JP2009212572A - Elastic wave resonator, and ladder filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an elastic wave resonator not generating spurious waves which apply excessive stress to an IDT electrode in a using band. <P>SOLUTION: The elastic wave resonator 1 is provided with the IDT electrode 2 on a piezoelectric substrate and grating reflectors 3a and 3b arranged on both sides of the propagation direction of elastic waves, and is used within the range of the stop band of the respective grating reflectors 3a and 3b, and the upper end frequency f<SB>H</SB>of the band wherein the elastic wave resonator 1 is used satisfies the condition of formula 1. The average speed of the elastic waves propagating through the piezoelectric substrate is denoted by v, the cycle length of the electrode fingers of the IDT electrode is denoted by λ<SB>0</SB>, the number of the electrode fingers of the IDT electrode is denoted by N, the distances between the IDT electrode and the respective reflectors are denoted by L<SB>1</SB>and L<SB>2</SB>, the upper end frequency of the band wherein the elastic wave resonator is used is denoted by f<SB>H</SB>, and a predetermined natural number ≥2 is denoted by n. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an acoustic wave resonator and a ladder filter including the acoustic wave resonator, and more particularly to a technique for extending the service life of the acoustic wave resonator.

  An elastic wave filter that is mounted on a mobile terminal such as a cellular phone and discriminates a high-frequency signal has a configuration in which a plurality of resonators are connected in a constant K shape as a series arm 101 and a parallel arm 102 as shown in FIG. Some of them employ a ladder type filter 100. The serial arm 101 and the parallel arm 102 of the ladder type filter 100 can be easily downsized and elastic waves such as SAW (Surface Acoustic Wave) resonators that can simultaneously form a plurality of resonators by photolithography. A resonator is often used.

For example, as shown in FIG. 12, the SAW resonator 10 is provided so as to be opposed to each other, and is an IDT electrode in which a large number of electrode fingers 231 and 232 are connected in a cross finger shape to bus bars 21 and 22 for inputting and outputting frequency signals. 2 and grating reflectors (hereinafter referred to as reflectors) 3a and 3b provided on both sides of the IDT electrode 2 in the SAW propagation direction. The IDT electrode 2 and the reflectors 3a and 3b are formed by patterning a metal film such as aluminum on a piezoelectric substrate (not shown) such as LiTaO ₃ , LiNbO ₃ or quartz. In FIG. 11, for example, the serial arm 101 and the parallel arm 102 including the SAW resonator 10 illustrated in FIG. 12 are simplified.

  In this SAW resonator 10, for example, when a frequency signal is input to the bus bar 21 on one side, this frequency signal is electro-mechanically converted by the electrode finger 231 to become SAW and propagates through the piezoelectric substrate in the left-right direction in FIG. When the electrode finger 232 connected to the opposite bus bar 22 is reached, the SAW is mechanical-electrically converted by the bus bar 22 and extracted as a frequency signal.

As schematically shown in FIG. 13A, the SAW resonator 10 has a resonance frequency “f _r ” at which the admittance is maximized with respect to the frequency signal, and an anti-resonance frequency “f _a ” at which the admittance is minimized. As shown schematically in FIG. 13 (b), the resonance frequency “f _rs ” of the series arm 101 and the anti-resonance frequency “f _ap ” of the parallel arm 102 are substantially matched. A bandpass filter in which a pass band is formed between the attenuation poles corresponding to each of the resonance frequency “f _rp ” of the parallel arm 102 and the anti-resonance frequency “f _as ” of the series arm 101 can be configured. Here, the SAW resonator 10 resonance frequency and anti-resonance frequency are, for example, the interval between the electrode fingers 232 connected to the bus bar 22 (also the interval between the electrode fingers 231) “λ ₀ ” (hereinafter referred to as the period length). Can be adjusted by changing.

However, in the ladder type filter 100, for example, as indicated by “▽ 1˜ ▽ 3” in the insertion loss characteristic diagram of FIG. 14, there is a ripple in the passband (where the attenuation becomes maximum in the passband). May occur and the frequency characteristics of the SAW resonator 10 may be deteriorated. For example, as shown in the admittance characteristic diagram of FIG. 15, in addition to the main resonance having the resonance frequency “f _rs ”, such ripples are generated at innumerable frequencies such as frequencies “f _spr1 , f _spr2 , f _spr3,. This is because spurious (sub-resonance) is excited.

  These spurious not only cause ripples as described above to worsen the insertion loss of the ladder type filter 100 but also reduce the power durability performance of the ladder type filter 100. For example, a ladder-type filter 100 provided as a transmission-side filter of a duplexer that separates a transmission signal and a reception signal at a front end unit of a mobile terminal may receive a signal having a power as high as 1 W, for example. . At this time, when the spurious is excited in the SAW resonator 10, the spurious increases the stress applied to the IDT electrode 2, and in particular causes the destruction of the IDT electrode 2 in the serial arm 101 of the ladder type filter 100.

Therefore, when ripples appear in the frequency characteristics of the ladder type filter 100, for example, avoid the frequency at which large ripples that deviate from the specifications of the insertion loss, and distinguish them from the pass band of the ladder type filter 100 (the above-mentioned “pass band”). Therefore, measures such as setting the upper end frequency “f _H ” of the passband specification value of the ladder filter 100 are expressed below. However, such a countermeasure not only has a problem that a sufficiently wide pass bandwidth cannot be set, but also, for example, when the positions and line widths of the electrode fingers 231 and 232 of the IDT electrode 2 are slightly shifted during photolithography, ripples are generated. May fall within the passband.

  In Patent Document 1, the electrode index of the IDT electrode and the crossing weight of the electrode fingers are adjusted to match the position (frequency) of the attenuation pole in the conductance characteristic of the IDT electrode with the frequency at which the spurious is generated. Techniques for reducing the size are described. However, the purpose of this technique is limited to reducing spurious noise, and since spurious noise still exists in the passband, the problem of reduction in service life cannot be solved.

Japanese Patent Application Laid-Open No. Sho 62-284509: page 60, lower right column, 3rd to 19th rows

  The present invention has been made based on such circumstances, and an object of the present invention is to provide an elastic wave resonator that does not generate spurious that applies excessive stress to the IDT electrode in the use band.

An elastic wave resonator according to the present invention includes an IDT electrode and a grating reflector disposed on both sides of the IDT electrode in the propagation direction of the elastic wave on a piezoelectric substrate.
Being used within the stopband of each grating reflector;
The upper end frequency of the band in which the elastic wave resonator is used satisfies the condition of the following formula 1.

但し、ｖは圧電基板を伝播する弾性波の平均速度、λ_０はＩＤＴ電極の電極指の周期長、ＮはＩＤＴ電極の電極指の本数、Ｌ_１、Ｌ_２はＩＤＴ電極と夫々の反射器との電極間距離、ｆ_Ｈは弾性波共振子が使用される帯域の上端周波数、ｎは予め決めた２以上の自然数であり、例えば当該ｎは３であることが好ましい。

Where v is the average velocity of the elastic wave propagating through the piezoelectric substrate, λ ₀ is the period length of the electrode fingers of the IDT electrode, N is the number of electrode fingers of the IDT electrode, and L ₁ and L ₂ are the IDT electrodes and their respective reflectors The inter-electrode distance, f _H is the upper end frequency of the band in which the acoustic wave resonator is used, and n is a predetermined natural number of 2 or more. For example, n is preferably 3.

At this time, the ladder filter according to the present invention includes an acoustic wave resonator that satisfies the above-described Equation 1 as a series arm,
The passband is within the stopband of each grating reflector;
Wherein the equation 1 f _H is provided with a be the upper end frequency of the pass band.

An elastic wave resonator according to another invention includes an IDT electrode and a grating reflector disposed on both sides of the IDT electrode in the propagation direction of the elastic wave on a piezoelectric substrate. In
Being used outside the stopband of each grating reflector;
The upper end frequency of the band in which the acoustic wave resonator is used satisfies the condition of Equation 2 below.

但し、ｖは圧電基板を伝播する弾性波の平均速度、λ_０はＩＤＴ電極の電極指の周期長、λ_０’はグレーティング反射器の電極指の周期長、ＮはＩＤＴ電極の電極指の本数、Ｍ_１、Ｍ_２は各グレーティング反射器の電極指の本数、Ｌ_１、Ｌ_２はＩＤＴ電極と夫々の反射器との電極間距離、ｆ_Ｈは弾性波共振子が使用される帯域の上端周波数、ｍは２以上の予め決めた自然数であり、例えば当該ｍは３であることが好ましい。

Where v is the average velocity of the elastic wave propagating through the piezoelectric substrate, λ ₀ is the period length of the electrode fingers of the IDT electrode, λ ₀ ′ is the period length of the electrode fingers of the grating reflector, and N is the number of electrode fingers of the IDT electrode , M ₁ and M ₂ are the number of electrode fingers of each grating reflector, L ₁ and L ₂ are distances between the electrodes of the IDT electrode and each reflector, and f _H is the upper end of the band where the acoustic wave resonator is used. The frequency m is a predetermined natural number of 2 or more. For example, the m is preferably 3.

At this time, the ladder filter according to the present invention includes an acoustic wave resonator that satisfies the above-described Expression 2 as a series arm,
The passband is outside the stopband range of each grating reflector;
Wherein the f _H of Equation 2 is provided with, and it is the upper frequency of the pass band.

  According to the present invention, an elastic wave resonator having a configuration satisfying Equations 1 and 2 includes two or more natural numbers “n” determined in advance in the above-described equation in the band in which the elastic wave resonator is used. ”Or“ m ”and there is no spurious less than the order. As a result, since the stress applied to the IDT electrode is reduced compared with the case where these spurious are generated, it is possible to extend the service life of the device including the elastic wave resonator according to the present invention, such as a ladder filter. It becomes.

Before describing the specific configuration of the SAW resonator according to the present embodiment, the principle of preventing a predetermined order of spurious from occurring in the SAW resonator will be described. FIG. 1 shows a result of simulating admittance characteristics of a conventional SAW resonator 10 in which spurious is generated in the vicinity of the main resonance. The horizontal axis represents frequency, and the vertical axis represents admittance in decibels (based on 1 [S]). Value). In the simulation, a model of the IDT electrode 2 having the same configuration as that shown in FIG. 12 was created, and the design conditions of the model SAW resonator 10 were set as follows.
Piezoelectric substrate: LiTaO ₃ (SAW propagation velocity v; 4,120 [m / s])
Period length of IDT electrode 2
: Λ ₀ = 4.63 [μm]
Total number of electrode fingers 231 and 232
: N = 160 [books]
Interelectrode distance between IDT electrode 2 and reflectors 3a and 3b
: L ₁ = L ₂ = L = 336.4 [μm]

  The inventor conducted a detailed study on the admittance characteristics shown in FIG. 1 obtained based on the above-described SAW resonator 10 model. As a result, the admittance characteristics are various contributing components based on the structure of the SAW resonator 10. It was found that the sum can be expressed by the following equation (1).

（１）式において、ＹはＳＡＷ共振子１０のアドミタンスを示し、Ｙ_０はＩＤＴ電極２の強制励振に基づく寄与（以下、強制励振項という）、Ｙ_１は反射器３ａ、３ｂの間に働くＳＡＷの音響的な寄与（以下、定常項という）、Ｙ_２は対向する電極指２３１、２３２間の静電容量に基づく電気的な寄与（以下、静電容量項という）を示している。図１には定常項「Ｙ_１」のシミュレーション結果を破線で示し、強制励振項と静電容量との和「Ｙ_０＋Ｙ_２」のシミュレーション結果を一点鎖線で示してある。

(1) In the formula, Y represents the admittance of the SAW resonator 10, _{Y 0} is the contribution based on the forced excitation of the IDT electrodes 2 (hereinafter, referred to as forced excitation section), _{Y 1} is exerted between the reflector 3a, 3b SAW acoustic contribution (hereinafter referred to as a steady term), Y ₂ represents an electrical contribution (hereinafter referred to as a capacitance term) based on the capacitance between the electrode fingers 231 and 232 facing each other. In FIG. 1, the simulation result of the steady term “Y ₁ ” is indicated by a broken line, and the simulation result of the sum “Y ₀ + Y ₂ ” of the forced excitation term and the capacitance is indicated by a one-dot chain line.

Looking at “Y ₀ + Y ₂ ” shown by the alternate long and short dash line in FIG. 1, it can be seen that these forced excitation terms and capacitance terms are contributing components that excite main resonance and anti-resonance. The local maximum corresponding to is not seen, and is not a contributing component for exciting spurious. On the other hand, in the steady term “Y ₁ ” indicated by the broken line, a local maximum value is observed at a position corresponding to each spurious. Therefore, the acoustic contribution of the SAW acting between the steady terms, that is, the reflectors 3 a and 3 b. It can be seen that is the cause of spurious.

Therefore, FIG. 2 shows an enlarged view of the frequency characteristics of the admittance “Y” and the steady term “Y ₁ ” of the SAW resonator 10 model in the frequency region where spurious is generated, and the relationship between these will be examined in detail. According to FIG. 2, when the frequency is lowered from the position where the main resonance occurs, in the steady term “Y ₁ ”, a maximum value appears at the same position as the main resonance, and then a minimum value appears. And local minimum appear alternately. As described above, since the frequency at which the local maximum value of the steady term “Y ₁ ” is generated coincides with the frequency at which the spurious of the admittance “Y” is generated, a condition in which these local maximum values do not appear can be obtained. If possible, it is considered that the corresponding spurious can be eliminated.

Although the stationary term “Y ₁ ” has already been described as an acoustic contribution component of the SAW between the reflectors 3 a and 3 b, the passband of the ladder filter 100 in which the SAW resonator 10 is incorporated is now a reflector. If the SAW excited by the SAW resonator 10 is considered to be totally reflected at the inner ends of the reflectors 3a and 3b, the following is true: It has been found that a SAW standing wave is formed between the reflectors 3a and 3b under the condition satisfying the expression (2).

但し、ｎは定在波の周期数（自然数）、ｖは圧電基板におけるＳＡＷの平均伝播速度［ｍ／ｓ］、λ_０はＩＤＴ電極２の周期長［×１０^−６ｍ］、Ｎは電極指２３１、２３２の総本数、ＬはＩＤＴ電極２と夫々の反射器３ａ、３ｂとの電極間距離［×１０^−６ｍ］、ｆはＳＡＷ共振子１０に入力される信号の周波数［×１０^６Ｈｚ］である。

Where n is the number of standing wave periods (natural number), v is the average propagation speed of SAW on the piezoelectric substrate [m / s], λ ₀ is the period length of the IDT electrode 2 [× 10 ⁻⁶ m], and N is the electrode The total number of fingers 231 and 232, L is the interelectrode distance between the IDT electrode 2 and each of the reflectors 3a and 3b [× 10 ⁻⁶ m], and f is the frequency of the signal input to the SAW resonator 10 [× 10 ⁶ Hz].

Based on the above equation (2), the frequency at which the number of standing wave periods becomes “n = 1, 2, 3” was calculated, and the result was plotted on the frequency characteristic diagram of FIG. When the plot of the number of periods of the standing wave shown in FIG. 2 (indicated by “■”) is compared with the frequency characteristic of the stationary term “Y ₁ ”, the number of periods n is between the reflectors 3a and 3b. The frequency at which the standing wave is formed substantially coincides with the frequency at which the minimum value appears in the stationary term “Y ₁ ”. Thus, if the frequency at which a standing wave having a period number n is generated between the reflectors 3a and 3b is defined as “f _spn (n is a natural number)”, the frequency at which the nth spurious (hereinafter referred to as the nth spurious) is generated. It can be said that “f _sprn (n is a natural number)”, that is, the frequency at which the n-th local maximum appears in the stationary term “Y ₁ ” exists between “f _spn ” and “f _{spn + 1} ”.

Therefore, if the design condition of the SAW resonator 10 and the natural number “n = 1, 2, 3,...” Are substituted into the following equation (3) obtained by solving equation (2) for f based on the relationship plotted in FIG. Thus, the frequency “f _spn ” at which the minimum value appears in the steady term “Y ₁ ” of the SAW resonator 10 can be known in advance.

また既述のようにスプリアスを発生する周波数「ｆ_ｓｐｒｎ」は周波数「ｆ_ｓｐｎ」と「ｆ_{ｓｐｎ＋１}」との間に存在するのであるから、当該ＳＡＷ共振子１０を組み込んだラダー型フィルタ１００の通過帯域の上端周波数「ｆ_Ｈ」よりも周波数「ｆ_ｓｐｎ」が低い場合に、周波数帯域内にｎ次のスプリアスが発生することもわかる。

Further, as described above, the frequency “f _sprn ” that generates the spurious signal exists between the frequencies “f _spn ” and “f _{spn + 1} ”, and _therefore , passes through the ladder filter 100 incorporating the SAW resonator 10. It can also be seen that when the frequency “f _spn ” is lower than the upper end frequency “f _H ” of the band, n-th order spurious is generated in the frequency band.

In this way, in the frequency characteristics of the SAW resonator 10, (a) the n-th order spurious occurs between the n-th minimum value and the (n + 1) -th minimum value of the steady term “Y ₁ ”. Using the characteristic that the frequency “f _spn ” at which the minimum value is generated can be predicted by the expression (3), the above-described period length λ ₀ , the number N of electrode fingers 231 and 232, the IDT electrode 2 and the reflectors 3 a and 3 b If the following equation (4) can be satisfied with the inter-electrode distance L as a design variable, a SAW resonator having no (n-1) th order spurious in the passband of the ladder filter 100 should be designed. Can do.

即ち（４）式の左辺は、定常項「Ｙ_１」のｎ番目の極小値が現れる周波数「ｆ_ｓｐｎ」に等しく、当該周波数が通過帯域の上端周波数「ｆ_Ｈ」よりも大きいということは、周波数「ｆ_ｓｐｎ」よりも高周波数側に存在する（ｎ−１）次のスプリアスもラダー型フィルタ１００の通過帯域内には存在しないことになる。本発明はこのような考えに基づいてなされている。以下、本実施の形態に係わるＳＡＷ共振子１の具体的な構成について説明する。

That is, the left side of the equation (4) is equal to the frequency “f _spn ” where the n-th local minimum value of the steady term “Y ₁ ” appears, and the frequency is higher than the upper end frequency “f _H ” of the passband. The (n−1) th order spurious that exists on the higher frequency side than the frequency “f _spn ” does not exist in the passband of the ladder type filter 100. The present invention has been made based on this idea. Hereinafter, a specific configuration of the SAW resonator 1 according to the present embodiment will be described.

  FIG. 3A is a plan view showing the configuration of the SAW resonator 1 according to the present embodiment. The same configuration as that of the conventional SAW resonator 10 shown in FIG. 12 is used in FIG. The same reference numerals are given. The SAW resonator 1 is provided with an IDT electrode 2 on a piezoelectric substrate (not shown) in the same manner as the SAW resonator 10 described above, and reflectors 3a and 3b are provided on both sides of the SAW propagation direction with respect to the IDT electrode 2. The arrangement is arranged.

IDT electrode 2 includes an electrode finger 231 and 232 of the total N present, periodic length of the electrode fingers 231 and 232 is lambda _0. Each of the reflectors 3a and 3b includes M electrode fingers 301, and these electrode fingers 301 are arranged at a pitch of “½λ ₀ ′”. Here, “λ ₀ ′” is a frequency characteristic related to the reflection coefficient of the reflectors 3 a and 3 b, and a center frequency “f ₀ ” in a frequency region (referred to as a stop band) in which a reflection coefficient of almost 100% is obtained by the M electrode fingers 301. ”And is defined by the following equation (5).

The frequency “f _s ” in the stop band can be expressed by the equation (6) in relation to the center frequency “f ₀ ”. As discussed in FIGS. 1 and 2, when the reflectors 3a and 3b are designed so that the pass band of the ladder filter 100 in which the SAW resonator 1 is incorporated is within the range of the stop band, Based on the equation (6), “λ ₀ ′” is determined so as to satisfy the following equation (7).

但し、ｒは１本の電極指３０１の反射係数、ｆ_Ｌはラダー型フィルタ１００の通過帯域の下端周波数である。

Here, r is the reflection coefficient of one electrode finger 301, and f _L is the lower end frequency of the passband of the ladder filter 100.

In the SAW resonator 1 having the above configuration, various design variables are selected so as to satisfy the above-described equation (4). As described above, innumerable spurious is generated in the frequency characteristics of the SAW resonator 10, but from the viewpoint of insertion loss of the ladder filter 100 and destruction of the IDT electrode 2, for example, spurious up to the second order becomes a problem. . Therefore, in the present embodiment, in order to prevent these primary and secondary spurs from being generated in the passband of the ladder filter 100, the expression (4) is satisfied under the condition of “n = 3”. As described above, various design variables “λ ₀ , N, L” are set.

In order to make it easy to capture an image in designing the SAW resonator 1, a specific example will be described. Now, there is a conventional SAW resonator 10 having the frequency characteristics shown in FIGS. 1 and 2 designed under the following conditions, and this is used as the series arm 101 of the ladder type filter 100.
Piezoelectric substrate: LiTaO ₃ (SAW propagation velocity v; 4,120 [m / s])
Period length of IDT electrode 2
: Λ ₀ = 4.63 [μm] (4.63 × 10 ⁻⁶ [m])
Total number of electrode fingers 231 and 232
: N = 160 [books]
Interelectrode distance between IDT electrode 2 and reflectors 3a and 3b
: L ₁ = L ₂ = L = 336.4 [μm] (336.4 × 10 ⁻⁶ [m])
Frequency at the upper end of the passband of the ladder filter 100
: F _H = 884 [MHz]

In the SAW resonator 10, for example, as shown in FIG. 2, there is a minimum value of the steady term “Y ₁ ” at a position of “f _sp3 = 878 MHz”, and the frequency “f _sp3 ” and the main resonance are reached. There are two spurious in between. In this case, it is necessary to redesign the values of the various design variables “λ ₀ , N, L” so that the expression (4) is satisfied under the condition “n = 3”. Here, the period length λ ₀ is a design variable that determines the frequency of the main resonance of the SAW resonator and may not be changed. For example, the case where the interelectrode distance is changed from L to L ′ will be considered.

As described above, the first term on the left side of the equation (4) corresponds to the frequency at which the main resonance occurs, and each design variable of the second term takes a positive value. Therefore, the main resonance occurs on the entire left side. The frequency is lower than the frequency. Therefore, in the SAW resonator 1 after the design change, a new frequency at which the third minimum value of the steady term “Y ₁ ” is generated is “f _sp3 ′ = 885 MHz”, and in the case of “f _H = 884 MHz” (4 ) The distance L ′ between the electrodes for which the equation is satisfied is obtained. When formula (3) is arranged for L under the condition of “n = 3”, the following formula (8) is obtained.

上記（８）式に設計変更前の「ｆ_ｓｐ３＝８７８×１０^６Ｈｚ」、変更後の「ｆ_ｓｐ３’＝８８５×１０^６Ｈｚ」を夫々代入して双方の計算結果の比を求めると、「（Ｌ’／Ｌ）＝３．２」となった。即ち、電極間距離を「Ｌ’＝１０７６ｍｍ」まで長くすることにより、ラダー型フィルタ１００の通過帯域（上端周波数「ｆ_Ｈ＝８８４ＭＨｚ」）に、設計変更前のＳＡＷ共振子１０にて存在していた２次までのスプリアスが現れないＳＡＷ共振子１を得ることができる。

Substituting “f _sp3 = 878 × 10 ⁶ Hz” before the design change and “f _sp3 ′ = 885 × 10 ⁶ Hz” before the design change into the above equation (8), respectively, “(L ′ / L) = 3.2”. That is, by increasing the distance between the electrodes to “L ′ = 1076 mm”, the SAW resonator 10 before the design change exists in the pass band of the ladder filter 100 (the upper end frequency “f _H = 884 MHz”). In addition, the SAW resonator 1 in which spurious up to the second order does not appear can be obtained.

Here, the contents of the above design change are illustrated to make it easier to imagine the design work of the SAW resonator 1, and actually the SAW resonator satisfying the expression (4) using only the interelectrode distance L as a variable. The design of 1 is not performed. For example, the number N of the electrode fingers 231 and 232 may be changed, or two variables of L and N may be changed. If the frequency of the main resonance can be changed, it is combined with the change of the period length λ ₀ and the upper end frequency f _H of the pass band, and the design is made on a trial and error basis while checking the filter characteristics of the ladder filter 100 and the like. It is good to do. Of course, the actual design conditions of the SAW resonator 10 are not limited to this example.

  In the plan view of FIG. 3A, a standing wave having a periodic length n = 3 is also displayed. This is displayed in order to make it easy to grasp the region where the standing wave is formed in the SAW resonator 1, and as described above, the SAW resonator 1 satisfying the equation (4) has a ladder type. Note that such a standing wave does not occur in the pass band of the filter 100. The same applies to the standing waves shown in FIGS. 3B, 4A, and 4B described later.

  The SAW resonator 1 according to the present embodiment has the following effects. Since the SAW resonator 1 satisfies the equation (4), in the pass band of the ladder-type filter 100 including the SAW resonator 1 as the series arm 101, a natural number n that is equal to or more than a predetermined number n in the equation (4). For example, the spurious of the order of 1 less than “n = 3” does not occur. As a result, since the stress applied to the IDT electrode becomes smaller than when spurious is generated, the service life of the ladder filter 100 can be extended.

Further, as described in the background art, in the ladder filter 100 using the conventional SAW resonator 10, the pass band is set avoiding the frequency at which a large spurious is generated, the SAW resonance according to the present invention. In the child 1, design variables on the left side, for example, the distance L between electrodes, the value of the number N of electrode fingers 231 and 232, and the like are appropriately selected so as to satisfy the expression (4), and the left side of the expression (4) is expressed as “v / By approaching λ ₀ ”, the restriction on the upper end frequency“ f _H ”of the pass band is relaxed, and there may be a case where a wider pass band than in the past can be set. However, as can be seen from the fact that the term “v−λ ₀ f” is included in the denominator of equation (8), for example, the closer the left side of equation (4) is to “v / λ ₀ ”, Since the value of N increases abruptly and the SAW resonator 1 increases in size, it is necessary to relax the restriction on the upper-end frequency in a possible range on the arrangement space of the SAW resonator 1.

Here, the distance between the electrodes is not limited to “L ₁ = L ₂ = L”, and may be “L ₁ ≠ L ₂ ” as shown in FIG. 3B, for example. In this case, by designing the SAW resonator 1 that satisfies the following expression (4) ′, which is a more general expression of the expression (4), it is possible to prevent (n−1) -order spurious from appearing in the passband.

  Next, the SAW resonator 1a according to the second embodiment will be described. The SAW resonator 1a according to the second embodiment has a point that the pass band of the ladder type filter 100 is outside the stop band of the reflectors 3a and 3b, for example, as shown in the following equation (9). This is different from the embodiment.

ラダー型フィルタ１００の通過帯域がこれらのストップバンドの範囲外にあるＳＡＷ共振子１ａの一例としては、ＩＤＴ電極２の周期長λ_０と反射器３ａ、３ｂの周期長λ_０’とが等しいＳＡＷ共振子１ａが挙げられる。即ち、ＳＡＷはＩＤＴ電極２を透過できるように設計されているので、この場合にはＳＡＷは反射器３ａ、３ｂも透過することができるからである。また、式中のｎには半整数も含まれる。

An example of a SAW resonator 1a passband of the ladder type filter 100 is outside the range of the stop band, the cycle length lambda ₀ and the reflector 3a of the IDT electrode 2, 3b cycle length lambda ₀ 'are equal SAW of The resonator 1a is mentioned. That is, the SAW is designed so as to be able to pass through the IDT electrode 2, and in this case, the SAW can also pass through the reflectors 3a and 3b. Further, n in the formula includes a half integer.

In such a SAW resonator 1a, for example, as shown in FIG. 4A, the SAW penetrates into the reflectors 3a and 3b to form a standing wave. It is necessary to rewrite the expression so that the denominator of the second term on the left side of the expression (4) indicating the width of the reflectors 3a and 3b is included. Therefore, for example, the period length “λ ₀ ′” of the electrode fingers 301 of the reflectors 3a and 3b, the number of electrode fingers 301 “M ₁ = M ₂ = M”, and the interelectrode distance “L ₁ = L ₂ = L”. Then, when the following expression (10) is satisfied, the SAW resonator 1a does not have (m−1) th order spurious.

Furthermore, the number of electrode fingers 301 is not limited to the conditions such as the number of electrode fingers 301 “M ₁ = M ₂ = M” and the distance between electrodes “L ₁ = L ₂ = L”, as shown in FIG. When “M ₁ ≠ M ₂ ” and the inter-electrode distance “L ₁ ≠ L ₂ ”, when the condition of the following expression (10) ′ obtained by correcting the expression (10) is satisfied, the (m−1) th order The SAW resonator 1a does not have the following spurious.

  The elastic wave resonator to which the present invention can be applied is limited to an elastic wave resonator using a surface acoustic wave such as the SAW resonators 1 and 1a according to the first and second embodiments described above. It is not something. For example, it is needless to say that the present invention can be applied to a resonator using a boundary acoustic wave. Further, in the ladder type filter 100, if the SAW resonators 1 and 1a according to the present embodiment are employed in at least one of the plurality of series arms 101, for example, the effect of reducing the insertion loss of the ladder type filter 100 is exhibited. be able to.

In addition, the use of the acoustic wave resonator according to the present invention is not limited to the series arm 101 of the ladder type filter 100. For example, it may be used as the parallel arm 102 of the ladder type filter 100 or may be used for a lattice type filter. Further, it may be used as an oscillator or a delay element. In these cases, “f _H ” in the equation (4) or the like may be read as the upper end frequency of the use band of the oscillator or the delay element.

(Simulation 1)
In the conventional SAW resonator 10 shown in FIG. 12, the frequency of a signal input to the SAW resonator 10 was changed, and a change in the stress distribution applied to the IDT electrode 2 was simulated.
The configuration of the SAW resonator 10 is as follows. An admittance characteristic diagram of the SAW resonator 10 is shown in FIG.
Piezoelectric substrate: LiTaO ₃ (SAW propagation velocity v; 4,210 [m / s])
(The simulation result shown in FIG. 5 is different in SAW propagation speed because the cutting direction of LiTaO ₃ is different. The same applies to (simulation 2) described later.)
Period length of IDT electrode 2
: Λ ₀ = 4.30 [μm]
Total number of electrode fingers 231 and 232
: N = 160 [books]
Interelectrode distance between IDT electrode 2 and reflectors 3a and 3b
: L ₁ = L ₂ = L = 1.075 [μm]

(Example 1-1)
A frequency signal that does not generate spurious corresponding to the point A in FIG. 5 was input, and the stress applied to the IDT electrode 2 was simulated.
Input power: 30 [dBm]
(Example 1-2)
A frequency signal that does not generate spurious corresponding to point B in FIG. 5 was input, and the stress applied to the IDT electrode 2 was simulated. The input power was the same as (Example 1-1).
(Comparative Example 1-1)
A frequency signal generating spurious corresponding to the point C in FIG. 5 was input, and the stress applied to the IDT electrode 2 was simulated. The input power was the same as (Example 1-1).

The results of (Example 1-1, 1-2) are shown in FIGS. 6A and 6B, and the result of (Comparative Example 1-1) is shown in FIG. 6C. 6A to 6C, the horizontal axis indicates the relative length “x / λ ₀ ” with respect to the period length “λ ₀ ” on the IDT electrode 2 and the reflectors 3 a and 3 b. The vertical axis indicates the stress [GPa] applied to the IDT electrode 2 and the reflectors 3a and 3b at that position. Here, the relative length “x / λ ₀ ” indicates the distance from the origin (“x / λ ₀ = 0”) on the horizontal axis shown below the SAW resonator 10 in FIG. The left end of the electrode 2 is defined as the origin.

  According to the result of (Example 1-1) shown in FIG. 6A, when a frequency signal corresponding to a point A where spurious is not generated is input, a stress distribution having a peak at the center of the IDT electrode 2 appears. The maximum value was approximately “0.1 GPa”. Further, according to the result of (Example 1-2) shown in FIG. 6B, a stress distribution having two peaks appears with respect to the frequency signal corresponding to the point B where spurious is not generated. The maximum value was approximately “0.15 GPa”.

  In response to these results, when a frequency signal corresponding to point C on the spurious is input, a stress distribution having three peaks appears, the maximum value of which is “0.3 GPa”, and the maximum value at a frequency at which no spurious is generated. Even if compared with 2 times, the stress of 2 times-3 times will be added. As discussed in the background art from the above, if a spurious signal is present in the passband of the ladder filter 100, the IDT electrode 2 is normally connected to a frequency signal that causes the spurious signal. It was confirmed that an excessive stress was applied than in the case of.

(Experiment 1)
A signal was input by changing the frequency to the ladder filter 100 having the frequency characteristics shown in FIG. 7, and the service life until the IDT electrode 2 was destroyed was measured. The service life was measured by an accelerated test under an environment where the deterioration rate was about 64 times the normal speed by setting the experimental temperature to 85 ° C. with respect to the normal use temperature of 25 ° C.
(Example 2-1)
The service life was measured by inputting a frequency signal (D point in FIG. 7) that does not generate spurious.
Input power: 32.0, 32.5, 33.0 [dBm]
(Comparative Example 2-1)
The service life was measured by inputting a frequency signal (point E in FIG. 7) that generates spurious. The input power was the same as in (Example 2-1).

The results of (Example 2-1) and (Comparative example 2-1) are shown in FIG. The horizontal axis of FIG. 8 indicates the input power [dBm] to the ladder filter 100, and the vertical axis indicates the logarithm of the destruction time [H] from the start of frequency signal input to the IDT electrode 2 to the destruction. Is shown. The result of (Example 2-1) is plotted with a triangle “Δ”, and the result of (Comparative Example 2-1) is plotted with a diamond “◇”.
According to the result of (Example 2-1), in the semilogarithmic plot of FIG. 8, the breakdown time draws a proportional straight line having a negative slope with respect to the input power, and the power input to the ladder filter 100 is The smaller it is, the longer the time until the IDT electrode 2 is destroyed.

  On the other hand, in the experimental result of (Comparative Example 2-1), the breakdown time is constant at about 0.02 [H] (about 1.2 [min]) at any input power, and the input power is changed. However, the destruction time did not change. For this reason, even if the input power is reduced to a normal value, the destruction time does not change, and the service life is expected to be extremely shortened, and the occurrence of spurious greatly increases the service life of the ladder filter 100. It was confirmed that it shortened.

(Simulation 2)
The ladder filter 100 including the SAW resonator 1 according to the embodiment having the configuration satisfying the expression (4) as the series arm 101, and the SAW resonator 10 having a configuration not satisfying the condition as the series arm 101. A model of the ladder filter 100 was created, and the frequency characteristics of the attenuation amount of each ladder filter 100 were simulated. The pass band of each ladder filter 100 was set to be within the stop band range of the reflectors 3a and 3b.
(Example 3-1)
Piezoelectric substrate: LiTaO ₃ (SAW propagation velocity v; 4,213 [m / s])
Period length of IDT electrode 2
: Λ ₀ = 4.74 [μm]
Total number of electrode fingers 231 and 232
: N = 94 [book]
Interelectrode distance between IDT electrode 2 and reflectors 3a and 3b
: L ₁ = L ₂ = L = 1.327 [μm]
Passband
Upper frequency: f _H = 831 [MHz]
Lower end frequency: f _L = 823 [MHz]
Under the above conditions, f _sp3 = 832.8 [MHz]> f _H and satisfies the expression (4).

(Comparative Example 3-1)
Piezoelectric substrate: LiTaO ₃ (SAW propagation velocity v; 4,213 [m / s])
Period length of IDT electrode 2
: Λ ₀ = 4.745 [μm]
Total number of electrode fingers 231 and 232
: N = 84 [books]
Interelectrode distance between IDT electrode 2 and reflectors 3a and 3b
: L ₁ = L ₂ = L = 1.186 μm
Passband
Upper frequency: f _H = 831 MHz
Lower frequency: f _L = 823 MHz
Under the above conditions, f _sp3 = 825.2 [MHz] <f _{H and} Equation (4) is not satisfied.

  The simulation results of (Example 3-1) are shown in FIGS. 9 (a) and 9 (b), and the simulation results of (Comparative Example 3-1) are shown in FIGS. 10 (a) and 10 (b). In each figure, the horizontal axis indicates the frequency [MHz] of the input signal, and the vertical axis indicates the attenuation amount of the ladder filter 100. FIG. 9B and FIG. 10B are enlarged views of regions surrounded by broken lines in FIG. 9A and FIG. 10A, respectively.

  Comparing FIG. 9A and FIG. 10A, it appears that (Example 3-1) and (Comparative example 3-1) show similar frequency characteristics. However, according to the enlarged view of FIG. 9B, in (Example 3-1), the maximum value of the attenuation (insertion loss) is not observed in the passband of the ladder filter 100, and ripples are generated. Absent. On the other hand, looking at the enlarged view of FIG. 10B, it can be seen that in (Comparative Example 3-1), a ripple is formed in the passband, and spurious is generated at this position. . In order to avoid the effect of spurious, it is necessary to narrow the passband width or move the entire passband to the high frequency side. For example, it is not possible to set the passband in the necessary frequency range. Often occurs.

  From the above, the SAW resonator 1 according to the embodiment having the configuration satisfying the expression (4) has a high performance in which excessive stress is not applied to the IDT electrode 2 and the degree of freedom in setting the passband is high. It can be said that it is an elastic wave resonator.

It is an admittance characteristic view of a conventional SAW resonator. FIG. 4 is a second admittance characteristic diagram of the SAW resonator. It is a top view of the SAW resonator which concerns on this Embodiment. It is a top view of the SAW resonator which concerns on 2nd Embodiment. It is a 2nd admittance characteristic figure concerning a conventional type SAW resonator. It is a characteristic view which shows the stress distribution added to an IDT electrode according to the presence or absence of generation | occurrence | production of a spurious. It is a frequency characteristic figure which shows the attenuation amount of the ladder type filter provided with the conventional type SAW resonator. It is a characteristic view which shows the destruction time of the IDT electrode according to the presence or absence of generation | occurrence | production of a spurious. It is a frequency characteristic figure which shows the attenuation amount of the ladder type filter provided with the SAW resonator which concerns on an Example. It is a frequency characteristic figure which shows the attenuation amount of the ladder type filter provided with the SAW resonator which concerns on a comparative example. It is a schematic block diagram of the ladder type filter provided with the SAW resonator. It is a top view which shows the structure of a SAW resonator. It is a characteristic view which shows the relationship between the admittance characteristic of a SAW resonator, and the attenuation amount of a ladder type filter. It is a frequency characteristic figure concerning the amount of attenuation of a ladder type filter provided with the conventional type SAW resonator. It is a 3rd admittance characteristic figure concerning a conventional type SAW resonator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1a SAW resonator 2 IDT electrode 3a, 3b Reflector 10 SAW resonator 21, 22 Bus bar 100 Ladder type filter 101 Series arm 102 Parallel arm 231, 232
Electrode finger 301 Electrode finger

Claims (6)

In an acoustic wave resonator including an IDT electrode and a grating reflector disposed on both sides of the IDT electrode in the propagation direction of the elastic wave on the piezoelectric substrate,
Being used within the stopband of each grating reflector;
An elastic wave resonator comprising: an upper end frequency of a band in which the elastic wave resonator is used satisfies a condition of the following formula 1.

Where v is the average velocity of the elastic wave propagating through the piezoelectric substrate, λ ₀ is the period length of the electrode fingers of the IDT electrode, N is the number of electrode fingers of the IDT electrode, and L ₁ and L ₂ are the IDT electrodes and their respective reflectors The inter-electrode distance, f _H is the upper end frequency of the band in which the acoustic wave resonator is used, and n is a predetermined natural number of 2 or more.   The elastic wave resonator according to claim 1, wherein n in Formula 1 is 3. Comprising the acoustic wave resonator according to claim 1 as a series arm;
The passband is within the stopband of each grating reflector;
Ladder filter, wherein the equation 1 f _H is provided with a be the upper end frequency of the pass band. In an acoustic wave resonator including an IDT electrode and a grating reflector disposed on both sides of the IDT electrode in the propagation direction of the elastic wave on the piezoelectric substrate,
Being used outside the stopband of each grating reflector;
An elastic wave resonator comprising: an upper end frequency of a band in which the elastic wave resonator is used satisfies a condition of the following Equation 2.

Where v is the average velocity of the elastic wave propagating through the piezoelectric substrate, λ ₀ is the period length of the electrode fingers of the IDT electrode, λ ₀ ′ is the period length of the electrode fingers of the grating reflector, and N is the number of electrode fingers of the IDT electrode , M ₁ and M ₂ are the number of electrode fingers of each grating reflector, L ₁ and L ₂ are distances between the electrodes of the IDT electrode and each reflector, and f _H is the upper end of the band where the acoustic wave resonator is used. The frequency m is a predetermined natural number of 2 or more.   The elastic wave resonator according to claim 4, wherein m in Formula 2 is 3. 5. Including the acoustic wave resonator according to claim 4 as a series arm;
The passband is outside the stopband range of each grating reflector;
Ladder filter, characterized in that f _H of Equation 2 is provided with, and it is the upper frequency of the passband.

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