JPS6126246B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface wave element such as a surface wave filter that utilizes surface waves.

This type of surface wave element, as shown in FIG.
Input electrodes 2 and output electrodes 3 each having a pair of comb-teeth electrodes arranged opposite to each other are adhered to each other.
A ground electrode 4 is arranged between the electrodes and 3.

In this type of surface wave element, the opposing width W of each pair of comb-shaped electrodes of electrodes 2 and 3 is made as large as possible, so that the surface waves excited and generated by the input electrode 2 are directly transmitted to the output electrode 3. The energy of the effective surface waves directed toward is increased, and the influence of unnecessary waves due to reflection of unnecessary surface waves that spread in other directions due to diffraction is minimized.

However, when the opposing width W of the comb-teeth electrodes is increased in this way, it is especially important to use piezoelectric ceramics with a high dielectric constant ε of around 1000, such as PZT (solid solution of lead titanate and lead zirconate)-based piezoelectric ceramics. In a surface wave element using a surface wave propagation medium having , the impedance is low and the load on the external circuit becomes heavy.

Therefore, in order to increase the impedance of a surface acoustic wave element using such piezoelectric ceramic, it is desired that the opposing width W of the comb-teeth electrodes be made as small as possible. Now, using piezoelectric ceramics
Regarding the 10.7MHz surface wave filter,
In this case, if the opposing width W of the comb-teeth electrodes is 1.5 mm or more, the energy of the surface waves propagating through the original propagation path is large, so the influence of unnecessary surface waves will hardly be a problem, but this width should be set to 1.0 mm or less. In this case, the influence of unnecessary surface waves becomes large. This is because the surface waves spread by diffraction reach the mismatched areas of acoustic impedance, that is, the edges of each electrode 2, 3, and 4, especially in the direction of surface wave propagation (in this specification, the surface wave propagation direction is the original surface wave direction). Reflection at both outer edges 2a, 2b, 3a, 3b and 4a, 4a extending along the direction in which the electrodes 2 and 3 should be propagated has a particularly large influence. There is.

In the present invention, as described above, the device is arranged on the surface wave propagation medium at predetermined intervals in the propagation direction of the surface wave, extends along a direction perpendicular to the propagation direction of the surface wave, and has a predetermined length. In a surface wave element comprising a pair of comb-teeth electrodes arranged alternately so as to face each other across the surface, a tip of each comb-teeth portion of each electrode;
The distance d between each electrode and each outer edge of the other electrode is selected to be at least three times the wavelength of the surface wave.

In Figures 2 to 8, the surface wave velocity is 2300, respectively.
Using the base 1 made of piezoelectric ceramic of m/sec
It shows the characteristics (each frequency - attenuation gain) of a 10.7 MHz (wavelength 215 μm) surface wave filter. Figure 2 shows the opposing width W when the opposing width W is 2.5 mm and the distance d is 550 μm, and Figure 3 shows the opposing width W. When is 1.7mm and distance d is 550μm, Fig. 4 shows that opposing width W is 1.5mm and distance d is 550μm.
In the case of Fig. 5, the opposing width W is 1.0 mm and the distance d is 550 mm.
In the case of μm, Figure 6 shows that the facing width W is 0.7 mm and the distance d
Fig. 7 shows the characteristics when W is 550 μm.
is 1.0 mm, distance d is 600 μm (approximately 2.8 of the surface wave wavelength)
8), the facing width W is 0.7 mm and the distance d
is 750 μm (approximately 3.5 times the surface wave wavelength).As is clear from these characteristics, when the opposing width W is 1 mm or less, the 5th
Diffraction and
Due to the influence of reflection, the characteristics according to theory could not be obtained, and the opposing width W was set to 1 mm as shown in Figures 7 and 8.
Even if the distance d is less than 2.8 of one wavelength of the surface wave,
It can be seen that when the value is set to 3 times or more, and 3 times or more is selected in consideration of reliability, the influence of unnecessary waves is removed.

Incidentally, since the inner edges 2c, 2d and 3c, 3d of each comb-teeth electrode of each electrode 2 and 3 have a zigzag pattern due to the comb-teeth, unnecessary waves are not generated at these edges. Since it is scattered, there is almost no effect from this.

As described above, in the present invention, the distance d from the effective part of the electrode, that is, the opposing part of the comb-shaped electrode, to the outer edge of the surface wave element is set to be three times or more the wavelength of the surface wave to be propagated. At this time, the influence of unnecessary waves can be avoided, but if necessary, in order to reliably eliminate the influence of unnecessary waves, a surface wave propagation medium, that is, an electrode coated on a piezoelectric substrate, is used. It is also possible to provide surface wave absorption or scattering means on the outer edge extending substantially along the surface wave propagation direction.

[Brief explanation of the drawing]

FIG. 1 is a schematic top view of a surface wave element for explaining the present invention, and FIGS. 2 to 8 are frequency-attenuation gain curves for explaining the invention. 1 is a piezoelectric substrate as a surface wave propagation medium, 2 is an input electrode, 2a and 2b are its outer edges, 2c and 2
d is its inner edge, 3 is its output electrode, 3a and 3b are its outer edges, 3c and 3d are its inner edges, 4 is a ground electrode, and 4a and 4b are its outer edges.

Claims (1)

[Claims] 1 Disposed on the surface wave propagation medium at a predetermined interval in the propagation direction of the surface wave, extending along a direction perpendicular to the propagation direction of the surface wave and facing each other over a predetermined length. In a surface wave element having pairs of comb-teeth electrodes arranged alternately, the distance d between the tip of each comb-teeth portion of each electrode and each outer edge of the other electrode is defined as the wavelength of the surface wave. Surface wave element selected to be more than 3 times more efficient.