1 of the series arm circuit SA and the ground potential GND.

[0030] The inductor LP1 is connected between the connection node N1 and the ground potential GND, that is, the inductor LP1 is connected in parallel to the parallel arm resonator P1 and the capacitor CP1, which are connected in series.

[0031] The resonator circuit RC2 includes a parallel arm resonator P2 and an inductor LP2. Each of the parallel arm resonator P2 and the inductor LP2 is connected between the connection node N2 of the series arm circuit SA and the ground potential GND. That is, the parallel arm resonator P2 and the inductor LP2 are connected in parallel between the connection node N2 and the ground potential GND.

[0032] The resonator circuit RC3 includes a parallel arm resonator P3, an inductor LP3, and a capacitor CP3. One end of the parallel arm resonator P3 is connected to the connection node N3, and the other end is connected to the ground potential GND via the capacitor CP3. That is, the parallel arm resonator P3 and the capacitor CP3 are connected in series between the connection node N3 of the series arm circuit SA and the ground potential GND.

[0033] The inductor LP3 is connected between the connection node N3 and the ground potential GND, that is, the inductor LP3 is connected in parallel to the parallel arm resonator P3 and the capacitor CP3, which are connected in series.

[0034] The resonator circuit RC4 includes a parallel arm resonator P4 and an inductor LP4. Each of the parallel arm resonator P4 and the inductor LP4 is connected between the connection node N4 of the series arm circuit SA and the ground potential GND. That is, the parallel arm resonator P4 and the inductor LP4 are connected in parallel between the connection node N4 and the ground potential GND.

[0035] 2 shows an example in which capacitors are provided in addition to parallel arm resonators and inductors in the resonator circuits RC1 and RC3. However, instead of or in addition to this configuration, capacitors may also be provided in the resonator circuits RC2 and / or RC4. Furthermore, as indicated by the dashed line in the resonator circuit RC4, an inductor LP41 may be provided instead of a capacitor. Thus, in the filter device 100 of the first embodiment, at least one of the resonator circuits is provided with an impedance element in addition to the resonator circuit and inductor. A line of a predetermined length may be provided as the impedance element. By adding the impedance element, the fractional bandwidth of each resonator circuit can be adjusted.

[0036] Specifically, the impedance element connected between the parallel arm resonator and the ground potential GND is used to adjust the interval between the resonant frequency and anti-resonant frequency in the resonator circuit, i.e., the effective fractional bandwidth. Generally speaking, when a capacitive element such as a capacitor is used as the impedance element, the fractional bandwidth becomes narrower. On the other hand, when an inductive element such as an inductor or a line is used as the impedance element, the fractional bandwidth becomes wider. In the stopband, which is lower in frequency than the passband of the filter device, narrowing the fractional bandwidth improves the steepness at the lower limit frequency of the passband. On the other hand, widening the fractional bandwidth slightly reduces the steepness, but allows the passband to be broadened.

[0037] In the filter device 100, the resonator circuits RC1 and RC3 are provided with capacitors as impedance elements, which increases the resonant frequencies of the resonator circuits RC1 and RC3 compared to when no impedance elements are provided, thereby improving the steepness of attenuation in the stopband, which is lower in frequency than the passband.

[0038] The inductors included in each resonator circuit are magnetically coupled to each other, and an LC parallel resonant circuit is formed by this magnetic coupling and capacitive coupling between the resonator circuits due to the capacitors.

[0039] In the filter device 100, by adjusting the resonant frequency and anti-resonant frequency of each of the resonator circuits RC1 to RC4 of the parallel arm circuit PA and the resonant frequency and anti-resonant frequency of the series arm circuit SA, it is possible to adjust the pass band of the filter device 100 and also to adjust the attenuation characteristics in the non-pass band (stop band) adjacent to the pass band.

[0040] (2) Substrate Structure Next, the substrate structure on which the filter device 100 is formed will be described with reference to Fig. 3. Fig. 3 is a side perspective view of the filter device shown in Fig. 1. Note that Fig. 3 is intended to provide a schematic explanation of the configuration of the substrate, and does not depict some elements of the circuit in Fig. 2.

[0041] Referring to FIG. 3, the filter apparatus 100 includes a dielectric substrate 300 and an acoustic wave device 200 disposed on the dielectric substrate 300 .

[0042] The dielectric substrate 300 is formed of ceramic, such as low-temperature co-fired ceramics (LTCC), or resin. Connection electrodes for connection to a mounting board or other devices are arranged on the bottom surface of the dielectric substrate 300. The connection electrodes include a terminal for connection to a ground potential GND and a terminal for connection to a signal line SIG. Furthermore, connection terminals for mounting the acoustic wave device 200 are arranged on the top surface (the main surface in the positive direction of the Z axis) of the dielectric substrate 300.

[0043] Vias connected to the connection terminals, and plate electrodes for constituting the inductor ID and capacitor CP are arranged inside the dielectric substrate 300. At least a portion of each inductor and each capacitor in the equivalent circuit of FIG. 2 is arranged inside the dielectric substrate 300.

[0044] The acoustic wave device 200 includes a piezoelectric layer made of a piezoelectric material, and a functional layer including a functional element is disposed on the piezoelectric layer to form an acoustic wave resonator. The acoustic wave resonator may be a surface acoustic wave (SAW) resonator as illustrated in FIG. 4 or a bulk acoustic wave (BAW) resonator as illustrated in FIGS. 5 and 6. The acoustic wave resonators disposed in the acoustic wave device 200 form the series arm resonator S1 of the series arm circuit SA and the parallel arm resonators P1 to P4 of the parallel arm circuit PA. Each of the series arm resonator S1 and the parallel arm resonators P1 to P4 may be a single resonator, or may be a configuration in which multiple resonators are connected in series and / or parallel.

[0045] The acoustic wave device 200 is mounted on the dielectric substrate 300 by conductive connecting members such as solder bumps. The upper surface of the dielectric substrate 300 is entirely covered with a protective molding resin 350. As shown in Fig. 3, a space 355 is formed between the main surface of the acoustic wave device 200 on which the resonators are arranged and the dielectric substrate 300 to allow the resonators to vibrate.

[0046] (3) Structure of Acoustic Wave Device (3-1) SAW Resonator Fig. 4 is a partial cross-sectional view of the acoustic wave device 200. In the acoustic wave device 200, the acoustic wave resonator is a SAW resonator.

[0047] 4 , the acoustic wave device 200 includes a support substrate 130, a functional layer 115, and an intermediate layer 150. The functional layer 115 includes a piezoelectric film 170, a comb-shaped (IDT: Inter Digital Transducer) electrode 180 serving as a functional element (transducer electrode), a high acoustic velocity film 161, and a low acoustic velocity film 162. The functional layer 115 is stacked on the support substrate 130 via the intermediate layer 150. In the functional layer 115, the high acoustic velocity film 161, the low acoustic velocity film 162, and the piezoelectric film 170 are stacked in this order from the support substrate 130 side toward the negative direction of the Z axis in FIG.

[0048] The support substrate 130 is a semiconductor substrate made of a material such as silicon (Si), gallium arsenide (GaAs), gallium nitride (GaN), silicon carbide (SiC), etc. In the example of Fig. 4, the support substrate 130 is silicon.

[0049] 4, lithium tantalate (LT) is used for the piezoelectric film 170, and an IDT electrode 180 is disposed on an upper surface 171 of the piezoelectric film 170. A SAW resonator is formed by the piezoelectric film 170 and the IDT electrode 180. Note that lithium niobate (LN), aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), or the like may also be used for the piezoelectric film 170.

[0050] The IDT electrode 180 is configured as a low-density metal layer or a laminate of a low-density metal layer and a high-density metal layer. The low-density metal layer is formed using an electrode material such as an elemental metal composed of at least one of aluminum (Al), titanium (Ti), copper (Cu), silver (Ag), nickel (Ni), and chromium (Cr), or an alloy containing these as a main component. The high-density metal layer is formed using an electrode material such as an elemental metal composed of at least one of gold (Au), platinum (Pt), tantalum (Ta), molybdenum (Mo), and tungsten (W), or an alloy containing these as a main component.

[0051] The intermediate layer 150 is an insulating film formed of, for example, a silicon oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxide film, or an aluminum nitride film.

[0052] The high acoustic velocity film 161 is formed of a material in which the velocity of a bulk wave propagating through the high acoustic velocity film 161 is higher than the velocity of an elastic wave propagating through the piezoelectric film 170. In other words, the high acoustic velocity film 161 is formed of a material having a higher acoustic impedance than the piezoelectric film 170. The high acoustic velocity film 161 is formed of a material such as aluminum nitride, silicon nitride, aluminum oxide (alumina), silicon oxynitride, silicon carbide, diamond-like carbon (DLC), or diamond. In the example of FIG. 4, the high acoustic velocity film 161 is formed of aluminum nitride (AlN).

[0053] The low acoustic velocity film 162 is made of a material that causes the bulk wave propagation velocity through the low acoustic velocity film 162 to be lower than the bulk wave propagation velocity through the piezoelectric film 170. In other words, the low acoustic velocity film 162 is made of a material that has a lower acoustic impedance than the piezoelectric film 170. The low acoustic velocity film 162 is made of, for example, a dielectric material such as silicon dioxide, glass, silicon oxynitride, or tantalum oxide, or a compound in which fluorine, carbon, boron, or the like is added to silicon dioxide. In the example of FIG. 4, the low acoustic velocity film 162 is made of silicon dioxide (SiO ２ ) is formed.

[0054] By configuring the high acoustic velocity film 161 and the low acoustic velocity film 162 to be laminated below the piezoelectric film 170, the high acoustic velocity film 161 and the low acoustic velocity film 162 function as a reflective layer (mirror layer) 160. The reflective layer 160 is a so-called acoustic Bragg reflector.

[0055] That is, surface acoustic waves leaking from the piezoelectric film 170 in the direction of the support substrate 130 are reflected by the high acoustic velocity film 161 due to the difference in the propagation speed of sound, and are confined as standing waves within the low acoustic velocity film 162. In this way, the reflective layer 160 suppresses the loss of acoustic energy of the surface acoustic waves propagating through the piezoelectric film 170, allowing the surface acoustic waves to propagate efficiently. Note that, although an example in which the high acoustic velocity film 161 and the low acoustic velocity film 162 are each single-layered as the reflective layer 160 has been described in FIG. 4, the reflective layer 160 may also be configured with a plurality of high acoustic velocity films 161 and low acoustic velocity films 162 arranged alternately.

[0056] (3-2) BAW Resonator FIGS. 5 and 6 are partial cross-sectional views of an acoustic wave device in which the acoustic wave resonator is a BAW resonator.

[0057] a) FBAR Fig. 5 shows an example in which the acoustic wave resonator is a film bulk acoustic resonator (FBAR) type BAW resonator. Referring to Fig. 5, an acoustic wave device 200A includes a support substrate 130, a functional layer 115A, and an intermediate layer 150. The functional layer 115A includes a piezoelectric film 170 and functional elements (transducer electrodes) 185 and 186. The acoustic wave device 200A is an FBAR type BAW resonator, and the functional layer 115A functions as a resonator.

[0058] The piezoelectric film 170 is made of lithium tantalate (LT) or lithium niobate (LN) and includes opposing upper and lower surfaces 171A and 172A. Flat electrodes 185 and 186 are formed on the upper and lower surfaces 171A and 172A, respectively. The piezoelectric film 170 may be made of aluminum nitride, zinc oxide, lead zirconate titanate, or the like, and may be doped with scandium (Sc), ytterbium (Yb), or the like.

[0059] Electrodes 185, 186 and electrode 187, which will be described later with reference to Fig. 6, are formed using an electrode material such as an elemental metal made of at least one of aluminum, copper, silver, gold, titanium, tungsten, platinum, chromium, nickel, and molybdenum, or an alloy containing any of these as a main component. These electrodes may also be laminates made of the above electrode materials. In the example of Fig. 5, electrodes 185, 186 are made of molybdenum (Mo).

[0060] In the FBAR type BAW resonator, a cavity 190 is formed between the portion of the functional layer 115A where the electrodes 185 and 186 are formed and the support substrate 130. This cavity 190 allows the piezoelectric film 170 to vibrate freely.

[0061] In the portion of the acoustic wave device 200A where the cavity 190 is not formed, the piezoelectric film 170 is supported by the support substrate 130. As described above, the intermediate layer 150 is formed between the piezoelectric film 170 and the support substrate 130.

[0062] b) SMR Fig. 6 shows an example in which the acoustic wave resonator is a BAW resonator of a solid mounted resonator (SMR). Referring to Fig. 6, an acoustic wave device 200B includes a support substrate 130, a functional layer 115B, and an intermediate layer 150. The functional layer 115B of the acoustic wave device 200B includes a piezoelectric film 170 and electrodes 185 and 187 that form a BAW resonator, and a plurality of high acoustic velocity films 161A and low acoustic velocity films 162A that form a reflecting layer 160A.

[0063] 6, the piezoelectric film 170 is made of lithium tantalate or lithium niobate, and the electrodes 185 and 187 are made of molybdenum. As in FIG. 4, the low acoustic velocity film 162A is made of silicon dioxide, and the high acoustic velocity film 161A is made of aluminum nitride.

[0064] In the acoustic wave device 200B, the reflecting layer 160A has a configuration in which a plurality of high acoustic velocity films 161A and low acoustic velocity films 162A are alternately stacked, and a BAW resonator is stacked above the reflecting layer 160A. An intermediate layer 150 is formed between the functional layer 115B and the support substrate 130.

[0065] The acoustic wave devices shown in FIGS. 4 to 6 are merely examples, and the number of layers of piezoelectric films, high acoustic velocity films, and low acoustic velocity films that form the functional layers may be different from those shown.

[0066] (4) Operating Principle of the Filter Device Next, the operating principle of the filter device 100 according to the first embodiment will be described with reference to FIG. 7 . FIG. 7 is a diagram illustrating the attenuation characteristics of the low-frequency stopband of the filter device 100. In FIG. 7 , the upper part (A) shows the attenuation characteristics of a single series arm resonator and the attenuation characteristics of each parallel arm circuit. The middle part (B) of FIG. 7 shows the attenuation characteristics (solid line LN12) of an LC parallel resonant circuit formed by a resonant circuit composed of the capacitive component of the parallel arm resonator and an inductor, and magnetic coupling between the capacitor of the series arm circuit and the resonator circuit of the parallel arm circuit, the attenuation characteristics (dash-dotted line LN13) of the parallel arm circuit, and the attenuation characteristics (dashed line LN14) of the series arm resonator. The lower part (C) of FIG. 7 shows the total attenuation characteristics of the filter device 100.

[0067] 7A, the solid line LN10 represents the attenuation characteristics of the parallel arm resonators P1 to P4, and the dashed line LN11 represents the attenuation characteristics of the series arm resonator S1. As shown in the figure, the lower limit frequency of the passband of the filter device 100 is in a frequency range higher than the resonant frequencies of the parallel arm resonators P1 to P4.

[0068] In a typical ladder-type filter device, the anti-resonance frequencies of the series arm resonators are set to be higher than the pass band, but in the filter device 100, the anti-resonance frequency of the series arm resonator S1 (f2 in FIG. 7) is set to be lower than the pass band and lower than the resonant frequencies of the parallel arm resonators P1 to P4 (f3 in FIG. 7). In this case, as indicated by the dashed line LN14 and the dashed-dotted line LN13 in the middle (B) of FIG. 7, attenuation poles occur at the anti-resonant frequency of the series arm resonator S1 and the resonant frequencies of the parallel arm resonators P1 to P4.

[0069] Generally, a resonator has a minimum impedance at the resonant frequency and a maximum impedance at the anti-resonant frequency. The resonator exhibits inductive characteristics in the frequency range between the resonant frequency and the anti-resonant frequency, and capacitive characteristics in other frequency ranges.

[0070] To broaden the bandwidth of a ladder-type filter device using resonators, it is necessary to set a wider interval between the antiresonant frequency of the series arm resonator and the resonant frequency of the parallel arm resonator. In this case, as described above, the region between the antiresonant frequency of the parallel arm resonator and the resonant frequency of the series arm resonator becomes capacitive, and the pass characteristics in this region may be degraded.

[0071] In the filter device 100 of the first embodiment, an inductor is connected in parallel to each parallel arm resonator of the parallel arm circuit, thereby generating parallel resonance between the inductor and a capacitance component in a frequency range higher than the anti-resonance frequency of the parallel arm resonator, thereby improving the pass characteristics in the pass band and achieving attenuation characteristics in a frequency range higher than the pass band.

[0072] Furthermore, due to parallel resonance between the inductor and a capacitance component in a frequency range lower than the resonant frequency of the parallel arm resonator, attenuation characteristics are obtained in a frequency range lower than the resonant frequency of the parallel arm resonator.

[0073] Furthermore, an LC parallel resonant circuit is formed by the magnetic coupling between the inductors M12, M23, and M34 in the parallel arm circuit and the capacitors CS12, CS23, and CS34 in the series arm circuit SA, which results in an attenuation pole on the lower frequency side of the pass band, as shown by f1 in FIG. 7.

[0074] In this way, the inductors in the parallel arm circuit provide the bandpass filter characteristics shown by the solid line LN12. In addition, the attenuation pole (dashed line LN11) due to the antiresonant frequency of the series arm resonator S1 and the attenuation pole (dashed line LN13) due to the resonant frequencies of the parallel arm resonators P1 to P4 improve the steepness of the attenuation, thereby improving the attenuation characteristics of the filter device 100 as a whole in the stopband, which is lower in frequency than the passband, as shown in the lower part (C) of FIG.

[0075] In the example of FIG. 7 , the antiresonant frequency of the series arm resonator is set to be lower than the resonant frequencies of all the parallel arm resonators. However, it is sufficient that the antiresonant frequency of the series arm resonator is set to be lower than the resonant frequency of at least one of the parallel arm resonators.

[0076] (Filter Characteristics) In FIGS. 8 to 13, the filter characteristics of the filter device 100 according to the first embodiment will be described in comparison with the configurations of three comparative examples.

[0077] (1) Comparative Example 1 The filter device of Comparative Example 1 is a filter device having a configuration in which the characteristics of the series arm resonators are the same as those of a general ladder-type filter device. In other words, the equivalent circuit of the filter device of Comparative Example 1 is the same as the equivalent circuit of Embodiment 1 shown in Fig. 2, but with regard to the characteristics of each resonator, as shown in Fig. 8, the anti-resonance frequency of the series arm resonator is set higher than the pass band of the filter device (dashed line LN11A in Fig. 8). In other words, the attenuation pole due to the series arm resonator occurs on the higher frequency side than the pass band (f4 in Fig. 8).

[0078] Fig. 9 is a diagram illustrating the attenuation characteristics of the filter devices of the first embodiment and the first comparative example. In Fig. 9 and Figs. 11, 13, 17, and 18 described below, the horizontal axis represents frequency, and the vertical axis represents the attenuation characteristics of each filter device. Note that the lower graph in each diagram is an enlarged view of the frequency range from 3.5 GHz to 8.0 GHz in the upper graph.

[0079] In Fig. 9, the solid line LN20 indicates the attenuation characteristics of the filter device 100 of the first embodiment, and the dashed line LN21 indicates the attenuation characteristics of the filter device of the first comparative example. Note that the solid line LN20A and dashed line LN21A in the graph at the bottom are enlarged versions of the vertical axes of the solid line LN20 and dashed line LN21, respectively. In the graph at the bottom, the solid line LN20 and dashed line LN21 refer to the left-hand coordinate axis, and the solid line LN20A and dashed line LN21A refer to the right-hand coordinate axis. Note that the same applies to Figs. 11, 13, 17, and 18, which will be described later.

[0080] 9, in the filter device of Comparative Example 1, the series arm resonator S1 contributes to the attenuation characteristics on the higher frequency side than the pass band, and an attenuation pole occurs near about 8 GHz as shown by the dashed lines LN21 and LN21A. On the other hand, in the stop band on the lower frequency side than the pass band, an attenuation of about 30 dB occurs near 3 GHz to 4 GHz.

[0081] In the filter device 100 of embodiment 1, as shown by the solid lines LN20 and LN20A, the steepness of the attenuation decreases on the higher frequency side than the pass band, but an attenuation of approximately 40 dB is achieved throughout the stop band on the lower frequency side than the pass band.

[0082] In this way, in the filter device 100 of the first embodiment, the anti-resonance frequency of the series arm resonator S1 is set to be lower than the lower limit frequency of the desired pass band and lower than the resonant frequencies of the parallel arm resonators P1 to P4, thereby expanding the attenuation band on the lower frequency side of the pass band and improving the attenuation steepness by adding the capacitors CP1 and CP3. Therefore, the attenuation characteristics in the stop band on the lower frequency side of the overband can be improved compared to a typical ladder-type filter device.

[0083] (2) Comparative Example 2 Fig. 10 is an example of an equivalent circuit diagram of a filter device 100X according to Comparative Example 2. Fig. 11 is a diagram for explaining the attenuation characteristics of the filter devices according to Embodiment 1 and Comparative Example 2. In Fig. 11, solid lines LN30 and LN30A indicate the attenuation characteristics of the filter device 100 according to Embodiment 1, and dashed lines LN31 and LN31A indicate the attenuation characteristics of the filter device 100X according to Comparative Example 2.

[0084] 10, the filter device 100X has a configuration in which the series arm resonator S1 of the series arm circuit SA in the filter device 100 in Fig. 2 is omitted. Therefore, in the filter device 100X, no attenuation pole is generated by the series arm resonator S1.

[0085] 11, the attenuation characteristics of the filter device 100X in the stopband, which is on the lower frequency side of the passband, in the vicinity of 3 GHz to 4 GHz, are worse than those of the filter device 100. In other words, by adopting a configuration like that of the filter device 100 of the first embodiment, it is possible to improve the attenuation characteristics compared to the filter device 100X of the second comparative example.

[0086] (3) Comparative Example 3 As described in Fig. 3 , the filter apparatus 100 according to the first embodiment includes a dielectric substrate 300 and an acoustic wave device 200 disposed on the dielectric substrate 300. The inductors included in the parallel arm circuit PA are typically disposed within the dielectric substrate 300. Therefore, the inductors included in the resonant circuits of the parallel arm circuit PA are magnetically coupled to each other to some extent. As described in the operation principle of Fig. 7 , the attenuation pole of the LC parallel resonant circuit formed by the magnetic coupling between the resonant circuits in the parallel arm circuit PA and the capacitor of the series arm circuit SA can affect the attenuation characteristics in a frequency band lower than the pass band.

[0087] In Comparative Example 3, the effect of magnetic coupling between resonant circuits is explained by comparing a theoretical configuration in which magnetic coupling between inductors included in each resonant circuit in the parallel arm does not occur with the configuration of the filter device 100 of embodiment 1.

[0088] Fig. 12 is an example of an equivalent circuit diagram of a filter device 100Y according to comparative example 3. Fig. 13 is a diagram for explaining the attenuation characteristics of the filter devices according to embodiment 1 and comparative example 3. In Fig. 13, solid lines LN40 and LN40A show the attenuation characteristics of the filter device 100 according to embodiment 1, and dashed lines LN41 and LN41A show the attenuation characteristics of the filter device 100Y according to comparative example 3.

[0089] The equivalent circuit of the filter device 100Y has a circuit configuration similar to that of the equivalent circuit of the first embodiment shown in FIG. 2 , except that the inductors of the resonator circuits RC1 to RC4 are not magnetically coupled to each other. In this case, an LC parallel resonant circuit is not formed between the resonator circuits due to the magnetic coupling between the inductors and the capacitors of the series arm circuits SA. Therefore, the attenuation pole of the LC parallel resonant circuit shown as f1 in FIG. 7 does not occur in the stopband on the lower frequency side of the passband. Therefore, the filter device 100Y does not achieve the attenuation effect due to the attenuation pole of the LC parallel resonant circuit. As shown by the dashed line LN41 in FIG. 13 , the attenuation characteristics are significantly degraded, particularly in the vicinity of 2 GHz to 3 GHz. Furthermore, because the filter device 100Y does not form an LC parallel resonant circuit, the attenuation characteristics are degraded in the passband near 6 GHz to 7 GHz, as shown by the dashed line LN41A.

[0090] As described above, in the filter device 100 of the first embodiment, an LC parallel resonant circuit is formed by the magnetic coupling between the parallel resonant circuits and the capacitor of the series arm circuit SA, thereby improving the attenuation characteristics in the stopband, which is on the lower frequency side of the passband.

[0091] 13, even in the case of Comparative Example 3, attenuation characteristics equivalent to those of the filter device 100 of Embodiment 1 are obtained in the frequency band of 4 GHz to 5 GHz. Therefore, magnetic coupling between parallel resonant circuits is not necessarily required to improve the attenuation characteristics in the stopband very close to the passband. In this case, for example, some or all of the inductors included in the resonator circuit may be configured as chip components and disposed outside the dielectric substrate 300.

[0092] (Piezoelectric Body and Euler Angles) As described above, by devising a circuit configuration for a filter device, it is possible to improve the attenuation characteristics in the stopband, which is on the lower frequency side of the passband. Meanwhile, in recent years, there has been an increase in communication standards in which the fractional bandwidth of the passband exceeds 10%, and there is a demand to increase the fractional bandwidth of the resonators themselves that make up the parallel arm circuit and series arm circuit. The fractional bandwidth of a resonator is expressed as 100 × (fa - fr) / fr, where fr is the resonant frequency and fa is the antiresonant frequency.

[0093] 14 and 15, in the filter device 100 of the first embodiment, lithium niobate (LiNbO ３ ) or lithium tantalate (LiTaO ３ ) as a piezoelectric body, the setting of Euler angles will be described.

[0094] (1) In the case of lithium niobate Figure 14 shows the case where lithium niobate (LiNbO ３ 14 is a diagram showing the relationship between the Euler angles and the fractional bandwidth for a vibration mode when using Euler angles (φ, θ, ψ). In FIG. 14, the horizontal axis shows the value of θ among the Euler angles (φ, θ, ψ), and the vertical axis shows the fractional bandwidth. Note that both φ and ψ are 0°.

[0095] 14, the solid line LN50 indicates the change in the fractional bandwidth when the main vibration mode is thickness-extensional vibration of the piezoelectric body, and the dashed line LN51 indicates the change in the fractional bandwidth when the main vibration mode is thickness-shear vibration of the piezoelectric body.

[0096] 14, when an elastic wave resonator with thickness extensional vibration as its primary vibration mode is used (solid line LN50), a relative bandwidth of 10% or more is achieved in the region D1 where θ = 107° to 140°. However, in the region where θ = 107° to 123°, the influence of unwanted waves due to thickness shear vibration (i.e., transverse waves) becomes relatively large.

[0097] Therefore, in the case of a BAW resonator that uses lithium niobate as a piezoelectric material and utilizes a vibration mode in which thickness longitudinal vibration is the main mode, by setting the Euler angles to (φ, θ, ψ) = (0°, 123° to 140°, 0°), as in region D10 in Figure 14, it is possible to reduce the relative bandwidth of undesired transverse waves to 0.5% or less, while achieving a relative bandwidth of 10% or more.

[0098] On the other hand, when an elastic wave resonator with thickness-shear vibration as its main vibration mode is used (dashed line LN51), a large relative bandwidth of 18% or more is achieved in the region D2 where θ = 67.5° to 92°. However, in the region where θ = 79° to 92°, the influence of unwanted waves due to thickness-extensional vibration (i.e., longitudinal waves) becomes relatively large.

[0099] Therefore, in the case of a BAW resonator that uses lithium niobate as a piezoelectric material and utilizes a vibration mode in which thickness-shear vibration is the main mode, by setting the Euler angles to (φ, θ, ψ) = (0°, 67.5° to 79°, 0°), as in region D20 in Figure 14, it is possible to reduce the relative bandwidth of longitudinal waves, which are unwanted waves, to 0.5% or less, while achieving a relative bandwidth of 18% or more.

[0100] (2) In the case of lithium tantalate Figure 15 shows the case where lithium tantalate (LiTaO ３ 15 is a diagram showing the relationship between the Euler angles and the fractional bandwidth for a vibration mode when using Euler angles (φ, θ, ψ). In FIG. 15, the horizontal axis shows the value of θ among the Euler angles (φ, θ, ψ), and the vertical axis shows the fractional bandwidth. Note that both φ and ψ are 0°.

[0101] In FIG. 15, the solid line LN60 indicates the case of a vibration mode in which the thickness longitudinal vibration of the piezoelectric body is the main mode, and the broken line LN61 indicates the case of a vibration mode in which the thickness shear vibration of the piezoelectric body is the main mode.

[0102] 15, when lithium tantalate is used as the piezoelectric material, the achievable relative bandwidth tends to be smaller overall than when lithium niobate is used. However, when using a vibration mode in which thickness-shear vibration is the primary mode (dashed line LN61), a relative bandwidth of 6% or more can be achieved in the region D3 where θ = 66° to 95°. However, in the region where θ = 80° to 95°, the influence of spurious waves due to thickness-extensional vibration (i.e., longitudinal waves) becomes relatively large.

[0103] Therefore, in the case of a BAW resonator that uses lithium tantalate as the piezoelectric material and utilizes a vibration mode in which thickness-shear vibration is the main mode, by setting the Euler angles to (φ, θ, ψ) = (0°, 67° to 80°, 0°), as in region D30 in Figure 15, it is possible to reduce the relative bandwidth of longitudinal waves, which are unwanted waves, to 0.5% or less, while achieving a relative bandwidth of 6% or more.

[0104] One of the "resonator circuits RC1 to RC4" in the first embodiment corresponds to the "first resonator circuit" in the present disclosure, and the other corresponds to the "second resonator circuit" in the present disclosure. Also, in the series arm circuit SA in the first embodiment, a capacitor disposed between the resonator circuits corresponding to the above-mentioned "first resonator circuit" and "second resonator circuit" corresponds to the "first capacitor" in the present disclosure.

[0105] The "series arm resonator S1" and the "series arm resonator S2" in the first embodiment correspond to the "first series arm resonator" and the "second series arm resonator," respectively, in the present disclosure. The "filter device 100C" and the "filter device 100D" in the first embodiment correspond to the "first filter device" and the "second filter device," respectively, in the present disclosure. The "output terminal T5" and the "output terminal T6" in the first embodiment correspond to the "first output terminal" and the "second output terminal," respectively, in the present disclosure.

[0106] [Embodiment 2] In the first embodiment, a configuration has been described in which an impedance element (inductor) is connected in series to at least one of the parallel arm resonators of each resonator circuit included in the parallel arm circuit. However, the impedance element is not an essential component for the features of the present disclosure, and even a configuration without the impedance element can achieve certain effects of the features of the present disclosure. Furthermore, the reduction in the number of components contributes to the miniaturization of the device.

[0107] 16 is an example of an equivalent circuit diagram of a filter device 100A according to Embodiment 2. The filter device 100A has a configuration in which the capacitors CP1 and CP3 in the filter device 100 according to Embodiment 1 shown in FIG. 2 are omitted. That is, each of the resonator circuits RC1 to RC4 in the parallel arm circuit PA is a parallel circuit of a parallel arm resonator and an inductor.

[0108] As described above, the impedance element connected in series to the parallel arm resonator can adjust the fractional bandwidth of the resonator circuit, thereby adjusting the pass characteristics and attenuation characteristics of the filter device. The necessity and type of the impedance element connected in series to the parallel arm resonator are determined appropriately depending on the required specifications of the filter device.

[0109] (Filter Characteristics) Next, the filter characteristics of the filter device 100A according to the second embodiment and a comparative example will be compared with each other using FIGS. 17 and 18. FIG.

[0110] 17, like Comparative Example 1 in Embodiment 1, the filter device of Comparative Example 4 has the same circuit configuration as the filter device 100A, but the anti-resonant frequency of the series arm resonator S1 is set higher than the pass band. In Fig. 17, solid lines LN70 and LN70A indicate the attenuation characteristics of the filter device 100A of Embodiment 2, and dashed lines LN71 and LN71A indicate the attenuation characteristics of the filter device of Comparative Example 4.

[0111] 17 , in the filter device of Comparative Example 4, the series arm resonator S1 contributes to the attenuation characteristics on the higher frequency side than the pass band, and an attenuation pole occurs near about 7.7 GHz as shown by the dashed lines LN71 and LN71A. On the other hand, in the stop band on the lower frequency side than the pass band, the attenuation is less than 30 dB around 4 GHz to 4.3 GHz.

[0112] In the filter device 100A of embodiment 2, as shown by the solid lines LN70 and LN70A, the steepness of the attenuation decreases on the higher frequency side of the pass band, but an attenuation of 30 dB or more is achieved throughout the stop band on the lower frequency side of the pass band.

[0113] In this way, also in the filter device 100A of the second embodiment, by setting the anti-resonance frequency of the series arm resonator S1 to be lower than the lower limit frequency of the desired pass band and lower than the resonant frequencies of the parallel arm resonators P1 to P4, it is possible to improve the attenuation characteristics in the stop band on the lower frequency side of the pass band compared to a typical ladder-type filter device.

[0114] 18 , similar to Comparative Example 2 in Embodiment 1, Comparative Example 5 has a configuration in which the series arm resonator S1 is deleted from the filter device 100A of Embodiment 2. In Fig. 18 , solid lines LN80 and LN80A indicate the attenuation characteristics of the filter device 100A of Embodiment 2, and dashed lines LN81 and LN81A indicate the attenuation characteristics of the filter device of Comparative Example 5.

[0115] 18 , the filter device of Comparative Example 5 does not include the series arm resonator S1, and therefore no attenuation pole due to the series arm resonator S1 is generated in the stop band on the lower frequency side of the pass band. In other words, no attenuation effect due to the attenuation pole of the series arm resonator S1 is obtained. As a result, the filter device of Comparative Example 5 has an attenuation of less than 30 dB in the stop band on the lower frequency side of the pass band around 4 GHz to 4.3 GHz.

[0116] On the other hand, in the filter device 100A of the second embodiment, as shown by the solid lines LN80 and LN80A, an attenuation of 30 dB or more can be achieved throughout the stop band on the lower frequency side of the pass band.

[0117] As described above, even when an impedance element is not connected to the parallel arm resonator of each resonator circuit included in the parallel arm circuit, by setting the antiresonant frequency of the series arm resonator to be lower than the lower limit frequency of the desired pass band and lower than the resonant frequency of the parallel arm resonator, it is possible to improve the attenuation characteristics in the stop band on the lower frequency side of the pass band.

[0118] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the description of the above embodiments, and is intended to include all modifications within the meaning and scope of the claims.

[0119] 10 Communication device, 20 High frequency front-end circuit, 30 RFIC, 40 BBIC, 100, 100A to 100D, 100X, 100Y Filter device, 105 Diplexer, 110A to 110D, 120 Matching device, 115, 115A, 115B Functional layer, 130 Support substrate, 150 Intermediate layer, 160, 160A Reflection layer, 161, 161A High acoustic velocity film, 162, 162A Low acoustic velocity film, 170 Piezoelectric film, 171, 171A Upper surface, 172A Lower surface, 180 IDT electrode, 185 to 187 Electrode, 190 Cavity, 200, 200A, 200B Acoustic wave device, 300 Dielectric substrate, 350 Molding resin, ANT Antenna, C13, C14, C24, CP, CP1, CP3, CS12, CS23, CS34 capacitors, GND ground potential, ID inductor, LNA1, LNA2 low noise amplifier, LP1 to LP4, LP41 inductor, M12, M23, M34 magnetic coupling, N1 to N4 connection node, P1 to P4 parallel arm resonator, PA parallel arm circuit, PA1, PA2 power amplifier, RC1 to RC4 resonator circuit, S1, S2 series arm resonator, SA series arm circuit, SIG signal line, SW1 switch, T1 to T3, TA terminals, T4, Tin input terminal, T5, T6, Tout output terminal.

Claims

1. A filter device configured to pass signals in a predetermined frequency band, comprising: an input terminal; an output terminal; a series arm circuit connected between the input terminal and the output terminal; and a parallel arm circuit including a first resonator circuit and a second resonator circuit connected in parallel between the series arm circuit and a ground potential, wherein the series arm circuit includes a first series arm resonator and a first capacitor connected in series between the input terminal and the output terminal, the first resonator circuit includes a first parallel arm resonator and a first inductor, each connected between one end of the first capacitor and the ground potential, the second resonator circuit includes a second parallel arm resonator and a second inductor, each connected between the other end of the first capacitor and the ground potential, each of the first series arm resonator, the first parallel arm resonator and the second parallel arm resonator is an acoustic wave resonator, and the anti-resonance frequency of the first series arm resonator and the resonant frequency of each parallel arm resonator are lower than a lower limit frequency of the predetermined frequency band.

2. The filter device according to claim 1, wherein the first series arm resonator is connected to the output terminal.

3. The filter device according to claim 2, wherein the series arm circuit further includes a second series arm resonator connected to the input terminal.

4. The filter device according to claim 1, wherein the first series arm resonator is connected to the input terminal.

5. A filter device according to any one of claims 1 to 4, wherein the anti-resonance frequency of the first series arm resonator is lower than the resonant frequency of at least one of the first parallel arm resonator and the second parallel arm resonator.

6. A filter device according to any one of claims 1 to 5, wherein at least one of the first resonator circuit and the second resonator circuit further includes an impedance element connected between the parallel arm resonator and the ground potential.

7. A filter device according to any one of claims 1 to 6, further comprising: a dielectric substrate formed of a dielectric material and on which the first inductor and the second inductor are arranged; and an acoustic wave device mounted on the dielectric substrate and having a piezoelectric body on which each resonator is arranged.

8. A filter device according to any one of claims 1 to 7, wherein each resonator is a BAW (Bulk Acoustic Wave) resonator.

9. The BAW resonator is disposed on a piezoelectric body, and the piezoelectric body is made of lithium niobate (LiNbO ３ ) or lithium tantalate (LiTaO ３ 9. The filter device of claim 8, wherein 10. The piezoelectric material is LiNbO ３ Each resonator is made of LiNbO ３ 10. The filter device according to claim 9, wherein the filter vibrates in thickness extensional vibration as a main mode, and the Euler angles are (φ, θ, ψ) = (0°, 107° to 140°, 0°).

11. The piezoelectric material is LiNbO ３ Each resonator is made of LiNbO ３ 10. The filter device according to claim 9, wherein the filter vibrates in a thickness-shear vibration mode of the above formula (1), and the Euler angles are (φ, θ, ψ) = (0°, 67.5° to 92°, 0°).

12. The piezoelectric material is LiTaO ３ Each resonator is made of LiTaO ３ 10. The filter device according to claim 9, wherein the filter vibrates in a thickness-shear vibration mode of the above formula (1), and the Euler angles are (φ, θ, ψ) = (0°, 66° to 95°, 0°).

13. A filter device according to any one of claims 1 to 7, wherein each resonator is a SAW (Surface Acoustic Wave) resonator.

14. A high-frequency front-end circuit comprising a filter device according to any one of claims 1 to 13.

15. A filter device comprising: an input terminal; first and second output terminals; a first filter device connected between the input terminal and the first output terminal and configured to pass signals in a first frequency band; and a second filter device connected between the input terminal and the second output terminal and configured to pass signals in a second frequency band different from the first frequency band, wherein each of the first filter device and the second filter device includes: a series arm circuit connected between the input terminal and the corresponding output terminal; and a parallel arm circuit including a first resonator circuit and a second resonator circuit connected in parallel between the series arm circuit and a ground potential, the series arm circuit including a first series arm resonator and a first capacitor connected in series between the input terminal and the output terminal, the first resonator circuit including a first parallel arm resonator and a first inductor each connected between one end of the first capacitor and the ground potential, and the second resonator circuit including a second parallel arm resonator and a second inductor each connected between the other end of the first capacitor and the ground potential, each of the first series arm resonator, the first parallel arm resonator, and the second parallel arm resonator is an acoustic wave resonator; and an antiresonant frequency of the first series arm resonator and a resonant frequency of each parallel arm resonator are lower than a lower limit frequency of a corresponding frequency band.

16. A high frequency front end circuit comprising the multiplexer of claim 15.

17. A communication device comprising the high-frequency front-end circuit according to claim 14 or 16.

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