JP2005072992A

JP2005072992A - Longitudinally coupled dual mode surface acoustic wave filter

Info

Publication number: JP2005072992A
Application number: JP2003300739A
Authority: JP
Inventors: Shozo Matsumoto; 省三松本
Original assignee: Toyo Communication Equipment Co Ltd
Current assignee: Toyo Communication Equipment Co Ltd
Priority date: 2003-08-26
Filing date: 2003-08-26
Publication date: 2005-03-17

Abstract

<P>PROBLEM TO BE SOLVED: To provide a longitudinally coupled dual mode surface acoustic wave filter comprising three IDTs arranged sequentially on a piezoelectric substrate along the propagating direction of a surface acoustic wave and having a narrow band and a steep attenuation gradient of pass band. <P>SOLUTION: In the longitudinally coupled dual mode surface acoustic wave filter comprising a first, second, third IDTs arranged sequentially on a piezoelectric substrate along the propagating direction of a surface acoustic wave and provided with first and second reflectors on the opposite sides of the first through the third IDTs, the distance L1 between the centers of closest electrode fingers of the first IDT and the second IDT is differentiated from the distance L2 between the centers of closest electrode fingers of the second IDT and the third IDT. Furthermore, period of the electrode fingers of the first and third IDTs is differentiated from the period of electrode finger of the second IDT. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

本発明は、圧電基板上の弾性表面波の伝搬方向に沿って複数のＩＤＴを順次配置した縦結合二重モード弾性表面波フィルタにおいて、狭帯域で通過域の減衰傾度を急峻にした縦結合二重モード弾性表面波フィルタに関する。 The present invention relates to a longitudinally coupled double mode surface acoustic wave filter in which a plurality of IDTs are sequentially arranged along a propagation direction of a surface acoustic wave on a piezoelectric substrate. The present invention relates to a heavy mode surface acoustic wave filter.

近年、弾性表面波（Surface Acoustic Wave：以下、ＳＡＷ）フィルタは移動体通信分野で広く利用され、高性能、小型、量産性等の優れた特徴があることから特に携帯電話等に多く用いられている。日本の携帯電話に使われているＰＤＣ方式（Personal Digital Cellular）のＲＦ部においては、隣接する送受信波を分離するために狭帯域で通過帯域の減衰傾度の急峻なＳＡＷフィルタが要求されている。この理由としては、図８に示すようにＰＤＣ方式において周波数割り当てが送信周波数帯８９３ＭＨｚ〜８９８ＭＨｚ（Ｔｘ１）と、受信帯８７０ＭＨｚ〜８８５ＭＨｚ（Ｒｘ２）とが近接しているためであり、双方が混信しないよう狭帯域で減衰傾度が急峻な特性が必要なのである。 In recent years, surface acoustic wave (SAW) filters have been widely used in the field of mobile communication, and have been used particularly in mobile phones because of their excellent characteristics such as high performance, small size, and mass productivity. Yes. In the RF part of the PDC (Personal Digital Cellular) used in Japanese mobile phones, a SAW filter with a narrow band and a steep inclination of the pass band is required to separate adjacent transmission and reception waves. This is because, as shown in FIG. 8, in the PDC system, the frequency allocation is close to the transmission frequency band 893 MHz to 898 MHz (Tx1) and the reception band 870 MHz to 885 MHz (Rx2), and both do not interfere with each other. Therefore, it is necessary to have such a characteristic that the attenuation gradient is steep in a narrow band.

携帯電話のＲＦ用フィルタとしてよく用いられる縦結合二重モードＳＡＷフィルタの平面図を図９に示す。圧電基板上にＳＡＷの伝搬方向に沿ってＩＤＴ３２、３３、３４を近接配置すると共に、これらのＩＤＴの両側にグレーティング反射器（以下、反射器と称す）３５ａ、３５ｂを配設する。前記ＩＤＴ３２、３３、３４は互いに間挿し合う複数の電極指を有する一対のくし形電極から形成され、中央のＩＤＴ３３の一方のくし形電極は入力端子ＩＮに接続し他方のくし形電極は接地する。さらに、両側のＩＤＴ３２、３４のそれぞれ一方のくし形電極を接続して出力端子ＯＵＴに接続すると共に、他方のくし形電極はそれぞれ接地して縦結合二重モードＳＡＷフィルタ３１が構成される。 FIG. 9 shows a plan view of a longitudinally coupled double mode SAW filter that is often used as an RF filter for a cellular phone. IDTs 32, 33, and 34 are disposed close to each other along the SAW propagation direction on the piezoelectric substrate, and grating reflectors (hereinafter referred to as reflectors) 35a and 35b are disposed on both sides of these IDTs. The IDTs 32, 33, and 34 are formed of a pair of comb electrodes having a plurality of electrode fingers that are interleaved with each other. One comb electrode of the central IDT 33 is connected to the input terminal IN and the other comb electrode is grounded. . Further, one comb electrode of each of the IDTs 32 and 34 on both sides is connected to the output terminal OUT, and the other comb electrode is grounded to constitute a longitudinally coupled double mode SAW filter 31.

前述の縦結合二重モードＳＡＷフィルタ３１において、狭帯域な特性を実現するにはＩＤＴ３２〜３４の電極指対数を増やせばよい。しかしながら、電極指対数を増やすと狭帯域になるものの挿入損失が劣化しまう問題があった。また、他の手段として、ＩＤＴ３２、３３及びＩＤＴ３３、３４の最も近接する電極指の中心間距離Ｌを大きくすることにより狭帯域化できることが知られている。図１０は、図９の縦結合二重モードＳＡＷフィルタにおいて、ＩＤＴの電極指周期をλとした時、ＩＤＴ間の最隣接電極指中心間距離Ｌを０．８２λ、０．８４λ、０．８６λと変化させたときの通過域特性を示している。同図より、Ｌを大きくすると通過帯域幅が減少し狭帯域になることが分かる。しかし、Ｌを大きくして狭帯域にするほどフィルタのインピーダンスがずれてしまい通過帯域の低域側の肩が垂れて挿入損失が大きく劣化してしまう。 In the above-described longitudinally coupled double mode SAW filter 31, in order to realize a narrow band characteristic, the number of electrode finger pairs of the IDTs 32 to 34 may be increased. However, when the number of electrode finger pairs is increased, there is a problem that the insertion loss is deteriorated although the band is narrowed. As another means, it is known that the band can be narrowed by increasing the distance L between the centers of the electrode fingers closest to the IDTs 32 and 33 and the IDTs 33 and 34. FIG. 10 shows the longitudinally coupled double-mode SAW filter of FIG. 9 where the distance L between adjacent electrode finger centers between IDTs is 0.82λ, 0.84λ, 0.86λ, where λ is the electrode finger period of the IDT. It shows the passband characteristics when changing. From the figure, it can be seen that when L is increased, the passband width is reduced and the band becomes narrower. However, the larger the L is, the narrower the band, the more the impedance of the filter shifts, and the low band side shoulder of the pass band hangs down, greatly degrading the insertion loss.

更に狭帯域化の別の方法として、図１１（ａ）に示すように縦結合二重モードＳＡＷフィルタ３１に１端子対ＳＡＷ共振子４１を直列接続する方法がある。これは、１端子対ＳＡＷ共振子４１を直列接続して該１端子対ＳＡＷ共振子４１の***振点を通過帯域内に位置させることにより減衰極を発生させて狭帯域化にする方法である。図１１（ｂ）は図１１（ａ）に示す縦結合二重モードＳＡＷフィルタ３１に１端子対ＳＡＷ共振子４１を直列接続した場合の通過域特性である。なお、中心周波数をＰＤＣ方式の送信（Ｔｘ１）用ＳＡＷフィルタに適用される８９５．５（ＭＨｚ）、中央ＩＤＴ３３の電極指対数を２７．５対、両側ＩＤＴの電極指対数を２０．５対、交差幅を１５λ、電極膜厚を約４．４％λ、ＩＤＴ間の最隣接電極指中心間距離Ｌを０．８４λとした。同図より、１端子対ＳＡＷ共振子の***振点による減衰極Ｅを通過帯域内に発生させることにより従来より狭帯域化することができる。ところで、ＰＤＣ方式の送信（Ｔｘ１）用ＳＡＷフィルタにおいては、同図に示すように挿入損失３．５（ｄＢ）、通過域低域側の受信側（Ｒｘ１）阻止域減衰量３５（ｄＢ）、通過域高域側減衰量３．５（ｄＢ）の仕様が要求されているが、この要求仕様を満足するには１端子対ＳＡＷ共振子の***振周波数を低く設定して、減衰極Ｅを減衰極Ｅ'に大きくシフトさせる必要がある。しかしながら、要求仕様に応じるべく減衰極Ｅを減衰極Ｅ'にシフトさせると通過帯域の高域側の肩が大きく垂れてしまい挿入損失が劣化してしまう問題があった。
特許第３３２４４２４号公報特開平９−３３１２２６号公報 Furthermore, as another method for narrowing the band, there is a method in which a one-terminal pair SAW resonator 41 is connected in series to a longitudinally coupled double mode SAW filter 31 as shown in FIG. This is a method of narrowing the band by generating an attenuation pole by connecting the one-terminal-pair SAW resonator 41 in series and positioning the anti-resonance point of the one-terminal-pair SAW resonator 41 in the passband. . FIG. 11B shows passband characteristics when the one-terminal pair SAW resonator 41 is connected in series to the longitudinally coupled double mode SAW filter 31 shown in FIG. The center frequency is 895.5 (MHz) applied to a PDC transmission (Tx1) SAW filter, the number of electrode finger pairs of the central IDT 33 is 27.5 pairs, the number of electrode finger pairs of both-side IDTs is 20.5 pairs, The crossing width was 15λ, the electrode film thickness was about 4.4% λ, and the distance L between adjacent electrode finger centers between IDTs was 0.84λ. From the figure, it is possible to make the band narrower than before by generating the attenuation pole E in the pass band due to the antiresonance point of the one-terminal pair SAW resonator. By the way, in the SAW filter for transmission (Tx1) of the PDC system, as shown in the figure, an insertion loss of 3.5 (dB), a reception side (Rx1) stopband attenuation 35 (dB) on the low pass band side, The specification of a passband high band side attenuation of 3.5 (dB) is required. To satisfy this required specification, the antiresonance frequency of the one-terminal-pair SAW resonator is set low, and the attenuation pole E is set to be low. It is necessary to shift greatly to the attenuation pole E ′. However, if the attenuation pole E is shifted to the attenuation pole E ′ in accordance with the required specifications, there is a problem that the shoulder on the high frequency side of the pass band droops and the insertion loss is deteriorated.
Japanese Patent No. 3324424 JP-A-9-33226

本発明が解決しようとする課題は、縦結合二重モードＳＡＷフィルタにおいて、ＰＤＣ方式のＲＦフィルタに要求される狭帯域な仕様を満足しようとすると通過帯域が大きく劣化してしまう点である。 The problem to be solved by the present invention is that, in a longitudinally coupled double mode SAW filter, the passband is greatly deteriorated if an attempt is made to satisfy a narrowband specification required for a PDC RF filter.

上記課題を解決するために本発明に係る縦結合二重モードＳＡＷフィルタの請求項１に記載の発明は、圧電基板上の弾性表面波の伝搬方向に沿って第１、第２、第３のＩＤＴを順次配置し、前記第１〜第３のＩＤＴの両側に第１及び第２の反射器を備えた縦結合二重モード弾性表面波フィルタにおいて、前記第１のＩＤＴと前記第２のＩＤＴの最も隣接する電極指の中心間距離Ｌ１と、前記第２のＩＤＴと前記第３のＩＤＴの最も隣接する電極指の中心間距離Ｌ２とを異ならせ、且つ、前記第１及び第３のＩＤＴの電極指周期と前記第２のＩＤＴの電極指周期とを異ならせたことを特徴とする縦結合二重モード弾性表面波フィルタである。 In order to solve the above-mentioned problems, the invention according to claim 1 of the longitudinally coupled double mode SAW filter according to the present invention is characterized in that the first, second and third are along the propagation direction of the surface acoustic wave on the piezoelectric substrate. In a longitudinally coupled double mode surface acoustic wave filter having IDTs sequentially arranged and having first and second reflectors on both sides of the first to third IDTs, the first IDT and the second IDT The distance L1 between the centers of the electrode fingers closest to each other and the distance L2 between the centers of the electrode fingers closest to each other of the second IDT and the third IDT, and the first and third IDTs This is a longitudinally coupled double mode surface acoustic wave filter characterized in that the electrode finger period of the second IDT and the electrode finger period of the second IDT are different.

請求項２に記載の発明は、前記縦結合二重モード弾性表面波フィルタにおいて、前記第１のＩＤＴと前記第２のＩＤＴの最も隣接する電極指の中心間距離Ｌ１と、前記第２のＩＤＴと前記第３のＩＤＴの最も隣接する電極指の中心間距離Ｌ２との差を｜Ｌ１−Ｌ２｜、ＩＤＴの電極指周期をλとした時、０．０５λ≦｜Ｌ１−Ｌ２｜≦０．１０λとしたことを特徴とする請求項１に記載の縦結合二重モード弾性表面波フィルタである。 According to a second aspect of the present invention, in the longitudinally coupled double-mode surface acoustic wave filter, the distance L1 between the electrode fingers closest to each other between the first IDT and the second IDT, and the second IDT When the difference between the center distance L2 of the electrode fingers closest to the third IDT is | L1-L2 | and the electrode finger period of the IDT is λ, 0.05λ ≦ | L1-L2 | ≦ 0. The longitudinally coupled double mode surface acoustic wave filter according to claim 1, wherein the length is 10λ.

請求項３に記載の発明は、前記縦結合二重モード弾性表面波フィルタにおいて、前記第１及び第３のＩＤＴの電極指周期をλ、前記第２のＩＤＴの電極指周期をλ'とした時、１．００４λ≦λ'≦１．０１１λ、あるいは、１．００４λ'≦λ≦１．０１１λ'としたことを特徴とする請求項１及び２に記載の縦結合二重モード弾性表面波フィルタである。 According to a third aspect of the present invention, in the longitudinally coupled double mode surface acoustic wave filter, the electrode finger period of the first and third IDTs is λ, and the electrode finger period of the second IDT is λ ′. 3. The longitudinally coupled double-mode surface acoustic wave filter according to claim 1, wherein 1.004λ ≦ λ ′ ≦ 1.011λ or 1.004λ ′ ≦ λ ≦ 1.011λ ′. It is.

請求項４に記載の発明は、前記縦結合二重モード弾性表面波フィルタにおいて、前記第１の反射器と前記第１のＩＤＴの最も隣接する電極指の中心間距離Ｌ３と、前記第２の反射器と前記第３のＩＤＴの最も隣接する電極指の中心間距離Ｌ４とを異ならせたことを特徴とする請求項１乃至３のいずれかに記載の縦結合二重モード弾性表面波フィルタである。 According to a fourth aspect of the present invention, in the longitudinally coupled double-mode surface acoustic wave filter, a distance L3 between centers of the electrode fingers closest to the first reflector and the first IDT, and the second The longitudinally coupled double mode surface acoustic wave filter according to any one of claims 1 to 3, wherein a distance L4 between centers of the electrode fingers closest to the reflector and the third IDT is made different. is there.

請求項５に記載の発明は、前記縦結合二重モード弾性表面波フィルタにおいて、Ｌ１＞Ｌ２の関係にあるときはＬ３＜Ｌ４となるように設定し、また、Ｌ１＜Ｌ２の関係にあるときはＬ３＞Ｌ４となるように設定したことを特徴とする請求項１乃至４のいずれかに記載の縦結合二重モード弾性表面波フィルタである。 According to a fifth aspect of the present invention, in the longitudinally coupled double-mode surface acoustic wave filter, when L1> L2 is set, L3 <L4 is set, and when L1 <L2 is satisfied. 5. The longitudinally coupled double mode surface acoustic wave filter according to claim 1, wherein L3> L4 is set.

請求項６に記載の発明は、前記縦結合二重モード弾性表面波フィルタに１端子対弾性表面波共振子を直列接続したこと特徴とする請求項１乃至５のいずれかに記載の縦結合二重モード弾性表面波フィルタである。 The invention according to claim 6 is characterized in that a longitudinally coupled two-mode surface acoustic wave filter has a one-terminal-pair surface acoustic wave resonator connected in series to the longitudinally coupled double mode surface acoustic wave filter. It is a heavy mode surface acoustic wave filter.

本発明の縦結合二重モードＳＡＷフィルタは、通過帯域を劣化することなく狭帯域で減衰特性が急峻な特性を実現したので、ＰＤＣ方式のＲＦ用フィルタとして優れた効果を奏するものである。 The longitudinally coupled double mode SAW filter according to the present invention realizes a narrow band and a steep attenuation characteristic without degrading the pass band, and therefore has an excellent effect as a PDC RF filter.

以下、発明を図面に図示した実施の形態例に基づいて詳細に説明する。図１（ａ）は、本発明に係る縦結合二重モードＳＡＷフィルタ１の平面図を示している。圧電基板上にＳＡＷの伝搬方向に沿ってＩＤＴ２、３、４を近接配置すると共に、これらのＩＤＴの両側に反射器５ａ、５ｂを配設する。前記ＩＤＴ２、３、４は互いに間挿し合う複数の電極指を有する一対のくし形電極から形成され、中央ＩＤＴ３の一方のくし形電極は入力端子ＩＮに接続し、他方のくし形電極は接地する。さらに、両側ＩＤＴ２、４のそれぞれ一方のくし形電極を接続して出力端子ＯＵＴに接続すると共に、他方のくし形電極はそれぞれ接地して縦結合二重モードＳＡＷフィルタ１を構成する。また、図１（ｂ）は、図１（ａ）の両側ＩＤＴ２、４のそれぞれ最内側の電極指をバスバーから切り離し、ＩＤＴ３のバスバーに接続した上で該電極指と元のＩＤＴ３の最外側の電極指とのスペースを電極にて埋めて設けて一体（幅広の電極指）とした縦結合二重モードＳＡＷフィルタ１１である。本発明の特徴の一つとして、ＩＤＴ２とＩＤＴ３（ＩＤＴ１２とＩＤＴ１３）の最も隣接する電極指の中心間距離Ｌ１とＩＤＴ３とＩＤＴ４（ＩＤＴ１３とＩＤＴ１４）の最も隣接する電極指の中心間距離Ｌ２とを異ならせたことである。 Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings. FIG. 1A shows a plan view of a longitudinally coupled double mode SAW filter 1 according to the present invention. IDTs 2, 3, and 4 are arranged close to each other along the SAW propagation direction on the piezoelectric substrate, and reflectors 5a and 5b are arranged on both sides of these IDTs. The IDTs 2, 3, and 4 are formed of a pair of comb electrodes having a plurality of electrode fingers that are interleaved with each other. One comb electrode of the central IDT 3 is connected to the input terminal IN, and the other comb electrode is grounded. . Further, one comb electrode of each of the IDTs 2 and 4 on both sides is connected to the output terminal OUT, and the other comb electrode is grounded to constitute the longitudinally coupled double mode SAW filter 1. FIG. 1 (b) shows that the innermost electrode fingers of both sides IDT2 and 4 of FIG. 1 (a) are separated from the bus bar and connected to the bus bar of IDT3, and then the electrode fingers and the outermost electrode of the original IDT3 are connected. This is a longitudinally coupled double mode SAW filter 11 that is integrated (wide electrode fingers) by filling a space with electrode fingers with electrodes. As one of the features of the present invention, the distance L1 between the centers of the electrode fingers closest to each other in IDT2 and IDT3 (IDT12 and IDT13) and the distance L2 between the centers of the electrode fingers closest to each other in IDT3 and IDT4 (IDT13 and IDT14). It is different.

ここで、図１（ａ）の縦結合二重モードＳＡＷフィルタ１のＬ１とＬ２とを異ならせた時の通過域特性を図２に示す。なお、縦結合二重モードＳＡＷフィルタ１の設計条件は、中心周波数ｆｏを８９５．５ＭＨｚ、ＩＤＴ３の電極対数を２７．５対、ＩＤＴ２及びＩＤＴ４の電極対数を２０．５対とした。また、ＩＤＴの電極指周期をλとした時、電極指交差幅を１５λ、電極膜厚を約４．４％λとし、Ｌ１とＬ２を０．８４λから異ならせた。同図より、Ｌ１＝０．８４λ、Ｌ２＝０．７９λとすることにより通過帯域中にディップＡが発生して、該ディップＡにより通過帯域が狭帯域となることが分かる。また、Ｌ１＝０．８９λ、Ｌ２＝０．７９λのグラフに示すように、Ｌ１とＬ２の差を大きくすると、ディップＡは低周波数側にシフトすると共に高減衰になるので更に狭帯域化することできる。 Here, FIG. 2 shows passband characteristics when L1 and L2 of the longitudinally coupled double mode SAW filter 1 of FIG. The design conditions for the longitudinally coupled double mode SAW filter 1 were a center frequency fo of 895.5 MHz, an IDT3 electrode pair number of 27.5 pairs, and an IDT2 and IDT4 electrode pair number of 20.5 pairs. When the electrode finger period of the IDT is λ, the electrode finger crossing width is 15λ, the electrode film thickness is about 4.4% λ, and L1 and L2 are different from 0.84λ. From the figure, it can be seen that by setting L1 = 0.84λ and L2 = 0.79λ, a dip A is generated in the passband, and the dipA narrows the passband. Further, as shown in the graphs of L1 = 0.89λ and L2 = 0.79λ, when the difference between L1 and L2 is increased, dip A shifts to a lower frequency side and becomes higher attenuation, so that the band is further narrowed. it can.

しかしながら、Ｌ１とＬ２とを大きく異ならすと図２のＢ部に示すように通過帯域の低域側において不要なリップルが発生してしまい特性が大きく劣化してしまう問題があった。この劣化の原因としては、Ｌ１とＬ２とを大きく異ならせるとＩＤＴ２〜４上をＳＡＷが励振する際にＬ１部及びＬ２部において不連続点が発生し、この不連続点でＳＡＷがバルク波へ変換されてしまうので特性が劣化してしまうものと推測した。 However, if L1 and L2 are greatly different from each other, there is a problem that unnecessary ripples are generated on the low band side of the pass band and the characteristics are greatly deteriorated as shown in part B of FIG. As a cause of this deterioration, if L1 and L2 are greatly different, when the SAW excites on the IDTs 2 to 4, discontinuous points are generated in the L1 part and the L2 part, and the SAW becomes a bulk wave at the discontinuous point. It was estimated that the characteristics would deteriorate because of the conversion.

そこで、中央ＩＤＴ３の電極指周期と両側ＩＤＴ２及び４の電極指周期とを双方異ならせることによって、Ｌ１部及びＬ２部におけるＳＡＷの励振の不連続部を緩和できるのではないかと考えた。図３は、両側ＩＤＴ２及び４の電極指周期λに対し、中央ＩＤＴ３の電極指周期λ'を１．００４λ、１．００８λ、１．０１１λと大きくしたときの通過域特性を示している。なお、このときのＬ１を０．８９λ、Ｌ２を０．７９λとしている。同図に示すように、ＩＤＴ３の電極指周期をＩＤＴ２及びＩＤＴ４の電極指周期より大きくすると、通過帯域低域側のリップルが除去され滑らかになっていることが分かる。これは、中央ＩＤＴの電極指周期を両側ＩＤＴの電極指周期と異ならせたことによりＳＡＷの励振の連続性が保たれた為と考えられる。しかしながら、同図に示すように中央ＩＤＴの電極指周期をあまり大きくしすぎると通過帯域の両肩が垂れてしまうので適切な値に設定する必要がある。具体的には、１．００４λ≦λ'≦１．０１１λに設定すれば良好な通過域特性が得られる。また、前述では両側ＩＤＴ２及び４の電極指周期を変えずに中央ＩＤＴ３の電極指周期を変化させたが、逆に中央ＩＤＴ３の電極指周期を変えずに両側ＩＤＴ２及び４の電極指周期を変化させた場合も同様な効果が得られるのは言うまでもない。 Therefore, it was considered that the discontinuous part of the SAW excitation in the L1 part and the L2 part can be alleviated by making the electrode finger period of the central IDT 3 different from the electrode finger periods of the both-side IDTs 2 and 4. FIG. 3 shows passband characteristics when the electrode finger period λ ′ of the central IDT 3 is increased to 1.004λ, 1.008λ, and 1.011λ with respect to the electrode finger period λ of the IDTs 2 and 4 on both sides. At this time, L1 is 0.89λ, and L2 is 0.79λ. As shown in the figure, when the electrode finger period of IDT3 is made larger than the electrode finger period of IDT2 and IDT4, it can be seen that the ripple on the low pass band side is removed and smoothed. This is presumably because the continuity of SAW excitation was maintained by making the electrode finger period of the central IDT different from the electrode finger period of the double-sided IDT. However, as shown in the figure, if the electrode finger period of the central IDT is made too large, both shoulders of the pass band hang down, so it is necessary to set to an appropriate value. Specifically, good passband characteristics can be obtained by setting 1.004λ ≦ λ ′ ≦ 1.011λ. In the above description, the electrode finger cycle of the central IDT 3 is changed without changing the electrode finger cycle of the both-side IDTs 2 and 4, but conversely, the electrode finger cycle of the both-side IDTs 2 and 4 is changed without changing the electrode finger cycle of the central IDT 3. It goes without saying that the same effect can be obtained even if it is used.

更に、各ＩＤＴ間の電極指周期を調整するだけでなく、反射器とＩＤＴの最も隣接する電極指中心間距離Ｌ３及びＬ４を調整することにより特性改善ができないか検討した。図４は、図１（ａ）における反射器とＩＤＴの最隣接電極指中心間距離Ｌ３及びＬ４を従来の０．５０λから変化させた時のフィルタ特性を示している。なお、Ｌ１＝０．８９λ、Ｌ２＝０．７９λ、中央ＩＤＴ３の電極指周期をλ'＝１．００８λとしている。同図に示すように、Ｌ３＝０．４８λ、Ｌ４＝０．４６λの条件においては通過帯域低域側にリップルが発生して特性が劣化してしまうが、Ｌ３＝０．４６λ、Ｌ４＝０．４８λとした条件においては通過帯域低域側の減衰傾度が急峻になる。これは、ＩＤＴ２とＩＤＴ３の最隣接電極指中心間距離Ｌ１を広くした時は反射器５ａとＩＤＴ２の最隣接電極指中心間距離Ｌ３を狭くし、また、ＩＤＴ３とＩＤＴ４の最隣接電極指中心間距離Ｌ２をＬ１より小さくした時は反射器５ｂとＩＤＴ４の最隣接電極指中心間距離Ｌ４を大きくすることによりＳＡＷの励振の連続性がより円滑になり、通過帯域の減衰傾度が急峻になったと考えられる。以上から、Ｌ１＞Ｌ２の関係にあるときはＬ３＜Ｌ４となるように設定し、また、Ｌ１＜Ｌ２の関係にあるときはＬ３＞Ｌ４となるように設定すればＳＡＷの励振の連続性がより円滑となり、通過帯域の減衰傾度が急峻になることが判明した。 Furthermore, not only the electrode finger period between the IDTs but also the distance between the electrode finger centers L3 and L4 that are closest to each other between the reflector and the IDT were adjusted to investigate whether the characteristics could be improved. FIG. 4 shows the filter characteristics when the distances L3 and L4 between the reflector and IDT closest electrode finger centers in FIG. 1A are changed from the conventional 0.50λ. Note that L1 = 0.89λ, L2 = 0.79λ, and the electrode finger period of the center IDT3 is λ ′ = 1.008λ. As shown in the figure, under the conditions of L3 = 0.48λ and L4 = 0.46λ, ripples are generated on the low pass band side and the characteristics deteriorate, but L3 = 0.46λ and L4 = 0. Under the condition of .48λ, the attenuation gradient on the low passband side becomes steep. This is because when the distance L1 between the nearest electrode finger centers of IDT2 and IDT3 is increased, the distance L3 between the nearest electrode finger centers of the reflector 5a and IDT2 is reduced, and between the nearest electrode finger centers of IDT3 and IDT4. When the distance L2 is made smaller than L1, the continuity of SAW excitation becomes smoother by increasing the distance L4 between the adjacent electrode finger centers of the reflector 5b and IDT4, and the attenuation gradient of the passband becomes steep. Conceivable. From the above, when the relationship of L1> L2 is established, L3 <L4 is set, and when the relationship of L1 <L2 is established, L3> L4 is set so that the continuity of SAW excitation is increased. It has been found that the smoothness becomes smoother and the attenuation gradient of the passband becomes steep.

以上説明したように、本発明に係る縦結合二重モードＳＡＷフィルタは、中央ＩＤＴと両側ＩＤＴの最も隣接する電極指の中心間距離を異ならせ、且つ、中央ＩＤＴと両側ＩＤＴの電極指周期を異ならせ、更に、両側ＩＤＴと反射器の最も隣接する電極指の中心間距離を調節することにより狭帯域で通過帯域の減衰傾度を急峻にした縦結合二重モードＳＡＷフィルタを実現した。図５は、本発明に係る縦結合二重モードＳＡＷフィルタをＰＤＣ方式のＲＦフィルタに適用した時の通過域特性であり、中心周波数を８９５．５ＭＨｚ、中央ＩＤＴの電極指対数を２７．５対、両側ＩＤＴの電極指対数を２０．５対、交差幅を１５λ、電極膜厚を約４．４％λ、Ｌ１＝０．８９λ、Ｌ２＝０．７９λ、Ｌ３＝０．４６λ、Ｌ４＝０．４８λとし、中央ＩＤＴの電極指周期を１．００８λに設定している。同図に示すように、ＰＤＣ方式のＲＦ用フィルタに要求される仕様を満足するには更なる狭帯域化と通過帯域の高域側に位置するレスポンスＣの高減衰化が必要である。 As described above, the longitudinally coupled double mode SAW filter according to the present invention varies the distance between the centers of the electrode fingers closest to the center IDT and the both-side IDTs, and the electrode finger period between the center IDT and the both-side IDTs. Further, a longitudinally coupled double mode SAW filter having a narrow band and a steep attenuation in the passband was realized by adjusting the distance between the centers of the electrode fingers closest to the IDT on both sides and the reflector. FIG. 5 shows passband characteristics when the longitudinally coupled dual mode SAW filter according to the present invention is applied to a PDC RF filter. The center frequency is 895.5 MHz, and the number of electrode finger pairs of the center IDT is 27.5 pairs. The number of electrode fingers on both sides IDT is 20.5, the crossing width is 15λ, the electrode film thickness is about 4.4% λ, L1 = 0.89λ, L2 = 0.79λ, L3 = 0.46λ, L4 = 0 .48λ, and the electrode finger period of the central IDT is set to 1.008λ. As shown in the figure, in order to satisfy the specifications required for the PDC-type RF filter, it is necessary to further narrow the band and to increase the attenuation of the response C located on the high band side of the pass band.

ここで、図６（ａ）に示すように前記縦結合二重モードＳＡＷフィルタ１に一端子対ＳＡＷ共振子２２を直列接続することにより、狭帯域化する共に通過帯域高域側のレスポンスの高減衰化を図った。図６（ｂ）に示すように、一端子対ＳＡＷ共振子の***振点Ｄをレスポンスに重ね、更に共振点を通過帯域内に位置させるように該一端子対ＳＡＷ共振子の周波数を設定することにより、通過帯域を狭帯域にすると共にレスポンスを高減衰としたので、ＰＤＣ方式のＲＦフィルタに要求される仕様を全て満足することができた。図７は、前記縦結合二重モードＳＡＷフィルタを試作した時の実測の通過域特性である。実測においても通過帯域が狭帯域な特性が得られ、要求仕様を全て満足できることが判明した。 Here, as shown in FIG. 6A, a one-terminal pair SAW resonator 22 is connected in series to the longitudinally coupled double mode SAW filter 1, thereby narrowing the band and increasing the response on the high passband side. Attenuation was attempted. As shown in FIG. 6B, the anti-resonance point D of the one-terminal pair SAW resonator is superimposed on the response, and the frequency of the one-terminal pair SAW resonator is set so that the resonance point is positioned in the pass band. As a result, the pass band was narrowed and the response was highly attenuated, so that all the specifications required for the PDC RF filter could be satisfied. FIG. 7 shows measured passband characteristics when the above-described longitudinally coupled double mode SAW filter is prototyped. In the actual measurement, it was found that the characteristics with a narrow pass band were obtained, and all the required specifications could be satisfied.

（ａ）に本発明に係る縦結合二重モードＳＡＷフィルタの平面図を示し、（ｂ）に本発明の第二の実施例に係る縦結合二重モードＳＡＷフィルタを示す。(A) shows a plan view of a longitudinally coupled double mode SAW filter according to the present invention, and (b) shows a longitudinally coupled double mode SAW filter according to a second embodiment of the present invention. 本発明に係る縦結合二重モードＳＡＷフィルタのＩＤＴ間の最隣接電極指中心間距離Ｌ１及びＬ２を変化させた時の通過域特性を示す。The passband characteristic when changing distance L1 and L2 between the nearest electrode finger centers between IDTs of the longitudinally coupled double mode SAW filter according to the present invention is shown. 本発明に係る縦結合二重モードＳＡＷフィルタの中央ＩＤＴの電極指周期を変化させた時の通過域特性を示す。The passband characteristic when the electrode finger period of the center IDT of the longitudinally coupled double mode SAW filter according to the present invention is changed is shown. 本発明に係る縦結合二重モードＳＡＷフィルタのＩＤＴ−反射器間の最隣接電極指中心間距離Ｌ３及びＬ４を変化させた時の通過域特性を示す。6 shows passband characteristics when the distance L3 and L4 between the adjacent electrode finger centers between the IDT and the reflector of the longitudinally coupled double mode SAW filter according to the present invention is changed. 本発明に係る縦結合二重モードＳＡＷフィルタをＰＤＣ方式のＲＦフィルタに適用した時の通過域特性を示す。The passband characteristic when the longitudinally coupled double mode SAW filter according to the present invention is applied to a PDC RF filter is shown. 本発明に係る縦結合二重モードＳＡＷフィルタに一端子対ＳＡＷ共振子を直列接続した時の平面図を（ａ）に、通過域特性を（ｂ）に示す。A plan view when a one-terminal pair SAW resonator is connected in series to the longitudinally coupled double mode SAW filter according to the present invention is shown in FIG. 本発明に係る縦結合二重モードＳＡＷフィルタの実測の通過域特性を示す。The measured pass band characteristic of the longitudinally coupled double mode SAW filter according to the present invention is shown. ＰＤＣ方式に用いられる搬送周波数帯の割付を示す。The allocation of the carrier frequency band used for the PDC system is shown. 従来の縦結合二重モードＳＡＷフィルタの平面図を示す。The top view of the conventional longitudinal coupling double mode SAW filter is shown. 従来の縦結合二重モードＳＡＷフィルタのＩＤＴ間の最隣接電極指中心間距離を変化させた時の通過域特性を示す。The passband characteristics when the distance between the nearest electrode finger centers between IDTs of a conventional longitudinally coupled double mode SAW filter is changed are shown. 従来の縦結合二重モードＳＡＷフィルタに一端子対ＳＡＷ共振子を直列接続した時の平面図を（ａ）に、通過域特性を（ｂ）に示す。A plan view when a one-terminal-pair SAW resonator is connected in series to a conventional longitudinally coupled double mode SAW filter is shown in (a), and a passband characteristic is shown in (b).

符号の説明Explanation of symbols

１、１１：縦結合二重モードＳＡＷフィルタ
２、３、４、１２、１３、１４：ＩＤＴ電極
５ａ、５ｂ、１５ａ、１５ｂ：反射器
２２：１端子対ＳＡＷ共振子
DESCRIPTION OF SYMBOLS 1, 11: Longitudinal coupling double mode SAW filter 2, 3, 4, 12, 13, 14: IDT electrode 5a, 5b, 15a, 15b: Reflector 22: 1 Terminal pair SAW resonator

Claims

圧電基板上の弾性表面波の伝搬方向に沿って第１、第２、第３のＩＤＴを順次配置し、前記第１〜第３のＩＤＴの両側に第１及び第２の反射器を備えた縦結合二重モード弾性表面波フィルタにおいて、
前記第１のＩＤＴと前記第２のＩＤＴの最も隣接する電極指の中心間距離Ｌ１と、前記第２のＩＤＴと前記第３のＩＤＴの最も隣接する電極指の中心間距離Ｌ２とを異ならせ、且つ、前記第１及び第３のＩＤＴの電極指周期と前記第２のＩＤＴの電極指周期とを異ならせたことを特徴とする縦結合二重モード弾性表面波フィルタ。 First, second, and third IDTs are sequentially arranged along the propagation direction of the surface acoustic wave on the piezoelectric substrate, and first and second reflectors are provided on both sides of the first to third IDTs. In the longitudinally coupled double mode surface acoustic wave filter,
The distance L1 between the centers of the electrode fingers closest to the first IDT and the second IDT is made different from the distance L2 between the centers of the electrode fingers closest to the second IDT and the third IDT. A longitudinally coupled double mode surface acoustic wave filter characterized in that the electrode finger period of the first and third IDTs is different from the electrode finger period of the second IDT.

前記縦結合二重モード弾性表面波フィルタにおいて、前記第１のＩＤＴと前記第２のＩＤＴの最も隣接する電極指の中心間距離Ｌ１と、前記第２のＩＤＴと前記第３のＩＤＴの最も隣接する電極指の中心間距離Ｌ２との差を｜Ｌ１−Ｌ２｜、ＩＤＴの電極周期をλとした時、０．０５λ≦｜Ｌ１−Ｌ２｜≦０．１０λとしたことを特徴とする請求項１に記載の縦結合二重モード弾性表面波フィルタ。 In the longitudinally coupled double-mode surface acoustic wave filter, the distance L1 between the centers of the electrode fingers closest to each other in the first IDT and the second IDT, and the most adjacent to the second IDT and the third IDT. When the difference between the distance L2 between the center of the electrode fingers to be applied is | L1−L2 | and the electrode period of the IDT is λ, 0.05λ ≦ | L1−L2 | ≦ 0.10λ is satisfied. 2. The longitudinally coupled double-mode surface acoustic wave filter according to 1.

前記縦結合二重モード弾性表面波フィルタにおいて、前記第１及び第３のＩＤＴの電極指周期をλ、前記第２のＩＤＴの電極指周期をλ'とした時、１．００４λ≦λ'≦１．０１１λ、あるいは、１．００４λ'≦λ≦１．０１１λ'としたことを特徴とする請求項１及び２に記載の縦結合二重モード弾性表面波フィルタ。 In the longitudinally coupled double-mode surface acoustic wave filter, when the electrode finger period of the first and third IDTs is λ and the electrode finger period of the second IDT is λ ′, 1.004λ ≦ λ ′ ≦ 3. The longitudinally coupled double mode surface acoustic wave filter according to claim 1, wherein 1.011λ or 1.004λ ′ ≦ λ ≦ 1.011λ ′.

前記縦結合二重モード弾性表面波フィルタにおいて、前記第１の反射器と前記第１のＩＤＴの最も隣接する電極指の中心間距離Ｌ３と、前記第２の反射器と前記第３のＩＤＴの最も隣接する電極指の中心間距離Ｌ４とを異ならせたことを特徴とする請求項１乃至３のいずれかに記載の縦結合二重モード弾性表面波フィルタ。 In the longitudinally coupled double-mode surface acoustic wave filter, the distance L3 between the electrode fingers closest to the first reflector and the first IDT, the second reflector, and the third IDT 4. The longitudinally coupled double mode surface acoustic wave filter according to claim 1, wherein the distance L4 between the centers of the adjacent electrode fingers is made different.

前記縦結合二重モード弾性表面波フィルタにおいて、Ｌ１＞Ｌ２の関係にあるときはＬ３＜Ｌ４となるように設定し、また、Ｌ１＜Ｌ２の関係にあるときはＬ３＞Ｌ４となるように設定したことを特徴とする請求項１乃至４のいずれかに記載の縦結合二重モード弾性表面波フィルタ。 In the longitudinally coupled double-mode surface acoustic wave filter, L3 <L4 is set when L1> L2, and L3> L4 is set when L1 <L2. The longitudinally coupled double mode surface acoustic wave filter according to claim 1, wherein the longitudinally coupled double mode surface acoustic wave filter is provided.

前記縦結合二重モード弾性表面波フィルタに１端子対弾性表面波共振子を直列接続したこと特徴とする請求項１乃至５のいずれかに記載の縦結合二重モード弾性表面波フィルタ。 6. The longitudinally coupled double-mode surface acoustic wave filter according to claim 1, wherein a one-terminal-pair surface acoustic wave resonator is connected in series to the longitudinally coupled double-mode surface acoustic wave filter.