JPH04284013A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To solve the problem of lowering a filter characteristic due to the distortion of exciting strength distribution and to offer the surface acoustic wave filter having the satisfactory filter characteristic, at the surface acoustic wave filter used for a filter for digital car telephone or a filter for timing extraction. CONSTITUTION:The two couples of a pair of interlaced electrodes are provided on a piezoelectric substrate 4, a pair of the interlaced electrodes are taken as uniform normal type electrodes (b), the other pair of interlaced electrodes are taken as apodization type electrode (a) changing the interlaced length in respect to the direction of arranging interlaced electrodes 1a, plural reflectors 2b and 2a are provided between the normal type electrode (b) and the apodization type electrode (a) and for the reflector 2a on the side of the electrode (a), the length is changed corresponding to the change of the interlaced length of the interlaced electrode 1a. Therefore, the satisfactory filter characteristics lowering a loss, enlarging the attenuation amount of a side lobe, enlarging the range of a group delay characteristic and making it flat can be obtained.

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 provided with a reflector between the intersecting electrode fingers of the intersecting electrodes. Conventionally, this type of surface acoustic wave filter is shown in Fig. 4.
The configuration shown in is common. In the figure, a2
is a regular type electrode on the input side, b is a regular type electrode on the output side, and the regular type electrode a2 on the input side and the regular type electrode b on the output side each have a large number of crossed electrode fingers 1a2, 1b, reflectors 2a2,
2b and extraction electrodes 3a2 and 3b, which are formed on the piezoelectric substrate 4. Reference numeral 5 denotes an absorbing material, which is provided at both ends of the piezoelectric substrate 4.

Next, its operation will be explained. The extraction electrode 3a2 of the regular type electrode a2 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. Extracting electrode 3a
The signal inputted from the piezoelectric substrate 4 is converted from an electrical signal to a mechanical signal by the intersecting electrode fingers 1a2 of the regular type electrode a2 on the input side formed on the piezoelectric substrate 4. The excited surface waves are reflected by the reflector 2a.
2 gives directionality and propagates as shown by the arrow in FIG.
b, the mechanical signal is again converted into an electrical signal, and is 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, the intersecting electrodes in which the intersecting length of the intersecting electrode fingers 1a2 and 1b of the regular type electrode a2 on the input side and the regular type electrode b on the output side as shown in FIG. 4 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. 5, it is possible to use one of the intersecting electrodes as an apodized electrode a3 to ensure attenuation, but the reflector 2 on the apodized electrode a3 side
If a3 is configured with the same constant length as the conventional one without apodizing, the excitation intensity distribution will be disturbed by the reflector 2a3, a shoulder will be formed in the frequency characteristics as shown in Fig. 6, and the group delay characteristics will also be disturbed. There is a problem with this.

[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 a method in which the reflector length of the reflector provided on the apodized electrode side is adjusted to a length corresponding to the intersecting length of the intersecting electrode fingers of the apodized electrode. It is a very important thing.

[0008]

[Operation] Therefore, according to the present invention, since the length of the reflector is changed according to the intersecting length of the intersecting electrode fingers of the apodized electrode, the excitation intensity distribution is not disturbed, and the original apodized electrode This makes it possible to obtain filter characteristics with low loss, large sidelobe attenuation, and flat group delay characteristics over a wide range.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to FIGS. 1 to 3 as well as to FIGS. 4 to 6, in which the same parts are given the same reference numerals, detailed explanations 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 of the apodized intersecting electrode a, and 2 is an apodized electrode on the input side.
a is a reflector, and 3a is an extraction electrode.

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 waves are given directionality by a reflector 2a having a length that varies according to the intersecting length of the apodized intersecting electrode fingers 1a,
The signal propagates as indicated by the arrow in FIG. 1, is converted from a mechanical signal to an electrical signal again by the crossed electrode fingers 1b of the regular type electrode b on the output side, and is transferred from the extraction electrode 3b to an external electrode (not shown).
Output to.

As described above, according to the above embodiment, since the length of the reflector 2a of the input side apodized electrode a is changed in accordance with the length of the apodized intersecting electrode fingers 1a, the excited As shown in FIG. 3, it is possible to obtain the effects of low loss, large sidelobe attenuation, and flat group delay characteristics over a wide range, without disturbing the distribution of the excitation intensity of the surface wave.

FIG. 2 shows the structure of a surface acoustic wave filter according to another embodiment of the present invention. In the figure, a1 is an apodized electrode on the input side, and 1a1 is the apodized electrode a.
The crossed electrode fingers 1a1 and 2a1 are the half of the regular electrode b side on the output side from the center of the apodized electrode a1.
The output side reflector 2a11 has a constant length that is the same as the maximum crossing length of or slightly longer than the maximum crossing length, and the other half of the apodized electrode a1 is adjusted to the change in the crossing length of the crossed electrode fingers 1a1. This is a long reflector on the opposite output side, and the same effect as in the first embodiment can be obtained.

In the above embodiment, the apodized electrode a or a1 was described as being on the input side and the regular electrode b as the output side, but the apodized electrode a or a1 was on the output side and the regular electrode b as the input side. It is also possible to use it as a crossed electrode.

[0015]

Effects of the Invention As is clear from the above embodiments, the present invention provides an apodized electrode as one electrode of two intersecting electrodes each having a built-in reflector formed on a piezoelectric substrate. By adjusting the length of a part to match the change in the intersecting length of the intersecting electrode fingers of the apodized electrode, an excellent structure with low loss, large sidelobe attenuation, and flat group delay characteristics over a wide range can be achieved. It is possible to obtain filter characteristics.

[Brief explanation of the drawing]

FIG. 1 is a plan view of a surface acoustic wave filter according to an embodiment of the present invention.

FIG. 2 is a plan view of a surface acoustic wave filter according to another embodiment of the present invention.

[Fig. 3] Filter characteristic diagram of the surface acoustic wave filter in the embodiment of the present invention

[Figure 4] Plan view of a conventional surface acoustic wave filter

[Fig. 5] Plan view of another conventional surface acoustic wave filter

[Figure 6] Filter characteristic diagram of conventional surface acoustic wave filter

[Explanation of symbols]

a Apodized electrode (crossed electrode) 1a Cross electrode fingers 2a Reflector 3a Extraction electrode b Regular type electrode (crossed electrode) 4 Piezoelectric substrate

Claims (2)

[Claims] Claim 1: Two sets of crossed electrodes are provided on a piezoelectric substrate,
One set of intersecting electrodes is a regular type electrode with a uniform crossing length, and the other set of intersecting electrodes is an apodized type electrode whose crossing length is varied with respect to the arrangement direction of the intersecting electrode fingers. A surface acoustic wave filter in which a plurality of reflectors are provided between the crossed electrodes of each apodized electrode, and the reflector length of the reflector on the apodized electrode side is changed in accordance with the change in the crossing length of the crossed electrode fingers. . 2. A plurality of reflectors provided in an apodized electrode are arranged so that the center of the apodized electrode is the boundary, and half of the side where the regular electrode is located (surface wave propagation direction side) is connected to the apodized electrode. The output side reflector has a uniform reflector length that is equal to or slightly longer than the maximum crossing length of the crossed electrode fingers, and the other half on the opposite side of the surface wave propagation direction is used as the output side reflector to change the crossing length. 2. The surface acoustic wave filter according to claim 1, wherein the reflector on the opposite output side has reflector lengths that are equal in length to each other.