Filter device

A filter device includes an antenna terminal to be connected to an antenna, first inductors connected between the antenna terminal and a ground potential and defined by parallel divided inductors, and first and second acoustic wave filters commonly connected to the antenna terminal and including second inductors, respectively. The first inductors and the second inductors are electromagnetically coupled to each other mainly in a one-to-one relationship.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a filter device including a plurality of pass bands.

2. Description of the Related Art

Various kinds of filter devices including a plurality of pass bands and transmitting and receiving a plurality of communication signals in different frequency bands by using a shared antenna have been proposed. One example of a filter device including a ladder filter is described in Japanese Unexamined Patent Application Publication No. 2014-180060. The ladder filter includes an inductor between a signal terminal and a series arm resonator. The filter device further includes a single inductor connected between an antenna terminal and a ground potential. The inductor connected to the antenna terminal and the inductor in the ladder filter are electromagnetically coupled to each other. This configuration increases the attenuation in a stop band.

The filter device described in Japanese Unexamined Patent Application Publication No. 2014-180060, however, is difficult to sufficiently support a structure that includes a plurality of ladder filters including inductors. That is, when the plurality of inductors included in the plurality of ladder filters and the inductor connected to the antenna terminal are electromagnetically coupled to each other, unnecessary coupling may occur between the inductors in the plurality of ladder filters with the inductor connected to the antenna terminal interposed therebetween, and the isolation characteristics may degrade. In addition, it is difficult to adjust the coefficient of coupling between the inductor connected to the antenna terminal and each of the inductors.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention provide filter devices each capable of improving isolation characteristics and easily adjusting coupling coefficients.

A filter device according to a preferred embodiment of the present invention includes an antenna terminal to be connected to an antenna, a plurality of first inductors connected between the antenna terminal and a ground potential and defined by a plurality of parallel divided inductors, and a plurality of band-pass filters commonly connected to the antenna terminal and including respective second inductors. The plurality of first inductors and the plurality of second inductors are electromagnetically coupled to each other mainly in a one-to-one relationship.

In a filter device according to a preferred embodiment of the present invention, the plurality of first inductors are electromagnetically shielded from each other. In this case, electromagnetic coupling between the parallel divided first inductors is effectively reduced or prevented. Accordingly, the isolation characteristics are further improved.

In a filter device according to a preferred embodiment of the present invention, at least one of the second inductors is connected to the ground potential. In this case, the isolation characteristics are effectively improved.

In a filter device according to a preferred embodiment of the present invention, the second inductors include at least one inductor not connected to the ground potential. In this case, the isolation characteristics are effectively improved.

In a filter device according to a preferred embodiment of the present invention, at least one of the band-pass filters is a ladder filter.

In a filter device according to a preferred embodiment of the present invention, at least one of the band-pass filters is a longitudinally coupled resonator acoustic wave filter.

In a filter device according to a preferred embodiment the present invention, the plurality of band-pass filters are included in three or more band-pass filters commonly connected to the antenna terminal. In this case, the isolation characteristics are effectively improved.

According to preferred embodiments of the present invention, the filter devices each capable of improving the isolation characteristics and easily adjusting the coefficients of coupling are provided.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention described in this specification are illustrative, and it is noted that configurations may be replaced in part or combined between different preferred embodiments.

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

A filter device1includes an antenna terminal3to be connected to an antenna. The filter device1includes first to fourth acoustic wave filters2ato2dcommonly connected to the antenna terminal3. Parallel divided first inductors L1aand L1bare connected between the antenna terminal3and a ground potential. The first inductors L1aand L1bprovide impedance matching.

In the present preferred embodiment, the first acoustic wave filter2ais preferably, for example, a transmission filter having a pass band in the range from about 1710 MHz to about 1785 MHz, which is the transmission band of Band3. The second acoustic wave filter2bis preferably, for example, a reception filter having a pass band in the range from about 2110 MHz to about 2170 MHz, which is the reception band of Band1. The third acoustic wave filter2cis preferably, for example, a transmission filter having a pass band in the range from about 1920 MHz to about 1980 MHz, which is the transmission band of Band1. The fourth acoustic wave filter2dis preferably, for example, a reception filter having a pass band in the range from about 1805 MHz to about 1880 MHz, which is the reception band of Band3. The pass bands of the first to fourth acoustic wave filters2ato2dare not limited to the above-described ranges.

The first and second acoustic wave filters2aand2bare preferably ladder filters, for example. The third and fourth acoustic wave filters2cand2dindicated with blocks inFIG. 1are not limited to any particular configurations.

The first acoustic wave filter2aincludes an input terminal4. The first acoustic wave filter2aincludes series arm resonators S1to S4connected in series to each other between the input terminal4and the antenna terminal3and parallel arm resonators P1to P4connected to the ground potential. The first acoustic wave filter2aincludes a second inductor L2aconnected between the parallel arm resonator P1and ground potential. As indicated by the broken line inFIG. 1, the second inductor L2ais electromagnetically coupled to the first inductor L1a.

The second acoustic wave filter2bincludes an output terminal5. The second acoustic wave filter2bincludes series arm resonators S11to S15connected in series to each other between the output terminal5and antenna terminal3and parallel arm resonators P11to P15connected to the ground potential. The second acoustic wave filter2bincludes a second inductor L12bconnected between the parallel arm resonator P15and ground potential. As indicated by the broken line inFIG. 1, the second inductor L12bis electromagnetically coupled to the first inductor L1b. As in the present preferred embodiment, the first inductors L1aand L1bmay preferably be electromagnetically shielded from each other. The inductance of the first inductor L1ais not limited to a particular value, and it may preferably be about 3.24 nH, for example. The inductance of the first inductor L1bis not limited to a particular value, and it may preferably be about 3.24 nH, for example. The details of the circuit configurations of the first and second acoustic wave filters2aand2bare described below.

The characteristics and features of the present preferred embodiment are configurations described below. The first inductors L1aand L1bare parallel divided inductors. The first inductor L1ais electromagnetically coupled mainly to the second inductor L2a, and the first inductor L1bis electromagnetically mainly coupled to the second inductor L12b. Thus, the isolation characteristics are improved, and the coefficients of coupling are easily adjusted. This will be described together with the detailed configurations of the first and second acoustic wave filters2aand2b.

In the first acoustic wave filter2a, the parallel arm resonator P1is connected between the junction of the input terminal4and series arm resonator S1and the ground potential. The parallel arm resonator P2is connected between the junction of the series arm resonators S1and S2and the ground potential. The parallel arm resonator P3is connected between the junction of the series arm resonators S2and S3and the ground potential. The parallel arm resonator P4is connected between the junction of the series arm resonators S3and S4and the ground potential.

The second inductor L2adescribed above is connected between the parallel arm resonator P1and ground potential. A third inductor L3ais connected between the parallel arm resonator P2and the ground potential. A third inductor L3bis connected between the parallel arm resonator P3and the ground potential. A third inductor L3cis connected between the parallel arm resonator P4and the ground potential.

In the second acoustic wave filter2b, the parallel arm resonator P11is connected between the junction of the series arm resonators S11and S12and the ground potential. The parallel arm resonator P12is connected between the junction of the series arm resonators S12and S13and the ground potential. The parallel arm resonator P13is connected between the junction of the series arm resonators S13and S14and the ground potential. The parallel arm resonator P14is connected between the junction of the series arm resonators S14and S15and the ground potential. The parallel arm resonator P15is connected between the junction of the series arm resonator S15and output terminal5and the ground potential.

A third inductor L13ais connected between the parallel arm resonator P11and the ground potential. A third inductor L13bis connected between the parallel arm resonator P12and the ground potential. End portions of the parallel arm resonators P13and P14that are near the ground potential are commonly connected to a third inductor L13c. The third inductor L13cis connected to the ground potential. The second inductor L12bis connected between the parallel arm resonator P15and the ground potential.

In the present preferred embodiment, as described above, the first inductor L1ais electromagnetically coupled mainly to the second inductor L2a. Thus, propagation of unnecessary signals to the antenna terminal3is reduced or prevented. On the other hand, the first inductor L1bis electromagnetically coupled mainly to the second inductor L12b. Thus, outputting of unnecessary signals from the output terminal5is reduced or prevented. Accordingly, the isolation characteristics are effectively improved.

The word “mainly” in the expression of “the first inductor L1ais electromagnetically coupled mainly to the second inductor L2a” includes a configuration in which the first inductor L1ais electromagnetically coupled to the second inductor L2aand is also more weakly electromagnetically coupled to another second inductor than the electromagnetic coupling with the second inductor L2a. That is, in preferred embodiments of the present invention, a plurality of first inductors and a plurality of second inductors are electromagnetically coupled to each other mainly in a one-to-one relationship. This configuration, in which they are electromagnetically coupled mainly in a one-to-one relationship, includes a configuration in which a first inductor is electromagnetically coupled to a second inductor mainly in a one-to-one relationship and is also more weakly electromagnetically coupled to another second inductor than the electromagnetic coupling with the second inductor in a one-to-one relationship. In other words, the configuration in which they are electromagnetically coupled mainly in a one-to-one relationship means that each of the plurality of first inductors has a single combination with a corresponding one of the plurality of second inductors that is most strongly electromagnetically coupled thereto and that each of the plurality of first inductors may be electromagnetically coupled to a second inductor other than the second inductor most strongly electromagnetically coupled thereto.

Further, the first inductors L1aand L1bare parallel divided inductors. Thus, in a state in which the total inductance of the first inductors L1aand L1bis an optimal value, the inductances of the first inductors L1aand L1bare able to be individually adjusted. Accordingly, in the optimal state, the coefficient of coupling between the first inductor L1aand the second inductor L2ais able to be adjusted, and the coefficient of coupling between the first inductor L1band second inductor L12bis also able to be adjusted.

In the present preferred embodiment, the coefficient of coupling between the first inductor L1aand second inductor L2ais preferably about −0.0017, for example. The coefficient of coupling between the first inductor L1band second inductor L12bis preferably about −0.0037, for example. In this manner, in the filter device1, the coefficients of coupling are able to be easily and suitably adjusted.

The first acoustic wave filter2amay have any circuit configuration that includes the second inductor L2aelectromagnetically coupled to the first inductor L1a. Similarly, the second acoustic wave filter2bmay have any circuit configuration that includes the second inductor L12belectromagnetically coupled to the first inductor L1b.

The advantages of the present preferred embodiment will be described in detail by comparison with a comparative example.

FIG. 2is a schematic circuit diagram of a filter device in a comparative example.

A filter device101according to the comparative example is different from the first preferred embodiment in that it includes only one inductor L101connected to the antenna terminal3and ground potential. The inductor L101is electromagnetically coupled to the second inductor L2ain the first acoustic wave filter2a.

FIG. 3illustrates isolation characteristics of the first and fourth acoustic wave filters in the first preferred embodiment and in the comparative example.FIG. 4illustrates isolation characteristics of the second and third acoustic wave filters in the first preferred embodiment and in the comparative example.FIG. 5illustrates cross-isolation characteristics of the third and fourth acoustic wave filters in the first preferred embodiment and in the comparative example.FIG. 6illustrates cross-isolation characteristics of the first and second acoustic wave filters in the first preferred embodiment and in the comparative example. The solid lines represent results for the first preferred embodiment, and the broken lines represent results for the comparative example. The cross-isolation characteristics indicate isolation characteristics between different Bands.

FIG. 3shows that the isolation characteristics of the first and fourth acoustic wave filters in the first preferred embodiment are improved in comparison with those in the comparative example.FIG. 4shows that the isolation characteristics of the second and third acoustic wave filters in the first preferred embodiment are also improved in comparison with those in the comparative example.FIG. 5shows that the cross-isolation characteristics of the third and fourth acoustic wave filters are equal or substantially equal in the first preferred embodiment and comparative example.FIG. 6shows that the cross-isolation characteristics of the first and second acoustic wave filters in the first preferred embodiment are improved in comparison with those in the comparative example.

In the comparative example, as illustrated inFIG. 2, because only the second inductor L2ain the first acoustic wave filter2aand the inductor L101are electromagnetically coupled to each other, the isolation characteristics are not sufficiently improved.

In contrast, in the present preferred embodiment, as illustrated inFIG. 1, the second inductor L2ain the first acoustic wave filter2aand the second inductor L12bin the second acoustic wave filter2bare electromagnetically coupled to the first inductors L1aand L1b, respectively, in a one-to-one relationship. Thus, the out-of-band attenuations for the first and second acoustic wave filters2aand2bare able to be increased, and the isolation characteristics are effectively improved.

For the first and second acoustic wave filters2aand2b, in particular, the out-of-band attenuation on a higher frequency side of each of the pass bands is further increased by the electromagnetic coupling.

As illustrated inFIGS. 5 and 6, the cross-isolation characteristics of the first and second acoustic wave filters2aand2band those of acoustic wave filters other than the first and second acoustic wave filters2aand2bare improved. Accordingly, preferred embodiments of the present invention are applicable to, in particular, cases in which three or more acoustic wave filters are included.

As in the present preferred embodiment, the first and second acoustic wave filters2aand2b, whose pass bands are different Bands, may preferably include the electromagnetically coupled second inductors L2aand L12b, respectively, as described above. Thus, the cross-isolation characteristics are effectively improved.

In the transmission filter, the second inductor L2amay preferably be an inductor connected between the input terminal4and ground potential. This configuration provides an effective improvement in impedance matching. In the reception filter, the second inductor L12bmay preferably be an inductor connected between the output terminal5and ground potential. This configuration provides an effective improvement in impedance matching.

The arrangement of the second inductors L2aand L12b, which are electromagnetically coupled to the first inductors L1aand L1b, respectively, is not limited to the above-described arrangement.

The first inductors L1aand L1bmay preferably be electromagnetically shielded from each other. This configuration provides an effective reduction or prevention of electromagnetic coupling between the first inductors L1aand L1b. Accordingly, the isolation characteristics are further improved.

The configuration in which the first inductors L1aand L1bare electromagnetically shielded from each other is not limited to a particular configuration. One example is illustrated inFIG. 7. In the case in which the first inductors L1aand L1bare disposed on the same substrate, they may be electromagnetically shielded by arranging wiring7between them. Another example is illustrated inFIG. 8. In the case in which the filter device includes an element8and a multilayer body on which the element is mounted, the first inductors L1aand L1bmay be electromagnetically shielded by providing them on different layers. In this case, a metal layer9may preferably be disposed between the first inductors L1aand L1b. This arrangement achieves more stable electromagnetic shielding between the first inductors L1aand L1b.

Three or more acoustic wave filters may include second inductors. The second inductors may be electromagnetically coupled to three or more parallel divided first inductors in a one-to-one relationship. In this case, the isolation characteristics are effectively enhanced.

In the present preferred embodiment, because only two first inductors L1aand L1bare included, the above-described advantages are obtainable, while the filter device is able to be reduced in size.

As in a first variation of the present preferred embodiment illustrated inFIG. 9, the first acoustic wave filter may include a second inductor L22aconnected between the series arm resonator S1and input terminal4. As in this case, the plurality of second inductors in the filter device may include the second inductor L22a, which is not directly connected to the ground potential.

As in a second variation of the present preferred embodiment illustrated inFIG. 10, a first acoustic wave filter32amay include a longitudinally coupled resonator acoustic wave filter36. In this case, the first inductor L1aand second inductor L2aare electromagnetically coupled to each other. InFIG. 10, a portion of the first acoustic wave filter32ais indicated with a block.

The filter device may include a plurality of filters other than acoustic wave filters. In this case, a plurality of parallel divided first inductors and a plurality of second inductors are electromagnetically coupled to each other mainly in a one-to-one relationship.