Acoustic wave filter device

In an acoustic wave filter device, a first filter circuit portion includes a first inductor provided in a series arm that couples an input terminal and an output terminal is serially connected to a first acoustic wave resonator, and a second filter circuit portion includes a second inductor provided in the series arm and second and third acoustic wave resonators and connected between one end and the other end of the second inductor and a ground potential. When the pass band center frequency is set as a first center frequency and the center frequency of a filter defined by the inductance of the first and second inductors and the capacitance of the first to third acoustic wave resonators is set as a second center frequency f2, f1 is less than f2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to acoustic wave filter devices, such as surface acoustic wave filter devices and boundary acoustic wave filter devices. More specifically, the present invention relates to an acoustic wave filter device in which a plurality of inductors are connected between an input terminal and an output terminal and in which a plurality of resonators are connected between a series arm coupling the input terminal and output terminal and the ground potential.

2. Description of the Related Art

Conventionally, various surface acoustic wave filter devices are used as trap filters in RF stages of communication apparatuses, such as mobile phones. For example, Japanese Unexamined Patent Application Publication No. 2004-129238 discloses a trap filter including a plurality of inductances serially connected between an input terminal and an output terminal and parallel arm resonators each connected between the input terminal and the ground potential, between the output terminal and the ground potential, and between a node of adjacent inductances and the ground potential.

FIG. 12is a circuit diagram illustrating a circuit configuration of the trap filter mentioned above. In a trap filter501, an inductance L1is connected between an input terminal IN and an output terminal OUT. A parallel arm resonator P1is connected between the input terminal IN and a ground potential. In addition, a parallel arm resonator P2is connected between the output terminal OUT and the ground potential. In this filter, each of the parallel arm resonators P1and P2is defined by a surface acoustic wave resonator in which an IDT electrode is disposed on a piezoelectric substrate. In addition, the resonant frequencies of the parallel arm resonators P1and P2are set to be substantially equal and located in a stop band.

On the other hand, Japanese Unexamined Patent Application Publication No. 2003-332881 discloses a surface acoustic wave filter device in which first one-terminal-pair surface acoustic wave resonators having a predetermined anti-resonant frequency are connected to a series arm coupling an input terminal and an output terminal, and a second one-terminal-pair surface acoustic wave resonator is connected between first ends of the first one-terminal-pair boundary acoustic wave resonators and a ground potential. In this filter, the resonant frequency of the second one-terminal-pair surface acoustic wave resonator is set to be substantially equal to the anti-resonant frequency of the first one-terminal-pair surface acoustic wave resonators, and as a result, a trap providing a large amount of attenuation is formed.

In the trap filter501disclosed in Japanese Unexamined Patent Application Publication No. 2004-129238, the inductance L1and the parallel arm resonators P1and P2define a stop band, i.e., a trap. However, the attenuation characteristic in a frequency band lower than the trap is flat, and thus, characteristics as a band-pass filter cannot be obtained on the lower frequency side.

Also in the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 2003-332881, a trap is provided by setting the anti-resonant frequency of the first one-terminal-pair surface acoustic wave resonators and the resonant frequency of the second one-terminal-pair surface acoustic wave resonator to be substantially equal, as described above. In this case, a characteristic as a trap filter is obtained by series resonance of the inductors serially connected with the first one-terminal-pair surface acoustic wave resonators and capacitance of the one-terminal-pair surface acoustic wave resonators. However, in the characteristic as a trap filter, the width of the passband is relatively small on the frequency side lower than the trap.

On the other hand, in mobile phones equipped with recording functions of ground-wave digital television DVB-H, a pass band with a sufficiently broad band width on the frequency side lower than the trap is required in order to enable recording during transmission. However, with the trap filters disclosed in Japanese Unexamined Patent Application Publication No. 2004-129238 and in Japanese Unexamined Patent Application Publication No. 2003-332881, as described above, it is difficult to provide a pass band having a sufficient band width on the frequency side lower than the trap.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of the present invention provide an acoustic wave filter device which has a circuit configuration in which an acoustic wave resonator and an inductor facilitate downsizing, and which cannot only provide a trap providing a sufficient amount of attenuation, but also provide a pass band having a sufficient band width on the frequency side lower than the trap.

Preferred embodiments of the present invention provide an acoustic wave filter device having a first attenuation band, a pass band which is located on the frequency side lower than the first attenuation band and has a first center frequency, and a second attenuation band which is located on the frequency side higher than the first attenuation band. The acoustic wave filter device includes first filter circuit portions having first inductors provided in a series arm coupling an input terminal and an output terminal and first acoustic wave resonators serially connected to the first inductors, and a second filter circuit portion having at least one second inductor provided in the series arm, a second acoustic wave resonator connected between one end of the second inductor and a ground potential, and a third acoustic wave resonator connected between the other end of the second inductor and the ground potential. In the acoustic wave filter device, the first filter circuit portions and the second filter circuit portion are serially connected in the series arm, and when the center frequency of a frequency characteristic of an LC filter formed by capacitance components of the first to third acoustic wave resonators and inductance components of the first and second inductors is set as a second center frequency, the second center frequency is set to be higher than the first frequency.

Preferably, the first filter circuit portion is connected to both the input side and output side of the second filter circuit portion.

Preferably, a plurality of second inductors are provided in the second filter circuit portion, and the second acoustic wave resonator connected to one end of one of the second inductors and the third acoustic wave resonator connected to the other end of the other one of the second inductors are disposed between the adjacent second inductors.

Preferably, the second center frequency is located at the high frequency side end of the pass band.

Preferably, an anti-resonant frequency of the first acoustic wave resonators and a resonant frequency of the second acoustic wave resonator are different from each other.

Preferably, the first and second inductors are defined by a chip-type inductance component.

Preferably, an acoustic wave filter chip is provided which includes a piezoelectric substrate having the first, second and third acoustic wave resonators provided thereon and a mounting substrate having the acoustic wave filter chip mounted thereon. In the acoustic wave filter device, the first and second inductors are built in the mounting substrate.

With an acoustic wave filter device according to preferred embodiments of the present invention, first filter circuit portions include first inductors provided in a series arm coupling an input terminal and an output terminal and first acoustic wave resonators serially connected to the first inductors, and a second filter circuit portion includes at least one second inductor provided in the series arm, a second acoustic wave resonator connected between one end of the second inductor and a ground potential, and a third acoustic wave resonator connected between the other end of the second inductor and the ground potential. The first filter circuit portions and the second filter circuit portion are connected in series. Thus, by determining frequency characteristics of the first acoustic wave resonators provided in the series arm and the second and third acoustic wave resonators, a first attenuation band as a trap providing a large amount of attenuation can be obtained.

In addition, in the low frequency side of the first attenuation band, an LC filter is provided by capacitance components of the first to third acoustic wave resonators and inductance components of the first and second inductors, and the second center frequency of the frequency characteristic of the LC filter is set to be higher than the first center frequency which is the center frequency of the pass band located on the frequency side lower than the first attenuation band. Therefore, the amount of attenuation in a portion adjacent to the stop band in the high frequency side of the pass band is sufficiently small, and thus, the band width can be sufficiently expanded. Accordingly, a pass band having a sufficient band width is securely provided on the frequency side lower than the trap. Therefore, for example, an acoustic wave filter device preferable as a band-pass filter for performing recording during transmission can be provided in a mobile phone provided with a recording function for the ground-wave digital television broadcasting.

When the first filter circuit portions are connected to both the input side and output side of the second filter circuit portion, this enables the amount of attenuation in the first and second attenuation bands to be further increased.

A plurality of the second inductors may be provided in the second filter circuit portion. In this case, the third acoustic wave resonator connected in the one end of one of the second inductors and the third acoustic wave resonator connected to the other end of the second inductors may be incorporated between the adjacent second inductors. In this case, the number of components can be reduced, and due to the multi-stage configuration of the second filter circuit portion, the amount of attenuation in the first and second attenuation bands can be further increased.

When the second center frequency is located at the high frequency side end of the pass band, the amount of attenuation at the second center frequency is sufficiently small, and thus, the amount of attenuation at the high frequency side of the pass band can be sufficiently decreased. Therefore, the pass band width can be more effectively expanded.

When the anti-resonant frequency of the first acoustic wave resonators and the resonant frequency of the second acoustic wave resonator are different from each other, the width of the second attenuation band can be expanded.

When the first and second inductors are defined by a chip-type inductance component, the first and second inductors can be surface-mounted on a mounting substrate, which facilitates downsizing of the acoustic wave filter device.

When an acoustic wave filter chip including a single piezoelectric substrate having the first to third acoustic wave resonators provided thereon and a mounting substrate having the acoustic wave filter chip mounted thereon are further included, and the first and second inductors are built in the mounting substrate, no external components defining the first and second inductors are required. Accordingly, further downsizing of the acoustic wave filter device can be facilitated and the reduction of the number of components can be achieved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detailed preferred embodiments of the present invention with reference to the drawings.

FIG. 1is a circuit diagram of a surface acoustic wave filter device according to a preferred embodiment of the present invention. A surface acoustic wave filter device1of the present preferred embodiment is preferably used, for example, as a trap filter used in an RF stage of a mobile phone provided with a recording function for ground-wave digital television broadcasting. The surface acoustic wave filter device1preferably is configured to have a trap band around 880-915 MHz and a pass band including a frequency band around 470-750 MHz of ground-wave digital television DVB-H in a frequency range below about 800 MHz. The center frequency of the pass band is set as a first center frequency. The surface acoustic wave filter device1also has a second attenuation band on the frequency side higher than the first attenuation band set as the trap band, i.e., in a frequency range of about 1300 MHz and higher, for example.

As illustrated inFIG. 1, in the surface acoustic wave filter device1, a first filter circuit portion2, a second filter circuit portion3and a first filter circuit portion4are serially connected in that order between an input terminal IN and an output terminal OUT. The first filter circuit portion2includes the input terminal IN, a first inductor L1aprovided in a series arm coupling the input terminal IN and the output terminal OUT, and a first acoustic wave resonator S1aserially connected to the first inductor L1a.

The second filter circuit portion3includes at least one second inductor L2provided in the series arm, a second acoustic wave resonator P32connected between one end of the second inductor L2and a ground potential, and a third acoustic wave resonator P33connected between the other end of the second inductor L2and the ground potential.

The first filter circuit portion4includes a first inductor L1bprovided in the series arm and a first acoustic wave resonator S1bserially connected to the first inductor L1b.

In the acoustic wave filter device of the present preferred embodiment, a serial resonance circuit is defined by capacitance components of the first acoustic wave resonators S1aand S1band the first inductors L1aand L1b, and a phase-shift circuit is defined by capacitance components of the second and third acoustic wave resonators arranged in parallel arms and the second inductor L2in the series arm. With this arrangement of the above serial resonance circuit and the phase-shift circuit, a band-pass type filter characteristic with a pass band can be obtained. Specifically, an LC filter is defined by the capacitance components of the first to third acoustic wave resonators S1a, S1b, P32, and P33and inductance components of the first and second inductors L1a, L1b, and L2. When the center frequency of the pass band located on the frequency side lower than the first attenuation band as the trap band is set as a first center frequency, a second center frequency which is the center frequency of the LC filter is set to be higher than the first center frequency. With this arrangement, the band width of the pass band is expanded. This will be described in more detail below.

FIG. 2shows an attenuation-frequency characteristic of a surface acoustic wave filter device according to the present preferred embodiment. The anti-resonant frequency of the acoustic wave resonators S1aand S1bare set to be substantially identical to the resonant frequency of the second and third acoustic wave resonators P32and P33arranged in the parallel arms. Thus, an attenuation trap indicated by an arrow A is formed. The trap A has a relatively small width and a large amount of attenuation.

On the other hand, while the amount of attenuation is set to be small at both sides of the trap A, the low frequency side of the trap A is set as the pass band in the acoustic wave filter device1of the present preferred embodiment. As described above, this pass band is utilizes the filter characteristic provided by the serial resonance circuit and the phase-shift circuit each defined by the capacitance components of the acoustic wave resonators and the inductors. Specifically, when the acoustic wave resonators in the surface acoustic wave filter device1illustrated inFIG. 1are all replaced with capacitive elements, a circuit illustrated inFIG. 6is obtained. Capacitive elements C1aand C1bare arranged in the series arm in place of the acoustic wave resonators S1aand S1b, and capacitive elements C2and C3are arranged in the parallel arms in place of the acoustic wave resonators P32and P33. The attenuation-frequency characteristic of the circuit illustrated inFIG. 6is illustrated inFIG. 7. Specifically, an LC filter circuit is defined by the capacitance of the capacitive elements C1a, C1b, C2, and C3and the inductance of the inductors L1a, L2, and L1barranged in the series arm. The frequency characteristic of the filter circuit is illustrated inFIG. 7.

In the present preferred embodiment, the amount of attenuation at the trap A as the first attenuation band is set to be sufficiently large. Further, the pass band having a sufficient band width is provided on the frequency side lower than the trap A, and the second attenuation band is provided on frequency side higher than the trap A. This will be described in more detail below.

In the present preferred embodiment, the center frequency of the LC filter defined by the capacitance components and the inductance components as described above is set as a second center frequency f2, and the center frequency of the pass band provided on the frequency side lower than the trap A of the surface acoustic wave filter device1is set as a first center frequency f1, f2is higher than f1. Thus, the amount of attenuation at a portion indicated by an arrow B inFIG. 2, i.e., near the high end of the pass band of the surface acoustic wave filter device1, is sufficiently small. In the other words, the amount of attenuation near the high end of the pass band is sufficiently small, and the pass band is sufficiently expanded on the high frequency side.

In addition, in order to broaden the band width of the LC filter to reduce insertion loss and to broaden the band width of the pass band in its lower frequency side, which is provided at the low frequency side of the trap A, it is necessary to decrease the inductance value of the first inductors L1aand L1bin the first filter circuit portion and increase the capacitance of the acoustic wave resonators S1aand S1b. Thus, the inductance value of the inductors L1aand L1bare decreased, which decreases the resistance. In addition, when the capacitance of the first acoustic wave resonators S1aand S1bis increased, the Q value is increased. Therefore, the insertion loss can be further reduced.

Thus, according to the present preferred embodiment, not only can the width of the pass band be expanded, the insertion loss can also be further reduced. Accordingly, a trap and a pass band with low insertion loss and a broad band width can be provided at the frequency side lower than the trap.

Now, a specific experimental example will be described.

The specifications of the first acoustic wave resonators S1a, S1b, and the second and third acoustic wave resonators P32and P33are shown in Table 1 below. The inductance value of the first inductors L1aand the L1bis preferably set to about 27 nH, and the inductance value of the second inductor L2is set to about 24 nH, for example. In this case, the frequency characteristic of the surface acoustic wave filter device1is as illustrated inFIG. 2.

In this example, the amount of attenuation at the trap A at around 880-915 MHz is as large as about 55 dB. In addition, a pass band was provided on the frequency side lower than the trap A. Note that the band width of a pass band refers to the width of a band in which the amount of attenuation is equal to or less than about 3 dB. The band width of the pass band was about 380 MHz. Therefore, low insertion loss can be achieved at about 470-750 MHz.

In addition, on the frequency side higher than the trap A, a second attenuation band was provided in a frequency range of about 1500-2000 MHz.

For comparison, a filter circuit illustrated inFIG. 8was prepared. In this filter circuit, an inductor L51a, acoustic wave resonators S52aand S52b, and an inductor L51bare connected in that order between an input terminal IN and an output terminal OUT, and a acoustic wave resonator P53is provided in a parallel arm connecting the node between the acoustic wave resonators S52aand S52band a ground potential. That is, this filter circuit is fabricated in accordance with the circuit configuration disclosed in Japanese Unexamined Patent Application Publication No. 2003-332881 described above. The specifications of the acoustic wave resonators S52a, S52b, and P53in the surface acoustic wave filter device of this comparative example are shown in Table 2. The inductance value of the inductor L51ais set to about 22 nH, and the inductance value of the inductor L51bis set to about 23 nH.

The specifications shown in Table 2 and the above inductance values were selected so that a trap was formed at around 880-915 MHz and a pass band was formed around 470-750 MHz, similarly to the above-described preferred embodiment.

A frequency characteristic of the surface acoustic wave filter device of the conventional example fabricated as described above is shown inFIG. 9.

As shown inFIG. 9, the amount of attenuation near each end of the pass band around 470 MHz and around 750 MHz was greater than about 3 dB. Thus, the band width of a frequency range with about 3 dB attenuation amount was not more than about 270 MHz. This presents a problem in that the insertion loss is large at around the high and low ends of the frequency band about 470-750 MHz of the ground digital television DVB-H.

As is apparent from the comparison ofFIG. 2andFIG. 9, according to the above-described preferred embodiment, a pass band with low loss and a broad band width can be provided on the frequency side lower than the trap.

While in the second filter circuit portion3, the single second inductor L2is connected, it is also possible for a plurality of second inductors L2aand L2bto be connected in the series arm in the second filter circuit portion. In this case, between the adjacent second inductors L2aand L2b, the second acoustic wave resonator connected to one end of the inductor L2band the third acoustic wave resonator connected to the other end of the second inductor L2amay be included. This reduces the number of parts. At the same time, the amount of attenuation at the first and second attenuation bands can be increased due to the multi-stage configuration of the second filter circuit portion.

Now, an example of the detailed structure of the surface acoustic wave filter device according to the above-described preferred embodiment will be provided.

FIG. 3is a schematic plan view illustrating a surface acoustic wave filter chip11used in the surface acoustic wave filter device1of the present preferred embodiment. This surface acoustic wave filter chip11preferably includes a substantially rectangular piezoelectric substrate12. InFIG. 3, while the piezoelectric substrate12is illustrated in a schematic plan view, electrodes provided below the piezoelectric substrate12are perspectively illustrated. This is because the piezoelectric substrate12is mounted on a package substrate14illustrated inFIG. 4Awith its orientation as illustrated in theFIG. 4A, such that the main components of the surface acoustic wave filter device1illustrated in a schematic cross-sectional view inFIG. 5Bare configured.

As illustrated inFIG. 3, on the lower surface of the piezoelectric substrate12, the first acoustic wave resonators S1aand S1band the second and third acoustic wave resonators P32and P33are provided. Each of the acoustic wave resonators is defined by a one-terminal-pair surface acoustic wave resonator having an IDT electrode and reflectors arranged at opposite sides of the IDT electrode in the surface acoustic wave propagation direction. InFIG. 3, these electrodes are not specifically illustrated, and portions on which IDT electrodes and pairs of reflectors are arranged are schematically illustrated.

An electrode structure of an IDT electrode and reflectors defining an acoustic wave resonator is schematically illustrated inFIG. 4B. As illustrated inFIG. 4B, an IDT electrode15includes several electrode fingers interdigitated with each other. Reflectors16and17are arranged at opposite sides of the IDT electrode15.

Referring back toFIG. 3, the acoustic wave resonator S1ais connected to an electrode land12aat the input terminal side through a wiring pattern. A bump13ais attached on the lower surface of the electrode land12a. The bump13ais provided to electrically connect to an electrode land14aon the package substrate14which will be described below. One end of the acoustic wave resonators S1aand P32are commonly connected and electrically connected to the electrode land12bthrough the wiring pattern.

An end of the acoustic wave resonator P32which is at the opposite side to the end connected to the electrode land12bis connected to an electrode land12cthrough the wiring pattern. The electrode land12cis an electrode land to be connected to a ground potential.

Another end of the acoustic wave resonator P33is electrically connected to an electrode land12d. The electrode land12dis an electrode land to be connected to the ground potential.

One end of the acoustic wave resonator P33and the acoustic wave resonator S1bare commonly connected and connected to an electrode land12ethrough the wiring pattern.

An end of the acoustic wave resonator S1bwhich is at the opposite side to the end connected to the electrode land12eis connected to an electrode land12fthrough the wiring pattern. The electrode land12fis electrically connected to the output terminal via the first inductor L1b.

As illustrated inFIG. 4A, electrode lands14ato14eare provided on the package substrate14. The electrode land14dis connected to the electrode lands12cand12dvia the bumps13cand13d. The electrode lands14b,14c, and14eare connected to the electrode lands12b,12c, and12fvia the bumps13d,13c, and13f, respectively. The electrode land14ais the electrode land to be connected to the input terminal via the first inductor L1a, and the electrode land14eis the electrode land to be connected to the output terminal via the first inductor L1b. The electrode land14dis the only electrode land that is connected to the ground potential. Each of the electrode lands14band14cdefines a terminal to be connected to the inductor L2.

Specifically, as illustrated inFIG. 5A, the first inductors L1aand L1band the second inductor L2are defined by a chip-type inductance component which is independent of the package substrate14. The inductor L1aand the inductor L1bare electrically connected to the electrode land14aand the electrode land14eof the package substrate14, respectively, and the inductor L2is electrically connected between the electrode lands14band14c.

In this configuration, as illustrated inFIG. 5B, the surface acoustic wave filter chip11includes the single piezoelectric substrate12and is mounted on the electrode lands on the package substrate14through the bumps by bump bonding. However, according to preferred embodiments of the present invention, the inductors may be defined by inductance elements other than a chip-type inductance component.

FIGS. 10A to 10Care a schematic plan views for describing a modified example in which an inductor is built in a mounting substrate, a fragmentary elevational cross-sectional view, and a schematic plan view illustrating an example of an inductance.

As illustrated inFIG. 10A, in the modified example, a surface acoustic wave filter chip (not shown) is mounted on the package substrate14by bump bonding, similarly to the above-described example. The package substrate14is mounted on a mounting substrate21defined by a multi-layer substrate. The above-described first and second inductors L1a, L1b, and L2are built in the mounting substrate21. In this example, a portion in which the first inductor L1ais disposed will be described as a representative example.

As illustrated inFIGS. 10A and 10B, the mounting substrate21includes electrode lands22aand22bon its upper surface. The electrode land22ais electrically connected to a terminal electrode23disposed on the mounting substrate21. The terminal electrode23defines an input terminal. The electrode22bis electrically connected to the electrode land14aconnected to an input terminal provided in the package substrate14.

The electrode lands22aand22bare connected to through-hole electrodes24aand24b, respectively. The through-hole electrodes24aand24bextend to an intermediate-height location in the mounting substrate21and are connected to a coil pattern25defining the first inductor L1adisposed at an intermediate-height location. As illustrated inFIG. 10C, the coil pattern25preferably has a winding pattern shape so as to have a plurality of turns on the mounting substrate21. One end of the coil pattern25is connected to the through-hole electrode24a, and the other end of the coil pattern25is connected to the through-hole electrode24b. In this manner, the first inductor L1ais built in the mounting substrate21. The second inductor L2and the other first inductor L1bare similarly built in the mounting substrate1.

Accordingly, the number of parts is reduced and further downsizing of the surface acoustic wave filter device can be achieved by building the inductors in the mounting board.

As is apparent from the above-described preferred embodiment and the modified example, the first and second inductors may be defined by a chip-type inductance component or inductors built in a mounting board. Further, in the above-described preferred embodiment, the surface acoustic wave filter chip11configured using the piezoelectric substrate12is mounted on the substrate through the bumps by a face bonding technique. However, the surface acoustic wave filter chip may also be bonded to the electrode lands on the package substrate through bonding wires.

In the above-described preferred embodiment, the first and second acoustic wave resonators preferably are defined by surface acoustic wave resonators. However, they may be defined by boundary acoustic wave resonators.