Absorber for piezoelectric surface acoustic wave device

A surface acoustic wave device includes a piezoelectric substrate, transmitting and receiving transducers provided on the surface of the substrate for propagating acoustic waves between the transducers, and absorbers provided between an end of the substrate and respective transducers for absorbing unwanted acoustic waves. The shape of the absorber is in relation to the energy distribution of the acoustic waves.

BACKGROUND OF THE INVENTION 
The present invention relates to a surface acoustic wave device for use in 
communication system, such as filter, and more particularly, to a surface 
acoustic wave device provided with an absorber for absorbing and 
suppressing reflected waves. 
Generally, a surface acoustic wave (SAW) device comprises, as shown in FIG. 
1, a transmitting, or launching, transducer 2 and a receiving transducer 
4, which are formed from interdigitated comb-like multi-electrode elements 
and disposed on a piezoelectric substrate 6. When an alternating 
electrical potential is applied to the electrodes of the transmitting 
transducer 2 through terminal plates 8 and 10 located at opposite side of 
the transducer 2, an alternating electric field is generated that causes 
localized vibrations in the substrate material 6. 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 4. The received 
acoustic waves can be removed as an electric signal, from terminal plates 
12 and 14. 
In the SAW device described above, the transmitting transducer 2 launches 
the surface waves in opposite directions simultaneously while the 
receiving transducer 4 receives the waves traveling in either direction. 
This is a significant problem in most SAW devices because in addition to 
responding to surface waves traveling directly from the transmitting 
transducer 2 to receiving transducer 4, the transducers respond to surface 
waves reflected from the ends of the substrate. These reflected surface 
waves produce unwanted signals, for example, as spurious signal in the 
time domain and/or ripples of the frequency response domain that distort 
the main, desired signal, adversely affecting the performance of the SAW 
device. 
There have been proposed various methods for suppressing the reflected 
waves. One method is, as shown in FIG. 1, to apply a rectangular absorbent 
material 16, 18 to the edges of the substrate adjacent to each transducer. 
Since the larger the absorbent material area, the higher the reduction of 
the surface waves, each absorbent material 16, 18 has a length, measured 
in the direction of propagation of the surface wave, sufficiently long to 
absorb the unwanted surface waves launched towards the edges of the 
substrate and reflected from such edges. 
Another method is shown in FIG. 2 in which the side edges are skewed 
relative to the fingers of transducer combs, and thus relative to the 
principal acoustic response axis, to change the direction of travel of the 
reflected surface waves for reducing the influence of the reflected 
surface wave on the transducer. 
Although the above described methods effectively suppress the unwanted 
waves and reflected waves, the application of absorbent material 16, 18 in 
the manner described above, or the change of the configuration of the 
substrate result in high manufacturing cost. More particularly, according 
to the SAW device of FIG. 1, each of the absorbers 16 and 18 occupies a 
large space in the substrate 6 to substantially enlarge the size of the 
SAW device. Thus, it is necessary to provide a substrate having a large 
size. Furthermore, since the absorbers 16 and 18 cover large area over the 
substrate, they require a large amount of absorbent material. Also, 
according to the SAW device of FIG. 2, since the substrate 6 is formed in 
the shape of parallelogram, a parent plate from which the substrates are 
cut out may not be fully utilized, thus wasting the substrate material. 
These and other conventional SAW devices are disclosed in Toda et al's U.S. 
Pat. No. 4,090,153 issued May 16, 1978, Drummond's U.S. Pat. No. 4,096,455 
issued June 20, 1978, and Yamada et al's Japanese Laid Open Patent 
Publication No. 100253/1976 (corresponding to U.S. Pat. No. 4,139,791 
issued Feb. 13, 1979). 
Accordingly, a primary object of the present invention is to provide a SAW 
device in which the unwanted waves and reflected waves can be effectively 
suppressed with a small amount of absorbent material. 
It is another object of the present invention to provide a SAW device of 
the above described type which is compact in size and can readily be 
manufactured at low cost. 
BRIEF DESCRIPTION OF THE INVENTION 
In accomplishing these and other objects, a SAW device according to the 
present invention comprises a substrate of a material capable of 
propagating acoustic waves along a surface of the substrate. A transducer 
means is deposited on the surface of the substrate and includes an 
interdigitated electrode arranged in accordance with a weighting function. 
The transducer means is adapted to launch, along a path on the surface of 
the substrate, acoustic waves having nonuniform energy distribution along 
the widthwise direction of its propagation. An absorber means is deposited 
on the surface of the substrate over the path for absorbing the acoustic 
waves propagating along the path towards the absorber means. The absorber 
means has an absorbent material spanning a long distance along the path 
where the acoustic waves having high energy propagate, and short distance 
along the path where the acoustic waves having low energy propagate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 3, one embodiment of a surface acoustic wave (SAW) device 
of the present invention comprises an elongated rectangular substrate 20 
of a piezoelectric material, such as PZT, having opposite end edges 20a 
and 20b and opposite side edges 20c and 20d. The substrate 20 can be 
formed by the deposition of a film of piezoelectric material over a 
dielectric plate, for example, a glass plate. The rectangular substrate 20 
has a transmitting transducer 22 and a receiving transducer 24 applied to 
its upper surface in a known manner. In the embodiment shown, the 
transmitting transducer 22 includes a pair of thin-film electrodes 
arranged in the shape of combs with interdigitated teeth. The tips of the 
comb-shaped electrodes of the transmitting transducer 22 as shown by an 
envelope line are arranged in a shape similar to rugby balls in accordance 
with a desired mathematical weighting function. A pair of terminal plates 
26 and 28 extend outwardly from the electrodes of the transducer 22 in the 
direction parallel to the interdigitated teeth and are deposited on the 
peripheral portions adjacent to the side edges 20c and 20d, respectively, 
of the substrate 20. 
The receiving transducer 24 includes a pair of comb-shaped electrodes of 
the uniform interdigitated type. A pair of terminal plates 30 and 32 
extend outwardly from the electrodes of the transducer 24 in the same 
manner as the terminal plates 26 and 28 and are also deposited on the 
peripheral portions adjacent to the side edges 20c and 20d, respectively, 
of the substrate 20. 
Provided on the substrate 20 between the transmitting transducer 22 and the 
edge 20a of the substrate 20 is an absorber 34 formed by an epoxy resin 
and deposited thereon by any known method, such as printing. A similar 
absorber 36 is deposited on the substrate 20 between the receiving 
transducer 24 and the edge 20b of the substrate 20. Before describing the 
details of the absorbers, the characteristic of the surface acoustic waves 
will be described. 
When an alternating electrical potential is applied between the terminal 
plates 26 and 28, an alternating electric field is generated that causes 
localized vibration in the substrate 20. In the case where the substrate 
20 is formed by the piezoelectric film laminated over the dielectric 
plate, the vibration is produced in the piezoelectric film. These 
vibrations give rise to acoustic waves, which propagate along a path 
defined on the surface of the substrate 20 in a direction perpendicular to 
the teeth of the electrodes. When the radiated energy of surface acoustic 
waves from the transmitting transducer 22 is measured, the central portion 
between the comb-shaped electrodes (that is, where the number of 
interdigitated teeth is greatest) generates surface acoustic waves having 
highest energy. In other words, the energy of the surface acoustic waves A 
(FIG. 4) propagating at the central portion of the path is highest and is 
reduced towards the opposite sides of the path. Therefore, the surface 
acoustic waves B and C propagating along the opposite sides of the path 
have relatively small amount of energy. A graph shown in FIG. 4 shows the 
distribution of surface wave energy along the widthwise direction of the 
path. 
Since the degree of reduction of surface wave energy is in relation to the 
distance of travel of the surface wave through the absorbent material, the 
absorber 34 is so shaped as to cover a wide area over the path on the 
substrate 20 where the surface waves having high energy travel, and cover 
a narrow area of the path where the surface waves having low energy 
travel. According to the SAW device of FIG. 3, the absorber 34 is so 
shaped that the curvature defining the edge thereof facing the end edge 
20a of the substrate 20 is similar to the distribution curvature shown in 
FIG. 4, while the edge of the absorber 34 facing the transmitting 
transducer 22 has a straight line parallel to the teeth of the comb-shaped 
electrodes. Accordingly, the absorber 34 has a shape similar to the nipple 
end portion of a feeding bottle. 
In the above arrangement, the surface wave A having high energy looses much 
of its energy during its travel through the long span portion of the 
absorber 34, and the surface waves B and C also loose their energy during 
their travel through the short span portion of the absorber 34. Therefore, 
the surface waves launched from the transmitting transducer 22 towards the 
end edge 20a of the substrate 20 will be attenuated to have a considerably 
smaller energy when they reach the end edge 20a. When the surface waves 
reaching the end edge 20a are reflected by the end edge 20a back towards 
the transducer 22, they will be further attenuated by the time they reach 
the transducer 22 to such a degree that they will not adversely affect the 
performance of the transducer 22. 
Although it is described that the comb-shaped electrodes of the receiving 
transducer 24 are interdigitated uniformly, they can be interdigitated in 
accordance with a desired weighting function. In this case, the 
transmitting transducer 22 may be so designed as to have a uniform 
interdigitated electrodes. 
It is to be noted that the absorber 36 provided adjacent to the receiving 
transducer 24 functions in a similar manner to the absorber 34. 
Furthermore, it is to be noted that either one of the absorbers 34 and 36 
can be eliminated or can be replaced by an absorber having a rectangular 
shape or by an absorber having one side edge skewed relative to the 
direction of travel of the surface wave. Moreover, it is possible to 
deposite each of the absorbers in a turned round position in which the 
nipple end portion points to the respective transducers. 
Referring to FIG. 5, there is shown a modification of the SAW device having 
terminal plates 26, 28, 30 and 32 deposited adjacent to the absorbers 34 
and 36 within the width of the transducers 22 and 24. More particularly, 
the terminal plates 26 and 28 are located adjacent to the nipple ended tip 
of the absorber 34 while the terminal plates 30 and 32 are located 
adjacent to the nipple ended tip of the absorber 36. The electrical 
connection to the respective terminal plates can be effected by the lead 
wire (not shown) soldered thereto or by the terminal leg (not shown) 
soldered thereto directly. According to the SAW device of FIG. 5, since no 
terminal plate is deposted along the side edges 20c and 20d, it is 
possible to narrow the width of the substrate 20, thus compacting the size 
of the SAW device. 
Moreover, it is possible to apply the absorbent material over the 
transmitting transducer 22 at portions X and Y where the teeth are not 
interdigitating. In this case the absorbent material can be integrally 
extended from the absorber 34. The absorbent material deposited on the 
transducer 22 advantageously absorbs the unwanted surface wave reflected 
by each of the teeth in the comb-shaped electrodes. 
Referring to FIG. 6, the transmitting transducer 22 in this embodiment has 
another pattern of interdigitated teeth. The tips of the comb-shaped 
electrodes as shown by an envelope line are arranged in a shape similar to 
mountains in accordance with a desired mathematical weighting function. In 
this case, the energy of surface acoustic waves is greatest at one side 
portion of the path of the surface wave and is gradually reduced towards 
the other side portion of the path, as shown by an energy distribution 
curve in FIg. 7. Accordingly, the absorber 34 of the SAW device of FIG. 6 
is so shaped that the curvature defining the edge thereof facing the end 
edge 20a of the substrate 20 is similar to the distribution curvature 
shown in FIG. 7, while the edge of the absorber 34 facing the transmitting 
transducer 22 has a straight line parallel to the teeth of the comb-shaped 
electrodes. The absorber 36 provided adjacent to the receiving transducer 
24 has a shape similar to the absorber 34 described above. 
Although the present invention has been fully described with reference to 
several preferred embodiments, 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.