JP2013229641A - Elastic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance steepness of filter characteristics while suppressing increase of insertion loss in the passband, without enlarging an elastic wave filter.SOLUTION: An elastic wave filter 1 includes first and second signal terminals 12, 13a, 13b, a longitudinal coupling resonator elastic wave filter 20 connected between the first signal terminal 12 and second signal terminals 13a, 13b, and elastic wave resonators 30a, 30b. The elastic wave resonators 30a, 30b are connected between the connection point of the longitudinal coupling resonator elastic wave filter 20 and signal terminals 13a, 13b, and the ground potential. The elastic wave resonators 30a, 30b have a piezoelectric substrate 10, and IDT electrodes 31a, 31b formed thereon. The IDT electrodes 31a, 31b are subjected to apodization weighting.

Description

  The present invention relates to an elastic wave filter.

  Conventionally, in a communication device such as a mobile phone, an elastic wave filter using an elastic wave such as a surface acoustic wave or a boundary acoustic wave has been widely used. For example, Patent Document 1 below discloses a surface acoustic wave filter including a longitudinally coupled resonator type surface acoustic wave filter section as a type of the acoustic wave filter. FIG. 11 is a schematic plan view of the surface acoustic wave filter described in Patent Document 1. As shown in FIG. 11, the surface acoustic wave filter 100 includes a longitudinally coupled resonator type surface acoustic wave filter unit 103 connected between an input terminal 101 and an output terminal 102. A surface acoustic wave resonator 104 a is connected between a connection point between the longitudinally coupled resonator type surface acoustic wave filter unit 103 and the input terminal 101 and the ground potential. On the other hand, a surface acoustic wave resonator 104b is connected between a connection point between the longitudinally coupled resonator type surface acoustic wave filter unit 103 and the output terminal 102 and the ground potential. By positioning the anti-resonance frequency of these surface acoustic wave resonators 104a and 104b in the pass band of the surface acoustic wave filter 100, and by positioning the resonance frequency in the attenuation region near the low pass band side, While suppressing the deterioration of the insertion loss, it is possible to increase the attenuation near the passband low band side.

Japanese Patent Laid-Open No. 7-30366

  By the way, in the surface acoustic wave filter 100, the steepness of the filter characteristic in the low band side transition band between the attenuation band located on the lower side of the pass band and the pass band is the resonance of the surface acoustic wave resonators 104a and 104b. Determined by return loss between frequency and anti-resonance frequency. From the viewpoint of obtaining high steepness of the filter characteristics in the low band side transition band, it is preferable to reduce the return loss by increasing the crossing width of the surface acoustic wave resonators 104a and 104b. However, when the crossing width of the surface acoustic wave resonators 104a and 104b is increased, the capacity also increases, resulting in a problem that the insertion loss in the passband increases.

  For example, two surface acoustic wave resonators may be connected in series between a connection point between the longitudinally coupled resonator type surface acoustic wave filter unit 103 and the input terminal 101 or the output terminal 102 and the ground potential. It is done. In this case, since the cross width of each surface acoustic wave resonator can be increased without increasing the overall capacity, the return loss between the resonance frequency and the antiresonance frequency can be improved. As a result, it is possible to increase the steepness of the filter characteristics in the low band side transition band while reducing the insertion loss in the pass band. However, in this case, it is necessary that each of the two surface acoustic wave resonators has a crossing width twice that of the case where one surface acoustic wave resonator is provided. Accordingly, the area occupied by each surface acoustic wave resonator is about twice that of the case where one surface acoustic wave resonator is provided, and a total area of about four times is required. Therefore, there is a problem that the surface acoustic wave filter is increased in size.

  The present invention has been made in view of the above points, and aims to improve the steepness of the filter characteristics without increasing the size of the elastic wave filter and suppressing an increase in insertion loss in the passband. is there.

  The elastic wave filter according to the present invention includes first and second signal terminals, a longitudinally coupled resonator type elastic wave filter unit, and an elastic wave resonator. The longitudinally coupled resonator type acoustic wave filter unit is connected between the first signal terminal and the second signal terminal. The acoustic wave resonator is connected between a connection point between the longitudinally coupled resonator type acoustic wave filter section and the first or second signal terminal and the ground potential. The acoustic wave resonator includes a piezoelectric substrate and an IDT electrode formed on the piezoelectric substrate. Apodization weighting is applied to the IDT electrode.

  In a specific aspect of the acoustic wave filter according to the present invention, the acoustic wave resonator is a surface acoustic wave resonator, and the longitudinally coupled resonator type acoustic wave filter unit is a longitudinally coupled resonator type surface acoustic wave filter unit. is there.

  In another specific aspect of the elastic wave filter according to the present invention, the elastic wave resonator is a boundary acoustic wave resonator, and the longitudinally coupled resonator type elastic wave filter unit is a longitudinally coupled resonator type boundary acoustic wave filter unit. It is.

  In the present invention, the IDT electrode of the acoustic wave resonator connected between the connection point between the longitudinally coupled resonator type acoustic wave filter section and the first or second signal terminal and the ground potential is apodized weighted. Is given. For this reason, it is possible to improve the steepness of the filter characteristics without excessively increasing the size of the acoustic wave filter and suppressing an increase in insertion loss in the passband.

1 is a schematic plan view of a surface acoustic wave filter according to an embodiment of the present invention. 1 is a schematic cross-sectional view of a part of a surface acoustic wave filter according to an embodiment of the present invention. It is a typical top view for demonstrating the shape of IDT electrode 31a of the surface acoustic wave resonator 30a in one Embodiment which implemented this invention. 6 is a schematic plan view for explaining the shape of an IDT electrode 31a in Example 2. FIG. It is a typical top view for explaining the shape of IDT electrode 31a in a comparative example. It is a graph showing the return loss of the surface acoustic wave resonator 30a in Examples 1 and 2 and a comparative example. It is a graph showing the insertion loss of the surface acoustic wave filter in Example 1 and a comparative example. It is a graph showing the insertion loss of the surface acoustic wave filter in Example 1 and a comparative example. It is a schematic sectional drawing of a part of surface acoustic wave filter concerning the 1st modification. It is a typical top view for explaining the shape of IDT electrode 31a in the 2nd modification. 1 is a schematic plan view of a surface acoustic wave filter described in Patent Document 1. FIG.

  Hereinafter, a preferred embodiment in which the present invention is implemented will be described taking the surface acoustic wave filter 1 shown in FIG. 1 as an example. However, the surface acoustic wave filter 1 is merely an example. The elastic wave filter according to the present invention is not limited to the surface acoustic wave filter 1.

  The surface acoustic wave filter 1 is a filter used as a reception-side filter for UMTS-Band 2 having a balanced-unbalanced signal conversion function. The transmission frequency band of UMTS-Band2 is 1850 MHz to 1910 MHz, and the reception frequency band is 1930 MHz to 1990 MHz.

  The surface acoustic wave filter 1 has an unbalanced signal terminal 12 as an input terminal and first and second balanced signal terminals 13a and 13b as output terminals. In this embodiment, the impedance of the unbalanced signal terminal 12 is 50Ω, and the impedances of the first and second balanced signal terminals 13a and 13b are 100Ω.

  A longitudinally coupled resonator type surface acoustic wave filter unit 20 is connected between the unbalanced signal terminal 12 and the first and second balanced signal terminals 13a and 13b. The longitudinally coupled resonator type surface acoustic wave filter unit 20 includes first and second longitudinally coupled resonator type surface acoustic wave element units 20a and 20b. The first longitudinally coupled resonator type surface acoustic wave element unit 20a is connected between the unbalanced signal terminal 12 and the first balanced signal terminal 13a. On the other hand, the second longitudinally coupled resonator type surface acoustic wave element unit 20b is connected between the unbalanced signal terminal 12 and the second balanced signal terminal 13b.

  Each of the first and second longitudinally coupled resonator type surface acoustic wave element portions 20a, 20b has three IDT electrodes 21a, 21b, 22a, 22b, 23a, 23b arranged along the surface acoustic wave propagation direction. And a pair of reflectors 24a, 24b, 25a, 25b provided on both sides of the surface acoustic wave propagation direction in the region where the three IDT electrodes 21a, 21b, 22a, 22b, 23a, 23b are provided. In each of the first and second longitudinally coupled resonator type surface acoustic wave element portions 20a and 20b, the IDT electrodes 21a, 21b, 22a, 22b, 23a and 23b are adjacent to the adjacent IDT electrodes. The edge part which is made into the narrow pitch electrode finger part whose period of an electrode finger is smaller than a center part.

  A surface acoustic wave resonator 19 a is connected between the first longitudinally coupled resonator type surface acoustic wave element unit 20 a and the unbalanced signal terminal 12. On the other hand, a surface acoustic wave resonator 19 b is connected between the second longitudinally coupled resonator type surface acoustic wave element portion 20 b and the unbalanced signal terminal 12. The surface acoustic wave resonators 19a and 19b have a resonance frequency located in the pass band of the longitudinally coupled resonator type surface acoustic wave filter unit 20 and an anti-resonance frequency located in an attenuation region near the high side of the pass band. It is configured.

  A surface acoustic wave resonator 30a is connected between a connection point between the first longitudinally coupled resonator type surface acoustic wave element unit 20a and the first balanced signal terminal 13a and the ground potential. A surface acoustic wave resonator 30b is connected between a connection point between the second longitudinally coupled resonator type surface acoustic wave element unit 20b and the second balanced signal terminal 13b and the ground potential. Each of the surface acoustic wave resonators 30a and 30b includes an IDT electrode 31a and 31b and a pair of reflectors 34a, 34b, 35a and 35b disposed on both sides of the IDT electrodes 31a and 31b in the surface acoustic wave propagation direction. Prepare.

  The surface acoustic wave resonators 30a and 30b are arranged such that the resonance frequency is located in the attenuation region near the low pass side of the longitudinally coupled resonator type surface acoustic wave filter unit 20, and the anti-resonance frequency is located in the pass band. It is configured. The surface acoustic wave resonators 30a and 30b and the surface acoustic wave resonators 19a and 19b increase the attenuation in the vicinity of the passband.

As shown in FIG. 2, the surface acoustic wave filter 1 of this embodiment includes a piezoelectric substrate 10 and an electrode 11 formed on the piezoelectric substrate 10. The electrode 11 constitutes the IDT electrode, reflector, wiring, and the like. In the present embodiment, the piezoelectric substrate 10 is composed of a 40 ± 5 ° Y-cut X-propagating LiTaO ₃ substrate and uses a leaky surface acoustic wave. However, the piezoelectric substrate 10 may be constituted by other piezoelectric substrate, such as LiNbO ₃ substrate or a quartz.

  The electrode 11 is made of Al. However, the electrode 11 may be made of a conductive material other than Al. The electrode 11 can also be formed of, for example, a metal such as Al, Pt, Au, Ag, Cu, Ni, Ti, Cr, and Pd, or an alloy containing one or more of these metals. Moreover, the electrode 11 can also be comprised with the laminated body of the some electrically conductive film which consists of the above metals and alloys.

  For example, a dielectric film such as a silicon oxide film or a silicon nitride film may be formed on the piezoelectric substrate 10 for the purpose of improving frequency temperature characteristics or protecting the IDT electrode and the reflector. .

  Next, the configuration of the IDT electrode 31a of the surface acoustic wave resonator 30a in the present embodiment will be described in detail with reference to FIG. In the present embodiment, the IDT electrode 31a and the IDT electrode 31b have substantially the same configuration. For this reason, suppose here that the following description regarding the IDT electrode 31a is also applied to the IDT electrode 31b.

  As shown in FIG. 3, the IDT electrode 31a has a pair of comb-like electrodes 32a and 33a that are interleaved with each other. Each of the comb-like electrodes 32a and 33a has a plurality of electrode fingers 32a1 and 33a1 and bus bars 32a2 and 33a2. The plurality of electrode fingers 32a1 and 33a1 are connected to the bus bars 32a2 and 33a2. Each of the comb-like electrodes 32a and 33a further includes a plurality of dummy electrodes 32a3 and 33a3 connected to the bus bars 32a2 and 33a2. The dummy electrode 32a3 of one comb-shaped electrode 32a faces the electrode finger 33a1 of the other comb-shaped electrode 33a in the cross width direction perpendicular to the elastic wave propagation direction. Further, the dummy electrode 33a3 of the other comb-shaped electrode 33a faces the electrode finger 32a1 of the one comb-shaped electrode 32a in the cross width direction.

  In the present embodiment, apodization weighting is applied to the IDT electrode 31a. Here, the apodization weighting means that the shape of the electrode fingers 32a1 and 33a1 is set so that there are portions where the cross widths of the electrode fingers 32a1 and 33a1 differ in the elastic wave propagation direction. In the present embodiment, specifically, an envelope L1 formed by connecting the tips of the electrode fingers 32a1 and an envelope L2 formed by connecting the tips of the electrode fingers 33a1 are in the elastic wave propagation direction. The shape of the electrode fingers 32a1 and 33a1 is set so as to be inclined with respect to the surface. In the present embodiment, one point where the maximum point of the crossing width, that is, the distance between the envelope L1 and the envelope L2 in the crossing width direction is maximum is formed.

  As described above, in this embodiment, apodization weighting is applied to the IDT electrodes 31a and 31b. For this reason, the apparent crossing width can be increased even with the same capacity as compared with the IDT electrode that is not apodized. Therefore, the return between the resonance frequency and the anti-resonance frequency of the surface acoustic wave resonators 30a and 30b is achieved without increasing the number of the surface acoustic wave resonators 30a and 30b and suppressing the deterioration of the insertion loss in the passband. Loss can be improved. Hereinafter, this effect will be described in detail based on examples and comparative examples.

  As Example 1, a surface acoustic wave filter having the same configuration as that of the surface acoustic wave filter 1 described in the above embodiment was manufactured with the following design parameters.

(Design parameters of Example 1)
First longitudinally coupled resonator type surface acoustic wave element unit 20a:
Cross width: 30.4λI (where λI is the wavelength of the surface acoustic wave determined by the pitch of the electrode fingers of the IDT electrode)
Number of electrode fingers of IDT electrodes 22a and 23a: 39 (of which, the number of electrode fingers in the narrow pitch electrode finger portion is 5)
Number of electrode fingers of the IDT electrode 21a: 43 (of which, the number of electrode fingers in the narrow pitch electrode finger portion on the IDT electrode 22a side: 3, the number of electrode fingers in the narrow pitch electrode finger portion on the IDT electrode 23a side: 7)
Number of electrode fingers in reflectors 24a and 25a: 65 Metallization ratio of IDT electrodes 21a-23a: 0.68
Electrode film thickness: 0.091λI
The period of the electrode fingers in the narrow pitch electrode finger portion of the IDT electrode 22a is smaller by 0.09 μm than the cycle of the electrode fingers in the narrow pitch electrode finger portion of the IDT electrode 23a.

Second longitudinally coupled resonator type surface acoustic wave element unit 20b:
In order to invert the phase of the output signal by 180 °, the configuration is the same as that of the first longitudinally coupled resonator type surface acoustic wave filter unit 20a except that the IDT electrode located at the center is inverted.

Surface acoustic wave resonators 19a and 19b:
Cross width: 11.0λII (where λII is the surface acoustic wave wavelength determined by the period of the electrode fingers of the IDT electrode)
Number of electrode fingers of IDT electrode: 71 Number of electrode fingers of reflector: 18 IDT metallization ratio: 0.60
Electrode film thickness: 0.095λI
Surface acoustic wave resonators 30a and 30b:
Maximum crossing width: 30.0λIII (where λIII is the surface acoustic wave wavelength determined by the period of the electrode fingers of the IDT electrode)
Number of electrode fingers of IDT electrodes 31a, 31b: 111 Number of electrode fingers of reflectors 34a, 35a, 34b, 35b: 18 Metallization ratio of IDT electrodes 31a, 31b: 0.60
Electrode film thickness: 0.091λI
Apodization: 100% (intersection width at both ends of IDT electrodes 31a and 31b in the elastic wave propagation direction is 0% of the maximum value of the intersection width)
Further, as Example 2, as shown in FIG. 4, the apodization in the surface acoustic wave resonators 30a and 30b is set to 50%, that is, the cross widths at both ends of the IDT electrodes 31a and 31b in the elastic wave propagation direction are cross widths. A surface acoustic wave filter having the same configuration as in Example 1 was prepared except that the maximum value was 50%.

  As a comparative example, as shown in FIG. 5, the surface acoustic wave resonators 30 a and 30 b are configured by regular IDT electrodes not subjected to apodization weighting, and the intersection width is the maximum value of the intersection width in the first embodiment. A surface acoustic wave filter having the same configuration as that of Example 1 was prepared, except that the half (15.0λIII) was used.

  In the description of Examples 1 and 2 and the comparative example, members having substantially the same functions as those of the above-described embodiment are referred to by common reference numerals, and the description thereof is omitted.

  FIG. 6 is a graph showing the return loss of the surface acoustic wave resonator 30a in Examples 1 and 2 and the comparative example. In addition, in FIG. 6, the graph shown as a continuous line is a graph showing the return loss of the surface acoustic wave resonator 30a in Example 1. FIG. A graph indicated by an alternate long and short dash line is a graph representing a return loss of the surface acoustic wave resonator 30a according to the second embodiment. A graph indicated by a broken line is a graph representing a return loss of the surface acoustic wave resonator 30a in the comparative example.

  From the graph shown in FIG. 6, it can be seen that the return loss between the resonance frequency and the antiresonance frequency of the surface acoustic wave resonator 30a is improved by applying apodization weighting to the surface acoustic wave resonator 30a.

  FIG. 7 is a graph showing the insertion loss of the surface acoustic wave filter in Example 1 and the comparative example. FIG. 8 is an enlarged graph of a part of FIG. 7 and 8, the graph shown by the solid line is a graph showing the insertion loss of the surface acoustic wave filter in Example 1, and the graph shown by the alternate long and short dash line shows the insertion loss of the surface acoustic wave filter in the comparative example. It is a graph.

  From the results shown in FIGS. 7 and 8, the insertion loss in the passband is the same in Example 1 in which the surface acoustic wave resonators 30a and 30b are apodized weighted and in the comparative example in which the apodized weight is not applied. It was. Regarding the steepness of the filter characteristics in the low band side transition band of the pass band, Example 1 was superior to the comparative example. Specifically, the difference between the frequency at which the insertion loss is 3.5 dB and the frequency at 40 dB is 14.5 MHz in the comparative example, whereas it is as small as 13.9 MHz in the first embodiment. It was. From the above results, by applying apodization weighting to the surface acoustic wave resonators 30a and 30b, the deterioration of the insertion loss in the passband is suppressed, and the resonance frequencies and antiresonance frequencies of the surface acoustic wave resonators 30a and 30b are determined. The return loss during can be improved. And by the improvement of the return loss, the steepness of the filter characteristic in the low band side transition band of the pass band can be realized. Note that the surface acoustic wave resonators 30a and 30b of the first embodiment have an area approximately twice that of the comparative example. However, in order to obtain the same filter characteristics by connecting two surface acoustic wave resonators in series, the area needs to be about four times that of the comparative example. Therefore, Example 1 is not larger than this case.

(First modification)
FIG. 9 is a schematic cross-sectional view of a part of the surface acoustic wave filter according to the first modification.

  In the above-described embodiment, the preferred embodiment of the present invention has been described by taking the surface acoustic wave filter 1 using the surface acoustic wave as an example. However, in the present invention, the acoustic wave resonator and the longitudinally coupled resonator type acoustic wave filter section are not limited to those using surface acoustic waves. For example, as shown in FIG. 9, the first and second dielectric layers 40 and 41 are formed on the piezoelectric substrate 10 so as to cover the electrode 11, thereby causing the acoustic wave filter to generate the boundary acoustic wave. A boundary acoustic wave filter including a boundary acoustic wave resonator and a longitudinally coupled resonator type boundary acoustic wave filter unit may be used. Of the first and second dielectric layers 40 and 41, only the first dielectric layer 40 may be provided. The material of the first and second dielectric layers 40 and 41 is not particularly limited. For example, the first dielectric layer 40 can be formed of silicon oxide, and the second dielectric layer 41 can be formed of silicon nitride.

(Second modification)
FIG. 10 is a schematic plan view for explaining the shape of the IDT electrode 31a in the second modification.

  In the above-described embodiment, the example in which the apodization weighting is applied to the acoustic wave resonator so that only one maximum point of the intersection width is formed has been described. However, the present invention is not limited to this configuration. For example, as shown in FIG. 10, apodization weighting may be applied to the acoustic wave resonator so that a plurality of maximum points of the intersection width are formed. In the above embodiment, the envelopes L1 and L2 are apodized so as to be linear, but may be apodized so as to be curved.

DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave filter 10 ... Piezoelectric substrate 11 ... Electrode 12 ... Unbalanced signal terminal 13a ... 1st balanced signal terminal 13b ... 2nd balanced signal terminal 19a, 19b ... Surface acoustic wave resonator 20 ... Longitudinal coupled resonator Type surface acoustic wave filter section 20a ... first longitudinally coupled resonator type surface acoustic wave element section 20b ... second longitudinally coupled resonator type surface acoustic wave element sections 21a, 21b, 22a, 22b, 23a, 23b ... IDT electrodes 24a, 24b, 25a, 25b ... reflectors 30a, 30b ... surface acoustic wave resonators 31a, 31b ... IDT electrodes 32a, 33a ... comb-like electrodes 32a1, 33a1 ... electrode fingers 32a2, 33a2 ... bus bars 32a3, 33a3 ... dummy electrodes 34a, 34b, 35a, 35b ... reflector 40 ... first dielectric layer 41 ... second dielectric layer

Claims (3)

First and second signal terminals;
A longitudinally coupled resonator type acoustic wave filter unit connected between the first signal terminal and the second signal terminal;
An acoustic wave resonator connected between a connection point between the longitudinally coupled resonator type acoustic wave filter section and the first or second signal terminal and a ground potential;
With
The acoustic wave resonator includes a piezoelectric substrate and an IDT electrode formed on the piezoelectric substrate,
An elastic wave filter in which the IDT electrode is apodized weighted.   The elastic wave filter according to claim 1, wherein the elastic wave resonator is a surface acoustic wave resonator, and the longitudinally coupled resonator type surface acoustic wave filter unit is a longitudinally coupled resonator type surface acoustic wave filter unit.   The elastic wave filter according to claim 1, wherein the elastic wave resonator is a boundary acoustic wave resonator, and the longitudinally coupled resonator type elastic wave filter unit is a longitudinally coupled resonator type boundary acoustic wave filter unit.

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