Abstract:
A surface acoustic wave filter (cascaded dual mode SAW filter) includes three IDT electrodes disposed adjacent to each other along the propagation direction of a surface wave on a piezoelectric substrate and two primary-tertiary longitudinally-coupled dual mode SAW fliers cascaded and constructed by arranging grating reflectors on both sides of three IDT electrodes. At least a pair of electrode fingers are thinned out from the IDT electrodes disposed outside, and floating electrodes are then disposed.

Description:
BACKGROUND  
       [0001]     1. Technical Field  
         [0002]     The present invention relates to a surface acoustic wave filter, and in particular, to a narrow-band surface acoustic wave filter which is reduced in size.  
         [0003]     2. Related Art  
         [0004]     In recent years, a surface acoustic wave filter (SAW filter) has been widely used in telecommunications field. In other words, because of its excellent features such as high performance, small size, mass production, and the like, the SAW filter is frequently used in a mobile phone or the like.  FIG. 9  is a plan view illustrating a construction of a cascaded dual mode SAW filter which is constructed by cascading two primary-tertiary longitudinally-coupled dual mode SAW filters (hereinafter, referred to as the dual mod SAW filter). The cascaded dual mode SAW filter is used in an RF filter of a mobile phone or the like. On the main surface of a piezoelectric substrate  31 , IDT electrodes  32 ,  33 , and  34  are disposed adjacent to each other along the propagation direction of a surface wave. On both sides of the IDT electrodes  32 ,  33 , and  34 , grating reflectors  35   a  and  35   b  (hereinafter, referred to as a reflector) are arranged, thereby forming a first dual mode SAW filter F 1 . In this case, the IDT electrodes  32 ,  33 , and  34  are respectively formed with a pair of comb-shaped electrodes each having a plurality of electrode fingers which are fitted into each other.  
         [0005]     On the same piezoelectric substrate  31 , a second dual mode SAW filter F 2  composed of IDT electrodes  32 ′,  33 ′, and  34 ′ and reflectors  35 ′ a  and  35 ′ b  is formed in the same way as the first dual mode SAW filter F 1 . The first and second dual mode SAW filter F 1  and F 2  are cascaded, thereby constructing a cascaded dual mode SAW filter.  
         [0006]      FIG. 10  illustrates filter characteristics obtained by simulation using the electrode pattern of the cascaded dual mode SAW filter shown in  FIG. 9  in order to design an RF filter (in which the center frequency is 1.57542 MHz, the bandwidth is +1 MHz, and the terminal impedance is 50 Ω) for a GPS which has recently been used in an in-vehicle telephone or the mobile phone, with the piezoelectric substrate set to a 38.7° Y—XLiTaO 3  substrate, the center frequency set to 1.5 GHz, 14.5 pairs of IDT electrodes  33  and  33 ′, 9.5 pairs of respective IDT electrodes  32 ,  34 ,  32 ′, and  34 ′, the intersection width set to 35λ (λ is a wavelength of the surface wave), each number of reflectors  35   a,    35   b,    35 ′ a,  and  35 ′ b  set to 100, and the electrode film thickness set to 7.7% λ. Within the passband, a large ripple appears since a broad-band electrode pattern is used.  
         [0007]     A technique for improving the ripple within the passband has been disclosed in JP-A-4-40705 and JP-A-7-74588. That is, as shown in  FIG. 11 , IDT electrodes  42 ,  43 , and  44  are disposed adjacent to each other along the propagation direction of a surface wave on the main surface of a piezoelectric substrate  41 . Further, on both sides of the IDT electrodes  42 ,  43 , and  44 , reflectors  45   a  and  45   b  are arranged, thereby forming a first dual mode SAW filter F 1 . On the same piezoelectric substrate  41 , a second dual mode SAW filter F 2  including IDT electrodes  42 ′,  43 ′, and  44 ′ and reflectors  45 ′ a  and  45 ′ b  is formed in the same way as the first dual mode SAW filter F 1 . The first and second dual mode SAW filters F 1  and F 2  are cascaded. Further, between the first and second dual mode SAW filters F 1  and F 2 , capacity electrodes  46   a  and  46   b  are disposed to be orthogonal to the propagation direction of a surface wave. Lead electrodes extending from one side of each of the comb-shaped electrodes of the capacity electrodes  46   a  and  46   b  are respectively connected to lead electrodes which cascade the filters F 1  and F 2 . The other sides of the comb-shaped electrodes are respectively grounded, thereby constructing a narrow-band cascaded dual mode SAW filter. With the capacity electrodes  46   a  and  46   b  functioning as capacity elements, the ripple within the band disappears.  
         [0008]      FIG. 12  shows filter characteristics obtained by simulation in a state where the constants of the dual mode SAW filter are set to be the same as those shown in  FIG. 10  and the capacitance values which are formed by the capacity electrodes  46   a  and  46   b  are commonly set to 0.64 pF. As a result, a ripple does not exist within the passband, and the passband becomes flat.  
         [0009]     JP-A-2002-353777 has disclosed a filter in which at least a pair of electrode fingers are thinned out from a center IDT electrode of a primary-tertiary longitudinally-coupled dual mode SAW flier and short-circuit-type floating electrodes are then disposed and a filter in which grating reflectors are disposed after thinning out such that the impedance of the filter can be matched with desired impedance by thinning out the electrode fingers.  
         [0010]     In the narrow-band cascaded dual mode SAW filter according to the related art as shown in  FIG. 11 , however, the capacity electrodes for forming capacity elements should be provided between two of the dual mode SAW filters F 1  and F 2 . Therefore, there is a problem that the cascaded dual mode SAW filter is enlarged as much as the space of where the capacity electrodes are provided.  
       SUMMARY  
       [0011]     An advantage of some aspects of the invention is that it provides a surface acoustic filter in which electrode fingers included in IDT electrodes are suitably thinned out, and floating electrodes or electrode fingers connected to a comb-shaped electrode which is topologically reversed are then disposed, thereby constructing the IDT electrodes. Therefore, a capacity ratio of a SAW resonator which is formed by the IDT electrodes becomes larger than that of the construction according to the related art. In other words, the capacity ratio can be deteriorated. Accordingly, when a narrow-band cascaded dual mode SAW filter is constructed, capacity elements do not need to be inserted between stages, and the filter can be reduced in size.  
         [0012]     According to an aspect of the invention, there is provided a surface acoustic wave filter (cascaded dual mode SAW filter) including three IDT electrodes disposed adjacent to each other along the propagation direction of a surface wave on a piezoelectric substrate and two primary-tertiary longitudinally-coupled dual mode SAW fliers cascaded and constructed by arranging grating reflectors on both sides of three IDT electrodes. At least a pair of electrode fingers are thinned out from the IDT electrodes disposed outside, and floating electrodes are then disposed.  
         [0013]     It is preferable that two floating electrodes be disposed so as to be substituted for the pair of thinned-out electrode fingers, and be short-circuited with respect to each other.  
         [0014]     According to another aspect of the invention, there is provided a surface acoustic wave filter (cascaded dual mode SAW filter) including three IDT electrodes disposed adjacent to each other along the propagation direction of a surface wave on a piezoelectric substrate and two primary-tertiary longitudinally-coupled dual mode SAW fliers cascaded and constructed by arranging grating reflectors on both sides of three IDT electrodes. At least one electrode finger is thinned out from the IDT electrodes disposed outside, and electrode fingers connected to the comb-shaped electrode which is topologically reversed are then disposed.  
         [0015]     According to a still further aspect of the invention, a surface acoustic wave filter (cascaded dual mode SAW filter) including three IDT electrodes disposed adjacent to each other along the propagation direction of a surface wave on a piezoelectric substrate; and two primary-tertiary longitudinally-coupled dual mode SAW fliers cascaded and constructed by arranging grating reflectors on both sides of three IDT electrodes. At least two electrode fingers are thinned out from the IDT electrodes disposed outside, and electrode fingers connected to the comb-shaped electrode which is topologically reversed are then disposed.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.  
         [0017]      FIG. 1  is a schematic plan view illustrating the construction of a first embodiment according to the invention.  
         [0018]      FIG. 2  shows passband characteristics obtained by simulation based on  FIG. 1 .  
         [0019]      FIG. 3  is a plan view illustrating the construction of a first dual mode SAW filter F 1  of  FIG. 1 .  
         [0020]      FIG. 4  is a circle diagram showing impedance characteristic of the dual mode SAW filter of  FIG. 3 .  
         [0021]      FIG. 5  is a schematic plan view illustrating the construction of a second embodiment according to the invention.  
         [0022]      FIG. 6  shows passband characteristics obtained by simulation based on  FIG. 5 .  
         [0023]      FIG. 7  is a schematic plan view illustrating the construction of a third embodiment according to the invention.  
         [0024]      FIG. 8  shows passband characteristics obtained by simulation based on  FIG. 7 .  
         [0025]      FIG. 9  is a plan view illustrating the construction of a broad-band cascaded dual mode SAW filter.  
         [0026]      FIG. 10  shows passband characteristics obtained by simulation based on  FIG. 9 .  
         [0027]      FIG. 11  is a plan view illustrating the construction of a narrow-band cascaded dual mode SAW filter.  
         [0028]      FIG. 12  shows passband characteristics obtained by simulation based on  FIG. 11 .  
         [0029]      FIG. 13  is a plan view illustrating the construction of the first dual mode SAW filter F 1  of  FIG. 9 .  
         [0030]      FIG. 14  is a circle diagram showing impedance characteristic of the dual mode SAW filter of  FIG. 13 .  
         [0031]      FIG. 15  is a plan view illustrating the construction of the first dual mode SAW filter F 1  of  FIG. 11 .  
         [0032]      FIG. 16  is a circle diagram showing impedance characteristic of the dual mode SAW filter of  FIG. 15 . 
     
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0033]      FIG. 1  is a plan view illustrating a cascaded dual mode SAW filter according to an embodiment of the invention. On the main surface of a piezoelectric substrate  1 , IDT electrodes  2 ,  3 , and  4  are disposed to be adjacent to each other along the propagation direction of a surface wave. On both sides of the IDT electrodes  2 ,  3 , and  4 , reflectors  5   a  and  5   b  are arranged, thereby forming a first dual mode SAW filter F 1 . Further, on the same piezoelectric substrate  1 , a second dual mode SAW filter F 2  including IDT electrodes  2 ′,  3 ′, and  4 ′ and reflectors  5 ′ a  and  5 ′ b  is formed in the same way as the first dual mode SAW filter F 1 . Then, the first and second dual mode SAW filters F 1  and F 2  are cascaded to construct the cascaded dual mode SAW filter.  
         [0034]     The feature of the invention is shown in the construction of the IDT electrodes  2 ,  3 , and  4  ( 2 ′,  3 ′, and  4 ′). Like a typical IDT electrode, the IDT electrode  3  ( 3 ′) is formed of a pair of comb-shaped electrodes each having a plurality of electrode fingers which are fitted into each other. In other words, the electrode fingers alternately connected to the plus comb-shaped electrode and the minus comb-shaped electrode. In the IDT electrodes  2  and  4  ( 2 ′ and  4 ′), at least a pair of electrode fingers are thinned out from a pair of comb-shaped electrodes each having a plurality of electrode fingers which are fitted into each other, and short-circuited floating electrodes are then disposed so as to construct the IDT electrodes  2  and  4  ( 2 ′ and  4 ′). Referring to the IDT electrode  2  of the embodiment shown in  FIG. 1 , three pairs of electrode fingers α, β, and γ are thinned out, and three short-circuit-type floating electrodes are then disposed so as to be substituted for the thinned-out electrode fingers. The IDT electrodes  4 ,  2 ′, and  4 ′ are constructed in the same way as the IDT electrode  2 .  
         [0035]      FIG. 2  shows filter characteristics obtained by simulation using the electrode pattern shown in  FIG. 1 , with the piezoelectric substrate set to a 38.7° Y—XLiTaO 3  substrate, the center frequency set to 1.5 GHz, 14.5 pairs of respective IDT electrodes  3  and  3 ′, 9.5 pairs of IDT electrodes  2 ,  4 ,  2 ′, and  4 ′ (which are set before thinning out and in which three electrodes are thinned out to dispose three short-circuit-type floating electrodes), the intersection width set to 35λ (λ is a wavelength), each number of reflectors  5   a,    5   b,    5 ′ a,  and  5 ′ b  set to  100 , and the electrode film thickness set to 7.7% λ. As shown in  FIG. 2 , the flatness and insertion loss within the passband are comparable to those of  FIG. 12 .  
         [0036]     The reason why the passband becomes flat without the capacity electrodes  46   a  and  46   b  functioning as capacity elements, which are included in the narrow-band cascaded dual mode SAW filter shown in  FIG. 11 , is examined.  FIG. 3  illustrates a filter which is constructed by taking out only the dual mode SAW filter F 1  from the cascaded dual mode SAW filter shown in  FIG. 1 .  FIG. 4  is a circle diagram (smith chart) which is obtained by simulating the impedance characteristic of the dual mode SAW filter of  FIG. 3 . Within the passband of the filter, the impedance changes in the vicinity of the real axis, if the frequency is changed. On the contrary, the impedance changes at the position separated from the real axis, outside the band.  
         [0037]     Only the dual mode SAW filter F 1  is taken out from the broad-band cascaded dual mode SAW filter shown in  FIG. 9 , and is constructed as shown in  FIG. 13 .  FIG. 14  is a circle diagram obtained by simulating the impedance characteristic of the filter shown in  FIG. 13 . If the frequency is changed within the passband of the filter, the impedance changes at the position separated from the real axis. Even at the frequency outside the band, the impedance changes at the position separated from the real axis.  
         [0038]     Only the dual mode SAW filter F 1  is taken out from the narrow-band cascaded dual mode SAW filter shown in  FIG. 11 , and the IDT electrodes  46   a  and  46   b  functioning as capacity elements are respectively connected in parallel to the output lead electrode, thereby constructing a filter as shown in  FIG. 15 .  FIG. 16  is a circle diagram obtained by simulating the impedance characteristic of the filter shown in  FIG. 15 . As shown in  FIG. 16 , the impedance changes in the vicinity of the real axis within the passband, if the frequency is changed within the passband. The impedance changes at the position separated from the real axis, if the frequency is changed outside the passband.  
         [0039]     In a filter of which the passband and attenuation band characteristic are excellent, the impedance shown within the passband substantially matches the terminal impedance. The impedance shown outside the passband does not match the terminal impedance. In other words, within the passband, the terminal impedance and the impedance of the filter match each other and the maximum power is supplied to the load, so that the loss of the filter is minimized. On the other hand, in a state where the terminal impedance and the impedance of the filter significantly mismatch outside the passband, the power supplied to the load is minimized, so that the loss of the filter increases. That is, the attenuation is shown.  
         [0040]     When the dual mode SAW filters F 1  and F 2  shown in  FIG. 1  are set to a SAW resonator, a capacity ratio assumed by the SAW resonator becomes larger (deteriorates) then the capacity ratio assumed by the dual mode SAW filters shown in  FIGS. 9 and 11 , and the frequency interval between the resonant frequency and the antiresonant frequency is narrowed. Therefore, without a capacity element being added, predetermined frequency allocation is constructed on the basis of a filter theory. Accordingly, it is considered that the passband characteristic of the filter becomes flat and the insertion loss decreases.  
         [0041]      FIG. 5  is a plan view illustrating a second embodiment according to the invention. On the main surface of a piezoelectric substrate  11 , IDT electrodes  12 ,  13 , and  14  are disposed adjacent to each other along the propagation direction of a surface wave. On both sides of the IDT electrodes  12 ,  13 , and  14 , reflectors  15   a  and  15   b  are arranged, thereby forming a first dual mode SAW filter F 1 . Further, on the same piezoelectric substrate  11 , a second dual mode SAW filter F 2  including IDT electrode  12 ′,  13 ′, and  14 ′ and reflectors  15 ′ a  and  15 ′ b  is formed in the same way as the first dual mode SAW filter F 1 . Then, the first and second dual mode SAW filters F 1  and F 2  are cascaded, thereby constructing a cascaded dual mode SAW filter.  
         [0042]     Like a typical IDT electrode, the IDT electrode  13  ( 13 ′) is formed of a pair of comb-shaped electrodes each having a plurality of electrode fingers which are fitted into each other. The feature of the second embodiment is present in the construction of the IDT electrodes  12  and  14  ( 12 ′ and  14 ′). At least one electrode finger is thinned out from the pair of comb-shaped electrodes each having a plurality of electrode fingers which are fitted into each other, and electrode fingers connected to the comb-shaped electrode which is topologically reversed are then disposed, thereby constructing the IDT electrodes  12  and  14  ( 12 ′ and  14 ′). Referring to the IDT electrode  12  of  FIG. 5 , two electrode fingers a and P are thinned out, and electrode fingers connected to the comb-shaped electrode which is topologically reversed are then disposed. The IDT electrodes  14 ,  12 ′ and  14 ′ are constructed in the same way as the IDT electrode  12 . If the IDT electrodes are constructed in such a manner, a capacity ratio of the SAW resonator formed by the IDT  12 ,  13 , and  14  ( 12 ′,  13 ′, and  14 ′) becomes larger (deteriorates) than a capacity ratio of the construction according to the related art. When the cascaded dual mode SAW filter is constructed, capacity elements do not need to be added between stages.  
         [0043]      FIG. 6  shows filter characteristics obtained by simulation using the electrode pattern shown in  FIG. 5 , with the piezoelectric substrate set to a 38.7° Y—XLiTaO 3  substrate, the center frequency set to 1.5 GHz, 14.5 pairs of IDT electrodes  13  and  13 ′, 9.5 pairs of respective IDT electrodes  12 ,  14 ,  12 ′, and  14 ′ (which are set before thinning out and in which two electrode fingers are thinned out and two electrode fingers connected to the comb-shaped electrode which is topologically reversed are disposed), the intersection width set to 35λ (λ is a wavelength), each number of reflectors  15   a,    15   b,    15 ′ a,  and  15 ′ b  set to  100 , and the electrode film thickness set to 7.7%λ. As shown in  FIG. 6 , the flatness and insertion loss within the passband are comparable to those of  FIG. 12 .  
         [0044]      FIG. 7  is a plan view illustrating a third embodiment according to the invention. On the main surface of a piezoelectric substrate  21 , the IDT electrodes  22 ,  23 , and  24  are disposed adjacent to each other along the propagation direction of a surface wave. On both sides of the IDT electrodes  22 ,  23 , and  24 , reflectors  25   a  and  25   b  are arranged to form a first dual mode SAW filter F 1 . Further, on the same piezoelectric substrate  21 , a second dual mode SAW filter F 2  including IDT electrodes  22 ′,  23 ′, and  24 ′ and reflectors  25 ′ a  and  25 ′ b  is formed in the same way as the first dual mode SAW filter F 1 . The first and second dual mode SAW filters F 1  and F 2  are cascaded, thereby constructing the cascaded dual mode SAW filter.  
         [0045]     The feature of the third embodiment is the construction of the IDT electrodes  22  and  24  ( 22 ′ and  24 ′). From a pair of comb-shaped electrodes each having a plurality of electrode fingers which are fitted into each other, at least one pair of electrode fingers are thinned out, and two electrode fingers connected to the comb-shaped electrode which is topologically reversed are then disposed, thereby constructing the IDT electrodes  22  and  24  ( 22 ′ and  24 ′). Referring to the IDT electrode  22  of  FIG. 7 , two pairs of electrode fingers α and β are thinned out, and electrode fingers connected to the comb-shaped electrode which is topologically reversed are respectively disposed. The IDT electrodes  24 ,  22 ′, and  24 ′ are constructed in the same way as the IDT electrode  22 . If the IDT electrodes are constructed in such a manner, a capacity ratio of a SAW resonator formed by the IDT  22 ,  23 , and  24  ( 22 ′,  23 ′, and  24 ′) becomes larger (deteriorates) than a capacity ratio of the construction according to the related art. When the cascaded dual mode SAW filter is constructed, a capacity element does not need to be added between stages.  
         [0046]      FIG. 8  shows filter characteristics obtained by simulation using the electrode pattern shown in  FIG. 7 , with the piezoelectric substrate set to a 38.7° Y—XLiTaO 3  substrate, the center frequency set to 1.5 GHz, 14.5 pairs of IDT electrodes  23  and  23 ′, 9.5 pairs of respective IDT electrodes  22 ,  24 ,  22 ′, and  24 ′ (which are set before thinning out and in which two pairs of electrode fingers are thinned out and four electrode fingers connected to the comb-shaped electrode which is topologically reversed are disposed), the intersection width set to 35λ (λ is a wavelength), each number of reflectors  25   a,    25   b,    25 ′ a,  and  25 ′ b  set to 100, and the electrode film thickness set to 7.7%λ. As shown in  FIG. 8 , the flatness and insertion loss within the passband are comparable to those of  FIG. 12 .  
         [0047]     Although the invention has been described by using lithium tantalate in which the cutting angle is 38.7°, the invention can be applied to lithium tantalate having a different cutting angle. Further, the invention can be applied to other piezoelectric materials such as lithium niobate and the like.  
         [0048]     In addition, the cascaded dual mode SAW filter has been described. However, if the invention is applied to a one-stage dual mode SAW filter, the passband characteristic thereof becomes flat.