Abstract:
A surface acoustic wave device having a given impedance includes multimode type filters connected in series. A composite impedance of the multimode type filters defines the given impedance of the surface acoustic wave device.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention generally relates to surface acoustic wave devices, and more particularly, to a surface acoustic wave device of a multimode type. 
   2. Description of the Related Art 
   Recently, filters formed by surface acoustic wave devices have been used in RF circuits of radio communications devices such as portable phones. The filters formed by the surface acoustic wave devices (SAW devices) may be used as a transmit filter, a reception filter and a duplexer in which the transmit filter and the receive filters are mounted in a single package. Various types of SAW filters are known, and are typically a ladder type SAW filter has SAW resonators arranged in a ladder formation, and a multimode type SAW filter. Generally, the ladder type SAW filter is used as a transmit filter, and the multimode type SAW filter is used as a receive filter. 
   As described in Japanese Patent Application Publication No. 2004-194269, the fundamental structure of the multimode type SAW filter has a pair of reflection electrodes formed on a piezoelectric substrate, and input and output interdigital transducers (IDTs) arranged between the reflection electrodes. When a drive voltage is applied across the input IDT, SAWs are propagated between the reflection electrodes, and multiple standing waves are produced between the reflection electrodes. A voltage based on the standing waves develops across the output IDT. The multimode type SAW filter thus formed functions as a bandpass filter. 
     FIG. 1A  shows a unit of the multimode type SAW filter (fundamental structure), and  FIG. 1B  shows a cascade type in which two units are cascaded. A multimode type SAW filter  10  of the unit type includes an input IDT  12 , output IDTs  14  and  16  arranged at opposing sides of the input IDT  12 , and reflection electrodes  18  and  20  located further out than the output IDTs  14  and  16 . Each of the input IDT  12 , and the output IDTs  14  and  16  has a pair of comb-like electrodes in which electrode fingers are interleaved. The input IDT  12 , the output IDTs  14  and  16 , and the reflection electrodes  18  and  20  are formed by metal patterns formed on a piezoelectric substrate, which may be LN (lithium niobate) or LT (lithium tantalate). The impedance Fs of the unit type is designed so as to be equal to the characteristic impedance of a transmission line (for example, 50 Ω) connected to the SAW filter. The cascade type of multimode type SAW filter shown in  FIG. 1B  has two multimode type SAW filters that are cascaded. More specifically, two output IDTs of the multimode type SAW filter  10 A are connected to two input IDTs of the multimode type SAW filter  10 B through signal patterns  22  and  24 , respectively. A comb-like electrode  12   a  of the input IDT of the multimode type SAW filter  10 A and a comb-like electrode  12   b  of the output IDT of the multimode type SAW filter  10 B are grounded. Since the comb-lie electrodes  12   a  and  12   b  are grounded, no signals are transmitted therebetween. The impedance Fi of the multimode type SAW filter  10 A and the impedance Fo of the multimode type SAW filter  10 B are both equal to the characteristic impedance of a transmission line connected to the SAW filter shown in  FIG. 1B , and may be equal to, for example, 50 Ω (Fs=Fi=50 Ω). 
     FIG. 2  shows an arrangement in which three cascaded multimode type SAW filters, each shown in  FIG. 1B , are connected in parallel. 
   The multimode type SAW filters shown in  FIGS. 1A and 1B  may be used as shown in  FIG. 3A . A transmit filter Tx and a receive filter Rx are connected to an antenna Ant. The receive filter Rx is the multimode type SAW filter. A transmitted signal is output to the antenna Ant through the transmit filter Tx. A signal received via the antenna Ant is applied to a next-stage circuit through the receive filter Rx. 
   However, the inventors found out the following problem. If a Jammer (a frequency component in a desired receive frequency range) is received while the signal is being sent, a leakage component of the transmitted signal excites the multimode type SAW filter of the receive filter Rx together with the Jammer. This problem is shown in  FIG. 3B , in which the horizontal axis (MHz) denotes the frequency and the vertical axis denotes the output power (dBm) of the receive filter Rx. As shown in  FIG. 3B , only the Jammer appears at the output port of the receive filter Rx in the absence of the transmitted signal. In contrast, when a leakage component of the transmitted signal exists, power components appear at both sides of the peak of the Jammer resulting from the leakage component. The power components increase the intermodulation level and may degrade the single tone defense (STD), which is described in the standardized specification of portable phones. This problem occurs in not only an arrangement in which the transmit filter Tx and the receive filter Rx are formed by separate piezoelectric substrates but also an arrangement with a single piezoelectric substrate on which the transmit filter Tx and the receive filter Rx are formed. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a SAW device having a multimode type SAW filter having excellent filter characteristics even in the presence of an external leakage signal. 
   This object of the present invention is achieved by a surface acoustic wave device having a given impedance including: multimode type filters connected in series, a composite impedance of the multimode type filters defining the given impedance of the surface acoustic wave device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
       FIG. 1A  shows a unit of multimode type SAW filter; 
       FIG. 1B  shows a conventional multimode type SAW filter having two multimode type SAW filters of unit type that are cascaded; 
       FIG. 2  shows another conventional multimode type SAW filter having three cascaded structures, each being as shown in  FIG. 1B ; 
       FIG. 3A  shows an application of the multimode type SAW filter; 
       FIG. 3B  shows a problem of the application; 
       FIG. 4A  shows the unit of the multimode type filter; 
       FIG. 4B  shows a multimode type SAW filter according to a first embodiment of the present invention; 
       FIG. 5  is a graph of intermodulation levels of the multimode type SAW filter shown in  FIG. 4B  and the conventional multimode type SAW filter; 
       FIG. 6  shows a variation of the structure shown in  FIG. 4B ; 
       FIG. 7  is a graph showing a possibility of a spurious component in the pass band in the structure shown in  FIG. 6 ; 
       FIG. 8A  shows the principle of a structure designed to suppress the spurious component shown in  FIG. 7 ; 
       FIG. 8B  is a graph showing effects of the structure shown in  FIG. 8A ; 
       FIG. 9A  shows a structure of the first embodiment of the present invention; 
       FIG. 9B  shows a multimode type SAW filter according to a second embodiment; 
       FIG. 10A  shows the structure of the second embodiment; 
       FIG. 10B  shows a multimode type SAW filter according to a third embodiment of the present invention; 
       FIG. 11A  shows the structure of a third embodiment; 
       FIG. 11B  shows a multimode type SAW filter according to a fourth embodiment of the present invention; 
       FIG. 12  shows a multimode type SAW filter according to a fifth embodiment of the present invention; 
       FIG. 13  shows a variation of the fifth embodiment; 
       FIG. 14  shows another variation of the fifth embodiment; 
       FIG. 15  is a graph of the filter characteristic of the fifth embodiment; 
       FIG. 16A  shows a unit of a balanced type multimode SAW filter according to a sixth embodiment; 
       FIG. 16B  shows a cascade type balanced multimode SAW filter according to the sixth embodiment; and 
       FIG. 17  shows a duplexer according to a seventh embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A description will now be given, with reference to the accompanying drawings, of preferred embodiments of the present invention. 
   [First Embodiment] 
     FIG. 4B  shows a multimode type SAW filter according to a first embodiment of the present invention, and  FIG. 4A  shows the aforementioned unit of the multimode type SAW filter that is illustrate for comparison with the first embodiment. The multimode type SAW filter shown in  FIG. 4B  has two multimode type SAW filters  10 C and  10 D, which are formed on a piezoelectric substrate  100  and are connected in series. In the following, the multimode type SAW filters  10 C and  10 D may be referred to as first and second filters, respectively. The first filter  10 C has a pair of reflection electrodes  18 A and  20 A, and input and output IDTs  12 A,  14 A and  16 A interposed between the reflection electrodes  18 A and  20 A. Similarly, the second filter  10 D has a pair of reflection electrodes  18 B and  20 B, and input and output ITDs  12 B,  14 B and  16 B interposed between the reflection electrodes  18 B and  20 B. 
   In order to connect the first and second filters  10 C and  10 D in series, patterned interconnection patterns  22 ,  24  and  26  are connected as shown. The interconnection patterns  22  and  24  connect the output IDTs of the first filter  10 C and the output IDTs of the second filter  10 D. The interconnection pattern  26  connects the input IDT of the first filter  10 C and the input IDT of the second filter  10 D. That is, the corresponding IDTs of the first and second filters  10 C and  10 D are connected in series. In operation, the potential of the interconnection pattern  26 , in other words, the potentials of the comb-like electrodes  12   a  and  12   b  are an intermediate potential between a drive voltage applied to an input terminal T 1  and ground potential. The input IDT of the first filter  10 C excites SAW by the difference between the drive potential and the intermediate potential. The input IDT of the second filter  10 D excites SAW by the difference between the intermediate potential and the ground potential. It is to be noted that the comb-like electrodes  12   a  and  12   b  are not at the ground potential, and this is quite different from the structure shown in  FIG. 1B . Output signals in phase are available at output terminals T 2  and T 3 . 
   Since the first and second filters  10 C and  10 D are connected in series, it is required to design the impedance so that the composite impedance of the impedance F 1  of the first filter  10 C and the impedance F 2  of the second filter  10 D has a desired impedance (for example, 50 Ω). The desired impedance may be equal to the characteristic impedance of transmission lines to which the multimode type SAW filter shown in  FIG. 4B  is connected. When the unit type shown in  FIG. 4A  has the impedance Fs equal to 50 Ω, the aperture length AP 1  (the interleaving width of electrode fingers) of the first filter  10 C and the aperture length AP 2  of the second filter  10 D are set larger than the aperture length AP 0  of the unit type in order to equal F 1 +F 2  to Fs (=50 Ω). Thus, the IDT areas of the first and second filters  10 C and  10 D are both greater than the IDT area of the unit type. In the example shown in  FIG. 4B , AP 1 =AP 2  (=2AP 0 ), and the IDT area of the first filter  10 C and the IDT area of the second filter  10 D are equal to each other. It may be said that the structure shown in  FIG. 4B  is formed by dividing the structure of the unit type into two. The multimode type SAW filters having different aperture lengths may have different electrostatic capacitances. 
   According to the structure shown in  FIG. 4B , the series connection of multiple stages (two stages in  FIG. 4B ) decreases the voltages applied across the IDTs and enables divided voltages to be applied across the IDTs. The enlarged IDT areas reduce SAW excitation energy per unit area. As a result, as shown in a graph of  FIG. 5 , the “invention” device having the structure shown in  FIG. 4B  has a restrained intermodulation level, as compared to the “conventional” device having the structure shown in  FIG. 1B . 
   In the structure shown in  FIG. 4B , the input terminal T 1  and the output terminals T 2  and T 3  are arranged so as to sandwich the electrode patterns. In contrast, as shown in  FIG. 6 , the input terminal T 1  and the output terminals T 2  and T 3  are arranged at an identical side of the electrode patterns. In the multimode type SAW filter shown in  FIG. 6  has two multimode type SAW filters  10 E (first filter) and  10 F (second filter) connected in series. The interconnection patterns  22 ,  24  and  26  for making the series connections are also bus bars of the IDTs. That is, the bus bars of the first filter  10 E and the corresponding bus bars of the second filter  10 F are connected in series by the common bus bars  22 ,  24  and  26 . The input terminal T 1  and the output terminals T 2  and T 3  are respectively provided to the three IDTs of the first filter  10 E. The composite impedance of the impedance F 11  of the first filter  10 E and the impedance F 12  of the second filter  10 F is equal to the characteristic impedance of the transmission lines to which the multimode type SAW filter shown in  FIG. 6  are connected (for example, 50 Ω). The output IDTs  14 B and  16 B are arranged so that the signals at the terminals T 2  and T 3  are in phase (identical polarization). More specifically, the electrode finger of the IDT  14 B adjacent to the IDT  12 B is connected to the bus bar  22 , and the electrode filter of the IDT  16 B adjacent to the IDT  12 B is connected to the bus bar  24 . 
     FIG. 7  shows a frequency characteristic of the structure shown in  FIG. 6  (AP 1 =AP 2 ). The horizontal axis of  FIG. 7  denotes the frequency (MHz), and the vertical axis thereof denotes the amount of attenuation (dB). As shown in  FIG. 7 , a spurious component appears in the low-frequency side of the pass band. A second embodiment of the present invention that will be described below is directed to a multimode type SAW filter having a structure capable of reducing the spurious component. 
   [Second Embodiment] 
   The second embodiment pays attention to an interpattern capacitance C in order to reduce the spurious component. As shown in  FIG. 8A , there is an interpattern capacitance C between the interconnection patterns  22  and  26 , which also serve as bus bars, and there is another interpattern capacitance C between the interconnection patterns  24  and  26 , which also serve as bus bars. The electrode fingers connected to the interconnection patterns  22 ,  24  and  26  are also related to the interpattern capacitances C.  FIG. 8B  shows an experimental result that describes how the frequency characteristic of the multimode type SAW filter shown in  FIG. 8A  is affected by the magnitudes of the interpattern capacitances C. As is seen from  FIG. 8B , the spurious component can be reduced by decreasing the interpattern capacitance C. 
     FIG. 9B  shows a multimode type SAW filter equipped with a structure directed to reduction in the interpattern capacitance C. In  FIG. 9A , the multimode type SAW filter according to the aforementioned first embodiment is illustrated for comparison with the second embodiment. The multimode type SAW filter shown in  FIG. 9B  includes the first filter  10 E and a second filter  10 G. At least one electrode finger is provided between electrode fingers extending from the bus bars  22 ,  24  and  26 , which are adjacent interconnection patterns, in which the at least one electrode finger is at a potential different from the potentials of the adjacent interconnection patterns. More specifically, electrode fingers Ic 1  at the ground potential different from the potentials of the bus bars  22  and  26  are arranged between an electrode finger Ia 1  extending from an end of the bus bar  22  and an electrode finger Ib 1  extending from an end of the bus bar  26  adjacent to the bus bar  22 . The bus bars  22  and  26  are at the intermediate potential between the ground potential and the drive voltage applied to the input terminal T 1 . Similarly, electrode fingers Ic 2  at the ground potential different from the potentials of the bus bars  22  and  24  are arranged between an electrode finger Ia 2  extending from an end of the bus bar  22  and an electrode finger Ib 2  extending from an end of the bus bar  24  adjacent to the bus bar  22 . Although the structure shown in  FIG. 9B  has two electrode fingers Ic 1  or Ic 2  are arranged between the electrode fingers extending from the different bus bars, only one electrode finger at the ground potential may be arranged therebetween. As described above, the ground potential is formed between the electrode fingers Ia 1  and Ib 1  and between the electrode fingers Ia 2  and Ib 2 , so that the structure shown in  FIG. 9B  has a smaller interpattern capacitance C than that of the structure shown in  FIG. 9A  and has a reduced spurious component. The distance d 22  between the electrode fingers Ia 1  and Ib 1  is longer than the corresponding distance d 21  in the structure shown in  FIG. 9A . It is thus possible to further reduce the interpattern capacitance C. The composite impedance of the impedance F 11  of the first filter  10 E and the impedance F 12  of the second filter  10 G may be equal to the characteristic impedances (for example, 50 Ω) of the transmission lines connected to the multimode type SAW filter. 
   The multimode type SAW filter of the second embodiment is capable of restraining the intermodulation level and reducing the spurious component in the bass band as shown in  FIG. 7B . 
   [Third Embodiment] 
     FIG. 10B  shows a multimode type SAW filter according to a third embodiment of the present invention. In  FIG. 10A , the multimode type SAW filters of the above-mentioned second embodiment is illustrated for comparison with the third embodiment. In the structure shown in  FIG. 10A , the lengths of the electrode fingers extending from the adjacent IDTs of the first filter  10 E are involved in the interpattern capacitance C (for example, the electrode fingers Id 1  and Ie 1 ). Thus, as shown in  FIG. 10B , the interpattern capacitance C can be reduced by shortening the length of the electrode fingers of the first filter  10 E. 
   In the structure shown in  FIG. 10B , a first filter  10 H has shortened electrode fingers. Thus, the aperture length AP 11  of the first filter  10 H is smaller than the aperture length AP 1  of the first filter  10 E shown in  FIG. 10A . Thus, the impedance F 21  of the first filter  10 H is greater than the impedance F 11  of the first filter  10 E shown in  FIG. 10A . In order to set the composite impedance of the first filter  10 H and a second filter  10 I equal to the desired impedance (for example, 50 Ω), the aperture length AP 12  of the second filter  10 I is made greater. Even in this case, the interpattern capacitance C is not increased because the second filter  10 I is configured so that the electrode fingers Ic 1  and Ic 2  at the ground potential different from the potentials of the bus bars  22 ,  24  and  26 , like the second filter  10 G shown in  FIG. 9B . 
   The multimode type SAW filter according to the third embodiment can restrain the intermodulation level and further reduces the spurious component in the pass band. 
   [Fourth Embodiment] 
     FIG. 11B  shows a multimode type SAW filter according to a fourth embodiment of the present invention. In  FIG. 11A , the multimode type SAW filter of the aforementioned third embodiment is illustrate for comparison with the fourth embodiment. The multimode type SAW filter of the fourth embodiment includes a first filter  10 J and a second filter  10 K, which have common bus bars  22 A,  24 A and  26 A also serving as interconnection patterns via which these filters are connected in series. A distance d 34  (interpattern distance) between the adjacent edges of the bus bars  22 A and  26 A and another distance d 34  between the adjacent edges of the bus bars  24 A and  26 A are longer than a corresponding distance d 33  shown in  FIG. 11A . In order to increase the distance d 34  to thus reduce the interpattern capacitance C, the edges of the bus bars  22 A,  24 A and  26 A are tapered. Widths L 34  of the bus bars  22 A,  24 A and  26 A are made narrower than the widths L 33  of the bus bars  22 ,  24  and  26  shown in  FIG. 11A , so that the interpattern capacitance C can be reduced. The composite impedance of the impedance F 31  of the first filter  10 J and the impedance F 32  of the second filter  10 K is equal to the characteristic impedance of the transmission lines to which the multimode type SAW filters of the fourth embodiment are connected. The reflection electrodes  18 C and  18 D shown in  FIG. 11B  have a common bus bar, and the reflection electrodes  20 C and  20 D have a common bus bar. 
   The multimode type SAW filter according to the fourth embodiment can restrain the intermodulation level and further reduce the spurious component in the pass band. 
   [Fifth Embodiment] 
     FIG. 12  shows a multimode type SAW filter according to a fifth embodiment of the present invention. As indicated by reference numerals  50 A,  50 B and  50 C, three multimode type SAW filters, each shown in  FIG. 11B , are connected in parallel, and multimode type SAW filters  52 A,  52 B and  52 C of unit type as shown in  FIG. 1A  or  4 A are cascaded to the filters  50 A,  50 B and  50 C, respectively. The terminals T 11 , T 12  and T 13  function as input terminals, and terminals T 21 , T 22  and T 23  function as output terminals. The identical signal is applied to the input terminals T 11 , T 12  and T 13 , and output signals in phase are available at the output terminals T 21 , T 22  and T 23 . 
   The multimode type SAW filter according to the fifth embodiment is capable of restraining the intermodulation level, effectively reducing the spurious component in the pass band, and improving the power durability. 
     FIG. 13  shows a variation of the multimode type SAW filter shown in  FIG. 12 . The outputs of the filters  50 A,  50 B and  50 C are commonly connected to an output terminal T 24 . 
     FIG. 14  shows a second variation of the multimode type SAW filter shown in  FIG. 12 . The SAW filter shown in  FIG. 14  includes a group of filters  50 A, SOB and  50 C, and another group of filters  50 D,  50 E and  50 F, which are respectively cascaded thereto. The filters  50 D,  50 E and  50 F are configured as shown in  FIG. 11B . 
     FIG. 15  shows a frequency characteristic of the multimode type SAW filter according to the fifth embodiment, in which the horizontal axis denotes the frequency (MHz) and the vertical axis denotes the amount of attenuation (dB). A line indicated by a reference numeral  60  shows an amount of attenuation of −3 dB, which corresponds to the amount of insertion loss generally required. Another line indicated by a reference numeral  62  shows an amount of attenuation of −50 dB, which corresponds to the amount of out-of-band attenuation generally required. It can be seen from  FIG. 15  that the multimode type SAW filter of the fifth embodiment has a small insertion loss and a large out-of-band attenuation. 
   [Sixth Embodiment] 
   A six embodiment of the present invention is a balanced filter.  FIG. 16A  shows a unit of balanced type multimode SAW filter according to the sixth embodiment. Referring to  FIG. 16A , signals B 1  and B 2  are balanced inputs or balanced outputs. In order to realize the balanced filter, the IDT  14 B and the IDT  16 C have different electrode structures. More specifically, adjacent electrode fingers of the IDTs  12 B and  14 B are connected to the bus bars  26  and  22 , respectively, while adjacent electrode fingers of the IDTs  12 B and  16 C are connected to the bus bars  26  and  28 , respectively. 
     FIG. 16B  shows a cascaded type balanced multimode SAW filter according to the sixth embodiment. Referring to this figure, multimode type SAW filters of unit type  52 D,  52 E and  52 F are respectively cascaded to the multimode type SAW filters  50 A,  50 B and  50 C connected in parallel. Each of the filters  52 D,  52 E and  52 F has balanced outputs or inputs indicated by B 1  and B 2 . 
   Even in the balanced type multimode SAW filter, the multimode type SAW filters  50 A,  50 B and  50 C have the aforementioned structures, functions and effects. It is thus possible to restrain the intermodulation level and further reduce the spurious component in the pass band. 
   [Seventh Embodiment] 
     FIG. 17  shows a duplexer according to a seventh embodiment of the present invention. This duplexer is a SAW device having a single package in which the transmit filter Tx and the receive filter Rx shown in  FIG. 3A  are built. The transmit filter Tx has a ladder arrangement of SAW resonators, and the receive filter Rx has the arrangement shown in  FIG. 12B . The transmit filter Tx and the receive filter Rx are formed on a single piezoelectric substrate  100 . The transmit filter Tx includes SAW resonators S 1 –S 6  in series arms of the ladder arrangement, and SAW resonators P 1  and P 2  in parallel arms. The duplexer thus configured is capable of suppressing the intermodulation level, which is the influence of the leakage component from the transmit filter Tx to the receiver filter Rx, and the spurious component in the pass band of the receive filter Rx. 
   The present duplexer may be used to separate transmit signals in the range of, for example, 1850 MHz to 1910 MHz and receive signals in the range of, for example, 1930 MHz to 1990 MHz from each other in communications apparatuses such as PCS portable phones. 
   The present invention is not limited to the specifically disclosed embodiments, and 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. 2004-252644 filed on Aug. 31, 2004, the entire disclosure of which is hereby incorporated by reference.