GB2050742A

GB2050742A - Surface acoustic wave filter

Info

Publication number: GB2050742A
Application number: GB8015689A
Authority: GB
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 1979-05-15
Filing date: 1980-05-12
Publication date: 1981-01-07
Also published as: DE3018080C2; GB2050742B; JPS55151810A; DE3018080A1

Abstract

A surface acoustic wave filter includes a substrate 1 made of piezoelectric material and at least one transducer deposited on the substrate. The transducer has a pair of comb- shaped electrodes with their teeth interdigitated. An absorbent material 5b is deposited on a portion of the transducer where the teeth are interdigitated for changing the frequency characteristic of the surface acoustic wave filter. As described the device has a transmitting transducer 2 and a receiving transducer 3, and the material 5b forms part of a larger epoxy resin absorber 5. A further absorber 6 is provided adjacent the transducer 3. <IMAGE>

Description

SPECIFICATION Surface acoustic wave filter The present invention relates to a surface acoustic wave filter for use in communication system and, more particularly, to a surface acoustic wave filter provided with an absorber not only for absorbing and suppressing reflected waves but also for adjusting frequency characteristics of the propagated surface waves.

Generally, a surface acoustic wave (SAW) filter comprises a transmitting, or launching, transducer and a receiving transducer, both being formed by comb-like multi-electrode elements, having their teeth interdigitated, and disposed on a piezoelectric substrate. When an alternating electricai potential is applied to the electrodes of the transmitting transducer, an alternating electric field is generated that causes localized vibration in the substrate. These vibrations give rise to acoustic waves, which propagate along the surface of the substrate in a well defined path orthogonal to the electrodes, and may be detected at any point along the path by the receiving transducer.

The frequency characteristics of the filtered signal obtained from the receiving transducer is determined by the pattern of interdigitated electrodes in the transmitting and receiving transducer. Therefore, the adjustment of the frequency characteristics can be achieved by changing the degree of overlapping, or interdigitating, teeth, by changing the width of each tooth, by changing the pitch of the aligned teeth, by changing the nurnber of the teeth, and/or by dropping out one or more teeth from the aligned electrodes.

The interdigitated electrodes are usually formed by the use of an etching method having the steps of depositing a metal film over the substrate, placing a mask having a predetermined electrode pattern over the metal film, and removing the unwanted film portions through any known etching technique. Therefore, for adjusting or changing the frequency characteristics of the transducer, it is a usual and general practice to change the pattern on the mask. Sinec the pattern on the mask has a fine structure, it is very difficult to reform the pattern on the given mask.

Therefore, for obtaining a different pattern on the mask, the mask itself is formed again from the beginning. However, this is a time consuming task and requires an additional manufacturing cost.

Even if the pattern on the mask were reformed by a very high skilled worker, it may not be as precise as desired, since the teeth in the electrodes has a size in the order of micron.

Accordingly, a primary object of the present invention is to provide a SAW device which can be manufactured without the necessity of using a different pattern of interdigitated electrode when the frequency characteristic is desired to be changed.

It is another object of the present invention to provide a method for effecting the adjustment of the frequency characteristics of the SAW device without reforming the pattern of interdigitated electrodes on the substrate.

To this end, the present invention provides a surface acoustic wave filter which comprises a substrate made of a material capable of propagating acoustic waves along a surface of the substrate, a transducer including a pair of combshaped electrodes having their teeth interdigitated in accordance with a predetermined weighting function and deposited on said surface of the substrate, and an absorbent material. In accordance with the present invention, the absorbent material is deposited on the surface of the substrate so as to cover a portion of the combshaped electrodes where the teeth are interdigitated.

These and other objects and features of the present invention will become apparent from the following description of a preferred embodiment thereof with reference to the accompanying drawings, in which Fig. 1 is a top plan view of a SAW filter according to the present invention; and Fig. 2 is a graph showing an amplitude frequency characteristic and a group-delay-time frequency characteristic of the SAW filter.

Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings.

Referring to Fig. 1 , there is shown a surface acoustic wave (SAW) filter according to the present invention, which comprises an elongated rectangular substrate 1 of a piezoelectric material, such as PZT, LiNbO3 or a similar substrate such as glass plate covered with ZnO film, having opposite end edges 1 a and 1 b and opposite side edges 1 c and id. The rectanguiar substrate 1 has a transmitting transducer 2 and a receiving transducer 3 both applied to an upper surface of the substrate in a known manner. In the embodiment shown, the transmitting transducer 2 includes a pair of thin-film metal electrodes arranged in the shape of combs with interdigitated teeth.The tips of the comb-shaped electrodes of the transmitting transducer 2 as shown by an envelope line are arranged in accordance with a desired mathematical weighting function. The receiving transducer 3 includes a pair of combshaped electrodes of the uniform overlap, or interdigitated, type.

The SAW filter of the above described type is used, for example, as a video intermediate frequency (VIF) filter for a television receiver having a sound trap for trapping sound signal in the pass band.

The electrodes in the transducers 2 and 3 are formed by a method of etching having the steps of laminating a metal film, such as an aluminum film, entirely over one surface of the substrate 1, applying a mask over the laminated surface, and removing the unwanted film portions by a photo- etching technique or any other known etching technique. The reference numeral 4 designates a shield electrode which is also formed by the etching method together with the electrodes of the transducers 2 and 3. The reference numerals 5 and 6 designate surface wave absorbers, which are made of an absorbent material, such as epoxy resin, and are deposited on the surface of the substrate 1 by any known method, such as a printing method.The absorber 5 consists of a portion 5a located on the surface of the substrate 1 between the end edge 1 a and the transmitting transducer 2, and a portion 5b overlapping a part of the transmitting transducer 2. The absorber 6 and the portion 5a of the absorber 5 serve to suppress unwanted surface acoustic waves. The portion 5b of the absorber 5 includes a section which is placed on the interdigitated teeth portion and a section which is placed on the noninterdigitated teeth portion. The absorbent material deposited on the interdigitated teeth portion reduces the energy of the surface acoustic waves thereat, thereby causing a change in the frequency characteristics of the SAW filter.For example, in the case of SAW filter having a weighted transducer for video intermediate frequency, its frequency characteristics without the deposition of the absorbent material on the interdigitated teeth portion is illustrated by a bold line in the graph of Fig. 2. When the absorbent material is deposited on the interdigitated teeth portion, the sound carrier region and the righthand corner region of the pass band in the amplitude frequency characteristic curve are changed in a manner shown by the arrows to reform the characteristic curve depicted by a fine line.

Furthermore, the application of the absorbent material over the interdigitated teeth portion of the transducer suppresses irregular ripples in the group delay time (GDT) characteristic as shown in the same graph of Fig. 2.

The absorbent material deposited on the noninterdigitated teeth portion of the transducer eliminates unwanted reflected surface acoustic waves to improve the performance of the SAW filter.

The deposition of the absorbent portion 5b can be carried out at the same time or after the deposition of the absorbent portion 5a. In both cases, the area and/or amount of the transducer to be covered by the absorbent portion 5b is determined in consideration of the degree to which the frequency characteristic is desired to be changed.

It is to be noted that the absorbent portion 5b can be applied so as to extend from the absorbent portion 5a as shown in Fig. 1, or can be applied separately from the absorbent portion 5a.

It is also to be noted that the absorber 6 can also be in the form as extending so as to partly overlap a portion of the receiving transducer 3.

According to the present invention, the frequency characteristics of the SAW filter can be simply changed or adjusted by the deposition of the absorbent material over the transducer where the teeth are interdigitated.

Although the present invention has fully been described with reference to the embodiment, many modifications and variations thereof will now be apparent to those skilled in the art.

Therefore, unless such modifications and variations depart from the true scope of the present invention, they should be construed as included therein.

Claims

1. A surface acoustic wave filter comprising: a substrate of a material capable of propagating acoustic waves along a surface of the substrate; a transducer including a pair of comb-shaped electrodes having their teeth interdigitated in accordance with a weighting function and deposited on said surface; an absorbent material covering a portion of said comb-shaped electrodes where the teeth are interdigitated.

2. A surface acoustic wave filter as claimed in Claim 1, further comprising an absorber deposited on said surface adjacent the transducer.

3. A surface acoustic wave filter as claimed in Claim 2, wherein said absorbent material is provided extendingly from said absorber.

4. A method for controlling a frequency characteristics of a surface acoustic wave filter having a substrate of a material capable of propagating acoustic waves along a surface of the substrate and a transducer including a pair of comb-shaped electrodes having their teeth interdigitated in accordance with a weighting function and deposited on said surface, said method comprising the step of: depositing an absorbent material over a predetermined area of said comb-shaped electrodes where the teeth are interdigitated.

5. A surface acoustic wave filter substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

6. A method for controlling a frequency characteristics of a surface acoustic wave filter substantially as hereinbefore described.