Abstract:
A SAW resonator filter which generates Shear Horizontal type surface acoustic waves includes a piezoelectric substrate, and first and second resonators arranged on the piezoelectric substrate. The first and second resonators include first and second interdigital transducers having electrode fingers, respectively. The first and second interdigital transducers are acoustically coupled to form a filter, and are divided into a plurality of sub-interdigital transducer portions, respectively. By dividing the interdigital transducers to have multiple sub-IDT portions, the effective electromechanical coupling coefficient is reduced, thereby enabling the bandwidth to be made narrower. Further, the improvement in the electrode structure allows for the filter to use a piezoelectric substrate having excellent temperature characteristics, so that it is possible to achieve a SAW resonator filter having a narrow bandwidth and superior temperature characteristics. Moreover, when an edge reflection type SAW resonator filter using SH-type surface acoustic waves is made, it is possible to provide a compact bandpass filter having low insertion loss and excellent selectability.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a SAW resonator filter and more specifically, the present invention relates to a SAW resonator filter having a narrow bandwidth and having a compact size. 
     2. Description of the Related Art 
     A SAW (surface acoustic wave) filter is widely used as a bandpass filter in communications devices and other electronic devices. SAW filters include a transverse-type SAW filter, which has two interdigital transducers (IDTs) arranged on a piezoelectric substrate with a predetermined distance therebetween, and a SAW resonator filter having a resonator including an IDT provided on a piezoelectric substrate. 
     An edge reflection type SAW resonator filter using Shear Horizontal (SH) surface acoustic waves such as Love waves and Bleustein-Gulyaev-Shimuzu (BGS) waves and other similar waves, is known as a SAW resonator filter. In an edge reflection type SAW resonator filter, the resonator is provided between two opposite edges of the piezoelectric substrate, and the edges are used to reflect the SH waves. Since reflectors are not needed, a compact filter can be realized. 
     Further, an edge reflection type longitudinally coupled SAW resonator filter, and a transversely coupled SAW resonator filter, which are made by coupling two SAW resonators, is already known (e.g. Japan Unexamined Patent Publication Nos. 9-69751 and 10-261938). 
     FIG. 19 is a perspective view showing an example of a conventional edge reflection type, longitudinally coupled SAW resonator filter. The longitudinally coupled SAW resonator filter  51  includes IDTs  53  and  54  which are provided on a rectangular piezoelectric substrate  52 . The IDTs  53  and  54  are edge reflection type surface acoustic wave resonators, and are aligned along the direction of propagation of the surface acoustic wave. 
     Alternatively, FIG. 20 shows a conventional edge reflection type, transversely coupled SAW resonator filter. The transversely coupled SAW resonator filter  61  includes IDTs  63  and  64  which are provided on a rectangular piezoelectric substrate  62 . Each of the IDTs  63  and  64  is an edge reflection type surface acoustic wave resonator, but here the IDTs  63  and  64  are aligned perpendicular to the direction of propagation of the surface acoustic wave. 
     There is a great demand for a filter having a narrow bandwidth in order to improve the degree of selection. In the conventional SAW resonator filters  51  and  61 , the bandwidth is reduced by methods that include: (1) using piezoelectric substrates  52  and  62  that have small electromechanical coupling coefficients; (2) making the thickness of the electrodes of the IDTs smaller, thereby lowering the effective electromechanical coupling coefficients thereof; or (3) adjusting the distance between the IDTs, and bringing the resonant frequencies of the two resonators closer together; and other similar methods. 
     The problem with the first method is that it is necessary to select a piezoelectric material for the piezoelectric substrate that is suitable for its intended purpose, and it is difficult to obtain a material which has an electromechanical coupling coefficient appropriate for the required bandwidth. Further, it is extremely difficult to find materials that are suitable for the necessary bandwidth and which also have excellent temperature characteristics 
     The problem with the second method is that when the thickness of the electrodes of the IDTs is decreased, vibration energy caused by the piezoelectric effect leads to conversion of the waves to bulk waves and deterioration the desired filter characteristics. Further, there are limits to the amount of narrowing of the bandwidth that can be achieved. 
     The problem with the third method is that when the resonant frequencies of the two resonance modes are brought too close together, the two resonance modes become almost joined, thus increasing insertion loss. 
     SUMMARY OF THE INVENTION 
     To overcome the problems described above, preferred embodiments of the present invention provide a SAW resonator filter having a narrow bandwidth without increasing the insertion loss, having excellent temperature characteristics, and IDTs that have sufficient electrode thickness. 
     A preferred embodiment of the present invention provides a SAW resonator filter which generates SH-type surface acoustic waves and that includes a piezoelectric substrate, and first and second resonators arranged on the piezoelectric substrate, the first and second resonators including first and second interdigital transducers having electrode fingers, respectively, wherein the first and second interdigital transducers are acoustically coupled to form a filter, and wherein the first and second interdigital transducers are divided into a plurality of sub-interdigital transducer portions, respectively. 
     In one preferred embodiment of the present invention, the first and second IDTs are preferably divided to have two to four sub-IDT portions. 
     Also, in other preferred embodiments, the first and second interdigital transducers may either be longitudinally coupled or transversely coupled. 
     Note that by dividing the interdigital transducers to have multiple sub-IDT portions, the effective electromechanical coupling coefficient is reduced, thereby enabling the bandwidth to be made narrower. Further, the improvement in electrode structure allows for the filter to use a piezoelectric substrate having excellent temperature characteristics, so that it is possible to achieve a SAW resonator filter having a narrow bandwidth and superior temperature characteristics. Moreover, when an edge reflection type SAW resonator filter using SH-type surface acoustic waves is made, it is possible to provide a compact band filter having low loss and excellent selectability. 
     In another preferred embodiment of the present invention, a communication apparatus includes a duplexer, wherein the duplexer includes the above-described resonator filter having first and second interdigital transducers that are divided into a plurality of sub-interdigital transducers, respectively. 
     Other elements, features and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus do not limit the present invention and wherein: 
     FIG. 1 is a perspective view of a transversely coupled SAW resonator filter according to a preferred embodiment of the present invention; 
     FIG. 2 is a diagram showing the relationship between the number of divisions of an IDT and the 10-dB attenuation bandwidth in a transversely coupled SAW resonator filter; 
     FIG. 3 is a diagram showing the relationship between number of divisions of an IDT and the insertion loss in a transversely coupled SAW resonator filter; 
     FIG. 4 is a perspective view showing another preferred embodiment of the transversely coupled SAW resonator filter of the present invention; 
     FIG. 5 is a perspective view showing still another preferred embodiment of the transversely coupled SAW resonator filter of the present invention; 
     FIG. 6 is a perspective view of a preferred embodiment of a transversely coupled SAW resonator filter having multiple stage connections according to the present invention; 
     FIG. 7 is a perspective view of another preferred embodiment of a transversely coupled SAW resonator filter having multiple stage connections according to the present invention; 
     FIG. 8 is a perspective view of a longitudinally coupled SAW resonator filter according to a preferred embodiment of the present invention; 
     FIG. 9 is a perspective view of a longitudinally coupled SAW resonator filter according to another preferred embodiment of the present invention; 
     FIG. 10 is a diagram showing the frequency vs. amplitude characteristics of a conventional longitudinally coupled SAW resonator filter; 
     FIG. 11 is a diagram showing the frequency vs. amplitude characteristics of a longitudinally coupled SAW resonator filter according to a preferred embodiment of the present invention show in FIG. 8; 
     FIG. 12 is a diagram showing the frequency vs. amplitude characteristics of a longitudinally coupled SAW resonator filter according to a preferred embodiment of the present invention shown in FIG. 9; 
     FIG. 13 is a diagram showing the frequency vs. amplitude characteristics of a longitudinally coupled SAW resonator filter when an IDT is divided into four according to a preferred embodiment of the present invention; 
     FIG. 14 is a diagram showing frequency amplitude characteristics of a longitudinally coupled SAW resonator filter when an IDT is divided into five according to a preferred embodiment of the present invention; 
     FIG. 15 is a diagram showing the relationship between the number of divisions of an IDT and a 10 dB attenuation bandwidth in a longitudinally coupled SAW resonator filter of a preferred embodiment of the present invention; 
     FIG. 16 is a diagram showing the relationship between the number of divisions of an IDT and insertion loss in a longitudinally coupled SAW resonator filter of a preferred embodiment of the present invention; 
     FIG. 17 is a diagram showing the relationship between the number of divisions of a SAW resonator of one IDT and an effective electromechanical coupling coefficient K in a longitudinally coupled SAW resonator filter of a preferred embodiment of the present invention; 
     FIG. 18 is a diagram showing the relationship between a number of divisions of an IDT, and the frequency difference ΔF between a resonant frequency and an anti-resonant frequency, evaluated from the IDT on one side of a longitudinally coupled SAW resonator filter; 
     FIG. 19 is a perspective view of a conventional longitudinally coupled SAW resonator filter; 
     FIG. 20 is a perspective view of a conventional transversely coupled SAW resonator filter; and 
     FIG. 21 is a block diagram showing a communication apparatus according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective view of a transversely coupled SAW resonator filter according to a preferred embodiment of the present invention. Referring to FIG. 1, the transversely coupled SAW resonator filter  1  preferably includes a substantially rectangular piezoelectric substrate  2 . The piezoelectric substrate  2  is preferably made of a piezoelectric single-crystal such as LiTaO 3 , LiNbO 3 , and quartz, or a piezoelectric ceramic such as a lead titanate zirconate ceramic. When the piezoelectric substrate  2  is made of a piezoelectric ceramic, the piezoelectric substrate  2  is polarized in a direction that is substantially parallel to a direction in which electrode fingers of IDTs  3  and  4  extend. 
     The piezoelectric substrate  2  has edges  2   a  and  2   b  that face each other. The direction that a line connecting the edges  2   a  and  2   b  extends is the direction in which SH-type surface acoustic waves propagate. 
     The IDTs  3  and  4  are provided on a top surface  2   c  of the substrate  2 , and extend in a direction that is substantially perpendicular to the direction of propagation of the surface acoustic wave and define first and second SAW resonators, respectively. Furthermore, the first and second resonators are acoustically coupled to define a transversely coupled SAW resonator filter. 
     One of the novel features of the SAW filter of preferred embodiments of the present invention resides in the structure of the IDT that constitutes each resonator. The IDT of each SAW resonator is divided into a plurality of sub-IDTs, which are connected in series and aligned in a direction in which the excited SH-type waves propagate. 
     More specifically, the IDT  3  is preferably divided into sub-IDT portions  3 A and  3 B so that sub-lDT portions  3 A and  3 B are arranged along the surface acoustic wave propagation direction. In addition, the sub-IDT portions  3 A and  3 B are electrically connected in series via a common bus bar  5  that is located between a pair of input terminals IN. The IDT  4  is also divided so that sub-IDT portions  4 A and  4 B are arranged along the surface acoustic wave propagation direction, and, the sub-IDT portions  4 A and  4 B are electrically connected in series via a common bus bar  5  that is located between a pair of output terminals OUT. 
     The IDT  3  has a plurality of electrode fingers  3   a  to  3   n , and the IDT  4  has a plurality of electrode fingers  4   a  to  4   n . Since the IDT  4  preferably has approximately the same constitution as the IDT  3 , the structure of the IDTs  3  and  4  will be explained with reference to IDT  3 . 
     The electrode fingers  3   a  to  3   n  are arranged in sequence along the direction of propagation of the surface acoustic wave. Of these, the sub-IDT portion  3 A has electrode fingers  3   a  to  3   g , and the sub-IDT portion  3 B has electrode fingers  3   h  to  3   n.    
     In the sub-IDT portion  3 A, the electrode fingers  3   a ,  3   c ,  3   e , and  3   g  are electrically connected to the common bus bar  5  that extends along the surface acoustic wave propagation direction at the approximate center of the top surface  2   c  of the piezoelectric substrate  2 . One end of each of the electrode fingers  3   b ,  3   d ,  3   e , and  3   f  is electrically connected to a bus bar  3   o . The electrode fingers  3   a ,  3   c ,  3   e , and  3   g  are arranged so as to interdigitate with the electrode fingers  3   b ,  3   d , and  3   f . In the sub-IDT portion  3 B, ends of the electrode fingers  3   h ,  3   j ,  3   l , and  3   n  are connected to the common bus bar  5 , and one end of the electrode fingers  3   i ,  3   k , and  3   m  are electrically connected to a bus bar  3   p.    
     Note that the adjacent outermost electrode fingers  4   g  and  4   h  of the sub-IDTs  4 A and  4 B and the adjacent outermost electrode fingers  3   g  and  3   h  of the sub-IDTs  3 A and  3 B are electrically connected, respectively. 
     The electrode fingers  3   a  and  3   n  have a width of approximately λ/8, and the width of the remaining electrode fingers  3   b  to  3   n  is about λ/4, where λ denotes a wavelength of the SH-type wave that is excited on the piezoelectric substrate  2 . Furthermore, the space or gap between the outermost electrode finger  4   g  of the sub-IDT portion  4 A and the outermost electrode finger  4   h  of the sub-IDT portion  4 B, and the space between the outermost electrode finger  3   g  of the sub-IDT portion  3 A and the outermost electrode finger  3   h  of the sub-IDT portion  3 B, and the spaces between other adjacent electrode fingers are set at about λ/4. That is, the space between the sub-lDTs  3 A and  3 B and the space between the sub-IDTs  4 A and  4 B are set at about λ/4. 
     The distance between the edge  2   a  and the edge  2   b  is generally set approximately to be equal to an integral multiple of λ/2. 
     In the SAW resonator filter  1 , when an input voltage is applied to the bus bars  3   o  and  3   p , SH-type waves are excited by the IDT  3  including the sub-IDT portions  3 A and  3 B. The excited SH-type waves are reflected by the edges  2   a  and  2   b  to form standing waves and generate resonance modes. Outputs based on the standing waves are detected by the IDT  4  including the sub-IDT portions  4 A and  4 B to also generate resonance modes. These resonance modes of the IDTs  3  and  4  are coupled together, to operate as a transversely coupled SAW resonator filter. The output is extracted from the bus bars  4   o  and  4   p , which the electrode fingers  4   b ,  4   d ,  4   f ,  4   i ,  4   k , and  4   m  of the IDT  4  are connected. 
     In the SAW resonator filter  1  of preferred embodiments of the present invention, the IDTs  3  and  4  are preferably divided in two as described above, and therefore the effective electromechanical coupling coefficient is lower than in the conventional transversely coupled SAW resonator filter  61  (see FIG.  20 ), and consequently the bandwidth can be made narrower. This will be explained with reference to FIG.  2  and FIG.  3 . 
     To compare, the conventional transversely coupled SAW resonator filter  61  shown in FIG. 20, and the transversely coupled SAW resonator filter  1  according to an example of preferred embodiments of the present invention, were made using a piezoelectric substrate comprising a piezoelectric ceramic of 2.0 mm×1. 0mm×0.5 mm. The number of pairs of the electrode fingers of the IDT  63  and the IDT  3  was 34.5, the logarithm of the electrode fingers of the IDT  64  and the IDT  4  was 34.5, and the aperture (overlapping length) of the electrode fingers in each case was about 1.5 λ. 
     Furthermore, various SAW resonator filters were made in which the number of divisions of the IDTs  3  and  4  in the SAW resonator filter  1  was greater than 3. 
     In these SAW resonator filters, the relationship between the number of divisions of the IDTs  3  and  4 , and the 10 dB attenuation bandwidth (which is a ratio expressed as the % of attenuation relative to the resonant frequency fr), and the relationship between the number of divisions and the insertion loss, was evaluated. The results are shown in FIG.  2  and FIG. 3, respectively. 
     As is clear from FIG.  2  and FIG. 3, the bandwidth can be made narrower in the transversely coupled SAW resonator filter  1  by dividing the IDTs  3  and  4 . Furthermore, it can be seen that as bandwidth becomes narrower, the number of divisions increases. But, note that when the number of divisions was increased from 4 to 5, insertion loss increased rapidly. 
     Therefore, it can be seen that the number of divisions should preferably be within a range of 2 to 4 in order to avoid a rapid increase in the insertion loss. 
     FIG. 4 to FIG. 7 are perspective views of the SAW resonator filter according to preferred embodiments of the present invention with modifications to the electrode structure. 
     In the transversely coupled SAW resonator filter  6  shown in FIG. 4, the first and second IDTs  3  and  4  are each divided into two, but the way that the IDTs are divided differs from that of the SAW resonator filter of FIG.  1 . 
     That is, in the SAW resonator filter  6 , the first IDT  3  has sub-IDT portions  3 A and  3 B, but while the first sub-IDT portion  3 A has multiple electrode fingers  3   a  to  3   f , the second sub-IDT portion  3 B only has electrode fingers  3   g  to  3   l . The electrode fingers  3   a  to  3   l  are arranged in this sequence along the surface acoustic wave propagation direction. 
     Ends of the electrode fingers  3   a ,  3   c , and  3   e  are connected to the common bus bar  5 , and the other ends of the electrode fingers  3   b ,  3   d , and  3   f  are electrically connected to a bus bar  7   a . Moreover, the electrode finger  3   f  is electrically connected to a bus bar  7   c , which is located near the common bus bar  5  and extends substantially parallel to the common bus bar  5 . That is, one end of the electrode finger  3   f  is connected to the bus bar  7   a , and the other end to the bus bar  7   c . Similarly, in the sub-IDT portion  3 B, one end of the electrode finger  3   g  is connected to the bus bar  7   a , and the other end to the bus bar  7   c . One end of the electrode fingers  3   h ,  3   j , and  3   l  are connected to a bus bar  7   b , and one end of the electrode fingers  3   i  and  3   k  are connected to the bus bar  7   c . As a result of this arrangement of the electrode fingers, the adjacent outermost electrode fingers  3   f  and  3   g  of the sub-IDTs  3 A and  3 B and are electrically connected. 
     The electrode fingers  4   a  to  4   l  in the sub-IDT portion  4 A have the same arrangement as the IDT  3 . In the IDTs  3  and  4 , the bus bars  7   b  and  7   d  are provided on the side of the sub-IDT portions  3 B and  4 B, and the bus bars  7   b  and  7   d  are electrically insulated from the common bus bar  5 . 
     FIG. 5 is a perspective view of another modification of the transversely coupled SAW resonator filter  1  wherein the first and second IDTs are divided into two parts. Here, it has substantially the same electrode structure as the SAW resonator filter  6  of FIG. 4, but the common bus bar  5  does not extend between the sub-IDT portions  3 B and  4 B. 
     As shown in FIG.  4  and FIG. 5, in the transversely coupled SAW resonator filters according to preferred embodiments of the present invention, the structure of the sub-IDTs may be varied. In these cases also, since the IDTs  3  and  4  are divided, the bandwidth can be made narrow as in the SAW resonator filter  1 . 
     Moreover, as shown in FIG. 6, a multi-stage SAW resonator filter can be formed by arranging a plurality of transversely coupled SAW resonator filters on a piezoelectric substrate  12 , and connecting them transversely. In the SAW resonator filter  11  shown in FIG. 6, a first SAW resonator filter portion  11 A and a second SAW resonator filter portion  11 B are provided between opposite ends  12   a  and  12   b  of the piezoelectric substrate  12 . Each of the SAW resonator filter portions  11 A and  11 B have the same arrangement as the SAW resonator filter  1  of FIG.  1 . Then, the SAW resonator filter portions  11 A and  11 B are electrically connected by connected lead portions  13  and  14 , and the transversely coupled SAW resonator filter portions  11 A and  11 B are transversely connected. 
     In this way, in the transversely coupled SAW resonator filter according to preferred embodiments of the present invention, a SAW resonator filter having a multiple-stage arrangement can be provided by connecting a plurality of transversely coupled SAW resonator filter portions on a single piezoelectric substrate  12 . 
     Further, FIG. 7 similarly shows a two-stage transversely connected resonator filter, in which two transversely coupled SAW resonator filters are provided on a single piezoelectric substrate. The SAW resonator filter  21  shown in FIG. 7 has an arrangement such that two of the SAW resonator filters  2  shown in FIG. 5 are arranged substantially perpendicular to the direction of propagation of the surface acoustic wave. The SAW resonator filter portions  21 A and  21 B are electrically connected, and transversely connected, by connection lead portions  23  and  24 . 
     FIG. 8 is a perspective view showing a longitudinally coupled SAW resonator filter according to another preferred embodiment of the present invention. 
     The SAW resonator filter  31  is provided by using a substantially rectangular piezoelectric substrate  32 . The piezoelectric substrate  32  is preferably made of the same material as the piezoelectric substrate  2 . Similarly, the piezoelectric substrate  32  has opposing edges  32   a  and  32   b . Further, first and second IDTs  33  and  34  are provided on the top surface  32   c  of the piezoelectric substrate  32 . The IDTs  33  and  34  are preferably defined by patterning a metal film such as aluminum. 
     The IDT  33 , which defines a first SAW resonator has multiple electrode fingers  33   a  to  33   j , which extend substantially perpendicular to the direction of propagation of the surface acoustic wave, and is divided into two sub-IDT portions  33 A and  33 B. That is, the sub-IDT portion  33 A has electrode fingers  33   a  to  33   e , and the sub-IDT portion  33 B has the electrode fingers  33   f  to  33   j.    
     In the sub-IDT portion  33 A, ends of the electrode fingers  33   a ,  33   c , and  3   e  are electrically connected to a common bus bar  33   k  that extends along the surface acoustic wave propagation direction. On the other hand, the electrode fingers  33   b  and  33   d  are connected to a bus bar  31 . Therefore, the electrode fingers  33   a ,  33   c  and  33   e  are arranged so that they are interdigitated with the electrode fingers  33   b  and  33   d.    
     In the sub-IDT portion  33 B, ends of the electrode fingers  33   f ,  33   h  and  33   j  are connected to the bus bar  33   k . Furthermore, ends of the electrode fingers  33   g  and  33   i  are electrically connected to a bus bar  33   m . Therefore, the electrode fingers  33   f ,  33   h  and  33   j  are arranged so as to be interdigitated with the electrode fingers  33   g  and  33 i. 
     As a result of the connection, sub-IDTs  33 A and  33 B are connected in series between a pair of input terminals IN and aligned along the surface acoustic wave propagation direction. 
     The electrode finger  33   a  is provided along the end edge defined by the edge  32   a  and the top surface  32   c , and has a width of approximately λ/8 when the wavelength of the excited surface acoustic wave is λ. The width of the remaining electrode fingers  33   b  to  33   j  is approximately λ/4. Furthermore, the width of the gaps extending along the surface acoustic wave propagation direction between the electrode fingers is approximately λ/4. That is, the space between the sub-IDTs  33 A and  33 B is set at about λ/4. 
     The IDT  34  defining the second resonator is provided along the surface acoustic wave propagation direction with respect to the IDT  33 . Like the IDT  33 , the IDT  34  has two sub-IDT portions  34 A and  34 B. The sub-IDT portion  34 A is provided on the side of the IDT  33 , and has electrode fingers  34   a  to  34   e . The sub-IDT portion B has electrode fingers  34   f  to  34   j.    
     In the sub-IDT portion  34 A, ends of the electrode fingers  34   a ,  34   c  and  34   e  are connected to a bus bar  34   k . On the other hand, ends of the electrode fingers  34   b  and  34   d  are connected to a bus bar  34   l , on the opposite side of the bus bar  34   k . Similarly, in the sub-IDT portion  34 B, ends of the electrode fingers  34   f ,  34   h  and  34   j  are connected to a bus bar  34   k , and ends of the electrode fingers  34   g  and  34   i  are connected to the bus bar  34   m.    
     In the sub-IDT portion  34 A, the electrode fingers  34   a ,  34   c  and  34   e  are arranged so as to be interdigitated with the electrode fingers  34   b  and  34   d . In the sub-IDT portion  34 B, the electrode fingers  34   f ,  34   h  and  34   j  are arranged so as to be interdigitated with the electrode fingers  34   g  and  34   i.    
     As a result of the connection, sub-IDTs  34 A and  34 B are connected in series between a pair of output terminals OUT and aligned along the direction of propagation of the surface acoustic wave. 
     Further, the outermost side electrode finger  34   j  is provided along the edge defined by the edge  32   b  and the top surface  32   c  of the piezoelectric substrate  32 . This electrode finger  34   j  has a width of approximately λ/8, similar to the electrode finger  33   a . Further, the width of the remaining electrode fingers  34   a  to  34   i , and the width of the gaps extending along the surface acoustic wave propagation direction between the electrode fingers, is approximately λ/4. That is, the space between the sub-IDTs  34 A and  34 B is about λ/4. In addition, the space between the IDT  33  and the IDT  34  is about λ/4. 
     In the longitudinally coupled SAW resonator filter  31  of the present preferred embodiment, the above-mentioned IDTs  33  and  34  are provided between the two opposite edges  32   a  and  32   b , thereby defining an edge reflection type SAW resonator filter. During operation, SH-type surface acoustic waves, for instance BGS waves or Love waves, are excited and resonance modes created by the resonators including the IDTs  33  and  34  are coupled, whereby the filter operates as a longitudinally coupled SAW resonator filter. 
     According to the SAW resonator filter  31  of the present preferred embodiment, in the edge reflection type SAW resonator filter having the IDTs  33  and  34  as described above, since each of the IDTs  33  and  34  is divided so as to have sub-IDT portions  33 A and  33 B, and sub-IDT portions  34 A and  34 B respectively, the effective electromechanical coupling coefficient is decreased. Therefore, the frequency differences Δf between the anti-resonant frequency and the resonant frequency of each of the two resonators are both greatly reduced. Consequently, with the longitudinally coupled resonator filter created by the coupling of the two resonators, the bandwidth can be made narrower than in the conventional longitudinally coupled SAW resonator filter  51  shown in FIG.  19 . This will be explained based on a detailed test example. 
     As shown in FIG. 19, a conventional SAW resonator filter  51  having a center frequency of 41.3 MHz was made using a substantially rectangular piezoelectric substrate including ceramic material and having dimensions of 2.1 mm×1.2 mm with a thickness of 0.5 mm, as the piezoelectric substrate  52 , the logarithms of the electrode fingers of the IDTs  53  and  54  being, respectively 20 pairs and 15 pairs, and the width of the intersections of the electrode fingers being 4 λ in each case. FIG. 10 shows the frequency amplitude characteristics of this SAW resonator filter  51 . 
     For comparison, a SAW resonator filter  31  of the present preferred embodiment with a center frequency of 41.3 MHz was made using the same piezoelectric substrate as above, the logarithms of the electrode fingers of the IDTs  33  and  34  being, respectively 20 pairs and 15 pairs, the width of the intersections of the electrode fingers being 4 λ in each case, but the IDTs  33  and  34  being divided into two as shown in FIG.  8 . FIG. 11 shows the attenuation frequency characteristics of the SAW resonator filter  31 . 
     In the characteristics of the SAW resonator filter  51  as shown in FIG. 10, the width of the 3 dB attenuation band where the attenuation amount is 3 dB was 1170 kHz, but in the present preferred embodiment, it was narrowed to 860 kHz. Therefore, it can be seen that according to the SAW resonator filter  31  of the present preferred embodiment, the bandwidth is narrower than in the conventional SAW resonator filter  51  which uses undivided IDTs  53  and  54 . 
     FIG. 9 is a perspective view showing a longitudinally coupled SAW resonator filter according to another preferred embodiment of the present invention. In the SAW resonator filter  41  of FIG. 9, a substantially rectangular piezoelectric substrate  42  has opposing edges  42   a  and  42   b . The piezoelectric substrate  42  includes the same material as the piezoelectric substrate  2  of FIG.  1 . First and second IDTs  43  and  44  are provided on the top surface  42   c  of the piezoelectric substrate  42  along the surface acoustic wave propagation direction. 
     The SAW resonator filter  41  of the present preferred embodiment differs from the SAW resonator filter  31  of FIG. 8 in that the first and second IDTs  43  and  44  each have three sub-IDT portions  43 A to  43 C, and  44 A to  44 C, which are connected in series, respectively. 
     That is, taking the IDT  43  as an example, the IDT  43  has electrode fingers  43   a  to  43   l  extending substantially perpendicular to the direction of propagation of the surface acoustic wave, but the sub-IDT portion  43 A has electrode fingers  43   a  to  43   d , the sub-IDT portion  43 B has electrode fingers  43   e  to  43   h , and the sub-IDT portion  43 C has electrode fingers  43   i  to  43   l.    
     The width of the electrode finger  43   a  is approximately λ/8, and the width of the remaining electrode fingers  43   b  to  43   l  is approximately λ/4. Furthermore, the width of the gaps extending along the surface acoustic wave propagation direction between the electrode fingers is approximately λ/4. 
     The IDT  44  has electrode fingers  44   a  to  44   l , and like the IDT  43  it has three sub-IDT portions  44 A to  44 C. 
     In the IDT  44 , the electrode finger  44   l  is provided along the edge defined by the edge  42   b  and the top surface  42   c , has a width of approximately λ/8, and the remaining electrode fingers  44   a  to  44   k  have a width of approximately λ/4. 
     As described above, in the SAW resonator filter  41 , the IDTs  43  and  44  each define SAW resonators, and the IDTs  43  and  44  are divided so as to have three sub-IDT IDT portions  43 A to  43 C, and  44 A to  44 C. Consequently, the effective electromechanical coupling coefficient can be reduced even further than in the SAW resonator filter  31  of the previously described preferred embodiment, thus further narrowing the bandwidth. This will be explained based on a detailed test example. 
     FIG. 12 shows frequency amplitude characteristics of the SAW resonator filter  41  made in the same way as the SAW resonator filter  31  in the preceding experimental example, characteristics of which are shown, respectively in FIG.  10  and FIG. 11 above, with the exception that the IDTs  43  and  44  are divided into three so as to have sub-IDT portions  43 A to  43 C, and  44 A to  44 C, as described above. As is clear from a comparison of FIG. 12 with FIGS. 10 and 11, by dividing the IDTs  43  and  44  into three portions, the bandwidth can be narrowed even further. 
     Furthermore, the inventors of the present invention made longitudinally coupled SAW resonator filters where the first and second IDTs were divided into four and five portions, while being identical in other respects to the above test example, and measured the frequency amplitude characteristics. The results are shown in FIG.  13  and FIG.  14 . 
     As is clear by comparing FIG.  13  and FIG. 14 with FIG.  11  and FIG. 12, it is possible to obtain an even narrower band by increasing the number of divisions. 
     Considering the results of FIG. 12 to FIG. 14, various SAW resonator filters having different numbers of IDT divisions were made, and the relationship between the number of divisions, the 10 dB attenuation bandwidth, and the insertion loss, were determined. The results are shown in FIG.  15  and FIG.  16 . Furthermore, the relationship between the number of divisions, the electromechanical coupling coefficient, and the frequency difference Δf between the resonant frequency and the anti-resonant frequency, determined from the resonance characteristics of the IDT on one side of the resonator filter, was determined. The results are shown in FIG.  17  and FIG.  18 . 
     As is clear from FIGS. 15 to  18 , the width of the 10 dB attenuation band becomes narrower as the number of divisions of the IDT increases, making it possible to narrow the bandwidth. However, it can be seen that insertion loss increases as the number of division increases. In particular, when the number of divisions is increased from four to five, the insertion loss is increased rapidly as seen in FIG.  16 . 
     Therefore, when forming an edge reflection type longitudinally coupled SAW resonator filter using SH-type surface acoustic waves, the number of divisions of the first and second IDTs should preferably be within a range of two to four, thereby allowing the bandwidth to be very narrow while preventing an increase in the insertion loss. 
     The present invention can be suitably applied to various electronic components or devices utilizing a surface acoustic wave filter in which the unique features of preferred embodiments of the present invention are successfully employed. For example, the present invention may be applied to a duplexer and communication apparatus including the duplexer. 
     FIG. 21 is a block diagram of a communication apparatus  80  having a duplexer  70 . The communication apparatus  80  may be, for example, a cellular phone since a cellular phone usually requires a small handy body and a high selectivity of signals and so is suitable to enjoy the benefits of a filter made according to the present invention. 
     The communication apparatus  80  includes a duplexer  70 , an antenna  81 , a receiver  82  and a transmitter  83 . The duplexer  70  preferably includes a SAW filter  71  and a SAW filter  72 , where one end of the SAW filter  71  and the SAW filter  72  are connected in parallel to define a first terminal  73 . The other ends of the SAW filter  71  and the SAW filter  72  are connected to a second terminal  74  and a third terminal  75 . The SAW filter  71  and the SAW filter  72  may be any one of the SAW filters of preferred embodiments of the present invention. The antenna  81 , the receiver  82  and the transmitter  83  are connected to the first terminal  73 , the second terminal  74  and the third terminal  75  of the duplexer  70 . 
     The pass bands of the SAW filters  71  and  72  of the duplexer  70  are selected such that the signals received through the antenna  81  passes through the SAW filter  71  and are blocked by the SAW filter  72  and that the signals to be transmitted from the transmitter  83  passes through the SAW filter  72 . 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.