Ladder filter

This application claims the benefit of priority to Japanese Patent Application No. 2016-189983 filed on Sep. 28, 2016 and is a Continuation Application of PCT Application No. PCT/JP2017/027313 filed on Jul. 27, 2017. The entire contents of each application are hereby incorporated herein by reference.

1. Field of the Invention

The present invention relates to a ladder filter including multiple serial arm resonators and multiple parallel arm resonators.

2. Description of the Related Art

Ladder filters have been used widely as band pass filters in RF stages of cellular phones. Ladder filters may achieve reduction in the loss and use in a higher frequency band. In addition, ladder filters may increase attenuation near the pass band.

In the ladder filter described in a PCT Publication WO 2011/065199 A1, multiple stages of ladder circuit units are connected to one another. Each ladder circuit unit includes a single serial arm resonator and a single parallel arm resonator. To reduce reflection between adjacent ladder circuit units, one of the adjacent ladder circuit units is connected to the other in a mirror-reversed manner.

In a band pass filter such as a ladder filter, the impedance of the input terminal and the impedance of the output terminal are typically set to 50Ω.

On the other hand, in a radio frequency circuit used in an RF stage or the like or other suitable structure of a cellular phone, a component, such as an amplifying device, a switch, or an LNA, is connected to a ladder filter. The impedance of such peripheral a component is sometimes not 50Ω. Therefore, to achieve impedance matching in such a peripheral component, an impedance matching circuit needs to be provided. An impedance matching circuit is a circuit that typically includes an inductor and a capacitor, and is generally a circuit having includes two devices or three devices. Accordingly, a large loss due to the impedance matching circuit is incurred. In addition, there arises a problem that the footprint is made large.

Preferred embodiments of the present invention provide ladder filters each of which may reduce a number of impedance matching devices, thus enabling a reduction or prevention of an increase in the loss due to the impedance matching devices and enabling the footprint to be small.

A ladder filter according to a preferred embodiment of the present invention includes a serial arm and a plurality of parallel arms. The serial arm connects a first end to a second end. The plurality of parallel arms are connected between the serial arm and a ground potential. The ladder filter includes a plurality of serial arm resonators disposed along the serial arm and a plurality of parallel arm resonators disposed along the respective parallel arms. Ladder circuit units whose number is n (n is an integer of two or more, hereinafter “n ladder circuit units”) are disposed from the first end to the second end. Each of the n ladder circuit units includes a corresponding one of the serial arm resonators and a corresponding one of the parallel arm resonators. The n ladder circuit units are mirror-connected to one another. An impedance at the first end is different from an impedance at the second end.

In a ladder filter according to a preferred embodiment of the present invention, in the n ladder circuit units, when impedances of adjacent ladder circuit units are Xa and Xb, the adjacent ladder circuit units being mirror-connected to one another, and when the ladder circuit unit with the impedance Xa is positioned on the first end side relative to the ladder circuit unit with the impedance Xb, Xa≤Xb or Xa≥Xb, in which not all impedances of all of the ladder circuit units are equal or substantially equal to one another.

In a ladder filter according to a preferred embodiment of the present invention, when the impedances of the n ladder circuit units are Xa, Xb, Xc, . . . , Xn from the first end side to the second end side, Xa≤Xb≤Xc≤ . . . ≤Xn or Xa≥Xb≥Xc≥ . . . ≥Xn is satisfied, but Xa=Xb=Xc= . . . =Xn is not satisfied.

In a ladder filter according to a preferred embodiment of the present invention, an inductor that is connected between at least one of ends and the ground potential is included. The ends are the first end and the second end.

In a ladder filter according to a preferred embodiment of the present invention, the impedance at the first end or the second end is preferably about 50Ω.

In a ladder filter according to a preferred embodiment of the present invention, the serial arm resonators and the parallel arm resonators are defined by elastic wave resonators.

Preferred embodiments of the present invention may provide ladder filters which may reduce the number of impedance matching devices, thus decreasing the loss and decreasing the footprint.

The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.

FIG. 1 is a circuit diagram illustrating a ladder filter according to a preferred embodiment of the present invention.

FIG. 2 is a diagram illustrating bandpass characteristics of ladder filters of an example of a preferred embodiment of the present invention and a comparative example.

FIG. 3 is an impedance Smith chart on the input terminal side defining and functioning as first ends of ladder filters of an example of a preferred embodiment of the present invention and a comparative example.

FIG. 4 is an impedance Smith chart on the output terminal side defining and functioning as second ends of ladder filters of an example of a preferred embodiment of the present invention and a comparative example.

FIG. 5 is a diagram illustrating a circuit configuration obtained when adjacent resonators of adjacent ladder circuit units in the circuit diagram illustrated in FIG. 1 are combined in terms of a capacitance.

Referring to the drawings, specific preferred embodiments of the present invention will be described below so as to clarify the present invention.

It is noted that the preferred embodiments described herein are exemplary, and that some components in different preferred embodiments may be replaced or combined with one another.

FIG. 1 is a circuit diagram of a ladder filter according to a preferred embodiment of the present invention.

A ladder filter 1 includes an input terminal 901 defining and functioning as a first end and an output terminal 902 defining and functioning as a second end. Multiple serial arm resonators 101 to 107 are disposed along a serial arm connecting the input terminal 901 to the output terminal 902 . A single parallel arm resonator is disposed along each of the multiple parallel arms connecting the serial arm to the ground potential. In FIG. 1 , parallel arm resonators 201 to 207 are disposed along the seven respective parallel arms.

An inductor 301 defining and functioning as an impedance matching device is connected between the input terminal 901 and the ground potential. An inductor 302 defining and functioning as an impedance matching device is connected also between the output terminal 902 and the ground potential.

The ladder filter 1 includes seven stages of ladder circuit units each including one serial arm resonator and one parallel arm resonator. For example, the first-stage ladder circuit unit includes the serial arm resonator 101 and the parallel arm resonator 201 . The second-stage ladder circuit unit includes the parallel arm resonator 202 and the serial arm resonator 102 . In the portion in which the first-stage and second-stage ladder circuit units are adjacent to each other, the first-stage ladder circuit unit includes a mirror-reversed configuration with respect to that of the second-stage ladder circuit unit. Thus, the connection configuration, in which one of adjacent ladder circuit units includes a mirror-reversed configuration with respect to that of the other ladder circuit unit, is called mirror connection. In the ladder filter 1 , for example, seven stages of ladder circuit units are preferably mirror-connected to one another.

The reason why mirror connection is used as described above is to reduce or prevent reflection between ladder circuit units, as described in the PCT publication WO 2011/065199 A1.

A feature of the ladder filter 1 according to the present preferred embodiment is that the impedance at the input terminal 901 , defining and functioning as a first end, is different from the impedance at the output terminal 902 defining and functioning as the second end. As described above, the impedance at the input terminal and the impedance at the output terminal have been typically set to, for example, about 50Ω in a band pass filter, such as a ladder filter. Therefore, when the impedance of a peripheral component that is to be connected is different from, for example, about 50Ω, an impedance matching circuit including two devices or three devices needs to be used to achieve impedance matching.

In contrast, in the ladder filter 1 , the impedance at the input terminal 901 is preferably different from the impedance at the output terminal 902 , that is, not all of the impedances are set to, for example, about 50Ω. Therefore, the ladder filter 1 may reduce the impedance difference from the impedance of a peripheral component. Consequently, the ladder filter 1 may reduce the number of impedance matching devices in an impedance matching circuit. Accordingly, an impedance matching circuit, which is not always necessary, including a single inductor 301 is provided on the input terminal 901 side. Similarly, an impedance matching circuit, which is not always necessary, including a single inductor 302 is also provided on the output terminal 902 side.

As described above, the number of impedance matching devices necessary for an impedance matching circuit may be reduced, and thus, a reduction or prevention of an increase of the loss due to an impedance matching circuit is achieved. In addition, since the size of the impedance matching circuit is decreased, the footprint of the ladder filter 1 may be also decreased.

Preferably, in a ladder filter including ladder circuit units whose number is n (n is an integer of two or more), in the case in which the impedances of adjacent ladder circuit units are Xa and Xb, and where the ladder circuit unit of the impedance Xa is positioned on the first end side relative to the ladder circuit unit of the impedance Xb, Xa≤Xb or Xa≥Xb, for example, is preferably satisfied. In this case, the impedances of all of the ladder circuit units are not the same or substantially same. In the case of this preferable configuration, in the ladder filter 1 , the impedance preferably remains the same or substantially same, or the impedance decreases sequentially along the path from the first end, that is, the input terminal 901 side, to the output terminal 902 side. Alternatively, the impedance preferably remains the same or substantially same, or the impedance increases sequentially. Therefore, the impedance difference between the stages may be reduced. Accordingly, the loss may be further decreased.

The preferable configuration described above is expressed in another representation as follows: when the impedances of ladder circuit units, whose number is n, are Xa, Xb, Xc, . . . , Xn from the first end side to the second end side, for example, Xa≤Xb≤Xc≤ . . . ≤Xn or Xa≥Xb≥Xc≥ . . . ≥Xn is preferably satisfied, but Xa=Xb=Xc= . . . =Xn is not satisfied.

The physical structure of the serial arm resonators 101 to 107 and the parallel arm resonators 201 to 207 described above is not particularly limiting. However, preferably, the serial arm resonators 101 to 107 and the parallel arm resonators 201 to 207 are defined by elastic wave resonators.

While achieving impedance matching, the ladder filter 1 may reduce the number of impedance matching devices and thus reduce the loss. This will be specifically described with reference to an example of a preferred embodiment and a comparative example, which are described below.

A ladder filter 1 defining and functioning as a band pass filter for Band 41 was manufactured as an example of the above-described preferred embodiment.

The design parameters of the serial arm resonators 101 to 107 and the parallel arm resonators 201 to 207 are illustrated in Table 1 described below.

In the example of a preferred embodiment described above, seven stages of ladder circuit units are mirror-connected to one another as described above. Therefore, for example, when it is assumed that the impedances of the ladder circuit units are X 1 , X 2 , X 3 , . . . , X 7 from the input terminal 901 side to the output terminal 902 side, the impedances of the ladder circuit units are preferably configured so that X 1 ≤X 2 ≤X 3 ≤ . . . ≤X 7 is satisfied, but X 1 =X 2 =X 3 = . . . =X 7 is not satisfied.

A ladder filter as the comparative example in which seven stages of ladder circuit units are mirror-connected to one another similarly to the above-described example of a preferred embodiment was prepared. However, in the ladder filter of the comparative example, the impedances of the stages are equal or substantially equal to one another. In the example of a preferred embodiment described above, the inductance of the inductor 301 and the inductance of the inductor 302 are preferably set to about 5 nH, for example. In the comparative example, the inductances of the inductors connected between the input and output terminals and the ground potential are set to about 5 nH, for example.

FIG. 2 is a diagram illustrating bandpass characteristics of the ladder filters of the example of a preferred embodiment and the comparative example described above. The solid line indicates the results of the example of a preferred embodiment. The broken line indicates the result of the comparative example.

As is clear from FIG. 2 , the bandpass characteristics of the example of a preferred embodiment in the pass band between about 2545 MHz and about 2655 MHz are equivalent or substantially equivalent to those of the comparative example.

FIGS. 3 and 4 illustrate an impedance Smith chart on the input terminal side defining and functioning as the first ends of the ladder filters of the example of a preferred embodiment and the comparative example described above, and an impedance Smith chart on the output terminal side defining and functioning as the second ends.

Also in FIGS. 3 and 4 , the solid lines indicate the results of the example of preferred embodiment. The broken lines indicate the results of the comparative example. FIG. 3 shows that both of the impedances of the example of a preferred embodiment and the comparative example are about 50Ω. In contrast, FIG. 4 shows that the impedance of the example of a preferred embodiment deviates from 50Ω, and is about 70Ω. FIG. 4 shows that the impedance of the comparative example is also about 50Ω.

As described above, the design parameters of the multiple serial arm resonators 101 to 107 and parallel arm resonators 201 to 207 are adjusted. Thus, the impedances of the stages of ladder circuit units may be easily adjusted. This enables the impedance on the input terminal 901 side, which is the first end, to be different from the impedance on the output terminal 902 side which is the second end. Therefore, the impedances on the input terminal 901 side and the output terminal 902 side are adjusted. Thus, the impedance difference from the impedance of a peripheral component that is to be connected may be reduced. Therefore, it was discovered that impedance matching may preferably be achieved by providing an impedance matching circuit disposed of the single inductor 301 or the single inductor 302 .

The serial arm resonators 101 to 107 and the parallel arm resonators 201 to 207 are connected to one another as illustrated in FIG. 1 . In the actual design of a ladder filter, many circuits include a configuration in which the capacitances of adjacent resonators are combined with each other.

FIG. 5 is a diagram illustrating the circuit configuration obtained when, in the ladder filter 1 illustrated in FIG. 1 , adjacent resonators of adjacent ladder circuit units are combined in terms of capacitance.

That is, as serial arm resonators in a ladder filter 1 A illustrated in FIG. 5 , the serial arm resonator 101 , a serial arm resonator 102 A, a serial arm resonator 104 A, and a serial arm resonator 106 A are disposed in this sequence from the input terminal 901 side to the output terminal 902 side. The parallel arm resonators 201 A, 203 A, and 205 A, and the parallel arm resonator 207 are disposed. The capacitance of the serial arm resonator 102 A corresponds to the combined capacitance of the serial arm resonator 102 and the serial arm resonator 103 in the ladder circuit units in FIG. 1 . Therefore, if the electrostatic capacity of the serial arm resonator 102 is equal or substantially equal to that of the serial arm resonator 103 , the capacitance of the serial arm resonator 102 A is half or substantially half of the capacitance of the serial arm resonator 102 or the capacitance of the serial arm resonator 103 .

In contrast, the capacitance of the parallel arm resonator 201 A is the combined capacitance of the capacitance of the parallel arm resonator 201 and the capacitance of the parallel arm resonator 202 of the ladder filter in FIG. 1 . Therefore, if the electrostatic capacity of the parallel arm resonator 201 is equal or substantially equal to the electrostatic capacity of the parallel arm resonator 202 , the capacitance of the parallel arm resonator 201 A is a double or substantially double the capacitance of the parallel arm resonator 201 or the capacitance of the parallel arm resonator 202 .

The configuration of the ladder filter 1 A corresponds to the configuration in which, in the ladder filter 1 , adjacent serial arm resonators between stages, for example, the serial arm resonators 102 and 103 , are combined to define the serial arm resonator 102 A, and in which adjacent parallel arm resonators between stages, for example, the parallel arm resonators 201 and 202 , are combined to define the parallel arm resonator 201 A. Therefore, in FIG. 5 , the parallel arm resonator 201 A is shared between a first-stage ladder circuit unit C 1 and a second-stage ladder circuit unit C 2 . Similarly, the serial arm resonator 102 A is shared between the second-stage ladder circuit unit C 2 and a third-stage ladder circuit unit. In this case, to make impedances different from one another between stages, the capacitance of the parallel arm resonator 201 A and the capacitance of the serial arm resonator 102 A may preferably be set to the combined capacitances obtained in accordance with the expressions described below.

The capacitance of serial arm resonator 102 A=(the capacitance of serial arm resonator 102 ×the capacitance of serial arm resonator 103 )/(the capacitance of serial arm resonator 102 +the capacitance of serial arm resonator 103 ).

The capacitance of the parallel arm resonator 201 A=(the capacitance of the parallel arm resonator 201 +the capacitance of the parallel arm resonator 202 ).

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.