Balanced acoustic wave filter and acoustic wave filter

In an acoustic wave filter, first and second filter elements are connected between an unbalanced terminal and first and second balanced terminals on an acoustic wave substrate. The filter elements are longitudinally-coupled surface wave filters including at least three IDTs. One end of the IDTs is mutually connected and is connected to the unbalanced terminal. The other ends of the IDTs are connected by a second connecting line so as to define an unbalanced-side grounded portion. Grounded ends of the IDTs are connected to each other by a first connecting line so as to define a common midpoint grounded portion. The common midpoint grounded portion and the unbalanced-side grounded portion are electrically separated from each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acoustic wave filter including a plurality of acoustic wave elements connected to each other and an acoustic wave filter device. More specifically, the present invention relates to a midpoint-grounded balanced acoustic wave filter having a balance-unbalance converting function and an acoustic wave filter device.

2. Description of the Related Art

Conventionally, various surface acoustic wave (SAW) filters have been used as band-pass filters for communication apparatuses, such as mobile phones. In particular, a SAW filter used in an RF (radio frequency) stage of a mobile phone is required to have a balance-unbalance converting function. When having the balance-unbalance converting function, a SAW filter does not need to include a balun functioning as a balance-unbalance converting element. Accordingly, the mobile phone or other suitable device can be miniaturized.

Japanese Unexamined Patent Application Publication No. 2004-88551 (Patent Document 1) discloses an example of a midpoint-grounded balanced SAW filter having a balance-unbalance converting function.FIG. 24is a schematic plan view showing the SAW filter described in Patent Document 1, andFIG. 25shows the circuit configuration thereof.

As shown inFIG. 24, a SAW filter200includes a surface wave substrate301. First and second SAW filter elements201and202and first and second SAW resonators203and204are disposed on the surface wave substrate301. As shown inFIG. 25, the first filter element201is a longitudinally-coupled SAW filter element including IDTs (interdigital transducers)205to207and reflectors208and209aligned in a propagation direction of surface waves. The filter element202has the same configuration, including IDTs210to212and reflectors213and214. Note that the phase of the second filter element202is 180 degrees different from that of the first filter element201.

The center IDT206of the first filter element201and the center IDT211of the second filter element202connect to an unbalanced terminal215. One end of the IDTs205and207connect to a first balanced terminal216via the SAW resonator203. On the other hand, one end of the outer IDTs210and212of the second filter element202connect to a second balanced terminal217via the SAW resonator204. With this configuration, the SAW filter200functions as a balanced SAW filter including the unbalanced terminal215and the first and second balanced terminals216and217.

In the first and second filter elements, the ends opposite to the ends connected to the unbalanced terminal215of the IDTs206and211are grounded. As shown inFIG. 24, the IDTs206and211connect to electrode pads302and303provided on the surface wave substrate301, respectively. The electrode pads302and303are electrode pads connected to a ground potential.

During manufacturing, metallic bumps are provided on the electrode pads302and303, and the metallic bumps are electrically connected to an electrode land connecting to a ground potential on a package side. Accordingly, the electrode pads302and303, that is, the grounded ends of the IDTs206and211, are mutually connected on the electrode land connecting to the ground potential on the package side.

On the other hand, the ends opposite to the ends connected to the balanced signal terminal216of the IDTs205and207of the first filter element are also grounded. That is, inFIG. 24, the grounded end of the IDT205is electrically connected to an electrode pad311. In addition, the IDT207is electrically connected to an electrode pad312. The electrode pads311and312are electrode pads connected to the ground potential.

Likewise, on the side of the second filter element202, the ends opposite to the ends connected to the balanced signal terminal217of the IDTs210and212are grounded. More specifically, as shown inFIG. 24, the grounded end of the IDT210is connected to the electrode pad312, whereas the grounded end of the IDT212is connected to an electrode pad313. Similar to the above-described electrode pads302and303, the electrode pads311to313are connected to an electrode land that is provided on the package substrate and that is connected to the ground potential. That is, the electrode pads311to313are mutually connected as shown in the package substrate.

The SAW filter200is mounted on the package substrate by a face-down method. In this case, metallic bumps are provided on a plurality of electrode pads including the above-described electrode pads302,303, and311to313. Then, the SAW filter200is mounted on the package substrate such that the surface on which the metallic bumps are provided is a lower surface, so that the metallic bumps enable the electrode pads302,303, and311to313to be connected to the electrode lands on the package substrate.

Since the above-described SAW filter200is assembled using a face-down method and does not need to include a balun because of having a balance-unbalance converting function, the SAW filter200can be miniaturized, so that the RF stage including the SAW filter200can also be miniaturized.

However, when the SAW filter200is actually used as a balanced SAW filter having a balance-unbalance converting function while being mounted on the package substrate as described above, out-of-band attenuation is not sufficient.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of the present invention provide a further miniaturized balanced acoustic wave filter having efficiently improved out-of-band attenuation, thus, having a favorable filter characteristic, and also provide an acoustic wave filter device including the acoustic wave filter.

According to a preferred embodiment of the present invention, a balanced acoustic wave filter includes an unbalanced terminal and first and second balanced terminals. The filter includes an acoustic wave substrate, a first longitudinally-coupled filter element that is located on the acoustic wave substrate and that includes at least three IDTs connected between the unbalanced terminal and the first balanced terminal, a second longitudinally-coupled filter element that is located on the acoustic wave substrate and that includes at least three IDTs connected between the unbalanced terminal and the second balanced terminal through which a signal of an opposite phase to that in the first balanced terminal of the first filter element flows, a first connecting line that connects a grounded end of the IDT connected to the first balanced terminal of the first filter element to a grounded end of the IDT connected to the second balanced terminal of the second filter element so as to define a common midpoint grounded portion and that is provided on the acoustic wave substrate, and a second connecting line that connects grounded ends of the IDTS connected to the unbalanced terminal in the first filter element to grounded ends of the IDTs connected to the unbalanced terminal in the second filter element so as to define an unbalanced-side grounded portion and that is provided on the acoustic wave substrate. The common midpoint grounded portion and the unbalanced-side grounded portion are electrically separated from each other on the acoustic wave substrate.

According to another preferred embodiment of the present invention, a balanced acoustic wave filter includes an acoustic wave substrate, a first longitudinally-coupled filter element that is disposed on the acoustic wave substrate and that includes an unbalanced terminal, first and second balanced terminals, and at least three IDTs connected between the unbalanced terminal and the first and second balanced terminals, a second longitudinally-coupled filter element that includes third and fourth balanced terminals connected to the first and second balanced terminals of the first filter element, fifth and sixth balanced terminals, and a plurality of IDTs connected between the third and fourth balanced terminals and the fifth and sixth balanced terminals, a first connecting line that connects grounded ends of the IDTs connected to the first and second balanced terminals of the first filter element to grounded ends of the IDTs connected to the third and fourth balanced terminals of the second filter element, that defines a common midpoint grounded portion, and that is provided on the acoustic wave substrate, and a second connecting line that is connected to the grounded ends of the IDTs connected to the unbalanced terminal of the first filter element and that is provided on the acoustic wave substrate so as to define an unbalanced-side grounded portion. The common midpoint grounded portion and the unbalanced-side grounded portion are electrically separated from each other on the acoustic wave substrate.

According to another preferred embodiment of the present invention, the balanced acoustic wave filter further includes a third connecting line that connects the unbalanced terminal to at least one of the IDTs connected to the unbalanced terminal and that crosses the first connecting line on the acoustic wave substrate, and interlayer insulating films disposed between the first and third connecting lines at intersections of the first and third connecting lines.

According to another preferred embodiment of the present invention, the balanced acoustic wave filter further includes a package. The package includes a first electrode land connected to the common midpoint grounded portion by a bump and a second electrode land connected to the unbalanced-side grounded portion by a bump. The first and second electrode lands are separated from each other on the package.

According to another preferred embodiment of the balanced acoustic wave filter according to the present invention, a surface acoustic wave is used as the acoustic wave, so that a balanced surface acoustic wave filter is provided. In the present invention, an “acoustic wave” includes not only a surface acoustic wave, but also other types of acoustic waves, such as a boundary acoustic wave.

According to another preferred embodiment of an acoustic wave filter device according to the present invention, a first balanced acoustic wave filter as the balanced acoustic wave filter according to a preferred embodiment of the present invention and a second balanced acoustic wave filter as the balanced acoustic wave filter according to another preferred embodiment of the present invention are disposed on an acoustic wave substrate. A center frequency of the first balanced acoustic wave filter is different from a center frequency of the second balanced acoustic wave filter. A common midpoint grounded portion of the first balanced acoustic wave filter and a common midpoint grounded portion of the second balanced acoustic wave filter are mutually connected, and an unbalanced-side grounded portion of the first balanced acoustic wave filter and an unbalanced-side grounded portion of the second balanced acoustic wave filter are mutually connected. The mutually connected common midpoint grounded portions and the mutually connected unbalanced-side grounded portions are electrically separated from each other on the acoustic wave substrate.

According to another preferred embodiment of the acoustic wave filter device according to the present invention, a first balanced acoustic wave filter as the balanced acoustic wave filter according to a preferred embodiment of the present invention and a second balanced acoustic wave filter as the balanced acoustic wave filter according to a preferred embodiment of the present invention are disposed on an acoustic wave substrate. A center frequency of the first balanced acoustic wave filter is different from a center frequency of the second balanced acoustic wave filter. A common midpoint grounded portion of the first balanced acoustic wave filter and a common midpoint grounded portion of the second balanced acoustic wave filter are mutually connected. The mutually connected common midpoint grounded portions, an unbalanced-side grounded portion of the first balanced acoustic wave filter, and an unbalanced-side grounded portion of the second balanced acoustic wave filter are electrically separated from each other on the acoustic wave substrate. The acoustic wave filter device further includes a package, the package including a first electrode land connected to the common midpoint grounded portion by a bump, a second electrode land connected to the unbalanced-side grounded portion of the first balanced acoustic wave filter by a bump, and a third electrode land connected to the unbalanced-side grounded portion of the second balanced acoustic wave filter by a bump. The first electrode land, the second electrode land, and the third electrode land are separated from each other on the package.

According to another preferred embodiment of the acoustic wave filter device according to the present invention, the acoustic wave filter device defines a duplexer or a multi-band filter.

In the balanced acoustic wave filter according to preferred embodiments of the present invention, the first connecting line connects the grounded end of the IDT connected to the first balanced terminal of the first filter element to the grounded end of the IDT connected to the second balanced terminal of the second filter element so as to define the common midpoint grounded portion, the second connecting line connects the grounded ends of the IDTS connected to the unbalanced terminal in the first filter element to the grounded ends of the IDTs connected to the unbalanced terminal in the second filter element so as to define the unbalanced-side grounded portion, and the common midpoint grounded portion and the unbalanced-side grounded portion are electrically separated from each other on the acoustic wave substrate. Therefore, in the midpoint-grounded balanced acoustic wave filter having a balance-unbalance converting function, out-of-band attenuation is effectively improved. The reasons are as follows. That is, ground current in the midpoint-grounded balanced acoustic wave filter flows only in the ground side of the IDT connected to the unbalanced terminal. That is, when the grounded ends of the IDTs connected to the balanced terminal are connected to each other and when the midpoint thereof is grounded, the two IDTs are connected in series between two balanced signal terminals. In this case, voltages of the same magnitude and opposite phases are generated in the two IDTs on the balanced side, current flows from the balanced terminal of one of the IDTs to the balanced terminal of the other IDT, and currents of opposite directions flow into the midpoint grounded portion and cancel each other.

However, in the configuration in which the first and second filter elements are connected, the midpoint grounded portions of the IDTs of the two filter elements cannot usually be combined on an acoustic wave substrate. Therefore, the midpoint grounded portions are mutually connected on the ground electrode side on the package substrate, as described in Patent Document 1. However, achieving miniaturization is difficult because the SAW substrate is connected to the package substrate by bumps, and thus, the distance between the ground electrode on the package substrate and the common midpoint grounded portion of the IDTs on the SAW substrate is large, the area of bumps is large, and the distances between the bumps are large. Furthermore, out-of-band attenuation disadvantageously deteriorates.

That is, in an electrode land of the packaged substrate, current due to surface acoustic waves flows from a connecting point of a bump connected to a grounded potential to a connecting point of a bump connected to another grounded potential, an electrical midpoint exists at a point between two bump connecting points of an electrode land on the package substrate, and the impedance between the point and the midpoint grounded portion of the IDTs of the first and second filter elements increases.

On the other hand, according to preferred embodiments of the present invention, common connection of the common midpoint grounded portion is achieved on the acoustic wave substrate. Thus, an electrical midpoint exists on the acoustic wave substrate, the distance between the common midpoint grounded portion and the electrical midpoint is reduced, the impedance between them is reduced, balance is improved, and out-of-band attenuation of the filter is improved. That is, the attenuation on a high-frequency side of a pass band is improved by connecting the separated common midpoint grounded portion and the unbalanced-side grounded portion by impedance or inductance. The inductance value in this case is not limited, but may be about 0.1 nH to about 10 nH. In addition, on the acoustic wave substrate, the grounded ends of the IDTs of the first filter element connected to the unbalanced terminal and the grounded ends of the IDTs of the second filter element connected to the unbalanced terminal are mutually connected by the second connecting line so that the unbalanced-side grounded portion is provided. That is, the ground on the unbalanced side is shared on the acoustic wave substrate. Accordingly, flow of ground current can be appropriately controlled, whereby out-of-band attenuation is expanded.

In the balanced acoustic wave filter according to preferred embodiments of the present invention, the first filter element that includes the unbalanced terminal and the first and second balanced terminals and that has a balance-unbalance converting function, and the second filter element including the third and fourth balanced terminals connected to the first and second balanced terminals of the first filter element and the fifth and sixth balanced terminals are provided on the acoustic wave substrate. The first connecting line connects the grounded ends of the IDTs connected to the first and second balanced terminals to the grounded ends of the IDTs connected to the third and fourth balanced terminals of the second filter element, so as to define the common midpoint grounded portion. The second connecting line is connected to the grounded ends of the IDTs connected to the unbalanced terminal of the first filter element so that the unbalanced-side grounded portion is provided. The common midpoint grounded portion and the unbalanced-side grounded portion are electrically separated from each other. In that case, the acoustic wave filter is miniaturized and out-of-band attenuation is effectively expanded, so that a favorable filter characteristic is obtained.

In preferred embodiments of the present invention, the third connecting line connecting at least one of the IDTs connected to the unbalanced terminal to the unbalanced terminal crosses the first connecting line on the acoustic wave substrate. At the intersections therebetween, interlayer insulating films are disposed between the first and third connecting lines. In that case, the interlayer insulating films reliably prevent short-circuits between the first and third connecting lines. Also, since the first and third connecting lines cross each other, the length of the first and third connecting lines is shortened, and connecting lines and electrodes are provided at high density on the acoustic wave substrate. Accordingly, the acoustic wave filter is further miniaturized.

In preferred embodiments of the present invention, the package includes the first electrode land connected to the common midpoint grounded portion by a bump and the second electrode land connected to the unbalanced-side grounded portion by a bump. The first and second electrode lands are separated from each other on the package. In that case, a configuration in which the balanced acoustic wave filter element is connected to the package by bumps is obtained. In this balanced acoustic wave filter, the common midpoint grounded portion is provided on the acoustic wave substrate and out-of-band attenuation is expanded. Therefore, a favorable filter characteristic is achieved in a miniaturized configuration in which the acoustic wave filter is mounted on the package substrate by bump bonding in a face-down method. When surface acoustic waves are used as acoustic waves, a miniaturized balanced surface acoustic wave filter having expanded out-of-band attenuation is provided according to preferred embodiments of the present invention.

In the acoustic wave filter device, the first and second balanced acoustic wave filters as the balanced acoustic wave filters according to preferred embodiments of the present are provided on the acoustic wave substrate, the center frequency of the first balanced acoustic wave filter is different from the center frequency of the second balanced acoustic wave filter, the common midpoint grounded portions of both of the filters are mutually connected, and the unbalanced-side grounded portions of the both filters are mutually connected. In this case, a plurality of acoustic wave filters having a balance-unbalance converting function can be configured into a chip component using an acoustic wave substrate according to preferred embodiments of the present invention. Accordingly, a miniaturized duplexer or a multi-band filter having expanded out-of-band attenuation is provided.

In the acoustic wave filter device, the first and second balanced acoustic wave filters as the balanced acoustic wave filters according to preferred embodiments the present are provided on the acoustic wave substrate, the center frequency of the first balanced acoustic wave filter is different from the center frequency of the second balanced acoustic wave filter, the common midpoint grounded portions of both of the filters are mutually connected, and the unbalanced-side grounded portions of the both filters are mutually connected. Furthermore, the acoustic wave filter device includes the package including the first to third electrode lands, the common midpoint grounded portion is connected to the first electrode land by a bump, the unbalanced-side grounded portion of the first balanced acoustic wave filter is connected to the second electrode land by a bump, the unbalanced-side grounded portion of the second balanced acoustic wave filter is connected to the third electrode land by a bump, and the first to third electrode lands are separated from each other. In this case, a plurality of acoustic wave filters having a balance-unbalance converting function can be configured into a chip component including an acoustic wave substrate. Furthermore, a miniaturized acoustic wave filter device in which the acoustic wave filter chip is connected to the package substrate by bumps is provided. In this acoustic wave filter device, the first to third electrode lands are separated from each other on the package, so that isolation between the first and second balanced acoustic wave filters is improved.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, the present invention is clarified by describing specific preferred embodiments of the present invention.

FIG. 1is a schematic plan view illustrating a circuit configuration of a surface acoustic wave (SAW) filter according to a first preferred embodiment of the present invention.FIG. 2is a schematic plan view showing a specific electrode configuration thereof.

As shown inFIG. 1, a SAW filter1includes a SAW substrate2defining an acoustic wave substrate. The SAW substrate2is preferably made of piezoelectric single crystal or piezoelectric ceramic and is preferably rectangular-shaped in this preferred embodiment. Various electrodes and connecting lines that are described below are provided on one principal surface2aof the SAW substrate2, so that the circuit configuration shown inFIG. 1is achieved.

As shown inFIG. 1, the SAW filter1includes an unbalanced terminal3and first and second balanced terminals4and5, thereby having a balance-unbalance converting function.

A single-port SAW resonator6is connected to the unbalanced terminal3. First and second filter elements7and8are connected in the subsequent stage of the one-port SAW resonator6. The first and second filter elements7and8are 3-IDT longitudinally-coupled SAW filter elements including three IDTs7a,7b, and7cor8a,8b, and8c.

One end of the outer IDTs7aand7cof the first filter element7is connected to the unbalanced terminal3via the SAW resonator6. One end of the center IDT7bis connected to the first balanced terminal4via a SAW resonator9. Likewise, one end of the outer IDTs8aand8cof the second filter element8is connected to the unbalanced terminal3via the SAW resonator6. One end of the center IDT8bis connected to the second balanced terminal5via a SAW resonator10.

The grounded end opposite to the ends connected to the first and second balanced terminals4and5of the IDTs7band8bare mutually connected by a first connecting line11, so that a common midpoint grounded portion is provided.

On the other hand, the ends opposite to the ends connected to the unbalanced terminal of the IDTs7a,7c,8a, and8care mutually connected by a second connecting line12, so that an unbalanced-side grounded portion is provided.

One of the unique characteristics of the SAW filter1according to this preferred embodiment is that the common midpoint grounded portion and the unbalanced-side grounded portion are provided as described above, and that the common midpoint grounded portion and the unbalanced-side grounded portion are separated from each other on the SAW substrate2. This is specifically described below with reference toFIG. 2.

InFIG. 2, elements that are the same as those inFIG. 1are denoted by the same reference numerals, and the description that has been made with reference toFIG. 1is appropriately omitted.

As shown inFIG. 2, an electrode pad corresponding to the unbalanced terminal3is disposed on the principal surface2aof the SAW substrate2, and a bump13is connected to the electrode pad.

An electrode pad14is disposed at the end of the first connecting line provided on the SAW substrate2. A bump15is connected to the electrode pad14. The electrode pad14is connected to an electrode land connecting to a ground potential on a package side (described below) via the bump15.

Although not shown inFIG. 1, each of the SAW resonator6, the filter elements7and8, and the SAW resonators9and10includes a pair of reflectors disposed on the outer sides in the propagation direction of surface waves, as shown inFIG. 2. For example, in the SAW resonator6, reflectors6band6care provided on both sides of an IDT6a. Note that the reflectors are not shown inFIG. 1for simple illustration.

Likewise, illustration of reflectors is omitted as necessary in circuit diagrams of other preferred embodiments described below.

Referring toFIG. 2, a third connecting line16is provided to connect the IDT6aof the SAW resonator6to the IDTs7a,7c,8a, and8cof the filter elements7and8.

In the filter elements7and8, the first connecting line11is connected to the center IDTs7band8b, and the third connecting line16is connected to the IDTs7a,7c,8a, and8con the both sides. The first connecting line11is connected to the electrode pad14, which is disposed on the side of one edge2bof the SAW substrate2with respect to the filter elements7and8. The third connecting line16is connected to the unbalanced terminal3disposed on the side of the edge2bvia the SAW resonator6. Therefore, the first connecting line11and the third connecting line16inevitably cross each other.

In this preferred embodiment, interlayer insulating films17ato17care provided at intersections between the first connecting line11and the third connecting line16in order to prevent a short-circuit between the two lines. That is, the interlayer insulating films17ato17care provided at the intersections so as to cover the third connecting line16. The first connecting line11passes over the interlayer insulating films17ato17b, so that electrical insulation between the first connecting line11and the third connecting line16is provided.

An appropriate insulating material, such as SiO2, can be used for the interlayer insulating films17ato17c, and the forming method is not limited.

In this preferred embodiment, the first connecting line11includes a first straight portion11aextending from the center IDT7bof the first filter element7onto the electrode land14, a second straight portion11bextending straight from the center IDT8bof the second filter element8, and a third straight portion11cthat bends at a right angle toward the first filter element7at the end opposite to the IDT8bof the straight portion11band that extends straight. The end of the third straight portion11cconnects to the first straight portion11a. Thus, the first connecting line11includes the above-described plurality of straight portions11ato11c, so that the IDTs7band8bconnect to the electrode pad14with relatively short distances therebetween.

On the other hand, the third connecting line16includes straight portions16a,16b,16c, and16dexternally extending from the ends of the IDTs7a,7c,8a, and8cin the direction that electrode fingers of the IDTs extend, and also include a straight portion16econnecting external ends of those straight portions16to16d. The straight portion16eis connected to the unbalanced terminal3via the SAW resonator6. Thus, the third connecting line16also includes the above-described straight portions16ato16e, which enable the IDTs7a,7c,8a, and8cto connect to the SAW resonator6and the unbalanced terminal3with relatively short distances therebetween.

The ends opposite to the above-described common midpoint grounded portion of the IDTs7band8bare connected to the SAW resonators9and10via connecting lines19and20, respectively, and thus are connected to the first and second balanced terminals4and5, respectively. On the other hand, the ends opposite to the ends connected to the unbalanced terminal3of the IDTs7a,7c,8a, and8care connected to the second connecting line12. The second connecting line12allows the grounded ends of the IDTs7a,7c,8a, and8cto be mutually connected and connects to an electrode pad22. A bump23is attached to the electrode pad22. The bump23is connected to an electrode land connecting to the ground potential of the package. Thus, the second connecting line12defines an unbalanced-side grounded portion. The unbalanced-side grounded portion is separated from the common midpoint grounded portion defined by the first connecting line11on the SAW substrate2.

Interlayer insulating films18aand18b, formed in the same manner as the interlayer insulating films17ato17c, are disposed at positions where the second connecting line12crosses the connecting lines19and20.

The first and second balanced terminals4and5are defined as electrode pads having a predetermined area. Metallic bumps24and25are attached to the respective electrode pads.

When the SAW filter1is to be produced, the SAW filter1is attached to a package31shown in a plan view inFIG. 3such that the principal surface2aof the SAW substrate2is a lower surface. That is, as shown in the simplified front view in FIG.4, the SAW filter1is attached to the package31by bump bonding in a face-down method, so that the SAW filter1is made into a product.

As shown inFIG. 3, the package31is formed by using a plate substrate, and a plurality of electrode lands32to36are placed on an upper surface31athereof. The electrode lands32to36can be formed by forming a film made of an appropriate conductive material on an entire surface and then performing patterning, or by applying a conductive material in a printing method, or other suitable method.

The metallic bump13(seeFIG. 2) attached to the input terminal3is attached to the electrode land32. On the other hand, the bump15provided in the common midpoint grounded portion is attached to the electrode land33connecting to the ground potential.

On the other hand, the metallic bumps23,24, and25shown inFIG. 2are attached to the electrode lands34,36, and35, respectively.

As described above, in the SAW filter1according to this preferred embodiment, the IDTs7band8bare mutually connected by the first connecting line11so that the common midpoint grounded portion is provided, and the common midpoint grounded portion is separated from the unbalanced-side grounded portion provided by the second connecting line12on the SAW substrate2. With this configuration, out-of-band attenuation is effectively improved as compared to that of a conventional equivalent SAW filter. This is clarified below by a comparison between the above-described preferred embodiment and a comparative example shown inFIGS. 5 and 6.

FIG. 5is a schematic plan view showing a circuit configuration of a SAW filter corresponding to a known SAW filter having a balance-unbalance converting function as described in Patent Document 1. In this SAW filter, neither a midpoint grounded portion nor an unbalanced-side grounded portion has a common connection.FIG. 6is a plan view showing a package on which this SAW filter is mounted by the face-down method. InFIG. 5, elements corresponding to those shown inFIG. 1are denoted by400+the reference numerals shown inFIG. 1.

In a SAW401, grounded ends of an IDT407bof a first filter element407and an IDT408bof a second filter element408are independently grounded. That is, a common midpoint grounded portion is not provided in a SAW substrate402.

Also, grounded ends of IDTs407aand407care separately grounded. Likewise, grounded ends of IDTs408aand408care separately grounded. Note that grounded ends of the IDTs407cand408aare mutually connected.

Other than that, the SAW filter401has the same configuration as that of the SAW1.

In a package431shown inFIG. 6, an unbalanced terminal403shown inFIG. 5is attached to an electrode land432provided on an upper surface431a. Bumps attached to the grounded ends of the IDTs407band408bare attached to an L-shaped electrode land433. In other words, a common midpoint grounded portion is provided on the electrode land433. That is, the common midpoint grounded portion is provided by using the electrode land433.

Likewise, a bump attached to the IDT407a, a bump connected in common to the grounded ends of the IDTs407cand408a, and a metallic bump connected to the grounded end of the IDT408care attached to an electrode land434. That is, the plurality of bumps connected to the grounded ends of the IDTs407a,407c,408a, and408con the unbalanced side are mutually connected on the electrode land434on the package431.

On the other hand, balanced terminals405and404are attached to electrode lands435and436, respectively.

Frequency characteristics were measured in a state in which the SAW filter1according to the above-described preferred embodiment and the SAW filter401according to the comparative example are mounted on the packages31and431, respectively.FIGS. 7 to 10show measurement results.FIG. 7shows characteristics of phase balance to frequency;FIG. 8shows characteristics of amplitude balance to frequency;FIG. 9shows characteristics of insertion loss to frequency; andFIG. 10shows characteristics of common-mode attenuation to frequency. In each figure, the solid line indicates a result in the preferred embodiment and the broken line indicates a result in the comparative example.

The above-mentioned amplitude balance and phase balance indicate the level of balance of an amplitude characteristic between the pair of balanced terminals4and5and the level in which the phase is inverted by 180 degrees. The amplitude balance and phase balance are defined as follows.

That is, the amplitude balance and the phase balance are defined as follows: amplitude balance=[A], A=|20 log(S21)|−|20 log(S31)|; and phase balance=B-180, B=|∠S21−∠S3|, assuming that the SAW filter having a balance-unbalance converting function is a 3-port device, in which an unbalanced input terminal is a first port and balanced output terminals are second and third ports. S21indicates a transfer coefficient from the first port to the second port, S31indicates a transfer coefficient from the first port to the third port, and || in the above expressions indicates an absolute value.

Between the pair of balanced signal terminals, an ideal amplitude balance is 0 dB and an ideal phase balance is 0 degree in a pass band.

As shown inFIGS. 7 to 10, a pass band is about 2.10 to about 2.20 (GHz) in the SAW filter1according to the present preferred embodiment. Within this pass band, the SAW filter1is superior to the SAW filter according to the comparative example in the phase balance and the amplitude balance.

Also, as shown inFIG. 9, out-of-band attenuation is significantly improved in the present preferred embodiment, especially at a higher-frequency side, as compared to the SAW filter401according to the comparative example.

Furthermore, as shown inFIG. 10, common mode attenuation is significantly improved in the preferred embodiment, especially out of band at a higher-frequency side, as compared to the SAW filter according to the comparative example.

As described above, in the SAW filter1according to the present preferred embodiment, out-of-band attenuation is improved and balance such as phase balance is improved as compared to the SAW filter401according to the comparative example. The possible reasons are as follows.

That is, since the common midpoint grounded portion on the acoustic wave substrate achieves a common connection, an electrical midpoint exists on the acoustic wave substrate. Thus, the distance between the common midpoint grounded portion and the electrical midpoint is small, the impedance between the both sides is low, the balance is improved, and out-of-band attenuation of the filter is expanded. That is, the common midpoint grounded portion and the unbalanced-side grounded portion separated from each other are connected by impedance or inductance, so that the attenuation in a high-frequency side of the pass band is improved. The inductance value in this case is not limited, but may be about 0.1 nH to about 10 nH. In addition, on the acoustic wave substrate, the grounded ends of the IDTs of the first filter element connected to the unbalanced terminal and the grounded ends of the IDTs of the second filter element connected to the unbalanced terminal are mutually connected by the second connecting line, so that the unbalanced-side grounded portion is defined. That is, the ground on the unbalanced side is shared on the acoustic wave substrate. Accordingly, flow of ground current is appropriately controlled and the out-of-band attenuation is expanded accordingly.

In addition, in the SAW filter1according to the above-described preferred embodiment, each of the first and third connecting lines is formed by combining straight lines, and connects the electrode land14or the unbalanced terminal3to the IDTs with relatively short distances therebetween. Furthermore, at intersections between the first connecting line11and the third connecting line16, electrical insulation is achieved by the interlayer insulating films17ato17c. Also, the second connecting line12includes a plurality of straight portions so as to connect the grounded ends of the plurality of IDTs7a,7c,8a, and8cwith a short distance therebetween. In the second connecting line12, interlayer insulating films17aand18aare provided at the intersections with the connecting lines19and20so as to achieve electrical insulation.

As described above, the connecting lines11,12, and16including a plurality of straight portions and the interlayer insulating films17ato17cand18ato18bare provided, and the plurality of connecting lines cross each other such that they are electrically insulated from each other. With this configuration, a density of electrodes and connecting lines on the principal surface2aof the SAW substrate2is effectively increased in the SAW filter1. That is, the SAW filter1can be further miniaturized as compared to the SAW filter401according to the comparative example.

In addition to the above-described miniaturization, the common midpoint grounded portion and the unbalanced-side grounded portion are provided on the SAW substrate2, not on the package31, and are separated from each other. Thus, the out-of-band attenuation is expanded and the balance is improved as shown inFIGS. 7 to 10.

According to this preferred embodiment, a miniaturized balanced SAW filter device having an excellent frequency characteristic is easily provided.

FIG. 11is a plan view showing a circuit configuration of a SAW filter according to a second preferred embodiment of the present invention, andFIG. 12is a schematic plan view showing an electrode configuration thereof.

In a SAW filter51according to the second preferred embodiment, various electrodes and connecting lines are disposed on a principal surface52aof a surface wave substrate52, so that the SAW filter is provided. More specifically, first and second filter elements54and55are connected to an unbalanced terminal53. Also, filter elements56and57are disposed in the subsequent stages of the filter elements54and55.

The filter elements54to57are 3-IDT longitudinally-coupled SAW filters including three IDTs54ato54c,55ato55c,56ato56c, or57ato57c. Although not shown inFIG. 11, in each of the SAW filter elements54to57, a pair of reflectors is provided on both sides of the three IDTs in the propagation direction of surface waves. The reflectors are schematically shown inFIG. 12. The third filter element56and the fourth filter element57are connected to first and second balanced terminals58and59.

More specifically, one end of the center IDT54bof the first filter element54and one end of the center IDT55bof the second filter element55are mutually connected as an unbalanced terminal and are connected to the unbalanced terminal53.

Grounded ends of the IDTs54band55bare mutually connected by a second connecting line60, so that an unbalanced-side grounded portion is provided.

On the other hand, ends of the IDTs54a,54c,55a, and55care connected to the IDTs56a,56c,57a, and57c, respectively. The other ends of the IDTs54a,54c,55a, and55care grounded ends and are mutually connected by a first connecting line61, so that a common midpoint grounded portion is provided. The first connecting line61is also connected to grounded ends of the IDTs56a,56c,57a, and57c.

On the other hand, the grounded ends of the IDTs56band57bconnect to the first connecting line61.

In this preferred embodiment, the common midpoint grounded portion and the unbalanced-side grounded portion are disposed on the principal surface52aof the SAW substrate52and are electrically separated from each other.

In this preferred embodiment, as in the SAW filter1according to the first preferred embodiment, miniaturization and expansion of out-of-band attenuation are achieved.

InFIG. 12, elements common to those inFIG. 11are denoted by the same reference numerals. As is clear fromFIG. 12, in this preferred embodiment, the first connecting line61and a third connecting line62cross each other at a plurality of positions, and interlayer insulating films63aand63bare provided at the intersections. That is, the interlayer insulating films63aand63bprovide electrical insulation between the first and third connecting lines61and62.

Likewise, as shown inFIG. 12, interlayer insulating films64to67are provided at intersections between other connecting lines. As a package for mounting the SAW filter51according to this preferred embodiment, a package similar to the package31shown inFIG. 3may be used.

In the present invention, as is clear from the SAW filter51according to the second preferred embodiment, a midpoint-grounded balanced SAW filter having a two-stage and four-element configuration may be used, unlike in the first preferred embodiment.

FIG. 13is a schematic plan view showing a circuit configuration of a SAW filter according to a third preferred embodiment of the present invention, andFIG. 14is a schematic plan view showing an electrode configuration thereof.

In a third SAW filter101, various electrodes and connecting lines are disposed on a principal surface2aof a surface wave substrate2. That is, a first filter element106and a second filter element107are connected between an unbalanced terminal103and first and second balanced terminals104and105. Herein, the first filter element106is connected in a former stage, whereas the second filter element107is connected in a latter stage. Thus, a midpoint-grounded balanced SAW filter having a two-stage configuration, each stage including one element, is provided.

The first filter element106is a 3-IDT longitudinally-coupled resonator SAW filter including three IDTs106ato106c. As shown inFIG. 14, reflectors106dand106eare disposed on both sides of an area provided with the IDTs106ato106cin the propagation direction of surface waves. The IDTs106aand106chave inverted phases.

The second filter element107includes three IDTs107ato107c. Although not shown inFIG. 13, reflectors107dand107eare disposed on both sides of an area provided with the IDTs107ato107cin the propagation direction of surface waves, as shown inFIG. 14.

The unbalanced terminal103is connected to one end of the IDT106bof the first filter element106. The other end of the IDT106bis a grounded end and is connected to a second connecting line108. The second connecting line108is connected to an electrode pad109shown inFIG. 14and defines an unbalanced-side grounded portion. A bump110is attached to the electrode pad109.

On the other hand, ends of the IDTs106aand106care connected to the IDTs107aand107cof the filter element107in the subsequent stage. The other ends of the IDTs106aand106care midpoint grounded ends and are mutually connected by a first connecting line111. Thus, the first connecting line111defines a common midpoint grounded portion. The first connecting line111also provides a common connection of the grounded ends of the IDTs107aand107c. The first connecting line111is connected to an electrode land112. A metallic bump113is attached onto the electrode land112.

The IDT107bof the second filter element107includes IDT portions107b1and107b2arranged in parallel. That is, the IDT portions107b1and107b2and a comb electrode that is engaged with the IDT portions171b1and171b2define the IDT107b. The balanced terminals104and105are electrically connected to the IDT portions107b1and107b2, respectively.

As shown inFIG. 14, a metallic bump114is attached to an electrode land corresponding to the unbalanced terminal103. Likewise, bumps115and116are attached to electrode pads corresponding to the first and second balanced terminals104and105, respectively.

In the midpoint-grounded balanced SAW filter having a 1 element×2 stage configuration as in this preferred embodiment, a common midpoint grounded portion is defined by the first connecting line111and the common midpoint grounded portion is separated from the unbalanced-side grounded portion on the principal surface2aof the SAW substrate2according to the present invention. With this configuration, out-of-band attenuation is expanded and balance is improved.

That is, the SAW filter101can be mounted on a package in the face-down method using the above-described bumps110,113, and114to116. In this preferred embodiment, the midpoint grounded portion functions as a common midpoint grounded portion by using the first connecting line111on the SAW substrate2of the SAW filter101, and is separated from the unbalanced-side grounded portion. Thus, midpoint grounded portions need not be mutually connected on the package side. Accordingly, out-of-band attenuation is expanded and balance is improved as in the first preferred embodiment.

Furthermore, the first connecting line111and the second connecting line108are disposed on the principal surface2aof the surface wave substrate2, and these lines are arranged at high density so that they cross each other. Accordingly, the density of electrodes and the arrangement density of connecting lines are increased in the SAW filter101, which enables miniaturization.

FIG. 15is a schematic plan view showing a circuit configuration according to a fourth preferred embodiment, andFIG. 16is a schematic plan view showing an electrode pattern thereof.

In a SAW filter device151according to the fourth preferred embodiment, two SAW filters, each being the SAW filter1according to the first preferred embodiment, are disposed on a surface wave substrate2.

An electrode configuration that is the same as that of the SAW filter1according to the first preferred embodiment is disposed on a principal surface2aof the SAW substrate2. That is, the electrode configuration that is the same as that of the first preferred embodiment is disposed between an unbalanced terminal3and first and second balanced terminals4and5. In addition to the unbalanced terminal3, an unbalanced terminal3A is provided. A pair of first and second balanced signal terminals4A and5A is provided for the unbalanced terminal3A. The electrode configuration that is the same as that of the SAW filter1according to the first preferred embodiment is provided between the unbalanced terminal3A and the balanced terminals4A and5A.

That is, the SAW filter device151corresponds to a configuration including two SAW filters1and1A according to the first preferred embodiment.

InFIG. 16, the same elements of the electrodes and connecting lines in the SAW filter1A as those in the SAW filter1are denoted by corresponding reference numerals attached with A, and the detailed description thereof is omitted.

Note that the center frequency of the SAW filter1A is different from that of the SAW filter1.

According to this preferred embodiment, the SAW filter device151, in which the two SAW filters1and1A having different frequencies and having a 1 element×1 stage configuration are integrated, is obtained.

As shown inFIGS. 15 and 16, the common midpoint grounded portions of the SAW filters1and1A are mutually connected by coupling the first connecting lines11and11A. Likewise, the unbalanced-side grounded portions are coupled.

In the present invention, a plurality of SAW filter elements according to the present invention can be disposed on a single SAW substrate, as in this preferred embodiment. Since each SAW filter element is configured in accordance with the present invention, out-of-band attenuation is expanded and balance is improved, and further miniaturization achieved. Furthermore, by providing a plurality of SAW filter elements on a single SAW substrate, a more miniaturized high-density filter device is obtained.

As described above, by using the plurality of SAW filters1and1A having different frequencies, the SAW filter device151can be used as a duplexer or a multi-band filter.

FIG. 17is a schematic plan view showing a circuit configuration of a SAW filter according to a fifth preferred embodiment of the present invention, andFIG. 18is a schematic plan view showing an electrode configuration thereof.

In a SAW filter device161, two SAW filter elements configured according to the present invention are disposed on one SAW substrate2, as in the SAW filter device151. In the fifth preferred embodiment, one of the SAW filter elements is the SAW filter101having a 1 element×2 stage configuration according to the above-described third preferred embodiment, and other SAW filter element is the SAW filter1according to the first preferred embodiment.

The center frequency of the SAW filter101is different from the center frequency of the SAW filter1.

As in the fourth preferred embodiment, the common midpoint grounded portion of the first SAW filter101and the common midpoint grounded portion of the second SAW filter1are mutually connected by connecting the first connecting line. Also, the unbalanced-side grounded portions of the SAW filters1and101are mutually connected by common connection of connecting lines.

InFIG. 18, the respective elements are denoted by the reference numerals used in the description about the electrode configuration of the first and third preferred embodiments. The description about the first and third preferred embodiments is used for detailed description about the respective elements.

As in the fifth preferred embodiment, two SAW filter elements may be configured as a single chip by combining the SAW filter elements according to the first and third preferred embodiments. In this case, out-of-band attenuation is expanded and balance is improved in each of the SAW filters1and101. In addition, further miniaturization is achieved because the two SAW filters101and1are integrated.

FIG. 19is a schematic plan view showing an electrode configuration of a SAW filter chip in an acoustic wave device according to a sixth preferred embodiment of the present invention. In the acoustic wave device according to the sixth preferred embodiment, a SAW filter chip181shown in the plan view inFIG. 19is attached to a package substrate as a package (described below) using bumps.

In the SAW filter chip181used in this preferred embodiment, two SAW filters, each being the SAW filter1according to the first preferred embodiment, are disposed on a surface wave substrate2, as in the SAW filter device151according to the fourth preferred embodiment.

That is, the SAW filter chip181has a configuration including two SAW filters1and1A according to the first preferred embodiment provided on a surface wave substrate.

InFIG. 19, the same elements of the electrodes and connecting lines in the SAW filter1A as those in the SAW filter1are denoted by corresponding reference numerals attached with A, and the detailed description thereof is omitted.

As in the SAW filter device151according to the fourth preferred embodiment, the center frequency of the SAW filter1A is different from the center frequency of the SAW filter1.

Therefore, according to this preferred embodiment, the SAW filter chip181, in which the first and second SAW filters1and1A having a 1 element×1 stage configuration and having different frequencies are integrated, is used. With this SAW filter chip181, a duplexer or a multi-band filter can be provided.

For example, a DCS receiving filter can be provided by the first SAW filter and a PCS receiving filter can be provided by the second SAW filter.

As shown inFIG. 19, the common midpoint grounded portions of the SAW filters1and1A are mutually connected by common connection of the first connecting lines11and11A.

In this preferred embodiment, however, the unbalanced-side grounded portions12and12A are not mutually connected unlike in the fourth preferred embodiment.

In this preferred embodiment, the above-described SAW filter chip181is mounted on a package substrate182as a package shown inFIG. 20. The package substrate182shown in the plan view inFIG. 20is provided with a plurality of electrode lands301to309to which various electric connecting portions of the SAW filter chip181are attached by bumps. Herein, the electrode lands308and309correspond to second and third electrode lands to which the unbalanced-side grounded portions12and12A of the SAW filter chip181are attached by bumps, and the electrode land307corresponds to an electrode land to which the common midpoint grounded portion is attached by a bump. In this preferred embodiment, the first to third electrode lands307to309are separated from each other.

As a reference example, the above-described SAW filter chip was mounted on a package substrate shown inFIG. 22by bumps so as to make an acoustic wave device according to the reference example. A package substrate183shown inFIG. 22has the same configuration as that of the package substrate182except that the second and third electrode lands are integrated into an electrode land308b. Thus, in the acoustic wave device according to the reference example, the unbalanced-side grounded portions12and12A are not electrically separated on the package substrate.

Characteristics of isolation to frequency of the acoustic wave device according to the sixth preferred embodiment and the acoustic wave device according to the above-described reference example were measured. That is, a level (isolation) of a signal input from a second SAW filter as a PCS receiving filter output to a first SAW filter as a DCS receiving filter (a pass band of 1805 to 1880 MHz) was measured. The result is shown inFIG. 21. InFIG. 21, the solid line indicates the result of the sixth preferred embodiment and the broken line indicates the result of the reference example.

The isolation characteristic is improved as the attenuation is increased.

As shown inFIG. 21, the isolation characteristic is enhanced by separating the first to third electrode lands by using the package substrate182.

As described above, in the present invention, a plurality of SAW filter elements configured in accordance with preferred embodiments of the present invention may be provided on a single SAW substrate, unbalanced-side grounded portions of the plurality of SAW filters may be separated from each other on the single SAW substrate, and electrode lands connected to the unbalanced-side grounded portions of the plurality of SAW filters may be separated from each other. With this configuration, since each SAW filter element is configured in accordance with the present invention, out-of-band attenuation is expanded and balance is improved, and also miniaturization is achieved. Furthermore, isolation among the plurality of SAW filter elements provided on the single SAW substrate is improved.

In the SAW filter chip181used in the sixth preferred embodiment, the common midpoint grounded portions11and11A of the SAW filters1and1A may be separated from each other, and the unbalanced-side grounded portions12and12A may be separated from each other.

It has been verified that the isolation characteristic obtained when a SAW filter chip having such a configuration is mounted on the package substrate181is substantially the same as the isolation characteristic obtained in the sixth preferred embodiment.

In the first to fifth preferred embodiments, SAW filter elements and SAW filter devices using surface acoustic waves have been described. However, the present invention may be applied to a boundary acoustic wave filter or a boundary acoustic wave filter device using boundary acoustic waves.

FIG. 23is a front cross-sectional view schematically showing an example of such a boundary acoustic wave filter. In the boundary acoustic wave filter171, IDT electrodes174ato174cdefining a filter are disposed at the boundary between a first medium layer172and a second medium layer173. Through-hole electrodes175ato175cenable the IDT electrodes174ato174cto be connected to electrode pads176ato176cdisposed on a lower surface of the medium layer172.

Bumps177ato177care attached to the electrode pads176ato176c.

In this boundary acoustic wave filer171, an electrode configuration similar to that in the above-described first to fifth preferred embodiments is obtained by using the IDT electrodes174ato174c, so that a boundary wave filter having a balance-unbalance converting function is provided. As in the first to fifth preferred embodiments, by providing a common midpoint grounded portion and an unbalanced-side grounded portion on the boundary acoustic wave filter171, out-of-band attenuation is expanded, balance is improved, and miniaturization is achieved.

In the boundary acoustic wave filter171, vibration is confined between the first and second medium layers172and173, and the lower surface of the second medium layer172is a mounted surface. Therefore, the boundary acoustic wave filter171is directly mounted on a circuit board, not on a package, by using the bumps177ato177c. That is, since a package having a cavity is not required, the boundary acoustic wave filter171can be mounted on a circuit board of an apparatus to be directly incorporated by using the bumps177ato177c. In this case, a ground electrode need not be shared on the circuit board, so that the incorporated apparatus is further miniaturized.