JPH0317461Y2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a surface acoustic wave resonator used as an oscillator of an oscillator in various communication devices.

Conventional technology Surface acoustic wave resonator (hereinafter referred to as SAW resonator)
utilizes surface acoustic waves due to the piezoelectric effect of a piezoelectric substrate such as crystal, and is used as a SAW resonator for VTR-RF modulators because it provides stable fundamental wave direct oscillation with little subresonance in the VHF to UHF band. ing. This SAW resonator has an electrode called IDT that generates surface acoustic waves on a piezoelectric substrate, and an electrode that reflects this surface acoustic wave.Due to the electrode structure, the SAW resonator has many pairs of electrodes.
A multi-pair IDT type resonator in which the IDTs are arranged horizontally in a row and the IDTs on both sides have a surface wave reflection function, and the IDT in the center.
It is broadly classified into reflector-type resonators, which have reflective electrodes on both sides.

For example, a basic example of a reflector-type SAW resonator will be explained with reference to FIGS. The IDT is formed in a pattern in which a pair of comb-shaped electrodes 1a and 1b having comb teeth of a constant pitch λ are interlocked with each other. 3,
3 is a reflector for surface waves excited by IDT2,
This is a diffraction grating consisting of grating electrodes 4, 4 formed on the piezoelectric substrate 1 on both sides of the IDT 2 in a blind pattern perpendicular to the propagation direction of the surface waves.
The IDT 2 and grating electrodes 4, 4 are formed by Al vapor deposition.

When an AC voltage is applied to the IDT2 of this SAW resonator, distortion with opposite phases occurs on the substrate surface between adjacent electrodes due to the piezoelectric effect, and a surface wave is excited.
This surface wave propagates on the piezoelectric substrate 1 and is divided into a reflected wave and a transmitted wave each time it reaches each grating electrode 4, 4. Therefore, each grating electrode 4,4
If the spacing between the grating electrodes 4, 4 is determined so that the phases of the reflected waves from the gratings are aligned, a standing wave will be excited between the reflectors 3, 3 and resonance will occur, as shown by the broken line in Figure 5. Ru. The resonance frequency at this time is determined by the wavelength determined by the electrode spacing λ of the IDT 2 and the surface wave propagation velocity determined by the material of the piezoelectric substrate 1.

Problems that the invention aims to solve By the way, SAW resonators apply an AC voltage to the IDT to excite a standing wave. Ideally, this standing wave is a so-called single mode, but in reality it is a constant transverse mode. It is a multi-mode system where existing waves occur. Therefore, if the operation of an oscillation circuit incorporating a SAW resonator is unstable, the desired oscillation frequency of the SAW resonator may change to a different frequency due to a transverse mode standing wave.

Since the transverse mode that deteriorates the stability of the oscillation of the SAW resonator occurs at a location laterally shifted from the central axis l of the resonator, in order to suppress the oscillation of this transverse mode, for example, conventional The method is to weight the IDT (apodise) or to reduce the IDT intersection width. The weighting of the former IDT is
Transverse mode oscillation is suppressed by setting lengths and shortenings in the widths of the many interlocking portions of a pair of IDT comb-shaped electrodes, but this reduces the interlocking portions of the IDT electrodes and reduces the capacitance between the IDT electrodes. This reduction in capacitance requires an increase in the number of interlocking electrodes to compensate for the reduction in capacitance, resulting in the problem of an increase in the size of the entire resonator. In addition, reducing the latter IDT crossing width means reducing the width of the interlocking portion as a whole, and in this case as well, the capacitance between the IDT electrodes decreases, so it is necessary to increase the number of interlocking electrodes of the IDT, and the overall width decreases. There was a problem that was getting bigger.

Therefore, the purpose of the present invention is to provide a SAW resonator with good oscillation stability without increasing the number of electrodes of the IDT.

Means for Solving the Problems Therefore, in order to achieve the above object, the present invention forms surface acoustic wave reflecting electrodes on a piezoelectric substrate in a convex substantially circular arc pattern orthogonal to each other at the center thereof in the surface acoustic wave propagation direction. It is characterized by being formed by.

Effect When the surface wave reflecting electrode is formed into a convex, substantially arc shape facing the surface wave propagation direction as in the present invention, this electrode has a high reflectance only in the central part perpendicular to the surface wave, and the electrode has a high reflectance. The standing wave of the fundamental mode due to the reflected wave at the center of the lever is most effectively excited, and the oscillation of the transverse mode is suppressed. In addition, there is no need to change the design of the surface wave excitation electrode (IDT).
The resonator does not become large.

Embodiments Hereinafter, embodiments of the present invention applied to, for example, a reflective SAW resonator will be explained based on FIGS. 1, 2, and 3.

1 and 2, 5 is a piezoelectric substrate such as crystal, 6 is an IDT formed in the center of the piezoelectric substrate 5,
7, 7 are grating electrodes formed on the piezoelectric substrate 6 on both sides of the IDT 6 for reflecting surface waves excited by the IDT 6. Figure 4 of this example
The only difference from the SAW resonator is the shape of the two grating electrodes 7, 7, which have a blind-shaped part.
It is formed in a substantially arc-shaped pattern convex toward the propagation direction of the surface waves excited by the IDT 6. For example, a plurality of blind portions of each grating electrode 7, 7 are made into an arc shape with the same curvature, width, and length, and the center line l is
Align it with the center line l of the IDT 6 and point its apex (center point) toward the IDT 6.

Now, when an AC voltage is applied to the IDT 6 to excite a surface wave, the surface wave reaches the grating electrodes 7, 7 and is reflected, but this reflection occurs at the center of the grating electrodes 7, 7, which is orthogonal to the surface wave. The standing wave caused by the reflected wave in this part becomes the fundamental mode standing wave and resonance occurs. Also
The surface waves from the IDT 6 are generated by the grating electrodes 7, 7.
However, as the grating electrodes 7 move away from the center, the angle of inclination of the grating electrodes 7, 7 increases with respect to the surface wave, and the reflectance deteriorates, so the resonance of the transverse mode naturally occurs. As a result, the SAW resonator resonates close to a single mode, and the resonant frequency becomes stable. The radius of curvature r of the grating electrodes 7, 7 that exhibits such a function must be such that r≧W/2, where W is the width of the grating electrodes 7, 7.

FIG. 3 shows an application example of the above embodiment, in which an IDT 9 and grating electrodes 10a and 10b are arranged on both sides of the IDT 9 on a piezoelectric substrate 8. The difference from the above embodiment is that one grating electrode By making only the grating electrode 10a arc-shaped, the other grating electrode 10b is formed in a comb-like pattern perpendicular to the surface wave propagation direction, as in the conventional case. In this embodiment as well, transverse mode oscillation is suppressed by the arcuate grating electrode 10a as in the above embodiment.

Note that this invention is not limited to reflector-type SAW resonators;
The same can be applied to a multi-pair IDT type SAW resonator. Further, the shape of the surface wave reflecting electrode is not limited to a circular arc pattern, but may be a parabolic pattern, a pattern in which a plurality of electrodes are arranged concentrically, or the like.

Effects of the invention According to the invention, transverse mode resonance can be suppressed without increasing the number of surface wave excitation electrodes, and the resonant frequency can be stabilized, resulting in a compact and highly reliable SAW.
Resonators can be provided.

[Brief explanation of the drawing]

1 and 2 are a plan view and a sectional view taken along the line A--A of one embodiment of the present invention, and FIG. 3 is a plan view of another embodiment of the present invention. 4 and 5 are a plan view and a sectional view taken along the line B--B of a conventional SAW resonator. 5... Piezoelectric substrate, 7, 7... Electrode for surface wave reflection, 8... Piezoelectric substrate, 10a, 10b... Electrode for surface wave reflection.

Claims (1)

[Scope of utility model registration request] 1. A surface acoustic wave resonator, characterized in that surface acoustic wave reflecting electrodes on a piezoelectric substrate are formed in a substantially arcuate convex pattern orthogonal to each other at the center in the direction of surface acoustic wave propagation.