JP3291912B2 - Surface acoustic wave device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave device used for communication equipment and the like.

[0002]

2. Description of the Related Art FIG. 3A shows a conventional resonator type surface acoustic wave device. In FIG. 3A, 1 is a piezoelectric substrate, 2 is a comb-shaped electrode, and 3 is a reflector. That is, the surface acoustic wave excited by the comb-shaped electrode 2 propagates to both sides and is confined between the reflectors 3 to function as a resonator or a filter.

[0003]

In the conventional resonator-type surface acoustic wave device, the reflector 3 includes a strip electrode having a constant line width of about one quarter of the wavelength corresponding to the resonance frequency.
The reflectors 3 are arranged at a constant pitch of about one half of the wavelength, and the frequency characteristics of the reflector 3 itself have large spurs on both sides of the pass band as shown in FIG. Spurious characteristics also appeared in the characteristics, and the out-of-band characteristics deteriorated.

It is an object of the present invention to prevent the deterioration of the out-of-band characteristics.

[0005]

According to the present invention, the cause of the occurrence of the spurious is pursued and studied. The cause of the occurrence of the spurious is that the reflectance of each strip electrode of the reflector is uniform. It has been found that the response outside the band increases, and in order to achieve the above object, in the surface acoustic wave device of the present invention, the reflectivity of each of the strip electrodes constituting the reflector differs depending on the location. It is.

[0006]

With this configuration, the comb electrodes are weighted so as to apply apodization, the spurious response outside the band is suppressed by the frequency characteristics of the reflector itself, and the elastic surface having excellent out-band characteristics. A wave device is obtained.

[0007]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1A is a schematic diagram showing a resonator type surface acoustic wave filter according to a first embodiment of the present invention. In FIG. 1A, quartz is used for the piezoelectric substrate 1,
The input and output comb electrodes 2 on the surface are 100
It is composed of pairs, and is apodized to suppress spurious due to the transverse mode. On the piezoelectric substrate 1 on both sides of the comb-shaped electrode 2, each of the reflectors 3 is composed of 200 short strip electrodes 3a. When the reflector 3 is positioned on the horizontal axis and the reflectance per strip electrode on the vertical axis as shown in FIG. 1B, the reflector 3 is configured to have a Hamming window function symmetrically about the center. By weighting the reflectivity in this manner, the frequency characteristic of the reflector 3 itself becomes as shown in FIG. 1C, and the characteristic in which the out-of-band is suppressed as compared with the conventional reflector having the same line width and the same pitch. Is obtained.
As a method of changing the reflectivity, first, a method of changing the line width of the strip electrode is used. Usually, in the reflector 3, the line width of the strip electrode is one quarter of the wavelength corresponding to the resonance frequency.
In the case of, the reflectance is almost maximum, and as the line width is reduced, the reflectance gradually decreases, and becomes nearly zero when the ratio is 1/8. Therefore, if the line width is set according to the required reflectance, a predetermined frequency characteristic can be obtained. FIG.
As shown in (d), the position of the strip electrode 3a is paired with two adjacent ones, and the position is shifted so as to widen or narrow the distance between the two without changing the average position of the conventional strip electrode 3A. Thus, the reflectance can be reduced without changing the resonance frequency. Further, when a piezoelectric substrate 1 having a small electromechanical coupling coefficient such as quartz is used, the reflectance is almost the same regardless of whether it is an open strip electrode or a short strip electrode, but lithium tantalate or niobate is used. When a piezoelectric substrate having a large electromechanical coupling coefficient, such as lithium, is used, the reflectivity differs between the short strip electrode and the open strip electrode, which can be used. By using the above methods alone or in combination, a resonator-type surface acoustic wave device having excellent out-of-band characteristics can be obtained.

FIG. 2A is a schematic view showing a surface acoustic wave resonator according to a second embodiment of the present invention. The basic configuration is the same as that of the first embodiment, but here, the distribution of the reflectance is different. That is, as shown in FIG. 2B, the reflectance increases as the distance from the comb-shaped electrode 2 increases. In general, it is desirable that the Q value of the resonator be higher except for a special case. In the conventional surface acoustic wave resonator, in order to increase the Q value, the thickness of the electrode is reduced to reduce the reflectance per strip electrode. By doing so, the resonance length can be increased, and the Q value increases. However, in this case, more reflective electrodes are required to sufficiently reduce the resonance resistance, resulting in an increase in the size of the device and an increase in cost. On the other hand, in this embodiment, the reflectance is small near the comb-shaped electrode 2 and becomes larger as the distance increases, so that the reflection center of the reflector 3 shifts outward from the conventional surface acoustic wave resonator, and the resonance length becomes longer. Since the length is longer, the Q value increases. Further, as in the first embodiment, the spurious is suppressed, so that the oscillation frequency does not easily jump.

[0009]

As described above, the present invention provides a surface acoustic wave device having excellent out-of-band characteristics or a surface acoustic wave resonance having a high Q value by changing the reflectivity of the strip electrode constituting the reflector depending on the location. You can get a child.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a resonator type surface acoustic wave filter according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram of a surface acoustic wave resonator according to a second embodiment of the present invention.

FIG. 3A is a plan view of a conventional resonator-type surface acoustic wave filter, and FIG.

[Explanation of symbols]

 Reference Signs List 1 piezoelectric substrate 2 comb-shaped electrode 3 reflector

Continuation of the front page (56) References JP-A-62-256512 (JP, A) JP-A-57-204620 (JP, A) JP-A-54-87457 (JP, A) JP-A-54-100238 (JP) JP-A-3-278606 (JP, A) JP-A-64-23131 (JP, U) JP-A-7-505989 (JP, A) International Publication 93/22831 (WO, A1) (58) Survey Field (Int.Cl. ⁷ , DB name) H03H 9/25 H03H 9/145

Claims (1)

(57) [Claims] With a 1. A piezoelectric substrate, a comb electrode provided on the piezoelectric substrate, and reflectors consisting of a large number of strip electrodes provided on both sides of the piezoelectric substrate of this comb-shaped electrodes, two of the strip electrode As a pair, the two flats
By changing the position of each other without changing the average position
A surface acoustic wave device, wherein the reflectance per one of the strip electrodes is changed .