Laminated balanced filter

A laminated balanced filter includes one unbalanced terminal, two balanced terminals, a ground terminal, an unbalanced-side LC parallel resonator, an intermediate LC parallel resonator, and a balanced-side inductor. The inductor electrode pattern of the balanced-side inductor includes a first portion extending from one end portion to a connection portion and a second portion extending from the other end portion to the connection portion. A direction of a current flowing through the first portion is opposite to a direction of a current flowing through the inductor electrode pattern of the intermediate LC parallel resonator. A direction of a current flowing through the second portion of the balanced-side inductor is the same as a direction of a current flowing through the inductor electrode pattern of the intermediate LC parallel resonator. The laminated balanced filter further includes a coupling capacitor connected between a balanced terminal connected to the first portion and an unbalanced terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laminated balanced filter including a laminated body including a plurality of dielectric layers and a plurality of electrode patterns located on the dielectric layers and having a balanced-unbalanced transforming function.

2. Description of the Related Art

A known laminated balanced filter including a laminated body including a plurality of dielectric layers and a plurality of electrode patterns located on the dielectric layers and having a balanced-unbalanced transforming function is described in Japanese Unexamined Patent Application Publication No. 2011-124880.FIG. 8is the exploded perspective view of the laminated balanced filter described in Japanese Unexamined Patent Application Publication No. 2011-124880, andFIG. 9is the equivalent circuit diagram of this laminated balanced filter.

As illustrated inFIG. 8, this laminated balanced filter is configured using a laminated body where dielectric layers251,242,241,271,211,201, and221and electrode patterns143,142,141,131,121,120,123, and122are laminated, the electrode patterns being formed on the dielectric layers. In addition, as illustrated inFIG. 9, this laminated balanced filter includes an unbalanced terminal T1and balanced terminals T2and T3. In addition, an LC parallel resonator is configured using an inductor L1and a capacitor C1, configured by the electrode pattern131, and one end portion of the corresponding LC parallel resonator is connected to the unbalanced terminal T1. Both end portions of an inductor L4are individually connected to the balanced terminals T2and T3.

In this circuit, the inductor L1and the inductor L4are coupled to each other, and accordingly, the corresponding laminated balanced filter turns out to have a balanced-unbalanced transforming function. In addition, inductors L2and L3and capacitors C2and C3function as a circuit for impedance matching and impedance conversion.

SUMMARY OF THE INVENTION

In the laminated balanced filter described in Japanese Unexamined Patent Application Publication No. 2011-124880, since it is difficult to adjust the position of an attenuation pole formed on a frequency side higher than a pass band, there has been a problem that it has been difficult to freely design a characteristic of a region of a frequency higher than the pass band. Therefore, in order to make it possible to adjust the position of an attenuation pole formed on a frequency side higher than a pass band, the present inventor designed a laminated balanced filter including two LC parallel resonators and an inductor. In addition, in order to cause an attenuation pole to occur in a desired frequency band, the present inventor performed design to connect a coupling capacitor between an input terminal and output terminal. In addition, during the designing, the inventor discovered that it was possible to obtain a laminated balanced filter whose phase characteristic is good.

Based on these discoveries, preferred embodiments of the present invention were developed to provide a laminated balanced filter that makes it possible to freely design an attenuation pole in a region whose frequency is higher than a pass band and to achieve a phase characteristic that is good.

According to a preferred embodiment of the present invention, a laminated balanced filter includes a laminated body including a plurality of dielectric layers and a plurality of electrode patterns located on the dielectric layers, the laminated balanced filter including one unbalanced terminal, two balanced terminals, a ground terminal, an unbalanced-side LC parallel resonator including one end portion connected to the ground terminal and the other end portion connected to the unbalanced terminal, a balanced-side inductor including two end portions individually connected to the balanced terminals, and at least one intermediate LC parallel resonator including one end portion connected to the ground terminal and disposed between the unbalanced-side LC parallel resonator and the balanced-side inductor on an equivalent circuit, wherein the unbalanced-side LC parallel resonator and an intermediate LC parallel resonator disposed nearest to the unbalanced-side LC parallel resonator among the intermediate LC parallel resonators on an equivalent circuit are electromagnetic-field-coupled to each other, the balanced-side inductor and an intermediate LC parallel resonator disposed nearest to the balanced-side inductor among the intermediate LC parallel resonators on an equivalent circuit are electromagnetic-field-coupled to each other, the unbalanced terminal, the balanced terminal, and the ground terminal are located on the surface of the laminated body, the balanced-side inductor includes a substantially helical-shaped inductor electrode pattern in a predetermined dielectric layer of the laminated body, a connection portion in a central or approximately central portion of the inductor electrode pattern is connected to the ground terminal, and includes a first portion leading from one end portion thereof to the connection portion and a second portion leading from the other end portion thereof to the connection portion, the intermediate LC parallel resonator includes a substantially helical-shaped inductor electrode pattern located in a layer in the laminated body, the layer being different from the inductor electrode pattern of the balanced-side inductor, a direction of a current flowing through the first portion of the inductor electrode pattern of the balanced-side inductor is opposite to a direction of a current flowing through the inductor electrode pattern of the intermediate LC parallel resonator, a direction of a current flowing through the second portion of the inductor electrode pattern of the balanced-side inductor is the same as a direction of the current flowing through the inductor electrode pattern of the intermediate LC parallel resonator, and the laminated balanced filter further includes a coupling capacitor connected between the balanced terminal connected to the first portion side and the unbalanced terminal.

In addition, in the laminated balanced filter according to a preferred embodiment of the present invention, it is desirable that at least portions of an aperture of the inductor electrode pattern of the intermediate LC parallel resonator and an aperture of the inductor electrode pattern of the balanced-side inductor overlap with each other when being viewed from a lamination direction.

In addition, in the laminated balanced filter according to a preferred embodiment of the present invention, it is desirable that positions of central axes of the aperture of the inductor electrode pattern of the intermediate LC parallel resonator and the aperture of the inductor electrode pattern of the balanced-side inductor coincide with each other when being viewed from the lamination direction.

In addition, in the laminated balanced filter according to a preferred embodiment of the present invention, it is desirable that the unbalanced-side LC parallel resonator includes a substantially helical-shaped inductor electrode pattern, wherein the circling direction of the inductor electrode pattern of the unbalanced-side LC parallel resonator and the circling direction of the inductor electrode pattern of the intermediate LC parallel resonator are opposite to each other when being viewed from the lamination direction.

In addition, in the laminated balanced filter according to a preferred embodiment of the present invention, it is desirable that each inductor electrode pattern and a capacitor electrode pattern of the laminated balanced filter are separately disposed on one main surface side and the other main surface side in the lamination direction of the laminated body.

In addition, in the laminated balanced filter according to a preferred embodiment of the present invention, it is desirable that the inductor electrode pattern and the capacitor electrode pattern of the unbalanced-side LC parallel resonator are separately disposed on one main surface side and the other main surface side in the lamination direction of the laminated body and the inductor electrode pattern of the intermediate LC parallel resonator and the inductor electrode pattern of the balanced-side inductor are disposed therebetween.

Since, in the laminated balanced filter according to a preferred embodiment of the present invention, the coupling capacitor is connected between the balanced terminal connected to the first portion side and the unbalanced terminal, such that it is possible to provide the laminated balanced filter in which it is possible to freely design an attenuation pole in a region whose frequency is higher than a pass band and the phase characteristic thereof is good.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is the equivalent circuit diagram of a laminated balanced filter according to a preferred embodiment of the present invention. A terminal T1is an unbalanced terminal to which an unbalanced signal is input. In addition, terminals T2and T3are balanced terminals from which a balanced signal is output. Between the terminal T1and the terminals T2and T3, an unbalanced-side LC parallel resonator LC1, an intermediate LC parallel resonator LC2, a balanced-side inductor L3, inductors L4and L5, capacitors C3and C4, and a coupling capacitor C5are disposed.

A capacitor C1and an inductor L1are connected in parallel to each other, and hence, the unbalanced-side LC parallel resonator LC1is configured. In addition, a capacitor C2and an inductor L2are connected in parallel to each other, and hence, the intermediate LC parallel resonator LC2is configured. Owing to the resonant circuit of the unbalanced-side LC parallel resonator LC1and the resonant circuit of the intermediate LC parallel resonator LC2, the pass band width and the center frequency of the filter are designed.

One end portion of the unbalanced-side LC parallel resonator LC1is connected to a ground, and the other end portion thereof is connected to the unbalanced terminal T1. In addition, the other end portion of the intermediate LC parallel resonator LC2is connected to the ground. The unbalanced-side LC parallel resonator LC1and the intermediate LC parallel resonator LC2are electromagnetic-field-coupled to each other through the inductor L1and the inductor L2. In other words, the inductor L1and the inductor L2are magnetic-field-coupled to each other, and hence, the unbalanced-side LC parallel resonator LC1and the intermediate LC parallel resonator LC2are coupled to each other. At this time, between the inductor L1and the inductor L2, mutual inductance M1occurs.

In the intermediate LC parallel resonator LC2and the balanced-side inductor L3, the inductor L2and the balanced-side inductor L3are electromagnetic-field-coupled to each other. Between the inductor L2and the balanced-side inductor L3, mutual inductance M2occurs.

A connection portion in the middle of the balanced-side inductor L3is connected to the ground. In addition, between one end portion of the balanced-side inductor L3and the terminal T2, the inductor L4is connected. In the same way, between the other end portion of the balanced-side inductor L3and the terminal T3, the inductor L5is connected. One end portion of the capacitor C3is connected to a connection point between the terminal T2and the inductor L4, and the other end portion thereof is connected to the ground. In the same way, one end portion of the capacitor C4is connected to a connection point between the terminal T3and the inductor L5, and the other end portion thereof is connected to the ground. By setting the inductors L4and L5and the capacitors C3and C4to predetermined values, it is possible to set, to a desired value, impedance between the terminals T2and T3from which the balanced signal is output.

Furthermore, between the terminal T1and the terminal T2, the coupling capacitor C5is connected. The coupling capacitor C5is arranged so as to shift an attenuation pole to a desired frequency on a frequency side higher than a pass band.

In the laminated balanced filter having such a configuration, for example, when an unbalanced signal is input from the terminal T1, the unbalanced signal is subjected to balanced-unbalanced transformation, and a balanced signal having a predetermined phase difference is output from the terminals T2and T3.

FIG. 2is a perspective view illustrating a laminated balanced filter according to a preferred embodiment of the present invention. The laminated balanced filter includes a laminated body1including a plurality of dielectric layers and electrode patterns laminated on each other. This laminated body1preferably has a substantially rectangular parallelepiped shape, and includes two main surfaces perpendicular or substantially perpendicular to a lamination direction (refer to an arrow inFIG. 2) and four side surfaces connecting the two main surfaces. An unbalanced terminal2, a ground terminal3, and balanced terminals4and5are external terminals, and located on the surfaces of the laminated body1. In more detail, the unbalanced terminal2, the ground terminal3, and the balanced terminals4and5are arranged on the two main surfaces and in side surfaces so as to connect portions located on the two main surfaces. The unbalanced terminal2, the ground terminal3, the balanced terminal4, and the balanced terminal5correspond to the terminal T1inFIG. 1, the ground inFIG. 1, the terminal T2inFIG. 1, and the terminal T3inFIG. 1, respectively.

FIG. 3is a plan view illustrating dielectric layers and electrode patterns, which configure a laminated balanced filter according to a preferred embodiment of the present invention. The laminated balanced filter includes a laminated body including a plurality of dielectric layers11ato11kand a plurality of electrode patterns located on the dielectric layers11ato11k.

A terminal electrode43aconfiguring a portion of the balanced terminal4, a terminal electrode44aconfiguring a portion of the unbalanced terminal2, a terminal electrode45aconfiguring a portion of the balanced terminal5, and a terminal electrode46aconfiguring a portion of the ground terminal3are individually provided on the dielectric layer11a. An inductor electrode pattern21bis located on the dielectric layer11b. An inductor electrode pattern21cis located on the dielectric layer11c. Inductor electrode patterns22dand24dare located on the dielectric layer11d. An inductor electrode pattern23eis located on the dielectric layer11e. An inductor electrode pattern25fis located on the dielectric layer11f. A capacitor electrode pattern32gis located on the dielectric layer11g. A ground electrode pattern41his located on the dielectric layer11h. Capacitor electrode patterns31i,33i, and34iare located on the dielectric layer11i. A coupling electrode pattern42jis located on the dielectric layer11j. Terminal electrodes43k,44k,45k, and46kare located on the dielectric layer11k.

Each of the inductor electrode patterns21b,22d,23e,24d, and25fpreferably has a substantially helical shape, for example. The term “helical shape” substantially means a shape continuously arranged so that an electrode pattern circles around a predetermined central axis. In addition, a portion surrounded by an inductor electrode pattern when being viewed from a lamination direction is referred to as an aperture.

In addition, while not illustrated, in the side surfaces of the dielectric layer, electrode patterns are preferably formed after lamination so as to connect the terminal electrode patterns43a,44a,45a, and46ato the terminal electrode patterns43k,44k,45k, and46k, respectively.

After being laminated in the order of “a” to “k” and fixed by applying pressure, for example, the dielectric layers11ato11kare fired to define a laminated body. The dielectric layer11kis laminated with respect to the other dielectric layers11ato11jso that the terminal electrodes43k,44k,45k, and46kare located in the surface of the laminated body after lamination.

Via electrodes61,62,63, and64are arranged in the lamination direction of the laminated body so as to individually penetrate the dielectric layers11b,11d,11dto11f, and11e.

A correspondence relationship betweenFIG. 1toFIG. 3will be described. The inductor electrode patterns21band21care connected to each other through the via electrode61. One end portion of the inductor electrode pattern21cis connected to the unbalanced terminal2inFIG. 2(T1inFIG. 1). In addition, one end portion of the inductor electrode pattern21bis connected to the ground terminal3inFIG. 2(the ground inFIG. 1). The inductor L1is configured by the inductor electrode patterns21band21cand the via electrode61.

The capacitor electrode pattern31iis extracted to a side surface on one long-side side of the laminated body1, and connected to the unbalanced terminal2inFIG. 2(T1inFIG. 1) and the inductor electrode pattern21c. The capacitor electrode pattern31iand the ground electrode pattern41hface each other through the dielectric layer11h, and configure the capacitor C1. In addition, the ground electrode41his extracted to a side surface on the other long-side side of the laminated body1, and connected to the ground terminal3inFIG. 2(the ground inFIG. 1). In this way, the inductor L1and the capacitor C1are connected in parallel to each other through the unbalanced terminal2and the ground terminal3, and configure the unbalanced-side LC parallel resonator LC1.

One end portion of the inductor electrode pattern22dis connected to the ground terminal3inFIG. 2(the ground inFIG. 1). In addition, the other end portion of the inductor electrode pattern22dis connected to the capacitor electrode pattern32gthrough the via electrode63. The capacitor electrode pattern32gand the ground electrode pattern41hface each other through the dielectric layer11g, and configure the capacitor C2. In this way, the inductor L2and the capacitor C2are connected in parallel to each other through the via electrode63and the ground terminal3, and configure the intermediate LC parallel resonator LC2.

The inductor L1and the inductor L2are disposed with being adjacent to each other in the lamination direction so as to be electromagnetic-field-coupled to each other. In addition, between the inductor electrode pattern21cand the inductor electrode pattern22d, no electrode pattern exists.

The aperture of the inductor L1and the aperture of the inductor L2are arranged so that at least portions thereof overlap with each other when being viewed from the lamination direction. Accordingly, between the inductor L1and the inductor L2, capacitive coupling mainly occurs. In addition, owing to the intensity of the electromagnetic field coupling between the inductor L1and the inductor L2, it is possible to design the pass band width of a filter.

In addition, when being viewed from the lamination direction, the positions of the central axes of the inductor L1and the inductor L2coincide with each other. In addition, when being viewed from the lamination direction, portions of the electrode patterns of the inductor L1and the inductor L2overlap with each other.

The inductor electrode pattern23econfigures the balanced-side inductor L3. The middle of the inductor electrode pattern23eis connected to the ground terminal3inFIG. 2(the ground inFIG. 1).

One end portion of the inductor electrode pattern23eis connected to the inductor electrode pattern24dthrough the via electrode62, and the other end portion thereof is connected to the inductor electrode pattern25fthrough the via electrode64. The inductor electrode pattern24dconfigures the inductor L4, and one end portion thereof is extracted to a side surface on one long-side side of the laminated body1, and connected to the balanced terminal4inFIG. 2(T2inFIG. 1). In addition, the inductor electrode pattern25fconfigures the inductor L5, and one end portion thereof is extracted to a side surface on the other long-side side of the laminated body1, and connected to the balanced terminal5inFIG. 2(T3inFIG. 1).

The capacitor electrode pattern33iis extracted to a side surface on one long-side side of the laminated body1, and connected to the unbalanced terminal4inFIG. 2(T2inFIG. 1). The capacitor electrode pattern33iand the ground electrode pattern41hface each other through the dielectric layer11h, and configure the capacitor C3.

The capacitor electrode pattern34iis extracted to a side surface on the other long-side side of the laminated body1, and connected to the unbalanced terminal5inFIG. 2(T3inFIG. 1). The capacitor electrode pattern34iand the ground electrode pattern41hface each other through the dielectric layer11h, and configure the capacitor C4.

The coupling electrode pattern42jand the capacitor electrode patterns31iand33iface each other through the dielectric layer11i, and configure the coupling capacitor C5. The coupling electrode pattern42jis a floating electrode that is not connected to any of the external terminals.

The inductor L2and the balanced-side inductor L3are disposed adjacent to each other in the lamination direction so as to be electromagnetic-field-coupled to each other. In addition, between the inductor electrode pattern22dconfiguring the inductor L2and the inductor electrode pattern23econfiguring the balanced-side inductor L3, no electrode pattern exists. In the present preferred embodiment, the aperture of the inductor electrode pattern22dand the aperture of the inductor electrode pattern23eare arranged so that at least portions thereof overlap with each other when being viewed from the lamination direction. Accordingly, electromagnetic field coupling occurs between the inductor L2and the balanced-side inductor L3.

In addition, when being viewed from the lamination direction, the central axes of the inductor L2and the balanced-side inductor L3coincide with each other. In addition, when being viewed from the lamination direction, portions of the electrode patterns of the inductor L2and the balanced-side inductor L3overlap with each other.

As described above, the unbalanced-side LC parallel resonator LC1, the intermediate LC parallel resonator LC2, the balanced-side inductor L3, the inductors L4and L5, the capacitors C3and C4, and the coupling capacitor C5are provided within the laminated body.

Arrows inFIG. 3indicate the directions of current flowing to the ground terminal3. The inductor electrode patterns21b,21c,22d, and23eare connected to the ground terminal3, in the lower right portions of the dielectric layers11b,11c,11d, and11eon which the inductor electrode patterns21b,21c,22d, and23eare provided, respectively. Accordingly, when being viewed from the lamination direction from the dielectric layer11atoward the dielectric layer11g, the inductor electrode patterns21band21cconfiguring the inductor L1circle in a clockwise fashion with respect to the ground terminal3. In the same way, the inductor electrode pattern22dconfiguring the inductor L2circles in a counterclockwise fashion. Accordingly, it is understood that the circling directions of the inductor electrode patterns21band21cand the circling direction of the inductor electrode pattern22dare opposite to each other when being viewed from the lamination direction. Therefore, with respect to the ground terminal3, directions in which the currents of the inductor L1and the inductor L2flow are opposite to each other. At this time, since mainly capacitive electromagnetic field coupling between the unbalanced-side LC parallel resonator LC1and the intermediate LC parallel resonator LC2is further strengthened, it is not necessary to separately provide a coupling capacitor, and an advantage that a laminated balanced filter is miniaturized is obtained.

FIG. 4is an enlarged view illustrating the inductor electrode pattern22dof the intermediate LC parallel resonator LC2and the inductor electrode pattern23eof the balanced-side inductor L3inFIG. 3. In addition, arrows in the drawing indicate the directions of currents flowing to the ground terminal3.

A connection portion53located in the middle of the inductor electrode pattern23eof the balanced-side inductor L3is connected to the ground terminal3. In addition, the inductor electrode pattern23eincludes a first portion51leading from one end portion thereof connected to the via electrode62to the connection portion53and a second portion52leading from the other end portion thereof connected to the via electrode64to the connection portion53.

The direction of a current flowing through the first portion51of the inductor electrode pattern23eis opposite to the direction of a current flowing through the inductor electrode pattern22dof the intermediate LC parallel resonator LC2. In addition, the direction of a current flowing through the second portion52of the inductor electrode pattern23eis the same as the direction of a current flowing through the inductor electrode pattern22dof the intermediate LC parallel resonator LC2.

In the present preferred embodiment, the inductor electrode pattern22dand the inductor electrode pattern23eare preferably arranged at positions roughly overlapping with each other when being viewed from the lamination direction. In addition, even if the inductor electrode pattern22dand the inductor electrode pattern23eare arranged at positions deviated from each other when being viewed from the lamination direction, when the central axes of the pivots of the inductor electrode patterns roughly coincide with each other and a point exists where the directions of currents in corresponding portions are the same as or opposite to each other, the advantageous effects of a preferred embodiment of the present invention are obtained.

In the present preferred embodiment, the coupling capacitor C5preferably is connected between the balanced terminal T2connected to the first portion51side and the unbalanced terminal T1. In this case, a phase characteristic is about 180° and becomes good, compared with a case where the coupling capacitor C5is connected between the balanced terminal T3connected to the second portion52side and the unbalanced terminal T1.

In the present preferred embodiment, the inductor electrode patterns21b,21c,22d,24d,23e, and25fare preferably located on one main surface side in the lamination direction of the laminated body. On the other hand, the capacitor electrode patterns32g,31i,33i,34i, and42jare formed on the other main surface side in the lamination direction of the laminated body. In this way, by separately disposing the inductor electrode patterns and the capacitor electrode patterns on one main surface side and the other main surface side, respectively, it is possible to further downsize the laminated balanced filter.

In addition, in the present preferred embodiment, the capacitor electrode pattern32gconfiguring the intermediate LC parallel resonator LC2faces to31iconfiguring the unbalanced-side LC parallel resonator LC1,33iand34iconfiguring the balanced-side LC parallel resonator, and the coupling electrode pattern42jconfiguring the coupling capacitor C5, with the ground electrode pattern41hdisposed therebetween. In this case, it is possible to prevent unnecessary capacitance between the capacitor electrode pattern32gof the intermediate LC parallel resonator LC2and the capacitor electrode pattern33iand34iof the balanced-side LC parallel resonator from occurring, and also to prevent unnecessary capacitance between the capacitor electrode pattern32gof the intermediate LC parallel resonator LC2and the coupling electrode pattern42jof the coupling capacitor C5from occurring.

In addition, in the present preferred embodiment, a case has been described where the number of intermediate LC parallel resonators preferably is one, for example. The number of intermediate LC parallel resonators may be more than one. In that case, among the plural intermediate LC parallel resonators, an intermediate LC parallel resonator disposed nearest to the unbalanced-side LC parallel resonator on an equivalent circuit and the unbalanced-side LC parallel resonator are electromagnetic-field-coupled to each other. In addition, the plural intermediate LC parallel resonators are electromagnetic-field-coupled to each other. In addition, among the plural intermediate LC parallel resonators, an intermediate LC parallel resonator disposed nearest to the balanced-side inductor on the equivalent circuit and the balanced-side inductor are electromagnetic-field-coupled to each other.

So as to confirm the advantageous effects of a preferred embodiment of the present invention, the simulation results of laminated balanced filters of an experimental example, a first comparative example, and a second comparative example were compared with one another.

First, it was assumed that the laminated balanced filter described inFIG. 1toFIG. 4was the experimental example.

FIGS. 5A and 5Bare the equivalent circuit diagrams of laminated balanced filters of the comparative examples. It was assumed that a laminated balanced filter having no coupling capacitor as illustrated inFIG. 5A(refer to the C5inFIG. 1) was the first comparative example. In addition, it was assumed that a laminated balanced filter where the coupling capacitor C5was connected to the second portion side as illustrated inFIG. 5B(refer toFIG. 3) was the second comparative example. In other words, in the second comparative example, the coupling electrode pattern42jand the capacitor electrode patterns31iand34iface each other through the dielectric layer11i, and configure the coupling capacitor C5. In addition, with respect to the laminated balanced filters of the experimental example, the first comparative example, and the second comparative example, the transmission characteristics thereof and the phase characteristics thereof were compared in simulations.

FIGS. 6A,6B, and6C illustrate the simulation results of the transmission characteristics.FIG. 6Ais the result of the experimental example,FIG. 6Bis the result of the first comparative example, andFIG. 6Cis the result of the second comparative example. It is understood that, compared with the first comparative example, in the experimental example, it is possible to design an attenuation pole in the vicinity of about 3.2 GHz desired, by providing the coupling capacitor. In addition, in the second comparative example where the coupling filter C5is connected to the second portion side, an attenuation pole supposed to exist on the high frequency side of a pass band disappears, and a filter characteristic is degraded.

FIGS. 7A,7B, and7C illustrate the simulation results of the phase characteristics.FIG. 7Ais the result of the experimental example,FIG. 7Bis the result of the first comparative example, andFIG. 7Cis the result of the second comparative example. The phase differences of the balanced signals of the experimental example, the first comparative example, and the second comparative example were 177 degrees, 183 degrees, and 202 degrees, respectively. For this reason, the experimental example where the coupling capacitor is connected to the first portion side has a phase characteristic comparable with the first comparative example having no coupling capacitor. On the other hand, in the second comparative example where the coupling capacitor is connected to the second portion side, it has become clear that a phase characteristic is degraded.