Surface acoustic wave filter and duplexer using the same

A multimode type SAW filter includes a piezoelectric substrate, and IDT groups formed on the piezoelectric substrate, the IDT groups each having multiple IDTs connected in series in which an input/output electrode and a ground electrode are coupled via a floating conductor. Adjacent electrode fingers between adjacent IDTs among the multiple IDTs of the IDT groups include a first electrode finger connected to the floating conductor in one of the IDT groups and a second electrode finger connected to one of the input/output electrode and the ground electrode in another one of the IDT groups.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to surface acoustic wave filters and duplexers using the same, and more particularly to a surface acoustic wave filter in which one IDT (InterDigital Transducer) is divided in multiple IDTs connected in series and a duplexer using such a filter.

2. Description of the Related Art

Recently, a filter of a surface acoustic wave device has been employed in the RF circuit of radio equipment such as a portable phone. The filter of the surface acoustic wave device (hereinafter referred to as SAW filter) may be a transmit filter, a receiver filter or an antenna duplexer equipped with the transmit filter and the receive filter that are packaged as a single device.

FIG. 1is a block diagram of a conventional antenna duplexer used in a portable phone. A transmit signal applied via a transmit terminal13passes through a transmit filter16, and is output via an antenna terminal14. A receive signal received via the antenna terminal14passes through a matching circuit12and a receive filter11, and is output via a receive terminal15.

Antenna duplexers are disclosed in FIG. 6 of Japanese Patent Application Publication No. 2003-249842 (hereinafter referred to as Document 1) and FIG. 30 of Japanese Patent Application Publication No. 2004-194269 (hereinafter referred to as Document 2). These antenna duplexers have a transmit filter formed by a ladder type SAW filter, and a receive filter formed by a multimode type SAW filter. The transmit filter may receive high power and is therefore formed by the SAW filter having high power durability. The receive filter is required to have a high out-of-band attenuation and a steep cutoff characteristic, and is therefore formed by the multimode type SAW filter.

The basic structure of the multimode type SAW filter is disclosed in, for example, FIG. 5 of Document 2, and is composed of a pair of reflectors formed on a piezoelectric substrate, an input IDT (composed of comb-like electrodes) and an output IDT. The input and output IDTs are interposed between the pair of reflectors. A drive voltage is applied to the input IDT, and resultant SAWs are propagated between the reflectors. There are multiple standing waves between the reflectors. Voltages that correspond to the frequencies of the standing waves appear at the output IDT. The multimode type SAW filter functions as a bandpass filter.

However, the duplexers disclosed in Documents 1 and 2 have the following problems. Referring toFIG. 1, input power of the transmit signal applied via the transmit terminal13passes through the transmit filter16and reaches the antenna terminal14, as indicated by the solid-line arrow. However, as indicated by a dotted-line arrow, some input power passes through the matching circuit12and reaches the receive filter11as leakage power. The multimode type SAW filter has poor power durability, and the leakage power may break the receive filter or may cause non-linearity of the receive filter11. This degrades the receive sensitivity.

SUMMARY OF THE INVENTION

The present invention has been made in terms of the above-mentioned circumstances, and has an object to provide a multimode type SAW filter having improved power durability and suppressed non-linearity and an antenna duplexer using such a multimode type SAW filter.

According to an aspect of the present invention, there is provided a multimode type SAW filter including: a piezoelectric substrate; and IDT groups formed on the piezoelectric substrate, the IDT groups each having multiple IDTs connected in series in which an input/output electrode and a ground electrode are coupled via a floating conductor, adjacent electrode fingers between adjacent IDTs among the multiple IDTs of the IDT groups including a first electrode finger connected to the floating conductor in one of the IDT groups and a second electrode finger connected to one of the input/output electrode and the ground electrode in another one of the IDT groups.

According to another aspect of the present invention, there is provided a multimode type SAW filter including: a piezoelectric substrate; multiple filters connected in parallel and formed on the piezoelectric substrate; each of the multiple filters including: IDT groups formed on the piezoelectric substrate, the IDT groups each having multiple IDTs connected in series in which an input/output electrode and a ground electrode are coupled via a floating conductor, adjacent electrode fingers between adjacent IDTs among the multiple IDTs of the IDT groups including a first electrode finger connected to the floating conductor in one of the IDT groups and a second electrode finger connected to one of the input/output electrode and the ground electrode in another one of the IDT groups.

According to yet another aspect of the present invention, there is provided a multimode type SAW filter including: a first filter; and a second filter that follows the first filter and is a multimode type SAW filter, the first filter including: IDT groups formed on the piezoelectric substrate, the IDT groups each having multiple IDTs connected in series in which an input/output electrode and a ground electrode are coupled via a floating conductor, adjacent electrode fingers between adjacent IDTs among the multiple IDTs of the IDT groups including a first electrode finger connected to the floating conductor in one of the IDT groups and a second electrode finger connected to one of the input/output electrode and the ground electrode in another one of the IDT groups.

According to a further aspect of the present invention, there is provided an antenna duplexer including: a transmit filter connected to a common terminal; and a receive filter connected to the common terminal, the receive filter including: a piezoelectric substrate; and IDT groups formed on the piezoelectric substrate, the IDT groups each having multiple IDTs connected in series in which an input/output electrode and a ground electrode are coupled via a floating conductor, adjacent electrode fingers between adjacent IDTs among the multiple IDTs of the IDT groups including a first electrode finger connected to the floating conductor in one of the IDT groups and a second electrode finger connected to one of the input/output electrode and the ground electrode in another one of the IDT groups.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to an aspect of the present invention, multiple IDTs connected in series via a floating conductor are provided between an input or output electrode and a ground electrode. With this arrangement, the power durability of the multimode type SAW can be improved.

FIG. 2Ashows a conventional double mode type SAW (DMS) filter, which is one of the multimode type SAW filters. Referring toFIG. 2A, three IDTa21a,21band21care provided side by side in a SAW propagation direction between reflectors20aand20bon a piezoelectric substrate10. An input/output electrode22bof the central IDT21bis connected to an input terminal22, and input/output electrodes22aand22cof the left and right IDTs21aand21care respectively connected to output terminals. Ground electrodes23a,23band23cof the IDTs21a,21band21care connected to ground. A symbol AP denotes an aperture length that is the lengths of the interleaving portions of the electrode fingers of the ground and input/output electrodes.

FIG. 2Bshows a comparative multimode SAW filter, which a double mode type SAW filter having multiple IDT groups, each of which has multiple IDTs connected in series. Referring toFIG. 2B, IDT groups31a,31band31care provided side by side in a SAW propagation direction between reflectors30aand30b. The IDT group31ahas IDTs32aand33aconnected in series. The IDT32ais composed of an input/output electrode34aand a floating conductor35a. The IDT33ais composed of the floating conductor35aand a ground electrode36a. Similarly, the IDT group31bhas IDTs32band33bconnected in series. The IDT32bis composed of an input/output electrode34band a floating conductor35b. The IDT33bis composed of the floating conductor35band a ground electrode36b. Similarly, the IDT group31chas IDTs32cand33cconnected in series. The IDT32cis composed of an input/output electrode34cand a floating conductor35c. The IDT33cis composed of the floating conductor35cand a ground electrode36c. The aperture length between the input/output electrode and the floating conductor is denoted as AP1, and the aperture length between the floating conductor and the ground electrode is denoted by AP2.

In order to replace the conventional DMS filter shown inFIG. 2Awith the comparative DMS filter shown inFIG. 2B, the comparative DMS filter has the same input/output impedances as those of the conventional DMS filter. In order to achieve this impedance relationship, preferably, the composite electrostatic capacitances of the IDT groups31a,31band31cof the comparative DMS filter are respectively equal to those of the IDTs21a,21band21c.

In the IDT groups of the comparative DMS filter, it is assumed that the electrostatic capacitance of the upper IDT having the aperture length AP1is denoted as C1, and the electrostatic capacitance of the lower IDT having the aperture length AP2is denoted as C2. The composite electrostatic capacitance of each of the IDT groups is expressed as 1/(1/C1+1/C2). Assuming that the electrostatic capacitance of IDT is proportional to the aperture length, the following equation should be satisfied in order to achieve such a relationship that the electrostatic capacitance of the IDT is equal to that of the IDT group:
AP1×AP2=(AP1+AP2)×AP(1).

A series fragmentation is defined as a way to convert one IDT into an IDT group composed of multiple IDTs connected in series. In the comparative filter shown inFIG. 2b, AP1=1.5×AP and AP2=3×AP. The voltage applied to the upper IDT of the comparative filter is equal to 0.66 times the voltage applied to the IDT of the conventional filter, namely, AP2/(AP1+AP2)=0.66. The voltage applied to the lower IDT of the comparative filter is equal to 0.33 times the voltage applied to the IDT of the conventional filter, namely, AP1/(AP1+AP2)=0.33.

When the voltage applied to the IDT is reduced as mentioned above, the strength of SAW per unit area in IDT can be reduced. This improves the power durability and suppresses the non-linearity.

The inventors changed the aperture lengths AP1and AP2of the comparative filter within the range of the above-mentioned expression (1) and investigated the bandpass characteristics thereof.FIG. 3Ashow bandpass characteristics of a first comparative filter where AP1=1.5AP and AP2=3.0 AP, a second comparative filter where AP1=1.8AP and AP2=2.25AP, and a third comparative filter where AP1=2.0AP and AP2=2.0AP, and the bandpass characteristic of the conventional filter.FIG. 3Bis an enlarged view of a portion surrounded by a circle shown inFIG. 3Ain which notches take place.

In the comparative examples, notches that increase the insertion loss at about 1,926 MHz. The notch increases as the aperture length AP1increases. The mostly improved power durability and mostly suppressed non-linearity are available in the third comparative filter where the voltage equal to half the voltage used in the conventional filter is applied between the upper and lower IDTs arranged in the series fragmentation. However,FIGS. 3A and 3Bshow that the third comparative filter has a large notch. In case where the notch exists in the pass band, different receive sensitivities are available in different channels of, for example, the portable phone. This prevents users from being provided with equal services. In terms of the above consideration, there is no way to use the first comparative filter having the smallest notch.

The notch in the pass band is caused by the electrostatic capacitances between the floating conductors35a,35band35c. For example, the electrostatic capacitance between the floating conductor35aof the IDT group31aand the floating conductor35bof the IDT group31bis mainly formed between the electrode finger of the floating conductor35acloser to (facing) the IDT group31band the electrode finger of the floating conductor35bcloser to (facing) the IDT group31a. According to an aspect of the present invention, there is provided a multimode type SAW filter having multiple IDTs connected in series in which the electrostatic capacitances between the floating conductors can be reduced.

First Embodiment

FIG. 4shows a structure of a first embodiment, which is an exemplary double mode type SAW filter that is one of the multimode type SAW filters and has IDT groups in which multiple IDTs are connected in series. Referring toFIG. 4, IDT groups41,42and43are arranged in a SAW propagation direction between reflectors40and45formed on a piezoelectric substrate10.

The IDT group41has IDTs51and56connected in series. The IDT51is composed of an input/output electrode61and a floating conductor71. The IDT56is composed of a ground electrode66and the floating conductor71. That is, the IDT41is structured so that the IDTs51and56are connected in series via the floating conductor71. Similarly, the IDT group42has an IDT52composed of an input/output electrode62and a floating conductor72, and an IDT57composed of a ground electrode67and the floating conductor72. The IDTs52and57are connected in series via the floating conductor72. The IDT group43has an IDT53composed of an input/output electrode63and a floating conductor73, and an IDT58composed of a ground electrode68and the floating conductor73. The IDTs53and58are connected in series via the floating conductor73. In each of the IDT groups, the input/output electrode and the ground electrode are connected in series via the floating conductor.

The upper IDTs are the IDT51(first IDT), the IDT52(second IDT), and the IDT53(third IDT) arranged in the SAW propagation direction. The lower IDTs are the IDT56(first IDT), the IDT57(second IDT), and the IDT58(third IDT). The input/output electrode62of the IDT group42is connected to an input terminal, and the input/output electrodes61and63of the IDT groups41and43are connected to output terminals. The ground electrodes66,67and68are connected to ground terminals. The piezoelectric substrate10may be a LiNbO3substrate or LiTaO3, and the input/output electrodes, ground electrodes and floating conductors may be made of Al.

When an input signal is applied to the input terminal (that is, the IDT group42), SAWs excited between the reflectors40and45are propagated towards these reflectors, so that multiple standing waves take place between the reflectors40and45. The voltages that correspond to the frequencies of the standing waves appear between the IDTs51and56of the IDT group41and between the IDTs53and58of the IDT group43. Thus, only the desired frequencies that correspond to the standing waves can be output to the output terminals. That is, the DMS in accordance with the first embodiment functions as a bandpass filter.

In the IDT groups41and42, an electrode finger74aof the floating conductor71of the IDT51is closest to the IDT52. An electrode finger65aof the input/output electrode62of the IDT52is closet to the IDT51. An electrode finger75aof the floating conductor71of the IDT56is closet to the IDT57. An electrode finger69aof the ground electrode67of the IDT57is closet to the IDT56. The same relationship as that of the IDTs51and52is available in the IDT groups42and43.

As mentioned above, in a case where one of the adjacent electrode fingers between the adjacent IDTs is connected to the floating conductor, the other electrode finger is connected to a conductor (input/output electrode or ground electrode) other than the floating conductor. Thus, the electrode finger of the input/output electrode or ground electrode is arranged between the floating conductors71and72, so that the electrostatic capacitance therebetween can be reduced. It is thus possible to restrain the notch in the pass band, as will be described in detail later.

As the connecting lines that connect the electrode fingers of each of the floating conductors71,72and73are shorter, the electrostatic capacitances between the floating conductors71,72and73become smaller. Preferably, the lengths of the connecting lines are equal to or less than five times the electrode pitch or period of the IDTs.

In the IDT51shown inFIG. 4, the floating conductor71has an electrode finger74bin addition to the electrode finger74a. The electrode finger74bis adjacent to the electrode finger74ain the SAW propagation direction. Similarly, in the IDT56, the floating conductor71has an electrode finger75bin addition to the electrode finger75aso that the electrode fingers75aand75bare adjacent in the SAW propagation direction. That is, one of two kinds of adjacent electrode fingers located between the adjacent IDTs is composed of multiple adjacent electrode fingers that are at an identical potential. In other words, the multiple electrode fingers74aand74bat the equal potential are provided between the IDT51(first IDT) and the IDT52(second IDT), and the multiple electrode fingers78aand78bat the equal potential are provided between the IDT52(second IDT) and the IDT53(third IDT).

In each of all the IDTs that form the multimode type SAW filter, the input/output electrode and the ground electrode are coupled in series via the floating conductor. It is thus possible to improve the power durability and effectively suppress the non-linearity. The electrode fingers74aand74bare connected to the conductor71at the potential different from the potentials of the input/output electrode61and the ground electrode66, and the electrode fingers78aand78bare connected to the conductor73at the potential different from the potentials of the input/output electrode63and the ground electrode68. It is thus possible to widen the pass band range.

The IDTs51and52are adjacent to each other in the SAW propagation direction. The multiple fingers74aand74bat the equal potential are connected to the floating conductor71. Similarly, the multiple electrode fingers75aand75bat the equal potential are connected to the floating conductor71.

The two electrode finger patterns of the IDTs connected in series in each of the IDT groups have a mirror symmetry with respect to an axis that runs in the SAW propagation direction. For example, the IDT51and IDT56of the IDT group41has a mirror symmetry with respect to a line A-A. That is, the input/output electrode61and the ground electrode66have a mirror symmetry with respect to the line A-A, and the floating conductor71has a mirror symmetry with respect to the line A-A.

The input/output electrodes61and63connected to the output terminals are provided on the upper side ofFIG. 4, and the input/output electrode62connected to the input terminal is provided on the upper side thereof. The ground electrodes66,67and68are provided on the lower side ofFIG. 4. That is, the input/output electrodes61and63connected to the output terminals and the input/output electrode62connected to the input terminal are provided in the same direction. It is thus possible to add coupling capacitances between the input terminal and the output terminals and to improve the out-of-band suppression.

The aperture lengths AP1of the IDTs51,52and53and the aperture lengths AP2of the input IDTs56,57and58are equal to 2.0×AP. With this arrangement, the power durability is mostly improved and the non-linearity is mostly suppressed.

The inventors simulated the passband characteristics of the conventional filter shown inFIG. 2A, the comparative filter shown inFIG. 2Bwhere AP1=AP2=2×AP, and the first embodiment shown inFIG. 4where AP1=AP2=2×AP. The results of the simulation are shown inFIGS. 5A and 5B.FIG. 5Ais a graph of the frequency characteristics of the passbands of the simulated filters, andFIG. 5Bis an enlarged view of a portion surrounded by a circle shown inFIG. 5A.

As in the case ofFIGS. 3A and 3B, a notch that increases the insertion loss takes place at about 1,926 MHz in the comparative filter. In contrast, no notch takes place in the first embodiment. This is because the first embodiment is capable of suppressing the electrostatic capacitance between the floating conductors71and72and the electrostatic capacitance between the floating conductors72and73. Thus, the multimode type SAW filter having the IDT groups each having multiple IDTs connected in series is allowed to have the relationship AP1=AP2that results in the mostly improved power durability and the mostly suppressed non-linearity without degrading the filter characteristics. In is thus possible to improve the power durability and the non-linearity without degrading the filter characteristics. Preferably, AP1=AP2should be satisfied. However, it is not essential to satisfy the relationship AP1=AP2. Even when AP1is not equal to AP2, the filter characteristics having no notch can be achieved.

The electrostatic capacitance between the floating conductors of the adjacent IDTs may be reduced by not only the first embodiment but also variations thereof. Now, these variations will be described.

FIG. 6shows a first variation of the first embodiment, which differs from the first embodiment in that floating conductors71a,72aand73aare different from the floating conductors71,72and73. The other structures of the variation are the same as those of the first embodiment. In the adjacent IDTs51and52, the electrode finger of the IDT51adjacent to the IDT52is an electrode finger64aof the input/output electrode61, and the electrode finger of the IDT52adjacent to the IDT51is an electrode finger76aof the floating conductor72a. Similarly, in the adjacent IDTs56and57, the electrode finger of the IDT56adjacent to the IDT57is an electrode finger69bof the ground electrode66, and the electrode finger of the IDT57adjacent to the IDT56is an electrode finger77aof the floating conductor72a. Additionally, an electrode finger77bis connected to the floating conductor72aand is arranged close to the electrode finger77a. The adjacent electrode fingers of the IDT groups42and43have a relationship similar to the above.

In the above-mentioned manner, the multiple electrode fingers that are at the equal potential may be connected to the floating conductor72aof the IDT group42. A mutual connection is made between the multiple electrode fingers76aand76bthat are at the equal potential and are located between the IDT51(first IDT) and the IDT52(second IDT), and multiple electrode fingers78cand78dthat are at the equal potential and are located between the IDT52(second IDT) and the IDT53(third IDT).

FIG. 7shows a second variation of the first embodiment, which differs from the first embodiment in that floating conductors71b,72band73bhave a structure different from that of the first embodiment. In the adjacent IDTs51and52, the electrode finger of the IDT51adjacent to the IDT52is an electrode finger74cof the floating conductor71b, and an electrode finger74dis additionally provided close to the electrode finger74c. The electrode finger of the IDT52adjacent to the IDT51is an electrode finger65aof the input/output electrode. Similarly, in the adjacent IDTs56and57, the electrode finger of the IDT56adjacent to the IDT57is an electrode finger69bof the ground electrode66, and the electrode finger of the IDT57adjacent to the IDT56is an electrode finger77cof the floating conductor72b, and an electrode finger77dis additionally provided close to the electrode finger77c. The adjacent electrode fingers of the IDT groups42and43have a relationship similar to that mentioned above. As described above, the multiple electrode fingers that are located on the input/output side and are at the equal potentials may be connected to the floating conductors71band73bon the opposite sides, and the multiple electrode fingers that are located on the ground side and are at the equal potential may be connected to the central floating conductor72b.

FIG. 8shows a third variation of the first embodiment, which has an arrangement in which the electrode finger patterns of the two IDTs among the IDTs connected in series are shifted in parallel in the direction perpendicular to the SAW propagation direction. The IDTs56,57and58have electrode patterns obtained by moving the IDTs51,52and53in parallel in the direction perpendicular to the SAW propagation direction. That is, a ground electrode66ahas an electrode pattern obtained by moving an upper portion of a floating conductor71cin parallel, and a lower portion of the floating conductor71chas an electrode pattern obtained by moving the input/output electrode61. Similarly, a ground electrode67ahas an electrode pattern obtained by moving an upper portion of a floating conductor72c, and a lower portion of the floating conductor72chas an electrode pattern obtained by moving the input/output electrode62in parallel. A ground electrode68chas an electrode pattern obtained by moving an upper portion of a floating conductor73cin parallel, and a lower portion of the floating conductor73chas an electrode pattern obtained by moving the input/output electrode63in parallel.

With the above arrangement, in the adjacent IDTs51and52, the electrode finger of the IDT51adjacent to the IDT52is an electrode finger74eof the floating conductor71c, and an electrode finger74fis additionally provided close to the electrode finger74e. The electrode finger of the IDT52adjacent to the IDT51is an electrode finger65aof the input/output electrode52. Similarly, in the adjacent IDTs56and57, the electrode finger of the IDT56close to the IDT57is an electrode finger69cof the ground electrode66a, and the electrode finger of the IDT57close to the IDT56is an electrode finger77eof the floating conductor72c. As in the case of the IDT56, the multiple electrode fingers69cand69dat the equal potential are connected to the ground electrode66a. This shows that the multiple electrode fingers at the identical potential may be connected to a conductor other than the floating conductor as long as the conductor is at the same potential as the multiple electrode fingers.

FIG. 9shows a fourth variation of the first embodiment. The fourth variation has an arrangement in which the ground electrodes66,67and68are connected on the piezoelectric substrate10via a connecting line46. The other structures of the fourth variation are the same as those of the first embodiment. It should be noted that the ground electrodes of the IDTs are connected together on the piezoelectric substrate. This makes it easy to control the common ground inductance added to the IDT group42connected to the input terminal and the IDT groups41and43connected to the output terminals.

FIG. 10shows a fifth variation of the first embodiment. The ground electrodes66,67and68are unified, and the electrode fingers of the ground electrodes66,67and68are directly connected to a single ground bus bar47. The ground bus bar47is a straight line shaped electrode to which the electrode fingers are directly connected. All the electrode fingers of the ground electrodes of the IDTs56,57and58are connected to the single bus bar, and the other structures of the fifth variation are the same as those of the first embodiment. It is thus possible to make it easy to control the common ground inductance added to the IDT group42connected to the input terminal and the IDTs41and43connected to the output terminals.

FIG. 11shows a sixth variation of the first embodiment, in which input/output electrodes61aand63aconnected to the output terminals are provided in a different direction from that in which an input/output electrode62connected to the input terminal is provided. The IDT group42has a lower stage of the IDT57having the ground electrode67and the floating conductor72, and an upper stage of the IDT52having the input/output electrode connected to the input terminal and the floating conductor72. In contrast, the IDT groups41and43have lower stages of IDTs51aand53ahaving input/output electrodes61aand63aconnected to the output terminals and the floating conductors71and73, and upper stages of IDTs56aand58ahaving ground electrodes66band68b, and the floating conductors71and73, respectively. The other structures of the sixth embodiment are the same as those of the first embodiment. With the above arrangement, it is possible to improve the power durability and suppress the non-linearity.

FIG. 12shows a seventh variation of the first embodiment. As to the IDT groups41and43located on the opposite sides, the input/output electrode61of the IDT51and the input/output electrode63of the IDT53are connected to input terminals, and the input/output terminal62of the IDT52of the IDT group42is connected to the output terminal. The other structures of the seventh variation are the same as those of the first embodiment. As compared to the first embodiment, the seventh variation has more improved durability and more suppressed non-linearity.

FIG. 13shows an eighth variation of the first embodiment, in which the connecting lines of the floating conductors are eliminated. IDT groups41a,42aand43aare disposed between reflectors40aand45ain the SAW propagation direction. The IDT group41ahas the input/output electrode61and the ground electrode66, and electrode fingers79of the floating conductor are interposed between the electrode fingers of the electrodes61and66. Similarly, the IDT group42ahas the input/output electrode62and the ground electrode69, and electrode fingers79of the floating conductors are interposed between the electrode fingers of the electrodes62and67. The IDT group43ahas the input/output electrode63and the ground electrode68, and electrode fingers79of the floating conductor are interposed between the electrode fingers of the electrodes63and68. Further, two electrode fingers79serving as floating conductors are provided so as to run between the input/output electrodes61and62and the ground electrodes66and67. Similarly, two electrode fingers79serving as floating conductors are provided so as to run between the input/output electrodes62and63and between the ground electrodes67and68. The other structures of the eighth embodiment are the same as those of the first embodiment. The elimination of the connecting lines of the floating conductors further suppresses the notch in the pass band.

FIG. 14shows a ninth embodiment of the first embodiment, in which there is provided an electrode finger between the connecting lines of the adjacent floating conductors of the adjacent IDTs, the electrode finger being not connected to the above connecting lines. The other structures of the ninth embodiment are the same as those of the first embodiment. Electrode fingers65band69eare provided between the connecting line48aof the floating conductor71of the IDTs51and56and the connecting line48bof the floating conductor72of the IDTs52and57. Further, the electrode finger65bis connected to the input/output electrode62b, and the electrode finger69eis connected to the ground electrode67b. That is, the electrode fingers65band69eare not connected to the floating conductors71and72but are isolated therefrom. The connecting lines48band48cof the IDT groups42and43have the same relation as mentioned above. That is, the electrode fingers are provided between the connecting lines of the adjacent floating conductors of the two adjacent IDTs connected in series, these electrode fingers being not connected to the above connecting lines. It is thus possible to reduce the electrostatic capacitances between the connecting lines48aand48band between the connecting lines48band48c. More specifically, it is possible to reduce the electrostatic capacitance between the floating conductors71and72and that between the floating conductors72and73. Therefore, the notch in the pass band can be further suppressed.

The first embodiments and variations thereof are the exemplary DMS filters. However, the present invention is not limited to the DMS filters but may include multimode SAW filters such that multiple IDT groups are arranged in the SAW propagation direction. Even the multimode SAW filters have similar advantages to the above. The number of electrode fingers at the equal potential is not limited to two, but an arbitrary number of electrode fingers more than two may be used to obtain the similar advantages.

Second Embodiment

A second embodiment of the present invention is an exemplary filter having three IDTs connected in series.FIG. 15is a top view of the second embodiment. Three IDT groups131a,131band131care provided on the piezoelectric substrate10and are located between reflectors130aand130balso provided thereon. The IDT group131ahas IDTs132a,133aand134aconnected in series. The IDT132ahas an input/output electrode135aand a floating conductor137a. The IDT133ahas the floating conductor137aand another floating conductor138a. The IDT134ahas a ground electrode136aand the floating conductor138a. Similarly, the IDT group131bhas IDTs132b,133band134bconnected in series. The IDT132bhas an input/output electrode135band a floating conductor137b. The IDT134bhas the floating conductor137band another floating conductor138b. The IDT134bhas a ground electrode136band the floating conductor138b. The IDT group131chas IDTs132c,133cand134cconnected in series. The IDT132chas an input/output electrode135cand a floating conductor137c. The IDT134chas the floating conductor137cand another floating conductor138c. The IDT134chas a ground electrode136cand the floating conductor138c.

In the adjacent IDTs132aand132b, the electrode finger of the IDT132aadjacent to the IDT132bis an electrode finger139aof a floating conductor137a. The electrode finger of the IDT132badjacent to the IDT132ais an electrode finger140aof an input/output electrode135b. Similarly, in the adjacent IDTs133aand133b, the electrode finger of the IDT133aadjacent to the IDT133bis an electrode finger139cof a floating electrode137a. The electrode finger of the IDT133badjacent to the IDT133ais an electrode finger141cof a floating conductor138b. In the adjacent IDTs134aand134b, the electrode finger of the IDT134aadjacent to the IDT134bis an electrode finger142aof a ground electrode136a. The electrode finger of the IDT134badjacent to the IDT134ais an electrode finger141aof a floating conductor138b. The IDT groups131band131chave a relationship similar to the above.

The IDTs133a,133band133cat the middle stage have a mirror relationship with the IDTs132a,132band132cat the upper stage with respect to an axis running in the SAW propagation direction. The IDTs134a,134band134cat the lower stage have an electrode pattern obtained by moving the IDTs33a,133band133cat the middle stage in parallel in the direction perpendicular to the SAW propagation direction.

As described above, one of the adjacent electrode fingers between the adjacent IDTs is connected to the floating conductor, the other electrode finger is connected to a conductor other than the above floating conductor, such as the input/output electrode or ground electrode. The electrode finger140aof the input/output electrode135bis disposed between the adjacent floating conductors137aand137b, and the electrode finger142aof the ground electrode136ais disposed between the adjacent floating conductors138aand138b. It is thus possible to reduce the electrostatic capacitance between the floating conductors137aand137band the electrostatic capacitance between the floating conductors138aand138band to suppress the notch in the pass band, as in the case of the first embodiment.

An electrode finger139bof the floating conductor137aof the IDT132ais additionally provided close to the electrode finger139athereof. The electrode fingers139aand139bare the adjacent electrode fingers for the IDT132b. Similarly, the adjacent electrode fingers of the IDT133aadjacent to the IDT133bincludes an electrode finger139dof the floating conductor137ain addition to the electrode finger139cthereof so that the electrode fingers139cand139dare arranged side by side and are adjacent to each other. The adjacent electrode fingers of the IDT134badjacent to the IDT134aincludes an electrode finger141bof the floating conductor138bin addition to the electrode finger141athereof so that the electrode fingers141aand141bare arranged side by side and are adjacent to each other one of two kinds of adjacent electrode fingers located between the adjacent IDTs is composed of multiple adjacent electrode fingers that are at an identical potential.

Preferably, the second embodiment having three divided IDTs connected in series has the same impedance as the conventional filter shown inFIG. 2A. It is thus preferred that the composite electrostatic capacitances of the IDT groups131a,131band131care respectively equal to the electrostatic capacitances of the IDTs21aand22band23c. It is therefore preferable to satisfy the following equation:
AP1×AP2×AP3=(AP1×AP2+AP1×AP3+AP2×AP3)×AP
where AP1is the aperture lengths of the IDTs132a,132band132cof the upper stage, AP2is the aperture lengths of the IDTs133a,133band133cof the middle stage, and AP3is the aperture lengths of the IDTs134a,134band134cof the lower stage. When AP1=AP2=AP3=3.0AP, the voltage applied across each IDT is the lowest. It is thus possible to obtain the mostly improved power durability and the mostly suppressed non-linearity. The second embodiment assumes that AP1=AP2=AP3=3.0AP.

In the second embodiment having the relationship of AP1=AP2=AP3=3.0AP, the voltage applied across each IDT is equal to ⅓ of the voltage applied across each IDT of the conventional filter. The voltage in the first embodiment is equal to ½ of the conventional voltage. Therefore, the second embodiment has further improved power durability and further suppressed non-linearity. As the number of stages resulting from the series fragmentation increases, the power durability can be further improved and the non-linearity can be further suppressed. However, the filter area increases. The number of stages by the series fragmentation may be determined taking into account the chip area of the filter, the power durability and non-linearity.

Third Embodiment

A third embodiment has four multimode SAW filters that are configured in accordance with the first embodiment and are connected in parallel.FIG. 16shows the third embodiment. The filter shown inFIG. 16has four DMS filters91,92,93and94, each of which has the same configuration as the fifth variation of the first embodiment. The aperture length of the third embodiment is equal to ¼ of that of the fifth variation. The input terminals of the DMS filters91,92,93and94are connected in parallel, and are connected to a common input terminal90. The output terminals of the DMS filters91through94are connected in parallel, and are connected to a common output terminal95. The third embodiment with the four DMS filters connected in parallel will have an input/output impedance equal to that of the fifth variation of the first embodiment if the aperture length of each of the DMS filters91-94is equal to ¼ of that of the fifth variation of the first embodiment. The ¼ reduced aperture length reduces the resistances of the electrode fingers and the insertion loss.

The DMS filters91-94used in the third embodiment are not limited to the fifth variation of the first embodiment but may have another configuration. The number of the DMS filters used in the third embodiment is not limited to four but an arbitrary number of DMS filters may be employed. As an increased number of DMS filters connected in parallel, a reduced aperture length can be used and a lowered insertion loss is available. However, the filter size becomes larger. The number of DMS filters to be connected in parallel may be selected taking into account the filter size and insertion loss.

Fourth Embodiment

A fourth embodiment is configured by connecting a multimode type SAW filter having only IDTs that are not formed by the series fragmentation to the output terminal of the multimode type SAW filter configured in accordance with the third embodiment.FIG. 17shows the fourth embodiment, which has a first filter100of the third embodiment, and a second filter102having an input terminal107connected to the output terminal95of the first filter100. The first filter100is the multimode type SAW filter of the third embodiment, and the second filter102is composed of conventional DMS filters103,104,105and106connected in parallel and having no series fragmentation. The input terminals of the DMS filters103,104,105and106are connected to a common input terminal107of the second filter102, and output terminals of the DMS filters103,104,105and106are connected to a common output terminal108of the second filter102.

The first and second filters100and102are cascaded, so that the out-of-band suppression can be greatly improved. The first filter100connected to the input terminal90mainly affects the power durability and non-linearity. The first filter100formed by the third embodiment improves the power durability and suppresses the non-linearity. The second filter102has only the IDTs that are not subjected to the series fragmentation, and occupies a small area on the piezoelectric substrate.

As described above, preferably the second filter102has only the IDTS that are not subjected to the series fragmentation. Alternatively, the second filter102may have IDTs formed by the fragmentation such as the third embodiment. The first filter100may be the first embodiment or any of the variations thereof. The second filter102may be a multimode type SAW filter composed of one or multimode type SAW filters connected in parallel.

Fifth Embodiment

A fifth embodiment is an antenna duplexer for the portable phone to which the filter of the fourth embodiment is applied.FIG. 18shows the antenna duplexer of the fifth embodiment. The antenna duplexer has a receive filter120aformed by the fourth embodiment, and four DMS filters121aformed by the fifth variation of the first embodiment are connected in parallel and are connected to the input filter of the receive filter120a. Further, the receive filter120aincludes four conventional DMS filters122aconnected to the common output terminal of the four DMS filters121a.

A transmit terminal128is connected to an input terminal of a transmit filter126formed by a ladder type SAW filter, which has an output terminal connected to an antenna terminal125. A transmit signal applied to the antenna terminal128is applied to the transmit filter126, which allows only desired frequency components to pass therethrough. These frequency components are transmitted via the antenna terminal125to which an antenna is connected. The antenna terminal125is coupled to a matching circuit123, which is connected to the input terminal of the receive filter120a. The output terminal of the receive filter120ais connected to the receive terminal124. A receive signal received via the antenna terminal125passes through the matching circuit123, and only desired frequency components thereof are allowed to pass through the receive filter120a, and are applied to the receive terminal124.

The transmit filter126is formed by the ladder type SAW filter having high power durability, and is thus capable of withstanding transmit signals of high power. The receive filter120ais formed by the multimode type SAW filter of the present invention, whereby a high out-of-band attenuation and a steep cutoff characteristic can be obtained. Further, as has been described with reference toFIG. 1, even in a case where part of the transmit signal from the transmit filter126leaks to the receive filter120a, the receive filter120ahas improved power durability and is hardly broken by the leakage power. Furthermore, the receive filter120ahas suppressed non-linearity and an improved receive sensitivity.

The inventors performed a cross modulation test (CM test), which is one of factors used for evaluating the performance of the antenna duplexer in order to confirm the effects of suppression of non-linearity.

FIG. 19Ashows an environment of the CM test. The block structure of the antenna duplexer is the same as shown inFIG. 1. A modulated signal (transmit signal) having a large power and a transmit frequency fTxis applied to the transmit terminal13in order to obtain a power of 25 dBm at the antenna terminal14. Simultaneously, the antenna terminal14is provided with a non-modulated signal (interference wave) that is as weak as −30 dBm and has a receive frequency of fTx+ΔG+Δf. Then, the signal spectra available at the receive terminal15was measured.

FIG. 19Bshows conditions of test frequencies in the CM test. The horizontal axis denotes the frequency, and the vertical axis denotes the amount of signal passage. A solid line inFIG. 19Bdenotes the filter characteristic of the transmit filter16, and a broken line denotes the filter characteristic of the receive filter11. The transmit and receive frequency ranges of a communication system with the portable phone having the antenna duplexer are included in the pass bands of the transmit filter16and the receive filter11, respectively. The parameter ΔG denotes a difference (fixed value) between the center frequency of the transmit frequency range and that of the receive frequency range. The parameter Δf is a channel interval (equal to a few MHz) defined for each communication system. Under the above conditions, the signal spectra of the interference waves obtained at the receive terminal15are measured. In the CM test, the receive filter11is provided with not only the interference waves but also a part of the transmit signal (leaked power) that is not applied to the antenna terminal14.

FIG. 20shows a spectrum of the interference waves observed at the receive terminal15. The horizontal axis ofFIG. 20denotes the frequency, and the vertical axis denotes the output power (dBm) measured at the receive terminal15. A broken line inFIG. 20denotes a spectrum of the output power at the receive terminal15measured in the absence of the transmit signal. This corresponds to a situation in which the receive filter11has no non-linearity. In this situation, a steep peak appears only at the receive frequency fTx+ΔG+Δf (the interference wave of this frequency). In contrast, the solid line inFIG. 20is the spectrum of the output power obtained at the receive terminal15measured when the transmit signal is input. In this case, if the receive filter11has a non-linearity, the receive signal (interference waves) is modulated by the leakage power, and the spectrum is widen, so that a modulated signal is adversely generated. The frequency of the modulated signal extends to the adjacent channel, and the receive signal fTx+ΔG may be buried in the modulated signal. This degrades the receive sensitivity. In this manner, the non-linearity of the receive filter11is evaluated by the magnitude of the modulated signal. The receive filter having a small modulated signal has a small non-linearity and is suitable for the antenna duplexer.

In addition to the fifth embodiment, antenna duplexer shown inFIGS. 21 and 22were evaluated by the CM test.FIG. 21shows a conventional antenna duplexer having a receive filter120bdifferent from that of the fifth embodiment. The other structures of the filter shown inFIG. 21are the same as those of the fifth embodiment. A DMS filter121bon the input terminal side and a DMS filter122bon the output terminal side are both the conventional DMS filters that do not have the series fragmentation. The antenna duplexer shown inFIG. 22(comparative filter) has a receiver filter120cdifferent from that of the fifth embodiment, and the other structures thereof are the same as those of the fifth embodiment. A DMS filter121con the input terminal side is the comparative DMS filter shown inFIG. 2B, and a DMS filter122con the output terminal side is the conventional DMS filter shown inFIG. 2A, in which each IDT is composed of only a pair of comb-like electrodes.

FIG. 23shows the results of the CM test. The horizontal axis denotes the frequency, and the vertical axis denotes the output power available at the receive terminal. A dotted line, a broken line and a solid line inFIG. 23denote spectra of the output powers at the receive terminals of the conventional filter, the comparative filter and the fifth embodiment, respectively. The transmit frequency used in the test was the lowest frequency in the transmission frequency range. The antenna duplexer of the fifth embodiment has the mostly improved modulated signal intensity for the adjacent channel. This is because the aperture lengths of the two IDTs connected in series are equal to 2.0×AP, whereby a reduced voltage can be applied to each IDT and the SAW intensity excited per unit area can be reduced. This makes it difficult to cause the non-linearity.

FIGS. 24A and 24Bshow pass band characteristics of the above-mentioned three different antenna duplexers.FIG. 24Bis an enlarged view of the receive band inFIG. 24A. The insertion losses of the conventional filter, comparative filter and fifth embodiment are illustrated by a dotted line, broken line and solid line. The antenna duplexer of the fifth embodiment has no notch in the pass band and exhibits filter characteristics equivalent to those of the conventional and comparative filters.

From the above-mentioned experimental results, it is confirmed that the power durability and non-linearity of the antenna duplexer can be improved by the DMS filter121aon the input terminal side of the receive filter120ahaving the multiple IDTs connected in series. Further, the electrostatic capacitances between the adjacent floating conductors of the DMS filter121acan be reduced and the non-linearity in the pass band of the antenna duplexer can be suppressed by the arrangement in which one of the adjacent electrode fingers between the adjacent IDTs of the DMS filter121ais connected to the floating conductor and the other is connected to a conductor other than the floating conductor (the input/output electrode or the ground electrode). It is thus possible to set the aperture lengths of the IDTs of the DMS filter120aconnected in series by the series fragmentation equal to each other (AP1=AP2) at which the mostly improved power durability and the most suppressed non-linearity. According to the fifth embodiment, it is possible to improve the power durability and suppress the non-linearity without degrading the filter characteristics of the antenna duplexer.

In the fifth embodiment, the filter of the fourth embodiment is applied to the receive filter120a. Alternatively, the first embodiment, its variations, the second embodiment or the third embodiment may be applied to the receive filter120a, and advantages similar to the above can be obtained in these cases.

Sixth Embodiment

A sixth embodiment is a balanced type SAW filter.FIG. 25is a top view of the sixth embodiment. An upper portion of the sixth embodiment is configured so that the floating conductor71on the left-hand side in the sixth variation of the first embodiment is replaced by a floating conductor71d, and the input/output electrodes61aand63aare changed to floating conductors161and163. The other structures of the sixth embodiment are the same as those of the sixth variation of the first embodiment. Between the reflectors150and155, the floating conductor161and a ground electrode166form an IDT151, and a floating conductor163and a ground electrode168form an IDT153. Between the IDTs151and153, there are provided IDTs152aand152bcomposed of floating conductors162and input/output electrodes167aand167b, to which a balanced output terminal1and a balanced output terminal2are connected, respectively.

Only one electrode finger74aof the floating conductor71don the left-hand side is adjacent to the IDTs52and57at the center. In contrast, two electrode fingers78aand78bof the floating conductor73on the right-hand side are adjacent to the IDT52at the center. Thus, the signals respectively excited by the IDTs151and153are 180° out of phase. Thus, the signals excited by the IDTs152aand152bare also 180° out of phase. Thus, the signals available at the balanced output terminals1and2are 180° out of phase, so that the filter of the sixth embodiment functions as a balanced type filter.

As in the case of the sixth variation of the first embodiment, one of the adjacent electrode fingers between the adjacent IDTs is connected to the floating conductor and the other is connected to a conductor other than the floating conductor (the input/output electrode or ground electrode). It is thus possible to reduce the electrostatic capacitance between the floating conductors71dand72and that between the floating conductors72and73and to suppress the notch in the pass band.

Seventh Embodiment

A seventh embodiment is a balanced type SAW filter in which each IDT group is composed of three IDTs connected in series.FIG. 26shows the seventh embodiment, in which the input/output electrodes135aand135cof the second embodiment are connected to the input terminals, and the input/output electrode135bthereof is connected to the balanced output terminal1, the ground electrode134bthereof being replaced with the input/output electrode135dand being connected to the balanced output terminal2. The other structures of the seventh embodiment are the same as those of the second embodiment. The input/output electrode135band the floating conductor137bare 180° out of phase, and the floating conductors137band138bare 180° out of phase. Further, the floating conductor138band the input/output electrode135care 180° out of phase. Thus, the balanced output terminal1connected to the input/output electrode135band the balanced output terminal2connected to the input/output electrode135dare 180° out of phase, and the filter of the seventh embodiment functions as the balanced type filter.

As in the case of the second embodiment, one of the adjacent electrode fingers between the adjacent IDTs is connected to the floating conductor and the other is connected to a conductor other than the floating conductor (the input/output electrode or ground electrode). It is thus possible to suppress the notch in the pass band and to improve the power durability and suppress the non-linearity without degrading the filter characteristics.

As in the case of the sixth or seventh embodiment, the balanced type SAW filter may be formed by employing any of the first embodiment, its variations, the second, third or fourth embodiment. It is thus possible to improve the power durability and suppress the non-linearity without degrading the filter characteristics. The antenna duplexer with the balanced type SAW filter of the present invention has improved power durability and suppressed non-linearity in the absence of degradation of the filter characteristics.

The present invention is not limited to the specifically described embodiments and variations thereof, but other embodiments and variations may be made without departing from the scope of the present invention.

The present invention is based on Japanese Patent Application No. 2005-130988 filed on Apr. 28, 2005, and the entire disclosure of which is hereby incorporated by reference.