JP4506394B2 - Unidirectional surface acoustic wave transducer and surface acoustic wave device using the same - Google Patents

Description

  The present invention relates to a unidirectional surface acoustic wave transducer in which a floating electrode is disposed between excitation electrodes and SAW excitation is unidirectional, and a surface acoustic wave device using the same.

  In recent years, surface acoustic wave (hereinafter referred to as SAW) devices have been widely used in the communication field, and are often used particularly for cellular phones and the like because they have excellent characteristics such as high performance, small size, and mass productivity. In addition, with the increase in demand for data communication such as images, IF filters used in mobile phones are required to have a wider band, lower ripple, and better group delay time deviation. Transversal SAW is a filter that satisfies such strict specifications. A filter is suitable.

  FIG. 5 shows a plan view of a conventional transversal SAW filter. An input IDT electrode 102 and an output IDT electrode 103 are arranged on the main surface of the piezoelectric substrate 101 along the SAW propagation direction at a predetermined interval, and between the input / output terminals between the IDT electrodes 102 and 103. A shield electrode 104 for shielding the direct wave is arranged. 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 electrode. Is grounded, one comb electrode of the IDT electrode 103 is connected to the output terminal OUT, and the other comb electrode is grounded. Further, in order to suppress unnecessary reflected waves from the substrate end face, an adhesive material 105 is applied to both ends of the piezoelectric substrate 101 in the long side direction (SAW propagation direction).

  However, in the case of a so-called single (solid) type IDT electrode in which comb-shaped electrodes are arranged in order of positive, negative, and positive as shown in FIG. 5, SAW propagates equally to the left and right along the propagation direction. There was a problem that would become larger.

In order to solve this problem, M. Lewis: Low Loss SAW Devices Employing Single Stage Fabrication, IEEE Ultrason. Symp . Proc ., Pp . 104-108 (1983). A SAW converter (hereinafter referred to as a reflection bank type SAW converter) in which a floating electrode is disposed inside the IDT electrode and the SAW excitation direction is unidirectional is disclosed in Japanese Patent Publication No. 3345609. It was. FIG. 6 shows the reflection bank type SAW converter. The reflection bank type SAW converter 112 includes a single electrode 116 in which positive electrode fingers extending from the bus bar 113 connected to the external terminal 115 and negative electrode fingers extending from the grounded bus bar 114 are alternately arranged, and the bus bar 113, It is comprised from the open type floating electrode 117 which is not electrically connected to either of 114. FIG. When the wavelength of the surface acoustic wave is λ, the distance between the centers of the positive electrode finger and the negative electrode finger of the single electrode 116 and the distance between the centers of the adjacent electrode fingers of the floating electrode 117 are λ / 2. Then, by shifting the position of the floating electrode 117 to the right by λ / 8 from the position away from the center of the single electrode 116, the floating electrode 117 operates as a unidirectional SAW converter in which SAW is strongly excited on the left side in the figure. Deterioration of insertion loss can be prevented.

  However, when the single electrodes 116 are arranged at a period of λ / 2 as described above, there is a problem that SAW reflections are superimposed inside the single electrode 116. When SAW reflection is superimposed, the function as an ideal unidirectional SAW converter is impaired, and strong reflection occurs at either the lower end frequency or upper end frequency of the reflection band of the filter, resulting in an asymmetric transfer response. Therefore, this is a problem in a transversal SAW filter that requires a symmetrical transmission response with respect to the center frequency.

  Therefore, a unidirectional SAW converter using a split electrode as shown in FIG. 7 has been considered. The unidirectional SAW converter 122 is a split electrode in which two positive electrode fingers extending from the bus bar 123 connected to the external terminal 125 and two negative electrode fingers extending from the grounded bus bar 124 are alternately arranged. 126 and an open floating electrode 127 that is not electrically connected to either of the bus bars 123 and 124. The distance between the centers of the positive electrode finger and the negative electrode finger of the split electrode 126 and the distance between the centers of adjacent electrode fingers of the floating electrode 127 are λ / 2. Then, by shifting the position of the floating electrode 127 from the center position of the adjacent split electrode 126 by λ / 2 to the right side by λ / 8, the unidirectional SAW converter in which SAW is strongly excited on the left side in the figure. And a symmetric transfer response can be obtained.

  By the way, the electrode finger width of each of the split electrodes 126 shown in FIG. 7 is normally formed at λ / 8, but when the gap G between the split electrode 126 and the floating electrode 127 approaches zero, it is adjacent to the gap G due to the proximity effect. The SAW reflection at the electrode finger ends is rapidly reduced, and the function as a unidirectional transducer is greatly impaired. Therefore, the gap G needs to be at least λ / 16 or more, and as a result, the width of the floating electrode F1 adjacent to the gap G needs to be λ / 4 or less. However, if the width of the floating electrode F1 is set to λ / 4 or less, there is a problem that the reflection efficiency is deteriorated and the function as the unidirectional SAW converter cannot be sufficiently exhibited.

Further, if the gap G in FIG. 7 is arranged at a distance of 1λ or more, the width of the floating electrode F1 is not limited, but the split electrode 126 and the floating electrode 127 must be adjacent to each other to obtain a transmission response with a steep attenuation gradient. I must. Further, if the distance between the split electrode and the floating electrode is widened by 1λ or more, an increase in chip size is inevitable.
Japanese Examined Patent Publication No. 7-36501 Japanese Patent No. 2984523 Japanese Patent No. 3345609 Japanese Patent Application No. 2004-52322 JP-A-60-263505 M. Lewis: Low Loss SAW Devices Employing Single Stage Fabrication, IEEE Ultrason.Symp.Proc., Pp.104-108 (1983). M. Takeuchi and K. Yamanouchi: New Type of SAW Reflectors and Resonators Consisting of Reflecting Elements with Positive and Negative Reflection Coefficients, IEEE Trans.Ultrason. Ferroelec. Freq. Contr., Vol. 33, No. 4, pp. 369- 374 (1986).

  In order to solve the above problem, the applicant invented a PNR (Positive and Nagative Reflectivity) type unidirectional SAW converter in Japanese Patent Application No. 2004-52322. This is because the floating electrode 127 shown in FIG. No. 4, No. 4, pp. 369-374 (1986). And PNR structure disclosed in JP-A-60-263505. FIG. 8 shows the PNR type unidirectional SAW converter. Positive electrode fingers T1 and T2 extending from the bus bar 132 connected to the external terminal 134, and negative electrode fingers T3 extending from the grounded bus bar 133, The split electrode 135 made of T4 is used as the first basic section, and the PNR 136 made up of the open type floating electrodes O1, O2 and the short type floating electrodes S1, S2 that are not electrically connected to either of the bus bars 132, 133. Is a second basic section, the structure includes at least one of the first and second basic sections. Then, the position of the PNR 136 is shifted to the right by λ / 8 from a position away from the center position of the positive (negative) electrode finger of the adjacent split electrode 135 by λ / 2, that is, according to FIG. 8, the positive electrode finger T1, By setting the distance between the center of T2 and the center of the open type floating electrode O1 to 3λ / 8, it is operated as a unidirectional SAW converter in which SAW is strongly excited on the left side in the figure. Note that the electrode periods of the split electrode 135 and the PNR 136 at this time are equal to λ.

  According to the aforementioned Japanese Patent Application No. 2004-52322, by applying a PNR type unidirectional SAW converter, high reflection efficiency can be obtained even if the width of the floating electrode adjacent to the split electrode is narrowed to λ / 4 or less. It is disclosed that it is effective in improving the manufacturing efficiency and reducing the device size.

  By the way, in the PNR type unidirectional SAW converter, since the phase velocity is different between the split electrode and the PNR, when the unidirectional SAW transducer is formed by combining them, the phase velocity partially changes. As a result, the ripple in the passband and the deviation of the group delay time are deteriorated. Accordingly, an object of the present invention is to reduce the ripple in the passband and realize an excellent group delay time deviation in the PNR type unidirectional SAW converter.

In order to solve the above problems, the invention according to claim 1 of the unidirectional SAW converter according to the present invention and the SAW converter using the SAW converter is provided on a piezoelectric substrate to constitute a surface acoustic wave device. The surface acoustic wave converter has a configuration in which a plurality of basic sections are connected, and the first to fourth four floating electrodes are arranged in order in the first basic section. In addition, the first and third floating electrodes or the second and fourth floating electrodes are short-circuited, and the second basic section is a split electrode in which two positive electrode fingers and two negative electrode fingers are paired. when to that arranged, the first and contains at least one respective second base section, the electrode period of the first basic section .lambda.1, the electrode period of the second basic section and λ2 Λ1 and λ2 are different from each other, and the electrode period deviation The d is characterized by setting to a range of 0 <d ≦ 0.038 is taken as d = (λ1-λ2) / λ1.

  The invention according to claim 2 is characterized in that, in the unidirectional surface acoustic wave transducer, an electrode periodic deviation rate d is set in a range of 0 <d ≦ 0.029.

  The invention described in claim 3 is characterized in that a 128 ° rotated Y-cut X-propagating lithium niobate is used for the piezoelectric substrate.

  The invention described in claim 4 is characterized in that the unidirectional surface acoustic wave transducer is used in a surface acoustic wave device.

  According to the first aspect of the present invention, in the PNR type unidirectional SAW converter, the electrode periods of the PNR and the split electrode are made different from each other, and the electrode period deviation rate d is set to 0 <d ≦ 0.038. Therefore, an excellent group delay time deviation can be realized.

  According to the second aspect of the present invention, in the PNR type unidirectional SAW converter, the electrode periods of the PNR and the split electrode are made different from each other, and the electrode period deviation rate d is set to 0 <d ≦ 0.029. Therefore, excellent group delay time deviation and low ripple can be realized at the same time.

  According to the third aspect of the present invention, by using 128 ° rotated Y-cut X-propagating lithium niobate for the piezoelectric substrate, a wide band characteristic can be realized when a filter is configured.

  According to the invention described in claim 4, by using the unidirectional SAW converter for a SAW device, it is possible to provide a SAW device that realizes an excellent group delay time deviation and low ripple.

  Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings. FIG. 1 shows a part of a PNR type unidirectional SAW converter according to the present invention, and the basic electrode structure is the same as the conventional structure shown in FIG. That is, the PNR type unidirectional SAW converter 1 includes positive electrode fingers T1 and T2 extending from the bus bar 2 connected to the external terminal 4 and negative electrode fingers T3 and T4 extending from the grounded bus bar 3. The split electrode 10 is used as the first basic section, and the PNR 11 including the open type floating electrodes O1 and O2 and the short type floating electrodes S1 and S2 that are not electrically connected to either of the bus bars 2 and 3 is used as the second basic section. When a section is used, the structure includes at least one of the first and second basic sections. In this embodiment, a 128 ° rotated Y-cut X-propagating lithium niobate (LN) substrate capable of realizing a wide band characteristic is used for the piezoelectric substrate.

  A feature of the present invention is that a unidirectional SAW converter is configured by making the electrode period λ1 of the PNR 11 and the electrode period λ2 of the split electrode 10 different from each other. That is, since the phase speeds of the split electrode and the PNR are different, it is estimated that the filter characteristics can be improved by adjusting the electrode period so that the frequencies of both surface waves are the same.

  Here, the electrode period deviation rate d of the PNR 11 with respect to the electrode period λ1 was set to d = (λ1−λ2) / λ1, and the change in the passband ripple when the electrode period deviation rate d was changed was examined. FIG. 2 shows the relationship between the electrode period deviation rate d and the ripple (dB) in the passband. Here, d = 0 means a conventional structure in which the electrode period deviation rate is not changed. From the figure, it can be seen that the ripple is lower than that of the conventional structure when 0 <d ≦ 0.029, and takes the minimum value within the range of 0.01 ≦ d ≦ 0.02. In particular, when d = 0.015, the ripple is about 0.5 dB, and it can be seen that the ripple is improved by 0.8 dB compared with the conventional d = 0 ripple 1.3 dB.

  Therefore, in the PNR type unidirectional SAW converter, the electrode period λ1 of the PNR and the electrode period λ2 of the split electrode are made different from each other, and the electrode period deviation rate d is set to 0 <d ≦ 0.029. It was found that the ripple was smaller than the structure.

  FIG. 3 shows pass characteristics when the electrode period deviation rate d is d = 0.015 in the transversal SAW filter using the PNR type unidirectional SAW converter shown in FIG. The solid line is the characteristic when d = 0.015, and the dotted line is the characteristic of the conventional structure when d = 0 for comparison. The piezoelectric substrate is a 128 ° rotated Y-cut X-propagating LN substrate, and the center frequency is 40 (MHz). As shown in the figure, it can be seen that when d = 0.015, the ripple is smaller and the passband is flat.

  Next, the group delay time deviation of the pass band when the electrode period deviation rate d was changed was examined. FIG. 4 shows the relationship between the electrode period deviation rate d and the group delay time deviation (ns). Here, d = 0 means a conventional structure in which the electrode period deviation rate is not changed. From the figure, it can be seen that the group delay time deviation is smaller than that of the conventional structure in the range of 0 <d ≦ 0.038 and takes the minimum value in the range of 0.01 ≦ d ≦ 0.03. In particular, when d = 0.02, the group delay time deviation takes a minimum value of 96 (ns), and the group delay time deviation is improved by about 20 (ns) compared to the conventional structure where d = 0. I understand that

  From the above, in the PNR type unidirectional SAW converter, the electrode period λ1 of the PNR and the electrode period λ2 of the split electrode are made different from each other, and the electrode period deviation rate d is set to 0 <d ≦ 0.038. It was found that the group delay time deviation was smaller than that of the conventional structure. If the electrode period deviation rate d is set to 0 <d ≦ 0.029, both ripple and group delay time deviation can be reduced simultaneously.

  So far, an example using 128 ° rotated Y-cut X-propagating lithium niobate for the piezoelectric substrate has been described. However, the present invention is not limited to this, and the piezoelectric substrate is made of crystal, lithium tantalate, lithium tetraborate, Langa Needless to say, the present invention can be applied to a site or the like. Needless to say, the unidirectional SAW converter of the present invention can be applied to filter types other than the transversal SAW filter.

It is a figure explaining the PNR type unidirectional SAW converter concerning the present invention. It is the figure which showed the change of the ripple with respect to the electrode period deviation rate d. It is a figure explaining the comparison of the passage characteristic of the PNR type unidirectional SAW converter concerning the present invention, and the conventional structure. It is the figure which showed the change of the group delay time deviation with respect to the electrode period deviation rate d. It is a figure explaining the conventional transversal SAW filter. It is a figure explaining the conventional reflective bank type unidirectional SAW converter. It is a figure explaining the reflection bank type unidirectional SAW converter which used the split electrode for the conventional excitation electrode. It is a figure explaining the conventional PNR type | mold unidirectional SAW converter.

Explanation of symbols

1: PNR type unidirectional SAW converter 2, 3: Bus bar 4: External terminal 10: Split electrode 11: PNR
T1, T2: Positive electrode fingers T3, T4: Negative electrode fingers O1, O2: Open type floating electrodes S1, S2: Short-circuit type floating electrodes

Claims (4)

A surface acoustic wave converter for constituting a surface acoustic wave element by being arranged on a piezoelectric substrate,
The surface acoustic wave transducer has a configuration in which a plurality of basic sections are connected, and the first and fourth floating electrodes are arranged in order in the first basic section and the first and third floating electrodes are arranged in order. Or the second and fourth floating electrodes are short-circuited, and when the second basic section is a split electrode in which two positive electrode fingers and two negative electrode fingers are paired, Including at least one each of the first and second basic sections,
When the electrode period of the first basic section is λ1, and the electrode period of the second basic section is λ2, λ1 and λ2 are different from each other, and the electrode period deviation rate d is d = (λ1-λ2) / A unidirectional surface acoustic wave transducer, wherein λ1 is set in a range of 0 <d ≦ 0.038.   2. The unidirectional surface acoustic wave transducer according to claim 1, wherein the electrode periodic deviation rate d is set in a range of 0 <d ≦ 0.029 in the unidirectional surface acoustic wave transducer.   The unidirectional surface acoustic wave transducer according to claim 1 or 2, wherein a 128 ° rotated Y-cut X-propagating lithium niobate is used for the piezoelectric substrate.   A surface acoustic wave device using the unidirectional surface acoustic wave transducer according to claim 1.

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