JPH0630432B2 - Surface acoustic wave device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a surface acoustic wave multimode filter that utilizes acoustic coupling of surface acoustic waves, and more particularly to improving the performance of a multimode filter.

[Conventional technology]

Conventionally, various configurations of multi-mode filters utilizing acoustic coupling of surface acoustic waves have been proposed.
Tiersten et al. Reported in 1975, IEEE Ultrasonics Symposium, pages 293-294. The bandwidth of this type of filter (FIG. 2) is determined by the width W of the resonator and the distance D between the resonators.

[Problems to be solved by the invention]

Since the above parameters W and D determine not only the ratio band but also the input output impedance, there is a drawback that the degree of freedom is extremely low in reality.

[Means for Solving Problems] In order to solve the above-mentioned drawbacks, the surface acoustic wave device of the present invention is
An input side interdigital electrode provided on the piezoelectric substrate for exciting the surface acoustic wave, a first dummy electrode arranged on the surface acoustic wave propagation path, and laterally coupled to the surface acoustic wave propagation path. In the direction perpendicular to the propagation path and adjacent to the first dummy electrode, and the output-side interdigital electrode that receives the surface acoustic wave obtained by the lateral coupling and the lateral coupling. The second dummy electrode is arranged on the propagation path of the surface acoustic wave and adjacent to the input side interdigital electrode.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

First, the operating principle of the present invention will be described.

The interdigital electrode (hereinafter abbreviated as IDT) has an electrode finger width of λ / 4.
In case of a single electrode, the ratio of the acoustic impedances Z _m and Z ₀ of the electrode part and the part without electrode If the product Nε of the impedance discontinuity count ε and the number N of electrode fingers obtained by is large, for example, Nε> 0.55, the internal reflection generated at each electrode finger cannot be ignored and the IDT itself may become a resonator. Are known. This can be seen, for example, in Y. Koyamada et al., "Narrow-band filters e
mploying surface-acorstic wave resonators "(Narrow Band Filters Employing Surface-Acoustic Wave Resonators) Pro
c.of IEEE, vol.64, NO.5, pp685-687.

When two IDTs that operate as the resonator are arranged so as to face each other in the propagation direction of the surface acoustic wave, the two IDTs are coupled to each other and longitudinal mode resonance occurs, which can be used as a filter or a resonator. This is due to Yamada et al., “Many-to-IDT”.
Two-Terminal Pair Surface Acoustic Wave Resonator Using a Magnetic Field, Journal of Acoustical Society of Japan
33, No. 10 (1977), pp 557-564.

This configuration is shown in FIG. Although this diagram shows the most basic SAW filter configuration, it can be used as a low-loss resonator type filter by operating as a resonator by setting Nε to 0.55 or more.

Similarly, when the two IDTs are arranged close to each other in the lateral direction perpendicular to the propagation direction of the surface acoustic wave, acoustic lateral coupling occurs between the propagation paths of the two IDTs. This is a well-known property in microwave strip lines and optical waveguides. Here, when Nε of these IDTs is, for example, 0.55 or more, the IDT operates as a resonator, and the transverse mode coupling between the two IDTs results in a resonator having two IDTs as input and output. FIG. 2 shows this configuration. In FIG. 2, there are reflectors 22 and 23, which are ID
This is to contain the energy leaking from T and strengthen the resonance of the IDT itself.

The present invention aims to improve the performance of a surface acoustic wave filter by simultaneously using the longitudinal mode and the transverse mode described above.

FIG. 1 shows the basic configuration of the present invention. Two IDTs 12,
Beside 14 are arranged dummy electrodes 13 and 15 having the same structure as the IDT except that the electrode fingers of the IDT are electrically short-circuited. The dummy electrodes 13 and 15 are the other IDT1
It faces 4,12. In such a structure, for example, the surface acoustic wave excited by the IDT 12 repeats internal reflection in the IDT and is propagated through the grading waveguide on the upper side in FIG. 1, which is composed of the IDT 12 and the dummy electrode 15. , IDT 14 and dummy electrode 13 are coupled laterally to the grating waveguide on the lower side, and energy flows to IDT 14. Here, since the configuration of FIG. 1 produces the transverse mode configuration of FIG. 2 while maintaining the longitudinal mode configuration of FIG. 3, it is possible to simultaneously use the longitudinal mode and the transverse mode, which could only be used conventionally. As a result, it becomes possible to realize a high-performance filter with a wider band than ever before by combining multiple modes of vertical mode and horizontal mode.

〔The invention's effect〕

As described above, according to the present invention, the degree of freedom in filter design is increased and the band can be increased.

[Brief description of drawings]

FIG. 1 is a plan view showing an embodiment of the present invention, and FIGS. 2 and 3 are plan views showing a conventional surface acoustic wave filter. 11, 21, 31 ... Piezoelectric substrate, 12, 14, 24, 2
5, 32, 33 ... Interdigitated electrodes, 13, 15 ... Dummy electrodes, 22, 23 ... Grating electrodes.

Claims (1)

[Claims] 1. An input side interdigital electrode provided on a piezoelectric substrate for exciting a surface acoustic wave, a first dummy electrode arranged on a propagation path of the surface acoustic wave, and a propagation path of the surface acoustic wave. An output-side crossed finger-shaped electrode that is disposed in a direction perpendicular to the propagation path so as to be laterally coupled to and adjacent to the first dummy electrode, and that receives the surface acoustic wave obtained by the lateral coupling. A surface acoustic wave device comprising a second dummy electrode arranged on a propagation path of the surface acoustic wave obtained by coupling and adjacent to the input side interdigital electrode.