JP2004363824A - Surface acoustic wave filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transversal SAW (Surface Acoustic Wave) filter in which two IDT electrodes are adjacently arranged, ripples in a pass band are reduced and insertion loss is reduced. <P>SOLUTION: In the transversal surface acoustic wave filter, the two IDT electrodes are arranged on a piezoelectric substrate along the transmission direction of a surface wave by leaving a prescribed interval. When the waveguide length of the IDT electrode is set to be Wo and an electrode finger intersection length to be W, they are set to be 0.86≤W/Wo≤0.96. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
The present invention relates to a surface acoustic wave filter, and more particularly to a surface acoustic wave filter with improved ripple and insertion loss.
[0002]
2. Description of the Related Art In recent years, surface acoustic wave (SAW) filters have been widely used in the field of mobile communication and have excellent characteristics such as high performance, small size, mass productivity, and the like. Many are used. In recent years, mobile phones have been rapidly spreading, and in particular, due to an increase in demand for data communication such as images, a communication system using a narrow band carrier such as a conventional PDC or AMPS system has been adapted to high speed and large capacity. The W-CDMA system using a wideband carrier has begun to spread. In an IF filter used in such a W-CDMA mobile phone, a filter characteristic with low insertion loss and small ripple in a pass band is strongly required. Specifically, an insertion loss of 10 (dB) is required. Hereafter, a specification of ripple 1.0 (dB) or less is required.
[0003]
FIG. 4 is a plan view showing the structure of a transversal SAW filter used for a conventional IF filter. Two IDT electrodes 12 and 13 are arranged on the main surface of the piezoelectric substrate 11 along the SAW propagation direction with a predetermined gap therebetween, and a direct wave between the input and output terminals is provided between the IDT electrodes 12 and 13. Is provided with a solid electrode 14 for shielding the light. Each of the IDT electrodes 12 and 13 is formed of a pair of comb-shaped electrodes having electrode fingers interposed therebetween, and connects one of the IDT electrodes 12 to the input terminal IN and connects the other to the input terminal IN. Ground. Further, one of the IDT electrodes 13 is connected to the output terminal OUT, and the other IDT electrode 13 is grounded. Then, sound absorbing materials 15 for suppressing unnecessary reflected waves are applied to both ends of the piezoelectric substrate 11 in the long side direction (the SAW propagation direction) to form a transversal SAW filter.
[0004]
However, as shown in FIG. 4, in the case of a so-called regular IDT electrode in which comb electrodes are arranged in the order of positive, negative, and positive, SAW propagates equally to the left and right along the propagation direction, so that the insertion loss of the filter increases. There was a problem that it would. Therefore, in order to solve this problem, a so-called unidirectional electrode in which the electrode has directionality is used. As the unidirectional electrode, a single-phase unidirectional transducer (hereinafter, referred to as SPUDT) in which a plurality of electrodes having different electrode widths are combined to utilize SAW reflection is well known. ing. FIG. 5A shows a plan view and a sectional view of the basic section 20 of the electrode period λ of the SPUDT. It is composed of electrode fingers having a width, and by determining the positions of the 3λ / 8 electrode fingers and the λ / 8 electrode fingers as shown in the figure, the SAW is reflected and the excitation is made unidirectional. FIG. 5B shows a structure in which the SPUDT is incorporated in a transversal SAW filter.
[0005]
However, if the IDT electrodes 22 and 23 are composed of only SPUDTs as shown in FIG. 5B, the insertion loss is reduced, but the directionality of the IDT electrodes is too strong, causing a problem that phase distortion occurs in the filter characteristics. Was. Therefore, an input / output IDT electrode of a transversal SAW filter is composed of an electrode having no directionality by canceling the reflection of the SAW (hereinafter, referred to as a reflection thinning electrode) and a SPUDT having the directionality by reflecting the SAW. By combining the above, a design method has been attempted in which the IDT electrode as a whole has an appropriate directionality and realizes low insertion loss and phase linearity at the same time. FIG. 6A shows a plan view and a cross-sectional view of a basic section 30 of the reflective thinning electrode having an electrode period λ, and has a structure in which electrode fingers having an electrode width of λ / 8 are arranged as shown. The propagation of the SAW of the reflection thinning electrode is bidirectional, and no reflection of the SAW occurs in the electrode. FIG. 6B shows a structure of a transversal SAW filter in which the reflection thinning electrode and the SPUDT are applied to the IDT electrodes 32 and 33. By adopting such a structure, low insertion loss and Filter characteristics with excellent phase linearity can be obtained.
[0006]
Also, U.S. Pat. No. 5,703,427 discloses a converter in which SPUDT basic sections having different directions are combined to generate several local resonant cavities inside the IDT electrode, and the IDT electrode as a whole has one direction. A so-called internal resonance type unidirectional converter (hereinafter referred to as Resonant Single Phase Uni-Directional Transducer: RSPUDT) is disclosed. FIG. 7 shows a simplified diagram of the RSPUDT. Assuming that the propagation direction in the right arrow direction in the drawing is the forward direction, and the propagation direction in the left arrow direction is the reverse direction, the first group in which the SPUDT basic section 41 in which the IDT electrode is weighted in the forward direction is formed at 50λ, A second group is formed by forming a SPUDT basic section 42 in which the IDT electrodes are weighted in the direction by 20λ, and a third group is formed by forming a SPUDT basic section 43 in which the IDT electrodes are weighted in the forward direction by 20λ. Conventionally, only positive and zero reflection weights are applied to the IDT electrode. However, the IDT electrode is weighted with positive, negative and zero reflections by using an RSPUDT combining basic sections having different directions. And a filter characteristic with low insertion loss, high attenuation, and wide band can be obtained.
[0007]
Furthermore, in order to sharpen the cut-off characteristic of the transversal SAW filter, it is general that one or both of the input and output IDT electrodes are weighted by excitation in addition to the above-mentioned weighting of reflection. Is being done.
[0008]
[Problems to be solved by the invention]
FIG. 8 shows measured filter characteristics of a transversal SAW filter in which an IDT electrode is formed by using the above RSPUDT. It should be noted that Li ₂ B ₄ O ₇ propagating in the Z direction in a 45 ° rotation X plate is used for the piezoelectric substrate. As can be seen from the figure, periodic fine ripples are generated in the pass band, and there is no margin for the specification within 1.0 dB of the ripple required as the IF filter for W-CDMA. There was a problem that it deteriorated greatly.
[0009]
A possible cause of the ripple is considered to be the influence of higher-order transverse mode spurious generated when exciting the SAW. As means for suppressing the spurious response in the transverse mode, Japanese Patent Publication No. Hei 6-85492 and Japanese Patent Application Laid-Open No. 9-260996 disclose a second-order transverse mode spurious response by changing the ratio of the electrode finger cross length to the waveguide length of the IDT electrode. Discloses a SAW resonator and a filter that suppress the above. FIG. 9 shows an enlarged view of the IDT electrode disclosed in the prior art. In the waveguide portion 51 of the IDT electrode 50, dummy electrode portions 53, 53 are provided above and below an electrode finger intersection 52. . Assuming that the waveguide length of the waveguide portion 51 is Wo and the intersection length of the electrode finger intersection portion 52 is W, the ratio of 0.65 ≦ W / Wo ≦ 0.75 in JP-B-6-85492 is obtained. A SAW resonator in which spurious of a second-order transverse mode is suppressed is disclosed. Japanese Patent Application Laid-Open No. 9-260996 discloses a SAW device in which the ratio of 0.75 <W / Wo ≦ 0.85 is suppressed to suppress the spurious in the second-order transverse mode. However, in any of the prior arts, it has been difficult to satisfy the ripple specification required for the W-CDMA IF filter.
[0010]
SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problem, and an object of the present invention is to provide a transversal SAW filter that reduces ripple in a pass band and reduces insertion loss in a transversal SAW filter. .
[0011]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a SAW filter according to the present invention, comprising: a transformer in which two IDT electrodes are arranged on a piezoelectric substrate at a predetermined interval along a propagation direction of a surface acoustic wave. In the versal surface acoustic wave filter, when the waveguide length of the IDT electrode is Wo and the electrode finger intersection length is W, 0.86 ≦ W / Wo ≦ 0.96. is there. In the invention according to claim 2, the IDT electrode has a basic section having one direction, a basic section having a reflecting function, a basic section having no reflecting function, when a basic section is one wavelength of an electrode cycle. 2. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter has at least one or more basic sections. According to a third aspect of the present invention, in the surface acoustic wave filter according to any one of the first to second aspects, the piezoelectric substrate is made of Li ₂ B ₄ O ₇ propagated in a 45-degree rotation X-plate Z direction. is there. The invention according to claim 4 is the surface acoustic wave filter according to any one of claims 1 to 3, wherein the surface acoustic wave filter is an IF filter for W-CDMA.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail based on an embodiment illustrated in the drawings. FIG. 1 is a plan view of a transversal SAW filter according to the present invention. The two IDT electrodes 2 and 3 are arranged on the main surface of the piezoelectric substrate along the SAW propagation direction with a predetermined gap therebetween, and a direct wave between the input and output terminals is shielded between the IDT electrodes 2 and 3. The solid electrode 4 for performing the operation is provided. Each of the IDT electrodes 2 and 3 is constituted by a pair of comb-shaped electrodes having a plurality of electrode fingers interposed between each other. The electrode is grounded, and one comb-shaped electrode of the IDT electrode 3 is connected to the output terminal OUT, and the other comb-shaped electrode is grounded. Then, a sound absorbing material for suppressing unnecessary reflected waves is applied to both ends of the piezoelectric substrate in the SAW propagation direction to form a transversal SAW filter. The electrode design method of the IDT electrodes 2 and 3 is to combine SPUDT basic sections having different directions so as to generate some local resonance cavities inside the IDT electrode and to make the entire IDT electrode unidirectional. RSPUDT is used. A feature of the present invention is that, as shown in FIG. 1, in the waveguide portions 6 of the electrode fingers of the IDT electrodes 2 and 3, dummy electrode portions 8 are provided above and below the electrode finger intersection portions 7 and the waveguide length of the IDT electrodes Is Wo and the electrode finger intersection length is W, the range of 0.86 ≦ W / Wo ≦ 0.96 is set.
[0013]
Here, the relationship between the waveguide length and the electrode finger intersection length will be described. In the conventional transversal SAW filter, the waveguide length Wo and the electrode finger intersection length W are designed to be W = Wo, that is, W / Wo = 1.00, so that a higher-order transverse mode is generated and a pass band is formed. It is considered that the fundamental-order mode (0-order mode) and the higher-order transverse mode interfere with each other to generate ripples, thereby deteriorating insertion loss. Therefore, in the present invention, the ripple and the insertion loss are improved by changing the ratio W / Wo of the waveguide length Wo and the electrode finger intersection length W. FIG. 2 simulates the relationship between the ratio W / Wo of the waveguide length Wo and the electrode finger cross length W and the relationship between the ripple and the insertion loss in the transversal SAW filter of FIG. 1 with the center frequency set to 380 MHz used in the W-CDMA system. 6 is a graph obtained by the following. The ratio of W / Wo was changed by changing the length D of the dummy electrode while the width W of the electrode intersection was fixed at 400 (μm). (A) shows the relationship between W / Wo and the ripple. When W / W = 0.86, the ripple has a minimum value of 0.47 (dB), and is compared with the conventional W / Wo = 1.00. The ripple has been greatly improved. (B) shows the relationship between W / Wo and insertion loss. When W / Wo = 0.86 to 0.88, the insertion loss takes a minimum value of 7.8 (dB), and the conventional W / Wo = This is greatly improved compared to 1.00. In particular, it can be seen that within the range of 0.86 ≦ W / Wo ≦ 0.96, both the ripple and the insertion loss have a small variation in numerical values and take a stable low value.
[0014]
Here, paying attention to 0.65 ≦ W / Wo ≦ 0.75 and 0.75 <W / Wo ≦ 0.85 disclosed in the conventional Japanese Patent Publication No. 6-85492 and JP-A-9-260996, From FIG. 2, it can be seen that the ripple and the insertion loss are unstable due to variations in the numerical values, and in particular, there is no room for the ripple within 1.0 dB of the ripple required for the IF filter for W-CDMA. .
[0015]
When the electrode finger intersection length W is fixed to 400 (μm), the waveguide length Wo is reduced in the conventional cases of 0.65 ≦ W / Wo ≦ 0.75 and 0.75 <W / Wo ≦ 0.85. In the case of 0.86 ≦ W / Wo ≦ 0.96 of the present invention, the waveguide length Wo is as small as 416 to 465 (μm), whereas the chip length is 471 to 615 (μm). It is advantageous. Further, when the waveguide length Wo is fixed at 450 (μm), the electrode finger intersection length W is 0.65 ≦ W / Wo ≦ 0.75 and 0.75 <W / Wo ≦ 0.85 in the related art. In the case of 0.86 ≦ W / Wo ≦ 0.96 of the present invention, the electrode finger intersection length W is as large as 382 to 432 (μm), so that the impedance can be easily reduced. Therefore, there is an advantage that matching with peripheral circuits is easy.
[0016]
As described above, by setting the ratio of the waveguide length Wo of the IDT electrode to the electrode finger intersection length W in the transversal SAW filter in the range of 0.86 ≦ W / Wo ≦ 0.96, the ripple in the pass band can be reduced. It has been found that this is advantageous for reducing the insertion loss and for reducing the size and impedance.
[0017]
FIG. 3 shows measured filter characteristics of the transversal SAW filter according to the present invention shown in FIG. The center frequency is set to 380 MHz applied to the IF filter for W-CDMA, and the piezoelectric substrate is made of Li ₂ B ₄ O ₇ propagating in the Z direction in a 45 ° rotating X-plate. The IDT electrodes 2 and 3 were formed of RSSPUDT, and the electrode finger intersection length W was 400 (μm) and the waveguide length Wo was 440 (μm), that is, W / Wo = 0.91. As is apparent from FIG. 8, the ripple in the pass band is reduced as compared with the conventional filter characteristic shown in FIG. 8, and the minimum insertion loss is greatly improved from about 7.8 dB to about 6.6 dB. You can see that.
[0018]
In the present embodiment, the RSPUDT is used for the IDT electrode of the transversal SAW filter. However, other structures may be used, such as a normal type IDT without weighting, one of reflection and excitation, or reflection and excitation. Needless to say, the present invention can be applied to an IDT electrode in which both are weighted. Further, in the piezoelectric substrate, an example has been described in which a 45 ° rotation X-plate Z-direction propagation Li ₂ B ₄ O ₇ is applied. However, other cut angles may be used, and other piezoelectric materials such as lithium niobate, quartz It is needless to say that the present invention can be applied to, for example, langasite, lithium tetraoxide and the like.
[0019]
【The invention's effect】
As described above, according to the present invention, in the transversal SAW filter, the ratio between the waveguide length Wo of the IDT electrode and the electrode finger crossing length W is set to 0.86 ≦ W / Wo ≦ 0.96, thereby allowing the transit. Since the in-band ripple and insertion loss can be reduced, the manufacturing yield can be improved.
[Brief description of the drawings]
FIG. 1 shows a plan view of a transversal SAW filter according to the present invention.
FIGS. 2A and 2B show the relationship between W / Wo and ripple in the transversal SAW filter according to the present invention, and FIG. 2B shows the relationship between W / Wo and insertion loss.
FIG. 3 shows measured filter characteristics of a transversal SAW filter according to the present invention.
FIG. 4 shows a plan view of a conventional transversal SAW filter.
FIGS. 5A and 5B are diagrams illustrating SPUDT, wherein FIG. 5A is a plan view of a basic section of SPUDT, and FIG. 5B is a plan view of a transversal SAW filter using SPUDT.
6A and 6B are diagrams illustrating a reflection thinning electrode, wherein FIG. 6A is a plan view of a transversal SAW filter in which a basic section of the reflection thinning electrode is applied, and FIG.
FIG. 7 shows a schematic diagram of an RSPUDT.
FIG. 8 shows measured filter characteristics of a conventional transversal SAW filter.
FIG. 9 is an enlarged view of an IDT electrode provided with a dummy electrode.
[Explanation of symbols]
1: Piezoelectric substrate 2, 3: IDT electrode 4: Solid electrode 6: Waveguide portion 7: Electrode finger intersection 8: Dummy electrode portion

Claims (4)

In a transversal surface acoustic wave filter in which two IDT electrodes are arranged at predetermined intervals along a propagation direction of a surface acoustic wave on a piezoelectric substrate, the waveguide length of the IDT electrode is Wo, and the electrode finger intersection length is Where W is 0.85 ≦ W / Wo ≦ 0.96. The IDT electrode has at least one of a basic section having one direction, a basic section having a reflecting function, and a basic section having no reflecting function, when a basic section is one wavelength of the electrode period. The surface acoustic wave filter according to claim 1, wherein the surface acoustic wave filter has a section. The piezoelectric substrate, surface acoustic wave filter according to any one of claims 1 to 2, characterized in that a 45 ° rotation X plate Z-propagating Li ₂ B ₄ O _7. The surface acoustic wave filter according to any one of claims 1 to 3, wherein the surface acoustic wave filter is a W-CDMA IF filter.

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