Composite filter device

Abstract

The present invention provides a composite filter device having a first filter and a second filter, wherein the composite filter device can further reduce the insertion loss in the second filter having a relatively high passband. The composite filter device (1) comprises: a piezoelectric substrate (22) which is a LiNbO3 substrate; a first filter (2) which is formed on the piezoelectric substrate (22) and includes a plurality of resonators each consisting of an elastic wave resonator; and a second filter (3) wherein one end of the second filter (3) and the first filter (2) are commonly connected to each other, the passband of the second filter (3) is in a frequency band higher than the passband of the first filter (2), and the bulk wave radiation frequency of all the resonators constituting the first filter (2) is higher than the passband of the second filter (3).

Description

Technical Field

[0001] The present invention relates to a composite filter device in which one ends of a plurality of filters are commonly connected to each other. Background Art

[0002] Composite filter devices using elastic wave filters are widely used in mobile communication devices such as smartphones. For example, in the elastic wave duplexer described in Patent Document 1 below, one end of the first filter and one end of the second filter are commonly connected to the antenna terminal. The passband of the second filter is made higher than the passband of the first filter. The first filter is composed of a ladder-type filter. The ladder-type filter has a plurality of series-arm resonators composed of elastic wave resonators and a plurality of parallel-arm resonators composed of elastic wave resonators. The body wave radiation frequency of the series-arm resonator closest to the antenna terminal among the plurality of series-arm resonators is made higher than the passband of the second filter. As a result, the insertion loss of the second filter is reduced.

[0003] Prior art literature

[0004] Patent Literature

[0005] Patent Document 1: International Publication No. 2009 / 147787 Summary of the Invention

[0006] Problems to be solved by the invention

[0007] However, even when the elastic wave duplexer according to the invention described in Patent Document 1 is used, the insertion loss in the passband of the second filter may increase.

[0008] An object of the present invention is to provide a composite filter device including a first filter and a second filter, wherein the composite filter device can further reduce the insertion loss in the second filter having a relatively high passband.

[0009] Technical solutions to problems

[0010] The composite filter device according to the present invention is a composite filter device comprising: a piezoelectric substrate, which is a LiNbO3 substrate; a first filter, which is formed on the piezoelectric substrate and includes a plurality of resonators, each of which is an elastic wave resonator; and a second filter, the second filter and the first filter being commonly connected at one end, the passband of the second filter being in a frequency band higher than the passband of the first filter, and the bulk wave radiation frequency of all the resonators constituting the first filter being higher than the passband of the second filter.

[0011] Effects of the Invention

[0012] According to the composite filter device of the present invention, the insertion loss in the passband of the second filter having a higher passband than that of the first filter can be further reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a circuit diagram of a composite filter device as one embodiment of the present invention.

[0014] Figure 2 This is a front cross-sectional view for explaining an elastic wave resonator used in one embodiment of the present invention.

[0015] Figure 3 (a) and Figure 3 (b) is a graph showing the impedance characteristics and phase characteristics of a general elastic wave resonator using rotated Y-cut X-propagation LiNbO 3 .

[0016] Figure 4 Graphs showing filter characteristics of an embodiment, a comparative example, and a single filter.

[0017] Figure 5 (a) and Figure 5 (b) is a graph showing the impedance characteristics and phase characteristics of the elastic wave resonator according to the reference example.

[0018] Figure 6 (a) and Figure 6 (b) is a graph showing the impedance characteristics and phase characteristics of the elastic wave resonator used in one embodiment of the present invention.

[0019] Figure 7 Graph showing the relationship between the film thickness of the electrode layer containing Pt of the IDT electrode, that is, the Pt film thickness, and the frequency ratio.

[0020] Figure 8 Graph showing the relationship between the film thickness of the electrode layer containing Au of the IDT electrode, that is, the Au film thickness, and the frequency ratio.

[0021] Figure 9 Graph showing the relationship between the film thickness of the electrode layer containing W of the IDT electrode, that is, the W film thickness, and the frequency ratio.

[0022] Figure 10 Graph showing the relationship between the film thickness of the electrode layer containing Ir of the IDT electrode, that is, the Ir film thickness, and the frequency ratio.

[0023] Figure 11 This is a diagram showing the relationship between the cut angle and the electromechanical coupling coefficient of unwanted waves in a Y-cut X-propagation LiNbO 3 substrate. DETAILED DESCRIPTION

[0024] Hereinafter, specific embodiments of the present invention will be described with reference to the accompanying drawings to clarify the present invention.

[0025] In addition, it should be noted that the embodiments described in this specification are merely illustrative, and some structures can be replaced or combined among different embodiments.

[0026] Figure 1 This is a circuit diagram of a composite filter device as one embodiment of the present invention.

[0027] The composite filter device 1 is used as a multiplexer in the RF stage of a smartphone, for example.

[0028] In the composite filter device 1, one end of the first filter 2 through the fourth filter 5 is commonly connected to the antenna terminal ANT. Furthermore, in the composite filter device 1 according to the present invention, as long as the first filter 2 and the second filter 3 are included, the number of filters connected to the antenna terminal ANT is not particularly limited. Specifically, the number of filters may be two, three, or five or more.

[0029] The first filter 2 to the fourth filter 5 are bandpass filters. The passband of the second filter 3 is higher than the passband of the first filter 2.

[0030] The first filter 2 is a ladder-type filter having multiple elastic wave resonators. For example, the first filter 2 is a transmission filter having a transmission terminal 10. Multiple series-arm resonators S11 to S14 are provided in the series arm connecting the transmission terminal 10 and the antenna terminal ANT. Parallel-arm resonators P11 to P13 are provided in the parallel arms connecting the series arm to the ground potential. The series-arm resonators S11 to S14 and the parallel-arm resonators P11 to P13 are each composed of elastic wave resonators.

[0031] Figure 2 This is a front cross-sectional view for explaining an elastic wave resonator used in one embodiment of the present invention.

[0032] The elastic wave resonator 21 includes a piezoelectric substrate 22 which is a LiNbO 3 substrate.

[0033] An IDT electrode 24 and reflectors 25 and 26 are provided on the piezoelectric substrate 22. Thus, a one-port elastic wave resonator is formed.

[0034] The dielectric film 27 is stacked so as to cover the IDT electrode 24 and the reflectors 25 and 26. The dielectric film 27 includes silicon oxide. However, the dielectric film 27 may include a dielectric other than silicon oxide, for example, SiO x N y , glass, germanium oxide, etc.

[0035] Figure 1 The series arm resonators S11 to S14 and the parallel arm resonators P11 to P13 in the illustrated first filter 2 are formed of such elastic wave resonators 21 .

[0036] The second filter 3 is a bandpass filter. The structure of the second filter 3 is not particularly limited, but the frequency of the passband is higher than that of the first filter 2 .

[0037] Furthermore, the passbands of the third filter 4 and the fourth filter 5 are in a frequency band different from the passbands of the first filter 2 and the second filter 3. The passbands of the third filter 4 and the fourth filter 5 are also different from each other.

[0038] The composite filter device 1 is characterized in that the bulk wave radiation frequency of all resonators constituting the first filter 2, namely, the series-arm resonators S11 to S14 and the parallel-arm resonators P11 to P13, is set to a frequency higher than the passband of the second filter 3. This further reduces the insertion loss within the passband of the second filter 3. This will be described in more detail using the following embodiments and comparative examples.

[0039] As an embodiment, the following composite filter device is constructed: the first filter 2 is a transmission filter of Band 1 used in LTE, and the second filter 3 is a reception filter of Band 1.

[0040] The passband is as follows.

[0041] 1st filter 2: Band 1 transmit filter, passband 1920MHz to 1980MHz

[0042] Second filter 3: Band 1 receive filter, passband 2110MHz to 2170MHz

[0043] In the embodiment, the bulk wave radiation frequencies of all the resonators used, ie, the series arm resonators S11 to S14 and the parallel arm resonators P11 to P13 are set to be higher than the passband of the second filter 3 .

[0044] The design parameters of the resonator of the embodiment are set as follows.

[0045] Piezoelectric substrate 22: 17° Y-cut X-propagation LiNbO3 substrate

[0046] Series arm resonators S11 to S14: Wavelength λ = 1.440 μm, determined by the electrode finger pitch, and bulk wave radiation frequency position at 2799 MHz

[0047] Parallel arm resonators P11 to P13: Wavelength λ = 1.605 μm, determined by the electrode finger pitch, and body wave radiation frequency position is 2511 MHz

[0048] In the series arm resonators S11 to S14 and the parallel arm resonators P11 to P13 described above, the main response used is a response based on Love waves.

[0049] The bulk wave radiation frequency is a frequency equivalent to the acoustic velocity of the slow shear bulk wave in the piezoelectric substrate 22. When the wavelength, determined by the electrode finger pitch of the IDT electrode, is λ and the acoustic velocity of the slow shear bulk wave is Vb, the bulk wave radiation frequency is expressed as Vb / λ. In the case of the rotated Y-cut X-propagation LiNbO3 used in this embodiment, Vb is approximately 4030 m / s.

[0050] For comparison purposes, in the composite filter device of the comparative example, the bulk wave radiation frequency of the series arm resonator S14 closest to the antenna terminal ANT is set at a frequency position higher than the passband of the second filter 3. The bulk wave radiation frequencies of the other resonators, namely, the series arm resonators S11 to S13 and the parallel arm resonators P11 to P13, are set to frequencies within or below the passband of the second filter 3.

[0051] The design parameters of each resonator of the comparative example are as follows.

[0052] Piezoelectric substrate: -6° Y-cut X-propagation LiNbO3 substrate

[0053] Series arm resonator S14: wavelength λ = 1.832 μm, determined by the electrode finger pitch, and bulk wave radiation frequency position at 2200 MHz

[0054] Series arm resonators S11 to S13: Wavelength λ = 1.883 μm, determined by the electrode finger pitch, and bulk wave radiation frequency position at 2140 MHz

[0055] Parallel arm resonators P11 to P13: Wavelength λ = 1.965 μm, determined by the electrode finger pitch, and body wave radiation frequency position is 2051 MHz

[0056] exist Figure 4 The filter characteristics of the above-mentioned embodiment and comparative example are shown. The solid line shows the filter characteristics of the embodiment, and the dashed line shows the filter characteristics of the comparative example. Furthermore, the dashed line also shows the filter characteristics of the second filter 3 alone. Here, the filter characteristics of the second filter 3 alone are measured without the first filter 2 connected to the antenna terminal.

[0057] according to Figure 4It is clear that the insertion loss within the passband of the second filter in the composite filter device of the comparative example is greater than the insertion loss within the passband of the second filter in the composite filter device 1 of the embodiment. Furthermore, it is clear that the insertion loss within the passband of the second filter in the composite filter device 1 of the embodiment is approximately equal to the insertion loss within the passband of the second filter 3 alone. This is believed to be due to the following reasons.

[0058] Figure 3 (a) and Figure 3 (b) shows the impedance characteristics and phase characteristics of a general elastic wave resonator using Love waves formed on a rotating Y-cut X-propagating LiNbO3 substrate. Figure 3 (a) and Figure 3 In (b), the horizontal axis shows the speed of sound, which is the product of frequency and wavelength.

[0059] Like in Figure 3 (a) and Figure 3 As shown by arrow A in (b), in the frequency band higher than the anti-resonance frequency, disturbances occur in the impedance characteristics and phase characteristics. This disturbance is caused by the radiation of the body wave. Figure 3 Arrow A indicates the position of the acoustic velocity Vb corresponding to the frequency of the body wave radiation, which is approximately 4030 m / s, which is the acoustic velocity of the slow shear body wave of the rotating Y-cut X-propagating LiNbO3. From the position indicated by arrow A to the high acoustic velocity side, a response due to the radiation of the body wave appears. This is believed to be caused by the loss caused by the radiation of the elastic wave into the piezoelectric substrate. In other words, in a frequency band higher than the frequency corresponding to the acoustic velocity of the slow shear body wave of the piezoelectric substrate, the elastic wave cannot be confined to the surface of the piezoelectric substrate and its vicinity, but is radiated into the piezoelectric substrate. This is believed to be caused by the loss generated thereby. The frequency corresponding to the acoustic velocity Vb of the slow shear body wave of the piezoelectric substrate, that is, Vb / λ, is called the body wave radiation frequency as mentioned above. When the material, film thickness, and film thickness of the IDT electrode change, the resonant frequency and anti-resonant frequency will change, but the body wave radiation frequency is determined only by the wavelength λ determined by the spacing between the piezoelectric substrate and the electrode fingers, and therefore does not change. That is, the interval between the resonant frequency, the antiresonant frequency, and the bulk wave radiation frequency changes depending on the material and film thickness of the IDT electrode.

[0060] When a signal in the passband of the second filter 3 is input from the antenna terminal ANT, which serves as a common terminal, a portion of the signal also reaches the resonators that constitute the first filter 2. If the bulk wave radiation frequency of the series-arm resonators S11 to S14 and the parallel-arm resonators P11 to P13 that constitute the first filter 2 falls within the passband of the second filter 3 or falls below the passband of the second filter 3, a portion of the signal in the passband of the second filter 3 leaks into the resonators of the first filter 2 as bulk waves. This deteriorates the insertion loss in the passband of the second filter 3.

[0061] In the comparative example, the bulk wave radiation frequency of the series arm resonator S14 closest to the antenna terminal ANT is higher than the 2110 MHz to 2170 MHz passband of the second filter 3. Therefore, no loss occurs within the passband of the second filter 3 due to bulk wave radiation from the series arm resonator S14. However, the bulk wave radiation frequencies of the other series arm resonators S11 to S13 and the parallel arm resonators P11 to P13 are within the passband of the second filter 3 or in a frequency band lower than the passband. This is believed to result in a deterioration in insertion loss as described above.

[0062] In contrast, in the above embodiment, the bulk wave radiation frequencies of all resonators, namely, the series-arm resonators S11 to S14 and the parallel-arm resonators P11 to P13, are in a frequency band higher than the passband of the second filter 3. Therefore, the insertion loss within the passband of the second filter 3 can be reduced.

[0063] Furthermore, in the comparative example, the wavelengths and resonant frequencies of the series arm resonator S14 closest to the antenna end and the other series arm resonators S11 to S13 sometimes differ significantly. In other words, the resonant frequency of the series arm resonator S14 sometimes falls higher than the passband of the first filter, which can easily degrade the characteristics of the first filter. In contrast, in the embodiment, as described later, the ratio of the resonant frequency to the bulk wave radiation frequency is increased. This allows the resonant frequencies of all series arm resonators to be close to each other and arranged within the passband of the first filter 2. Consequently, the characteristics of the first filter 2 are less likely to degrade.

[0064] Furthermore, the first filter 2 and the second filter 3 may be formed on the same piezoelectric substrate, or the second filter 3 may be formed using a piezoelectric substrate different from the piezoelectric substrate forming the first filter 2. Furthermore, the second filter 3 may be a filter that does not use a piezoelectric substrate, or may be a filter that does not use elastic waves, such as an LC filter.

[0065] In addition, you can also Figure 1 As shown, in addition to the first filter 2 and the second filter 3, at least one other filter, such as a third filter 4 and a fourth filter 5, is also commonly connected to the first and second filters 2 and 3. In this case, preferably, at least three filters are commonly connected in the composite filter device 1, and the passband of the second filter 3 is located at the highest frequency side of the passbands of all the filters. In this case, the insertion loss degradation caused by the bulk wave radiation of the first filter 2 is less likely to occur in filters other than the first and second filters 2 and 3.

[0066] In addition, in addition to the first filter 2 and the second filter 3, at least one other filter may be connected to the first filter 2 and the second filter 3 in common. In the structure in which at least three filters are connected in common, the passband of the first filter 2 is the lowest among the passbands of all filters.

[0067] Preferably, in the composite filter device according to the present invention, in the resonator having the lowest resonant frequency among the multiple resonators constituting the first filter, the ratio of the bulk wave radiation frequency to the resonant frequency is greater than 1.4. More preferably, in the composite filter device according to the present invention, in all the resonators constituting the first filter, the ratio of the bulk wave radiation frequency to the resonant frequency is greater than 1.4. In addition, it is preferable that the ratio of the bulk wave radiation frequency to the resonant frequency is less than 2.0. This is because, as will be described later, although the ratio of the bulk wave radiation frequency to the resonant frequency increases as the thickness of the main electrode increases, if the thickness of the main electrode becomes too thick, it becomes difficult to form the electrode and the dielectric film.

[0068] If the ratio of the bulk wave radiation frequency to the resonant frequency is 1.4 or greater, the present invention can be applied to various combinations of the first filter and the second filter. This will be described below.

[0069] In recent years, carrier aggregation (CA) has been introduced. In carrier aggregation, bandpass filters for different frequency bands are connected together. In conventional duplexers, receive filters for the same frequency band are connected together. When bandpass filters for different frequency bands are connected together, there are combinations where the passband frequencies are quite far apart. Table 1 below shows representative examples of frequency band combinations used for carrier aggregation and the receive passbands for each frequency band.

[0070] [Table 1]

[0071]

[0072] In conventional duplexers, the frequency band with the largest separation between the transmit filter passband and the receive filter passband is Band 4. In Band 4, the ratio of the center frequency of the receive filter passband to the center frequency of the transmit filter passband is 1.23. In contrast, in carrier aggregation, two bandpass filters with a larger frequency ratio are sometimes connected to the antenna terminals. Therefore, for example, in the elastic wave resonator used for the first filter, it is preferable to increase the ratio of the bulk wave radiation frequency to the resonant frequency.

[0073] For example, when the ratio of the body wave radiation frequency to the resonant frequency is increased to 1.4, it can be applied to a combination of bandpass filters whose ratio of the center frequency of the passband is 1.4 or less. For example, it can cope with a combination of receiving filters such as Band1 and Band7, Band2 and Band7, or Band39 and Band41. In addition, when the ratio of the above-mentioned body wave radiation frequency to the resonant frequency of the main response is increased to 1.5, it can cope with a combination of frequency bands whose center frequency ratio is 1.5 or less. For example, it can cope with a combination of receiving filters such as Band11 and Band1, Band3 and Band38, Band3 and Band7. Therefore, it is preferable that the ratio of the body wave radiation frequency to the resonant frequency of the main response is 1.4 or more. Refer to Figure 5 as well as Figure 6 This will be described in more detail.

[0074] Figure 5 (a) and Figure 5 (b) shows the impedance characteristics and phase characteristics of the Love wave elastic wave resonator using the following design parameters.

[0075] Piezoelectric substrate: -6° Y-cut X-propagation LiNbO3 substrate

[0076] IDT electrode: Pt-containing electrode, Pt film thickness = 0.025λ

[0077] SiO2 film thickness: 0.30λ

[0078] The wavelength λ was set to 2.375 μm. The velocity of sound of the slow shear bulk wave of the piezoelectric substrate was 4030 m / s, which was the value of LiNbO 3 propagating in the rotation Y-cut X direction.

[0079] according to Figure 5 As can be seen from (a), the resonant frequency of the main response utilizing Love waves is 1486 MHz. The bulk wave radiation frequency is 1697 MHz. Therefore, the frequency ratio, which is the ratio of the bulk wave radiation frequency to the resonant frequency, is 1.142. In this case, if the first filter is a Band 11 reception filter and the second filter is a Band 1 reception filter, and the elastic wave resonator is used as an elastic wave resonator for the Band 11 reception filter, the bulk wave radiation frequency exists in a frequency band lower than the passband of Band 1. Therefore, the loss in the second filter, which is a Band 1 reception filter, may deteriorate.

[0080] On the other hand, an elastic wave resonator having a ratio of the bulk wave radiation frequency to the resonant frequency of 1.515 was manufactured using the following design parameters.

[0081] Piezoelectric substrate: 15° Y-cut X-propagation LiNbO3 substrate

[0082] IDT electrode: Pt-containing electrode, Pt film thickness = 0.095λ

[0083] SiO2 film thickness: 0.30λ

[0084] The impedance characteristics and phase characteristics of the elastic wave resonator are shown in FIG. Figure 6 (a) and Figure 6 (b). The Love wave wavelength λ is set to 1.790 μm. The resonant frequency of the Love wave response occurs at 1486 MHz. The bulk wave radiation frequency is 2252 MHz. Therefore, the frequency ratio, which is the ratio of the bulk wave radiation frequency to the resonant frequency, is 1.515.

[0085] If the elastic wave resonator described above is used as the elastic wave resonator of the Band 11 reception filter, the bulk wave radiation frequency will exist in a band higher than the passband of Band 1. Therefore, the loss in the Band 1 reception filter can be significantly reduced, and good filter characteristics can be obtained.

[0086] As described above, the ratio of the bulk wave radiation frequency to the resonant frequency of the main response of the elastic wave resonator is preferably 1.4 or greater, and more preferably 1.5 or greater.

[0087] Furthermore, to increase the frequency ratio, the IDT electrode can be made of a high-density metal and its thickness can be increased. Preferred examples of such electrode materials include Pt, Au, W, Ir, Ta, Mo, and Cu. In this case, the IDT electrode preferably contains these high-density metals. However, as long as the IDT electrode uses these metals as the primary electrode material, electrode layers containing other metals may also be stacked.

[0088] Here, the main electrode material refers to an electrode material that plays a major role in exciting elastic waves used in the IDT electrode, and refers to an electrode material that accounts for 50% by weight or more of the entire metal constituting the IDT electrode.

[0089] When Pt, Au, W, or Ir films are used as the material of the IDT electrode, the relationship between the Pt film thickness, the Au film thickness, the W film thickness, or the Ir film thickness and the above-mentioned frequency ratio is shown in FIG. Figures 7 to 10 .

[0090] according to Figure 7 It is clear that when Pt is used as the main electrode material, the frequency ratio can be made greater than 1.4 when the Pt film thickness is greater than 0.072λ. In addition, it is known that the thickness at which the frequency ratio becomes greater than 1.5 is greater than 0.092λ. Figure 8It is clear that when Au is used as the main electrode material, the frequency ratio becomes 1.4 or more when the Au film thickness is 0.072λ or more, and the frequency ratio becomes 1.5 or more when the thickness is 0.090λ or more. Figure 9 It is clear that when the main electrode material is W, the thickness at which the frequency ratio becomes 1.4 or greater is 0.088λ or greater, and the thickness at which the frequency ratio becomes 1.5 or greater is 0.116λ or greater. Figure 10 It is clear that when the main electrode material is Ir, the thickness at which the frequency ratio becomes 1.4 or greater is 0.076λ or greater, and the thickness at which the frequency ratio becomes 1.5 or greater is 0.10λ or greater.

[0091] As described above, it is understood that by correcting the film thickness according to the type of main electrode material, the frequency ratio can be set to 1.4 or more, or 1.5 or more.

[0092] Figure 11 The relationship between the cut angle and the electromechanical coupling coefficient of unwanted waves in a Y-cut, X-propagation LiNbO3 substrate is shown. In the elastic wave resonator utilizing Love waves, spurious waves caused by Rayleigh wave excitation may occur near the resonant frequency-antiresonant frequency band. These unwanted waves are spurious waves caused by these Rayleigh waves.

[0093] according to Figure 11 It is clear that changing the cut angle changes the electromechanical coupling coefficient of Rayleigh waves. Therefore, it is preferable to set the cut angle within a range of 3° to 26°. In this case, the electromechanical coupling coefficient of unwanted waves can be reduced to 0.1% or less. Using a LiNbO3 substrate with this cut angle can suppress degradation of the filter characteristics of the first filter.

[0094] In this specification, the orientation of the LiNbO3 used in the piezoelectric substrate is described as θ° rotated Y-cut X-propagation. If expressed using Euler angles, it becomes (0°, θ-90°, 0°). Here, the first and third Euler angles, which are 0°, can also be within the range of -5° to 5°. Here, even when the crystal axes of the LiNbO3 used in the piezoelectric substrate are oriented in opposite directions, the electrical characteristics become the same. Therefore, a piezoelectric substrate using LiNbO3 with Euler angles of (0°, θ+90°, 0°) can also be used. In this case, if notated using cutting angles, it becomes (θ-180)° rotated Y-cut X-propagation.

[0095] In addition, although Figure 1 In the embodiment, the first filter 2 includes series arm resonators S11 to S14 and parallel arm resonators P11 to P13 , but the number of resonators in the ladder-type filter is not limited thereto.

[0096] Description of Reference Numerals

[0097] 1: Composite filter device;

[0098] 2~5: 1st filter to 4th filter;

[0099] 10: Sending terminal;

[0100] 21: elastic wave resonator;

[0101] 22: piezoelectric substrate;

[0102] 24: IDT electrode;

[0103] 25, 26: reflector;

[0104] 27: dielectric film;

[0105] P11~P13: parallel arm resonator;

[0106] S11~S14: Series arm resonator.

Claims

1. A composite filter device comprising: The piezoelectric substrate is a LiNbO3 substrate; a first filter formed on the piezoelectric substrate and including a plurality of resonators each consisting of an elastic wave resonator; and a second filter, wherein one end of the second filter and the first filter are commonly connected to each other; The passband of the second filter is in a higher frequency band than the passband of the first filter. The bulk wave radiation frequencies of all the resonators constituting the first filter are higher than the passband of the second filter.

2. The composite filter device according to claim 1, wherein: In a resonator having the lowest resonance frequency among the plurality of resonators constituting the first filter, a ratio of a bulk wave radiation frequency to the resonance frequency is 1.4 or greater.

3. The composite filter device according to claim 1 or 2, wherein: In all the resonators constituting the first filter, a ratio of a bulk wave radiation frequency to a resonance frequency is 1.4 or greater.

4. The composite filter device according to claim 1 or 2, wherein: All the resonators constituting the first filter have the same IDT electrode provided on the piezoelectric substrate. The IDT electrode has multiple electrode fingers, and the wavelength determined by the electrode finger spacing is λ. The main electrode material of the IDT electrode contains a metal selected from the group consisting of Pt, Au, W, Ir, Ta, Mo, and Cu as a main component.

5. The composite filter device according to claim 4, wherein: The first filter includes a dielectric film provided to cover the IDT electrode. The composite filter device according to claim 4 , wherein: The main electrode material of the IDT electrode is Pt, and the thickness of the electrode layer containing the Pt is greater than or equal to 0.072λ.

7. The composite filter device according to claim 4, wherein: The main electrode material of the IDT electrode is Au, and the thickness of the electrode layer containing the Au is greater than or equal to 0.072λ.

8. The composite filter device according to claim 4, wherein: The main electrode material of the IDT electrode is W, and the thickness of the electrode layer containing W is greater than or equal to 0.088λ.

9. The composite filter device according to claim 4, wherein: The main electrode material of the IDT electrode is Ir, and the thickness of the electrode layer containing Ir is greater than or equal to 0.076λ.

10. The composite filter device according to claim 1 or 2, wherein: The piezoelectric substrate is a LiNbO3 substrate with a 3° to 26° rotation, Y-cut, and X-propagation.

11. The composite filter device according to claim 1 or 2, wherein: In addition to the first filter and the second filter, at least one other filter is commonly connected to the first filter and the second filter, and the passband of the second filter is located on the highest frequency side among the passbands of all filters.

12. The composite filter device according to claim 1 or 2, wherein: In addition to the first filter and the second filter, at least one other filter is commonly connected to the first filter and the second filter, and the passband of the first filter is the lowest among the passbands of all the filters.

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