JP4457914B2 - 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 having a floating electrode therein 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. Due to the increasing demand for data communication such as images, IF filters used in mobile phones are required to have a wider bandwidth, lower ripples, and a steep attenuation gradient. Transversal SAW filters are suitable as filters that satisfy such strict specifications. Yes.

  FIG. 15 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. In order to suppress unnecessary reflected waves from the substrate end face, the sound absorbing 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 regular (single) type IDT electrode in which comb-shaped electrodes are arranged in order of positive, negative, and positive as shown in FIG. 15, SAW propagates equally to the left and right along the propagation direction. There was a problem of getting bigger.

  In order to solve this problem, there is a unidirectional SAW converter in which floating electrodes are arranged inside a so-called reflective bank type as disclosed in Japanese Patent No. 2085072, Japanese Patent No. 2984523, and Japanese Patent No. 3345609. it was thought. FIG. 16 is a plan view of the reflective bank type unidirectional SAW converter. The unidirectional SAW converter 112 includes a single electrode 120 in which positive electrode fingers extending from the positive bus bar 113 connected to the external terminal 115 and negative electrode fingers extending from the grounded negative bus bar 114 are alternately arranged, It is composed of an open type floating electrode 121 that is not electrically connected to either of the bus bars 113 and 114. When the fundamental 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 120 and the distance between the centers of the adjacent electrode fingers of the floating electrode 121 are λ / 2. Then, by shifting the position of the floating electrode 121 from the position away from the center of the single electrode 120 by λ / 2 to the right by λ / 8, it operates as a unidirectional converter in which SAW is strongly excited on the left side in the figure. Loss deterioration can be prevented.

  However, when the single electrodes 120 are arranged at a period of λ / 2 as described above, there is a problem that SAW reflections are superimposed on the single electrodes 120. When SAW reflection is superimposed, the function as an ideal unidirectional converter is lost, 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. 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. 17 has been considered. The unidirectional SAW converter 132 alternates between two positive electrode fingers extending from the positive bus bar 133 connected to the external terminal 135 and two negative electrode fingers extending from the grounded negative bus bar 134. The split electrode 140 is disposed and an open floating electrode 141 that is not electrically connected to either of the bus bars 113 and 114. Further, the distance between the centers of the positive electrode finger and the negative electrode finger of the split electrode 140 and the distance between the centers of adjacent electrode fingers of the floating electrode 141 are λ / 2. Then, by shifting the position of the floating electrode 141 to the right side by λ / 8 from the position separated by λ / 2 from the center position of the adjacent split electrode, a unidirectional SAW converter in which SAW is strongly excited on the left side in the figure. Operate and obtain a symmetric transfer response.

  In the above description, the open type floating electrode is used. However, as shown in FIG. 18, the short-circuit type floating electrode in which the ends of the electrode fingers are electrically connected and the distance between the centers of the adjacent electrode fingers is λ / 2. A unidirectional SAW converter with 151 is also known.

  By the way, although the electrode finger width of the split electrode 140 shown in FIG. 17 is usually formed at λ / 8, when the gap G between the split electrode 140 and the floating electrode 141 approaches zero, the electric power adjacent to the gap G is caused by the proximity effect. Extreme SAW reflection 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.

Here, the relationship between the electrode width of the floating electrode, the reflection efficiency, and the film thickness dependency was examined. FIG. 19 shows the relationship between the open-type and short-circuit type floating electrode widths and the reflection efficiency when the electrode film thickness H is changed using 128 ° rotated Y-cut X-propagating LiNbO ₃ for the piezoelectric substrate. The electrode film thickness and electrode width are expressed in terms of wavelength, and the reflection efficiency is represented by the mode coupling coefficient κ ₁₂ ′ of the mode coupling theory. The solid line represents the electrode film thickness of 0.25% λ, and the broken line represents the electrode film. The characteristic is a thickness of 1.00% λ. As is apparent from the figure, the reflection efficiency magnitude | κ ₁₂ ′ | tends to increase as the electrode width increases in the open type, and as the electrode width decreases in the short-circuit type. As described above, in the background where the width of the floating electrode adjacent to the split electrode is limited to λ / 4 (= 0.25λ) or less, the short-circuit type has better reflection efficiency than the open type. However, it is understood that when the electrode film thickness is increased, the reflection efficiency of the short-circuit type floating electrode is greatly deteriorated.

  Therefore, when an open type floating electrode is used in a unidirectional SAW converter, the reflection efficiency is reduced due to the limitation of the electrode width, and when the short type floating electrode is used, the reflection efficiency is lowered when the film thickness is increased, There arises a problem that the function as the unidirectional SAW converter is impaired.

Further, if the gap G in FIG. 17 is spaced apart by 1λ or more, the width of the floating electrode F1 is not limited, but the split electrode 140 and the floating electrode 141 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, there arises a problem that an increase in chip size is unavoidable.
Japanese Patent No. 2085072 Japanese Patent No. 2984523 Japanese Patent No. 3345609 Japanese Patent No. 3175830 JP-A-60-263505 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).

  The problem to be solved by the present invention is that, in a conventional unidirectional SAW converter in which an open type or short type floating electrode is arranged, if an open type floating electrode is used, the reflection efficiency is reduced due to the limitation of the electrode width. If the short-circuit type floating electrode is used, the reflection efficiency decreases when the film thickness is increased. Therefore, there is a problem that the passband has a single peak characteristic and the bandwidth, flatness, and steepness deteriorate. It was. Further, when the distance between the excitation electrode and the floating electrode is increased, there is a problem that the chip size becomes large. In the present invention, in order to solve the above-described problem, in a unidirectional SAW converter having a floating electrode disposed therein and a SAW device using the SAW device, widening, flattening, and high steepness can be achieved by making the passband square. It aims at realizing and reducing the device size.

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 device using the same is provided on a piezoelectric substrate to constitute a surface acoustic wave element. A surface acoustic wave converter, wherein the surface acoustic wave converter has a configuration in which a plurality of basic sections having a width corresponding to the wavelength λ of the surface acoustic wave to be excited are connected. It is assumed that a split electrode having a pair of two positive electrode fingers and two negative electrode fingers is arranged in the section, and the first and fourth four floating electrodes are arranged in order in the second basic section and the first and Short-circuiting only the floating electrodes between the third floating electrodes or between the second and fourth floating electrodes, and including at least one of the first and second basic sections, Two positive electrode fingers or negative electrode fingers constituting one basic section The distance between the center position of the book and the center position of the first floating electrode is 3λ / 8 ± nλ / 2 (where n = 0, 1, 2, 3,...). It is said.

  According to a second aspect of the present invention, there is provided a surface acoustic wave converter for constituting a surface acoustic wave element disposed on a piezoelectric substrate, wherein the surface acoustic wave converter has a wavelength of the surface acoustic wave to be excited. It is assumed that a plurality of basic sections having a width corresponding to λ are connected, and the first basic section is provided with a split electrode in which two positive electrode fingers and two negative electrode fingers are paired. In the second basic section, the first to fourth four floating electrodes are arranged in order, the first and third floating electrodes are short-circuited, and the second and fourth floating electrodes are opened. When the first to fourth four floating electrodes are arranged in order in the basic section of 3 and the first and third floating electrodes are opened and the second and fourth floating electrodes are short-circuited, Includes at least one of first to third basic sections, and constitutes the first basic section The distance between the center position of two positive electrode fingers or two negative electrode fingers and the center position of the first floating electrode is 3λ / 8 ± nλ / 2 (where n = 0, 1, 2, 3,...・).

  According to a third aspect of the present invention, there is provided a surface acoustic wave converter for constituting a surface acoustic wave element disposed on a piezoelectric substrate, wherein the surface acoustic wave converter has a wavelength of the surface acoustic wave to be excited. It is assumed that a plurality of basic sections having a width corresponding to λ are connected, and the first basic section is provided with a split electrode in which two positive electrode fingers and two negative electrode fingers are paired. In the second basic section, the first to fourth four floating electrodes are arranged in order, the first and third floating electrodes are short-circuited, and the second and fourth floating electrodes are opened. The first to fourth four floating electrodes are arranged in order in the three basic sections, the first and third floating electrodes are opened, and the second and fourth floating electrodes are short-circuited. A split electrode in which the basic section is a pair of the two positive electrode fingers and the two negative electrode fingers, When the basic interval of 1 and the positive / negative arrangement are reversed, and the fifth basic interval is constituted by only positive electrode fingers or only negative electrode fingers while maintaining the electrode finger arrangement position of the split electrode, It includes at least two types of basic sections among the first, fourth, and fifth basic sections and includes at least one basic section of the second and third basic sections, and is a positive electrode constituting the split electrode. The distance between the center position of two electrode fingers or two negative electrode fingers and the center position of the first floating electrode is 3λ / 8 ± nλ / 2 (where n = 0, 1, 2, 3,... ).

  The invention described in claim 4 is characterized in that the split electrode and the first to fourth floating electrodes have an electrode finger width of λ / 8 ± λ / 40.

  The invention described in claim 5 is characterized in that the electrode finger width of the split electrode and the electrode finger widths of the first to fourth floating electrodes are different from each other.

  The invention described in claim 6 is characterized in that the electrode finger widths of the first and third floating electrodes are different from the electrode finger widths of the second and fourth floating electrodes.

  The invention described in claim 7 is characterized in that an electrode period of the split electrode and an electrode period of the first to fourth floating electrodes are different from each other.

  According to an eighth aspect of the present invention, the distance from the center position of the two positive electrode fingers of the split electrode or the center position of the two negative electrode fingers to the center position of the first floating electrode is 3λ / 8 ± nλ. / 2 + σ (where n = 0, 1, 2, 3,..., Σ ≦ λ / 16).

  The invention according to claim 9 is characterized in that both ends of the first and third floating electrodes or both ends of the second and fourth floating electrodes are conductively connected.

  The invention described in claim 10 is characterized in that, in the split electrode, weighting is applied in the cross width direction of the positive electrode finger and the negative electrode finger along the propagation direction of the surface acoustic wave.

  According to an eleventh aspect of the present invention, in the first to fourth floating electrodes, the length direction of the floating electrode is weighted along the propagation direction of the surface acoustic wave.

The invention described in claim 12 is characterized in that 128 ° rotated Y-cut X-propagating LiNbO ₃ is used for the piezoelectric substrate.

  A thirteenth aspect of the invention is a surface acoustic wave device using the unidirectional surface acoustic wave transducer according to any one of the first to twelfth aspects.

  According to the first aspect of the present invention, high reflection efficiency can be obtained without depending on the electrode film thickness, and the chip size can be reduced because it is not necessary to increase the distance between the split electrode and the floating electrode. In addition, compared with the conventional SAW device, since the pass band is square, the flatness is improved, and a low loss and a wide band can be realized.

  According to the second and third aspects of the invention, high reflection efficiency can be obtained without depending on the electrode film thickness, and the chip size can be reduced because it is not necessary to increase the distance between the split electrode and the floating electrode. Further, the steepness of the pass band is higher than that of the invention described in claim 1, and the manufacturing yield can be improved.

  According to the invention described in claim 4, since the high reflection efficiency can be obtained by setting the width of the split electrode and the floating electrode to λ / 8 ± λ / 40, it is necessary to keep the distance between the split electrode and the floating electrode by 1λ or more. The chip size can be reduced.

  According to the fifth to seventh aspects of the present invention, the unidirectionality can be further improved by making the electrode finger widths or electrode periods of the split electrode and the floating electrode different from each other.

  According to the eighth aspect of the present invention, since the distance between the excitation center and the reflection center of the unidirectional SAW converter is corrected, deterioration of the unidirectional characteristic can be prevented and high performance of the device can be realized. .

  According to the invention described in claim 9, even when both ends of the first and third floating electrodes or both ends of the second and fourth floating electrodes are conductively connected, The same effects as those of the invention described in (1) can be obtained.

  According to the tenth and eleventh aspects of the present invention, it is possible to improve the performance of the device by weighting the split electrode or the floating electrode.

According to the twelfth aspect of the invention, by using the 128 ° rotated Y-cut X-propagating LiNbO ₃ having a large electromechanical coupling coefficient for the piezoelectric substrate, a wide band characteristic can be realized when a filter or the like is configured.

  According to the thirteenth aspect of the present invention, when the SAW device is configured using the unidirectional SAW converter according to the first to twelfth aspects, the device size can be reduced, the pass characteristic is reduced, and the broadband And flattening and high steepness can be realized.

  Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings. FIG. 1 shows a unidirectional SAW transducer according to the present invention. The unidirectional SAW converter 1 has a pair of positive electrode fingers T1 and T2 extending from the positive bus bar 2 connected to the external terminal 4 and negative electrode fingers T3 and T4 extending from the negative bus bar 3 grounded. The split electrode 10 and the reflective electrode 11 made of floating electrodes O1, O2, S1, and S2 that are not electrically connected to either of the bus bars 2 and 3 are configured. When the fundamental wavelength of SAW is λ, the width of each electrode finger of the split electrode 10 and the reflective electrode 11 is λ / 8, the center positions of the positive electrode fingers T1, T2 of the split electrode 10 and the negative electrode fingers T3, T4 Is λ / 2, and the distance between the centers of the floating electrodes O1, O2, S1, and S2 of the reflective electrode 11 is λ / 4. Then, the position of the reflective electrode 11 is shifted to the right side by λ / 8 from the position away from the center position of the adjacent split electrode 10, that is, the center of the positive electrode fingers T1, T2 and the center of the floating electrode O1. Is made to operate as a unidirectional converter in which SAW is strongly excited on the left side in the figure. The feature of the present invention is that the split electrode 10 is used as the excitation electrode, the floating electrodes O1 and O2 of the reflective electrode 11 function as an open type grating, and the floating electrodes S1 and S2 are electrically connected to each other to resonate with each other. That is, it is made to function as a short-circuit grating so that the potential becomes zero in the vicinity.

  The reflective electrode 11 has the same structure as PNR (Positive and Nagative Reflectivity) disclosed in Non-Patent Document 1 and Patent Document 5. According to Non-Patent Document 1 and Patent Document 5, it is disclosed that by applying the PNR structure to a reflector of a SAW resonator, mode conversion to a bulk wave can be reduced and reflection efficiency can be increased. On the other hand, in the present invention, in the unidirectional SAW converter in which the conventional open type or short-circuit type floating electrode is arranged, the PNR structure is arranged instead of the open type or short-circuit type floating electrode. The reflection efficiency of the unidirectional SAW converter was increased to improve the pass characteristics.

  Here, the operation principle of the unidirectional SAW converter of the present invention will be described. The PNR 11 is formed of open floating electrodes O1 and O2 having a width of λ / 8 and short-circuit floating electrodes S1 and S2, and as is clear from the relationship of FIG. 19, the open floating electrodes O1 and O2 are The short-circuit type floating electrodes S1 and S2 have a negative reflection coefficient, and have a positive reflection coefficient. SAW reflections generated between these floating electrodes are overlapped, and the reflection center can be said to be on the center R of the floating electrode S1 in FIG. When a plurality of reflection sources are viewed from a certain reference position, if the plurality of reflection sources are on the period of λ / 2, the reflected waves are all added in phase, so that the reflection center R is R ′ shifted by λ / 2. It can be virtually assumed that there is a reflection center at the position. On the other hand, the excitation center T of the split electrode 10 is considered to be the center position of the positive electrode fingers T1 and T2 connected to the bus bar 2, and the interval between the excitation center T and the reflection center R ′ is λ / 8. In the figure, the left side functions as an ideal unidirectional SAW converter in which the SAW is strongly excited.

  By the way, when the split electrode 10 and the PNR 11 are adjacent to each other, that is, when the gap G between the positive electrode finger T1 and the floating electrode O1 approaches zero, the function as the unidirectional converter is greatly impaired, so the width of the floating electrode O1 is λ As described above, it is limited to / 4 (0.25λ) or less.

Here, the relationship between the electrode width of PNR11, the reflection efficiency, and the film thickness dependency was examined. FIG. 2 shows the relationship between the electrode width of the PNR 11 and the reflection efficiency κ ₁₂ ′ when the electrode film thickness H is changed using 128 ° rotated Y-cut X-propagating LiNbO ₃ for the piezoelectric substrate. The electrode film thickness and the electrode width are expressed in terms of wavelength, and the reflection efficiency is expressed by the mode coupling coefficient κ ₁₂ ′ of the mode coupling theory. The solid line is H = 0.25% λ, and the broken line is H = 1. The characteristic is 0.00% λ. As is apparent from the comparison with the characteristics of the conventional open type or short type floating electrode shown in FIG. 19, the PNR 11 has almost no reflection efficiency even when the electrode film thickness is increased from 0.25% λ to 1.00% λ. It does not change. That is, it can be seen that the PNR 11 has very little film thickness dependency. Furthermore, even when the electrode width is reduced to 0.25λ or less, a high reflection efficiency of | κ ₁₂ ′ | of about 0.010 is obtained.

  As described above, the unidirectional SAW converter according to the present invention does not deteriorate the reflection efficiency even when the film thickness changes, and the floating electrode O1 adjacent to the positive electrode finger T1 of the split electrode 10 does not deteriorate. Even when the width is 0.25λ or less, high reflection efficiency can be obtained. Therefore, it is not necessary to separate the gap G by 1λ or more, and an increase in chip size can be prevented.

  FIG. 3 shows a part of an IDT electrode of a transversal SAW filter using a unidirectional SAW converter according to the present invention. The IDT electrode 11 includes an electrode part A formed by split electrodes of two positive electrode fingers and two negative electrode fingers, an electrode part B composed of PNR, and an electrode in which the electrode part B is reversed left and right and the reflection phase is reversed. Part C, a combination of a plurality of electrode sections D, in which all electrode fingers are extended from the bus bar on only one side and SAW excitation is not performed, and an electrode section E in which the electrode section A is reversed left and right and the excitation phase is reversed Configured. In this way, when a transversal SAW filter is configured using a unidirectional SAW converter, the transmission response can be improved in performance by weighting the excitation and reflection on the electrodes. The arrangement method of the electrode parts A to E is obtained by solving an optimization problem using a genetic algorithm (GA) by a computer.

  Here, the pass characteristics of the transversal filter using the unidirectional SAW converter of the present invention and the conventional transversal filter were compared. 4 to 6 show the pass characteristics of the conventional transversal filter, FIG. 4 shows the pass characteristics of the transversal filter using the regular SAW converter shown in FIG. 15, and FIG. 5 shows the pass characteristics of FIG. FIG. 6 shows the pass characteristic of a transversal filter in which an open type floating electrode is arranged in the gap between the split electrodes, and FIG. 6 shows the pass characteristic of a transversal filter in which the short type floating electrode shown in FIG. 18 is arranged in the gap between the split electrodes. Yes. 7 and 8 show the pass characteristics of the transversal filter using the unidirectional SAW converter of the present invention, and FIG. 7 shows that the reflective electrode having the PNR structure shown in FIG. 1 is arranged in the gap between the split electrodes. FIG. 8 shows the pass characteristic of the transversal filter in which the PNR structure reflection electrode shown in FIG. 3 and the reflection electrode obtained by horizontally inverting the PNR structure are simultaneously arranged in the gap between the split electrodes. ing. FIGS. 4 to 8 all have a center frequency of 40 (MHz), (a) shows an enlarged view of the pass characteristics, and (b) shows the vicinity of the pass characteristics. FIG. 9 shows a comparison of the pass characteristics shown in FIGS. 4 to 8. The minimum loss of the pass band, the amplitude deviation at the center frequency ± 1.75 MHz, and the numerical value of the bandwidth at a position 3 dB down from the minimum loss. In addition, the standard of attenuation is set near the passbands of 30 to 35 MHz and 45 to 50 MHz, and when it satisfies 20 dB or more based on the minimum loss, it is OK, and when it is not satisfied, it is NG. Attenuation determination of attenuation was performed.

  First, the transversal filter using the conventional regular SAW converter shown in FIG. 4 satisfies the standard of out-of-band attenuation because the pass characteristic is a single peak, the bandwidth is narrow, and the steepness is poor. I understand that it is not. Next, the transversal filter in which the open type or short type floating electrode shown in FIGS. 5 and 6 is arranged has an improved minimum loss, amplitude deviation and pass bandwidth compared to the above-mentioned normal type, and an out-of-band attenuation. However, the pass characteristics are largely convex near the center frequency, indicating that the flatness is still poor. On the other hand, the transversal filter in which the reflective electrode having the PNR structure of the present invention shown in FIG. 7 is disposed has the minimum loss equivalent to that of the conventional transversal filter in which the open or short-circuited floating electrode is disposed. While realizing, it can be seen that the bandwidth is widened, the amplitude deviation is improved by nearly 1 dB, and the passband is flattened. Further, in the transversal filter in which the reflection electrode having the PNR structure shown in FIG. 8 and the reflection electrode in which the PNR structure is reversed left and right are simultaneously disposed inside, the insertion loss, bandwidth, It can be seen that a higher steepness is obtained while maintaining the amplitude deviation, and that there is room for the out-of-band attenuation standard.

  As described above, the transversal filter in which the reflective electrode of the PNR structure according to the present invention is disposed inside realizes the minimum loss equivalent to that of the conventional transversal filter in which the open type or short-circuit type floating electrode is disposed. However, the flatness of the bandwidth and passband could be improved. Furthermore, a transversal filter in which a reflective electrode having a PNR structure and a reflective electrode obtained by horizontally inverting the PNR structure are simultaneously disposed inside increases the steepness while maintaining good insertion loss, amplitude deviation, and bandwidth. Manufacturing yield can be improved.

  By the way, when attention is paid to the excitation center T of FIG. 1 in the present invention, the left electrode of the positive electrode finger T1 is a floating electrode O1 in an electrically open state, and the right electrode of the positive electrode finger T2 is electrically short-circuited. Since the negative electrode finger T3 is in a state, the charge distribution or potential is not symmetrical, the position of the excitation center T actually moves slightly, and the distance from the reflection center R ′ deviates from λ / 8. As a result, the unidirectionality may be impaired.

  In order to solve this problem, a one-way SAW converter in which the distance between the excitation center and the reflection center was corrected as shown in FIG. 4 was examined. Similar to the unidirectional SAW converter 1 shown in FIG. 1, the unidirectional SAW converter 21 includes a PNR formed of split electrodes T1 to T4 and floating electrodes O1, O2, S1, and S2. As described above, the excitation center T is considered to be at the center position of the positive electrode fingers T1 and T2, but actually the excitation center moves to T2 shifted by σ to the left in the drawing. Therefore, the positions of the floating electrodes O1, O2, S1, S2 are shifted to the left by σ, that is, the distance between the centers of the positive electrode fingers T1, T2 and the floating electrode O1 is set to 3λ / 8 + σ, and the reflection centers R2 and R2 ′. If the position is corrected, the distance between the excitation center T2 and the reflection center R2 ′ can be set to λ / 8, and deterioration of unidirectionality can be prevented. Although the value of σ varies depending on various conditions such as the piezoelectric substrate, the electrode material, and the electrode film thickness, it has been confirmed by simulation that the value σ falls within the range of σ ≦ λ / 16 under any condition.

  Although the width of the split electrode and the floating electrode has been λ / 8 so far, it is difficult to make the electrode width coincide with λ / 8 in actual manufacturing, and λ / 8 ± λ in consideration of manufacturing variations. If it is within the range of / 40, there is no problem in characteristics. Further, in order to increase the reflection efficiency of the reflective electrode, the electrode fingers of the floating electrodes O1, O2, S1, S2 and the split electrodes T1 to T4 are made different from each other, or the floating electrodes O1, O2 of the open type are short-circuited. The electrode finger widths of the electrodes S1 and S2 may be different. FIG. 5 shows the electrode fingers of the open type floating electrodes O1 and O2 with the widths of the short-circuit type floating electrodes S1 and S2 fixed to 0.15λ, 0.25λ and 0.35λ in the unidirectional SAW converter of FIG. The change in the reflection efficiency when the width is changed is shown. The electrode film thickness is 0.25% λ. As shown in the figure, it is understood that the reflection efficiency increases as the widths of the short-circuit type floating electrodes S1 and S2 are increased and the widths of the open type floating electrodes O1 and O2 are decreased. Thus, as in the unidirectional SAW converter shown in FIG. 6, if the width of the open floating electrode is reduced and the width of the short floating electrode is increased in the reflective electrode, the reflection efficiency is increased and the unidirectionality is further increased. Improved.

  Further, the electrode period of the split electrodes T1 to T4 and the electrode period of the floating electrodes O1, O2, S1, and S2 may be different from each other so that the resonance frequencies thereof are the same. By doing so, the continuity of the propagation response is maintained in the unidirectional SAW converter, and a low loss characteristic is obtained.

  As still another embodiment, both ends of the short-circuit type floating electrode in the reflective electrode may be electrically connected like a unidirectional SAW converter 41 shown in FIG. In addition, as in the unidirectional SAW converter 51 shown in FIG. 8, so-called apodization weighting is performed by changing the crossing width of the split electrodes and the length of the electrode fingers of the reflective electrodes along the SAW propagation direction. The dummy electrode 52 may be provided so as to fill a gap generated at the tip of each electrode finger on the extension of the finger.

  So far, a unidirectional SAW converter using a 128 ° rotated Y-cut X-propagating lithium niobate and a SAW device using the same have been described. However, the present invention is not limited to this, and the piezoelectric substrate Needless to say, the present invention can be applied to quartz, lithium tantalate, lithium tetraborate, langasite and the like.

It is a figure explaining the unidirectional SAW converter which concerns on this invention. The relationship of the reflective electrode width | variety of the unidirectional SAW converter which concerns on this invention, reflection efficiency, and film thickness dependence is shown. 2 shows an IDT electrode of a transversal SAW filter using a unidirectional SAW converter according to the present invention. It is a transmission characteristic of the conventional normal type transversal filter, (a) shows an enlarged view of the transmission characteristic, and (b) shows the vicinity of the transmission characteristic. It is a transmission characteristic of the transversal filter which has arrange | positioned the conventional open type floating electrode inside, (a) shows the enlarged view of a transmission characteristic, (b) shows the vicinity of a transmission characteristic. It is a transmission characteristic of the transversal filter which has arrange | positioned the conventional short-circuit type floating electrode inside, (a) shows the enlarged view of a transmission characteristic, (b) shows the vicinity of a transmission characteristic. FIG. 4A is a transmission characteristic of a transversal filter in which a reflective electrode having a PNR structure according to the present invention is arranged, FIG. 5A is an enlarged view of the transmission characteristic, and FIG. It is a transmission characteristic of the transversal filter which has arrange | positioned the reflection electrode of the PNR structure of this invention, and the reflection electrode which reversed this PNR structure right and left inside, (a) is an enlarged view of a transmission characteristic, (b) is a transmission characteristic The neighborhood is shown. The comparison data of each passing characteristic shown in Drawing 4-Drawing 8 are shown. It is a figure explaining the unidirectional SAW converter which correct | amended the position of the excitation center and the reflection center. The reflection efficiency when the short-circuit electrode width of the reflective electrode of the unidirectional SAW converter according to the present invention is fixed and the open electrode width is changed is shown. It is a figure explaining the unidirectional SAW converter which concerns on the other Example of this invention. The unidirectional SAW converter which connected the upper end and lower end of the short circuit type floating electrode of a reflective electrode is shown. Figure 3 shows a unidirectional SAW transducer with apodization weighting. 1 shows a conventional transversal SAW filter. 1 shows a conventional reflective bank type unidirectional SAW converter. A unidirectional SAW converter using a split electrode as a conventional excitation electrode is shown. A short-circuit type floating electrode is shown. The relationship between the electrode finger width, reflection efficiency, and film thickness dependence of the open type and short type floating electrodes is shown.

Explanation of symbols

1, 11, 21, 31, 41, 51: Unidirectional SAW converter 2: Positive bus bar 3: Negative bus bar 4: External terminal 10: Split electrode 11: Reflective electrode (PNR)
52: Dummy electrodes T1, T2: Positive electrode fingers T3, T4: Negative electrode fingers O1, O2: Open floating electrodes S1, S2: Short-circuit floating electrodes T: Excitation center R: Reflection center

Claims (13)

A surface acoustic wave converter for constituting a surface acoustic wave element by being arranged on a piezoelectric substrate,
The surface acoustic wave converter has a configuration in which a plurality of basic sections having a width corresponding to the wavelength λ of the surface acoustic wave to be excited are connected.
Assume that the first basic section is a split electrode in which two positive electrode fingers and two negative electrode fingers are paired, and the first to fourth floating electrodes are sequentially arranged in the second basic section. In addition, only one of the floating electrodes between the first and third floating electrodes or the second and fourth floating electrodes is short-circuited, and includes at least one of the first and second basic sections. And
An interval between the center position of two positive electrode fingers or two negative electrode fingers constituting the first basic section and the center position of the first floating electrode is 3λ / 8 ± nλ / 2 (where n = 0 , 1, 2, 3,..., A unidirectional surface acoustic wave transducer characterized by the above. A surface acoustic wave converter for constituting a surface acoustic wave element by being arranged on a piezoelectric substrate,
The surface acoustic wave converter has a configuration in which a plurality of basic sections having a width corresponding to the wavelength λ of the surface acoustic wave to be excited are connected.
The first basic section is a split electrode in which two positive electrode fingers and two negative electrode fingers are arranged, and the first basic section to the fourth four floating electrodes are sequentially arranged in the second basic section. In addition, the first and third floating electrodes are short-circuited and the second and fourth floating electrodes are opened, and the first to fourth four floating electrodes are arranged in order in the third basic section. When the first and third floating electrodes are opened and the second and fourth floating electrodes are short-circuited, the first to third basic sections are included, and
An interval between the center position of two positive electrode fingers or two negative electrode fingers constituting the first basic section and the center position of the first floating electrode is 3λ / 8 ± nλ / 2 (where n = 0 , 1, 2, 3,..., A unidirectional surface acoustic wave transducer characterized by the above. A surface acoustic wave converter for constituting a surface acoustic wave element by being arranged on a piezoelectric substrate,
The surface acoustic wave converter has a configuration in which a plurality of basic sections having a width corresponding to the wavelength λ of the surface acoustic wave to be excited are connected.
The first basic section is a split electrode in which two positive electrode fingers and two negative electrode fingers are arranged, and the first basic section to the fourth four floating electrodes are sequentially arranged in the second basic section. In addition, the first and third floating electrodes are short-circuited and the second and fourth floating electrodes are opened, and the first to fourth four floating electrodes are arranged in order in the third basic section. A split electrode in which the first and third floating electrodes are opened and the second and fourth floating electrodes are short-circuited, and the fourth basic section is a pair of the two positive electrode fingers and the two negative electrode fingers The first basic section and the positive and negative arrangements are reversed, and the fifth basic section is configured by only positive electrode fingers or only negative electrode fingers while maintaining the electrode finger arrangement position of the split electrode. When
Including at least two types of basic sections among the first, fourth, and fifth basic sections and including at least one basic section of the second and third basic sections;
The distance between the center position of two positive electrode fingers or two negative electrode fingers constituting the split electrode and the center position of the first floating electrode is 3λ / 8 ± nλ / 2 (where n = 0, 1, 2, 3... A unidirectional surface acoustic wave transducer characterized in that 4. The unidirectional device according to claim 1, wherein the split electrode and the first to fourth floating electrodes have an electrode finger width of λ / 8 ± λ / 40. 5. Surface acoustic wave transducer. 4. The unidirectional surface acoustic wave according to claim 1, wherein an electrode finger width of the split electrode and an electrode finger width of the first to fourth floating electrodes are different from each other. converter. 4. The electrode finger width of the first and third floating electrodes and the electrode finger width of the second and fourth floating electrodes are made different from each other. 5. Unidirectional surface acoustic wave transducer. 7. The unidirectional surface acoustic wave transducer according to claim 1, wherein an electrode period of the split electrode and an electrode period of the first to fourth floating electrodes are different from each other. . The distance between the center position of the two positive electrode fingers of the split electrode or the center position of the two negative electrode fingers and the center position of the first floating electrode is 3λ / 8 ± nλ / 2 + σ (where n = 0, The unidirectional surface acoustic wave transducer according to claim 1, wherein 1, 2, 3... Σ ≦ λ / 16). The one-way property according to any one of claims 1 to 8, wherein both ends of the first and third floating electrodes or both ends of the second and fourth floating electrodes are conductively connected. Surface acoustic wave transducer. 10. The split electrode according to any one of claims 1 to 9, wherein weighting is performed in a cross width direction of the positive electrode finger and the negative electrode finger along a propagation direction of the surface acoustic wave. Unidirectional surface acoustic wave transducer. The weighting is given to the length direction of the said floating electrode in the said 1st thru | or 4th floating electrode along the propagation direction of a surface acoustic wave, The Claim 1 thru | or 10 characterized by the above-mentioned. Unidirectional surface acoustic wave transducer. The unidirectional surface acoustic wave transducer according to claim 1, wherein 128 ° rotated Y-cut X-propagating LiNbO 3 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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