Abstract:
A surface acoustic wave (SAW) filter includes a piezoelectric substrate, a first interdigital transducer (IDT) for input and a second IDT for output that are provided on the piezoelectric substrate, the first IDT and the second IDT being arranged in a propagation direction, and a shield electrode arranged between the first IDT and the second IDT and/or between interconnection lines that connect the first IDT and the second IDT, at least one of the first IDT and the second IDT being of a longitudinal coupling multi-mode type having a balanced operation. Thus, it is possible to suppress a stray capacitance between the first IDT and the second IDT, and thereby to improve the symmetry of balanced operation signals.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention generally relates to a surface acoustic wave (hereinafter referred to as SAW) device that employs a piezoelectric material, and more particularly, to a SAW device having multiple interdigital transducers (hereinafter referred to as IDT) that are provided on a piezoelectric substrate, and to wireless devices that employ the same. This SAW device is of a longitudinal coupling multi-mode type. 
   2. Description of the Related Art 
   The SAW filters are widely used for high-frequency circuits on wireless devices as filters. The wireless devices are represented by mobile telephones or the like. In recent years, integrated circuits (ICs) that perform a balanced input and output operation have been employed for the high-frequency circuits of the wireless devices. Subsequently, the SAW filter is also required to perform the balanced input and output operation. Conventionally, as an example of realizing the balanced operation, Japanese Patent Application Publication No. 6-204781 (hereinafter referred to as Document 1) discloses a method of using electrodes as input and output terminals. One electrode that faces the IDT on the input side is used for an input terminal, and the other electrode that faces the IDT on the output side is used for an output terminal. Also, as another example, Japanese Patent Application Publication No. 11-97966 (hereinafter referred to as Document 2) discloses that the IDTs are divided into two groups, that is, input group and output group. The balanced operation is realized by operating those two groups that are 180° out of phase. 
   It is to be noted that there are problems in the conventional techniques disclosed in Documents 1 and 2. Specifically, a stray capacitance is generated between the IDT on the input side and that on the output side, and the symmetry is bad between balanced operation signals. Thus, electronic devices equipped with the SAW filters such as wireless devices may malfunction. 
   SUMMARY OF THE INVENTION 
   It is a general object of the present invention to solve the above-mentioned problems. 
   More specifically, the present invention intends to reduce the stray capacitance between the input-side IDT and the output-side IDT and improve the symmetry between the balanced signals. 
   This is achieved by suppressing the stray capacitance between the IDT on the input side and that on the output side to a small degree and improving the symmetry between the balanced operation signals. 
   According to an aspect of the present invention, there is provided a surface acoustic wave (SAW) filter includes, a piezoelectric substrate, a first interdigital transducer (IDT) for input and a second IDT for output that are provided on the piezoelectric substrate, the first IDT and the second IDT being arranged in a propagation direction, and a shield electrode arranged between the first IDT and the second IDT and/or between interconnection lines that connect the first IDT and the second IDT, at least one of the first IDT and the second IDT being of a longitudinal coupling multi-mode type having a balanced operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will be described in detail with reference to the following drawings, wherein: 
       FIG. 1  is a plan view of a SAW filter in accordance with a first embodiment of the present invention; 
       FIG. 2  shows an example of a stray capacitance; 
       FIG. 3  is a graph showing an amplitude symmetry of the first embodiment of the present invention and a comparative example; 
       FIG. 4  is a graph showing a phase symmetry of the first embodiment of the present invention and the comparative example; 
       FIG. 5  is a plan view of the SAW filter having a pad for earth potential provided on a piezoelectric substrate in accordance with the first embodiment of the present invention; 
       FIG. 6  is a plan view of the SAW filter having another pad for earth potential provided on the piezoelectric substrate in accordance with the first embodiment of the present invention; 
       FIG. 7  is a plan view of a SAW filter in accordance with a second embodiment of the present invention; 
       FIG. 8  is a graph showing the amplitude symmetry of the second embodiment of the present invention and the comparative example; 
       FIG. 9  is a graph showing the phase symmetry of the second embodiment of the present invention and the comparative example; 
       FIG. 10  is a plan view of the SAW filter having a pad for earth potential provided on the piezoelectric substrate in accordance with the second embodiment of the present invention; 
       FIG. 11  is a plan view of the SAW filter having another pad for earth potential provided on the piezoelectric substrate in accordance with the second embodiment of the present invention; 
       FIG. 12  is a plan view of a SAW filter in accordance with a third embodiment of the present invention; 
       FIG. 13  is a plan view of a SAW filter in accordance with a fourth embodiment of the present invention; 
       FIG. 14  is a plan view of a SAW filter in accordance with a fifth embodiment of the present invention; 
       FIG. 15  is a plan view of a SAW filter in accordance with a sixth embodiment of the present invention; 
       FIG. 16  is a plan view of a SAW filter in accordance with a seventh embodiment of the present invention; 
       FIG. 17  is a plan view of a SAW filter in accordance with an eighth embodiment of the present invention; 
       FIG. 18  is a plan view of a SAW filter in accordance with a ninth embodiment of the present invention; 
       FIG. 19  is a plan view of a SAW filter in accordance with a tenth embodiment of the present invention; and 
       FIG. 20  is a plan view of a SAW filter in accordance with an eleventh embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A description will now be given, with reference to the accompanying drawings, of embodiments of the present invention. 
   First Embodiment 
     FIG. 1  is a plan view of a SAW filter of a longitudinal coupling multi-mode type in accordance with a first embodiment of the present invention. The SAW filter has a unique shield electrode  12 , which will be described after basic components of the first embodiment. 
   This SAW filter includes a piezoelectric substrate  100  and three IDTs  4 ,  5 , and  6 . The piezoelectric substrate  100  is made of lithium tantalate or lithium niobate. The three IDTs  4 ,  5 , and  6  are adjacently arranged in a propagation direction on the piezoelectric substrate  100 . The IDTs  4 ,  5 , and  6  are schematically illustrated in  FIG. 1 . In fact, the IDT  4  is arranged at the center, and the IDTs  5  and  6  are arranged on both sides of the IDT  4 . Each of the IDTs  4 ,  5 , and  6  is composed of a pair of comb-like electrodes. Each comb-like electrode is composed of a bus bar and electrode fingers that extend from the bus bar. The bus bar serves as an interconnection line that connects the electrode fingers together. For instance, the pair of comb-like electrodes of the IDT  4  includes bus bars  4   a  and  4   b  and the electrode fingers that extend from the bus bars  4   a  and  4   b . The electrode fingers extending from the bus bar  4   a  and those extending from the bus bar  4   b  are alternately overlapped or interleaved, and overlapping parts of the adjacent electrode fingers are involved in excitation of the SAW. Similarly, a pair of comb-like electrodes of the IDT  5  includes bus bars  5   a  and  5   b  and the electrode fingers that extend from the bus bars  5   a  and  5   b . A pair of comb-like electrodes of the IDT  6  includes bus bars  6   a  and  6   b  and the electrode fingers that extend from the bus bars  6   a  and  6   b . The bus bar  5   a  of the IDT  5  and the bus bar  6   a  of the IDT  6  are connected to a signal line  15 , and the bus bar  5   b  of the IDT  5  and the bus bar  6   b  of the IDT  6  are connected to a ground  8 . On the other hand, the bus bars  4   a  and  4   b  of the IDT  4  are connected to balanced signal terminals  2   a  and  2   b  respectively. Signals that appear on the balanced signal terminals  2   a  and  2   b  are balanced signals, that is, the signals having the 180° phase difference. Along the propagation direction of the SAW, a reflector  3  is arranged adjacent to the IDT  5 , and a reflector  7  is arranged adjacent to the IDT  6 . 
   In addition, there are provided an IDT  10  and reflectors  9  and  11  that are arranged on both sides of the IDT  10 . The IDT  10  is composed of a pair of comb-like electrodes. One of the comb-like electrodes is connected to a signal terminal  1 , and the other is connected to a signal line  15 . The signal line  15  connects the IDT  10 , the IDT  5 , and the IDT  6 . For example, the signal terminal  1  serves as an input terminal. The balanced signal terminals  2   a  and  2   b  serve as output terminals. In contrast, the signal terminal  1  may serve as an output terminal. The balanced signal terminals  2   a  and  2   b  may serve as input terminals. 
   Next, a description will be given of the shield electrode  12 . The shield electrode  12  is arranged between the bus bar  4   a , the balanced signal terminal  2   a , a signal line  4   c  and the signal line  15 . Both edges of the shield electrode  12  are respectively arranged between the bus bar  4   a  and the bus bar  5   a , and between the bus bar  4   a  and the bus bar  6   a . The shield electrode  12  is also connected to the earth potential  13 . In the case where the signal terminal  1  serves as the input terminal, the shield electrode  12  is arranged between the input IDTs  5  and  6  and the output IDT  4 , and between the signal lines  4   c  and  15 . The signal line  4   c  is connected to the IDT  4 , and the signal line  15  is connected to the IDTs  5  and  6 . The shield electrode  12  operates in order to reduce affects on the stray capacitance between input and output. 
     FIG. 2  shows an example of the stray capacitance.  FIG. 2  shows a configuration obtained by omitting the shield electrode  12  and the earth potential  13  from  FIG. 1 . Hereinafter, this configuration is referred to as a comparative example. Referring to  FIG. 2 , a stray capacitance C appears between the adjacent bus bars  4   a  and  5   a , and between the adjacent bus bars  4   a  and  6   a . When an input voltage is applied to the signal terminal  1 , the input voltage passes through the signal line  15  and excites the IDTs  5  and  6 . At the same time, part of the current, which is leaked current, flows through the stray capacitance C to the bus bar  4   a  from the bus bars  5   a  and  6   a . This current degrades the symmetry of the balanced operation on the balanced signal terminals  2   a  and  2   b . In addition to the above-mentioned stray capacitance, there exist other stray capacitances. For example, there exist small stray capacitances, as compared to the above-mentioned stray capacitance, between the signal line  15  and the bus bar  4   a , between the signal line  15  and the signal line  4   c , and between the signal line  15  and the balanced signal terminal  2   a . Further, a stray capacitance also appears between the electrode fingers extending from the bus bar  5   a  and between those extending from the bus bar  6   a , although they are relatively small. 
   The shield electrode  12  functions to reduce the affects caused by the above-mentioned stray capacitance C and the other stray capacitances that appear between the signal line  15  and the bus bar  4   a , between the signal line  15  and the signal line  4   c , and between the signal line  15  and the balanced signal terminal  2   a . For example, the current leaked from the bus bars  5   a  and  6   a  flows to the edges of the shield electrode  12 , which are respectively provided between the bus bars  4   a  and  5   a , and between the bus bars  4   a  and  6   a , and then flows out of the earth potential  13  through the shield electrode  12 . Therefore, the above-mentioned current leaked from the bus bars  5   a  and  6   a  does not flow into the bus bar  4   a  of the IDT  4 . It is thus possible to improve the symmetry of the balanced operation on the balanced signal terminals  2   a  and  2   b . Similarly, other leaked currents flow into the shield electrode  12 , which currents may be the current leaked between the signal line  15  and the bus bar  4   a , between the signal line  15  and the signal line  4   c , and between the signal line  15  and the balanced signal terminal  2   a.    
   Referring to  FIGS. 3 and 4 , a description will be given that the balanced operation in accordance with the first embodiment of the present invention has a more excellent symmetry than the comparative example. The symmetry of the balanced operation may be judged whether is good or bad by examining the symmetries of amplitude and phase.  FIG. 3  is a graph showing the amplitude symmetries of the balanced output in accordance with the first embodiment of the present invention and the comparative example.  FIG. 4  is a graph showing phase symmetries in accordance with the first embodiment of the present invention and the comparative example. The horizontal axes in  FIGS. 3 and 4  denote frequency (MHz). The vertical axis in  FIG. 3  denotes frequency symmetry (dB) and the vertical axis in  FIG. 4  denotes phase symmetry (degree). On each axis, 0.0 defines the perfect symmetry. As shown in  FIGS. 3 and 4 , it has been found that both the frequency symmetry and phase symmetry are considerably improved. 
   The shield electrode  12  is connected to the earth potential  13 . The earth potential  13  is not connected to the ground potential  8  to which the IDTs  5  and  6  are connected. That is, the potential connected to the shield electrode  12 , which is the earth potential, is in a circuitry different from the circuitry in which the ground potential of the IDTs  5  and  6  is included. In other words, there exists no interconnection line to connect the shield electrode  12  and the IDT  4 , or to connect the shield electrode  12  and the IDTs  5  and  6 . With the above-mentioned configuration, the shield electrode  12  and the IDTs  5  and  6  are completely separate, and thus it is possible to further improve the symmetry of the balanced operation. The ground potential  8  and the earth potential  13  are fed from outside the SAW filter. For example, the ground potential  8  is fed through an external connection terminal that is attached to a package on which the piezoelectric substrate  100  is mounted, and the earth potential  13  is fed through another external connection terminal. A bonding wire, for example, is used to connect the external connection terminal and the shield electrode  12 . In the above-mentioned case, as shown in  FIG. 5 , preferably, a pad  12   a  to be connected to the shield electrode  12  is arranged on the piezoelectric substrate  100 . Also, referring to  FIG. 6 , the shield electrode  12  is separated into two parts  12 A and  12 B to provide pads  12   a  and  12   b  respectively. It is preferable that the earth potential  13  may be zero voltage, but may not necessarily be limited to zero voltage. 
   As shown in  FIG. 1 , preferably, the edges of the shield electrode  12  are arranged between the adjacent bus bars  4   a  and  5   a , and between the adjacent bus bars  4   a  and  6   a . It is to be noted that the symmetry of the balanced operation can be improved by simply arranging the edges of the shield electrode  12  between the signal line  4   c  or the balanced terminal  2   a  and the signal line  15 . 
   In addition, the IDT  10  and the reflectors  9  and  11  may be omitted. Here, the signal line  15  is directly connected to the signal terminal  1 . 
   Second Embodiment 
     FIG. 7  is a plan view showing a SAW filter in accordance with a second embodiment of the present invention. Hereinafter, in the second embodiment, the same components and configurations as those of the first embodiment have the same reference numerals. The shield electrode  12  includes two comb-like shield electrodes  14  that are connected to the earth potential  13 . One of the two comb-like shield electrodes  14  is arranged between IDTs  4  and  5 , and the other is arranged between IDTs  4  and  6 . The lengths of electrode fingers of the comb-like electrodes  14  are substantially equal to those of the IDTs  4  though  6 , which are vertical to the propagation direction of the SAW. The IDT  4  and the IDT  5  do not face each other. The IDT  4  and the IDT  6  do not face each other, either. The comb-like shield electrode  14  includes two electrode fingers. However, the number of electrode fingers may be one, or may be equal to or greater than three. The comb-like shield electrode  14  may be longer than that shown in  FIG. 7 . 
     FIG. 8  is a graph showing the amplitude symmetry of the second embodiment of the present invention and the comparative example in  FIG. 2 .  FIG. 9  is a graph showing the phase symmetry of the second embodiment of the present invention and the comparative example. The horizontal axes in  FIGS. 8 and 9  denote frequency (MHz). The vertical axis in  FIG. 8  denotes frequency symmetry (dB) and the vertical axis in  FIG. 9  denotes phase symmetry (degree). In each vertical axis, 0.0 defines the perfect symmetry. As shown in  FIGS. 8 and 9 , it has been found that both the frequency symmetry and phase symmetry are considerably improved. 
   It goes without saying that the pads  12   a  and  12   b  shown in  FIGS. 5 and 6  may be applied to  FIG. 7 .  FIGS. 10 and 11  show such configurations. 
   Third Embodiment 
     FIG. 12  is a plan view of a SAW filter in accordance with a third embodiment of the present invention. Two filters  30  and  130  are connected in parallel. The balanced operation is performed between the terminal  2   a  of the filter  30  and the terminal  2   b  of the filter  130 . The filter  30  has the same components and configuration as those of the SAW filter shown in  FIG. 1 , and includes the shield electrode  12 . The filters  30  and  130  have mutually different electrode finger patterns so as to operate in opposite phases. Specifically, the IDT  4  of the filter  30  has a different electrode finger pattern from that of the filter  130 . The filter  130  also includes the shield electrode  12  as shown. The distance between bus bars  4   a  and  5   a  of the filter  130  is longer than its corresponding distance of the filter  30 . This results from the electrode finger pattern of the filter  130 . An edge  12   e  of the shield electrode  12  of the filter  130  is widely arranged. Thus, an excellent symmetry of the balanced operation is obtainable by the function of the shield electrode  12 . 
   Two earth potentials  13  in  FIG. 12  are in circuitries different from those of ground potentials  8 . The two earth potentials  13  are connected together on a package on which the piezoelectric substrate  100  is mounted, and are connected to one external connection terminal for the earth potential. 
   Fourth Embodiment 
     FIG. 13  is a plan view of a SAW filter in accordance with a fourth embodiment of the present invention. In accordance with the fourth embodiment of the present invention, a shield electrode  12 A is added to the third embodiment of the present invention. Each of filters  30  and  130  includes the shield electrode  12 A on the opposite side of shield electrodes  12  in order to establish shielding between IDTs  4  and  5  and between IDTs  4  and  6 . The shield electrode  12 A is connected to the earth potential  13 A. It is thus possible to further improve the symmetry of the balanced operation, by providing the shield electrodes  12  and  12 A on both sides of the IDTs  4  through  6  that are arranged in line. The earth potentials  13 A are separately arranged and are respectively connected to the two shield electrodes  12 A. However, the earth potentials  13 A may be connected together on the piezoelectric substrate  100  and then may be connected to the earth potential. 
   In addition to  FIG. 13 , in  FIGS. 1 ,  5 , and  6 , the shield electrodes  12  and  12 A may be provided on both sides of the IDTs  4  through  6 , which are arranged in line. 
   Fifth Embodiment 
     FIG. 14  is a plan view of a SAW filter in accordance with a fifth embodiment of the present invention. The fifth embodiment of the present invention includes the two shield electrodes  12  in  FIG. 12 , and also includes the same type of comb-like electrodes as those in  FIG. 7 . 
   Sixth Embodiment 
     FIG. 15  is a plan view of a SAW filter in accordance with a sixth embodiment of the present invention. The sixth embodiment of the present invention includes the four shield electrodes  12  in  FIG. 13 , and also includes the same type of comb-like electrodes as those in  FIG. 7 . Adjacent electrode fingers of the shield electrode  12  are all connected together. Thus, the shield electrodes  12  and  12 A of the filter  30  are formed in a loop so as to surround the IDT  4 . Similarly, the shield electrodes  12  and  12 A of the filter  130  are arranged in a loop so as to surround another IDT  4 . Earth potentials  13 A are respectively connected to the shield electrode  12 A as shown in  FIG. 15 , but may be connected together on the piezoelectric substrate  100 , and then may be connected to the earth potential. 
   Seventh Embodiment 
     FIG. 16  is a plan view of a SAW filter in accordance with a seventh embodiment of the present invention. This filter includes two filters  40  and  140 . In the case where the signal terminal  1  is used as an input terminal, the filter  40  outputs balanced signals to the filter  140 , which outputs the balanced signals through balanced signal terminals  2   a  and  2   b . The IDT  4  of the filter  140  includes two IDTs, which are connected together to one of bus bars and are adjacently arranged in a propagation direction. The shield electrode  12  is provided to be connected by the filters  40  and  140 . Edges of the shield electrode  12  are arranged between adjacent bus bars. The shield electrode  12  is connected to the earth potential in a circuitry, which is different from that of the ground potential  8 . 
   Eighth Embodiment 
     FIG. 17  is a plan view of a SAW filter in accordance with an eighth embodiment of the present invention. The eighth embodiment of the present invention includes the shield electrodes  12  in accordance with the seventh embodiment of the present invention as shown in  FIG. 16 , and also includes the same type of comb-like electrodes as those in  FIG. 7 . 
   Ninth Embodiment 
     FIG. 18  is a plan view of a SAW filter in accordance with a ninth embodiment of the present invention. The ninth embodiment of the present invention is a modified example of the seventh embodiment of the present invention as shown in  FIG. 16 . The ninth embodiment of the present invention includes an IDT  4 , which has an electrode pattern different from that of the IDT  4  employed in the seventh embodiment of the present invention. The IDT  4  used in the ninth embodiment includes a pair of comb-like electrodes to which balanced signal terminals  2   a  and  2   b  are respectively connected. The shield electrode  12  has the same configuration as that in  FIG. 7 . 
   Tenth Embodiment 
     FIG. 19  is a plan view of a SAW filter in accordance with a tenth embodiment of the present invention. This embodiment includes the shield electrodes  12  employed in the seventh embodiment of as shown in  FIG. 18 , and also includes the same type of comb-like electrodes as those in  FIG. 7 . 
   The first through tenth embodiments of the present invention have been described so far. In the above-mentioned embodiments, the shield electrode  12  may be made of the same material as those of the IDTs  4  through  6 , for example, aluminum or aluminum-copper alloy, and may be made at the same time. In addition, the above-mentioned embodiments are employed together as necessary. For example, the pads, which are provided on the piezoelectric substrate  100  as shown in  FIGS. 5 and 6 , may be applied to the fifth through tenth embodiments. Further, with respect to other variations of the IDT configuration, the shield electrode may be arranged between the input IDT and the output IDT, or between the interconnection lines that connect the input and output IDTs. 
   Eleventh Embodiment 
     FIG. 20  is a block diagram illustrating a wireless device in accordance with an eleventh embodiment. This wireless device is equipped with some SAW filters of the present invention.  FIG. 20  shows transmission and reception systems of the wireless device. In the case where the wireless device is a mobile telephone or the like, the transmission and reception systems in  FIG. 20  are connected to a speech processing system or the like. 
   The wireless device includes an RF (Radio Frequency) unit  170 , a modulator  171 , and an IF (Intermediate Frequency) unit  172 . The RF unit  170  includes an antenna  173 , a separator  174 , a low noise amplifier  183 , an interstage filter  184 , a mixer (multiplier)  175 , a local oscillator  176 , an interstage filter  177 , a mixer (multiplier)  178 , an interstage filter  179 , and a power amplifier  180 . An audio signal applied from the speech processing system is modulated on the modulator  171 , and the frequency of the audio signal is converted or mixed on the mixer  178  of the RF unit  170  with the use of an oscillation signal generated by the local oscillator  176 . An output from the mixer  178  passes through the interstage filter  179  and the power amplifier  180 , and is given to the separator  174 . The separator  174  includes a transmission filter  174   1 , a reception filter  174   2 , and a matching circuit (not shown). The separator  174  utilizes the SAW filter(s) of the present invention. A signal transmitted from the power amplifier  180  is fed to the antenna  173  through the separator  174 . 
   The signal received from the antenna  173  passes through the reception filter  174   2  of the separator  174 , and is applied to the mixer  175  through the low noise amplifier  183  and the interstage filter  184 . The mixer  175  receives an oscillating frequency generated by the local oscillator  176  by way of the interstage filter  177 , converts the frequency of the received signal, and applies the signal to the IF unit  172 . The IF unit  172  receives the signal by way of the IF filter  181 , demodulates with a demodulator  182 , and outputs the demodulated audio signal to the speech processing system that is not shown. 
   The SAW filter of the present invention includes the above-mentioned separator  174  and the interstage filters  177 ,  179 , and  184 . The above-mentioned separator  174  includes the transmission filter  174   1  and the reception filter  174   2 . 
   The SAW filter of the present invention is capable of decreasing malfunctions caused by noises in a high-frequency circuit of the wireless device on which an integrated circuit (IC) having an input and output of the balanced operation. 
   The present invention is not limited to the above-mentioned first through eleventh embodiments, and other embodiments, variations and modifications may be made without departing from the scope of the present invention. 
   The present invention is based on Japanese Patent Application No. 2003-392832 filed on Nov. 21, 2003, the entire disclosure of which is hereby incorporated by reference.