Filter

Abstract

This filter enables reduction in frequency dependence of an attenuation amount in a stop band. The filter comprises a first resonator, a second resonator, and a third resonator which constitute a signal path for passing a signal of a passband therethrough. The first resonator is cascade-connected to the second resonator through a first connection. The second resonator is cascade-connected to the third resonator through a second connection. The third resonator is cascade-connected to the first resonator through a third connection. The coupling sign parameter of the first connection and the coupling sign parameter of the second connection are both one of positive and negative. The coupling sign parameter of the third connection is the other of positive and negative.

Description

filter

[0001] One aspect of the present disclosure relates to a filter.

[0002] Patent Document 1 listed below discloses an example of a filter configuration.

[0003] WO2014 / 156720

[0004] A filter according to one aspect of the present disclosure includes a first resonator, a second resonator, and a third resonator that form a signal path that passes a passband signal, wherein the first resonator is cascade-connected to the second resonator via a first connection, the second resonator is cascade-connected to the third resonator via a second connection, and the third resonator is cascade-connected to the first resonator via a third connection, and when Arg(S21 / S11) is defined as a coupling sign parameter using matrix elements of S parameters, both of the coupling sign parameter of the first connection and the coupling sign parameter of the second connection are either positive or negative, and the coupling sign parameter of the third connection is the other of positive and negative.

[0005] 11 is a graph showing an enlarged portion of FIG. 12. FIG. 13 is a circuit diagram illustrating the basic configuration of a filter according to a first embodiment. FIG. 14 is a circuit diagram illustrating the configuration of a filter according to a comparative example. FIG. 15 is a graph showing an example of the attenuation characteristics of the filter according to the first embodiment. FIG. 16 is a graph showing an enlarged portion of FIG. 3 together with the attenuation characteristics of a comparative example. FIG. 17 is a graph showing an enlarged portion of FIG. 18. FIG. 19 is a circuit diagram illustrating the basic configuration of a filter according to a second embodiment. FIG. 20 is a graph showing an example of the attenuation characteristics of the filter according to the second embodiment. FIG. 21 is a graph showing an enlarged portion of FIG. 22 together with the attenuation characteristics of a comparative example. FIG. 23 is a graph showing an enlarged portion of FIG. 24. FIG. 25 is a schematic diagram illustrating the basic configuration of a filter according to a fourth embodiment. FIG. 26 is a circuit diagram illustrating a circuit configuration applicable to connection. FIG. 27 is a circuit diagram illustrating a circuit configuration applicable to connection. FIG. 28 is a circuit diagram illustrating a circuit configuration applicable to connection.

[0006] It is desirable to reduce the frequency dependence of the attenuation in the stopband of a filter.

[0007] [Embodiment 1] Embodiment 1 will be described below. For ease of explanation, components having the same functions as those described in embodiment 1 will be denoted by the same reference numerals in the following embodiments, and their descriptions will not be repeated. For simplicity, descriptions of well-known technical matters will be omitted as appropriate. Unless otherwise specified, the components, materials, and numerical values ​​described in this specification are merely examples. Therefore, for example, unless otherwise specified, the positional relationships and connection relationships of the components are not limited to the examples in the figures. Furthermore, the figures are not necessarily drawn to scale.

[0008] 1 is a circuit diagram illustrating the basic configuration of a filter 100 according to embodiment 1. In embodiment 1, the filter 100 is illustrated as a ladder-type filter. However, as will be apparent to those skilled in the art, a filter according to one aspect of the present disclosure is not limited to a ladder-type filter.

[0009] The filter 100 may include a first resonator, a second resonator, and a third resonator. In the example of FIG. 1 , the filter 100 includes resonators Res1, Res2, Res3, Res4, and Res5. The resonators Res1, Res3, and Res5 are LC resonators. The resonators Res2 and Res4 are acoustic wave resonators. In this specification, the resonators Res1, Res2, Res3, Res4, and Res5 are also collectively referred to as resonators Res.

[0010] The first resonator, the second resonator, and the third resonator may form a signal path that passes signals in a pass band. In the example of Figure 1, the resonators Res1, Res2, and Res3 function as signal paths that pass signals in a pass band defined by the filter 100.

[0011] The first resonator may be cascade-connected to the second resonator via a first connection. The second resonator may be cascade-connected to the third resonator via a second connection. The third resonator may be cascade-connected to the first resonator via a third connection. In the example of FIG. 1 , resonator Res1 is cascade-connected to resonator Res2 via a connection CCa. Resonator Res2 is cascade-connected to resonator Res3 via a connection CCb. Resonator Res3 is cascade-connected to resonator Res1 via a connection CCc. That is, connection CCc connects resonators Res1 and Res3, skipping resonator Res2.

[0012] When Arg(S21 / S11) is defined as the coupling sign parameter using the matrix elements of the S parameters, the coupling sign parameter of the first connection and the coupling sign parameter of the second connection may both be either positive or negative, and the coupling sign parameter of the third connection may be either positive or negative. In the example of FIG. 1, as shown below, the coupling sign parameter of the connection CCa and the coupling sign parameter of the connection CCb may be positive, and the coupling sign parameter of the connection CCc may be negative. The connections CCa and CCb may correspond to the circuit CT1-2 described below, i.e., the circuit 1 group CT1 (positive coupling sign parameter), and the connection CCc may correspond to the circuit CT2-3 described below, i.e., the circuit 2 group CT2 (negative coupling sign parameter). Assume that the connections CCa and CCb (circuit CT1-2) have resonant frequencies f0a and f0b, and the connection CCc (circuit CT1-2) has a resonant frequency f0c. Here, f0a = 1 / [2 π (La Ca)] １／２ ], f0b=1 / [2・π・(Lb・Cb) １／２ ], f0c=1 / [2・π・(Lc・Cc) １／２]. As will be described later, the positive and negative signs of the coupling sign parameter are reversed at the resonance frequencies f0a, f0b, and f0c. At a frequency f lower than any of the resonance frequencies f0a, f0b, and f0c, the coupling sign parameter of the connections CCa and CCb is positive, and the coupling sign parameter of the connection CCc is negative. Furthermore, if the resonance frequencies f0a, f0b, and f0c are set higher than the frequency band in which the filter 100 functions as a filter, it is possible to make the coupling sign parameter of the connections CCa and CCb positive and the coupling sign parameter of the connection CCc negative throughout the entire frequency band.

[0013] At least one of the coupling sign parameters of the first connection, the second connection, and the third connection may be configured to have both positive and negative values ​​between 0.5 times the minimum frequency of the pass band and twice the maximum frequency of the pass band. For example, when the pass band of the filter 100 has a minimum frequency fmin and a maximum frequency fmax, any of the resonant frequencies f0a, f0b, and f0c described above is positioned between the frequency fmin·0.5 and the frequency fmax·2. In this case, any of the coupling sign parameters of the connections CCa, CCb, and CCc may have both positive and negative values.

[0014] The first connection may include a first coupling element. The second connection may include a second coupling element. The third connection may include a third coupling element. Both the first coupling element and the second coupling element may include one of an inductor and a capacitor, and the third coupling element may include the other of an inductor and a capacitor. In the example of Figure 1, connection CCa includes capacitor Ca as the first coupling element, connection CCb includes capacitor Cb as the second coupling element, and connection CCc includes inductor Lc as the third coupling element.

[0015] The first coupling element may include a first inductor and a first capacitor connected in series to the first inductor. The second coupling element may include a second inductor and a second capacitor connected in series to the second inductor. The third coupling element may include a third inductor and a third capacitor connected in parallel to the third inductor. In the example of FIG. 1 , the connection CCa includes, as the first coupling element, an inductor La and a capacitor Ca connected in series to the inductor La. The connection CCb includes, as the second coupling element, an inductor Lb and a capacitor Cb connected in series to the inductor Lb. The connection CCc includes, as the third coupling element, an inductor Lc and a capacitor Cc connected in parallel to the inductor Lc.

[0016] At least one of the first resonator, the second resonator, and the third resonator (resonator) may include an acoustic wave resonator. When the resonator including the acoustic wave resonator is located in a parallel arm, the resonator may further have a first end connected to the reference potential section. On the other hand, when the filter 100 includes, for example, a ladder-type filter and the resonator including the acoustic wave resonator is located in a series arm, the first end does not need to be connected to the reference potential section. In the example of FIG. 1 , the resonator Res2 is located in the parallel arm and has an acoustic wave resonator PL2 and a first end connected to the ground terminal GD-2 (reference potential section). More specifically, the resonator Res2 has an inductor L2 and an acoustic wave resonator PL2 connected in parallel to each other, and a first end connected to the ground terminal GD-2 (reference potential section).

[0017] The acoustic wave resonator may be located in at least one of the first resonator, the second resonator, and the third resonator. One of the resonant frequency and the anti-resonant frequency of the resonator may be located within the passband. The other of the resonant frequency and the anti-resonant frequency of the resonator may not be located within the passband. In the example of FIG. 1 , the acoustic wave resonator PL2 is located in the resonator Res2. The resonant frequency fr2 of the resonator Res2 is located in the attenuation band of the filter 100 (not located within the passband), and the anti-resonant frequency fa2 of the resonator Res2 is located within the passband of the filter 100. In the examples of FIGS. 3 to 5 described below, the anti-resonant frequency fa2 of the resonator Res2 is located within the passband A0 of the filter 100, and the resonant frequency fr2 of the resonator Res2 is not located within the passband A0 of the filter 100. As a result, an attenuation pole P11 (or an attenuation pole P12) is formed outside the pass band A0 in correspondence with the resonance frequency fr2.

[0018] Of the first, second, and third resonators, the other two resonators, excluding one, may be composed of an inductor and a capacitor. The other of the resonant frequency and anti-resonant frequency of one resonator may be located closer to the passband than the attenuation poles of the other two resonators (not located within the passband). In the example of FIG. 1 , resonator Res1 is composed of an inductor L1 and a capacitor C1 connected in series. Resonator Res3 is composed of an inductor L3 and a capacitor C3 connected in series. The other of the resonant frequency fr2 and anti-resonant frequency fa2 of resonator Res2 may be located closer to the passband than the attenuation pole fa1 of resonator Res1 and the attenuation pole fa3 of resonator Res3 (not located within the passband). In the examples of Figures 3 to 5 described below, the attenuation pole P11 (or attenuation pole P12) corresponding to the resonant frequency fr2 of the resonator Res2 is located closer to the passband A0 than the attenuation poles P13 and P14 of the resonators Res1 and Res2.

[0019] The filter may further include a fourth resonator and a fifth resonator that form a signal path together with the first resonator, the second resonator, and the third resonator. The third resonator may be cascade-connected to the fourth resonator via a fourth connection. The fourth resonator may be cascade-connected to the fifth resonator via a fifth connection. The fifth resonator may be cascade-connected to the third resonator via a sixth connection. In the example of FIG. 1 , the filter 100 includes resonators Res4 and Res5. The resonators Res4 and Res5 form a communication path together with the resonators Res1, Res2, and Res3. The resonator Res3 is cascade-connected to the resonator Res4 via a connection CCd. The resonator Res4 is cascade-connected to the resonator Res5 via a connection CCe. Resonator Res5 is cascaded with resonator Res3 via connection CCf.

[0020] The resonator Res4 may have an elastic wave resonator PL4 and a first end connected to the ground terminal GD-4 (reference potential portion). More specifically, the resonator Res4 may have an inductor L4 and an elastic wave resonator PL4 connected in parallel with each other, and a first end connected to the ground terminal GD-4 (reference potential portion). The resonator Res5 may be composed of an inductor L5 and a capacitor C5 connected in series with each other.

[0021] The coupling sign parameter of the fourth connection and the coupling sign parameter of the fifth connection may both be either positive or negative, and the coupling sign parameter of the sixth connection may be either positive or negative. In the example of FIG. 1, the coupling sign parameter of the connection CCd and the coupling sign parameter of the connection CCe may be positive, and the coupling sign parameter of the connection CCf may be negative. The connection CCd and the connection CCe may correspond to the circuit CT1-2 described below, i.e., the circuit 1 group CT1 (positive coupling sign parameter), and the connection CCf may correspond to the circuit CT2-3 described below, i.e., the circuit 2 group CT2 (negative coupling sign parameter). The reason for this is the same as in the case of the connection CCa, the connection CCb, and the connection CCc, so a detailed explanation will be omitted.

[0022] The fourth connection may include a fourth coupling element. The fifth connection may include a fifth coupling element. The sixth connection may include a sixth coupling element. Both the fourth coupling element and the fifth coupling element may include one of an inductor and a capacitor. The sixth coupling element may include the other of an inductor and a capacitor. In the example of FIG. 1 , the connection CCd includes a capacitor Cd as the fourth coupling element. The connection CCe includes a capacitor Ce as the fifth coupling element. The connection CCf includes an inductor Lf as the sixth coupling element. More specifically, the connection CCd includes an inductor Ld and a capacitor Cd connected in series as the fourth coupling element. The connection CCe includes an inductor Le and a capacitor Ce connected in series as the fifth coupling element. The connection CCf includes an inductor Lf and a capacitor Cf connected in parallel as the sixth coupling element.

[0023] The filter may include a matching network. In the example of FIG. 1 , the filter 100 includes a matching network MNg and a matching network MNh. The matching network MNg is composed of an inductor La and a capacitor Ca, and has a first end connected to a ground terminal GDa. The matching network MNh is composed of an inductor Lb and a capacitor Cb, and has a first end connected to a ground terminal GDb. A resonator Res1 is connected to the matching network MNg. A resonator Res5 is connected to the matching network MNh.

[0024] The filter may include a notch filter. In the example of Fig. 1, the filter 100 includes a notch filter NF. The notch filter NF is composed of an inductor Lc and a capacitor Cc, and has a first end connected to a ground terminal GDc.

[0025] 1, the filter 100 includes an input terminal Pin and an output terminal Pout. The notch filter NF is connected to the input terminal Pin, and the output terminal Pout is connected to the output terminal Pout.

[0026] The connection diagram TP1 is a connection diagram showing the connection state between resonators Res1 to Res5 in the filter 100. The coupling sign parameters of the connection CCa connecting resonator Res1 and resonator Res2, the connection CCb connecting resonator Res2 and resonator Res3, the connection CCd connecting resonator Res3 and resonator Res4, and the connection CCe connecting resonator Res4 and resonator Res5 are positive and represented by solid lines. On the other hand, the coupling sign parameters of the connection CCc connecting resonator Res3 and resonator Res1, and the connection CCf connecting resonator Res5 and resonator Res3 are negative and represented by dashed lines. In this example, the coupling sign parameters of the connections CCc and CCf that perform cross-coupling are negative.

[0027] 2 is a circuit diagram showing the configuration of a filter 100x of a comparative example. As shown in FIG. 2, the filter 100x includes a resonator R01, a resonator R02, an acoustic wave resonator SAW1, an acoustic wave resonator SAW2, an acoustic wave resonator SAW3, and an acoustic wave resonator SAW4 between an input terminal Pin and an output terminal Pout.

[0028] The resonator R01 has an inductor L01 and a capacitor C01 connected in series. A first end of the resonator R01 is connected to the input terminal Pin via a capacitor C03, and a second end is connected to the ground terminal GD-01. The resonator R02 has an inductor L02 and a capacitor C02 connected in series. A first end of the resonator R02 is connected to the first end of the resonator R01 via a capacitor C04, and a second end is connected to the ground terminal GD-01.

[0029] The acoustic wave resonators SAW1 to SAW4 are connected in series and connected to a first end of the resonator R02 via a capacitor C05.

[0030] An inductor L06 connects the gap between the acoustic wave resonators SAW1 and SAW2 and the ground terminal GD. An inductor L07 connects the gap between the acoustic wave resonators SAW2 and SAW3 and the ground terminal GD. An inductor L08 connects the gap between the acoustic wave resonators SAW3 and SAW4 and the ground terminal GD.

[0031] Connection diagram TP0 is a connection diagram showing the connection state of resonators Res01, Res02, and acoustic wave resonators SAW1 to SAW4 in filter 100x. In filter 100x, resonators Res01, Res02, and acoustic wave resonators SAW1 to SAW4 are simply cascaded in order, with no jump connection.

[0032] (Example of attenuation characteristics of each filter) Fig. 3 is a graph showing an example of the attenuation characteristics of each filter in embodiment 1. Fig. 4 is a graph showing an enlarged portion of Fig. 3 together with the attenuation characteristics of a comparative example. Fig. 5 is a graph showing an enlarged portion of Fig. 4.

[0033] 3 to 5 show the frequency characteristics of each filter in the first embodiment and the comparative example. In FIGS. 3 to 5, the horizontal axis (Frequency) represents frequency (unit: GHz), and the vertical axis (Transmission) represents attenuation (unit: dB). The attenuation can also be interpreted as the passing amount. Therefore, the attenuation characteristics can also be interpreted as the passing characteristics.

[0034] 3, 4, and 5 show the pass band A0 and stop band A1 of the filter 100. In this specification, the pass band of a filter refers to the frequency band from the low-frequency cutoff frequency fcl to the high-frequency cutoff frequency fch. The attenuation characteristics of the filter have a -3 dB attenuation point on both the low-frequency side and the high-frequency side relative to the peak frequency indicating the frequency at which the minimum attenuation is 0 dB.

[0035] The low-frequency cutoff frequency fcl is the frequency of the -3 dB attenuation point on the low-frequency side of the pass band. The high-frequency cutoff frequency fch is the frequency of the -3 dB attenuation point on the high-frequency side of the pass band of the filter. The low-frequency cutoff frequency fcl can also be expressed as the frequency at the low-frequency end of the pass band, or the low-frequency end. The high-frequency cutoff frequency fch can also be expressed as the frequency at the high-frequency end of the pass band A0, or the high-frequency end.

[0036] The stop band A1 is located at least one of the lower frequency side of the low frequency end of the pass band A0 and the higher frequency side of the high frequency end of the pass band A0, and is a frequency range in which the filter 100 blocks signals from passing through. Here, the stop band A1, which is located on the lower frequency side of the pass band A0, will be considered.

[0037] As shown in graph G1, in the example of embodiment 1, attenuation poles P11, P12, P13, and P14 are generated. The attenuation poles P11 and P12 correspond to the resonators Res2 and Res2, and the attenuation poles P13 and P14 correspond to two cross-couplings (connection CCc and connection CCf).

[0038] The attenuation poles P11 and P12 of the resonators Res2 and Res4 are located closer to the pass band than the attenuation poles P13 and P14 of the two jump couplings (connections CCc and CCf). The resonators Res2 and Res4 are respectively composed of acoustic wave resonators PL2 and PL4, and because they have large Q values, it is easy to place the attenuation poles P11 and P12 close to the pass band. If the resonator has a large Q value, it is possible to suppress loss in the pass band of the filter even if the attenuation poles are located close to the pass band.

[0039] As shown in graph G0, the comparative example exhibits attenuation poles P01, P02, P03, and P04. The attenuation poles P01 and P02 correspond to the acoustic wave resonators SAW1, SAW2, SAW3, and SAW4, respectively, and the attenuation poles P03 and P04 correspond to the resonators Res01 and Res02. As shown in graph G0, the comparative example exhibits attenuation greater than −40 dB in part of the stopband A1.

[0040] 4, in the first embodiment, in addition to the attenuation poles P13 and P14, attenuation poles P15 and P16 are formed, thereby reducing the amount of attenuation over a wide frequency range of the stop band A1. These attenuation poles P15 and P16 are formed by a combination of the attenuation poles of the notch filter NF and the high-pass filter characteristics of the matching network MNg.

[0041] As described above, in the first embodiment, cross-coupling is formed between the resonators Res1 and Res3, and between the resonators Res3 and Res5, by the connections CCc and CCf. By positioning the multiple attenuation poles resulting from this cross-coupling in the stop band A1, it becomes easy to reduce the attenuation in the entire stop band A1. In other words, the frequency dependence of the attenuation in the stop band A1 of the filter can be reduced. By positioning the attenuation poles P13 and P14 resulting from the cross-coupling in the stop band A1, which is away from the pass band, it becomes possible to reduce the attenuation on the low-frequency side of the stop band A1.

[0042] As shown below, when the signs of the coupling sign parameters in cross-coupling have a combination of positive and negative values, an attenuation pole may be generated. That is, when a filter has cross-coupling, a signal (1) that has passed through the cross-coupling and a signal (2) that has not passed through the cross-coupling are multiplexed. For example, in embodiment 1, a signal (1) that has passed through the cross-coupling (connection CCc) and a signal (2) that has not passed through the cross-coupling (connections CCa and CCb) are multiplexed in resonator Res3. Here, assume that the signs of the coupling sign parameters of the connections CCa, CCb, and CCc between resonators Res1, Res2, and Res3 have a combination of positive and negative values. In this case, signals (1) and (2) cancel each other out at frequencies where their amplitudes are equal and their phases are opposite. This signal cancellation results in an attenuation pole. On the other hand, if the signs of the coupling sign parameters of the connections CCa, CCb, and CCc are either positive or negative, the signals (1) and (2) do not cancel each other out, and no attenuation pole is generated. The same is true for the connections CCd, CCe, and CCf between the resonators Res3, Res4, and Res5.

[0043] Forming an attenuation pole by cross-coupling makes it easier to make the filter more compact than forming an attenuation pole without cross-coupling, for example, by using a resonator. This is because cross-coupling requires smaller inductance and capacitance than resonators. For example, cross-coupling is possible simply by connecting two resonators Res with a wire, due to the relatively small inductance of the wire. This coupling can correspond to the circuit CT2-1 described below, i.e., the circuit 2 group CT2 (negative coupling sign parameter).

[0044] [Embodiment 2] Embodiment 2 will be described below. For convenience of explanation, components having the same functions as those described in embodiment 1 are denoted by the same reference numerals, and the description thereof will not be repeated.

[0045] 6 is a circuit diagram illustrating a basic configuration of a filter 100a according to Embodiment 2. In Embodiment 1, the filter 100a is illustrated as a ladder filter. However, as will be apparent to those skilled in the art, an acoustic wave filter according to an aspect of the present disclosure is not limited to a ladder filter.

[0046] The filter 100a may include a first resonator, a second resonator, and a third resonator. In the example of Fig. 6, the filter 100a includes resonators Res1, Res2, Res3, Res4, and Res5. The resonators Res1, Res3, and Res5 are LC resonators. The resonators Res2 and Res4 are acoustic wave resonators. In this specification, the resonators Res1, Res2, Res3, Res4, and Res5 are also collectively referred to as resonators Res.

[0047] The first resonator, the second resonator, and the third resonator may form a signal path that passes signals in a pass band. In the example of Fig. 6, the resonators Res1, Res2, and Res3 function as a signal path that passes signals in a pass band defined by the filter 100a.

[0048] The first resonator may be cascaded with the second resonator via a first connection. The second resonator may be cascaded with the third resonator via a second connection. The third resonator may be cascaded with the first resonator via a third connection. In the example of FIG. 6 , resonator Res1 is cascaded with resonator Res2 via a connection CCa. Resonator Res2 is cascaded with resonator Res3 via a connection CCb. Resonator Res3 is cascaded with resonator Res1 via a connection CCc. Connection CCc couples resonators Res3 and Res1 in a cross-connection fashion.

[0049] When Arg(S21 / S11) is defined as the combined sign parameter using the matrix elements of the S parameters, the combined sign parameter of the first connection and the combined sign parameter of the second connection may both be either positive or negative, and the combined sign parameter of the third connection may be either positive or negative. In the example of Figure 6, as shown below, the combined sign parameter of the connection CCa and the combined sign parameter of the connection CCb may be positive, and the combined sign parameter of the connection CCc may be negative.

[0050] Resonator Res1 can be considered to be both a resonator and a connection CCa connecting resonators Res1 and Res2. This is because resonator Res1 is directly connected to resonator Res2 (a signal is extracted from resonator Res1 itself), and resonator Res1 corresponds to circuits CT1-2 and CT1-2, described below, for connecting resonators. That is, connection CCa corresponds to circuits CT1-2 and CT1-2. Therefore, connection CCa has a positive coupling sign parameter at frequencies lower than the resonant frequency fr1 of resonator Res1 (corresponding to circuit CT1-2). Similarly, resonator Res3 is both a resonator and a connection CCb connecting resonators Res2 and Res3 (corresponding to circuits CT1-2 and CT1-2, described below), and has a positive coupling sign parameter at frequencies lower than the resonant frequency fr3 of resonator Res3 (corresponding to circuit CT1-2). The connection CCc corresponds to a circuit CT2-14 described later, that is, the circuit 2 group CT2 (negative coupling sign parameter).

[0051] At least one of the first resonator, the second resonator, and the third resonator (resonators) may include an acoustic wave resonator. When a resonator including an acoustic wave resonator is located in a parallel arm, the resonator may further include a first end connected to a reference potential section. On the other hand, when a resonator including an acoustic wave resonator is located in a series arm, the first end does not need to be connected to the reference potential section. In the example of FIG. 6 , the resonator Res2 is located in the parallel arm and includes an acoustic wave resonator PL2 and a first end connected to a ground terminal GD-2 (reference potential section). More specifically, the resonator Res2 includes an inductor L2 and an acoustic wave resonator PL2 connected in parallel to each other, and a first end connected to the ground terminal GD-2 (reference potential section).

[0052] The acoustic wave resonator may be located in at least one of the first, second, and third resonators. One of the resonant frequency and anti-resonant frequency of the resonator may be located within the passband. The other of the resonant frequency and anti-resonant frequency of the resonator may not be located within the passband. In the example of FIG. 6 , the acoustic wave resonator PL2 is located in the resonator Res2. The anti-resonant frequency fa2 of the resonator Res2 is located within the passband of the filter 100a, and the resonant frequency fr2 is located in the attenuation band of the filter 100a (not located within the passband). In the examples of FIGS. 7 to 9 described below, the anti-resonant frequency fa2 of the resonator Res2 is located within the passband A0 of the filter 100a, and the resonant frequency fr2 of the resonator Res2 is not located within the passband A0 of the filter 100a. As a result, an attenuation pole P22 (or an attenuation pole P23) is formed outside the pass band A0 in correspondence with the resonance frequency fr2.

[0053] Of the first, second, and third resonators, the other two resonators, excluding one, may be composed of inductors and capacitors. The resonant frequency and the other anti-resonant frequency of one resonator may be located closer to the passband than the attenuation poles of the other two resonators (not located within the passband). In the example of FIG. 6 , resonator Res1 is composed of an inductor L1 and a capacitor C1 connected in series. Resonator Res3 is composed of an inductor L31, a capacitor C3, and an inductor L32 connected in series. The resonant frequencies fr1 and fr3 of resonators Res1 and Res3 are located within or near the passband. The mutual inductance between resonators Res1 and Res3 couples resonators Res1 and Res3, bypassing resonator Res2, resulting in signal cancellation and an attenuation pole.

[0054] The third connection may be a mutual inductance coupling between the first resonator and the third resonator. In the example of Fig. 6, the connection CCc is a mutual inductance coupling between the inductor L1 of the resonator Res1 and the inductor L31 of the resonator Res3. Since the inductor L1 and the inductor L31 are connected in series, the connection CCc corresponds to the circuit CT2-15 described below, i.e., the circuit 2 group CT2 (negative coupling sign parameter).

[0055] The filter may further include a fourth resonator and a fifth resonator that form a signal path together with the first resonator, the second resonator, and the third resonator. The third resonator may be cascade-connected to the fourth resonator via a fourth connection. The fourth resonator may be cascade-connected to the fifth resonator via a fifth connection. The fifth resonator may be cascade-connected to the third resonator via a sixth connection. In the example of FIG. 6 , the filter 100a includes resonators Res4 and Res5. The resonators Res4 and Res5 form a communication path together with the resonators Res1, Res2, and Res3. The resonator Res3 is cascade-connected to the resonator Res4 via a connection CCd. The resonator Res4 is cascade-connected to the resonator Res5 via a connection CCe. Resonator Res5 is cascaded with resonator Res3 via connection CCf.

[0056] The resonator Res4 may have an elastic wave resonator PL4 and a first end connected to the ground terminal GD-4 (reference potential portion). More specifically, the resonator Res4 may have an inductor L4 and an elastic wave resonator PL4 connected in parallel with each other, and a first end connected to the ground terminal GD-4 (reference potential portion). The resonator Res5 may be composed of an inductor L5 and a capacitor C5 connected in series with each other.

[0057] The combined code parameter of the fourth connection and the combined code parameter of the fifth connection may both be either positive or negative, and the combined code parameter of the sixth connection may be either positive or negative. In the example of Figure 6, the combined code parameter of the connection CCd and the combined code parameter of the connection CCe may be positive, and the combined code parameter of the connection CCf may be negative, as follows:

[0058] The resonator Res3 is a resonator and also a connection CCd (corresponding to circuits CT1-2 and CT1-2 described below) connecting the resonators Res3 and Res4, and has a positive coupling sign parameter at frequencies lower than the resonant frequency fr3 of the resonator Res3 (corresponding to circuit CT1-2). The resonator Res5 is a resonator and also a connection CCe (corresponding to circuits CT1-2 and CT1-2 described below) connecting the resonators Res4 and Res5, and has a positive coupling sign parameter at frequencies lower than the resonant frequency fr4 of the resonator Res4 (corresponding to circuit CT1-2). The sixth connection may be a mutual inductance coupling between the third and fifth resonators. In the example of FIG. 6, the connection CCf is a mutual inductance coupling between the inductor L5 of the resonator Res5 and the inductor L32 of the resonator Res3. Since the inductor L5 and the inductor L32 are connected in series, the connection CCf corresponds to the circuit CT2-15 described below, that is, the circuit 2 group CT2 (negative coupling sign parameter).

[0059] The second resonator may be cascade-connected to the fourth resonator via a seventh connection. The seventh connection may be a mutual inductance coupling between the second resonator and the fourth resonator. In the example of FIG. 6, the resonator Res2 is cascade-connected to the resonator Res4 via a connection CCg. The connection CCg is a mutual inductance coupling between the inductor L2 of the resonator Res2 and the inductor L4 of the resonator Res4. The connection CCg corresponds to the circuit CT2-14 described below, i.e., the circuit 2 group CT2 (negative coupling sign parameter).

[0060] The filter may include a matching network. In the example of Fig. 6, the filter 100 includes a matching network MNg and a matching network MNh. The matching network MNg is composed of an inductor La and a capacitor Ca, and has a first end connected to a ground terminal GDa. The matching network MNh is composed of an inductor Lb and a capacitor Cb, and has a first end connected to a ground terminal GDb. A resonator Res1 is connected to the matching network MNg. A resonator Res5 is connected to the matching network MNh.

[0061] The filter may include a notch filter. In the example of Fig. 6, the filter 100 includes a notch filter NF. The notch filter NF is composed of an inductor Lc and a capacitor Cc, and has a first end connected to a ground terminal GDc.

[0062] A filter may have an input terminal and an output terminal. In the example of Figure 1, the filter 100 has an input terminal Pin and an output terminal Pout.

[0063] The connection diagram TP2 is a connection diagram showing the connection state between resonators Res1 to Res5 in the filter 100a. The coupling sign parameters of connection CCa connecting resonator Res1 and resonator Res2, connection CCb connecting resonator Res2 and resonator Res3, connection CCd connecting resonator Res3 and resonator Res4, and connection CCe connecting resonator Res4 and resonator Res5 are positive and represented by solid lines. On the other hand, the coupling sign parameters of connection CCc connecting resonator Res3 and resonator Res1, connection CCf connecting resonator Res5 and resonator Res3, and connection CCg connecting resonator Res4 and resonator Res2 are negative and represented by dashed lines. In other words, the coupling sign parameters of connections CCc, CCf, and CCg, which perform cross-coupling, are negative.

[0064] (Examples of attenuation characteristics of each filter) Fig. 7 is a graph showing an example of the attenuation characteristics of a filter in embodiment 2. Fig. 8 is a graph showing an enlarged portion of Fig. 7 together with the attenuation characteristics of a comparative example. Fig. 9 is a graph showing an enlarged portion of Fig. 8.

[0065] As shown in graph G2, attenuation poles P21, P22, P23, P24, and P25 occur in the example of embodiment 2. The attenuation poles P22 and P23 correspond to the resonators Res2 and Res4, and the attenuation poles P21, P24, and P25 correspond to three cross-couplings (connections CCc, CCf, and CCg) due to mutual inductance.

[0066] The attenuation poles P24 and P25 due to cross-coupling are located within the stop band A1, along with the attenuation poles P21 and P22 due to the resonator. By locating the attenuation poles P24 and P25 due to cross-coupling on the lower frequency side than the attenuation poles P21 and P22 due to the resonator, it becomes easier to reduce the attenuation in the entire stop band A1. That is, the frequency dependence of the attenuation in the stop band A1 of the filter can be reduced. Multiple attenuation poles are formed by cross-coupling, and these attenuation poles are positioned farther away from the pass band A0 than the attenuation poles due to the resonator. As a result, the attenuation in the stop band A1 can be reduced overall while maintaining a high Q. The attenuation pole P21 contributes to reducing the attenuation in the stop band, which is located on the higher frequency side than the pass band A0.

[0067] This cross coupling due to mutual inductance uses the inductor that constitutes the resonator Res as an element of the connection CC, and the connection CC itself does not include any circuit elements, which makes it easier to make the filter more compact.

[0068] [Embodiment 3] Embodiment 3 will be described below. For convenience of explanation, components having the same functions as those described in embodiment 1 will be denoted by the same reference numerals in the following embodiments, and the description thereof will not be repeated.

[0069] 10 is a circuit diagram illustrating the basic configuration of a filter 100b according to embodiment 3. In embodiment 3, the filter 100b is illustrated as a ladder filter. However, as will be apparent to those skilled in the art, the filter according to one aspect of the present disclosure is not limited to a ladder filter.

[0070] The filter 100b may include a first resonator, a second resonator, and a third resonator. In the example of Fig. 10, the filter 100b includes resonators Res1, Res2, Res3, Res4, and Res5. The resonators Res1, Res3, and Res5 are LC resonators. The resonators Res2 and Res4 are acoustic wave resonators. In this specification, the resonators Res1, Res2, Res3, Res4, and Res5 are also collectively referred to as resonators Res.

[0071] The first resonator, the second resonator, and the third resonator may form a signal path that passes signals in a pass band. In the example of Fig. 10, the resonators Res3, Res4, and Res5 function as a signal path that passes signals in a pass band defined by the filter 100b.

[0072] The first resonator may be cascaded with the second resonator via a first connection. The second resonator may be cascaded with the third resonator via a second connection. The third resonator may be cascaded with the first resonator via a third connection. In the example of FIG. 10 , the resonator Res3 is cascaded with the resonator Res4 via a connection CCc. The resonator Res4 is cascaded with the resonator Res5 via a connection CCe. The resonator Res5 is cascaded with the resonator Res3 via a connection CCf. The connection CCf cross-couples the resonators Res5 and Res3.

[0073] When Arg(S21 / S11) is defined as the coupling sign parameter using the matrix elements of the S parameters, the coupling sign parameter of the first connection and the coupling sign parameter of the second connection may both be either positive or negative, and the coupling sign parameter of the third connection may be either positive or negative. In the example of Figure 10, the coupling sign parameter of the connection CCc and the coupling sign parameter of the connection CCe may be positive, and the coupling sign parameter of the connection CCf may be negative. The connection CCc and the connection CCe may correspond to the circuit CT1-1 described below, i.e., the circuit 1 group CT1 (positive coupling sign parameter), and the connection CCf may correspond to the circuit CT2-3 described below, i.e., the circuit 2 group CT2 (negative coupling sign parameter).

[0074] The first connection may include a first coupling element. The second connection may include a second coupling element. The third connection may include a third coupling element. Both the first coupling element and the second coupling element may include one of an inductor and a capacitor, and the third coupling element may include the other of an inductor and a capacitor. In the example of Figure 10, the connection CCc includes a capacitor Cc as the first coupling element, the connection CCe includes a capacitor Ce as the second coupling element, and the connection CCf includes an inductor Lf as the third coupling element.

[0075] The first coupling element may include a first inductor and a first capacitor connected in series to the first inductor. The second coupling element may include a second inductor and a second capacitor connected in series to the second inductor. The third coupling element may include a third inductor and a third capacitor connected in parallel to the third inductor. In the example of Fig. 10, the connection CCc may include, as the first coupling element, in addition to the capacitor Cc, an inductor connected in series to the capacitor Cc. The connection CCe may include, as the second coupling element, in addition to the capacitor Ce, an inductor connected in series to the capacitor Ce. The connection CCf may include, as the third coupling element, inductor Lf and capacitor Cf connected in parallel to the inductor L3.

[0076] At least one of the first resonator, the second resonator, and the third resonator (resonators) may include an acoustic wave resonator. When a resonator including an acoustic wave resonator is located in a parallel arm, the resonator may further include a first end connected to a reference potential section. On the other hand, when a resonator including an acoustic wave resonator is located in a series arm, the first end does not need to be connected to a reference potential section. In the example of FIG. 10 , the resonator Res4 is located in the parallel arm and includes an acoustic wave resonator PL4 and a first end connected to a ground terminal GD-4 (reference potential section). More specifically, the resonator Res4 includes an inductor L4 and an acoustic wave resonator PL4 connected in parallel with each other, and a first end connected to a ground terminal GD-2 (reference potential section).

[0077] The acoustic wave resonator may be located in at least one of the first, second, and third resonators. One of the resonant frequency and anti-resonant frequency of the resonator may be located within the passband. The other of the resonant frequency and anti-resonant frequency of the resonator may not be located within the passband. In the example of FIG. 10 , the acoustic wave resonator PL4 is located in the resonator Res4. One of the resonant frequency fr4 and anti-resonant frequency fa4 of the resonator Res4 may be located within the passband of the filter 100b, while the other of the resonant frequency fr4 and anti-resonant frequency fa4 may not be located within the passband of the filter 100b. In the examples of FIGS. 11 to 13 described below, the anti-resonant frequency fa4 of the resonator Res4 is located within the passband A0 of the filter 100b, and the resonant frequency fr4 of the resonator Res4 is not located within the passband A0 of the filter 100b. As a result, an attenuation pole P31 (or an attenuation pole P34) is formed outside the pass band A0 corresponding to the resonance frequency fr4.

[0078] Of the first, second, and third resonators, the other two resonators, excluding one, may be composed of an inductor and a capacitor. The other of the resonant frequency and anti-resonant frequency of one resonator may be located closer to the passband than the attenuation poles of the other two resonators. In the example of FIG. 10 , resonator Res3 is composed of an inductor L3 and a capacitor C3 connected in series. Resonator Res5 is composed of an inductor L5 and a capacitor C5 connected in series. The other of the resonant frequency fr4 and anti-resonant frequency fa4 of resonator Res4 may be located closer to the passband than the attenuation pole fa3 of resonator Res3 and the attenuation pole fa5 of resonator Res5 (not located within the passband). In the examples of FIGS. 11 to 13 described later, the attenuation pole P34 corresponding to the resonance frequency fr4 of the resonator Res4 is located closer to the pass band A0 than the attenuation poles P32 and P33 of the resonators Res3 and Res5.

[0079] The filter may further include a fourth resonator and a fifth resonator that form a signal path together with the first resonator, the second resonator, and the third resonator. The first resonator may be cascade-connected to the fourth resonator via a fourth connection. The fourth resonator may be cascade-connected to the fifth resonator via a fifth connection. The fourth resonator may be cascade-connected to the second resonator via a sixth connection. In the example of FIG. 10 , the filter 100b includes resonators Res1 and Res2. Resonators Res1 and Res2 form a communication path together with resonators Res3, Res4, and Res5. Resonator Res3 is cascade-connected to resonator Res2 via a connection CCb. Resonator Res2 is cascade-connected to resonator Res1 via a connection CCa. Resonator Res2 is cascaded with resonator Res4 via connection CCd.

[0080] The resonator Res2 may have an elastic wave resonator PL2 and a first end connected to the ground terminal GD-2 (reference potential portion). More specifically, the resonator Res2 has an inductor L2 and an elastic wave resonator PL2 connected in parallel with each other, and a first end connected to the ground terminal GD-2 (reference potential portion). The resonator Res1 may be composed of an inductor L1 and a capacitor C1 connected in series with each other.

[0081] (Examples of attenuation characteristics of each filter) Fig. 11 is a graph showing an example of attenuation characteristics of a filter in embodiment 3. Fig. 12 is a graph showing an enlarged portion of Fig. 11 together with the attenuation characteristics of a comparative example. Fig. 13 is a graph showing an enlarged portion of Fig. 12.

[0082] As shown in graph G3, in the example of embodiment 3, attenuation poles P31, P32, P33, P34, and P35 are generated. The attenuation poles P32, P33, and P34 correspond to the resonators Res2 and Res2, and the attenuation poles P31 and P35 correspond to two cross-couplings. The attenuation pole P31 corresponds to the cross-coupling between the resonators Res2, Res3, and Res4, and the attenuation pole P35 corresponds to the cross-coupling (connection CCf) between the resonators Res3, Res4, and Res5.

[0083] The attenuation pole P34 due to cross-coupling is located within the stop band A1, along with the attenuation poles P32, P33, and P34 due to the resonator. By locating the attenuation pole P35 due to cross-coupling on the lower frequency side than the attenuation poles P32, P33, and P34 due to the resonator, it becomes easier to reduce the attenuation in the entire stop band A1. In other words, the frequency dependence of the attenuation in the stop band A1 of the filter can be reduced. The attenuation pole P34 due to cross-coupling is located farther from the pass band A0 than the attenuation poles due to the resonator. As a result, the attenuation in the stop band A1 can be reduced overall while maintaining a high Q. The attenuation pole P31 contributes to reducing the attenuation in the stop band, which is on the higher frequency side than the pass band A0.

[0084] [Fourth Embodiment] The following describes a fourth embodiment. For convenience of explanation, components having the same functions as those described in the first embodiment are denoted by the same reference numerals, and the description thereof will not be repeated.

[0085] 14 is a schematic diagram illustrating the basic configuration of a filter 100c according to embodiment 4. As the filter 100c, a filter 100c1, a filter 100c2, and a filter 100c3 are shown, each having a different combination of positive and negative values ​​of the combined sign parameters of connections CC1 to CC7.

[0086] The filter 100c is an abstract representation of the filters 100, 100a, and 100b shown in FIGS. 1, 6, and 10. The filter 100c1 in FIG. 14 has resonators Res1 to Res5 that are cascade-connected between the input terminal Pin and the output terminal Pout and form a signal path. The resonators Res1 and Res2 are cascade-connected by a connection CC1. The resonators Res2 and Res3 are connected by a connection CC2. The resonators Res3 and Res4 are cascade-connected by a connection CC3. The resonators Res4 and Res5 are cascade-connected by a connection CC4.

[0087] On the other hand, resonators Res1 and Res3 are cascade-connected by a connection CC5. Resonators Res2 and Res4 are cascade-connected by a connection CC6. Resonators Res3 and Res5 are cascade-connected by a connection CC7. At least one of connections CC5, CC6, and CC7 may be omitted.

[0088] The resonators Res1, Res3, and Res5 may be LC resonators. The resonators Res2 and Res4 may be acoustic wave resonators. The resonators Res1 to Res5 may have the same configurations as the resonators Res1 to Res5 shown in FIGS. 1, 6, and 10, respectively.

[0089] Connections CC1 to CC7 can have the same configuration as connections CCa to CCg in Figures 1, 6, and 10. In addition to this, connections CC1 to CC7 can also have a variety of other configurations. Configurations that can be used for connections CC1 to CC7 are described below.

[0090] 15 to 18 are circuit diagrams illustrating circuit configurations applicable to connections CC1 to CC7. These configurations are divided into a circuit 1 group CT1 and a circuit 2 group CT2. Circuit 1 group CT1 includes circuits CT1-1 to CT1-2, circuits CT1-11 to CT1-14, circuits CT1-21 to CT1-22, and circuits CT1-31 to CT1-32. Circuit 2 group CT2 includes circuits CT2-1 to CT2-2, circuits CT2-11 to CT2-15, circuits CT2-21 to CT2-22, and circuits CT2-31 to CT2-32.

[0091] The first circuit group CT1 has a configuration in which the combined sign parameter Arg(S21 / S11) is positive, and the second circuit group CT2 has a configuration in which the combined sign parameter Arg(S21 / S11) is negative.

[0092] The joint sign parameter Arg(S21 / S11) will be explained below. The S parameter S11 is the input reflection coefficient as seen from the input side when the output side of the filter is terminated at Z0. The S parameter S21 is the transmission coefficient from the input side to the output side when the output side of the filter is terminated at Z0. In other words, the joint sign parameter Arg(S21 / S11) represents the positive or negative phase of the transmitted wave relative to the reflected wave.

[0093] Hereinafter, the circuits CT1-i and CT2-i (i: integer) will be described together, because the circuits CT1-i and CT2-i with the same number i correspond in configuration.

[0094] The circuit CT1-1 has a capacitor C, a first terminal T1, and a second terminal T2. The first terminal T1 is connected to the first terminal of the capacitor C and to one of the two resonators Res connected by the circuit CT2-1. The second terminal T2 is connected to the second terminal of the capacitor C and to the other of the two resonators Res.

[0095] The circuit CT2-1 has an inductor L, a first terminal T1, and a second terminal T2. The first terminal T1 is connected to the first terminal of the inductor L and to one of the two resonators Res connected by the circuit CT2-2. The second terminal T2 is connected to the second terminal of the inductor L and to the other of the two resonators Res.

[0096] In these circuits CT1-1 and CT2-1, the sign of the combined sign parameter Arg(S21 / S11) is determined by the difference in element configuration, i.e., the difference in the capacitor C and inductor L. In contrast, as will be shown below, there are cases where the sign of the combined sign parameter Arg(S21 / S11) cannot be determined by the element configuration alone.

[0097] Both the circuit CT1-2 and the circuit CT2-2 have an inductor L and a capacitor C connected in series, a first end T1, and a second end T2. The first end T1 is connected to a first end of the inductor L and capacitor C connected in series, and is connected to one of two resonators Res connected by the circuit CT2-2. The second end T2 is connected to a second end of the inductor L and capacitor C connected in series, and is connected to the other of these two resonators Res. In the following, the description of the first end T1 and the second end T2 will be omitted as appropriate.

[0098] The positive or negative value of the joint sign parameter for these circuits CT1-2 and CT2-2 is determined by its relationship with the frequency. That is, if the frequency f is small relative to the resonant frequency f0 in these configurations, the joint sign parameter will be positive (circuit CT1-2). On the other hand, if the frequency f is large relative to the resonant frequency f0, the joint sign parameter will be negative (circuit CT2-2).

[0099] The resonance frequency f0 of the circuit CT1-2 and the circuit CT2-2 is defined by the following formula: f0=1 / [2·π·(L·C)] １／２ ] L: inductance of inductor L C: capacitance of capacitor C

[0100] As described above, the frequency determines whether the circuits CT1-2 and CT2-2 belong to circuit group CT1 or circuit group CT2. Furthermore, even if the circuit configuration is substantially the same, for two circuits with different inductances L and capacitances C, one may belong to circuit group CT1 and the other to circuit group CT2.

[0101] Both the circuit CT1-3 and the circuit CT2-3 have an inductor L and a capacitor C connected in parallel, a first terminal T1, and a second terminal T2. In this case, if the frequency f is greater than the resonant frequency f0 in these configurations, the coupling sign parameter is positive (circuit CT1-3). On the other hand, if the frequency f is smaller than the resonant frequency f0, the coupling sign parameter is negative (circuit CT2-3).

[0102] The circuit CT1-11 has a capacitor C, a first terminal T1, a second terminal T2, and two third terminals T0. The first terminal T1 and the second terminal T2 are connected to a first terminal of the capacitor C. The two third terminals T0 are connected to a second terminal of the capacitor C. The first terminal T1 is connected to one of two resonators Res connected by the circuit CT1-11. The second terminal T2 is connected to the other of these two resonators Res. The two third terminals T0 are connected to each of these two resonators Res or to a ground terminal (reference potential portion). The circuit CT1-11 has a positive coupling sign parameter.

[0103] The circuit CT2-11 has an inductor L, a first terminal T1, a second terminal T2, and a third terminal T0. The first terminal T1 and the second terminal T2 are connected to the first terminal of the inductor L. The third terminal T0 is connected to the second terminal of the inductor L. The connections of the first terminal T1, the second terminal T2, and the third terminal T0 are the same as those of the circuit CT1-11, so a description thereof will be omitted. The circuit CT2-11 has a negative coupling sign parameter.

[0104] Both the circuit CT1-12 and the circuit CT2-12 have an inductor L and a capacitor C connected in series, a first terminal T1, a second terminal T2, and a third terminal T0. The first terminal T1 and the second terminal T2 are connected to a first terminal of the inductor L and the capacitor C connected in series. The third terminal T0 is connected to a second terminal of the inductor L and the capacitor C connected in series. If the frequency f is small with respect to the resonant frequency f0, the coupling sign parameter is positive (circuit CT1-12). On the other hand, if the frequency f is large with respect to the resonant frequency f0, the coupling sign parameter is negative (circuit CT2-12).

[0105] Both the circuit CT1-13 and the circuit CT2-13 have an inductor L and a capacitor C connected in parallel, a first terminal T1, a second terminal T2, and a third terminal T0. The first terminal T1 and the second terminal T2 are connected to a first terminal of the inductor L and the capacitor C connected in parallel. The third terminal T0 is connected to a second terminal of the inductor L and the capacitor C connected in parallel. If the frequency f is larger than the resonant frequency f0, the coupling sign parameter is positive (circuit CT1-13). On the other hand, if the frequency f is smaller than the resonant frequency f0, the coupling sign parameter is negative (circuit CT2-13).

[0106] Each of the circuits CT1-14 and CT2-14 includes an inductor L1 and an inductor L2 coupled by a mutual inductance, a first end T1, a second end T2, and a third end T0. The first end T1 is connected to the first end of the inductor L1, and the second end T2 is connected to the first end of the inductor L2. The third end T0 is connected to the second end of the inductor L1 and the second end of the inductor L2.

[0107] Here, the components of the circuit CT1-14 and the circuit CT2-14 are included in the two resonators Res connected by the circuit CT1-14 and the circuit CT2-14. That is, the inductor L1 is a component of one of the two resonators Res connected by the circuit CT1-14 and the circuit CT2-14. The inductor L2 is a component of the other of these two resonators Res. That is, the circuit CT1-14 and the circuit CT2-14 do not need to exist as independent circuits.

[0108] Here, if the polarity of the signal at the first end of inductor L1 and the first end of inductor L2, which are coupled by mutual inductance, is opposite, the coupling sign parameter is positive (circuit CT1-14). On the other hand, if the polarity of the signal at the first end of inductor L1 and the first end of inductor L2 is the same, the coupling sign parameter is negative (circuit CT2-14). This positive / negative relationship changes depending on the winding direction of the coils of inductor L1 and inductor L2.

[0109] The circuit CT2-15 has inductors L1 and L2 connected in series and coupled by a mutual inductance, a first end T1, a second end T2, and a third end T0. Like the circuits CT1-14 and CT2-14, the components of the circuit CT2-15 are included in two resonators Res connected by the circuit CT2-15.

[0110] On the other hand, the circuit CT2-15 differs from the circuits CT1-14 and CT2-14 in that the inductors L1 and L2 are connected in series between the first terminal T1 and the second terminal T2. Because the inductors L1 and L2 are connected in series, the polarity of these inductances affects the increase or decrease of the mutual inductance, but does not cause a reversal of the propagation phase. As a result, the coupling sign parameter of the circuit CT2-15 is negative.

[0111] Each of the circuits CT1-21 and CT2-21 has two inductors L, one capacitor C, a first terminal T1, a second terminal T2, and a third terminal T0. The two inductors L are connected in series between the first terminal T1 and the second terminal T2. The capacitor C has a first terminal connected to the connection point of the two inductors L and a second terminal connected to the third terminal T0.

[0112] In these circuits CT1-21 and CT2-21, "L / C>Z0 ２ / 2" or "L / C<Z0 ２ / 2" is added as a condition. ２ / 2, if the frequency f is small relative to the resonance frequency f0, the joint sign parameter becomes positive (circuit CT1-21), and if the frequency f is large relative to the resonance frequency f0, the joint sign parameter becomes negative (circuit CT2-21). ２ / 2, if the frequency f is large relative to the resonance frequency f0, the joint sign parameter is positive (circuit CT1-21), and if the frequency f is small relative to the resonance frequency f0, the joint sign parameter is negative (circuit CT2-21).

[0113] The resonance frequency f0 of the circuit CT1-21 and the circuit CT2-21 is defined by the following formula: f0=|2·L / C−Z0 ２ | １／２ / (2 π L) L: inductance of inductor L C: capacitance of capacitor C Z0: characteristic impedance of circuit CT1-21 and circuit CT2-21 Z0 = 50 [Ω]

[0114] Here, for ease of understanding, the inductances of the two inductors L are assumed to be equal, but the inductances of the two inductors L may be different.

[0115] Each of the circuits CT1-22 and CT2-22 has two capacitors C, one inductor L, a first terminal T1, a second terminal T2, and a third terminal T0. The two capacitors C are connected in series between the first terminal T1 and the second terminal T2. The inductor L has a first terminal connected to the connection point of the two capacitors C and a second terminal connected to the third terminal T0.

[0116] In these circuits CT1-22 and CT2-22, "L / C>Z0 ２ / 2, if the frequency f is small relative to the resonance frequency f0, the joint sign parameter becomes positive (circuit CT1-22), and if the frequency f is large relative to the resonance frequency f0, the joint sign parameter becomes negative (circuit CT2-22). ２ / 2, if the frequency f is large relative to the resonance frequency f0, the joint sign parameter is positive (circuit CT1-22), and if the frequency f is small relative to the resonance frequency f0, the joint sign parameter is negative (circuit CT2-22).

[0117] The resonance frequency f0 of the circuit CT1-22 and the circuit CT2-22 is defined by the following formula: f0=1 / [2·π·C·|2·L / C−Z0] ２ | １／２ ] L: inductance of inductor L C: capacitance of capacitor C Z0: characteristic impedance of circuit CT1-21 and circuit CT2-21 Z0 = 50 [Ω]

[0118] Here, for ease of understanding, the capacitances of the two capacitors C are assumed to be equal, but the capacitances of the two capacitors C may be different.

[0119] Both the circuit CT1-31 and the circuit CT2-31 have one inductor L, two capacitors C, a first terminal T1, a second terminal T2, and a third terminal T0. The one inductor L is connected between the first terminal T1 and the second terminal T2. One of the two capacitors C is connected between the first terminal T1 and the third terminal T0. The other of the two capacitors C is connected between the second terminal T2 and the third terminal T0.

[0120] In these circuits CT1-31 and CT2-31, "L / C>Y0 ２ / 2, if the frequency f is greater than the resonance frequency f0, the joint sign parameter becomes positive (circuit CT1-31), and if the frequency f is smaller than the resonance frequency f0, the joint sign parameter becomes negative (circuit CT2-31). ２ / 2", if the frequency f is small relative to the resonance frequency f0, the joint sign parameter is positive (circuit CT1-31), and if the frequency f is large relative to the resonance frequency f0, the joint sign parameter is negative (circuit CT2-31).

[0121] The resonance frequency f0 of the circuit CT1-31 and the circuit CT2-31 is defined by the following formula: f0=|2·C / L−Y0 ２ | １／２ / (2 π C) L: Inductance of inductor L C: Capacitance of capacitor C Y0: Characteristic admittance of circuit CT1-21 and circuit CT2-21 Y0 = 0.02 [S] (= 1 / 50 [Ω] －１ ])

[0122] Here, for ease of understanding, the capacitances of the two capacitors C are assumed to be equal, but the capacitances of the two capacitors C may be different.

[0123] Each of the circuits CT1-32 and CT2-32 has two inductors L, one capacitor C, a first terminal T1, a second terminal T2, and a third terminal T0. The one capacitor C is connected between the first terminal T1 and the second terminal T2. One of the two inductors L is connected between the first terminal T1 and the third terminal T0. The other of the two inductors L is connected between the second terminal T2 and the third terminal T0.

[0124] In these circuits CT1-32 and CT2-32, "L / C>Y0 ２ / 2, if the frequency f is small relative to the resonance frequency f0, the joint sign parameter becomes positive (circuit CT1-32), and if the frequency f is large relative to the resonance frequency f0, the joint sign parameter becomes negative (circuit CT2-32). ２ / 2", if the frequency f is large relative to the resonance frequency f0, the joint sign parameter is positive (circuit CT1-32), and if the frequency f is small relative to the resonance frequency f0, the joint sign parameter is negative (circuit CT2-32).

[0125] The resonance frequency f0 of the circuit CT1-32 and the circuit CT2-32 is defined by the following formula: f0=1 / [2·π·L·|2·C / L−Y0] ２ | １／２ ] L: inductance of inductor L C: capacitance of capacitor C Y0: characteristic admittance of circuit CT1-32 and circuit CT2-32 Y0=0.02 [S]

[0126] Here, for ease of understanding, the inductances of the two inductors L are assumed to be equal, but the inductances of the two inductors L may be different.

[0127] As described above, resonators can be connected using a variety of circuit 1 groups CT1 and circuit 2 groups CT2. For example, circuit 1 group CT1 can be applied to connections CCa, CCb, CCd, and CCd in embodiment 1, and circuit 2 group CT2 can be applied to connections CCc and CCf. Circuit 1 group CT1 can be applied to connections CCa, CCb, CCd, and CCd in embodiment 2, and circuit 2 group CT2 can be applied to connections CCc, CCf, and CCg. Circuit 1 group CT1 can be applied to connections CCa, CCb, CCc, CCd, and CCd in embodiment 3, and circuit 2 group CT2 can be applied to connection CCf.

[0128] Furthermore, in the filter connections CC, the circuit 1 group CT1 and the circuit 2 group CT2 may be interchanged. For example, as seen in filters 100c1, 100c2, and 100c3, all or some of the positive and negative values ​​of the coupling sign parameters may be interchanged. In this case, in the three connections constituting at least one of the following connection combinations (1) to (3), the signs of the coupling sign parameters of two connections may be either positive or negative, and the sign of the coupling sign parameter of the remaining connection may be either positive or negative.

[0129] [Summary] A filter according to aspect 1 of the present disclosure includes a first resonator, a second resonator, and a third resonator that form a signal path that passes a passband signal, wherein the first resonator is cascade-connected to the second resonator via a first connection, the second resonator is cascade-connected to the third resonator via a second connection, and the third resonator is cascade-connected to the first resonator via a third connection, and when Arg(S21 / S11) is defined as a coupling sign parameter using matrix elements of S parameters, both of the coupling sign parameter of the first connection and the coupling sign parameter of the second connection are either positive or negative, and the coupling sign parameter of the third connection is the other of positive and negative.

[0130] A filter according to aspect 2 of the present disclosure is the filter of aspect 1, wherein the first connection includes a first coupling element, the second connection includes a second coupling element, the third connection includes a third coupling element, both the first coupling element and the second coupling element include either an inductor or a capacitor, and the third coupling element includes the other of the inductor and the capacitor.

[0131] A filter according to aspect 3 of the present disclosure is the filter of aspect 2, wherein the first coupling element includes a first inductor and a first capacitor connected in series to the first inductor, the second coupling element includes a second inductor and a second capacitor connected in series to the second inductor, and the third coupling element includes a third inductor and a third capacitor connected in parallel to the third inductor.

[0132] A filter according to aspect 4 of the present disclosure is configured such that, in any of aspects 1 to 3, at least one of the combined code parameter of the first connection, the combined code parameter of the second connection, and the combined code parameter of the third connection has both positive and negative values ​​between 0.5 times the lowest frequency of the pass band and twice the highest frequency of the pass band.

[0133] A filter according to a fifth aspect of the present disclosure is the filter of any one of the first to fourth aspects, wherein at least one of the first resonator, the second resonator, and the third resonator includes an acoustic wave resonator.

[0134] A filter according to aspect 6 of the present disclosure is the same as that of aspect 5, except that the elastic wave resonator is located in one resonator which is at least one of the first resonator, the second resonator, and the third resonator, and one of the resonant frequency and the anti-resonant frequency of the one resonator is located within the pass band, and the other of the resonant frequency and the anti-resonant frequency of the one resonator is not located within the pass band.

[0135] A filter according to aspect 7 of the present disclosure is the same as that of aspect 6, except that two other resonators among the first resonator, the second resonator, and the third resonator, excluding the first resonator, are composed of an inductor and a capacitor, and the other of the resonant frequency and the anti-resonant frequency is located closer to the pass band than the attenuation poles of the two other resonators.

[0136] A filter according to an eighth aspect of the present disclosure is the filter of any one of the first to seventh aspects, wherein the third connection is coupling between the first resonator and the third resonator by mutual inductance.

[0137] A filter according to a ninth aspect of the present disclosure is the filter of any one of the first to eighth aspects, further comprising a fourth resonator and a fifth resonator that form the signal path together with the first resonator, the second resonator, and the third resonator, wherein the third resonator is cascade-connected to the fourth resonator via a fourth connection, the fourth resonator is cascade-connected to the fifth resonator via a fifth connection, and the fifth resonator is cascade-connected to the third resonator via a sixth connection, wherein both the coupling sign parameter of the fourth connection and the coupling sign parameter of the fifth connection are either positive or negative, and the coupling sign parameter of the sixth connection is the other of positive and negative.

[0138] A filter according to aspect 10 of the present disclosure is the same as that of aspect 9, wherein the fourth connection includes a fourth coupling element, the fifth connection includes a fifth coupling element, the sixth connection includes a sixth coupling element, both the fourth coupling element and the fifth coupling element include either an inductor or a capacitor, and the sixth coupling element includes the other of an inductor and a capacitor.

[0139] A filter according to an eleventh aspect of the present disclosure is the filter of the ninth aspect, wherein the sixth connection is coupling between the third resonator and the fifth resonator by mutual inductance.

[0140] A filter according to aspect 12 of the present disclosure is any one of aspects 9 to 11, wherein the second resonator is cascade-connected to the fourth resonator via a seventh connection, and the seventh connection is coupling between the second resonator and the fourth resonator by mutual inductance.

[0141] [Additional Notes] The invention according to the present disclosure has been described above based on the drawings and examples. However, the invention according to the present disclosure is not limited to the above-described embodiments. In other words, the invention according to the present disclosure can be modified in various ways within the scope of the present disclosure, and embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the invention according to the present disclosure. In other words, it should be noted that a person skilled in the art could easily make various modifications or corrections based on the present disclosure. It should also be noted that these modifications or corrections are included in the scope of the present disclosure.

[0142] 100, 100a, 100b, 100c, 100x Filter Res (Res1 to Res5) Resonator PL2, PL4, SAW1 to SAW4 Elastic wave resonator CC1 to CC7, CCa to CCg Connection CC1 (CC1-1 to CC1-32) Circuit 1 group CC2 (CC2-1 to CC2-32) Circuit 2 group Pin Input terminal Pout Output terminal

Claims

1. A filter comprising a first resonator, a second resonator, and a third resonator constituting a signal path for passing a passband signal, wherein the first resonator is cascade-connected to the second resonator via a first connection, the second resonator is cascade-connected to the third resonator via a second connection, and the third resonator is cascade-connected to the first resonator via a third connection, wherein, when Arg(S21 / S11) is defined as a coupling sign parameter using matrix elements of S parameters, both of the coupling sign parameter of the first connection and the coupling sign parameter of the second connection are either positive or negative, and the coupling sign parameter of the third connection is the other of positive and negative.

2. The filter of claim 1, wherein the first connection includes a first coupling element, the second connection includes a second coupling element, the third connection includes a third coupling element, both the first coupling element and the second coupling element include one of an inductor and a capacitor, and the third coupling element includes the other of an inductor and a capacitor.

3. The filter of claim 2, wherein the first coupling element includes a first inductor and a first capacitor connected in series to the first inductor, the second coupling element includes a second inductor and a second capacitor connected in series to the second inductor, and the third coupling element includes a third inductor and a third capacitor connected in parallel to the third inductor.

4. A filter as claimed in any one of claims 1 to 3, wherein at least one of the combined code parameter of the first connection, the combined code parameter of the second connection, and the combined code parameter of the third connection is configured to have both positive and negative values ​​between 0.5 times the lowest frequency of the pass band and twice the highest frequency of the pass band.

5. The filter according to any one of claims 1 to 4, wherein at least one of the first resonator, the second resonator and the third resonator has an elastic wave resonator.

6. The filter as described in claim 5, wherein the elastic wave resonator is located in one resonator which is at least one of the first resonator, the second resonator, and the third resonator, one of the resonant frequency and the anti-resonant frequency of the one resonator is located within the pass band, and the other of the resonant frequency and the anti-resonant frequency of the one resonator is not located within the pass band.

7. The filter according to claim 6, wherein two other resonators, excluding the one resonator, among the first resonator, the second resonator, and the third resonator are constituted by an inductor and a capacitor, and the other of the resonant frequency and the anti-resonant frequency is located on the side of the passband with respect to the attenuation poles of the two other resonators.

8. A filter according to any one of claims 1 to 7, wherein the third connection is coupling between the first resonator and the third resonator by mutual inductance.

9. The filter according to any one of claims 1 to 8, further comprising a fourth resonator and a fifth resonator which configure the signal path together with the first resonator, the second resonator, and the third resonator, wherein the third resonator is cascade connected to the fourth resonator via a fourth connection, the fourth resonator is cascade connected to the fifth resonator via a fifth connection, and the fifth resonator is cascade connected to the third resonator via a sixth connection, both of the coupling sign parameter of the fourth connection and the coupling sign parameter of the fifth connection are either positive or negative, and the coupling sign parameter of the sixth connection is the other of positive and negative.

10. The filter of claim 9, wherein the fourth connection includes a fourth coupling element, the fifth connection includes a fifth coupling element, the sixth connection includes a sixth coupling element, both the fourth coupling element and the fifth coupling element include one of an inductor and a capacitor, and the sixth coupling element includes the other of an inductor and a capacitor.

11. The filter of claim 9, wherein the sixth connection is a mutual inductance coupling between the third resonator and the fifth resonator.

12. A filter as claimed in any one of claims 9 to 11, wherein the second resonator is cascade-connected to the fourth resonator via a seventh connection, and the seventh connection is coupling between the second resonator and the fourth resonator by mutual inductance.

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