JP2005167571A - Transversal surface acoustic wave filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the amount of attenuation in the blocking band of a position, at which a frequency is separated from a pass band in a transversal SAW filter, where an IDT electrode for input and an IDT electrode for output are arranged on a piezoelectric substrate with a prescribed interval. <P>SOLUTION: The IDT electrode 2 for input and the IDT 3 for output are arranged with a prescribed gap on the piezoelectric substrate 1, and a shielding electrode 4 for shielding the direct wave between I/O terminals is arranged at the gap. The shielding electrode 4 is formed in a grating shape and is set so that the electrode cycle λs of the shielding electrode 4 becomes 0.964 times larger than the electrode cycle λi of the IDT electrode. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a transversal surface acoustic wave filter in which an input IDT electrode and an output IDT electrode are arranged on a piezoelectric substrate with a predetermined interval therebetween, and a stop band away from a pass band is highly attenuated. It relates to a wave filter.

  In recent years, surface acoustic wave (SAW) filters have been widely used in the field of mobile communication, and have many features such as high performance, small size, and mass productivity. It is used. The IF SAW filters used in these devices are small, light, broadband, and low loss, and also require a steep attenuation gradient to block adjacent carrier frequencies. Further, the IF SAW filter is required to have phase linearity in order to maintain high-speed digital data quality. A transversal SAW filter that can design amplitude characteristics and phase characteristics separately is most suitable as a filter type that satisfies such required specifications.

  FIG. 6 is a plan view showing a configuration of a conventional transversal SAW filter, in which the input IDT electrode 102 and the output IDT electrode 103 are spaced apart from each other along the SAW propagation direction on the main surface of the piezoelectric substrate 101. And a shield electrode 104 for shielding a direct wave between the input and output terminals is disposed between the IDT electrodes 102 and 103. The IDT electrodes 102 and 103 are composed of a pair of comb electrodes having a plurality of electrode fingers interleaved with each other, and one comb electrode of the IDT electrode 102 is connected to the input terminal IN and the other comb is formed. The electrode is grounded. Further, a comb electrode is connected to one of the IDT electrodes 103 to the output terminal OUT, and the other comb electrode is grounded. Further, in order to suppress unnecessary reflected waves from the end face of the substrate, the sound absorbing material 105 is applied to both ends of the piezoelectric substrate 101 in the long side direction (SAW propagation direction).

  Incidentally, a rectangular solid electrode as shown in FIG. 6 is generally used as the shape of the shield electrode 104. However, when such a solid electrode is used, SAW reflection occurs at the end face of the shield electrode, and the IDT. There has been a problem that unnecessary waves are mixed in the electrode and ripples are generated in the filter characteristics. In addition, an attempt has been made to reduce the ripple by making the shape of the shield electrode a parallelogram so that the end face is slanted to prevent unwanted waves caused by SAW reflection from being mixed into the IDT electrode. The effect of reflection could not be completely eliminated.

  In order to solve the above problem, a grating type shield electrode has been devised. As shown in FIG. 7, the shield electrode is an electrode having a width of λi / 8 so that the distance between the centers of adjacent electrode fingers is λi / 4 when the electrode period of the IDT electrodes 112 and 113 is λi. This is a structure in which fingers are sequentially arranged. The electrode period of the shield electrode and the electrode period of the IDT electrode are set equal. Thus, since the reflection which arises in the end surface of a shield electrode is canceled by forming a shield electrode with a grating, generation | occurrence | production of the ripple resulting from an unnecessary wave can be prevented.

Further, in the grating type shield electrode, the amount of attenuation in the vicinity of the passband is obtained by weighting the SAW reflection by shifting the distance between the electrode finger centers of the shield electrode or changing the width of the electrode finger. Can be increased.
Japanese Patent Application No. 7-142779

  The problem to be solved by the present invention is that, in a transversal SAW filter in which a shield electrode is disposed between an input IDT electrode and an output IDT electrode, the shield electrode is made a grating type so that the attenuation in the vicinity of the passband is obtained. However, the effect can be obtained only in the vicinity of the pass band, and it is difficult to highly attenuate the stop band whose frequency is away from the pass band.

  In order to solve the above-mentioned problems, the invention according to claim 1 of the transversal SAW filter according to the present invention is such that at least two IDT electrodes are arranged on the piezoelectric substrate with a predetermined gap therebetween, and direct contact between the input and output terminals. In the transversal surface acoustic wave filter in which a shield electrode for shielding waves is arranged in the gap between the IDT electrodes, the shield electrode is formed in a grating shape, and the electrode period λs of the shield electrode is the electrode of the IDT electrode The transversal surface acoustic wave filter is characterized by being different from the period λi.

  2. The transversal surface acoustic wave filter according to claim 1, wherein the electrode period λs of the shield electrode is approximately 0.964 times the electrode period λi of the IDT electrode. It is.

  The invention according to claim 3 is the shield electrode according to claim 1 or 2, wherein the shield electrode is a shield electrode in which electrode fingers are sequentially arranged with a distance between centers of adjacent electrode fingers being λs / 2. This is a transversal surface acoustic wave filter.

  According to a fourth aspect of the present invention, in the shield electrode, the distance between the centers of some adjacent electrode fingers of the shield electrode is 3λs / 4 or 3λs / 8, and the distance between the centers of the other electrode fingers is 3λs / 4. The transversal surface acoustic wave filter according to claim 1, wherein the shield electrodes are sequentially arranged as λs / 2.

  According to a fifth aspect of the present invention, in the shield electrode, the distance between the centers of some adjacent electrode fingers having the shield electrode is set to 3λs / 4, and the distance between the electrode fingers having the center distance of 3λs / 4 is set. 3. The shield electrode according to claim 1, wherein one electrode finger having a width of λs / 4 or less is arranged, and the other electrode fingers are shield electrodes sequentially arranged with a center-to-center distance of λs / 2. This is a transversal surface acoustic wave filter.

  According to a sixth aspect of the present invention, the shield electrode has a structure in which electrode fingers are sequentially arranged so that the distance between the centers of adjacent electrode fingers is λs / 2, and one of the electrode fingers is thinned out. 3. The transversal surface acoustic wave filter according to claim 1, wherein the transversal surface acoustic wave filter has a structure in which two electrode fingers having a width of λs / 4 or less are arranged at a position. 4.

  The present invention relates to a transversal SAW filter in which a shield electrode for shielding a direct wave between input and output terminals is arranged between an input IDT electrode and an output IDT electrode on a piezoelectric substrate, the shield electrode and the IDT electrode By making both electrode periods different from each other, the stop band far from the pass band can be highly attenuated.

  Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings. FIG. 1 is a plan view of a transversal SAW filter according to a first embodiment of the present invention. An input IDT electrode 2 and an output IDT electrode 3 are arranged on the main surface of the piezoelectric substrate 1 along the SAW propagation direction with a predetermined interval, and between the input / output terminals between the IDT electrodes 2 and 3. A grating type shield electrode 4 is disposed to shield the direct wave. The IDT electrodes 2 and 3 are composed of a pair of comb electrodes having a plurality of electrode fingers interleaved with each other, and one comb electrode of the IDT electrode 2 is connected to the input terminal IN and the other comb electrode is formed. The electrode is grounded. The comb electrode is connected to the output terminal OUT on one side of the IDT electrode 3, and the other comb electrode is grounded. Further, in order to suppress unnecessary reflected waves from the substrate end face, the sound absorbing material 5 is applied to both ends of the piezoelectric substrate 1 in the long side direction (SAW propagation direction). A feature of the present invention is that the electrode period λs of the shield electrode 4 and the electrode period λi of the input IDT electrode 2 and the output IDT electrode 3 are made different. The shield electrode 4 is set such that the electrode period λs of the shield electrode 4 is set to λs = 0.964λi with respect to the electrode period λi of the IDT electrodes 2 and 3, and an electrode finger having a width of 3λs / 8 is set to λs / 4. The structure is arranged sequentially with a gap of.

  FIG. 2 shows a comparison of the pass characteristics of the transversal SAW filter of the present invention product and the conventional product. The solid line in FIG. 2 shows the pass characteristic of the transversal SAW filter according to the present invention. As shown in FIG. 1, the shield electrode 4 has 3λs / 8 width electrode fingers arranged at intervals of λs / 4. In this structure, λs = 0.964λi. Also, the dotted line in FIG. 2 shows the pass characteristic of the conventional transversal SAW filter. As shown in FIG. 7, the shield electrode 114 has λs / 8 width electrode fingers arranged at intervals of λs / 8, The electrode period is the same as the electrode period of the IDT electrode. Both the present invention product and the conventional product use a quartz crystal substrate as the piezoelectric substrate, the center frequency fo is around 200 MHz, the number of electrode finger pairs of the input IDT electrodes 2 and 112 is 102, and the electrodes of the output IDT electrodes 3 and 113 are both The number of finger pairs was 177, the number of electrode fingers of the shield electrodes 4 and 114 was 80, and the electrode period λi was about 15 μm. From FIG. 2, it can be seen that the transversal SAW filter of the present invention has higher attenuation at the A portion where the frequency is away from the center frequency. This is considered to be due to the fact that the SAW excitation wave is changed by making the electrode period of the shield electrode different from the electrode period of the IDT electrode, and the effect of not selectively transmitting the frequency near the portion A is obtained.

  As described above, in the conventional structure, it was not possible to increase the attenuation with respect to the stop band at a position away from the pass band, but by separating the electrode period of the shield electrode from the electrode period of the IDT electrode, it is separated from the pass band. It was found that the attenuation can be increased with respect to the stopband at the position.

  Further, in the transversal SAW filter of the present invention, the structure of the shield electrode conventionally used for shielding the direct wave between the input and output terminals can be changed without increasing the number of IDT electrodes or increasing the chip size. As a result, it is possible to reduce the filter size.

  Even in the structure other than the shield electrode as shown in FIG. 1, it is possible to highly attenuate the stop band whose frequency is separated from the pass band by making the electrode periods of the IDT electrode and the shield electrode different. Other embodiments will be described in detail below.

  FIG. 3 shows a transversal SAW filter according to a second embodiment of the present invention. The shield electrodes 14 and 24 shown in the figure are electrode fingers having a width of about λs / 4 so that the distance between the centers of adjacent electrode fingers is λs / 2 when the electrode period of the shield electrode is λs. It is characterized by sequentially arranging and shifting the distance between some electrode finger centers. Here, the shield electrode 14 in (a) shifts the distance between some electrode finger centers from λs / 2 to 3λs / 4, and the shield electrode 24 in (b) sets the distance between some electrode finger centers to λs. / 2 to 3λs / 8. In both (a) and (b), the electrode period λs of the shield electrode is set so that λs = 0.964λi with respect to the electrode period λi of the IDT electrode. In this way, the reflection band can be widened by shifting the distance between the electrode finger centers of some of the shield electrodes and weighting the reflection of the SAW, so that the stop band at a position away from the pass band can be widened. It is possible to achieve high attenuation over the range.

  By the way, in the structure of the shield electrode shown in the second embodiment, the reflection band can be broadened, but there is a possibility that distortion is generated in the pass band due to strong reflection of SAW in the shield electrode. Therefore, the inventors of the present application have considered a shield electrode in which the SAW reflection amount in the shield electrode is adjusted by adjusting the width of some electrode fingers constituting the shield electrode. Hereinafter, a third embodiment will be described in detail.

  FIG. 4 shows a transversal SAW filter according to a third embodiment of the present invention. The shield electrodes 34 and 44 shown in the figure are electrode fingers having a width of about λs / 4 so that the distance between the centers of adjacent electrode fingers is λs / 2 when the electrode period of the shield electrode is λs. It is characterized by being sequentially arranged and changing the width of some electrode fingers. Here, the shield electrode 34 of (a) thins out one electrode finger in the B part, and two electrode fingers having a width of λs / 4 or less are arranged at the position, and the shield electrode 34 of (b) 44, the distance between the electrode finger centers is shifted from λs / 2 to 3λs / 4 in part C, and one electrode finger having a width of λs / 4 or less is arranged between the shifted electrode fingers. In both (a) and (b), the electrode period λs of the shield electrode is set to be λs = 0.964λi with respect to the electrode period λi of the IDT electrode. As described above, the amount of SAW reflection in the shield electrode can be adjusted by changing the width of a part of the electrode fingers of the shield electrode. Thus, high attenuation can be achieved and distortion can be prevented from entering the passband.

  So far, only the case where the electrode periods of the IDT electrode and the shield electrode are uniform has been mentioned, but in the transversal SAW filter, there are many methods for improving the filter characteristics by locally changing the electrode period. Used. In the present invention, it is possible to apply even when such an electrode period is not uniform. Hereinafter, a fourth embodiment will be described in detail.

  FIG. 5 shows a schematic diagram of a transversal SAW filter according to a fourth embodiment of the present invention. In the transversal SAW filter of FIG. 5A, the input IDT electrode 52 and the output IDT electrode 53 are composed of the section X composed of the electrode fingers 10 pairs of the electrode period λi1 and the electrode fingers 20 pairs of the electrode period λi2. Section Y. Thus, when the IDT electrodes are configured with different electrode periods, the electrode period λs of the shield electrode is 0.964 times the average electrode period λi ′ of the IDT electrodes, that is, λi ′ = (10 × λi1 + 20 × λi2) / 30. By setting to, it is possible to make the stop band at a position away from the pass band high attenuation.

  Further, in the transversal SAW filter of FIG. 5B, the shield electrode 64 is composed of a section S1 composed of a pair of electrode fingers having an electrode period λs1 and a section S2 composed of a pair of electrode fingers having an electrode period λs2. As described above, when the shield electrodes are configured with different electrode periods, the average electrode period λs ′ of the shield electrodes, that is, λs ′ = (10 × λs1 + 20 × λs2) / 30 is the input IDT electrode and the output IDT electrode. By setting the electrode period of the shield electrode so as to be 0.964 times the electrode period λi, it is possible to highly attenuate the stop band at a position away from the pass band.

1 is a plan view of a transversal SAW filter according to a first embodiment of the present invention. The comparison of the pass characteristic of the transversal SAW filter concerning the 1st example of the present invention and the conventional transversal SAW filter is shown. It is a figure explaining the shield electrode which changed the distance between some electrode finger centers concerning the 2nd example of the present invention. It is a figure explaining the shield electrode which changed a part of electrode finger width concerning the 3rd example of the present invention. It is a figure explaining the transversal SAW filter at the time of comprising the IDT electrode or shield electrode which concerns on the 4th Example of this invention with a different electrode period. It is a figure explaining the conventional transversal SAW filter. It is a figure explaining the conventional grating type shield electrode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Piezoelectric substrate 2, 52, 62 Input IDT electrode 3, 53, 63 Output IDT electrode 4, 14, 24, 34, 44, 54 Shield electrode 5 Sound absorbing material

Claims (6)

In a transversal surface acoustic wave filter in which at least two IDT electrodes are arranged on a piezoelectric substrate with a predetermined gap therebetween, and a shield electrode for shielding a direct wave between input and output terminals is arranged in the gap between the IDT electrodes. ,
The transversal surface acoustic wave filter according to claim 1, wherein the shield electrode is formed in a grating shape, and the electrode period λs of the shield electrode is different from the electrode period λi of the IDT electrode.   2. The transversal surface acoustic wave filter according to claim 1, wherein an electrode period λs of the shield electrode is approximately 0.964 times an electrode period λi of the IDT electrode.   The transversal surface acoustic wave filter according to claim 1, wherein the shield electrode is a shield electrode in which electrode fingers are sequentially arranged with a distance between centers of adjacent electrode fingers being λs / 2.   The shield electrode is a shield in which the distance between the centers of some adjacent electrode fingers with the shield electrode is set to 3λs / 4 or 3λs / 8, and the other electrode fingers are sequentially arranged with the distance between the centers set to λs / 2. The transversal surface acoustic wave filter according to claim 1, wherein the transversal surface acoustic wave filter is an electrode.   The shield electrode is an electrode having a width of λs / 4 or less between electrode fingers having a center-to-center distance of 3λs / 4 and a center-to-center distance of 3λs / 4. The transversal surface acoustic wave filter according to claim 1 or 2, wherein one finger is arranged and the other electrode fingers are shield electrodes arranged sequentially with a center-to-center distance of λs / 2. The shield electrode is based on a structure in which electrode fingers are sequentially arranged so that the distance between the centers of adjacent electrode fingers is λs / 2, and one of the electrode fingers is thinned out, and a width of λs / 4 or less is formed at the position. The transversal surface acoustic wave filter according to claim 1, wherein the transversal surface acoustic wave filter has a structure in which two electrode fingers are arranged.

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