Abstract:
A surface acoustic wave device supported by a package body. At least one surface acoustic wave element having interdigital electrodes disposed on a propagation path of a surface acoustic wave on the piezoelectric substrate. These interdigital electrodes include an input-side interdigital electrode connected to a ground pad on the package body and an output-side interdigital electrode connected to another ground pad on the package body.

Description:
This application is a divisional of application Ser. No. 08/760,097, filed Dec. 3, 1996, now U.S. Pat. No. 5,963,114. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to surface-acoustic-wave (SAW) devices and more particularly to a SAW device having an improved passband characteristic. Further, the present invention relates to a SAW device that is flexible in design for setting input and output impedances of the SAW device as desired. 
     SAW devices are used extensively for a filter or a resonator in compact radio telecommunication apparatuses operational in a VHF or UHF band, a typical example being a portable telephone apparatus operational in a MHz band or GHz band. 
     In such high frequency radio telecommunication apparatuses, it is required that the SAW filters or SAW resonators used therein have a wide pass-band and simultaneously a very sharp off-band attenuation. Further, the SAW filters and resonators should be able to achieve an impedance matching with a cooperating circuit, which may be an integrated circuit forming the electronic apparatus in which the SAW device is used. 
     FIGS. 1A and 1B show the construction of a typical conventional SAW filter. 
     Referring to FIG. 1A, the SAW filter is a device of the so-called double-mode type and includes a pair of reflectors  10 A and  10 B on a piezoelectric substrate I as usual in a SAW filter, wherein the piezoelectric substrate may be a Y-X cut single-crystal plate of LiTaO 3  or LiNbO 3 . Further, electrodes  11 A,  11 B and  11 C are provided consecutively between the foregoing reflectors  10 A and  10 B from the reflector  10 A to the reflector  10 B. 
     In the illustrated example of FIG. 1A, the substrate  1  is formed of a single-crystal plate of 36° Y-X LiTaO 3 , and the reflectors  10 A and  10 B, aligned in an X-direction of the substrate  1 , define a propagation path of a surface acoustic wave excited on the piezoelectric substrate  1 . Each of the electrodes  11 A,  11 B and  11 C includes a primary-side interdigital electrode such as an electrode ( 11 A) 1 , ( 11 B) 1  or ( 11 C) 1  and a secondary-side interdigital electrode such as an electrode ( 11 A) 2 , ( 11 B) 2  or ( 11 C) 2 , wherein the primary-side electrode and the secondary-side electrode are disposed such that the electrode fingers of the primary-side electrode and the electrode fingers of the corresponding secondary-side electrode extend in respective, mutually opposing directions, as usual in an interdigital electrode. Thereby, the electrode fingers of the primary-side electrode and the electrode fingers of the secondary-side electrode are repeated alternately in the X-direction on the substrate  1  and intersect the path of the surface acoustic wave traveling in the X-direction on the substrate  1 . The pitch of the electrode fingers is determined by a central frequency of the SAW filter to be formed as well as by the sound velocity of the surface acoustic wave traveling on the substrate  1  in the X-direction. When viewed in the X-direction, the electrode fingers of the primary-side electrode and the electrode fingers of the secondary-side electrode overlap with each other over an overlap width W. 
     In the construction of FIG. 1A, the primary-side electrode ( 11 A) 1 , of the electrode  11 A is connected to an input terminal commonly with the primary-side electrode ( 11 C) 1  of the electrode  11 C. On the other hand, the secondary-side electrodes ( 11 A) 2  and ( 11 C) 2  are both grounded. Thereby, the SAW filter of FIG. 1A forms a device of the so-called dual input single-output type. 
     The double-mode SAW filter of such a construction uses a first-order mode of surface acoustic wave formed between the foregoing reflectors  10 A and  10 B with a frequency f 1  and a third-order mode of surface acoustic wave formed also between the reflectors  10 A and  10 B with a frequency f 3 , wherein the SAW filter forms a pass-band characteristic as indicated in FIG.  2 . FIG. 2 shows the attenuation of the SAW filter as a function of the frequency. In FIG. 2, it should be noted that a pass-band is formed between the foregoing frequency f 1  of the first-order mode and the frequency f 3  of the third-order mode. FIG. 1B shows the energy distribution of the surface acoustic wave excited in the structure of FIG.  1 A. 
     Conventionally, it has been practiced to form the interdigital electrodes  11 A- 11 C to be generally symmetric about the center of the X-axis in view of the corresponding symmetricity of the first-order-mode and the third-order mode of the excited surface acoustic waves (see FIG.  1 B), so that the first order-mode surface acoustic wave and the third-order-mode surface acoustic wave are excited efficiently. Thus, it has been practiced conventionally to set a number N 1  indicating the number of the electrode finger pairs formed by the primary-side electrode fingers and the secondary-side electrode fingers in the interdigital electrode  11 A, to be equal to a number N 3  indicating the number of the electrode finger pairs formed by the primary-side electrode fingers and the secondary-side electrode fingers in the interdigital electrode  11 C (N 1 =N 3 ). 
     However, FIG. 2 clearly indicates that various spurious peaks exist in the SAW device outside the pass-band defined by the frequencies f 1  and f 3 . As a result of the existence of such spurious peaks, it should be noted that the sharpness of attenuation of surface acoustic wave outside the pass-band is reduced unwantedly, particularly in the frequency range between 1550 MHz and 1600 MHz. It should be noted that the attenuation of a SAW filter or resonator should be flat and minimum inside the pass-band and increase sharply outside the pass-band. In order to maximize the selectivity of the filter, it is desired to maximize the attenuation outside the passband. 
     In the conventional SAW filter of FIG. 1A, all of the interdigital electrodes  11 A,  11 B and  11 C have the same overlap width W of the electrode fingers. Thus, the input and the output impedances of the SAW filter are determined by the number of pairs of the electrode fingers in the electrodes  11 A- 11 C. Generally, it should be noted that the input and output impedances of a SAW filter are inversely proportional to the number of the electrode finger pairs N 1  and N 3  and the overlapping W for the electrodes  11 A- 11 C. As the number N 1  and the number N 3  of the electrode finger pairs are set equal to each other and the overlap width W is constant in conventional SAW devices, it has been difficult to set the input impedance and the output impedance independently and as desired. Thus, conventional SAW devices have failed to meet the demand for the capability of flexibly setting the input and output impedances, while such a demand of flexible setting of the input and output impedances is particularly acute in recent compact radio apparatuses for GHz applications such as a portable or mobile telephone apparatus. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general object of the present invention to provide a novel and useful SAW device wherein the foregoing problems are eliminated. 
     Another and more specific object of the present invention is to provide a SAW device capable of suppressing spurious peaks effectively outside the pass-band and simultaneously providing a sharp attenuation outside the pass-band. 
     Another object of the present invention is to provide a SAW device capable of setting an input impedance and an output impedance independently and flexibly. 
     Another object of the present invention is to provide a SAW device, comprising: 
     a piezoelectric substrate; 
     at least first and second SAW elements formed commonly on said piezoelectric substrate each along a predetermined propagation path of a surface acoustic wave on said piezoelectric substrate; 
     each of said first and second SAW elements including a plurality of interdigital electrodes disposed along said predetermined propagation path of said surface acoustic wave; 
     each of said plurality of interdigital electrodes including a primary-side electrode that in turn includes a plurality of mutually parallel electrode fingers extending in a first direction across said propagation path and a secondary-side electrode that in turn includes a plurality of mutually parallel electrode fingers extending in a second, opposite direction across said propagation path; 
     said electrode fingers of said primary-side electrode and said electrode fingers of said secondary-side electrode being disposed, in each of said interdigital electrodes in each of said first and second SAW elements, alternately along said propagation path so as to overlap with a predetermined overlap width when viewed in a direction of said propagating path; 
     said overlap width having a first value commonly in said plurality of interdigital electrodes forming said first SAW element and a second, different value commonly in said plurality of interdigital electrodes forming said second SAW element; 
     said first SAW element being cascaded to said second SAW element by connecting a secondary-side electrode of an interdigital electrode included in said first SAW element to a primary-side electrode of an interdigital electrode included in said second SAW element. 
     Another object of the present invention is to provide a SAW device, comprising: 
     a piezoelectric substrate; 
     first and second reflectors provided on said piezoelectric substrate along a propagation path of a surface acoustic wave excited on said piezoelectric substrate; and 
     a plurality of interdigital electrodes disposed on said piezoelectric substrate consecutively from said first reflector to said second reflector; 
     each of said plurality of interdigital electrodes including a primary-side electrode that includes a plurality of mutually parallel electrode fingers extending in a first direction across said propagation path of said surface acoustic wave and a secondary-side electrode that includes a plurality of mutually parallel electrode fingers extending in a second, opposite direction across said propagation path of said surface acoustic wave, said electrode fingers of said primary-side electrode and said electrode fingers of said secondary-side electrode being disposed, in each of said plurality of interdigital electrodes, alternately along said propagation path and overlapping with a predetermined overlap width when viewed in a direction of said propagation path of said surface acoustic wave; 
     said plurality of interdigital electrodes being cascaded by connecting a secondary-side electrode of an interdigital electrode to a secondary-side electrode of another interdigital electrode. 
     According to the present invention, it is possible to set the input impedance and output impedance of the SAW device as desired, by appropriately setting the overlap of the electrode fingers in the first SAW element and in the second SAW element or in a first interdigital electrode and a second interdigital electrode cascaded to the first interdigital electrode, without changing the pitch of the interdigital electrodes. As the pitch of the interdigital electrodes is not changed, the frequency characteristic of the SAW filter is not influenced, and only the input and output impedances are set independently and arbitrarily in the present invention according to the demand of the circuit design. 
     As a result of such an arbitrary setting of the input and output impedances, a number of such SAW filters can be cascaded successfully, resulting an improved suppression of spurious peaks outside the pass-band. In other words, a SAW filter having a very sharp selectivity is obtained. Further, by cascading a number of SAW filters to form a SAW filter assembly, it is possible to set the ratio between the input impedance and the output impedance of the SAW filter assembly to be a very large value not attainable by a single stage SAW filter. 
     Another object of the present invention is to provide a SAW device, comprising: 
     a package body supporting a piezoelectric substrate thereon; 
     at least one SAW element formed on said piezoelectric substrate; 
     said SAW element including a plurality of interdigital electrodes disposed along a propagation path of a surface acoustic wave on said piezoelectric substrate, each of said interdigital electrodes including an input-side interdigital electrode and an output-side interdigital electrode; 
     said input-side interdigital electrode being connected to a first ground pad provided on said package body; 
     said output-side interdigital electrode being connected to a second, different ground pad provided on said package body. 
     According to the present invention, the problem of interference between the ground electrode of the input-side interdigital electrode and the ground electrode of the output-side interdigital electrode is successfully eliminated, and the pass-band characteristics of the SAW device is improved substantially. 
     Another object of the present invention is to provide a double-mode SAW device, comprising: 
     a piezoelectric substrate; 
     first and second reflectors provided on said piezoelectric substrate along a propagation path of a surface acoustic wave on said piezoelectric substrate; 
     first, second and third interdigital electrodes provided on said piezoelectric substrate consecutively from said first reflector to said second reflector; 
     each of said first through third interdigital electrodes including first through third number of pairs of electrode fingers respectively; 
     wherein said first number of pairs of electrode fingers for said first interdigital electrode is different from said third number of pairs of electrode fingers for said third interdigital electrode. 
     According to the present invention, the symmetricity in the structure of the SAW device in the propagating direction of the SAW device is intentionally lost by setting the first and third number of pairs of the electrode fingers differently. Thereby, the surface acoustic wave reflected by the first reflector and the surface acoustic wave reflected by the second reflector cancel with each other, and the spurious peaks associated with such an interference of the reflected surface acoustic wave devices is successfully eliminated, 
     Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are diagrams respectively showing a construction and an operational principle of a conventional double-mode SAW filter; 
     FIG. 2 is a diagram showing a theoretical frequency characteristic of the SAW filter of FIGS. 1A and 1B; 
     FIG. 3 is a diagram showing a construction of a SAW filter according to a first embodiment of the present invention; 
     FIG. 4 is a diagram showing a theoretical frequency characteristic of the SAW filter of FIG. 3 in comparison with a corresponding theoretical frequency characteristic of the SAW filter of FIGS. 1A and 1B; 
     FIG. 5 is a diagram showing an observed frequency characteristic of the SAW filter of FIG. 3; 
     FIG. 6 is a diagram showing an observed frequency characteristic of SAW filter of FIGS.  1 A and  1 B;, 
     FIG. 7 is a diagram showing a construction of a SAW filter according to a second embodiment of the present invention; 
     FIG. 8 is a diagram showing a modification of the SAW filter of FIG. 7; 
     FIG. 9 is a diagram showing a further modification of the SAW filter of FIG. 7; 
     FIG. 10 is a diagram showing a frequency characteristic of the SAW filter of FIG. 9; 
     FIG. 11 is a diagram showing a construction of a SAW filter according to a third embodiment of the present invention; 
     FIG. 12 is a diagram showing a construction of a SAW filter according to a fourth embodiment of the present invention; 
     FIG. 13 is a diagram showing a frequency characteristic of the SAW filter of FIG. 12; 
     FIG. 14 is a diagram showing the construction of the SAW filter of FIG. 12 including a metal cap in an exploded state; 
     FIG. 15 is a diagram showing a frequency characteristic of the SAW filter of FIG. 14; 
     FIG. 16 is a diagram showing a construction of a SAW filter according to a sixth embodiment of the present invention; 
     FIG. 17 is a diagram showing a construction of a SAW filter according to a seventh embodiment of the present invention; 
     FIG. 18 is a diagram showing a modification of the SAW filter of FIG. 17; 
     FIG. 19 is a diagram showing another modification of the SAW filter of FIG. 17; and 
     FIG. 20 is a diagram showing a further modification of the SAW filter of FIG.  17 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIRST EMBODIMENT 
     First, the principle of the present invention will be described with reference to FIG. 3 showing a SAW filter  11  according to a first embodiment of the present invention, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 3, the SAW filter  11  has a double-mode construction similar to the conventional SAW filter of FIG. 1A, except that the number of the electrode finger pairs N 1  for the interdigital electrode  11 A and the number of the electrode finger pairs N 3  for the interdigital electrode  11 C, and further the number of the electrode finger pairs N 2  for the interdigital electrode  11 B, are changed from each other (N 1 ≠N 3 ≠N 2 ). 
     FIG. 4 shows a calculated, theoretical frequency characteristic of the SAW filter  11  of FIG. 3, wherein the continuous line of FIG. 4 indicates the result for a conventional case in which the numbers of the electrode finger pairs N 1 , N 2  and N 3  are set respectively to 20, 40 and 20. It should be noted that a relationship N 1 =N 3  holds in this case. Further, the broken line of FIG. 4 indicates the result for a case in which the numbers of the electrode finger pairs N 1 , N 2  and N 3  are set to 25, 35 and 45, respectively. In this case, a relationship N 1 ≠N 3 ≠N 2  holds. The dotted line of FIG. 4 indicates the result for a case in which the numbers of the electrode finger pairs N 1 , N 2  and N 3  are set respectively to 20, 40 and 30. In this case, too, the relationship N 1 ≠N 3 ≠N 2  holds. 
     In the calculation of FIG. 4, it should further be noted that a single crystal plate of 36° Y-X LiTaO 3  is assumed for the substrate  1 , and the calculation is made for the case in which the interdigital electrodes on the substrate  1  is formed of Al with a thickness corresponding to 8% the wavelength of the surface acoustic wave excited on the substrate  1 . 
     In the course of investigation including such a theoretical calculation of the frequency characteristic of the SAW filter  11 , the inventor of the present invention has discovered, as indicated in FIG. 4, that the height of the spurious peaks outside the pass-band decreases significantly and substantially when the numbers N 1  and N 3  of the electrode finger pairs for the interdigital electrode  11 A and  11 C are set asymmetric (N 1 ≠N 3 ) about the central interdigital electrode  11 B. While the reason of such a suppressing of the spurious peaks is not fully understood, it is thought that such an asymmetric construction of the SAW filter  11  facilitates cancellation of the surface acoustic waves excited by the interdigital electrode  11 A and returning to the interdigital electrode  11 B after reflection at the reflector  10 A and the surface acoustic waves excited by the interdigital electrode  11 C and returning to the interdigital electrode  11 B after reflection at the reflector  10 B. 
     FIG. 5 shows an actually observed frequency characteristic of the SAW filter  11  of FIG. 3 for the case in which a 42° Y-X LiTaO 3  single crystal plate is used for the piezoelectric substrate  1  and the numbers of the electrode finger pairs N 1 , N 2  and N 3  are set to 20, 40 and 30, respectively (N 1 :N 2 :N 3 =20:40:30). In FIG. 5, it should also be noted that the interdigital electrodes  11 A- 11 C are formed of A 1  with a thickness corresponding to 6% the wavelength of the surface acoustic wave excited on the substrate  1 . 
     FIG. 6, on the other hand, shows an actually observed frequency characteristic of the conventional SAW filter of FIG. 1A for the case in which a 42° Y-X LiTaO 3  single crystal plate is used for the piezoelectric substrate  1  similarly to the case of FIG.  5  and the numbers of the electrode finger pairs N 1 , N 2  and N 3  are set respectively to 21, 35 and 21 (N 1 :N 2 :N 3 =21:35:21). The interdigital electrodes  11 A- 11 C are formed of Al with a thickness corresponding to 6% the wavelength of the surface acoustic wave excited on the substrate  1 , similarly to the case of FIG.  5 . 
     Referring to FIGS. 5 and 6, it should be noted that the height of the predominant spurious peaks appearing at the lower-frequency side of the pass-band in the characteristic of FIG. 6 is reduced substantially in the characteristic of FIG.  5 . Further, the spurious peak appearing on the higher-frequency side of the pass-band is suppressed substantially. 
     It should be noted that the SAW filter  11  of the present embodiment is designed for use in a GHz band. In a SAW filter for use in such a ultra-high frequency band, it should be noted that the thickness of the interdigital electrode on the piezoelectric substrate  1  is no longer ignorable with respect to the wavelength of the excited SAW, and an added-mass effect of the electrode appears conspicuously. Such an added-mass effect causes a shift of the optimum cut angle of a LiTaO 3  or LiNbO 3  single-crystal substrate to a higher-angle side. In the case of a LiTaO 3  substrate, the optimum cut angle becomes 40° Y-44° Y, which is substantially higher than the conventionally used optimum cut angle of 36° Y. In the case of a LiNbO 3  substrate, the optimum cut angle falls in the range between 66° Y and 74° Y when the added-mass effect of the electrode is considered. 
     In the SAW filter  11 , it should be noted that the added-mass effect appears particularly conspicuous when the thickness of the interdigital electrodes  11 A- 11 C is in the range of 5-10% the wavelength of the excited SAW, provided that LiTaO 3  is used for the substrate  1  and the electrodes  11 A- 11 C are formed of Al or an Al alloy. When LiNbO 3  is used for the substrate  1 , on the other hand, the added-mass effect appears conspicuous when the thickness of the interdigital electrodes  11 A- 11 C falls in the range of 4-12% the wavelength of the excited surface acoustic wave. In this case, too, use of Al or an Al-alloy is assumed for the interdigital electrodes  11 A- 11 C. 
     SECOND EMBODIMENT 
     Next, a SAW filter circuit device according to a second embodiment of the present invention will be described with reference to FIG. 7, wherein those parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 7, the SAW filter circuit device of the present embodiment is constructed on the substrate  1  of 42° Y-X LiTaO 3  single crystal plate and includes, in addition to the SAW filter  11 , another SAW filter  21  that includes reflectors  20 A and  20 B aligned on the same substrate  1  in the X-direction, wherein the SAW filter  20  further includes interdigital electrodes  21 A,  21 B and  21 C disposed consecutively from the reflector  20 A to the reflector  20 B. Similarly as before, the SAW filter  11  includes the reflectors  10 A and  10 B as well as the interdigital electrodes  11 A- 11 C, all disposed on the same, common substrate  1 . 
     In the construction of FIG. 7, it should be noted that the secondary-side electrode ( 11 B) 2  forming a part of the interdigital electrode  11 B is connected to a corresponding primary-side electrode ( 21 B) 1  of the interdigital electrode  21 B. Thereby, the SAW filter  11  and the SAW filter  21  are cascaded. In each of the SAW filters  11  and  21 , the foregoing relationship of N 1 ≠N 2 ≠N 3  may hold similarly to case of the first embodiment. The present embodiment, however, includes also the case in which the foregoing relationship does not hold. 
     In the embodiment of FIG. 7, it should be noted that each of the primary-side electrodes ( 11 A) 1 , and ( 11 C) 1 , respectively of the interdigital electrodes  11 A and  11 C, are connected commonly to an input electrode pad. Further, the secondary-side electrodes ( 11 A) 2  and ( 11 C) 2  of the interdigital electrodes  11 A and  11 C as well as the primary side electrode ( 11 B) 1 , of the interdigital electrode  11 B are grounded. Thereby, the SAW filter  11  forms a filter of a so-called dual-input single-output type. On the other hand, secondary-side electrodes ( 21 A) 2  and ( 21 C) 2  respectively of the interdigital electrodes  21 A and  21 C are connected commonly to an output electrode pad in the SAW filter  21 . Further, primary-side electrodes ( 21 A) 1  and ( 21 C) 1  respectively of the interdigital electrodes  21 A and  21 C as well as a secondary-side electrode ( 21 B) 2  of the interdigital electrode  21 B are grounded. Thereby, the SAW filter  21  forms a filter of a single-input dual-output type. 
     In the embodiment of FIG. 7, the electrode fingers overlap with each other in the SAW filter  11  with an overlap width W 1  when viewed in the traveling direction of the surface acoustic wave in the SAW filter  11 . Similarly, the electrode fingers overlap with each other in the SAW filter  21  with an overlap width W 2  when viewed in the traveling direction of the surface acoustic wave in the SAW filter  21 , wherein the SAW filters  11  and  21  are formed such that the overlap width W 2  for the SAW filter  21  is different from the overlap width W 1  for the SAW filter  11 . Thereby, the SAW filter circuit device as a whole shows an input impedance equal to the input impedance of the SAW filter  11  and an output impedance equal to the output impedance of the SAW filter  21 , wherein the input impedance of the SAW filter  11  is determined by the foregoing overlap width W 1 , while the output impedance of the SAW filter  21  is determined by the overlap width W 2 . Thus, by setting the overlap widths W 1  and W 2  independently, it is possible to design the input impedance and the output impedance of the SAW filter circuit device independently and as desired. 
     FIG. 8 shows a modification of the SAW filter circuit device of FIG. 7, wherein those parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 8, it should be noted that the primary-side electrode ( 11 B) 1  of the interdigital electrode  11 B is connected to an input electrode pad and the secondary-side electrode ( 11 B) 2  is grounded. On the other hand, the primary-side electrodes ( 11 A) 1  and ( 11 C) 1  of the interdigital electrodes  11 A and  11 C are grounded, and the secondary-side electrodes ( 11 A) 2  and ( 11 C) 2  of the interdigital electrodes  11 A and  11 C are connected respectively to the primary-side electrode ( 21 A) 1  of the interdigital electrode  21 A and the primary-side electrode ( 21 C) 1  of the interdigital electrode  21 C. Thus, the SAW filter  11  of the embodiment of FIG. 8 has a single-input dual-output construction. 
     In the SAW filter  21 , on the other hand, the secondary electrodes ( 21 A) 2  and ( 21 C) 2  of the interdigital electrodes  21 A and  21 C are grounded, and the output is obtained at the secondary-side electrode ( 21 B) 2  of the interdigital electrode  21 B. Thus, the SAW filter  21  has a dual-input single-output construction. 
     In the SAW filter device of FIG. 8, too, it is possible to set the input impedance and the output impedance of the SAW filter circuit device as desired, by setting the overlap width W 1  and the overlap width W 2  independently in the SAW filter  11  and the SAW filter  21 . 
     FIG. 9 shows a further modification of the SAW filter circuit device of FIG. 7, wherein those parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 9, the SAW filter  11  has a dual-input single-output construction similarly to the embodiment of FIG.  7 . Further, the SAW filter  21  has a dual-input single-output construction similarly to the embodiment of FIG.  7 . Thus, the primary-side electrodes ( 11 A) 1 , and ( 11 C) 1  of the interdigital electrodes  11 A and  11 C are connected commonly to an input electrode pad and the secondary-side electrodes ( 11 A) 2  and ( 11 C) 2  of the interdigital electrodes  11 A and  11 C as well as the primary-side electrode ( 11 B) 1 , of the interdigital electrode  11 B are grounded. 
     In the SAW filter  21 , the primary-side electrodes ( 21 A) 1  and ( 21 C) 1  of the interdigital electrodes  21 A and  21 C are connected commonly to the secondary-side electrode ( 11 B) 2  of the interdigital electrode  11 B, and the secondary-side electrodes ( 21 A) 2  and ( 21 C) 2  are grounded. Further, the secondary-side electrode ( 21 B) 2  of the interdigital electrode  21 B is connected to an output electrode pad. In other words, the construction of FIG. 9 includes two dual-input single-output SAW filters  11  and  21  in a cascaded connection. 
     In the SAW filter circuit device of FIG. 9, it should be noted that the SAW filters  11  and  21  are cascaded such that an impedance matching is established between the output side of the SAW filter  11  and the input side of the SAW filter  21 , for minimizing the loss occurring as a result of such a cascaded connection. 
     More specifically, it is known that there holds a general relationship 
      Z 1 :Z 2 =Z 3 :Z 4   
     between the SAW filter  11  and the SAW filter  21 , where Z 1 , and Z 2  respectively stand for the input impedance and output impedance of the SAW filter  11 , Z 3  and Z 4  respectively stand for the input impedance and output impedance of the SAW filter  21 . 
     The present embodiment realizes an impedance matching between the SAW filters  11  and  21  as represented by a condition 
     
       
         Z 2 =Z 3   
       
     
     by setting the overlap widths W 1  and W 2  appropriately. 
     As a result, there holds a relationship between the impedances Z 1 , Z 2 , Z 3  and Z 4  as follows: 
     
       
         Z 2 =Z 3 =(Z 1 ·Z 4 ) 
       
     
     In the SAW filter circuit device of FIG. 9, the overlap width W 1  of the SAW filter  11  is set to 60λwhile the overlap width W 2  of the SAW filter  21  is set to 35λ, wherein λ represents the wavelength of the surface acoustic wave excited on the piezoelectric substrate  1  and has a value of about 4.3 μm in the present example. Further, there holds the following relationship for the SAW filters  11  and  21  in the SAW filter device of FIG.  9 : 
     
       
         N 1 :N 2 :N 3 =15:21:15 
       
     
     In this case, the SAW filter  11  has an input impedance of 50Ω, wherein this value of the input impedance of the SAW filter  11  provides the input impedance of the cascaded SAW filter circuit device of FIG.  9 . Further, the SAW filter  21  thus configured has an output impedance of 150Ω, wherein this output impedance of the SAW filter  21  provides the output impedance of the cascaded SAW filter circuit device. 
     In the SAW filter circuit device of FIG. 9 where there exists an impedance matching between the cascaded SAW filters  11  and  21 , it is possible to increase the number of the cascaded stages further, such that the output impedance of the SAW filter circuit device becomes very much larger than or very much smaller than the input impedance of the same SAW filter circuit device. 
     Further, it should be noted that such a cascaded SAW filter circuit device, which may include many cascaded SAW filters therein, is extremely effective for suppressing the spurious peaks outside the pass-band and for improving the selectivity of the filter. 
     FIG. 10 shows the pass-band characteristic of the cascaded SAW filter circuit device of FIG. 9 for the case in which the input side is terminated by a resistance of 50Ω and the output side is terminated by a resistance of 150Ω. 
     Referring to FIG. 10, it will be understood that the spurious peaks outside the pass-band are effectively suppressed by cascading the SAW filters  11  and  21  as such. In other words, FIG. 10 indicates clearly that an effect of suppressing spurious peaks similarly to the effect achieved by the SAW filter of the first embodiment, is achieved also in the present embodiment. 
     THIRD EMBODIMENT 
     FIG. 11 shows the construction of a SAW filter according to a third embodiment of the present invention, wherein those parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     In the present embodiment, the input impedance and the output impedance are changed for a single SAW filter. 
     Referring to FIG. 11, it should be noted that the primary-side electrode ( 11 B) 1  of the interdigital electrode  11 B is connected to a first input electrode pad while the secondary-side electrode ( 11 B) 2  of the interdigital electrode  11 B is connected to a second input electrode pad. Thus, the SAW filter of FIG. 11 operates as a differential filter device when different input signals are supplied respectively to the foregoing primary-side electrode ( 11 B) 1  and the secondary-side electrode ( 11 B) 2 . Alternatively, the electrode ( 11 B) 2  may be grounded. 
     In the SAW filter of FIG. 11, the secondary-side electrode ( 11 A) 2  and the secondary side electrode ( 11 C) 2  are connected to each other, and the primary-side electrode ( 11 A) 1  of the interdigital electrode  11 A is connected to a first output electrode pad, the primary-side electrode ( 11 C) 1  of the interdigital electrode  11 C is connected to a second output terminal. Thereby, the interdigital electrode  11 A and the interdigital electrode  11 C are cascaded. The electrode ( 11 A) 1  and the electrode ( 11 C) 1  may be grounded. 
     In the construction of FIG. 11, it should be noted that the interdigital electrodes  11 A- 11 C have a common overlap width W for the electrode fingers. Even in such a construction, the output impedance of the SAW filter as a whole is provided by a sum of an output impedance Z 1  of the interdigital electrode  11 A and an output impedance Z 3  of the interdigital electrode  11 C. In other words, the construction of FIG. 11 allows an adaptation of the output impedance of the SAW filter as a whole with respect to the input impedance, although the degree of freedom of such an adjustment is limited somewhat as compared with the previous embodiment. In the SAW filter of FIG. 11, the input impedance is provided by the input impedance Z 2  of the interdigital electrode  11 B. 
     FOURTH EMBODIMENT 
     FIG. 12 shows a construction of a SAW filter according to a fourth embodiment of the present invention including a package, wherein those parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 12, the piezoelectric substrate  1  carrying thereon a SAW filter similar to the SAW filter of FIG. 7 is held on a ceramic package body  100 , wherein the package body  100  carries thereon ground electrode pads  101  and  103  at a first side thereof together with an input electrode pad  102  such that the ground electrode pads  101  and  103  are located at both lateral sides of the input electrode pad  102 . Similarly, the package body  100  carries thereon ground electrode pads  104  and  106  on a second, opposite side thereof together with an output electrode pad  105  such that the ground electrode pads  104  and  106  are located at both lateral sides of the output electrode pad  105 . 
     In the construction of FIG. 12, the ground electrode of the interdigital electrode  11 A corresponding to the electrode ( 11 A) 2  of FIG. 7 is connected to the ground electrode pad  101  on the package body  100  by an Al wire  107 . Further, the ground electrode of the interdigital electrode  11 C corresponding to the electrode ( 11 C) 2  of FIG. 7 is connected to the ground electrode pad  103  on the package body  100  by another Al wire  107 . Similarly, the ground electrode of the interdigital electrode  11 B corresponding to the electrode ( 11 B) 1  of FIG. 7 is connected to the foregoing ground electrode  103  by a still another Al wire  107 . Further, the output electrodes of the-interdigital electrodes  11 A and  11 C corresponding to the electrodes ( 11 A) 1  and ( 11 C) 1  are connected commonly to the input electrode pad  102  disposed between the ground electrode pad  101  and the ground electrode pad  103  by way of respective Al wires  107 . 
     In the SAW filter  21  formed also on the same piezoelectric substrate  1 , it should be noted that the ground electrode of the interdigital electrode  21 A corresponding to the electrode ( 21 A) 1  of FIG. 7 is connected to the ground electrode pad  104  on the package body  100  by another Al wire  107 . Further, the ground electrode of the interdigital electrode  21 C corresponding to the electrode ( 21 C) 1  of FIG. 7 is connected to the ground electrode pad  106  on the package body  100  by still another Al wire  107 . Further, the ground electrode of the interdigital electrode  21 B corresponding to the electrode ( 21 B) 2  of FIG. 7 is connected to the ground electrode pad  104  by another Al wire  107 . Further, the output electrodes of the interdigital electrodes  21 A and  21 C corresponding to the electrodes ( 21 A) 2  and ( 21 C) 2  are connected commonly to the output electrode pad  105  provided between the foregoing ground electrode pads  104  and  106  by means of another Al wire  107 . Furthermore, the SAW filter  11  and the SAW filter  21  are cascaded by connecting the secondary-side electrode ( 11 B) 2  of the interdigital electrode  11 B to the primary-side electrode ( 21 B) 1  of the interdigital electrode  21 B. 
     Generally, electrodes provided on a ceramic package more or less form a capacitive coupling with each other, while the present invention avoids the problem associated with such a capacitive coupling of the electrodes by disposing the input-side ground electrode pads  101  and  103  on the first edge of the package body  100  and the output-side ground electrode pads  104  and  106  on the second, opposite edge of the package body  100 . By disposing the ground electrodes as such, it is possible to avoid interference between the input-side ground electrode pads and the output-side ground electrode pads, and the selectivity of the SAW filter as a whole is improved. In the construction of FIG. 12, it should further be noted that the ground electrode pads  101  and  103  are separated and the ground electrode pads  104  and  106  are separated for further suppressing of the interference. 
     FIG. 13 shows the pass-band characteristic of the SAW filter of FIG. 12 for the case in which the ground connections of the SAW filters  11  and  21  are made both to the input side where the ground electrode pads  101  and  103  are provided and the output side where the ground electrode pads  104  and  106  are provided. 
     Referring to FIG. 13 showing characteristic curves A and B, the characteristic curve B represents the pass-band characteristic for the SAW filter of FIG. 12 as it is, while the characteristic curve A represents the pass-band characteristic of the SAW filter of FIG. 12 for the case in which the secondary-side electrodes ( 11 C) 2  and ( 11 A) 2  of the interdigital electrodes  11 A and  11 C are connected respectively to the ground electrode pads  101  and  104  and further to the ground electrode pads  103  and  106  by respective Al wires  107 . Similarly, the secondary-side electrodes ( 21 C) 2  and ( 21 A) 2  of the interdigital electrodes  21 A and  21 C are connected respectively to the ground electrode pads  101  and  104  and further to the ground electrode pads  103  and  106  by respective Al wires  107  in the case of the characteristic curve A. 
     As will be seen clearly from FIG. 13, the suppression of spurious peaks outside the pass-band of the SAW filter is deteriorated in the case of the characteristic curve A, indicating the effectiveness of the construction of FIG. 12 that provides the characteristic curve B. 
     FIFTH EMBODIMENT 
     A SAW filter is used generally in the form of a package in which the SAW filter is accommodated in a package body. Thus, the package of the SAW filter also requires an improvement, particularly with respect to a metal protective cap used in the package for protecting the SAW device accommodated therein. 
     FIG. 14 shows a fifth embodiment of the present invention directed to such an improvement of the package, wherein FIG. 14 shows the package that accommodates the SAW filter of FIG. 12 in an exploded state. In FIG. 14, those parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 14, the package includes a package body corresponding to the package body  100  of FIG. 12, wherein the package body  100  in turn is formed of a base  100 A and a holder piece  100 B provided on the base  100 A, wherein the holder piece  100 B is formed with a central opening for accommodating therein a SAW filter which may have a construction of FIG. 12, for example. Further, the SAW package includes a spacer member  110  provided on the foregoing package body  100 , and a metal cap  120  is provided on the spacer member  110  thus provided on the package body  100  for protecting the SAW filter held in the package body  100 . 
     It should be noted that the base  100 A of the package body  100  is formed with chamfered surfaces  100 A 1 ,  100 A 2 ,  100 A 3  and  100 A 4  at four corners thereof and a ground electrode  100 G is formed on the top surface as indicated in FIG. 14, wherein the ground electrode  100 G extends in the direction of the output-side edge in the form of electrode leads  100 G a  and  100 G b . Further, electrodes leads  100   ga  and  100   gb  extend in a downward direction on the side wall of the base  100 A respectively from the electrode leads  100 G a  and  100 G b . Similarly, electrode leads  100 G c  and  100 G d  extend from the ground electrode  100 G toward the input-side edge of the base  10 A, and electrodes  100   gc  and  100   gd  not shown in FIG. 14 extend respectively from the electrode leads  100 G c  and  100 G d  on the side wall of the base  100 A in the downward direction similarly to the electrode leads  100   ga  and  100   gb.    
     The base  100 A carries thereon the piezoelectric substrate  1  of the SAW filter, and the holder piece  100 B is mounted upon the base  100 A as noted before, such that the SAW filter on the base  100 A is accommodated in the central opening formed in the holder piece  100 B. Thereby, the piezoelectric substrate  1  is adhered to a part of the ground electrode  100 G exposed by the central opening of the holder piece  100 B. 
     The holder piece  100 B is formed with chamfered surfaces  100 B 1 - 100 B 4  at four corners thereof respectively corresponding to the chamfered surfaces  100 Al 1 - 100 A 4 , and the electrode pads  101 - 103  are formed on the top surface of the holder piece  100 B along an input-side edge as indicated in FIG.  14 . Similarly, the electrode pads  104 - 106  are formed on the top surface of the holder piece  100 B along an output-side edge. 
     Further, electrode leads  104   a ,  105   a  and  106   a  extend on the side wall of the holder piece  100 B respectively from the electrodes  104 - 106  in the downward direction, wherein the electrode lead  104   a  is connected to the electrode lead  100   ga  on the side wall of the base  100 A. Similarly, the electrode lead  106   a  is connected to the electrode lead  100   gb , and the electrode lead  105   a  is connected to an electrode lead  100   o  provided on the side wall of the base  100 A between the electrode leads  100   ga  and  100   gb . Similar electrode leads are formed also on the input-side of the holder piece  100 B in correspondence to the electrode pads  101 - 103 . 
     The spacer member  110  has a ring-shaped form and exposes a part of the electrode pads  101 - 106  as well as a part of the piezoelectric substrate  1 , wherein it will be noted from FIG. 14 that the spacer member  110  includes chamfered surfaces  110   1 - 110   4  respectively corresponding to the chamfered surfaces  100 B 1 - 100 B 4  of the holder piece  100 B. The spacer member  100  carries thereon a ground electrode  110 A, and the metal cap  120  is brazed upon the ground electrode  110 A thus formed on the spacer member  110 . 
     It should be noted that the ground electrode  110 A includes a ground lead extending on the chamfered surface  110   1  in the downward direction, wherein the ground lead is connected to a corresponding ground lead  104   a  extending from the ground electrode pad  104  on the holder piece  100 B to the chamfered surface  100 B 1 , upon mounting of the spacer member  110  on the holder piece  100 B. In other words, the metal cap  120  is connected to the ground electrode pad  104  alone and not to other ground electrode pads  101 ,  103  or  106 . By configuring the package structure as such, it is possible to avoid the problem of deterioration of the SAW filter pass-band characteristic caused by the interference of ground electrode pads explained with reference to FIG.  13 . 
     FIG. 15 shows the pass-band characteristic of the SAW filter of FIG.  14 . 
     Referring to FIG. 15 showing characteristic curves A and B, it should be noted that the characteristic curve B indicates the pass-band characteristic of the SAW filter of FIG. 14, while the characteristic curve A indicates the pass-band characteristic of the SAW filter in which the metal cap  120  is grounded at all of the four corners thereof in the construction of FIG.  14 . 
     As will be seen clearly in FIG. 15, the spurious level outside the pass-band increases substantially in the case the metal cap  120  is grounded at all the four corners thereof as compared with the case of FIG. 14 in which the metal cap  120  is grounded only at one corner thereof. The result of FIG. 15 clearly demonstrates the existence of interference between different ground electrode pads acting over the metal cap  120 . 
     SIXTH EMBODIMENT 
     FIG. 16 shows the construction of a SAW filter according to a sixth embodiment of the present invention including the package body, wherein the SAW filter of FIG. 16 is a modification of the SAW filter of FIG.  12 . Thus, the parts of the SAW filter of FIG. 16 corresponding to those of FIG. 12 are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 16, the piezoelectric substrate  11  held on the package body  100  carries thereon only the SAW filter  11  or  21 . Thereby, the interdigital electrodes  11 A and  11 C of the output-side are grounded at the output-side ground electrode pads  104  and  106  respectively. Further, the interdigital electrode  11 B of the input-side is grounded at the input-side ground electrode pad  101 . In the SAW filter of FIG. 16, it should be noted that an input signal is supplied also to the ground electrode pad  101 . Thereby, the SAW filter of FIG. 16 operates as a differential type filter. 
     In the SAW filter of FIG. 16 the interaction of the input-side ground pad and the output-side ground pad via the capacitive coupling is successfully and effectively eliminated, and an excellent pass-band characteristic similar to the one shown in FIG. 12 is obtained. 
     SEVENTH EMBODIMENT 
     FIG. 17 shows the construction of a SAW filter according to a seventh embodiment of the present invention, wherein those parts described previously with reference to preceding drawings are designated by the same reference numerals and the description thereof will be omitted. 
     Similarly to the SAW filter of FIG. 16, the SAW filter of the present embodiment can be used also as a differential type filter, by supplying an input signal not only to the input electrode pad but also to the ground electrode pad. Thus, the SAW filter of FIG. 16 has the secondary-side electrodes ( 11 A) 2  and ( 11 C) 2  of the interdigital electrodes  11 A and  11 C not grounded but supplied with a second input signal IN 2  different from a first input signal IN 1  which is supplied to the primary-side electrodes ( 11 A) 1  and ( 11 C) 1  of the interdigital electrodes  11 A and  11 C. Thereby, it should be noted that primary-side electrode ( 11 B) 1  of the interdigital electrode  11 B is not grounded but produces an output signal OUT 2  that is different from an output signal OUT 1  obtained at the secondary-side electrode ( 11 B) 2  of the interdigital electrode  11 B. 
     In the construction of FIG. 17, it should be noted that the relationship 
     
       
         N 1 ≠N 2 ≠N 3   
       
     
     holds between the numbers N 1 , N 2  and N 3  of the electrode finger pairs, similarly to the embodiment of FIG.  3 . 
     FIG. 18 shows a modification of the embodiment of FIG. 17 in which the SAW filter is operated in a differential mode in FIG. 18, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted. 
     Referring to FIG. 18, the secondary-side electrodes ( 11 A) 2  and ( 11 C) 2  of the interdigital electrodes  11 A and  11 C are supplied commonly with an input signal IN 2  different from an input signal IN 1  supplied to the primary-side electrodes ( 11 A) 1  and ( 11 C) 1 . Further, an output signal OUT 2  different from an output signal OUT 1  obtained from the secondary-side electrodes ( 21 A) 2  and ( 21 C) 2  are obtained from the primary-side electrode ( 21 A) 1  of the interdigital electrode  21 A and the primary-side electrode ( 21 C) 1 . Similarly to the case of FIG. 7, it should be noted that the SAW filter of FIG. 18 has a construction in which the overlap width W 1  for the SAW filter  11  is different from the overlap width W 2  for the SAW filter  21 . 
     FIGS. 19 and 20 show respectively an example of modifying the SAW filters of FIGS. 8 and 9 to form differential mode SAW filters. In these examples, too, an input signal IN 2  different from the input signal IN 1  used in the example of FIG. 8 or FIG. 9 is supplied to the ground electrode, and an output signal OUT 2  different from the output signal OUT 1  is obtained at the ground electrode. As the construction of FIGS. 19 and 20 is obvious from the description heretofore, further description thereof will be omitted. In the construction of FIGS. 19 and 20, it should be noted that the differential construction may be provided only to one of the input-side and the output-side. 
     In each of the embodiments described heretofore, it is preferable to use a Y-cut single crystal plate of LiTaO 3  or LiNbo 3 , with a cut angle of 40° Y-44° Y when LiTaO 3  is used or with a cut angle of 66° Y-74° Y when LiNbO 3  is used. When LiTaO 3  is used for the piezoelectric substrate  1 , it is preferable to set the thickness of the interdigital electrodes on the substrate  1  to have a thickness of 5-10% the wavelength of the surface acoustic wave excited on the piezoelectric substrate  1 , provided that the interdigital electrodes are formed of Al or an Al-alloy. When the piezoelectric substrate  1  is formed of LiNbO 3 , on the other hand, it is preferable to form the interdigital electrodes with a thickness corresponding to 4-12% the wavelength of the surface acoustic wave excited on the piezoelectric substrate  1 . 
     Further, the present invention is not limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention.