Ladder filter

Abstract

Provided is a ladder filter capable of achieving both narrowness of a passband and improvement of reflection characteristics. A ladder filter (1) is provided with series arm resonators (S1, S3, S4) and parallel arm resonators (P1, P2, P4). The series arm resonators (S1, S3, S4) are disposed on the series arm (4). The parallel arm resonators (P1, P2, P4) are disposed on the plurality of parallel arms (5). The resonance frequency of at least one of the series arm resonators (S1, S3, S4) is lower than the corresponding frequency band. The anti-resonant frequency of at least one of the parallel arm resonators (P1, P2, P4) is higher than the corresponding frequency band. The ladder filter (1) is further provided with one or more capacitors (C1, C2) disposed on at least one of the plurality of arms including the series arm (4) and the plurality of parallel arms (5). The capacitors (C1, C2) are not disposed on the parallel arms (51, 52, 54) in which the parallel arm resonators (P1, P2, P4) are disposed among the plurality of parallel arms (5).

Description

Technical Field

[0001] The present invention relates to a ladder filter, and more particularly, to a ladder filter including series arm resonators and parallel arm resonators. Background Art

[0002] The ladder surface acoustic wave filter described in Patent Document 1 includes a plurality of resonators (series arm resonators) arranged in series arms and a plurality of resonators (parallel arm resonators) arranged in a plurality of parallel arms. In this ladder surface acoustic wave filter, by making the anti-resonant frequency of the parallel arm resonator coincide with the resonant frequency of the series arm resonator, a filter having a passband centered on the coincident frequency is formed.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2001-156586 Summary of the Invention

[0006] Problems to be Solved by the Invention

[0007] However, in the case of manufacturing a filter having an attenuation characteristic near a passband through which a signal included in a corresponding frequency band passes, using a resonator in which the frequency interval between the resonant frequency and the anti-resonant frequency is wider than the corresponding frequency band, in a filter in which the anti-resonant frequency of the parallel arm resonator coincides with the resonant frequency of the series arm resonator as in Patent Document 1, it is sometimes impossible to ensure the attenuation amount near the passband. That is, the passband of the filter is sometimes too wide compared to the corresponding frequency band. In addition, the above-mentioned "corresponding frequency band" is the communication frequency band of the signal passing through the filter. In this case, if the resonant frequency of the parallel arm resonator is increased and the resonant frequency of the series arm resonator is decreased in order to ensure the attenuation amount near the passband, although the above-mentioned attenuation characteristic is improved, the impedance characteristic of the passband of the filter becomes inductive and the reflection characteristic of the filter deteriorates.

[0008] An object of the present invention is to provide a ladder filter capable of achieving both narrowbanding of the passband and improvement of the reflection characteristic.

[0009] Technical Means for Solving the Problems

[0010] One aspect of the present invention relates to a ladder filter having one or more series-arm resonators and one or more shunt-arm resonators. The one or more series-arm resonators are disposed in a series arm connecting a first terminal and a second terminal. The one or more shunt-arm resonators are disposed in a plurality of shunt arms connecting the series arm and ground. The frequency intervals between the resonance frequencies and the anti-resonance frequencies of the one or more series-arm resonators and the one or more shunt-arm resonators are wider than the corresponding frequency band. The resonance frequency of at least one series-arm resonator among the one or more series-arm resonators is lower than the corresponding frequency band. The anti-resonance frequency of at least one shunt-arm resonator among the one or more shunt-arm resonators is higher than the corresponding frequency band. The ladder filter further includes one or more capacitors disposed in at least one of the plurality of arms including the series arm and the plurality of shunt arms. The one or more capacitors are not disposed in the shunt arms in which the one or more shunt-arm resonators are disposed among the plurality of shunt arms.

[0011] One aspect of the present invention relates to a ladder filter having a series arm connecting a first terminal and a second terminal and a plurality of shunt arms connected between the series arm and ground. The connection points of the series arm with the plurality of shunt arms are defined as a plurality of branch points. The series arm has a plurality of series-arm sections. The plurality of series-arm sections include: a section between the branch point adjacent to the first terminal among the plurality of branch points in the series arm and the first terminal; a section between the branch point adjacent to the second terminal among the plurality of branch points in the series arm and the second terminal; and a section between two adjacent branch points among the plurality of branch points. The ladder filter includes: one or more capacitors disposed in at least one of the plurality of series-arm sections and the plurality of shunt arms; and resonators disposed in the remainder of the plurality of series-arm sections and the plurality of shunt arms. The frequency interval between the resonance frequency and the anti-resonance frequency of the resonators is wider than the corresponding frequency band. The resonance frequency of at least one series-arm resonator among the one or more series-arm resonators disposed in the series arm is lower than the corresponding frequency band. The anti-resonance frequency of at least one shunt-arm resonator among the one or more shunt-arm resonators disposed in the shunt arm is higher than the corresponding frequency band.

[0012] Advantages of the Invention

[0013] One aspect of the present invention relates to a ladder filter having the advantage of being able to balance the narrowbanding of the passband and the improvement of the reflection characteristics. Description of the Drawings

[0014] Figure 1 is a circuit diagram of the ladder filter according to the embodiment.

[0015] Figure 2It is a top view of the same trapezoidal filter as above.

[0016] Figure 3 It is a circuit diagram of the trapezoidal filters related to Comparative Examples 1 and 2.

[0017] Figure 4 It is a graph showing the impedance characteristics of Comparative Example 1.

[0018] Figure 5 It is a graph showing the impedance characteristics of Comparative Example 2.

[0019] Figure 6 It is a graph showing the impedance characteristics of the trapezoidal filter according to the embodiment.

[0020] Figure 7 It is a graph showing the attenuation characteristics of the trapezoidal filters related to Comparative Example 1, Comparative Example 2, and the embodiment respectively. Detailed Embodiment

[0021] (Embodiment)

[0022] (1) Outline of the Trapezoidal Filter

[0023] Regarding the trapezoidal filter 1 according to the embodiment, refer to Figures 1 to 7 for description.

[0024] The trapezoidal filter 1 is, for example, a trapezoidal surface acoustic wave filter. For example, the trapezoidal filter 1 is used by the high-frequency module of a communication device. The trapezoidal filter 1 can be used as a receive filter, a transmit filter, or a transceiver filter provided in the receive path or the transmit path within the high-frequency module and passing the frequency components corresponding to the corresponding frequency band from the high-frequency signal flowing in the receive path or the transmit path. In addition, the so-called "corresponding frequency band" is the communication frequency band of the signal passing through the trapezoidal filter 1 and is a frequency band specified by the communication standard. In the present embodiment, for example, it is Band30Rx (2350 - 2360 MHz).

[0025] The trapezoidal filter 1 assumes, for example, a relatively narrow frequency band (such as Band30Rx) as the corresponding frequency band. However, the corresponding frequency band of the trapezoidal filter 1 is not limited to a relatively narrow frequency band. In addition, the trapezoidal filter 1 is configured by narrowing the passband in accordance with the corresponding frequency band.

[0026] In the following description, as an example, the case where the trapezoidal filter 1 is a receive filter is assumed for description.

[0027] (2) Circuit Structure of the Trapezoidal Filter

[0028] As Figure 1 shown, the trapezoidal filter 1 includes a first terminal 2, a second terminal 3, a series arm 4, and a plurality of (inFigure 1 In the example of, there are 4) parallel arms 5 (51 to 54), one or more (in Figure 1 the example of, there are 2) capacitors (capacitor C1 (first capacitor) and capacitor C2 (second capacitor)), one or more (in Figure 1 the example of, there are 3) series arm resonators S1, S3, S4, and one or more (in Figure 1 the example of, there are 3) parallel arm resonators P1, P2, P4.

[0029] The first terminal 2 is, for example, an input terminal for inputting a high-frequency signal. In a state where the ladder filter 1 is mounted on a high-frequency module, the first terminal 2 is connected, for example, to a signal path to an antenna terminal. The second terminal 3 is, for example, an output terminal for outputting a high-frequency signal that has passed through the ladder filter 1. In a state where the ladder filter 1 is mounted on a high-frequency module, the second terminal 3 is connected, for example, to a signal path to an output terminal of the high-frequency module.

[0030] The series arm 4 is a circuit connecting the first terminal 2 and the second terminal. The plurality of parallel arms 5 (51 to 54) are circuits connecting the series arm 4 and the ground. The plurality of parallel arms 51 to 54 correspond one-to-one with the plurality of branch points N1 to N4 of the series arm 4, and are connected between the corresponding branch points and the ground.

[0031] One or more (in Figure 1 the example of, there are 2) capacitors C1, C2 are arranged in one or more of the plurality of arms including the series arm 4 and the plurality of parallel arms 5 (in Figure 1 the example of, there are 2) the series arm 4 and the parallel arm 53). More specifically, the capacitor C1 is arranged between the branch points N1 and N2 of the series arm 4. The capacitor C2 is arranged in the parallel arm 53 connecting the branch point N3 of the series arm 4 and the ground. That is, the capacitor C2 is arranged in the parallel arm 53 among the plurality of parallel arms 5 where the parallel arm resonators P1, P2, P4 are not arranged. In other words, the capacitor C2 is not arranged in the parallel arms 51, 52, 54 among the plurality of parallel arms 5 where the parallel arm resonators P1, P2, P4 are arranged.

[0032] Three series arm resonators S1, S3, S4 are arranged in the series arm 4. More specifically, the series arm resonator S1 is arranged between the first terminal 2 and the branch point N1 in the series arm 4. The series arm resonator S3 is arranged between the branch points N2 and N3 in the series arm 4. The series arm resonator S4 is arranged between the branch points N3 and N4 in the series arm 4.

[0033] Three parallel-arm resonators P1, P2, and P4 are each arranged in three of the four parallel arms 5 where the capacitor C2 is not arranged. More specifically, the parallel-arm resonator P1 is arranged in the parallel arm 51 that is connected to the branching point N1 of the series arm 4 and grounded. The parallel-arm resonator P2 is arranged in the parallel arm 52 that is connected to the branching point N2 of the series arm 4 and grounded. The parallel-arm resonator P4 is arranged in the parallel arm 54 that is connected to the branching point N4 of the series arm 4 and grounded.

[0034] The frequency intervals between the resonance frequencies and anti-resonance frequencies of the series-arm resonators S1, S3, S4 and the parallel-arm resonators P1, P2, P4 are wider than the corresponding frequency bands.

[0035] As described above, the connection points of the series arm 4 with the respective parallel arms 5 are defined as the branching points N1 to N4. The series arm 4 has a plurality of series-arm intervals L. The plurality of series-arm intervals L includes the interval L1 between the branching point N1 adjacent to the first end 2 and the first end 2 among the plurality of branching points N1 to N4 in the series arm 4, the interval L5 between the branching point N4 adjacent to the second end 3 and the second end 3 among the plurality of branching points N1 to N4 in the series arm 4, and the intervals L2 to L4 between two adjacent branching points (for example, branching points N1 and N2, branching points N2 and N3, branching points N3 and N4) among the plurality of branching points N1 to N4. In addition, the interval L5 between the branching point N4 adjacent to the second end 3 and the second end 3 among the plurality of branching points N1 to N4 in the series arm 4 is included in the series-arm interval L in the present embodiment, but may not be included in the series-arm interval L.

[0036] The ladder filter 1 includes: one or more capacitors C1, C2 arranged in at least one of the plurality of series-arm intervals L and the plurality of parallel arms 5; and resonators S1, S3, S4, P1, P2, P4 arranged in the rest of the plurality of series-arm intervals L and the plurality of parallel arms 5. The resonance frequencies frs1, frs3, frs4 of at least one of the one or more series-arm resonators (for example, S1, S3, S4) among the series-arm resonators S1, S3, S4 arranged in the series arm 4 are lower than the corresponding frequency band. In addition, the anti-resonance frequencies fap1, fap2, fap4 of at least one of the one or more parallel-arm resonators (for example, P1, P2, P4) among the parallel-arm resonators P1, P2, P4 arranged in the parallel arm 5 are higher than the corresponding frequency band.

[0037] In the ladder filter 1, as described above, the resonance frequencies of the three series-arm resonators S1, S3, and S4 are set lower than the corresponding frequency bands. Further, as described above, the anti-resonance frequencies of the three shunt-arm resonators P1, P2, and P4 are set higher than the corresponding frequency bands. Accordingly, the passband of the ladder filter 1 is narrowed compared to the passband of the ladder filter of Comparative Example 1 described later.

[0038] Further, in the ladder filter 1, as described above, one or more capacitors C1 and C2 are arranged in at least one of the plurality of arms including the series arm 4 and the plurality of shunt arms 5 (for example, the series arm 4 and the shunt arm 53). Thereby, the impedance characteristic of the passband of the ladder filter 1 is set to be intermediate between inductive and capacitive. As a result, the reflection characteristic of the ladder filter 1 is improved.

[0039] In this way, in the ladder filter 1, it is possible to achieve both narrowing of the passband and improvement of the reflection characteristic.

[0040] (3) Structure of the ladder filter

[0041] As Figure 2 shown, in addition to the above-described circuit structure (series arm 4, plurality of shunt arms 5, first terminal 2, second terminal 3, two capacitors C1, C2, three series-arm resonators S1, S3, S4, and three shunt-arm resonators P1, P2, P4), the ladder filter 1 further includes a substrate 6 and a plurality of (three in the Figure 2 example) ground electrodes 71 to 73.

[0042] The substrate 6 is, for example, a piezoelectric substrate. The above piezoelectric substrate is formed of, for example, a piezoelectric single crystal or a piezoelectric ceramic. The above piezoelectric substrate is, for example, a substrate formed of a piezoelectric material such as lithium tantalate (LiTaO3) or lithium niobate (LiNbO3). Further, the above piezoelectric substrate may be a substrate in which a piezoelectric layer formed of a piezoelectric material such as lithium tantalate or lithium niobate is laminated on a support substrate such as silicon. Further, a dielectric layer or a conductor layer such as silicon oxide or silicon nitride may be formed between the support substrate and the piezoelectric layer. The substrate 6 is, for example, a plate-shaped substrate having a rectangular top view. The substrate 6 has a first main surface 6a and a second main surface 6b on both sides in its thickness direction. In the present embodiment, the longitudinal direction of the first main surface 6a is the propagation direction of the surface acoustic wave. The above-described circuit structure and the three ground electrodes 71 to 73 are provided on the first main surface 6a of the substrate 6.

[0043] The first terminal 2, the second terminal 3, and the plurality of ground electrodes 71 to 73 are each formed as, for example, rectangular pad electrodes.

[0044] For example, the first terminal 2 is disposed near the corner 6c of the first main surface 6a on the first main surface 6a. The second terminal 3 is disposed on the first main surface 6a at a distance from the first terminal 2 in the long side direction of the first main surface 6a. More specifically, for example, the second terminal 3 is disposed near the corner 6d of the first main surface 6a. For example, the ground electrode 71 is disposed on the first main surface 6a between the first terminal 2 and the second terminal 3. The ground electrode 72 is disposed on the first main surface 6a at a distance from the first terminal 2 in the short side direction of the first main surface 6a. More specifically, for example, the ground electrode 72 is disposed near the corner 6f of the first main surface 6a. The ground electrode 73 is disposed on the first main surface 6a at a distance from the ground electrode 72 in the long side direction of the first main surface 6a. More specifically, for example, the ground electrode 73 is disposed at the corner 6e of the first main surface 6a.

[0045] On the first main surface 6a, between the first terminal 2 and the ground electrode 72 (on one end side in the long side direction on the first main surface 6a), the series arm resonator S1, the shunt arm resonator P1, and the shunt arm resonator P2 are disposed. The series arm resonator S1, the shunt arm resonator P1, and the shunt arm resonator P2 are arranged and disposed in sequence from the first terminal 2 toward the ground electrode 72.

[0046] In addition, on the first main surface 6a, between the second terminal 3 and the ground electrode 73, the shunt arm resonator P4, the series arm resonator S4, and the series arm resonator S3 are disposed. The shunt arm resonator P4, the series arm resonator S4, and the series arm resonator S3 are arranged and disposed in sequence from the second terminal 3 toward the ground electrode 73.

[0047] In addition, at the center in the long side direction of the first main surface 6a (that is, between the series arm resonator S1, the shunt arm resonator P1, and the shunt arm resonator P2 and the shunt arm resonator P4, the series arm resonator S4, and the series arm resonator S3), the capacitor C1 and the capacitor C2 are disposed. The capacitor C2 and the capacitor C1 are arranged and disposed in sequence along the short side direction of the first main surface 6a from the ground electrode 71 side toward the side opposite to the ground electrode 71.

[0048] The series arm resonators S1, S3, S4 and the shunt arm resonators P1, P2, P4 each have a pair of first comb electrodes 8 (also called IDT electrodes) for acoustic surface wave propagation and a pair of reflectors 9 for reflecting the acoustic surface wave.

[0049] The pair of first comb electrodes 8 are formed as electrode patterns on the first main surface 6a of the substrate 6. The pair of first comb electrodes 8 each have a bus bar 8a and a plurality of electrode fingers 8b.

[0050] For example, bus bars 8a of a pair of first comb-shaped electrodes 8 each extend in the long side direction (i.e., the surface acoustic wave propagation direction) of the first main surface 6a, and are arranged at intervals from each other in the short side direction (i.e., the direction orthogonal to the surface acoustic wave propagation direction) of the first main surface 6a.

[0051] Multiple electrode fingers 8b of a pair of first comb-shaped electrodes 8 each extend from the bus bar 8a in a direction facing another bus bar 8a of the first comb-shaped electrode 8 (i.e., the short side direction of the first main surface 6a). Multiple electrode fingers 8b of a pair of first comb-shaped electrodes 8 each are arranged to be inserted between multiple electrode fingers 8b of another first comb-shaped electrode 8. Multiple electrode fingers 8b of a pair of first comb-shaped electrodes 8 each are arranged along the long side direction of the bus bar 8a (i.e., the long side direction of the first main surface 6a). Therefore, the arrangement direction M1 of the pair of first comb-shaped electrodes 8 is the same as the long side direction of the bus bar 8a. In addition, the arrangement direction M1 is the direction in which multiple electrode fingers 8b of the pair of first comb-shaped electrodes 8 are arranged. Furthermore, the inter-digital pitch PT1 of the pair of first comb-shaped electrodes 8 is set to be equal to half of the wavelength of the surface acoustic wave having a predetermined resonance frequency. In addition, the inter-digital pitch PT1 is the center-to-center distance between adjacent electrode fingers 8b.

[0052] A pair of reflectors 9 are arranged on both sides in the long side direction of the bus bar 8a in the pair of first comb-shaped electrodes 8. The pair of reflectors 9 are formed as electrode patterns on the first main surface 6a of the substrate 6.

[0053] Capacitors C1 and C2 each have a pair of second comb-shaped electrodes 10 (also referred to as IDT electrodes). The pair of second comb-shaped electrodes 10 are formed as electrode patterns on the first main surface 6a of the substrate 6. The pair of second comb-shaped electrodes 10 have bus bars 10a and multiple electrode fingers 10b.

[0054] Bus bars 10a of a pair of second comb-shaped electrodes 10 each extend in the short side direction of the first main surface 6a, and are arranged at intervals from each other in the long side direction (i.e., the surface acoustic wave propagation direction) of the first main surface 6a.

[0055] A plurality of electrode fingers 10b of each of the pair of second comb electrodes 10 extend from the bus bar 10a in a direction facing another bus bar 10a of the second comb electrode 10. A plurality of electrode fingers 10b of each of the pair of second comb electrodes 10 are configured to be inserted between the plurality of electrode fingers 10b of another second comb electrode 10. A plurality of electrode fingers 10b of each of the pair of second comb electrodes 10 are arranged along the long side direction of the bus bar 10a. Therefore, the arrangement direction M2 of the pair of second comb electrodes 10 coincides with the long side direction of the bus bar 10a. In the present embodiment, when viewed from above in the thickness direction of the substrate 6, the arrangement direction M2 of the pair of second comb electrodes 10 is orthogonal to the arrangement direction M1 of each of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4. In addition, the arrangement direction M2 is the direction in which the plurality of electrode fingers 10b of the pair of second comb electrodes are arranged.

[0056] In addition, the inter-digital pitch PT2 of the pair of second comb electrodes 10 is larger or smaller than the inter-digital pitch PT1 of each of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4. More specifically, the inter-digital pitch PT2 of the pair of second comb electrodes 10 is larger than the maximum inter-digital pitch PT1 among the inter-digital pitches PT1 of each of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4, or smaller than the minimum inter-digital pitch PT1 among the inter-digital pitches PT1 of each of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4. In addition, the inter-digital pitch PT2 is the center-to-center distance between adjacent electrode fingers 10b.

[0057] The series arm 4 (refer to Figure 1 ) has circuits 4a to 4e. The circuits 4a to 4e are formed as electrode patterns on the first main surface 6a of the substrate 6. The circuit 4a connects the first terminal 2 and one first comb electrode 8 of the series arm resonator S1. The circuit 4b connects the other first comb electrode 8 of the series arm resonator S1 and one second comb electrode 10 of the capacitor C1. The circuit 4c connects the other second comb electrode 10 of the capacitor C1 and one first comb electrode 8 of the series arm resonator S3. The circuit 4d connects the other second comb electrode 10 of the series arm resonator S3 and one first comb electrode 8 of the series arm resonator S4. The circuit 4e connects the other first comb electrode 8 of the series arm resonator S4 and the second terminal 3.

[0058] The parallel arm 51 (refer to Figure 1)(1) It has circuits 51a and 51b. The circuits 51a and 51b are formed as electrode patterns on the first main surface 6a of the substrate 6. The circuit 51a connects one first comb electrode 8 of the parallel arm resonator P1 to the branch point N1 of the circuit 4b. The circuit 51b connects the other first comb electrode 8 of the parallel arm resonator P1 to the ground electrode 72.

[0059] The parallel arm 52 (see Figure 1 )(1) It has circuits 52a and 52b. The circuits 52a and 52b are formed as electrode patterns on the first main surface 6a of the substrate 6. The circuit 52a connects one first comb electrode 8 of the parallel arm resonator P2 to the branch point N2 of the circuit 4c. The circuit 52b connects the other first comb electrode 8 of the parallel arm resonator P2 to the ground electrode 72. In Figure 2 the example of, the circuit 52b becomes a structure partially common with the circuit 51b.

[0060] The parallel arm 53 (see Figure 1 )(1) It has circuits 53a and 53b. The circuits 53a and 53b are formed as electrode patterns on the first main surface 6a of the substrate 6. The circuit 53a connects one second comb electrode 10 of the capacitor C2 to the branch point N3 of the circuit 4d. The circuit 53b connects the other second comb electrode 10 of the capacitor C2 to the ground electrode 71.

[0061] The parallel arm 54 (see Figure 1 )(1) It has circuits 54a and 54b. The circuits 54a and 54b are formed as electrode patterns on the first main surface 6a of the substrate 6. The circuit 54a connects one first comb electrode 8 of the parallel arm resonator P4 to the branch point N4 of the circuit 4e. The circuit 54b connects the other first comb electrode 8 of the parallel arm resonator P4 to the ground electrode 71.

[0062] In the ladder filter 1 configured as described above, when viewed from above in the thickness direction of the substrate 6, the arrangement direction M2 of the capacitors C1 and C2 is inclined with respect to the arrangement direction M1 of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4 (orthogonal in the Figure 2 example of). Thereby, a pair of second comb electrodes 10 having the same structure as the pair of first comb electrodes 8 can be used to form the capacitors C1 and C2 differently from the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4. Further, in this way, by using a pair of second comb electrodes 10 to form the capacitors C1 and C2, the capacitors C1 and C2 can be formed by using the manufacturing processes of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4.

[0063] In addition, by adjusting the pitch PT1 between the teeth of each of the three series-arm resonators S1, S3, and S4, the resonance frequencies of the three series-arm resonators S1, S3, and S4 are set lower than the corresponding frequency bands as described above. In addition, by adjusting the pitch PT1 between the teeth of each of the three parallel-arm resonators P1, P2, and P4, the anti-resonance frequencies of the three parallel-arm resonators P1, P2, and P4 are set higher than the corresponding frequency bands as described above.

[0064] (4) Narrowing of the passband and reflection characteristics of the ladder filter

[0065] Refer to Figures 3 to 6 to explain the narrowing of the passband and the reflection characteristics of the ladder filter 1 in comparison with Comparative Examples 1 and 2.

[0066] Figures 4 to 6 The impedance waveforms of the ladder filter 1 according to Comparative Example 1, Comparative Example 2, and the embodiment are shown respectively. In addition, in Figures 4 to 6 , the curves HS1 to HS4 represent the impedance waveforms of the series-arm resonators S1 to S4, and the curves HP1 to HP4 represent the impedance waveforms of the parallel-arm resonators P1 to P4.

[0067] In addition, in Figures 4 to 6 , the reference numerals frs1 to frs4 represent the resonance frequencies of the series-arm resonators S1 to S4, and the reference numerals fas1 to fas4 represent the anti-resonance frequencies of the series-arm resonators S1 to S4. The reference numerals frp1 to frp4 represent the resonance frequencies of the parallel-arm resonators P1 to P4, and the reference numerals fap1 to fap4 represent the anti-resonance frequencies of the parallel-arm resonators P1 to P4.

[0068] In addition, in Figures 4 to 6 , the reference numerals G1, G2, and G3 represent the schematic attenuation characteristics (i.e., filter pass characteristics) of the ladder filter 1 according to Comparative Example 1, Comparative Example 2, and the embodiment. In addition, in Figures 4 to 6 , the reference numeral W1 is an example of the corresponding frequency band (for example, the frequency band of Band30Rx: 2350 - 2360 MHz).

[0069] (4-1) Comparative Example 1

[0070] Comparative Example 1 is a ladder filter. As Figure 3 shown, Comparative Example 1 has a structure in which the capacitor C1 in the ladder filter 1 according to the embodiment is replaced with the series-arm resonator S2, and the capacitor C2 is replaced with the parallel-arm resonator P3. That is, Comparative Example 1 is a ladder filter having four series-arm resonators S1 to S4 and four parallel-arm resonators P1 to P4.

[0071] AsFigure 4 As shown, in Comparative Example 1, among the four series-arm resonators S1 to S4, the respective resonance frequencies frs1 to frs4 are substantially the same as each other, and the respective anti-resonance frequencies fas1 to fas4 are substantially the same as each other. Further, among the four parallel-arm resonators P1 to P4, the respective resonance frequencies frp1 to frp4 are substantially the same as each other, and the respective anti-resonance frequencies fap1 to fap4 are substantially the same as each other. Further, the resonance frequencies frs1 to frs4 of the four series-arm resonators S1 to S4 and the anti-resonance frequencies fap1 to fap4 of the four parallel-arm resonators P1 to P4 are substantially the same as each other and are included within the corresponding frequency band W1.

[0072] The high-frequency attenuation region Q1 of the attenuation characteristic G1 of Comparative Example 1 is formed at frequencies near the anti-resonance frequencies fas1 to fas4 of the four series-arm resonators S1 to S4. Further, the low-frequency attenuation region Q2 of the attenuation characteristic G1 of Comparative Example 1 is formed at frequencies near the resonance frequencies frp1 to frp4 of the four parallel-arm resonators P1 to P4. The passband Q3 of the attenuation characteristic G1 of Comparative Example 1 is formed between the high-frequency attenuation region Q1 and the low-frequency attenuation region Q2. In addition, the so-called "passband" is the frequency width at the position where the attenuation has decreased by -3 dB from the maximum value in the attenuation characteristic G1.

[0073] In Comparative Example 1, as described above, the resonance frequencies frs1 to frs4 of the four series-arm resonators S1 to S4 and the anti-resonance frequencies fap1 to fap4 of the four parallel-arm resonators P1 to P4 are substantially the same. Therefore, when the corresponding frequency band W1 is a relatively narrow frequency band, there is a problem that the frequency interval between the anti-resonance frequencies fas1 to fas4 of the series-arm resonators S1 to S4 and the resonance frequencies frp1 to frp4 of the parallel-arm resonators P1 to P4 in Comparative Example 1 is quite large compared to the corresponding frequency band W1. That is, when the corresponding frequency band W1 is a relatively narrow frequency band, there is a problem that the passband Q3 of Comparative Example 1 is quite large compared to the corresponding frequency band W1.

[0074] (4-2) Comparative Example 2

[0075] As Figure 5 shown, compared with Comparative Example 1, in Comparative Example 2, the resonance frequencies frs1 to frs4 of the four series-arm resonators S1 to S4 are lower than the corresponding frequency band W1, and the anti-resonance frequencies fap1 to fap4 of the four parallel-arm resonators P1 to P4 are higher than the corresponding frequency band W1. That is, Comparative Example 2 is a filter in which the resonance frequencies frs1 to frs4 of the four series-arm resonators S1 to S4 are shifted to the lower-frequency side compared to the corresponding frequency band W1 and the anti-resonance frequencies fap1 to fap4 of the four parallel-arm resonators P1 to P4 are shifted to the higher-frequency side of the corresponding frequency band W1 in Comparative Example 1.

[0076] Accordingly, the attenuation region Q1 on the high-frequency side of the attenuation characteristic G2 of Comparative Example 2 is shifted toward the low-frequency side as compared with the attenuation region Q1 on the high-frequency side of Comparative Example 1, and the attenuation region Q2 on the low-frequency side of the attenuation characteristic G2 of Comparative Example 2 is shifted toward the high-frequency side as compared with the attenuation region Q2 on the low-frequency side of Comparative Example 1. As a result, the frequency intervals between the anti-resonant frequencies fas1 to fas4 of the series-arm resonators S1 to S4 and the resonant frequencies frp1 to frp4 of the parallel-arm resonators P1 to P4 in Comparative Example 2 become narrower than the frequency intervals between the anti-resonant frequencies fas1 to fas4 of the series-arm resonators S1 to S4 and the resonant frequencies frp1 to frp4 of the parallel-arm resonators P1 to P4 in Comparative Example 1. Accordingly, the passband Q3 of the attenuation characteristic G2 of Comparative Example 2 becomes narrower than the passband Q3 of Comparative Example 1. As a result, the passband Q3 of Comparative Example 2 is improved so as not to become too large as compared with the corresponding frequency band W1. In this way, in Comparative Example 2, the problem points of Comparative Example 1 are improved.

[0077] However, in Comparative Example 2, as described above, the resonant frequencies frs1 to frs4 of the four series-arm resonators S1 to S4 are lower than the corresponding frequency band W1, and the anti-resonant frequencies fap1 to fap4 of the four parallel-arm resonators P1 to P4 are higher than the corresponding frequency band W1. As a result, the impedance characteristic of the passband Q3 of Comparative Example 2 becomes inductive, and as a result, the reflection characteristic of Comparative Example 2 deteriorates. In this way, in Comparative Example 2, since the impedance characteristic of the passband Q3 becomes inductive, there is a problem that the reflection characteristic (i.e., the impedance matching with circuit elements etc. connected to the ladder filter of Comparative Example 2) deteriorates.

[0078] (4-3) Narrowing of the passband and improvement of the reflection characteristic of the ladder filter according to the embodiment

[0079] As Figure 1 shown, the ladder filter 1 according to the embodiment has a structure in which at least one resonator (for example, the series-arm resonator S2 and the parallel-arm resonator P3) among the four series-arm resonators S1 to S4 and the four parallel-arm resonators P1 to P4 is replaced with a capacitor (capacitors C1, C2) in the structure of Comparative Example 2 (see Figure 3 ). Accordingly, the impedance characteristic of the passband Q3 (see Figure 6 ) of the ladder filter 1 according to the embodiment migrates from an inductive characteristic to a characteristic intermediate between an inductive and a capacitive characteristic, and as a result, the reflection characteristic of the ladder filter 1 according to the embodiment is improved. In this way, in the ladder filter 1 according to the embodiment, the problem points of Comparative Example 2 are improved. In addition, in the example of Figure 1 , an example is illustrated in Comparative Example 2 (see Figure 3In the case where the series-arm resonator S2 and the shunt-arm resonator P3 are replaced with capacitors C1 and C2 in ( ). However, in Comparative Example 2, at least one of the plurality of series-arm resonators S1 to S4 and the plurality of shunt-arm resonators P1 to P4 (for example, the series-arm resonator S2) may be replaced with the capacitor C1. In addition, at least one of the plurality of series-arm resonators S1 to S4 and the plurality of shunt-arm resonators P1 to P4 (for example, the shunt-arm resonator P3) may be replaced with the capacitor C2. In addition, a structure in which all the resonators among the four series-arm resonators S1 to S4 and the four shunt-arm resonators P1 to P4 are replaced with capacitors is excluded.

[0080] In addition, as Figure 6 shown, in the ladder filter according to the embodiment, similarly to Comparative Example 2, the resonance frequencies frs1, frs3, and frs4 of the three series-arm resonators S1, S3, and S4 are lower than the corresponding frequency band W1, and the anti-resonance frequencies fap1, fap2, and fap4 of the three shunt-arm resonators P1, P2, and P4 are higher than the corresponding frequency band W1. Accordingly, similarly to Comparative Example 2, the attenuation region Q1 on the high-frequency side of the attenuation characteristic G3 of the ladder filter 1 according to the embodiment is shifted to the low-frequency side as compared with the attenuation region Q1 on the high-frequency side of Comparative Example 1, and the attenuation region Q2 on the low-frequency side of the attenuation characteristic G3 of the ladder filter 1 according to the embodiment is shifted to the high-frequency side as compared with the attenuation region Q2 on the low-frequency side of Comparative Example 1. As a result, the frequency intervals between the anti-resonance frequencies fas1, fas3, and fas4 of the series-arm resonators S1, S3, and S4 and the resonance frequencies frp1, frp2, and frp4 of the shunt-arm resonators P1, P2, and P4 in the ladder filter 1 according to the embodiment become narrower than the frequency intervals between the anti-resonance frequencies fas1 to fas4 of the series-arm resonators S1 to S4 and the resonance frequencies frp1 to frp4 of the shunt-arm resonators P1 to P4 in Comparative Example 1. That is, the passband Q3 of the attenuation characteristic G3 of the ladder filter 1 according to the embodiment becomes narrower than the passband Q3 of Comparative Example 1. In this way, in the ladder filter 1 according to the embodiment, similarly to Comparative Example 2, the passband Q3 is improved so as not to become too large as compared with the corresponding frequency band W1.

[0081] In this way, in the ladder filter 1 according to the embodiment, the passband Q3 is narrowed according to the corresponding frequency band W1, and the reflection characteristic is improved.

[0082] (4-4) Comparison of attenuation characteristics (i.e., filter passing characteristics) between the embodiment and Comparative Examples 1 and 2

[0083] In Figure 7Among them, the curve graph G1 represents the attenuation characteristics of Comparative Example 1, the curve graph G2 represents the attenuation characteristics of Comparative Example 2, and the curve graph G3 represents the attenuation characteristics of the ladder filter 1 according to the embodiment. In addition, as a band-pass filter that allows the signal in the corresponding frequency band W1 to pass through, the required characteristic (lower limit of the attenuation amount) of the required insertion loss is denoted by the reference numeral IL1, the required characteristic of the attenuation amount on the low-frequency side is denoted by the reference numeral AT1, and the required characteristic of the attenuation amount on the high-frequency side is denoted by the reference numeral AT2.

[0084] As Figure 7 shown, in the attenuation characteristic G1 of Comparative Example 1, the required characteristic IL1 of the insertion loss is satisfied in the passband. That is, in the attenuation characteristic G1, the required characteristic IL1 of the insertion loss is exceeded in the passband. However, in the attenuation characteristic G1 of Comparative Example 1, neither the attenuation amount on the low-frequency side nor the attenuation amount on the high-frequency side satisfies the required characteristics AT1 and AT2. That is, in the attenuation characteristic G1, the attenuation amount on the low-frequency side is not lower than the required characteristic AT1 throughout the range of the required characteristic AT1, and the attenuation amount on the high-frequency side is not lower than the required characteristic AT2 throughout the range of the required characteristic AT2.

[0085] In contrast, in the attenuation characteristic G2 of Comparative Example 2, both the attenuation amount on the low-frequency side and the attenuation amount on the high-frequency side satisfy the required characteristics AT1 and AT2. That is, in the attenuation characteristic G2 of Comparative Example 2, the attenuation amount on the low-frequency side is lower than the required characteristic AT1 throughout the range of the required characteristic AT1, and the attenuation amount on the high-frequency side is lower than the required characteristic AT2 throughout the range of the required characteristic AT2. As a result, the attenuation region on the high-frequency side of the attenuation characteristic G2 of Comparative Example 2 shifts to the low-frequency side compared to the attenuation region on the high-frequency side of the attenuation characteristic G1 of Comparative Example 1. In addition, the attenuation region on the low-frequency side of the attenuation characteristic G2 of Comparative Example 2 shifts to the high-frequency side compared to the attenuation region on the low-frequency side of the attenuation characteristic G1 of Comparative Example 1. That is, the passband of the attenuation characteristic G2 of Comparative Example 2 becomes narrower than the passband of the attenuation characteristic G1 of Comparative Example 1.

[0086] However, in the attenuation characteristic G2 of Comparative Example 2, the required characteristic IL1 of the insertion loss is not satisfied in the passband. That is, in the attenuation characteristic G2 of Comparative Example 2, it is lower than the required characteristic IL1 of the insertion loss in the passband. This is because, in Comparative Example 2, the impedance characteristic in the passband becomes inductive (i.e., the reflection characteristic of Comparative Example 2 deteriorates).

[0087] In contrast, in the attenuation characteristic G3 of the ladder filter 1 according to the embodiment, similar to the attenuation characteristic G2 of Comparative Example 2, the attenuation amounts on the low-frequency side and the high-frequency side both satisfy the required characteristics AT1 and AT2. That is, the attenuation region on the high-frequency side of the attenuation characteristic G3 of the ladder filter 1 according to the embodiment shifts toward the low-frequency side compared to the attenuation region on the high-frequency side of the attenuation characteristic G1 of Comparative Example 1. In addition, the attenuation region on the low-frequency side of the attenuation characteristic G3 of the ladder filter 1 according to the embodiment shifts toward the high-frequency side compared to the attenuation region on the low-frequency side of the attenuation characteristic G1 of Comparative Example 1. As a result, the passband of the attenuation characteristic G3 of the ladder filter 1 according to the embodiment becomes narrower than the passband of the attenuation characteristic G1 of Comparative Example 1.

[0088] In addition, in the attenuation characteristic G3 of the ladder filter 1 according to the embodiment, the required characteristic IL1 of the insertion loss is satisfied in the passband. That is, in the ladder filter 1 according to the embodiment, the impedance characteristic in the passband shifts toward the property intermediate between capacitive and inductive. That is, compared with Comparative Example 2, the reflection characteristic of the ladder filter 1 according to the embodiment is improved.

[0089] In this way, the passband of the ladder filter 1 according to the embodiment is narrowed compared with Comparative Example 1, and the reflection characteristic is improved compared with Comparative Example 2.

[0090] (5) Specific examples of corresponding frequency bands

[0091] The ladder filter 1 according to the embodiment assumes a relatively narrow frequency band as the corresponding frequency band W1. Specifically, as the corresponding frequency band W1, for example, Band30Rx, Band30Tx, Band34TRx, and Band53TRx in the Band defined by 3GPP (registered trademark) are assumed. In addition, the corresponding frequency band W1 is not limited to the Bands exemplified above.

[0092] (6) Effects

[0093] The ladder filter 1 according to the embodiment includes one or more series-arm resonators S1, S3, S4 and one or more shunt-arm resonators P1, P2, P4. One or more series-arm resonators S1, S3, S4 are arranged in a series arm 4 connecting a first end 2 and a second end 3. One or more shunt-arm resonators P1, P2, P4 are arranged in a plurality of shunt arms 5 connecting the series arm 4 and the ground. Among one or more series-arm resonators S1, S3, S4, the resonance frequencies frs1, frs3, frs4 of at least one series-arm resonator S1, S3, S4 are lower than the corresponding frequency band W1. Among one or more shunt-arm resonators P1, P2, P4, the anti-resonance frequencies fap1, fap2, fap4 of at least one shunt-arm resonator P1, P2, P4 are higher than the corresponding frequency band W1. The ladder filter 1 further includes one or more capacitors C1, C2 arranged in at least one of a plurality of arms including the series arm 4 and the plurality of shunt arms 5. One or more capacitors C1, C2 are not arranged in the shunt arms 51, 52, 54 in which one or more shunt-arm resonators P1, P2, P4 are arranged among the plurality of shunt arms 5.

[0094] According to this structure, the resonance frequencies frs1, frs3, frs4 of the series-arm resonators S1, S3, S4 are lower than the corresponding frequency band W1, and the anti-resonance frequencies fap1, fap2, fap4 of the shunt-arm resonators P1, P2, P4 are higher than the corresponding frequency band W1. Thus, compared with the ladder filter of Comparative Example 1, the anti-resonance frequencies fas1, fas3, fas4 of the series-arm resonators S1, S3, S4 (i.e., the frequencies of the high-frequency attenuation region Q1 constituting the attenuation characteristic G3 of the ladder filter 1) become lower, and the resonance frequencies frp1, frp2, frp4 of the shunt-arm resonators P1, P2, P4 (i.e., the frequencies of the low-frequency attenuation region Q2 constituting the attenuation characteristic G3 of the ladder filter 1) become higher. Thus, the passband Q3 of the attenuation characteristic G3 of the ladder filter 1 can be made narrower than the passband Q3 of the attenuation characteristic G1 of Comparative Example 1.

[0095] In addition, one or more capacitors C1, C2 are arranged in at least one of the plurality of arms. Therefore, the impedance characteristic of the passband Q3 of the ladder filter 1 can be set to a characteristic intermediate between inductive and capacitive. Thus, the reflection characteristic of the ladder filter 1 can be improved. In addition, except for the case where one or more capacitors C1, C2 are arranged in all of the plurality of arms.

[0096] According to the above, it is possible to balance the narrowing of the passband Q3 of the ladder filter 1 and the improvement of the reflection characteristic.

[0097] (7) Modification

[0098] Describe the modification of the above embodiment.

[0099] (7-1) Modification 1

[0100] In the above embodiment, the following case is illustrated, that is, when viewed from above in the thickness direction of the substrate 6, the arrangement direction M2 of the capacitors C1 and C2 is orthogonal to the arrangement direction M1 of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4 (that is, inclined at 90 degrees).

[0101] However, the arrangement direction M2 of the capacitors C1 and C2 is not limited to being orthogonal to the arrangement direction M1 of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4. For example, the arrangement direction M2 of the capacitors C1 and C2 only needs to be inclined by more than 10 degrees relative to the arrangement direction M1 of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4. Thus, the capacitors C1 and C2 can be distinguished from the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4.

[0102] Preferably, the arrangement direction M2 of the capacitors C1 and C2 only needs to be inclined by more than 45 degrees relative to the arrangement direction M1 of the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4. Thus, the capacitors C1 and C2 can be more reliably distinguished from the series arm resonators S1, S3, S4 and the parallel arm resonators P1, P2, P4.

[0103] (7-2) Modification 2

[0104] In the above embodiment, the case where the resonance frequencies frs1, frs3, and frs4 of all the three series arm resonators S1, S3, and S4 are made lower than the corresponding frequency band W1 is exemplified. However, the resonance frequency of at least one of the three series arm resonators S1, S3, and S4 may be made lower than the corresponding frequency band W1. In this case, the series arm resonator set to be lower than the corresponding frequency band W1 among the three series arm resonators S1, S3, and S4 may also include at least the series arm resonator with the lowest resonance frequency among the three series arm resonators S1, S3, and S4. In addition, the series arm resonator set to be lower than the corresponding frequency band W1 among the three series arm resonators S1, S3, and S4 may also be only the series arm resonator with the lowest resonance frequency among the three series arm resonators S1, S3, and S4.

[0105] According to Modification Example 2, similarly to the case of the above-described embodiment, compared with the case where the resonance frequencies frs1, frs3, and frs4 of all the series-arm resonators S1, S3, and S4 are not lower than the corresponding frequency band W1 (for example, Comparative Example 1 of the above-described embodiment), the attenuation region on the high-frequency side of the ladder filter 1 according to Modification Example 2 can be shifted toward the low-frequency side. As a result, similarly to the above-described embodiment, the passband of the ladder filter 1 according to Modification Example 2 can be narrowed.

[0106] (7-3) Modification Example 3

[0107] In the above-described embodiment, the case where the resonance frequencies frp1, frp2, and frp4 of all three parallel-arm resonators P1, P2, and P4 are higher than the corresponding frequency band W1 is illustrated. However, the resonance frequency of at least one of the three parallel-arm resonators P1, P2, and P4 may be higher than the corresponding frequency band W1. In this case, the parallel-arm resonator among the three parallel-arm resonators P1, P2, and P4 that is set to be higher than the corresponding frequency band W1 may include at least the parallel-arm resonator having the highest resonance frequency among the three parallel-arm resonators P1, P2, and P4. Further, the parallel-arm resonator among the three parallel-arm resonators P1, P2, and P4 that is set to be higher than the corresponding frequency band W1 may be only the parallel-arm resonator having the highest resonance frequency among the three parallel-arm resonators P1, P2, and P4.

[0108] According to Modification Example 3, similarly to the case of the above-described embodiment, compared with the case where the resonance frequencies frp1, frp2, and frp4 of all the parallel-arm resonators P1, P2, and P4 are not higher than the corresponding frequency band W1 (for example, Comparative Example 1 of the above-described embodiment), the attenuation region on the low-frequency side of the ladder filter 1 according to Modification Example 3 can be shifted toward the high-frequency side. As a result, similarly to the above-described embodiment, the passband of the ladder filter 1 according to Modification Example 3 can be narrowed.

[0109] (7-4) Modification Example 4

[0110] In the above-described embodiment, the case where the capacitors C1 and C2 have a pair of second comb teeth electrodes 10 is illustrated. However, the capacitors C1 and C2 are not limited to the case of having a pair of second comb teeth electrodes 10. For example, the capacitors C1 and C2 may have a MIM (Metal-Insulator-Metal) structure in which a metal sandwiches an insulating layer. According to this structure, similarly to the above-described embodiment, the impedance characteristic of the passband of the ladder filter 1 according to Modification Example 4 can be improved from inductive to an intermediate characteristic between inductive and capacitive.

[0111] (7-5) Other Modification Examples

[0112] The above-described embodiments are merely one of various embodiments of the present invention. As long as the object of the present invention can be achieved, the above-described embodiments can be variously modified according to design and the like. In addition, the mutually different structural elements of the above-described embodiments and modified examples can be appropriately combined.

[0113] (Mode)

[0114] The following modes are disclosed in this specification.

[0115] The ladder filter (1) according to the first mode includes one or more series-arm resonators (S1, S3, S4) and one or more shunt-arm resonators (P1, P2, P4). One or more series-arm resonators (S1, S3, S4) are arranged in a series arm (4) connecting a first end (2) and a second end (3). One or more shunt-arm resonators (P1, P2, P4) are arranged in a plurality of shunt arms (5) connecting the series arm (4) and the ground. The frequency intervals between the resonance frequencies and the anti-resonance frequencies of one or more series-arm resonators (S1, S3, S4) and one or more shunt-arm resonators (P1, P2, P4) are wider than the corresponding frequency band. The resonance frequency (frs1, frs3, frs4) of at least one series-arm resonator (S1, S3, S4) among one or more series-arm resonators (S1, S3, S4) is lower than the corresponding frequency band (W1). The anti-resonance frequency (fap1, fap2, fap4) of at least one shunt-arm resonator (P1, P2, P4) among one or more shunt-arm resonators (P1, P2, P4) is higher than the corresponding frequency band (W1). The ladder filter (1) further includes one or more capacitors (C1, C2) arranged in at least one arm among a plurality of arms including the series arm (4) and the plurality of shunt arms (5). One or more capacitors (C1, C2) are not arranged in the shunt arms (51, 52, 54) in which one or more shunt-arm resonators (P1, P2, P4) are arranged among the plurality of shunt arms (5).

[0116] According to this structure, the resonance frequency (frs1, frs3, frs4) of the series-arm resonator (S1, S3, S4) is lower than the corresponding frequency band (W1), and the anti-resonance frequency (fap1, fap2, fap4) of the shunt-arm resonator (P1, P2, P4) is higher than the corresponding frequency band (W1). Thereby, compared with Comparative Example 1, the anti-resonance frequency (fas1, fas3, fas4) of the series-arm resonator (S1, S3, S4) becomes lower, and the resonance frequency (frp1, frp2, frp4) of the shunt-arm resonator (P1, P2, P4) becomes higher. Thereby, the passband (Q3) of the ladder filter (1) can be made narrower than the passband (Q3) of the ladder filter of Comparative Example 1.

[0117] In addition, at least one of the plurality of arms is provided with more than one capacitor (C1, C2). Therefore, the impedance characteristic of the passband (Q3) of the ladder filter (1) can be set to a characteristic intermediate between inductive and capacitive. Thereby, the reflection characteristic of the ladder filter (1) can be improved.

[0118] Based on the above, it is possible to balance the narrowbanding of the passband (Q3) of the ladder filter (1) and the improvement of the reflection characteristic.

[0119] The ladder filter (1) according to the second aspect is based on the first aspect, wherein the one or more capacitors (C1, C2) include a first capacitor (C1) disposed in the series arm (4) and a second capacitor (C2) disposed in any one of the plurality of parallel arms (5).

[0120] With this structure, by means of two capacitors (the first capacitor (C1) and the second capacitor (C2)), the impedance characteristic of the passband (Q3) of the ladder filter (1) can be set to a characteristic intermediate between inductive and capacitive. Thereby, the reflection characteristic of the ladder filter (1) can be improved.

[0121] The ladder filter (1) according to the third mode includes a series arm (4) connecting a first end (2) and a second end (3), and a plurality of parallel arms (5) connected between the series arm (4) and the ground. The connection points of the series arm (4) with the plurality of parallel arms (5) are defined as a plurality of branch points (N1 to N4). The series arm (4) has a plurality of series arm sections (L). The plurality of series arm sections (L) include: a section (L1) between the branch point (N1) adjacent to the first end (2) among the plurality of branch points (N1 to N4) in the series arm (4) and the first end (2); a section (L5) between the branch point (N4) adjacent to the second end (3) among the plurality of branch points (N1 to N4) in the series arm (4) and the second end (3); and sections (L2 to L4) between two adjacent branch points (branch point (N1) and branch point (N2), branch point (N2) and branch point (N3), branch point (N3) and branch point (N4)) among the plurality of branch points (N1 to N4). The ladder filter (1) includes: one or more capacitors (C1, C2) disposed in at least one of the plurality of series arm sections (L) and the plurality of parallel arms (5); and resonators (S1, S3, S4, P1, P2, P4) disposed in the rest of the plurality of series arm sections (L) and the plurality of parallel arms (5). The frequency interval between the resonance frequency and the anti-resonance frequency of the resonators (S1, S3, S4, P1, P2, P4) is wider than the corresponding frequency band (W1). The resonance frequency (frs1, frs3, frs4) of at least one of the one or more series arm resonators (S1, S3, S4) among the series arm resonators (S1, S3, S4) disposed in the series arm (4) is lower than the corresponding frequency band (W1). The anti-resonance frequency (fap1, fap2, fap4) of at least one of the one or more parallel arm resonators (P1, P2, P4) among the parallel arm resonators (P1, P2, P4) disposed in the parallel arm (5) is higher than the corresponding frequency band (W1).

[0122] According to this structure, the resonance frequency (frs1, frs3, frs4) of the series arm resonators (S1, S3, S4) is lower than the corresponding frequency band (W1), and the anti-resonance frequency (fap1, fap2, fap4) of the parallel arm resonators (P1, P2, P4) is higher than the corresponding frequency band (W1). Thus, compared with Comparative Example 1, the anti-resonance frequency (fas1, fas3, fas4) of the series arm resonators (S1, S3, S4) becomes lower, and the resonance frequency (frp1, frp2, frp4) of the parallel arm resonators (P1, P2, P4) becomes higher. Thus, the passband (Q3) of the ladder filter (1) can be made narrower than the passband (Q3) of the ladder filter of Comparative Example 1.

[0123] In addition, at least one of the plurality of series arm intervals (L) and the plurality of parallel arms (5) is provided with one or more capacitors (C1, C2). Therefore, the impedance characteristic of the passband (Q3) of the ladder filter (1) can be set to a property intermediate between inductive and capacitive. Thereby, the reflection characteristic of the ladder filter (1) can be improved.

[0124] Based on the above, it is possible to balance the narrowbanding of the passband (Q3) of the ladder filter (1) and the improvement of the reflection characteristic.

[0125] The ladder filter (1) according to the fourth aspect is based on the third aspect, wherein the one or more capacitors (C1, C2) include: a first capacitor (C1) disposed in one of the plurality of series arm intervals (L); and a second capacitor (C2) disposed in one of the plurality of parallel arms (5).

[0126] According to this structure, by means of two capacitors (the first capacitor (C1) and the second capacitor (C2)), the impedance characteristic of the passband (Q3) of the ladder filter (1) can be set to a characteristic intermediate between inductive and capacitive. Thereby, the reflection characteristic of the ladder filter (1) can be improved.

[0127] In the ladder filter (1) according to the fifth aspect, based on any one of the first aspect to the fourth aspect, wherein at least one series arm resonator (S1, S3, S4) includes the series arm resonator having the lowest resonance frequency among the one or more series arm resonators (S1, S3, S4).

[0128] According to this structure, it is possible to effectively shift the attenuation region (Q1) on the high-frequency side of the attenuation characteristic (G3) of the ladder filter (1) toward the low-frequency side.

[0129] In the ladder filter (1) according to the sixth aspect, based on the fifth aspect, wherein at least one series arm resonator (S1, S3, S4) includes all of the one or more series arm resonators (S1, S3, S4).

[0130] According to this structure, it is possible to further effectively shift the attenuation region (Q1) on the high-frequency side of the attenuation characteristic (G3) of the ladder filter (1) toward the low-frequency side.

[0131] In the ladder filter (1) according to the seventh aspect, based on any one of the first aspect to the sixth aspect, wherein at least one parallel arm resonator (P1, P2, P4) includes the parallel arm resonator having the highest resonance frequency among the one or more parallel arm resonators (P1, P2, P4).

[0132] According to this structure, it is possible to effectively shift the attenuation region (Q2) on the low-frequency side of the attenuation characteristic (G3) of the ladder filter (1) toward the high-frequency side.

[0133] In the ladder filter (1) according to the eighth aspect, according to the seventh aspect, at least one shunt-arm resonator (P1, P2, P4) includes all of more than one shunt-arm resonator (P1, P2, P4).

[0134] According to this structure, it is possible to further effectively shift the attenuation region (Q2) on the low-frequency side of the attenuation characteristic (G3) of the ladder filter (1) toward the high-frequency side.

[0135] The ladder filter (1) according to the ninth aspect is based on any one of the first to eighth aspects, and further includes a substrate (6) on which more than one capacitor (C1, C2), more than one series-arm resonator (S1, S3, S4), and more than one shunt-arm resonator (P1, P2, P4) are arranged. The more than one series-arm resonator (S1, S3, S4) and the more than one shunt-arm resonator (P1, P2, P4) each have a pair of first comb electrodes (8) with the same arrangement direction (M1). The more than one capacitor (C1, C2) each has a pair of second comb electrodes (10). When viewed from above in the thickness direction of the substrate (6), the arrangement direction (M2) of the pair of second comb electrodes (10) is inclined with respect to the arrangement direction (M1) of the pair of first comb electrodes (8).

[0136] According to this structure, the arrangement direction (M2) of the pair of second comb electrodes (10) is inclined with respect to the arrangement direction (M1) of the pair of first comb electrodes (8). Thereby, it is possible to form the capacitors (C1, C2) using a pair of second comb electrodes (10) having the same structure as the pair of first comb electrodes (8), and to distinguish them from the series-arm resonators (S1, S3, S4) and the shunt-arm resonators (P1, P2, P4). In addition, by forming the capacitors (C1, C2) using a pair of second comb electrodes (10) in this way, it is possible to form the capacitors (C1, C2) using the manufacturing processes of the series-arm resonators (S1, S3, S4) and the shunt-arm resonators (P1, P2, P4).

[0137] In the ladder filter (1) according to the tenth aspect, according to the ninth aspect, the pitch between the teeth (PT2) of the pair of second comb electrodes (10) is larger or smaller than the pitch between the teeth (PT1) of the pair of first comb electrodes (8).

[0138] According to this structure, it is possible to suppress the resonance of spurious waves generated by the capacitors (C1, C2) and the spurious waves generated by the series-arm resonators (S1, S3, S4) or the parallel-arm resonators (P1, P2, P4). As a result, it is possible to suppress the generation of ripples in the passband (Q3) of the ladder filter (1) due to this resonance.

[0139] The ladder filter (1) according to the 11th mode is based on the 10th mode, wherein the substrate (6) is a piezoelectric substrate.

[0140] According to this structure, it is possible to suppress the resonance of bulk waves (spurious waves) leaking from the capacitors (C1, C2) and the bulk waves (spurious waves) leaking from the series-arm resonators (S1, S3, S4) or the parallel-arm resonators (P1, P2, P4). As a result, it is possible to suppress the generation of ripples in the passband (Q3) of the ladder filter (1) due to this resonance.

[0141] Description of Reference Numerals

[0142] 1: Ladder filter;

[0143] 2: First terminal;

[0144] 3: Second terminal;

[0145] 4: Series arm;

[0146] 4a - 4e: Circuits;

[0147] 5: Parallel arm;

[0148] 6: Substrate;

[0149] 6a: First main surface;

[0150] 6b: Second main surface;

[0151] 6c - 6f: Corners;

[0152] 8: First comb-shaped electrode;

[0153] 8a: Bus bar;

[0154] 8b: Electrode finger;

[0155] 9: Reflector;

[0156] 10: Second comb-shaped electrode;

[0157] 10a: Bus bar;

[0158] 10b: Electrode finger;

[0159] 51 - 54: Parallel arms;

[0160] 51a - 54a, 51b - 54b: Circuits;

[0161] 71 to 73: Ground electrodes;

[0162] C1: Capacitance (first capacitance);

[0163] C2: Capacitance (second capacitance);

[0164] AT1, AT2: Desired characteristics of attenuation amount;

[0165] fap1 to fap4, fas1 to fas4: Anti-resonant frequencies;

[0166] frp1 to frp4, frs1 to frs4: Resonant frequencies;

[0167] G1 to G3: Attenuation characteristics;

[0168] HP1 to HP4, HS1 to HS4: Impedance waveforms;

[0169] IL1: Desired characteristics of insertion loss;

[0170] L: Series arm section;

[0171] L1 to L5: Sections;

[0172] M1, M2: Arrangement directions;

[0173] N1 to N4: Branch points;

[0174] PT1, PT2: Spacing between comb teeth;

[0175] P1 to P4: Shunt arm resonators;

[0176] Q1, Q2: Attenuation regions;

[0177] Q3: Passband;

[0178] S1 to S4: Series arm resonators;

[0179] W1: Corresponding frequency band.

Claims

1. A ladder filter includes: One or more series-arm resonators disposed in a series arm connecting a first terminal and a second terminal; and One or more shunt-arm resonators disposed in a plurality of shunt arms connecting the series arm and ground, The frequency intervals between the resonance frequencies and the anti-resonance frequencies of the one or more series-arm resonators and the one or more shunt-arm resonators are wider than the corresponding frequency band, The resonance frequency of at least one series-arm resonator among the one or more series-arm resonators is lower than the corresponding frequency band, The anti-resonance frequency of at least one shunt-arm resonator among the one or more shunt-arm resonators is higher than the corresponding frequency band, The ladder filter further includes: one or more capacitors disposed in at least one of a plurality of arms including the series arm and the plurality of shunt arms, The one or more capacitors are not disposed in the shunt arms in which the one or more shunt-arm resonators are disposed among the plurality of shunt arms.

2. The ladder filter according to claim 1, wherein The one or more capacitors include: A first capacitor disposed in the series arm; and A second capacitor disposed in any one of the plurality of shunt arms.

3. A ladder filter includes: A series arm connecting a first terminal and a second terminal; and A plurality of shunt arms connected between the series arm and ground, Defining connection points of the series arm with the plurality of shunt arms as a plurality of branch points, The series arm has a plurality of series-arm sections, The plurality of series-arm sections include: A section between the branch point adjacent to the first terminal among the plurality of branch points in the series arm and the first terminal; A section between the branch point adjacent to the second terminal among the plurality of branch points in the series arm and the second terminal; and A section between two adjacent branch points among the plurality of branch points, The ladder filter includes: One or more capacitors disposed in at least one of the plurality of series-arm sections and the plurality of shunt arms; and Resonators disposed in the rest of the plurality of series-arm sections and the plurality of shunt arms, The frequency interval between the resonance frequency and the anti-resonance frequency of the resonators is wider than the corresponding frequency band, The resonance frequency of at least one series-arm resonator among the one or more series-arm resonators disposed in the series arm is lower than the corresponding frequency band, The anti-resonance frequency of at least one shunt-arm resonator among the one or more shunt-arm resonators disposed in the shunt arm is higher than the corresponding frequency band.

4. The ladder filter according to claim 3, wherein The one or more capacitors include: A first capacitor disposed in one of the plurality of series-arm sections; and A second capacitor disposed in one of the plurality of shunt arms.

5. The ladder filter according to any one of claims 1 to 4, wherein The at least one series-arm resonator includes the series-arm resonator having the lowest resonance frequency among the one or more series-arm resonators.

6. The ladder filter according to claim 5, wherein The at least one series arm resonator includes all of the one or more series arm resonators.

7. The ladder filter according to any one of claims 1 to 6, wherein the at least one shunt arm resonator includes the shunt arm resonator having the highest resonance frequency among the one or more shunt arm resonators.

8. The ladder filter according to claim 7, wherein the at least one shunt arm resonator includes all of the one or more shunt arm resonators.

9. The ladder filter according to any one of claims 1 to 8, wherein the ladder filter further includes: a substrate on which the one or more capacitors, the one or more series arm resonators, and the one or more shunt arm resonators are disposed, the one or more series arm resonators and the one or more shunt arm resonators each have a pair of first comb teeth electrodes with the same arrangement direction as each other, the one or more capacitors each have a pair of second comb teeth electrodes, when viewed from above in the thickness direction of the substrate, the arrangement direction of the pair of second comb teeth electrodes is inclined with respect to the arrangement direction of the pair of first comb teeth electrodes.

10. The ladder filter according to claim 9, wherein the pitch between the teeth of the pair of second comb teeth electrodes is larger or smaller than the pitch between the teeth of the pair of first comb teeth electrodes.

11. The ladder filter according to claim 10, wherein the substrate is a piezoelectric substrate.

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