JPH08340234A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE: To improve the performance of a resonator coupling surface acoustic wave filter using a lithium tantalate substrate, a lithium niobate substrate or a lithium tetraborate substrate having a high acoustic velocity cutting face by clearly defining the optimum range of a necessary design parameter. CONSTITUTION: The propagation direction of the surface acoustic wave is controlled within the range of (40 deg. to 90 deg., 40 deg. to 90 deg. and 0 deg. to 60 deg.) by Euler's angle display and a frequency difference Δf between the resonance frequency (frs) of a series arm resonator and the resonance frequency (frp) of a parallel arm resonator is set within a prescribed range formulated by using center frequency, the electrostatic capacitance Cos of the series arm resonator and the electrostatic capacitance Cop of the parallel arm resonator as parameters.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resonator-coupled surface acoustic wave filter constructed by connecting surface acoustic wave resonators to a series arm and a parallel arm.

[0002]

2. Description of the Related Art In recent years, surface acoustic wave devices have been widely applied as circuit devices such as resonator filters and signal processing delay lines in communication devices such as automobile telephones. For example, the surface acoustic wave element shown in FIG. 9 is a substrate having piezoelectricity.
On the surface of (1), a blind-shaped electrode (2) and a grid-shaped reflector (3)
(3) is formed to form a resonator.

Further, as shown in FIG. 8, a series arm (4) of a ladder type circuit.
By arranging the resonators (6) and (7) on the parallel arm (5) and the parallel arm (5), respectively, a filter can be constructed. Such a resonator-coupled surface acoustic wave filter is widely used because it has advantages such as low insertion loss and no matching circuit. In the resonator coupled surface acoustic wave filter, the resonance frequency frs of the series arm resonator (6) is
By making the anti-resonance frequency fap of the parallel arm resonator (7) substantially coincide with each other, a bandpass filter characteristic is realized.

A parameter for designing a resonator-coupled surface acoustic wave filter is a difference Δf between the resonance frequency frs of the series arm resonator and the resonance frequency frp of the parallel arm resonator.
(Δf = frs−frp) and capacitances Cos and Cop determined from the number of electrode pairs and the aperture length of each resonator, and the substrate (36 °) conventionally used for surface acoustic wave filters.
Y-X LiTaO ₃ , 64 ° Y-X LiNbO ₃ , and 41
The optimum range of the design parameters has been clarified for (Y-X LiNbO ₃ ).
No. 0, JP-A-6-69750, etc.).

[0005]

By the way, in order to respond to the higher frequency of surface acoustic wave filters in recent years, research has been conducted on a cut surface for obtaining a higher sound velocity, and as a result, lithium tantalate (LiTaO ₃₎ has been investigated. ), Lithium niobate (LiNbO ₃ ), and lithium tetraborate (Li ₂ B ₄ O ₇ ).
As for, the cut surface of higher sonic velocity than before has been discovered. That is, for lithium tantalate, the propagation direction of the surface acoustic wave is (90 °, 90 °, 31
For lithium niobate, the propagation direction of the surface acoustic wave is the Euler angle display (90 °, 90 °, 37 °).
For lithium tetraborate, the propagation direction of the surface acoustic wave is (0 °, 45 °, 90 °) in the Euler angle display.

However, with respect to the resonator-coupled surface acoustic wave filter using the lithium tantalate substrate, the lithium niobate substrate, or the lithium tetraborate substrate having the above-mentioned cut surface, the optimum range of those design parameters has not yet been reached. Not revealed. The object of the present invention is a resonator-coupled surface acoustic wave filter using a lithium tantalate substrate, a lithium niobate substrate, or a lithium tetraborate substrate having a cut surface of high acoustic velocity, and required design parameters, In particular, the optimum range of the resonance frequency of the series arm resonator and the parallel arm resonator is clarified, and thereby the performance of the resonator coupled surface acoustic wave filter is improved. In the present invention, as the first index for judging the performance of the surface acoustic wave filter, the ripple and the insertion loss in the characteristics in the pass band are adopted, and as the second index, the voltage standing wave ratio representing the matching state is adopted. (VSWR; Voltage Stand
ing Wave Ratio).

[0007]

In the case of a resonator-coupled surface acoustic wave filter, as shown in FIG. 10, the difference Δf between the resonance frequency frs of the series arm resonator and the resonance frequency frp of the parallel arm resonator is shown.
With respect to (hereinafter, simply referred to as resonance frequency difference), the larger the value, the larger the ripple A in the passband characteristic,
It is known that the smaller the value, the larger the insertion loss B. On the other hand, regarding the electrostatic capacitance Cos of the series arm resonator and the electrostatic capacitance Cop of the parallel arm resonator, it is known that VSWR changes with changes in these values.

Generally, it is desirable in design that the ripple A is suppressed to 2.0 dB or less and the insertion loss B is suppressed to 5.0 dB or less. Also, regarding VSWR, 2.0
It is desirable in design to keep the value below. Therefore, in the present invention, a large number of prototypes given various design parameters are manufactured, and based on the experimental results of these prototypes, the resonance frequency at which the ripple and the insertion loss can be suppressed to the above limit values or less. The optimum range of the difference Δf was clarified, and further, the optimum range of the capacitances Cos and Cop of the series arm resonator and the parallel arm resonator capable of suppressing the value of VSWR to the above-mentioned limit value or less was clarified.

That is, in the surface acoustic wave filter using the lithium tantalate substrate, the propagation direction of the surface acoustic wave is the Euler angle display (40 ° to 90 °, 40 ° to 90 °).
(Degrees, 0 degrees to 60 degrees) and a range equivalent thereto, and the resonance frequency difference Δf falls within a range defined by the following formula 7 with the center frequency f ₀ and the capacitance ratio P as parameters. It is set.

[0010]

## EQU7 ## Δfmin ≦ Δf ≦ Δfmax where Δf = frs−frp Δfmin = (− 0.001588 + 0.0116P−0.00
421P ² ) × f ₀ Δfmax = (0.0778-0.0736P + 0.0252
P ² ) × f ₀ P = Cop / Cos

More specifically, the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator are set within a band-like range defined by the following equation 8.

## EQU00008 ## Cop = -0.49 Cos + (4978 ± 2850) / f ₀

In the surface acoustic wave filter using the lithium niobate substrate, the propagation direction of the surface acoustic wave is the Euler angle display (40 to 90 degrees, 40 to 90 degrees, 0 to 60 degrees), and The resonance frequency difference Δf is set within an equivalent range, and is set within a range defined by the following Expression 9 with the center frequency f ₀ and the capacitance ratio P as parameters.

[0013]

Δfmin ≦ Δf ≦ Δfmax where Δf = frs−frp Δfmin = (0.0262 + 0.0245P−0.000021)
P ² ) × f ₀ Δfmax = (0.303−0.269P + 0.082P ² ) ×
f ₀ P = Cop / Cos

More specifically, the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator are set within a band-like range defined by the following expression 10.

## EQU10 ## Cop = -0.93 Cos + (3895 ± 1425) / f ₀

In the surface acoustic wave filter using the lithium tetraborate substrate, the propagation direction of the surface acoustic wave is the Euler angle display (0 to 50 degrees, 15 to 75 degrees, 40 to 90 degrees), and The resonance frequency difference Δf is set within an equivalent range, and is set within a range defined by the following formula 11 with the center frequency f ₀ and the capacitance ratio P as parameters.

[0016]

Δfmin ≦ Δf ≦ Δfmax where Δf = frs−frp Δfmin = (− 0.00002 + 0.00642P−0.0)
021P ² ) × f ₀ Δfmax = (0.0305-0.0161P + 0.0021)
P ² ) × f ₀ P = Cop / Cos

More specifically, the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator are set within the band-like range defined by the following formula 12.

## EQU12 ## Cop = -0.47 Cos + (3078 ± 912) / f ₀

In designing a surface acoustic wave filter,
First, the center frequency f _{0 is} calculated from the required filter specifications.
And out-of-band suppression are determined, and further, the capacitance ratio P according to the out-of-band suppression is determined. Therefore, the determined center frequency f ₀ and the capacitance ratio P are substituted into the above formulas 7, 9 or 11 to calculate the optimum range (Δfmin to Δfmax) of the resonance frequency difference Δf according to the material of the substrate. Then, the resonance frequency difference Δf is determined as a value within this range.

Next, if the resonance frequency frs (≈f ₀ ) of the series arm resonator is determined as a value near the center frequency f ₀ , the resonance frequency frp of the parallel arm resonator is determined from the determined resonance frequency difference Δf. (= Frs−Δf) can be determined.

By designing the series arm resonator and the parallel arm resonator based on the resonance frequency frs of the series arm resonator and the resonance frequency frp of the parallel arm resonator thus obtained, the ripple of 2. The insertion loss can be suppressed to 0 dB or less and the insertion loss can be suppressed to 5.0 dB or less.

Further, satisfying the determined capacitance ratio P,
Further, the values of the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator, which are to be included in the band-shaped range defined by the above-mentioned equation 8, equation 10 or equation 12, are determined.

Based on the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator thus obtained,
By designing the series arm resonator and the parallel arm resonator, VSWR can be suppressed to 2.0 or less.

[0023]

According to the present invention, a lithium tantalate substrate, a lithium niobate substrate, which has a cut surface of high acoustic velocity,
Alternatively, in the resonator-coupled surface acoustic wave filter using the lithium tetraborate substrate, the ripple frequencies and the insertion loss can be suppressed at the same time by setting the resonance frequencies of the series arm resonator and the parallel arm resonator in the optimum range.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION A resonator-coupled surface acoustic wave filter according to the present invention is a series arm of a ladder type circuit as shown in FIG.
(4) and the parallel arm (5) are connected to one-port resonators (6) and (7), respectively. As shown in FIG. 9, each 1-port resonator (6) (7) has a blind electrode (2) and a grid reflector on the substrate (1).
(3) Formed from (3).

In the lithium tantalate substrate,
The propagation direction of the surface acoustic wave is the Euler angle display (40 degrees to 9
The electrodes are formed in the range of 0 degrees, 40 degrees to 90 degrees, 0 degrees to 60 degrees, and preferably (90 degrees, 90 degrees, 31 degrees). In addition, in the lithium niobate substrate, the propagation direction of the surface acoustic wave is the Euler angle display (40 degrees to 90 degrees).
The electrodes are formed so as to be in the range of 40 degrees to 90 degrees, 0 degrees to 60 degrees, and preferably (90 degrees, 90 degrees, 37 degrees). Further, in the lithium tetraborate substrate, the propagation direction of the surface acoustic wave is the Euler angle display (0 to 50 degrees, 15
The range of degrees to 75 degrees, 40 degrees to 90 degrees, preferably
The electrodes are formed so as to be (0 degrees, 45 degrees, 90 degrees).
As a result, supersonic surface acoustic waves (pseudo-surface acoustic waves in which the longitudinal wave component is more dominant than the transverse wave component) are obtained.

In the present embodiment, first, regarding the resonance frequency difference Δf, the optimum range for setting the ripple to 2.0 dB or less and the insertion loss to 5.0 dB or less was determined by an experiment using a plurality of prototypes. .

FIG. 1 shows the results of an experiment using a prototype of a resonator-coupled surface acoustic wave filter having a center frequency of 1.9 GHz using a lithium tantalate substrate. That is, in the graph of FIG. 1, the capacitance ratio P is plotted on the horizontal axis and the resonance frequency difference Δf is plotted on the vertical axis. Ripple and insertion loss were measured for a plurality of prototypes in which these values were gradually changed. A prototype that meets the conditions of ripple of 2.0 dB or less and insertion loss of 5.0 dB or less
The mark and the other prototypes are plotted with x marks. The optimum range of Δf (MHz) that satisfies the above conditions is
When P = 0.5, 6 ≦ Δf ≦ 90, and when P = 0.75, 9
≦ Δf ≦ 70, P = 1.0, 11 ≦ Δf ≦ 56, P
= 1.25, 12 ≦ Δf ≦ 48. The upper curve of the graph of FIG. 1 represents the limit of ripple, and the lower curve represents the limit of insertion loss. Therefore, these upper limit and lower limit, respectively,
By applying the least squares method, the resonance frequency difference Δf
The following Expression 13 is obtained as the optimum range of

[0028]

Equation 13] ^{-3 + 22P-8P 2 ≦ Δf} ≦ 148-14
0P + 48P ²

Further, the above formula 13 is used as the center frequency f ₀ (MH
z) gives the following formula 14.

## EQU14 ## Δfmin ≦ Δf ≦ Δfmax where Δfmin = (-0.0000158 + 0.0116P-0.00
421P ² ) × f ₀ Δfmax = (0.0778-0.0736P + 0.0252
P ² ) × f ₀

FIG. 2 shows the experimental results of a prototype of a resonator-coupled surface acoustic wave filter with a center frequency of 1.9 GHz using a lithium niobate substrate, as in FIG. The optimum range of Δf (MHz) for ripples of 2.0 dB or less and insertion loss of 5.0 dB or less is 72 ≦ Δf ≦ at P = 0.5.
359, P = 0.75, 83 ≦ Δf ≦ 280, P =
When 1.0, 92 ≦ Δf ≦ 220, P = 1.25, 1
02 ≦ Δf ≦ 180. Therefore, by applying the least squares method to these upper and lower limits, respectively,
The following Expression 15 is obtained as the optimum range of the resonance frequency difference Δf.

[0031]

[Equation 15] 50 + 47P-4P ² ≤ Δf ≤ 576-51
2P + 156P ²

Further, the above formula 15 is used as the center frequency f ₀ (MH
z) gives the following equation 16:

Δfmin ≦ Δf ≦ Δfmax where Δfmin = (0.0262 + 0.0245P−0.0021)
P ² ) × f ₀ Δfmax = (0.303−0.269P + 0.082P ² ) ×
f ₀

FIG. 3 shows the results of an experiment using a prototype of a resonator-coupled surface acoustic wave filter with a center frequency of 1.9 GHz using a lithium tetraborate substrate, as in FIG. The optimum Δ that satisfies the following conditions: ripple is 2.0 dB or less and insertion loss is 5.0 dB or less.
The range of f (MHz) is 5 ≦ Δf ≦ 4 at P = 0.5.
5, P = 0.75, 7 ≦ Δf ≦ 37, P = 1.0, 8 ≦ Δf ≦ 32, P = 1.25, 9 ≦ Δf ≦ 2
It becomes 6. Therefore, these upper limit and lower limit, respectively,
By applying the least squares method, the resonance frequency difference Δf
The following Expression 17 is obtained as the optimum range of

[0034]

[Number 17] ^{-0.05 + 12P-4P 2 ≦ Δf} ≦ 58-3
1P + 4P ²

Further, the above _equation 17 is applied to the center frequency f ₀ (MH
z) gives the following formula 18.

Δfmin ≦ Δf ≦ Δfmax where Δfmin = (-0.00002 + 0.00642P-0.0)
021P ² ) × f ₀ Δfmax = (0.0305-0.0161P + 0.0021)
P ² ) × f ₀

In this embodiment, the capacitance Cos and Cop of the series arm resonator and the parallel arm resonator are then VS.
The optimum range for setting the WR to 2.0 or less was determined.

FIG. 4 shows the results of an experiment using a prototype of a resonator-coupled surface acoustic wave filter with a center frequency of 1.9 GHz using a lithium tantalate substrate. That is, in the graph of FIG. 4, the horizontal axis represents the capacitance Cos (pF) of the series arm resonator, and the vertical axis represents the capacitance Cop of the parallel arm resonator.
(PF) is taken, and VSWR is measured for a plurality of prototypes in which these values are gradually changed, and the prototypes satisfying the condition that the measured value is 2.0 or less are marked with ○, and other prototypes Is a plot with x.

The capacitances Cos and Cop (pF) are shown in FIG.
When N is the number of electrode pairs and W (μm) is the opening length, it can be calculated from the following equation 19.

[Equation 19] C = 4 × 10 ⁻⁴ × N × W

As shown in the figure, the prototype satisfying the above conditions is
It exists in a band-shaped region indicated by a solid line in the figure, and the band-shaped range can be expressed by the following formula 20.

[0040]

[Equation 20] Cop = −0.49 Cos + 2.62 ± 1.5

Further, the above formula 20 is used as the center frequency f ₀ (MH
z) gives the following formula 21.

[Equation 21] Cop = −0.49 Cos + (4978 ± 2850) / f ₀

FIG. 5 shows the experimental results of the prototype of the resonator-coupled surface acoustic wave filter with a center frequency of 1.9 GHz using a lithium niobate substrate, as in FIG. The prototype that meets the VSWR measurement requirement of 2.0 or less exists in the strip-shaped area indicated by the solid line in the figure.
The band-shaped range can be represented by the following Expression 22.

[0043]

[Equation 22] Cop = -0.93 Cos + 2.05 ± 0.75

Further, the above formula 22 is used as the center frequency f ₀ (MH
z) gives the following equation 23.

[Equation 23] Cop = −0.93 Cos + (3895 ± 1425) / f ₀

FIG. 6 shows the experimental results of a prototype of a resonator-coupled surface acoustic wave filter with a center frequency of 1.9 GHz using a lithium tetraborate substrate, as in FIG. The prototype that satisfies the VSWR measurement of 2.0 or less exists in the band-shaped region indicated by the solid line in the figure, and the band-shaped range can be expressed by the following formula 24.

[0046]

## EQU23 ## Cop = -0.47 Cos + 1.62 ± 0.48

Further, the above formula 24 is used as the center frequency f ₀ (MH
z), the following equation 25 is obtained.

## EQU25 ## Cop = -0.47 Cos + (3078 ± 912) / f ₀

Therefore, by determining the resonance frequency difference Δf in the range defined by the above equations (14), (16) or (18) according to the material of the substrate, the ripple in the pass band characteristic is 2.0 dB or less. At the same time, the insertion loss is suppressed to 5.0 dB or less. Further, the electrostatic capacitances of the series arm resonator and the parallel arm resonator are calculated by
Alternatively, the VSWR can be suppressed to 2.0 or less by setting the band range defined by the equation (25).

FIG. 7 shows a procedure for actually designing a resonator-coupled surface acoustic wave filter based on the above-mentioned design parameter determination method. First, in step S1, the center frequency f ₀ and the out-of-band suppression D are determined based on the required filter specifications (see FIG. 10). Next, in step S2, the capacitance ratio P (for example, 20 dB) corresponding to the determined out-of-band suppression (for example, 20 dB) is determined from the relationship between the out-of-band suppression and the capacitance ratio (for example, a linear relationship) that is known in advance. 0.75) is decided.

Subsequently, in step S3, the series arm resonator satisfying the determined capacitance ratio P and being included in the band-like range defined by the above-mentioned formula 21, formula 23 or formula 25. The values of the electrostatic capacitance Cos and the electrostatic capacitance Cop of the parallel arm resonator are determined. For example, from FIG. 4, P =
At 0.75, it is possible to determine Cop = 1.2 and Cos = 1.6.

After that, in step S4, the aperture length W and the logarithm N of the electrode are determined by using the above equation (19). For example, for parallel arms, from Cop = 1.2, W = 80μ
m, N = 38 is obtained, and Cos = 1.6 for the series arm.
From the above, W = 40 μm and N = 100 are obtained.

On the other hand, in step S5, from the determined center frequency f ₀ and the capacitance ratio P, the above _equations 14 and 16 are obtained.
Alternatively, the resonance frequency difference Δf is determined based on the equation 18. For example, when P = 0.75, from FIG. 1, Δf = 3
0 MHz is obtained.

Next, in step S6, the electrode pitch is determined. At this time, the resonance frequency frs (≈f ₀ ) of the series arm resonator is determined as a value near the center frequency f ₀ , and the resonance frequency frp (= frs of the parallel arm resonator is determined from the determined resonance frequency difference Δf. -Δf) is determined. Then, the electrode pitch of the series arm resonator and the parallel arm resonator, the electrode width, and the space between the electrodes are determined from these resonance frequencies.

According to the above design method, the ripple is 2.0
It is possible to easily design a high-performance surface acoustic wave filter that can suppress the insertion loss to 5.0 dB or less and the VSWR to 2.0 or less at the same time.

The above description of the embodiments is for explaining the present invention, and should not be understood so as to limit the invention described in the claims or to reduce the scope.
Further, it goes without saying that the configuration of each part of the present invention is not limited to the above-described embodiment, and various modifications can be made within the technical scope described in the claims.

[Brief description of drawings]

FIG. 1 shows whether a surface acoustic wave filter using a lithium tantalate substrate has a capacitance ratio P on the horizontal axis and a resonance frequency difference Δf on the vertical axis, and shows whether or not the conditions regarding ripple and insertion loss are appropriate and their limits. It is a graph.

FIG. 2 is a graph of the above regarding a surface acoustic wave filter using a lithium niobate substrate.

FIG. 3 is a graph of the above regarding a surface acoustic wave filter using a lithium tetraborate substrate.

[FIG. 4] In a surface acoustic wave filter using a lithium tantalate substrate, the capacitance Co of the series arm resonator is plotted on the horizontal axis.
s, the vertical axis is the electrostatic capacitance Cop of the parallel arm resonator, and VS
6 is a graph showing suitability of a condition regarding WR and its limit.

FIG. 5 is a graph of the above regarding a surface acoustic wave filter using a lithium niobate substrate.

FIG. 6 is a graph of the above regarding a surface acoustic wave filter using a lithium tetraborate substrate.

FIG. 7 is a flowchart showing a design procedure of a surface acoustic wave filter according to the present invention.

FIG. 8 is a diagram showing a basic configuration of a resonator-coupled surface acoustic wave filter.

FIG. 9 is a diagram showing an electrode configuration of a 1-port resonator.

FIG. 10 is a graph illustrating the relationship between the resonance frequency difference Δf and the ripple and insertion loss.

[Explanation of symbols]

 (1) substrate (2) blind electrode (3) grid reflector (4) series arm (5) parallel arm (6) series arm resonator (7) parallel arm resonator

Front page continuation (72) Inventor Taizo Kobayashi 2-5-5 Keihan Hondori, Moriguchi City, Osaka Sanyo Electric Co., Ltd. (72) Inventor Kosuke Takeuchi 2-5-5 Keihan Hondori, Moriguchi City, Osaka Prefecture Sanyo Electric Co., Ltd. (72) Inventor Kenichi Shibata 2-5-5 Keihan Hondori, Moriguchi City, Osaka Prefecture Sanyo Electric Co., Ltd. (72) Masami Ikeda, 2-5 Keihan Hondori, Moriguchi City, Osaka Prefecture No. 5 in Sanyo Electric Co., Ltd. (72) Inventor Toshiharu Tanaka 2-5-5 Keihan Hondori, Moriguchi City, Osaka Prefecture Sanyo Electric Co., Ltd. (72) Inventor Yasutaka Shimizu 3-1-1 Umeoka, Setagaya-ku, Tokyo No. 10

Claims (6)

[Claims] 1. A surface acoustic wave resonator is connected to a series arm and a parallel arm, and each surface acoustic wave resonator has an electrode for excitation formed on a surface of a substrate made of lithium tantalate.
The propagation direction of the surface acoustic wave is the Euler angle display (40 degrees to 9
(0 degree, 40 degrees to 90 degrees, 0 degrees to 60 degrees) and a surface acoustic wave filter in a range equivalent thereto, the frequencies of the resonance frequency frs of the series arm resonator and the resonance frequency frp of the parallel arm resonator. The difference Δf is characterized in that it is set in a range defined by the following formula 1 with the center frequency f ₀ , the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator as parameters. Surface acoustic wave filter. [Formula 1] Δfmin ≦ Δf ≦ Δfmax where Δf = frs−frp Δfmin = (− 0.0000158 + 0.0116P−0.00
421P ² ) × f ₀ Δfmax = (0.0778-0.0736P + 0.0252
P ² ) × f ₀ P = Cop / Cos 2. The elastic surface according to claim 1, wherein the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator are set within a band-like range defined by the following equation 2. Wave filter. ## EQU00002 ## Cop = -0.49 Cos + (4978 ± 2850) / f ₀ 3. A surface acoustic wave resonator is connected to a series arm and a parallel arm, and each resonator has an electrode for excitation formed on the surface of a substrate made of lithium niobate to propagate surface acoustic waves. In the surface acoustic wave filter whose direction is the Euler angle display (40 ° to 90 °, 40 ° to 90 °, 0 ° to 60 °) and is in the range equivalent to this, parallel to the resonance frequency frs of the series arm resonator. The frequency difference Δf from the resonance frequency frp of the arm resonator is a range defined by the following formula 3 with the center frequency f ₀ , the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator as parameters. A surface acoustic wave filter characterized by being set to. Δfmin ≦ Δf ≦ Δfmax where Δf = frs−frp Δfmin = (0.0262 + 0.0245P−0.000021)
P ² ) × f ₀ Δfmax = (0.303−0.269P + 0.082P ² ) ×
f ₀ P = Cop / Cos 4. The elastic surface according to claim 3, wherein the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator are set within a band-shaped range defined by the following equation 4. Wave filter. ## EQU00004 ## Cop = −0.93 Cos + (3895 ± 1425) / f ₀ 5. A surface acoustic wave resonator is connected to a series arm and a parallel arm, each surface acoustic wave resonator having an electrode for excitation formed on the surface of a substrate made of lithium tetraborate, The direction of wave propagation is indicated by the oiler angle (0 to 50 degrees,
(15 degrees to 75 degrees, 40 degrees to 90 degrees) and a surface acoustic wave filter within the range equivalent thereto, in the resonance frequency frs of the series arm resonator and the resonance frequency f of the parallel arm resonator.
The frequency difference Δf from rp is set within the range defined by the following _{equation 5} with the center frequency f ₀ , the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator as parameters. Surface acoustic wave filter characterized by. Δfmin ≦ Δf ≦ Δfmax where Δf = frs−frp Δfmin = (− 0.00002 + 0.00642P−0.0)
021P ² ) × f ₀ Δfmax = (0.0305-0.0161P + 0.0021)
P ² ) × f ₀ P = Cop / Cos 6. The elastic surface according to claim 5, wherein the capacitance Cos of the series arm resonator and the capacitance Cop of the parallel arm resonator are set within a band-like range defined by the following equation 6. Wave filter. ## EQU00006 ## Cop = -0.47 Cos + (3078 ± 912) / f ₀