JPH04298112A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To realize a surface acoustic wave filter having an excellent filter characteristic by solving a problem of generation of a spurious signal due to a disturbed exciting intensity distortion resulting into deteriorating the filter characteristic in the surface acoustic wave filter used for a digital automobile telephone set filter or a timing extraction filter or the like. CONSTITUTION:The upper side of a non-crossing part 1ax of an interdigital electrode finger 1a and a non-crossing part 2ax of a reflector 2a present on an opposite side in the propagation direction of a surface acoustic wave from the center of an apodized electrode (a) in two sets of interdigital electrodes (a), (b), formed on a piezoelectric substrate 4 is covered by a sound absorption member 6. Thus, the excellent filter characteristic with a low loss, a large attenuation in the lobe, and a flat group delay characteristic over a wide range is realized.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter used in digital car telephone filters, timing extraction filters, and the like.

0002

2. Description of the Related Art In recent years, digitalization in the information and communication field has progressed, and a filter with a relatively narrow band and low loss is required for system configuration. However, in a transversal surface acoustic wave filter, the smaller the specific band, the larger the chip size becomes. Therefore, using a three-electrode configuration to realize a low-loss filter is extremely disadvantageous in terms of component size and cost, and it is desirable to use a two-electrode configuration using a unidirectional transducer.

[0003] Therefore, it is considered an effective means to use a single-phase unidirectional electrode having reflectors between the intersecting electrode fingers of the intersecting electrodes. Conventionally, this type of surface acoustic wave filter is shown in Fig. 3.
The configuration was as shown in . In the figure, a1 is a regular type electrode on the input side, b is a regular type electrode on the output side, and the regular type electrode a1 on the input side and the regular type electrode b on the output side are a number of crossed electrode fingers 1a1 and 1b, and a reflector, respectively. 2a1, 2b and extraction electrodes 3a1, 3b, the piezoelectric substrate 4
formed on top. Reference numeral 5 represents an absorbing material, which is provided at both ends of the piezoelectric substrate 4.

Next, its operation will be explained. The extraction electrode 3a1 of the regular type electrode a1 on the input side and the extraction electrode 3b of the regular type electrode b on the output side are connected to external electrodes for input or output (not shown) by wire bonding or the like, respectively, to input signals. Output is done. The signal inputted from the extraction electrode 3a1 is converted from an electrical signal to a mechanical signal by the intersecting electrode fingers 1a1 of the regular type electrode a1 on the input side formed on the piezoelectric substrate 4. The excited surface wave is given directionality by the reflector 2a1, propagates as shown by the arrow in FIG. It is converted and output from the extraction electrode 3b to the external electrode. Note that the absorbing material 5 is coated on both ends of the piezoelectric substrate 4 in order to prevent the adverse effects of surface waves generated during such operations.

[0005]

[Problem to be Solved by the Invention] However, as shown in FIG. 3, the intersecting electrodes in which the intersecting lengths of the intersecting electrode fingers 1a1 and 1b of the regular type electrode a1 on the input side and the regular type electrode b on the output side are a constant length. When using this filter, the obtained filter characteristic has a sidelobe attenuation of 26 to 33 dB at most, and cannot be used for applications requiring attenuation of 33 dB or more. Therefore, as shown in FIG. 4, it is conceivable to use one of the intersecting electrodes as an apodized electrode a2 to ensure attenuation, but the reflector 2a on the apodized electrode a2 side
If the reflector 2a2 is constructed with the same constant length as the conventional one without apodizing, the excitation intensity distribution will be disturbed by the reflector 2a2, creating a shoulder in the frequency characteristic as shown in Fig. 5, and the group delay characteristic will also be disturbed. There is the issue of putting it away.

[0006] The present invention is intended to solve these problems, and an object of the present invention is to provide a surface acoustic wave filter having excellent filter characteristics.

[0007]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides an intersecting electrode that exists on the opposite side of the propagation direction of a surface wave from the center of the intersecting electrode fingers and reflector constituting an apodized electrode. It has a structure in which the unintersected portions of the electrode fingers and the unintersected portion reflectors in the area of the unintersected electrode fingers are covered with a sound absorbing material.

[0008]

[Operation] Therefore, according to the present invention, among the crossed electrode fingers and the reflector constituting the apodized electrode, the uncrossed portions of the crossed electrode fingers that exist on the opposite side of the surface wave propagation direction from the center and the uncrossed portions Since the unintersected reflectors in the intersection area are covered with sound absorbing material, the distribution of excitation intensity is not disturbed.
It becomes possible to exhibit the original characteristics of the apodized electrode, and it is possible to obtain flat filter characteristics with low loss and large sidelobe attenuation, and a wide range of group delay characteristics.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the same reference numerals to the same parts as in FIGS. 1 and 2 as well as in FIGS. 3 and 4, detailed explanation thereof will be omitted, and differences will be explained.

FIG. 1 is a plan view showing the configuration of an embodiment of the present invention. In the figure, a is an apodized electrode on the input side, 1a is an intersecting electrode finger, and 1ax is an unintersecting part of the intersecting electrode finger 1a. , 2a is a reflector, 2ax is an unintersected reflector in the unintersected region of the crossed electrode fingers 1a, 3a is an extraction electrode,
Reference numeral 6 denotes a sound absorbing material made of epoxy resin, acrylic resin, silicone resin, etc., which covers the uncrossed portion 1ax of the crossed electrode fingers 1a and the uncrossed portion reflector 2ax.

Next, its operation will be explained. A signal inputted from the extraction electrode 3a of the input side apodized electrode a formed on the piezoelectric substrate 4 is converted from an electrical signal to a mechanical signal by the crossed electrode fingers 1a. The excited surface wave is given directionality by the reflector 2a, propagates as shown by the arrow in FIG.
b converts the mechanical signal into an electrical signal again, and outputs it from the extraction electrode 3b to an external electrode (not shown).

In the above operation, the disturbance in the excitation intensity distribution caused by the uncrossed part reflector 2ax in the area of the uncrossed part 1ax of the crossed electrode fingers 1a is caused by the sound absorbing material 6 coated on the uncrossed part reflector 2ax. This can be prevented by removing unnecessary reflections.

According to the embodiment described above, the upper surfaces of the unintersected portions 1ax of the crossed electrode fingers 1a of the input side apodized electrode a and the unintersected portions of the reflector 2ax of the reflector 2a are made of epoxy resin, acrylic resin, etc. Sound absorbing material 6 made of silicone resin etc.
Because the surface wave is covered by can be obtained.

In the above embodiment, the apodized electrode a is used as the input side and the regular electrode b is used as the output side. It is also possible to use

[0015]

Effects of the Invention As is clear from the above embodiments, the present invention provides that one set of electrodes of two sets of intersecting electrodes each having a built-in reflector formed on a piezoelectric substrate is an apodized type electrode. By covering the uncrossed parts of the crossed electrode fingers and the upper surface of the uncrossed reflector with sound absorbing material, it is an excellent filter with low loss, large sidelobe attenuation, and flat group delay characteristics over a wide range. properties can be obtained.

[Brief explanation of drawings]

FIG. 1 is a plan view showing the structure of a surface acoustic wave filter in an embodiment of the present invention.

[Figure 2] Filter characteristic diagram of the same surface acoustic wave filter

[Figure 3
] Plan view showing the structure of a conventional surface acoustic wave filter

[Figure 4
] Plan view showing the structure of another conventional surface acoustic wave filter

[Figure 5] Filter characteristic diagram of conventional surface acoustic wave filter

[Explanation of symbols]

a Apodized electrode (crossed electrode) b Regular type electrode (crossed electrode) 1a, 1b Cross electrode fingers 1ax　Uncrossed part of crossed electrode fingers 2a, 2b Reflector 2ax Unintersected reflector 3a, 3b Extraction electrode 4 Piezoelectric substrate 6 Sound absorbing material

Claims (1)

[Claims] Claim 1: Two sets of crossed electrodes are provided on a piezoelectric substrate,
One set of intersecting electrodes is a regular type electrode in which the intersecting length of the intersecting electrode fingers is uniform, and the other set of intersecting electrodes is an apodized electrode in which the intersecting length of the intersecting electrode fingers is varied in the arrangement direction of the intersecting electrode fingers. type electrode, both the regular type electrode and the apodized type electrode have a plurality of reflectors between their crossed electrode fingers, and the reflector length of the reflector on the apodized type electrode side is the maximum of the crossed electrode fingers of the apodized type electrode. The uncrossed part of the crossed electrode fingers that is equal to or longer than the maximum crossing length and is uniform and is located on the opposite side of the surface wave propagation direction from the center of the apodized electrode and the uncrossed part area. A surface acoustic wave filter characterized in that a certain non-intersecting reflector is covered with a sound absorbing material.