JPH029213A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To simplify the manufacture process, to improve the manufacture yield and to reduce the manufacture cost by constituting each comb-line electrode by the 1st area exciting a surface acoustic wave and the 2nd area whose electrode pitch is formed to the size due to the characteristic and adopting the electrode structure having a large bulk mode conversion. CONSTITUTION:The areas opposite to each other in the input side comb-tooth electrode 12 and the output side comb-tooth electrode 13 are constituted as so-called split electrode being nearly a lambda/8 electrode having the electrode pitch of nearly lambda/8 electrode, where lambda is a wavelength of a propagated surface acoustic wave SAW. No acoustic reflection of the SAW is caused in the split electrode as shown in figure (b). Similarly, the reflected wave caused by the electrodes 31-34 from each electrode end face is cancelled together by taking the middle point of the electrode 21 as a reference point even in the electrode structure of equal pitch as shown in figure (c). Thus, no acoustic reflection is caused in both the areas A, B. Then an undesired wave is eliminated without applying a sound absorbing agent to the area B.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Field of Application) The present invention relates to a surface acoustic wave filter, and particularly to a surface acoustic wave filter having an electrode structure for simplifying the manufacturing process.

(Prior Art) Surface acoustic wave filters have been widely used in the field of communication equipment such as radio equipment including receivers for televisions and the like.

FIG. 4 is a diagram showing the structure of the electrodes of such a conventional surface acoustic wave (hereinafter abbreviated as SAW) filter. l
Ta03.1. A pair of input/output comb-shaped electrodes for SAW excitation, for example, an input-side comb-shaped electrode 2 and an output-side comb-shaped electrode 3, are mounted on a piezoelectric substrate 1 made of lNb0 z or the like.
They are formed facing each other in the propagation direction (FIG. 4(a)).

In general, the electrode configuration of the input-side comb-like electrode 2 is such that the interval between the electrode fingers constituting the input-side comb-like electrode 2 is approximately equal to the wavelength λ of the SAW at the center frequency at which the SAW is most strongly excited. λ/8 electrode and approximately λ/8 electrode spacing,
It consists of a so-called split type electrode (Fig. 4 (
b)).

The electrical signal input to the input side comb tooth electrode 2 is converted into a SAW by the input side comb tooth electrode 2, and is propagated to both the left and right sides of the electrode.

Of the excited SAWs, a part of the 5AW 4 that advances in the direction of the output side comb tooth electrode 3, that is, to the right in the figure, is converted into an electric signal again by the output side comb tooth electrode 3.

Also, among the SAWs excited by the input-side comb-shaped electrode 2, 5AW5 proceeds in the opposite direction to the above-mentioned 5AW4, that is, to the left in the figure.
, the output side of the 5AW4 that moves to the right in the figure is not converted into an electrical signal by the toothed electrode 3, and this output side is less than the toothed electrode 3.
The SAW6 that passes through is an unnecessary wave that does not directly affect the characteristics of the SAW filter.

Generally, this unnecessary wave is reflected by the end face of the piezoelectric substrate, returns to the input/output electrode 2.3, and causes deterioration of the characteristics of the filter. Therefore, in such a conventional surface acoustic wave filter, a sound absorbing agent is applied to both sides of the input side comb-like electrode 2 and the output side comb-like electrode 3 to absorb the unnecessary SAW. This prevents characteristic deterioration due to unnecessary waves.

(Problems to be Solved by the Invention) By the way, since the application of the above-mentioned sound absorbing agent is generally performed by screen printing technology using a screen mask, in the conventional manufacturing process of surface acoustic wave filters, <
In addition to the process of forming the toothed electrode, a printing process, such as applying a sound absorbing agent, baking, etc., is required, which complicates the manufacturing process, lowers the manufacturing yield, and increases manufacturing costs. there were.

The present invention has been made to solve the above-mentioned problems, and by creating an electrode structure that can remove unnecessary waves without using sound absorbers, it is possible to simplify the manufacturing process, improve manufacturing yield, and reduce manufacturing costs. The purpose of this invention is to provide a surface acoustic wave filter.

[Structure of the Invention] (Means for Solving the Problem) The surface acoustic wave filter of the present invention includes a piezoelectric substrate, an input comb-shaped electrode and an output comb-shaped electrode formed on the piezoelectric substrate. In the surface acoustic wave filter, each of the comb-shaped electrodes has a first region that excites surface acoustic waves along the propagation direction of the surface acoustic waves, and a first region that does not excite surface acoustic waves and has an electrode period of λ/. 2 (where λ is the wavelength of the surface acoustic wave at the center frequency at which the surface acoustic wave is most strongly excited in the first region).
It is characterized by being composed of the following areas.

In addition, the comb-shaped electrodes in the first region are solid electrodes with an electrode width of approximately λ/4, split electrodes with an electrode width of approximately λ/8, or repeating structures with a width of approximately λ18 and approximately 5/8λ, and the interval between each electrode is approximately λ. /8 may be configured.

Furthermore, the second region may be formed of electrodes having a repeating structure of approximately 2λ15 electrode width and 4λ15 periods.

(Function) Generally, when there is periodic perturbation from an electrode or the like in the SAW propagation path, if the period is different from the wavelength of the SAW, a part of the SAW is converted to the bulk mode, and the intensity of the SAW is Attenuate.

Furthermore, when the perturbation is an electrode material, the thicker the electrode film generally is, the larger the amount of conversion to the bulk mode becomes. By using this property and creating an electrode structure where the area where the sound absorbing material should originally be applied is converted to bulk mode, the application of the sound absorbing material becomes unnecessary, simplifying the manufacturing process, improving manufacturing yield, and reducing manufacturing costs. Obtain a surface acoustic wave filter that enables this.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing the electrode configuration of the surface acoustic wave filter according to the embodiment. On a piezoelectric substrate 11, a pair of comb-shaped electrodes, for example, an input-end comb-shaped electrode 12 and an output-side comb I! 1 electrode 13 is SA
They are formed facing each other in the W propagation direction.

The area indicated by A in the figure of the comb-shaped electrode 12, 13, that is, the area on the side where the input side comb-tooth electrode 12 and the output side comb-tooth electrode 13 face each other is shown in FIG. 1(b). As shown, when the wavelength of the propagating SAW is λ, it is composed of a so-called split type electrode having approximately λ/8 electrodes and an electrode interval of approximately λ/8.

On the other hand, the region of the comb-shaped electrode indicated by B in the figure, that is, the region outside the input-end comb-shaped electrode 12 and the output-side comb-shaped electrode 13, is approximately 3λ/8, as shown in FIG. 1(c). Comb tooth-like electrodes with the same potential and width are arranged at an electrode spacing of 3λ.

Here, it is known that the split electrode shown in FIG. 1(b) above does not cause acoustic reflection of SAW, and similarly, even with the equal pitch electrode structure shown in FIG. 1(C), Each reflected component 1. generated at each electrode 31.32.33.34.
In 2.3.4, when the center of the electrode 21 is taken as a reference point, the reflected waves from each electrode end surface act to cancel each other out, as shown in FIG. Therefore, no acoustic reflection occurs in either area A or area B. However, in the electrode structure of region B, since the electrode period is not in units of λ, a part of the SAW is converted to the bulk mode, and the SAW is attenuated.

Figure 3 shows the Ag film thickness and the electrode in region B when the piezoelectric substrate is 128°Y-XLINb03 and AΩ is used as the electrode material.
FIG. 3 is a diagram showing the relationship with true mode conversion loss. Ag
If the film thickness is h/λ-0.05, the electrode is about 2d.
There is a loss of B. Therefore, the loss is about 10 dB with 5 electrodes.
That is, the SAW is attenuated to about 1/10. Therefore, unnecessary waves can be removed without applying sound absorbing material to region B.

[Effects of the Invention] As explained above, according to the surface acoustic wave filter of the present invention, unnecessary waves in the SAW filter are attenuated by an electrode with a large mode conversion loss, so that the electrode does not need to be coated with a sound absorbing agent. This structure simplifies the SAW filter manufacturing process, improves manufacturing yield, and reduces manufacturing costs.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the electrode configuration of the surface acoustic wave filter of the embodiment, FIG. 2 is a phase diagram of the surface acoustic wave generated at the electrode end surface in region B of the embodiment, and FIG. 3 is the piezoelectric substrate 1: 128' Y
-Xi, 1NbO3, electrode material 1. 4 is a diagram showing the relationship between the Ag film thickness and the mode conversion loss of the electrode in region B in the case of = 1. FIG. 4 is a diagram showing the electrode configuration of a conventional surface acoustic wave filter. 11...Piezoelectric substrate 12...Input end comb electrode 13...
...Output side comb tooth electrode 3], 32.33.34.
...Electrode applicant Toshiba Corporation Patent attorney Satoshi Suyama - Figures 1~1~ (a) (b)

Claims (1)

[Scope of Claims] A surface acoustic wave filter having a piezoelectric substrate, and input comb-shaped electrodes and output comb-shaped electrodes formed on the piezoelectric substrate, each of the comb-shaped electrodes comprising: A first region that excites surface acoustic waves along the propagation direction of surface acoustic waves, and a region that does not excite surface acoustic waves and has an electrode period of λ/2 (where λ is the area where surface acoustic waves are most active in the first region). and a second region formed to be larger than the wavelength of the surface acoustic wave at a center frequency that is strongly excited.