JPS63156412A - Surface wave filtering device - Google Patents

Abstract

PURPOSE:To dissolve the phase turbulence of a surface wave in the weight crossing part of an exciting transducer and to improve a filter characteristic, by forming an inner dummy electrode finger provided oppositely to each tip part of respective exciting electrode finger. CONSTITUTION:In the weight crossing part of the exciting transducer 2, the inner dummy electrode fingers 2d provided oppositely to each tip part of respective exciting electrode finger 2a or 2b in a main lobe A and side lobes B and C are linked and formed on a connection electrode 2c side keeping an interval (d) of regulated value. Since the surface wave S2 generated at the tip part of the exciting electrode finger 2a1 advances crossing the exciting electrodes 2b1 and 2b2, and the dummy electrode fingers 2d1 and 2d2, its wave surface is in line with the surface wave A passing a center part, thereby, the phase turbulence can be dissolved.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a surface wave filter device used in a VXF (video intermediate frequency) filter or the like of a television receiver.

(e) Conventional technology Figure 3 shows the basic structure of the surface wave filter device as mentioned above, in which (1) is a piezoelectric substrate such as LiTaO5JPLiNbos, (2) is its electrode fingers (2a) and (2b). (3) is a weighted excitation transducer in which the intersection part (aperture length) of the electrode fingers (4a) and (4b) are changed sequentially; (51 (51) is composed of the magnetic electrode fingers (5a) arranged at the same pitch as the excitation electrode fingers (2a) and (2b) of the excitation transducer (2) and electrically connected to the shield electrode (3). It's a nine dummy Denmaki.

A surface wave filter device having such a structure is described, for example, in U.S. Pat. No. 3,699,364. ) (C) The desired band selection characteristics are obtained by the shape, but in this case, the outer dummy electrode (51 (51) is provided for each rope (A) to
(CI) This is to align the wavefronts of each surface wave passing inside and outside, respectively, to prevent phase disturbance t1 from occurring.

By the way, the design of the excitation transducer (2) is as follows:
First, excitation electrode fingers [2a) (2b
), then connect the proximal side of each electrode finger to the connecting electrode (
20) so that they are connected one after another. For this reason, the excitation electrode fingers (2a) (2b) do not intersect (non-intersecting part), such as a part of the main rope pronation.
In areas where the length is large, the distance (d) between the connecting electrode (2c) and the tips of the excitation electrode fingers (2a) and (2b) exceeds the specified value. (2
d) are formed in series on the connection electrode (20) side, and the distance (d) between the tips of each dummy electrode finger (2d) and each excitation electrode finger (2a) (2b) is the specified value as described above. This prevents the wavefront from being disturbed at the above location.

However, the length of the non-intersecting part of the excitation electrode fingers (2a, r2b) is relatively short (many parts in the AJ and the length of the excitation electrode fingers (2a, 2b) themselves are originally short). Within the side rope (B) (0), the aforementioned spacing (d)
Since the inner dummy electrode finger (2d) can be set to a specified value, no inner dummy electrode finger (2d) is provided. Therefore, in FIG. 4, which shows such a point enlarged, the surface wave (82) excited at the tip of the excitation electrode finger (2a+) passes over the connection electrode (2c1), As a result, the wavefront is not aligned with the surface wave (Sl) passing through the center, resulting in phase disturbance, which causes deterioration of filter characteristics, such as a decrease in trap attenuation of the filter and waviness of group delay characteristics.

(c) Problems to be Solved by the Invention It is an object of the present invention to eliminate the above-mentioned drawback, that is, the phase disturbance of the surface wave within the weighted intersection of the excitation transducer, and to improve the filter characteristics.

B) Ninth Means for Solving the Problems In the surface wave filter device of the present invention, inner dummy electrode fingers are formed opposite to the tips of each of the excitation electrode fingers in the weighted intersection of the excitation transducer.

(E) Effect According to the above configuration, the surface waves generated at the tips of the excitation electrode fingers pass over the connecting electrodes less frequently, so the speed approaches the speed of the surface waves traveling through the center of the excitation electrode fingers. Therefore, the phase disturbance of the surface waves is reduced.

(f) Embodiment FIG. 1 shows an example of an electrode pattern configuration of a surface wave filter device according to the present invention, and parts corresponding to those in FIG. 3 are given the same figure numbers and will be explained. What differs from Fig. 3 in Fig. 1 is that in the weighted intersection of the excitation transducer (2), the excitation electrode fingers in its main lobe (~ and in the side ropes (B) and (0) (2a) (2b
), the spacing (d
), and inner dummy electrode fingers (2d) facing each other are formed in series on the connection electrode (2C) side as shown in FIG.

That is, in the same figure, the main lobe (A
), and all excitation electrode fingers (2a) in the main lobe (4) and side ropes (B) and (C).
) (2b) is characterized in that an inner dummy electrode finger (2d) is formed in a negative manner.

Therefore, in FIG. 2, which shows an enlarged view of a part of the fc excitation transducer (2d) formed in this way, the intersection part (e) of each excitation electrode finger (2a) (2b) is shown. teeth,
It is the same as the one in Figure 4 (therefore, its basic band characteristics are the same), but its total length (6) is the inner dummy electrode finger (2d
) is twice as long as the length of

In the device of the present invention, as shown in FIG. ) (2d2) Since it travels across the entire area, the wavefront aligns with the surface wave (A) passing through the center, eliminating phase disturbance.

In addition, as shown in Figure 1, inner dummy electrode fingers (2d) are attached to all excitation mfrL pole fingers (2a) r2b) in the main lobe ■ and side ropes (B) and (C) of the excitation transducer (2). Although it is not possible to completely eliminate surface waves passing over the connection electrode (2c), the generation of such surface waves can be sufficiently reduced, thereby improving the filter characteristics.

Needless to say, the present invention can also be applied to the case where the excitation electrode fingers are of the thin λ type (τ 壓) as shown in FIG.

(G) Effects of the Invention As described above, according to the surface wave filter of the present invention, filter characteristics such as a decrease in trap attenuation due to phase disturbance of the surface wave in the weighting pattern of the excitation transducer, and waviness in the group delay characteristic. deterioration can be improved.

[Brief explanation of the drawing]

FIG. 1 shows 1! of the surface wave filter device of the present invention. FIG. 2 is an enlarged view of the main parts of the excitation transformer, FIG. 3 is a plan view showing the electrode pattern configuration of a conventional surface wave filter device, and FIG. 4 is its FIG. 3 is an enlarged view of the main parts of the excitation transducer. (2): Excitation transducer, (2a) r2b)
: Excitation electrode finger, (2d): Inner dummy IE pole finger Figure 2

Claims (2)

[Claims] (1) In a surface wave filter device in which a weighted excitation transducer and a receiving transducer are formed facing each other on a piezoelectric substrate, each tip of an excitation electrode finger in a weighted intersection of the excitation transducer is A surface wave filter device characterized in that inner dummy electrode fingers are formed to face each other. (2) The inner dummy electrode fingers are for each excitation electrode finger in the main rope and side rope of the weighted intersection.
The surface wave filter device according to claim 1, characterized in that the surface wave filter device is formed substantially uniformly.