Band elimination filter, filter device, antenna duplexer and communication apparatus

A band elimination filter includes an input terminal and an output terminal. A capacitor is coupled between a first terminal connected to the input terminal and a second terminal connected to the output terminal. The first terminal is grounded only via a first grounding point. The second terminal is grounded only via a second grounding point. A first acoustic resonator is connected between the first terminal and the first grounding point and a second acoustic resonator is connected between the second terminal and the second grounding point.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a band elimination filter used in a communication apparatus, such as a cellular phone and car phone.

2. Related Art of the Invention

In the prior art a surface acoustic wave filter or piezoelectric filter has been used as an RF filter in communication apparatus. The surface acoustic wave filters mainly used include a longitudinal coupled mode filter having a plurality of interdigital transducer electrodes (IDT electrodes) closely arranged in the propagation direction and a ladder filter having surface acoustic wave resonators interconnected in a ladder-like arrangement. On the other hand, as the piezoelectric filter, a bulk wave filter is used. It has been desired that these filters are increased in performance and reduced in size.

In the following, a conventional band elimination filter will be described with reference to the drawings.

FIG. 19(a) shows a configuration of a surface acoustic wave resonator. In this drawing, the surface acoustic wave resonator comprises an IDT electrode1702formed on the piezoelectric substrate1701, reflector electrodes1703and1704.

FIG. 19(b) shows a configuration of a piezoelectric resonator. In this drawing, the piezoelectric resonator comprises a piezoelectric layer3011, an upper electrode3012formed on an upper principal plane of the piezoelectric layer3011, a lower electrode3013formed on a lower principal plane of the piezoelectric layer3011and a substrate3014. There is formed a depression in the surface of the substrate3014which is in contact with the lower electrode3013, and the depression constitutes a cavity3015. In this configuration, the upper electrode3012, the lower electrode3013, the piezoelectric layer3011sandwiched between the upper electrode3012and the lower electrode3013, and the part of the substrate3014which constitutes the cavity3015constitute the piezoelectric resonator.

The surface acoustic wave resonator and the piezoelectric resonator are each represented by an equivalent circuit shown inFIG. 19(c), and have electric characteristics that provide serial resonance and parallel resonance, respectively.

Connecting a plurality of such surface acoustic wave resonators in a ladder arrangement provides a ladder surface acoustic wave filter (for example, see Japanese Patent No. 3152418, the disclosure of which is incorporated herein by reference in its entirety).

Such a conventional acoustic resonator will now be described by taking a surface acoustic wave resonator as an example.

FIG. 20(a) shows, as a conventional example 1, a configuration of a surface acoustic wave filter formed by interconnecting three surface acoustic wave resonators1801,1802and1803in a π arrangement. As shown inFIG. 20(a), the surface acoustic wave resonators1801and1802each have one end grounded, and the other ends of the surface acoustic wave resonators are coupled to a transmission line at a predetermined interval, the transmission line1804having signal input and output terminals. The surface acoustic wave resonator1803is disposed in the predetermined interval on the transmission line1804.

In this configuration, as for the passing characteristics, the pass band and the attenuation band of the filter depend on the resonance and anti resonance frequencies of the parallel surface acoustic wave resonators1802and1803, which are placed in parallel with the serial surface acoustic wave resonator1801as shown inFIG. 20(b). However, there cannot be provided a band elimination filter having a low loss over a wide band.

FIG. 21(a) shows, as a conventional example 2, a circuit of a band elimination filter formed by connecting two surface acoustic wave resonators1901and1902in parallel. As shown inFIG. 21(a), the surface acoustic wave resonators1901and1902each have one end grounded, and the other ends of the surface acoustic wave resonators are coupled to a transmission line1903at a predetermined interval, the transmission line1903having signal input and output terminals. As shown inFIG. 21(b), although the filter has a low loss at frequencies higher than the stop band (attenuation pole), it has a high loss at frequencies lower than the stop band (attenuation pole).

As described above, with the surface acoustic wave filter composed of a plurality of acoustic resonators, such as surface acoustic wave resonators, used in communication apparatus or the like, it has been difficult to provide characteristics of a high attenuation within a desired frequency range and a low loss over wide frequency bands lower and higher than a stop band.

The present invention has been devised in view of the problem described above. An object of the invention is to provide a band elimination filter or the like that provides characteristics of a high attenuation within a desired frequency band and a low loss over wide frequency bands lower and higher than a stop band.

SUMMARY OF THE INVENTION

The 1staspect of the present invention is a band elimination filter, comprising:

a plurality of acoustic resonators each having one end grounded; and

a transmission line to which the other end of each of said plurality of acoustic resonators is connected,

wherein at least some of said other ends are coupled to the transmission line at predetermined intervals, and

at least one reactance element is provided on said transmission line in all or a part of said predetermined intervals.

The 2ndaspect of the present invention is the band elimination filter according to the 1staspect of the present invention, wherein said acoustic resonator is a surface acoustic wave resonator formed on a principal surface of a piezoelectric substrate.

The 3rdaspect of the present invention is the band elimination filter according to the 2ndaspect of the present invention, wherein a normalized impedance, which is obtained by normalizing the impedance of said reactance element with a characteristic impedance, is higher than 1.

The 4thaspect of the present invention is the band elimination filter according to the 3rdaspect of the present invention, wherein said normalized impedance is lower than 1.5.

The 5thaspect of the present invention is the band elimination filter according to any of the 1stto the 3rdaspects of the present invention, wherein said reactance element is an inductor.

The 6thaspect of the present invention is the band elimination filter according to the 5thaspect of the present invention, wherein said inductor includes a wire used in wire mounting.

The 7thaspect of the present invention is the band elimination filter according to the 1stor the 2ndaspects of the present invention, wherein said reactance element is a capacitor.

The 8thaspect of the present invention is the band elimination filter according to the 1stor the 2ndaspects of the present invention, wherein said reactance element includes a capacitor and an inductor.

The 9thaspect of the present invention is the band elimination filter according to the 8thaspect of the present invention, wherein said reactance element includes a parallel circuit of a capacitor and an inductor.

The 10thaspect of the present invention is the band elimination filter according to the 8thaspect of the present invention, wherein said reactance element includes a serial circuit of a capacitor and an inductor.

The 11thaspect of the present invention is the band elimination filter according to the 1staspect of the present invention, wherein said reactance element is a chip component.

The 12thaspect of the present invention is the band elimination filter according to the 1staspect of the present invention, wherein said reactance element is formed on a piezoelectric substrate.

The 13thaspect of the present invention is the band elimination filter according to the 1staspect of the present invention, wherein said reactance element is formed in a mounting substrate on which said band elimination filter is mounted.

The 14thaspect of the present invention is the band elimination filter according to the 13thaspect of the present invention, wherein said mounting substrate is a laminated body having a dielectric layer.

The 15thaspect of the present invention is the band elimination filter according to the 13thaspect of the present invention, wherein said acoustic resonators are face-down mounted on said mounting substrate.

The 16thaspect of the present invention is the band elimination filter according to the 2ndaspect of the present invention, wherein electrode pads of said surface acoustic wave resonators which are grounded are separated from each other on said piezoelectric substrate.

The 17thaspect of the present invention is the band elimination filter according to the 1staspect of the present invention, wherein said acoustic resonator is a piezoelectric resonator.

The 18thaspect of the present invention is the band elimination filter according to the 17thaspect of the present invention, wherein said piezoelectric resonator is a bulk wave resonator having an upper electrode, a lower electrode and a piezoelectric layer sandwiched between said upper electrode and said lower electrode.

The 19thaspect of the present invention is the band elimination filter according to the 18thaspect of the present invention, wherein said piezoelectric layer is composed of a piezoelectric thin film.

The 20thaspect of the present invention is the band elimination filter according to the 18thaspect of the present invention, wherein said reactance element is formed using said electrodes of said bulk wave resonator.

The 21staspect of the present invention is the band elimination filter according to the 1staspect of the present invention, wherein said surface acoustic wave resonators have different resonance frequencies.

The 22ndaspect of the present invention is the band elimination filter according to the 1staspect of the present invention, wherein said one end of each of said acoustic resonators is independently grounded wiring on a substrate.

The 23rdaspect of the present invention is the band elimination filter according to the 1staspect of the present invention, wherein said reactance element is an acoustic resonator having a resonance frequency different from the resonance frequencies of said acoustic resonators by a predetermined amount.

The 24thaspect of the present invention is a filter device comprising a band elimination filter according to the 1staspect of the present invention.

The 25thaspect of the present invention is an antenna duplexer, comprising:

a transmission filter; and

a receiving filter;

wherein a band elimination filter according to the 24thaspect of the present invention is used as said transmission filter or said receiving filter.

The 26thaspect of the present invention is a communication apparatus, comprising:

transmission means of transmitting a signal;

receiving means of receiving a signal, and

a band elimination filter according to the 1staspect of the present invention is used in said transmission means and/or said receiving means.

DESCRIPTION OF SYMBOLS

PREFERRED EMBODIMENTS OF THE INVENTION

Now, a surface acoustic wave filter according to an embodiment 1 of the present invention will be described with reference to the drawings.

FIG. 1shows a configuration of the surface acoustic wave filter and a passing characteristic according to the embodiment 1.FIG. 1(a) shows the configuration of the surface acoustic wave filter andFIG. 1(b) shows the passing characteristic. As shown inFIG. 1(a), the surface acoustic wave filter has first and second surface acoustic wave resonators101and102as acoustic resonators of the present invention and an inductor103as a reactance element of the present invention, which couples the resonators with each other. As shown inFIG. 19(a), the surface acoustic wave resonators101,102each have an IDT electrode formed on a piezoelectric substrate, which is equivalent to a piezoelectric substrate according to the present invention, and reflector electrodes disposed on both sides thereof.

More specifically, the surface acoustic wave resonators101and102each have one end grounded, and the other ends of the surface acoustic wave resonators are connected to a transmission line104at a predetermined interval, the transmission line104having signal input and output terminals. The inductor103is coupled in the predetermined interval between the ends connected to the transmission line104.

FIG. 1(b) shows the passing characteristic in the vicinity of a frequency of 900 MHz provided when the inductance of the inductor103is set at about 8 nH in the configuration shown inFIG. 1(a). The maximum attenuation is about 38 dB, and a low loss can be attained over wide frequency bands lower and higher a stop band.

FIG. 2(a) shows a maximum attenuation with respect to a normalized impedance (ωL/Zo). Here, reference character Zo denotes a characteristic impedance, reference character ω denotes an angular frequency, and reference character L denotes an inductance. The characteristic impedance Zo is set at about 50 Ω. The solid line, the dashed line and the dotted line indicate the maximum attenuation plotted with respect to the impedance for the arrangement according to this embodiment, the maximum attenuation for the arrangement according to the conventional example 1 shown inFIG. 20and the maximum attenuation for the arrangement according to the conventional example 2 shown inFIG. 21, respectively.FIG. 2(b) shows an out-of-band loss with respect to a normalized impedance. The solid line, the dashed line and the dotted line indicate the out-of-band loss plotted with respect to the normalized impedance for the arrangement according to this embodiment, the out-of-band loss for the arrangement according to the conventional example 1 shown inFIG. 20(b) and the out-of-band loss for the arrangement according to the conventional example 2 shown inFIG. 21(b), respectively. In the conventional example 1, while the in-band attenuation is higher than that in the conventional example 2, the out-of-band loss is also higher than that in the conventional example 2. In the conventional example 2, while the out-of-band loss is lower than that in the conventional example 1, the in-band attenuation is also lower than that in the conventional example 1.

In other words, both the characteristics of a higher attenuation within the attenuation band and a lower out-of band loss cannot be attained simultaneously.

As for the attenuation characteristic according to this embodiment, the attenuation is higher than that in the conventional example 2 shown inFIG. 21(b) over the whole range of Z/Zo, and is higher than that in the conventional example 1 shown inFIG. 20(b) within a range satisfying the relation of Z/Zo>1, and is higher than 40 dB. As for the loss characteristic according to this embodiment, the loss is improved over the whole range of Z/Zo compared to the conventional example 1 shown inFIG. 20(b), and is improved within a range satisfying the relation of Z/Zo<1.5 compared to the conventional example 2 shown inFIG. 21(b), and is equal to or less than about 1 dB. That is, within a range of Z between about Zo and 1.5Zo exclusive, both of the attenuation and the loss are improved compared to those in the conventional examples.

As described above, according to this embodiment, there can be provided a surface acoustic wave filter having bands top characteristics of a high attenuation and a low loss by coupling the two surface acoustic wave resonators with each other by the inductor as a reactance element.

While two surface acoustic wave resonators are used in this embodiment, three or more surface acoustic wave resonators may be used. In such a case, all the portions of the transmission line between the terminals of the surface acoustic wave resonators coupled thereto may have their respective inductors, or a part of them may have no inductor. It is essential only that at least one inductor is provided between the terminals, other than those grounded, of at least two surface acoustic wave resonators. In addition, in mounting, a serial circuit of a plurality of inductors, a parallel circuit of a plurality of inductors, or a combination thereof may be used. Furthermore, the configuration of the surface acoustic wave resonator itself is not limited to that described above.

The piezoelectric substrate in this embodiment may be a single crystal substrate, a substrate having a piezoelectric thin film formed thereon, a substrate having a dielectric thin film on a piezoelectric substrate. As far as the surface acoustic wave resonator constituting the surface acoustic wave filter has a characteristic that provides serial resonance and parallel resonance, the same effect as in this embodiment can be provided.

In the following, a surface acoustic wave filter according to an embodiment 2 of the present invention will be described with reference to the drawings.

The surface acoustic wave filter according to the embodiment 2 has the same configuration as that of the embodiment 1, except that the surface acoustic wave resonators101and102constituting the surface acoustic wave filter have different resonance frequencies. That is, the surface acoustic wave resonators101and102have different pitches of IDT electrodes, and as a result, they have different resonance frequencies and antiresonance frequencies.

FIG. 3(a) shows a passing characteristic in the vicinity of a frequency of 900 MHz provided when the inductance value of the inductor is about 8 nH. For comparison,FIG. 3(b) shows a passing characteristic in the vicinity of a frequency of 900 MHz provided in the case where the two surface acoustic wave resonators1901and1902in the circuit in the conventional example 2 have different resonance frequencies. In this embodiment 2, the stop band is expanded because of the different resonance frequencies. Furthermore, the attenuation is increased and the loss is reduced on both sides of the attenuation band, compared to the passing characteristic in the conventional example 2 shown inFIG. 3(b).

As described above, according to this embodiment, there can be provided a surface acoustic wave filter having bandstop characteristics of a high attenuation, a wide stop band high and a low loss by coupling the two surface acoustic wave resonators having different resonance frequencies with each other by the inductor serving as a reactance element.

While two surface acoustic wave resonators are used in this embodiment, three or more surface acoustic wave resonators may be used. In such a case, the inductor(s) can be arranged in the same manner as in the embodiment 1. Furthermore, the configuration of the surface acoustic wave resonator is not limited to that described above.

The piezoelectric substrate in this embodiment may be a single crystal substrate, a substrate having a piezoelectric thin film formed thereon, a substrate having a piezoelectric substrate and a dielectric thin film formed thereon. As far as the surface acoustic wave resonator constituting the surface acoustic wave filter has a characteristic that provides serial resonance and parallel resonance, the same effect as in this embodiment can be provided.

In the following, a surface acoustic wave filter according to an embodiment 3 will be described with reference to the drawings.

FIG. 4shows a configuration of a part of the surface acoustic wave filter according to the embodiment 3 which is formed on a piezoelectric substrate. InFIG. 4, the part of the surface acoustic wave filter formed on the piezoelectric substrate is composed of first and second surface acoustic wave resonators402and403formed on a piezoelectric substrate401, which is corresponding to the piezoelectric substrate of the present invention. An IDT electrode of the first surface acoustic wave resonator402is provided with electrode pads404and405, and an IDT electrode of the second surface acoustic wave resonator403is provided with electrode pads406and407.

FIG. 5(a) shows a configuration of the whole of the surface acoustic wave filter, according to this embodiment. In the surface acoustic wave filter shown inFIG. 5(a), the electrode pad404of the first surface acoustic wave resonator402is coupled to an input terminal501, and the electrode pad406of the second surface acoustic wave resonator403is coupled to an output terminal502. In addition, an inductor503is disposed between the first and second surface acoustic wave resonators402and403. The electrode pad405of the first surface acoustic wave resonator402is grounded via an inductance component504, which is assumed to be a parasitic component, such as a wire or a wiring on the mounting substrate. The electrode pad407of the second surface acoustic wave resonator403is grounded via an inductance component505, which is assumed to be a parasitic component, such as a wire or a wiring on the mounting substrate. That is, groundings of the surface acoustic wave resonators on the piezoelectric substrate401are provided separately and independently. That is, the surface acoustic wave resonators are prevented from having a common impedance, such as a wire or wiring on the mounting substrate, when the groundings thereof are drawn from the piezoelectric substrate401. Herein, only the parasitic component at the time when the groundings are drawn is considered.

FIG. 5(b) shows a passing characteristic according to this embodiment. InFIG. 5(a), the inductor503has an inductance of about 10 nH, and the inductors504,505serving as parasitic components have an inductance of about 1 nH.

FIG. 6shows an example for comparison, in which a common electrode pad601is coupled to the groundings of the first and second surface acoustic wave resonators402,403. That is, as shown inFIG. 7(a), a common electrode pad is coupled to the groundings of the surface acoustic wave resonators on the piezoelectric substrate and grounded via an inductor701which is assumed to be a parasitic component.FIG. 7(b) shows a characteristic thereof. Here, it is assumed that the inductor701has an inductance of 1 nH. From comparison ofFIGS. 5(b) and7(b), it can be seen that, in the passing characteristic in this embodiment, the attenuation is extremely increased. In other words, by providing the separate electrode pads coupled to the ground, that is, by grounding the surface acoustic wave resonators via the separate wirings at least on the piezoelectric substrate, the surface acoustic wave filters according to the embodiment 1 and 2 can be implemented without degrading the characteristics thereof.

In mounting, wire mounting or face down mounting may be used. For example,FIG. 8shows a configuration of a surface acoustic wave filter that is wire-mounted, in which four electrode pads404,406,405and407are independently coupled to terminals801a,801b,801cand801din the package via wires802a,802b,802cand802d,respectively. The inductor, which is to be disposed between surface acoustic wave resonators, is coupled between the terminals801aand801b.In addition, the terminals801aand801bare grounded inside or outside of the package.

FIG. 9shows a configuration of a surface acoustic wave filter face-down mounted on a mounting substrate, in which electrode pads404and406on a piezoelectric substrate401having two surface acoustic wave resonators901formed thereon are coupled to pads903aand903bon a mounting substrate906via bumps902aand902bin a face-down manner, respectively. The pads903aand903bon the mounting substrate906are electrically coupled to external terminals905aand905bon the lower surface of the substrate through via holes904aand904b,respectively. In addition, although not shown, electrode pads405and407are also coupled to the mounting substrate906and grounded in the same manner.

As described above, according to this embodiment, there can be provided a surface acoustic wave filter having bandstop characteristics of a high attenuation and a low loss by providing the separate electrode pads to be grounded for the two surface acoustic wave resonators on the piezoelectric substrate.

While two surface acoustic wave resonators are used in this embodiment, three or more surface acoustic wave resonators may be used as in the embodiment 1. Furthermore, the configuration of the surface acoustic wave resonator is not limited to that described above.

The piezoelectric substrate in this embodiment may be a single crystal substrate, a substrate having a piezoelectric thin film formed thereon, a substrate having a piezoelectric substrate and a dielectric thin film formed thereon. As far as the surface acoustic wave resonator constituting the surface acoustic wave filter has a characteristic that provides serial resonance and parallel resonance, the same effect as in this embodiment can be provided.

In the following, a surface acoustic wave filter according to an embodiment 4 of the present invention will be described with reference to the drawings.

FIGS. 10(a) and10(b) show a configuration of the surface acoustic wave filter and a passing characteristic according to the embodiment 4.FIG. 10(a) shows the configuration of the surface acoustic wave filter andFIG. 10(b) shows the passing characteristic. As shown inFIG. 10(a), the surface acoustic wave filter has first and second surface acoustic wave resonators1001and1002and a capacitor1003serving as a reactance element that couples the resonators with each other.

More specifically, the surface acoustic wave resonators1001and1002each have one end grounded, and the other ends of the surface acoustic wave resonators are connected to a transmission line1004at a predetermined interval, the transmission line1004having signal input and output terminals. The capacitor1003is coupled in the predetermined interval between the ends connected to the transmission line1004. As shown inFIG. 19(a), the surface acoustic wave resonators1001and1002each have an IDT electrode formed on a piezoelectric substrate and reflector electrodes disposed on both sides thereof.

FIG. 10(b) shows the passing characteristic in the vicinity of a frequency of 900 MHz provided when the capacitance of the capacitor1003is set at 8 pF. Compared to the passing characteristic in the conventional example 2 shown inFIG. 21(b), the attenuation is enhanced.

FIG. 11(a) shows a maximum attenuation with respect to a normalized impedance (Z=1/ωCZo). Here, reference character Zo denotes a characteristic impedance, reference character ω denotes an angular frequency, and reference character C denotes a capacitance. The characteristic impedance Zo is set at about 50 Ω. The solid line, the dashed line and the dotted line indicate the maximum attenuation with respect to the normalized impedance for the arrangement according to this embodiment, the maximum attenuation for the arrangement according to the conventional example 1 shown inFIG. 20(b) and the maximum attenuation for the arrangement according to the conventional example 2 shown inFIG. 21(b), respectively. As for the attenuation characteristic according to this embodiment, compared to the conventional example 1 shown inFIG. 20(b), the attenuation is increased within a range satisfying the relation of Z/Zo>1, and compared to the conventional example 2 shown inFIG. 21(b), the attenuation is increased over the whole range of Z/Zo.FIG. 11(b) shows an out-of-band loss with respect to the normalized impedance. Compared to the conventional example 1 shown inFIG. 20(b), the loss is improved within a range satisfying the relation of Z/Zo<1.5.

As described above, according to this embodiment, there can be provided a surface acoustic wave filter having bandstop characteristics of a high attenuation and a low loss by coupling the two surface acoustic wave resonators with each other by the capacitor serving as a reactance element.

While two surface acoustic wave resonators are used in this embodiment, three or more surface acoustic wave resonators may be used. In such a case, the capacitor (s) can be arranged in the same manner as the inductor(s) in the embodiment 1. In addition, in mounting, a serial circuit of a plurality of capacitors, a parallel circuit of a plurality of capacitors, or a combination thereof may be used. Furthermore, the configuration of the surface acoustic wave resonator itself is not limited to that described above.

The piezoelectric substrate in this embodiment may be a single crystal substrate, a substrate having a piezoelectric thin film formed thereon, a substrate having a dielectric thin film on a piezoelectric substrate. As far as the surface acoustic wave resonator constituting the surface acoustic wave filter has a characteristic that provides serial resonance and parallel resonance, the same effect as in this embodiment can be provided.

In the embodiments 1 to 4 described above, the inductor or capacitor is used as the reactance element to couple the two surface acoustic wave resonators with each other. However, a parallel circuit of an inductor and a capacitor or a serial circuit thereof may be used as shown inFIG. 12.FIGS. 12(a) and12(b) show a parallel circuit of an inductor and a capacitor and a serial circuit thereof, respectively. InFIG. 12(a), a surface acoustic wave resonators1201and1202are coupled to each other via a parallel circuit of a capacitor1204and an inductor1203. InFIG. 12(b), the surface acoustic wave resonators1201and1202are coupled to each other via a serial circuit of the capacitor1206and the inductor1205. Of course, a plurality of capacitors or inductors may be mounted.

While two surface acoustic wave resonators are used in this embodiment, three or more surface acoustic wave resonators may be used. In such a case, the serial circuit or parallel circuit can be disposed in the same manner as the inductors in the embodiment 1.

In the following, a surface acoustic wave filter according to an embodiment 5 of the present invention will be described with reference to the drawings.

FIG. 13shows a configuration of the surface acoustic wave filter according to the embodiment 5 on a piezoelectric substrate. InFIG. 13, the surface acoustic wave filter comprises first and second surface acoustic wave resonators402and403formed on a piezoelectric substrate401, which is corresponding to the piezoelectric substrate according to the present invention. An IDT electrode of the first surface acoustic wave resonator402is provided with electrode pads404and405, and an IDT electrode of the second surface acoustic wave resonator403is provided with electrode pads406and407. In addition, an inductor1301formed on the piezoelectric substrate is coupled between the electrode pads404and406. In this case, the inductor1301can be formed simultaneously with film deposition and patterning of the surface acoustic wave resonators. In addition, the electrode pads404,406are coupled to input and output terminals, and the electrode pads405,407are grounded. Such an arrangement can eliminate the need of connection of the inductor outside the package, thereby realizing downsizing.

In addition, the inductor coupling the two surface acoustic wave resonators may be formed in a mounting substrate, which is previously formed by laminating dielectric layers, and then the piezoelectric substrate having the surface acoustic wave resonators formed thereon may be face-down mounted on the mounting substrate.FIG. 14shows a configuration of a surface acoustic wave filter face-down mounted on a mounting substrate, in which electrode pads404and406on a piezoelectric substrate401having two surface acoustic wave resonators901formed thereon are coupled to pads903aand903bon a mounting substrate906via bumps902aand902bin a face-down manner, respectively. The pads903aand903bon the mounting substrate906are electrically coupled to external terminals905aand905bon the lower surface of the substrate through via holes904aand904b,respectively. In addition, although not shown, electrode pads405and407are also coupled to the mounting substrate and grounded in the same manner. An inductor1401is formed by an internal layer pattern of the mounting substrate906and electrically coupled between the input and output terminals of the filter, that is, between the surface acoustic wave resonators. In this case, the inductor1401can be formed on a larger area, and thus, a higher inductance can be provided. Alternatively, the inductor1401may be formed on the surface of the mounting substrate.

As described above, in the surface acoustic wave filter according to this embodiment, the inductor coupling the two surface acoustic wave resonators is formed on the piezoelectric substrate or in the mounting substrate, and therefore, downsizing of the surface acoustic wave filter can be attained.

While two surface acoustic wave resonators are used in this embodiment, three or more surface acoustic wave resonators may be used as in the embodiments 1 to 4. Furthermore, the configuration of the surface acoustic wave resonator is not limited to that described above.

The piezoelectric substrate in this embodiment may be a single crystal substrate, a substrate having a piezoelectric thin film formed thereon, a substrate having a dielectric thin film on a piezoelectric substrate. As far as the surface acoustic wave resonator constituting the surface acoustic wave filter has a characteristic that provides serial resonance and parallel resonance, the same effect as in this embodiment can be provided.

In addition, in this embodiment described above, the inductor is formed in an internal layer of the mounting substrate. However, it may be formed in the package.

In addition, while the inductor is formed on the piezoelectric substrate or in the mounting substrate, a wire used in wire mounting may be used to serve as an inductance component.

In this embodiment, while a method of forming the surface acoustic wave filter using an inductor as a reactance element has been described, a capacitor or a combination of an inductor and a capacitor may be used as the reactance element.

In the following, a band elimination filter according to an embodiment 6 of the present invention will be described with reference to the drawings.

FIG. 15shows an equivalent circuit diagram of the band elimination filter according to the embodiment6. In the embodiments described above, the band elimination filter according to the present invention is a surface acoustic wave filter having a surface acoustic wave resonator as an acoustic resonator. However, in this embodiment, two acoustic resonators are piezoelectric resonators of a bulk wave type, rather than the resonators of the surface acoustic wave type.

InFIG. 15, the band elimination filter comprises first and second piezoelectric resonators1501and1502and an inductor1503serving as a reactance element. The first and second piezoelectric resonators1501and1502have characteristics that provide serial resonance and parallel resonance, and the equivalent circuit of the piezoelectric resonator is the same as that of the surface acoustic wave resonator. That is, concerning the operation of the resonator, the piezoelectric resonator is the same as the surface acoustic wave resonator, and coupling such piezoelectric resonators using the inductor can provide a band elimination filter with a low loss and a high attenuation. For example, piezoelectric resonators having such characteristics include a bulk wave resonator using a piezoelectric thin film and a bulk wave resonator using a single crystal. In such bulk wave resonators, the frequency range that can be provided is limited. However, the Q value of the resonator can be improved by appropriately selecting the piezoelectric material, and the resonator can have a lower loss and a higher attenuation than those of the surface acoustic wave resonator. Besides, the normalized impedance of the reactance element as well as the characteristic impedance of the equivalent circuit is set, whereby the bulk wave resonator can be optimized in the same manner as the surface acoustic wave resonator.

FIG. 16(a) shows a specific configuration of a band elimination filter using a bulk wave type piezoelectric resonator. The band elimination filter comprises a substrate2001and a piezoelectric layer2002having resonator electrodes on both principal planes thereof, the piezoelectric layer being provided on the substrate2001and being corresponding to the piezoelectric layer according to the present invention.

In addition, an upper electrode2003aand a lower electrode2003bare provided on the upper principal plane of the piezoelectric layer2002at a predetermined interval. A lower electrode2004aand a lower electrode2004bare provided on the lower principal plane of the piezoelectric layer at positions opposite to the upper electrode2003aand the upper electrode2003b,respectively. The lower electrodes2004aand2004bare independently grounded, and the upper electrodes2003aand2003bare coupled to signal input and output terminals. In addition, an inductor2006, which connects the upper electrodes2003aand2003bto each other, is provided over the predetermined interval between the upper electrodes2003aand2003b.The upper and lower electrodes may be formed by patterning a material, such as molybdenum, aluminum and platinum.

On the other hand, depressions are formed in the surface of the substrate2001which is in contact with the lower electrodes2004aand2004b,and the depressions constitute cavities2007aand2007b.

In this configuration, the upper electrode2003a,the lower electrode2004a,the part of the piezoelectric layer2002sandwiched between the upper electrode2003aand the lower electrode2004a,and the part of the substrate2001which constitutes the cavity2007aconstitute a first resonator2008, which is corresponding to the piezoelectric resonator1501. Besides, the upper electrode2003b,the lower electrode2004b,the part of the piezoelectric layer2002sandwiched between the upper electrode2003band the lower electrode2004b,and the part of the substrate2001which constitutes the cavity2007bconstitute a second resonator2009, which is corresponding to the piezoelectric resonator1502. The inductor2006is corresponding to the inductor1503and formed by patterning the electrode, for example.

FIG. 16(b) shows another example of specific configuration of a band elimination filter using a bulk wave type piezoelectric resonator. Parts identical to or corresponding to those inFIG. 16(a) are assigned the same reference numerals, and detailed description thereof will be omitted. In the example shown inFIG. 16(b), a dielectric layer2014is formed directly below the inductor instead of the piezoelectric layer2002. This can improve the isolation between the resonators.

As described above, according to this embodiment, there can be provided a band elimination filter having bandstop characteristics of a high attenuation and a low loss by coupling the two piezoelectric resonators with each other by the inductor serving as a reactance element.

While two piezoelectric resonators are used in this embodiment, three or more piezoelectric resonators may be used. In such a case, all the portions between the upper electrodes may have their respective inductors, or a part of them may have no inductor. It is essential only that at least one inductor is provided between the electrodes, other than those grounded, of at least two piezoelectric resonators. In addition, in mounting, a serial circuit of a plurality of inductors, a parallel circuit of a plurality of inductors, or a combination thereof may be used. Furthermore, the configuration of the piezoelectric resonator itself is not limited to that described above.

While the inductor is used as a reactance element in the above description, a capacitor may be used.

FIGS. 17(a) to17(c) show specific configurations of a band elimination filter using piezoelectric resonators having a capacitor as a reactance element. Parts identical to or corresponding to those shown inFIGS. 16(a) and16(b) are assigned the same reference numerals and detailed description thereof will be omitted.

As shown inFIG. 17(a), in the band elimination filter, an upper electrode2010aand an upper electrode2010bhave different lengths in the lateral direction in this drawing, and the upper electrode2011ais longer than the lower electrode2011b.On the other hand, a lower electrode2011aand a lower electrode2011balso have different lengths, and the lower electrode2011bis longer than the upper electrode2010b.Furthermore, the lower electrode2011ais grounded, while the lower electrode2011bis not grounded. Instead, the upper electrode2010bis grounded.

In such a configuration, there are formed three parts each having an in between piezoelectric layer2002, that is, (A) a part having the upper electrode2010aand the lower electrode2011aopposite to each other, (B) a part having the upper electrode2010band the lower electrode2011bopposite to each other and (C) a part having the upper electrode2010aand the lower electrode2011bopposite to each other. The part (A) constitutes a first resonator2008, the part (B) constitutes a second resonator2009, and the part (C) constitutes a capacitor2012, which is equivalent to the capacitor1003in the embodiment 4.

In the example shown inFIG. 17(b), in the part (C) described above, the piezoelectric layer2002is thinned compared to those in the other parts (A) and (B). This can increase the capacitance of the capacitor2012.

In the example shown inFIG. 17(c), in the part (C) described above, a dielectric2013is inserted between the upper electrode2010aand the lower electrode2011binstead of the piezoelectric layer2002. This allows the capacitance of the capacitor2012to be set to a desired value and the isolation between the resonators to be improved.

Also in the case of using the capacitor, three or more piezoelectric resonators may be used. In such a case, all the sections between the electrodes of adjacent piezoelectric resonators may have an overlap of the upper electrode and the lower electrode and, therefore, their respective capacitors. Alternatively, a part of the sections between the electrodes of adjacent piezoelectric resonators may have no overlap of the upper electrode and the lower electrode and therefore no capacitor. It is essential only that at least one capacitor is provided between at least two piezoelectric resonators. In addition, in mounting, a serial circuit of a plurality of capacitors, a parallel circuit of a plurality of capacitors, or a combination thereof may be used. Furthermore, the configuration of the piezoelectric resonator itself is not limited to that described above.

As in the embodiment 2, the resonance frequency of each piezoelectric resonator can be varied to provide a wider stop band. As in the embodiment 4, a parallel circuit of an inductor and a capacitor or a serial circuit of an inductor and a capacitor may be used as a reactance element. As in the embodiment 5, the reactance element can be formed in the package or mounting substrate, thereby realizing downsizing.

In addition, the acoustic wave filter or the band elimination filter according to the present invention may be used in combination with a filter having another configuration.

In the above description, the piezoelectric resonator has the cavities2007aand2007bformed by forming depressions on the substrate2001. However, the cavities may be formed by forming through holes from the bottom of the substrate, or acoustic mirrors may be used instead of forming the cavities. Besides, in the above description, the first resonator2008and the second resonator2009each have the electrodes formed on the both principal planes of the piezoelectric layer. However, without being limited to this configuration, the resonators may have any configuration as far as they have resonance and antiresonance characteristics.

In the above description, the inductor2006and the capacitor2012are both formed on the substrate2001. However, they may be provided outside the substrate2001and coupled to the resonators by a bonding wire or the like.

In each embodiment described above, the reactance element is an inductor, a capacitor or a combination thereof. However, the reactance element in the present invention may be implemented by the surface acoustic wave resonator or piezoelectric resonator described above. In such a case, the configuration of the band elimination filter similar that of the π-connection bandpass filter shown inFIG. 20(a). However, the surface acoustic wave resonator or piezoelectric resonator can be made to function as a capacitive reactance element, if the resonance frequency thereof is shifted by a predetermined amount from those of the other acoustic resonators. Here, the predetermined amount is such an amount that the resonance and antiresonance frequencies of the acoustic resonators as primary resonators are equal to or lower than the resonance frequency of the acoustic resonator as the reactance element, or equal to or higher than the antiresonance frequency thereof. At the frequency satisfying this condition, the acoustic resonator as a reactance element has a capacitive characteristic, and thus, the same effect as in the embodiments described above can be attained.

In the following, a communication apparatus according to an embodiment 7 of the present invention will be described with reference to the drawings.

FIG. 18(a) is a block diagram showing a communication apparatus1601according to the present invention. InFIG. 18(a), a transmission signal output from a transmitter circuit is transmitted from an antenna1605via a transmission amplifier1602, a transmission filter1603and a switch1604. A reception signal received at the antenna1605is input to a receiver circuit via a switch1604, a receiving filter1606and a receiving amplifier1607. The transmission amplifier1602and the transmission filter1603correspond to transmission means according to the present invention, and the receiving filter1606and the receiving amplifier1607correspond to receiving means according to the present invention.

By applying the band elimination filter according to the present invention to a part of the transmission filter1603or a part of the receiving filter1603of the communication apparatus1601, the transmission efficiency or the receiving sensitivity can be improved, respectively. Thus, the communication apparatus can have a higher performance.

In the communication apparatus1601described above, the switch1604is used as means to switch between transmission and reception. However, as shown inFIG. 18(b), it may be replaced with an antenna duplexer1808. If the band elimination filter according to the present invention is applied to a part of the transmission filter or a part of the receiving filter of the antenna duplexer1608, an adequate amount of attenuation in the stop band can be assured, and an adequate isolation between the transmission and the reception can be assured. Here, the transmission filter and the receiving filter each correspond to a filter device according to the present invention. Furthermore, the antenna duplexer1608corresponds to an antenna duplexer according to the present invention. The filter device according to the present invention may be implemented as a combination of the band elimination filter according to the present invention and another filter as with the transmission filter and the receiving filter in this embodiment, or implemented solely by the band elimination filter according to the present invention.

According to the present invention, there can be provided a band elimination filter having a high attenuation within a desired band and a low loss at frequencies lower and higher than the stop band and a communication apparatus or the like having the same band elimination filter.