Surface acoustic wave resonator and surface acoustic wave filter using the same

A Q-factor of a resonator at a high frequency is improved. An insertion loss of a filter using such a resonator and steepness of the filter are improved. A plurality of surface acoustic wave resonators including an interdigital transducer and reflecting electrodes provided on both sides thereof are connected in parallel on a piezoelectric substrate. Resonance frequencies of the surface acoustic wave resonators are rendered equal among all the resonators connected in parallel. In this way, the Q-factor of the resonance can be improved. A surface acoustic wave filter using such surface acoustic wave resonators is formed in order to improve the insertion loss and the steepness.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a surface acoustic wave resonator and a surface acoustic wave filter using the same which are used in particular for cellular phones or the like.

2. Background Art

Conventionally, such surface acoustic wave filters have had a structure as shown inFIG. 13.

In a surface acoustic wave filter shown inFIG. 13, a series resonator2and a parallel resonator3which are surface acoustic wave resonators are formed on a piezoelectric substrate1and connected to each other to provide a filter characteristic. As the resonators, multiple pairs of interdigital transducers or an interdigital transducer with reflecting electrodes being provided on both sides have been employed.

As a document on the conventional art related to the present invention, Japanese Laid-Open Publication No. 2001-119260 is known, for example.

The structure described above has a problem that sufficient Q-factors of the resonators cannot be secured when the frequency increases. Furthermore, when a filter is formed, there is a limit on improving insertion loss and steepness.

BRIEF SUMMARY OF THE INVENTION

The present invention is to solve the above-described conventional problems, and objects thereof are to improve Q-factors of surface acoustic wave resonators and provide surface acoustic wave filters with low insertion loss and high steepness.

In order to achieve the above-described objects, in the present invention a plurality of surface acoustic wave resonators including an interdigital transducer (hereinafter, referred to as “IDT”) and reflecting electrodes provided on both sides of the IDT are connected in parallel, and resonance frequencies of the surface acoustic wave resonators connected in parallel are made equal among all the surface acoustic wave resonators connected in parallel.

According to the present invention, Q-factors of the surface acoustic wave resonators can be improved. The insertion loss of the surface acoustic wave filter can be reduced, and the steepness of the filter characteristic can be improved.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, Embodiment 1 of the present invention will be described.FIG. 1is a diagram showing an exemplary structure of a surface acoustic wave resonator10according to Embodiment 1 of the present invention. The surface acoustic wave resonator10shown inFIG. 1is a so-called one-port resonator. The surface acoustic wave resonator10includes a signal input terminal T1for receiving a signal input from the outside, a signal output terminal T2for outputting a signal to the outside, and a piezoelectric substrate11formed of 39° Y-cut, X-propagating lithium tantalate. On a surface of the piezoelectric substrate11, three surface acoustic wave resonators14(surface acoustic wave resonators) are formed in line on the same surface acoustic wave propagation path.

Each of the surface acoustic wave resonators14includes an IDT12and two reflectors13(reflecting electrodes) being adjacent to both ends of the IDT12. Three surface acoustic wave resonators14having the same structures, i.e., surface acoustic wave resonators14having substantially the same resonance frequencies are connected in parallel and are connected in series between the signal input terminal T1and the signal output terminal T2, i.e., a signal path. In this example, the number of the reflectors13provided between a plurality of IDTs12in the surface acoustic wave resonator10is two.

FIG. 2is an enlarged view of one of the surface acoustic wave resonators14shown inFIG. 1. In the surface acoustic wave resonator14, for example, a film thickness of electrodes of the IDT12and the reflectors13is about 0.4 μm, an overlap length W of the IDT12is about 40 μm, the number of electrode fingers15in the IDT12is 200, the number of electrodes in a reflector13is 50, pitch P1for the electrode fingers15in the IDT12is about 2.33 μm, and pitch P2for the electrodes in the reflectors13is about 2.38 μm. The surface acoustic wave resonator14shown inFIG. 2is simplified, and is shown as having 10 electrode fingers15in the IDT12and four electrodes in the reflector13.

The resonance frequency of the surface acoustic wave resonator14is determined based on mainly the film thickness of the electrodes and the pitch P1for the electrode fingers15. The three surface acoustic wave resonators14shownFIG. 1all have the same structure, such as the film thickness of the electrodes, the pitch P1for the electrode fingers15, and the like. Thus, the resonance frequencies of the surface acoustic wave resonators14connected in parallel are all the same.

FIG. 3shows a frequency property of the surface acoustic wave resonator10having the above-described structure and a frequency property of the series resonator2in the conventional surface acoustic wave filter shown inFIG. 13, which are obtained by simulation. The frequency property of the surface acoustic wave resonator10shown inFIG. 1is represented by a solid line as graph G1inFIG. 3. Graph G2shown by a broken line inFIG. 3represents the frequency property of the conventional surface acoustic wave resonator shown inFIG. 13which includes one IDT and reflectors provided on both sides of the IDT. This conventional surface acoustic wave resonator includes one IDT including 600 electrode fingers. This is the same number as the total of the electrode fingers15in the three IDTs12of the surface acoustic wave resonator14shown inFIG. 1. Other conditions are same as those of the surface acoustic wave resonator10.

By comparing the graph G1and the graph G2shown inFIG. 3, it can be seen that the graph G1is more highly raised from a resonance point to a non-resonance point. Now, the Q-factors of the resonances are compared. In the present invention as represented by the graph G1, the resonance frequency of the surface acoustic wave resonator10is about 840 MHz (the resonance frequency of each of the surface acoustic wave resonators14is also about 840 MHz), and the Q-factor of the resonance is about 870. Meanwhile, the Q-factor of the conventional surface acoustic resonator represented by the graph G2is about 830. It can be seen that the Q-factor of the resonance is improved in the surface acoustic wave resonator10of the present invention.

In this example, three surface acoustic wave resonators14having the same resonance frequency are connected in parallel to form the surface acoustic wave resonator10. Having the same resonance frequency includes the case where there is a difference in the resonance frequencies of a plurality of the surface acoustic wave resonators14connected in parallel due to error factors such as manufacturing variation and the like. For example, when a difference between the maximum resonance frequency and the minimum resonance frequency of the plurality of the surface acoustic wave resonators14connected in parallel is 0.03% or lower, the resonance frequencies are substantially the same.

FIG. 4is a graph representing the Q-factor of the resonance of the surface acoustic wave resonator10when there is a difference in the pitch P1for the electrode fingers15in the three surface acoustic wave resonators14of the surface acoustic wave resonator10shown inFIG. 1, which is obtained by simulation. In the graph shown inFIG. 4, a pitch difference shown by a horizontal axis is a difference between the maximum pitch and the minimum pitch among the three surface acoustic wave resonators14which is represented in percentage. For example, in the surface acoustic wave resonator10shown inFIG. 1, if pitch P1for the electrode fingers15in the surface acoustic wave resonator14at one end is narrower than pitch P1for the electrode fingers15in the central surface acoustic wave resonator14by 0.05% and pitch P1for the electrode fingers15in the surface acoustic wave resonator14at the other end is broader than the pitch P1for the electrode fingers15in the central surface acoustic wave resonator14by 0.05%, the pitch difference is represented as 0.1%.

In the surface acoustic wave resonators14used for the simulation ofFIG. 4, the film thickness of the electrodes of the IDTs12and the reflectors13is 0.4 μm, the overlap length W of the IDTs12is 40 μm, the number of the electrode fingers15in the IDTs12is 200, the number of electrodes of the reflectors13is 50, and pitch P2for the electrodes in the reflectors13is 2.38 μm. The pitch P1for the electrode fingers15in the IDT12provided in the central surface acoustic wave resonator14among the three surface acoustic wave resonators14is 2.33 μm.

As shown inFIG. 4, the Q-factor of the surface acoustic wave resonator10is 870 when the pitch difference is 0%. The Q-factor is 855 when the pitch difference is 0.02%. The Q-factor is 807 when the pitch difference is 0.04%. The Q-factor is 532 when the pitch difference is 0.1%. The Q-factor is 248 when the pitch difference is 0.2%. Thus, it is confirmed that the Q-factor of the resonance can be improved in the surface acoustic wave resonator10compared to that of the conventional surface acoustic wave resonator (series resonator2) shown inFIG. 13when the pitch difference among the plurality of surface acoustic wave resonators14is 0.03% or lower (corresponding to when the difference in the resonance frequencies is 0.03% or lower).

In this example, three surface acoustic wave resonators14having the same structure are connected in parallel to form the surface acoustic wave resonator10. The present invention is not limited to the example of connecting a plurality of surface acoustic wave resonators14having the same structure in parallel. The surface acoustic wave resonator10may be formed by connecting a plurality of surface acoustic wave resonators14having the same resonance frequency but different structures in parallel.

FIG. 5is a graph representing the Q-factor of a resonance when a plurality of the surface acoustic wave resonators14having the same resonance frequency but structures different from one another are connected in parallel, which is obtained by simulation.FIG. 5shows the Q-factor of resonance of the surface acoustic wave resonator10when the IDTs12in the three surface acoustic wave resonators14of the surface acoustic wave resonator10have different numbers of the electrode fingers15and the resonance frequencies of the surface acoustic wave resonators14are the same. In this example, the pitch P1is varied in order to compensate for a shift in the resonance frequency due to a difference in the numbers of the electrode fingers15.

First, the surface acoustic wave resonator10shown inFIG. 1is formed by connecting three surface acoustic wave resonators14which each have 200 electrode fingers15in parallel. This means that the combination of the numbers of electrode fingers15in the surface acoustic wave resonators14is 200, 200 and 200. InFIG. 5, the Q-factor 870 of such a resonator is shown on the vertical axis with the number of the electrode being 200.

The Q-factor 857 of the surface acoustic wave resonator10formed by connecting one surface acoustic wave resonator14having 300 electrode fingers15in the IDT12and the pitch P1of 2.330 μm and two surface acoustic wave resonators14having 150 electrode fingers15and the pitch P1of 2.329 μm in parallel is shown on the vertical axis with the number of the electrode being 300 inFIG. 5.

The Q-factor 838 of the surface acoustic wave resonator10formed by connecting one surface acoustic wave resonator14having 400 electrode fingers15in the IDT12and the pitch P1of 2.330 μm, one surface acoustic wave resonator14having 150 electrode fingers15and the pitch P1of 2.329 μm, and one surface acoustic wave resonator14having 50 electrode fingers15and the pitch P1of 2.321 μm in parallel is shown on the vertical axis with the number of the electrode being 400 inFIG. 5.

The Q-factor 833.5 of the surface acoustic wave resonator10formed by connecting one surface acoustic wave resonator14having 500 electrode fingers15in the IDT12and the pitch P1of 2.330 μm and two surface acoustic wave resonators14having 50 electrode fingers15and the pitch P1of 2.321 μm in parallel is shown on the vertical axis with the number of the electrode being 500 inFIG. 5.

The Q-factor 830 of the surface acoustic wave resonator of the conventional example which includes one IDT and 600 electrode fingers thereof, as in the series resonator2shown inFIG. 13, for example, is shown on the vertical axis with the number of the electrode fingers being 600 inFIG. 5. As shown inFIG. 5, the effect of improving the Q-factor is the highest when a plurality of like surface acoustic wave resonators14are connected in parallel (when the number of electrode fingers is 200). The Q-factor of resonance can be more improved compared to that of the surface acoustic wave resonator (the series resonator2) according to the conventional example shown inFIG. 13(when the number of electrode fingers is 600) even when a plurality of the surface acoustic wave resonators14having the same resonance frequency but different numbers of the electrode fingers15are connected in parallel to form the surface acoustic wave resonator10(when the number of electrode fingers is 300, 400 or 500).

It is known that in a one-port resonator as the surface acoustic wave resonator14, ripple tends to be generated at a frequency of a signal corresponding to the number of the electrode fingers15. When a plurality of surface acoustic wave resonators14respectively having the equal number of electrode fingers15as in the surface acoustic wave resonator10shown inFIG. 1are connected in parallel, the frequency at which ripple is generated in each of the surface acoustic wave resonators14is the same. Thus, these ripples overlapped and the peak value of the ripple may increase.

Therefore, by connecting a plurality of the surface acoustic wave resonators14having the same resonance frequency but the numbers of the electrode fingers15in the IDT12different from one another in parallel to form the surface acoustic wave resonator10, the frequency of the ripple generated at each of the surface acoustic wave resonators14can be varied and the ripples less overlapped. In this way, the peak value of the ripple can be reduced while the Q-factor of the resonance is improved.

In this example, three surface acoustic wave resonators14are connected in parallel to form the surface acoustic wave resonator10. However, the number of the surface acoustic wave resonators14connected in parallel is not limited to three, but it may be two, four or higher.

Conventionally, it is known that when the IDT and the reflectors are formed with the same electrode pitch, a peak frequency of a radiation conductance of the IDT is lower than a central frequency of a reflection property of the reflectors. Thus, usually, the pitch in the reflectors is made slightly larger than the pitch in the IDT to have the peak frequency of the radiation conductance of the IDT and the central frequency of the reflection property of the reflectors which approximately match one another to improve the Q-factor of resonance. However, when a piezoelectric material having a high reflectivity of the electrode fingers is used and the number of the electrode fingers of the IDT increases, the IDT itself functions as a reflector. This is substantially the same as providing a reflector having the same pitch as the IDT. The Q-factor of resonance is deteriorated.

Therefore, in the present invention, the IDT is divided and the number of the each of the IDTs is decreased to improve the Q-factor of the resonance. Also, the IDTs are connected in parallel and a desired property is achieved.

FIG. 6shows an exemplary structure of a surface acoustic wave resonator10awhich is another form of Embodiment 1 of the present invention. The surface acoustic wave resonator10ashown inFIG. 6includes a signal input terminal T1for receiving a signal input from the outside, a signal output terminal T2for outputting a signal to the outside, and a piezoelectric substrate11formed of 39° Y-cut, X-propagating lithium tantalate. On a surface of the piezoelectric substrate11, a plurality of, for example, three IDTs12are provided in a line on the same surface acoustic wave propagation path. Reflectors13aare respectively provided between the IDTs12. Two reflectors13are respectively provided near both ends of the line of the IDTs12. The plurality of the IDTs12are connected in parallel to each other, and are respectively connected between the signal input terminal T1and the signal output terminal T2. In this example, the reflectors13aprovided between the IDTs12in the surface acoustic wave resonator10aare also used by the IDTs12on the both sides.

A film thickness of electrodes of the IDT12and the reflectors13and13ais about 0.4 μm, an overlap length W of the IDT12is about 40 μm, the number of electrode fingers15in the IDTs12is 200, the number of electrodes in the reflectors13aprovided between the three IDTs12is 50, the number of electrodes in the reflectors13at the both ends is 50, pitch P1for the electrode fingers in the IDTs12is about 2.33 μm, and pitch P2for the electrodes in the reflectors13and13ais about 2.38 μm.

The Q-factor of the resonance of the surface acoustic wave resonator10aobtained as such is measured to be about 870. This figure is similar to the Q-factor of the surface acoustic wave resonator10having the separate surface acoustic wave resonators14connected in parallel as shown inFIG. 1. The two reflectors13provided between the IDTs12in the surface acoustic wave resonator10can be replaced with one reflector13ain the surface acoustic wave resonator10a. Thus, the size of the surface acoustic wave resonator10acan be small.

The higher the electrode number in the reflectors13abetween the IDTs12is, the more desirable. Since the size increases as the number increases, the number is may not be larger than the number of the electrodes in the reflectors13at both ends.

FIG. 7shows an exemplary structure of a surface acoustic wave resonator10bwhich is another form of Embodiment 1 of the present invention. The surface acoustic wave resonator10bshown inFIG. 7is a so-called one-port resonator, and includes a signal input terminal T1for receiving a signal input from the outside, a signal output terminal T2for outputting a signal to the outside, and a piezoelectric substrate11formed of 39° Y-cut, X-propagating lithium tantalate. On a surface of the piezoelectric substrate11, four surface acoustic wave resonators14aare formed.

Each of the surface acoustic wave resonators14aincludes an IDT12aand reflectors13provided near both ends of the IDT12a. Four like surface acoustic wave resonators14a, i.e., the surface acoustic wave resonators14ahaving the same resonance frequency, are connected in parallel, and are respectively connected between the signal input terminal T1and the signal output terminal T2. Two surface acoustic wave resonators14aare provided in line to form a pair. Two pairs are provided side by side instead of being aligned.

A film thickness of electrodes in the surface acoustic wave resonators14ais about 0.4 μm, an overlap length W of the IDTs12ais about 40 μm, the number of electrode fingers15in the IDTs12ais 150, the number of electrodes in the reflectors13is 50, and pitch P2for the electrodes in the reflectors13is about 2.38 μm. Pitch P1(interval) of the electrode fingers15in the IDTs12ais about 2.28 μm at the both ends. Pitch P1between the electrode fingers15from the fifteenth electrode fingers15from both ends of the IDT12ais 2.33 μm. This means that, the pitch P1between the electrode fingers15near the center of the IDT12ais 2.33 μm. The pitch P1for the electrode finger15from both ends of the IDT12ato the fifteenth electrode fingers15gradually increases from 2.28 μm to 2.33 μm.

Usually, as the number of electrode fingers15in the IDTs12ais decreased in a one-port resonator such as the surface acoustic wave resonator10bshown inFIG. 7, ripple tends to be generated near the resonance point. Thus, in the surface acoustic wave resonator10bshown inFIG. 7, the pitch P1for a part of the electrode fingers15near both ends of the IDTs12aand the pitch P1for the electrode fingers15near the center are varied. In this way, the Q-factor of resonance can be improved while the ripple is reduced.

FIGS. 8A and 8Bare graphs for illustrating an effect of reducing ripple achieved by the surface acoustic wave resonator10b.FIG. 8Ais a graph showing a result of simulation for obtaining a frequency property of the surface acoustic wave resonator10bshown inFIG. 7.FIG. 8Bis a graph showing a result of simulation for obtaining a frequency property when the pitch P1for any of the electrode fingers15in the IDTs12aof the surface acoustic wave resonator10bshown inFIG. 7is 2.33 μm. InFIGS. 8A and 8B, horizontal axes show the frequency of the signal input to the signal input terminal T1, and the vertical axes show an amount of transfer when the signal received by the signal input terminal T1is output from the signal output terminal T2which is represented in decibel.

As shown inFIG. 8B, when the pitch P1for the electrode fingers15in the IDTs12ais set to be the uniform value, 2.33 μm, ripple is generated around 825 MHz as denoted by reference numeral B. On the other hand, when the surface acoustic wave resonator10bshown inFIG. 7is used, ripple is not generated even around 825 MHz as denoted by reference numeral A. The effect of reducing the ripple achieved by the surface acoustic wave resonator10bshown inFIG. 7is confirmed.

In the surface acoustic wave resonator10bshown inFIG. 7, it is preferable to start varying the pitch P1in the IDTs12a, which is different from the pitch P1at the center of the IDTs12a, at the fifteenth electrode fingers15from both ends. It is not limited to the fifteenth electrode fingers from the ends. As long as the pitch P1for a part of the electrode fingers15among the plurality of electrode fingers15of the IDTs12aat the both ends is different from the pitch P1at the center of the IDTs12a, such a structure may be employed.

In this example, the difference between the pitch P1for the electrode fingers15near the center of the IDTs12aand the pitch P1for the electrode fingers15at both ends of the IDTs12ais 0.05 μm. The difference in the pitch P1between the portion near the center and both ends may be about 0.5% to 3% of the pitch P1near the center, for example.

In this example, the pitch P1for the electrode fingers15from the both ends of the IDTs12ato the fifteenth electrode fingers15gradually increases toward the center. The present invention is not limited to such a gradual increase. As long as the pitch at both ends is different from the pitch P1near the center, the pitches P1of the electrode fingers15at both ends of the IDTs12amay be substantially uniform.

FIG. 9is a graph showing a result of a simulation for obtaining a property when the pitch P1for the electrode fingers15from both ends of the IDTs12aof the surface acoustic wave resonator10bshown inFIG. 7to the fifteenth electrode fingers15is 2.31 μm, and the pitch P1for the rest of the electrode fingers15is 2.33 μm. As shown inFIG. 9, even when the pitches P1for the electrode fingers15at the both ends of the IDTs12aare uniform instead of gradually increasing, ripple is not generated near 825 MHz as denoted by reference numeral C. The effect of reducing ripple is confirmed.

Hereinafter, Embodiment 2 of the present invention will be described. In Embodiment 1 of the present invention, a structure of a surface acoustic wave resonator is shown. Embodiment 2 is different from Embodiment 1 in that a structure of a ladder-type surface acoustic wave filter using such a surface acoustic wave resonator is shown.

FIG. 10is a diagram showing an exemplary structure of a surface acoustic wave filter21according to Embodiment 2 of the present invention. The surface acoustic wave filter21shown inFIG. 10is an example of a ladder-type surface acoustic wave filter, and includes a signal input terminal T1for receiving a signal input from the outside, a signal output terminal T2for outputting a signal to the outside, a ground terminal T3for ground connection, and a piezoelectric substrate11formed of 39° Y-cut, X-propagating lithium tantalate. On a surface of the piezoelectric substrate11, a series resonator16and a parallel resonator17are formed.

The signal input terminal T1, the signal output terminal T2, and the ground terminal T3may be wiring patterns formed on the surface of the piezoelectric substrate11, connectors for connecting the surface acoustic wave filter21to an external circuit, or the like.

The series resonator16is a surface acoustic wave resonator which is provided between the signal input terminal T1and the signal output terminal T2, i.e., is serially connected to a signal path from the signal input terminal T1to the signal output terminal T2. For example, a surface acoustic wave resonator10ashown inFIG. 6is employed. The series resonator16may also be a surface acoustic wave resonator10shown inFIG. 1, or a surface acoustic wave resonator10bshown inFIG. 7. The parallel resonator17is a surface acoustic wave resonator which is provided between the signal output terminal T2and the ground terminal T3, i.e., which is connected between the signal path and ground.

The series resonator16is formed similarly to the surface acoustic wave resonator10ashown inFIG. 6. A plurality of, for example, three IDTs12are provided in a line on the same surface acoustic wave propagation path of the surface of the piezoelectric substrate11. A reflector13ais respectively provided between the IDTs12. Reflectors13are provided near both ends of the line of the IDTs12. The plurality of the IDTs12are connected in parallel between the signal input terminal T1and the signal output terminal T2.

A film thickness of electrodes in the series resonator16and the parallel resonator17is about 0.4 μm. In the series resonator16, an overlap length W of the IDT12is about 40 μm, the number of electrode fingers15in the IDTs12is 200, the number of electrodes in the reflectors13aprovided between the IDTs12is 20, the number of electrodes in the reflectors13at the both ends of the series resonator16is 50, pitch P1for the electrode fingers in the IDTs12is about 2.33 μm, and pitch P2for the electrodes in the reflectors13and13ais about 2.38 μm.

The parallel resonator17is formed by connecting one IDT18between the signal output terminal T2and the ground terminal T3on the piezoelectric substrate11, and providing reflectors19near both ends of the IDT18. An overlap length W at electrode fingers of the IDT18is about 40 μm, the number of electrode fingers in the IDT18is 200, pitch P1for the electrode fingers in the IDT18is about 2.44 μm, and pitch P2for the electrodes in the reflectors19is about 2.42 μm.

FIG. 11is a graph showing a comparison between a frequency property of the surface acoustic wave filter21having the above-described structure, and a frequency property of the conventional surface acoustic wave filter shown inFIG. 13. InFIG. 11, the frequency property of the surface acoustic wave filter21shown inFIG. 10is represented by a solid line as graph G3, and the frequency property of the conventional surface acoustic wave filter shown inFIG. 13is represented by a broken line as graph G4.

As can be seen fromFIG. 11, it is confirmed that the frequency property of the surface acoustic wave filter21represented by the graph G3has a broadened band and improved steepness in a band higher than a pass band (around 865 MHz) compared to the frequency property of the conventional surface acoustic wave filter represented by the graph G4.

In the surface acoustic wave filter21shown inFIG. 10, only the series resonator16is formed by connecting a plurality of the IDTs12in parallel. The parallel resonator17can also be formed by connecting a plurality of IDTs18, and for example, the surface acoustic wave resonator10,10a, or10bmay be employed.

Hereinafter, Embodiment 3 of the present invention will be described. Embodiment 3 is different from Embodiment 2 in that, while Embodiment 2 relates to a ladder-type surface acoustic wave filter using one terminal pair surface acoustic wave resonator, Embodiment 3 is applied to a surface acoustic wave filter using a multiple-port surface acoustic wave resonator.

FIG. 12shows an exemplary structure of a surface acoustic wave filter22according to Embodiment 3 of the present invention. The surface acoustic wave filter22shown inFIG. 12is an example of a ladder-type surface acoustic wave filter, and includes a signal input terminal T1for receiving a signal input from the outside, a signal output terminal T2for outputting a signal to the outside, a ground terminal T3for a ground connection, and a piezoelectric substrate11formed of 39° Y-cut, X-propagating lithium tantalate. On a surface of the piezoelectric substrate11, a multiple-port surface acoustic wave resonator23is formed.

In the multiple-port surface acoustic wave resonator23, IDTs12aand12band reflectors13,13a, and13bare provided in line on the same surface acoustic wave propagation path on the piezoelectric substrate11. In the multiple-port surface acoustic wave resonator23, a plurality of, for example, three IDTs12aare provided. A reflector13ais respectively provided between the IDTs12a. A reflector13is provided near one end of the line of the IDTs12a, and the reflector13bis provided near the other end. The plurality of the IDTs12aare connected in parallel to each other and are serially connected to the signal path between the signal input terminal T1and the signal output terminal T2.

The IDT12bis provided such that one end of the IDT12bis located near the reflector13b. A reflector13is provided near the other end of the IDT12b. The IDT12bis connected between the signal output terminal T2and the ground terminal T3, i.e., between the signal path and the ground.

The multiple-port surface acoustic wave resonator23having the above-described structure is a single surface acoustic wave resonator formed by aligning the IDTs12aand12band reflectors13,13a, and13bon the same surface acoustic wave propagation path on the piezoelectric substrate11, and also forms a multiple-port surface acoustic wave filter22including a signal input terminal T1, a signal output terminal T2, and a ground terminal T3.

In such a case, three IDTs12a, a reflector13anear the IDT12a, two reflectors13a, and a reflector13form a series resonator. The reflector13b, the IDT12b, and a reflector13near the IDT12bforms a parallel resonator. The reflector13bat one end of the series resonator is also used as the reflector13bat one and of the parallel resonator.

A film thickness of electrodes in the multiple-port surface acoustic wave filter22is about 0.4 μm. An overlap length W of the IDTs12aand12bis about 40 μm, the number of electrode fingers in the IDTs12ais 200, the number of electrode fingers in the IDT12bis 200, the number of electrodes in the reflectors13ais 20, the number of electrodes in the reflector13bis 20, the number of electrodes in the reflectors13is 50, pitch P1for the electrode fingers in the IDTs12ais about 2.33 μm, pitch P1for the electrode fingers in the IDT12bis about 2.44 μm, pitch P2in the reflectors13ais about 2.38 μm, pitch P2in the reflector13bis about 2.41 μm, pitch P2in the reflector13near the IDT12ais about 2.38 μm, and pitch P2in the reflector13near the IDT12bis about 2.42 μm.

In this way, the number of the reflectors can be decreased by one compared to that of the surface acoustic wave filter21shown inFIG. 10. Thus, the multiple-port surface acoustic wave filter22can be made smaller than the surface acoustic wave filter21. Similarly to the surface acoustic wave filter21shown inFIG. 10, the pass band for a signal is broadened and the steepness can be improved than in the conventional surface acoustic wave filter shown inFIG. 13.

It is preferable that the IDTs12ahave the same structure such as the sane number of electrode fingers and the pitch for the electrode fingers. The number of the electrode fingers in the IDT12bmay be selected as appropriate depending upon the design.

The IDT12bconnected between the signal path and the ground may be replaced with a plurality of the IDTs connected in parallel with reflectors being inserted between the IDTs.

The reflectors13abetween the IDTs12aare necessary for achieving the effect of the present invention. The reflector13bbetween the IDT12aand the IDT12bmay be omitted depending upon the design.

INDUSTRIAL APPLICABILITY

The surface acoustic wave resonator and the surface acoustic wave filter according to the present invention provide an effect of improving a Q-factor of a resonance and providing a surface acoustic wave filter with a low insertion loss and high steepness. The present invention is useful for filters in the field of communication such as cellular phones and the like, or the field of video images such as televisions and the like.