Surface acoustic wave transducer with internal acoustic reflection

A surface acoustic wave transducer comprises a piezoelectric substrate, a first comb-shaped electrode with strips having the same width of .lambda./8 (.lambda. is the wavelength of a surface acoustic wave), and a second comb-shaped electrode with strips having the same width of .lambda./8. The strips of these comb-shaped electrodes are spaced apart from each other for a distance of .lambda./8. The strips of either electrode form pairs. Of each pair of strips, one strip has a narrow portion having a width of .lambda./16 and the other strip has a broad portion having a width of 3.lambda./16, which extends along the narrow portion. The narrow and broad portions of the strips of either comb-shaped electrode both acoustically and electrically reflect surface acoustic waves, whereas the other portions of the strips only electrically reflect the waves.

BACKGROUND OF THE INVENTION 
The present invention relates to a surface acoustic wave transducer. 
A surface acoustic wave transducer comprises a substrate made of 
piezoelectric material, a first comb-shaped electrode formed on one 
surface of the substrate, and a second comb-shaped electrode formed on the 
surface of the substrate. The first and second comb-shaped electrodes have 
each a plurality of strips. The strips of the first electrode are 
interdigitated with those of the second electrode. 
When a high-frequency signal voltage is applied between the first and 
second electrodes, the substrate vibrates, thereby generating surface 
acoustic waves. As the waves propagate along the surface of the substrate, 
they are reflected from the edges of the strips of both electrodes. The 
reflected waves form spurious components generally called "acoustic 
reflection components". In the meantime, the waves propagating under the 
comb-shaped electrode connected to a prescribed external load will 
reinduce high-frequency signals on the comb-shaped electrode, thus 
producing spurious components generally called "electrical reflection 
components". Both types of reflection components will appear in the output 
signals of a filter or a delay line composed of the transducer, in the 
form of a triple transit echo. They deteriorate the characteristic of the 
filter or delay line. 
This problem will be solved if the transducer is so designed as to make the 
acoustic reflection components cancel out the electrical reflection 
components over a broad range of frequencies. Several attempts have been 
made to provide such a surface acoustic wave transducer, but have failed. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a surface acoustic wave 
transducer with a good spurious characteristic over a broad range of 
frequencies. 
According to the invention, there is provided a surface acoustic wave 
transducer which comprises: a piezoelectric substrate; a first comb-shaped 
electrode formed on the piezoelectric substrate, having a plurality of 
parallel strips extending at right angles to the direction in which 
surface acoustic waves propagate, each having a first portion which only 
electrically reflects the waves and a second portion which both 
electrically and acoustically reflects the waves; and a second comb-shaped 
electrode formed on the piezoelectric substrate and having a plurality of 
parallel strips interdigitated with the strips of the first comb-shaped 
electrode, extending at right angles to the direction in which the waves 
propagate, each having a first portion which only electrically reflects 
the waves and a second portion which both electrically and acoustically 
reflects the waves.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A few embodiments of the invention will be described in detail with 
reference to the attached drawings. 
FIG. 1 shows a surface acoustic wave transducer as a first embodiment of 
this invention. The transducer comprises a substrate 11 made of 
piezoelectric material, e.g., LiNbO.sub.3, a first transducer electrode 
assembly 12, and a second transducer electrode assembly 13. These 
assemblies 12, 13 are arranged on one surface of substrate 11, and are 
spaced apart from each other for a predetermined distance. 
First transducer electrode assembly 12 comprises a first comb-shaped 
electrode 12a and a second comb-shaped electrode 12b. Electrode 12a has 
pairs of parallel strips. The width of these strips is .lambda./8, where 
.lambda. is the wavelength of the surface acoustic wave. The strips of any 
pair are integrally formed with a base strip 12ab and extend at right 
angles from it. The first and second strips 12a-1, 12a-2 forming a first 
split electrode pair are spaced for a distance of .lambda./8. The third 
strip 12a-3 is arranged at a distance of 5.lambda./8 from the second strip 
12a-2. The fourth strip 12a-4 is arranged at a distance of .lambda./8 from 
the third strip 12a-3. These strips 12a-3, 12a-4 form a second pair. The 
middle portion 12a-31 of strip 12a-3 is .lambda./16 wide, narrower than 
both end portions. By contrast, the middle portion 12a-41 of strip 12a-4 
is 3.lambda./16 wide, broader than both end portions. The distance between 
these portions 12a-31, 12a-41 is .lambda./8. The fifth strip 12a-5 is 
placed at a distance of 5.lambda./8 from fourth strip 12a-4. The sixth 
strip 12a-6 is placed at a distance of .lambda./8 from fifth strip 12a-5. 
The strips 12a-5, 12a-6 form a third pair. The middle portion 12a-51 of 
strip 12a-5 is .lambda./16 wide, narrower than both end portions. By 
contrast, the middle portion 12a-61 of strip 12a-6 is 3.lambda./16 wide, 
broader than both end portions. The distance between these portions 
12a-31, 12a-41 is .lambda./8. Middle portion 12a-51 is longer than the 
portion 12a-31 of third strip 12-3, and middle portion 12a-61 is also 
longer than the portion 12a-41 of fourth strip 12a-4. The other pairs of 
strips (not shown) are similar to the first and third pair in position and 
shape. 
Second electrode 12b is similar in structure to electrode 12a. It comprises 
a base strip 12bb and pairs of parallel strips which are interdigitated 
with those of first electrode 12a. The width of these strips is 
.lambda./8. The strips of any pair are integrally formed with base strip 
12bb and extend at right angles from it. The strips 12b-1, 12b-2 forming 
the first pair are spaced for a distance of .lambda./8. The third strip 
12b-3 is arranged at a distance of 5.lambda./8 from second strip 12b-2. 
The fourth strip 12b-4 is arranged at a distance of .lambda./8 from strip 
12b-3. These strips 12b-3, 12b-4 form the second pair. The middle portion 
12b-31 of strip 12b-3 is .lambda./16 wide, narrower than both end 
portions. By contrast, the middle portion 12b-41 of strip 12b-4 is 
3.lambda./16 wide, broader than both end portions. The distance between 
these portions 12b -31, 12b-41 is .lambda./8. The fifth strip 12b-5 is 
placed at a distance of 5.lambda./8 from fourth strip 12b-4. The sixth 
strip 12b-6 is arranged at a distance of .lambda./8 from fith strip 12b-5. 
These strips 12b-5, 12b-6 form the third pair. The middle portion 12b-51 
of strip 12b-5 is .lambda./16 wide, narrower than both end portions. By 
contrast, the middle portion 12b-61 of strip 12b-6 is 3.lambda./16 wide, 
broader than both end portions. The distance between these portions 
12b-31, 12b-41 is .lambda./8. Middle portion 12b-51 is longer than the 
portion 12b-31 of third strip 12b-3, and middle portion 12b-61 is also 
longer than the portion 12b-41 of fourth strip 12a-4. The other pairs of 
strips (not shown) are similar to the first and third pair in position and 
shape. 
All the strips 12a-1, 12a-2, 12a-3 . . . of first electrode 12a are of the 
same length and extend from base strip 12ab to the position close to the 
base strip 12bb. Similarly, all the strips 12b-1, 12b-2, 12b-3 . . . of 
second electrode 12b extend from base strip 12bb to the positions near the 
base strip 12ab. Hence, those portions of the strips of electrode 12a, 
which extend along the corresponding portions of the adjacent strips of 
electrode 12b, have the same length, i.e., interdigitated length. 
Second transducer electrode assembly 13 comprises a first comb-shaped 
electrode 13a and a second comb-shaped electrode 13b. Electrode 13a has 
parallel strips 13a-1, 13a-2, 13a-3 and 13a-4, and electrode 13b also has 
parallel strips 13b-1, 13b-2, 13b-3 and 13b-4. The strips of both 
electrodes 13a, 13b have the same width .lambda./8 and have the same 
length. Strips 13a-1 and 13a-2 are interdigitated with strips 13b-1 and 
13b-2. 
With reference to FIG. 2, it will now be described how the transducer of 
FIG. 1 operates. Let us assume that the comb-shaped electrodes 12a, 12b 
are connected between the output terminals of a high-frequency wave signal 
load (not shown). The substrate 11 is excited by an output signal from the 
incoming wave and regenerates surface acoustic waves. Since the strips of 
both electrodes 12a and 12b have the same width of .lambda./8 and are 
spaced apart from each other for a distance of .lambda./8, except for the 
portions which are surrounded by the diamond-shaped envelope, those 
portions A of the strips which are outside the envelope produce no 
acoustic reflection components for the reason disclosed in U.S. Pat. No. 
3,727,155. Since those portions B of the strips which are surrounded by 
the envelope have widths less than .lambda./8 or more than .lambda./8, the 
impulse response characteristic RA with respect to the acoustic reflection 
component changes with time as illustrated by a broken line in FIG. 2. 
Although the surface acoustic waves are not reflected from portions A, 
electrical reflection compoments will be induced in these portions A by 
secondary signals generated when portions A are excited by the surface 
acoustic waves propagating under the electrodes 12a, 12b. The portions B 
of the strips generate not only the acoustic reflection components but 
electrical reflection components. The impulse response characteristic RE 
with respect to the electrical reflection components generated by the 
portions A and B of the strips changes with time as indicated by a chain 
line in FIG. 2. As clearly understood from FIG. 2, the characteristic RE 
is defined by superposing the signal voltages regenerated by the strips as 
portions A and B are excited by the surface acoustic waves. In short, this 
characteristic RE is the self-convolution of the interdigitated pattern of 
the strips of electrodes 12a and 12b. As shown in FIG. 1, the strips of 
electrodes 12a and 12b have the same interdigitated length. The 
interdigitated pattern h(t) of these strips is shown in FIG. 3(a). The 
self-convolution of pattern h(t) is obtained by the superposing integral 
of the pattern h(t) with another identical pattern h'(t) shown in FIG. 
3(b) and by plotting the union of both patterns h(t) and h'(t) on the time 
axis. The self-convolution thus obtained is shown in FIG. 3(c). 
Accordingly, the characteristic RE changes to the same extent as the 
characteristic RA but in the opposite direction. 
Since the reason why the response RE is in opposite direction with respect 
to the response RA is, for example, shown by Kentaro Hanma et al, "A 
Tripple Transit Suppression Technique", 76IEEE Ultrasonics Symp. Proc. 
P-328, the more detailed explanation can be omitted here. 
If the interdigitated pattern of portions B is designed in such a manner as 
to make the response RA cancel out the response RE, the transducer will 
have good spurious characteristics over a broad range of frequency, and 
will not be influenced by acoustic reflection or electrical reflection. 
The transducer electrode assembly 12 shown in FIG. 1 has the reflection 
impulse response characteristic illustrated in FIG. 2. Therefore, the 
surface acoustic waves reemitted from assembly 12 to assembly 13 have the 
two types of reflection spurious components (FIG. 2). It should be noted 
that the acoustic reflection components cancel out the electrical 
reflection components in the transducer electrode assembly 13. 
FIG. 4 illustrates another surface acoustic wave transducer, a second 
embodiment of the invention. This transducer comprises a first comb-shaped 
electrode 21 and a second comb-shaped electrode 22. Electrode 21 has 
parallel strips 21a, 21b, 21c . . . which have the same width of 
.lambda./8. Electrode 22 has parallel strips 22a, 22b 22c . . . which have 
the same width of 5.lambda./8. The strips of electrode 21 are 
interdigitated with those of electrode 22. The strips of electrodes 21, 22 
are spaced apart from each other for a distance of .lambda./8. The strips 
22b, 22c, 22d, 22e . . . have each a slit having a width of .lambda./8. 
The slits of these strips have such different lengths that the acoustic 
reflection components may have the diamond-shaped envelope shown by broken 
lines in FIG. 4. 
Those portions A of the strips 21a, 21b . . . and 22a, 22b . . . , which 
are located outside the envelope, produce electrial reflection components, 
but no acoustic reflection components. By contrast, those portions B of 
strips 21a, 21b . . . and 22a, 22b . . . , which are enclosed by the 
envelope, produce both electrical and acoustic reflection components. 
Strips 21a, 21b . . . having a width of .lambda./8 and the strips 22a, 22b 
. . . having a width of 5.lambda./8, which are arranged with gaps of 
.lambda./8 among them, produce no acoustic reflection components for the 
known reason which is stated in U.S. Pat. No. 3,990,023. 
As in the embodiment of FIG. 1, the interdigitated lengths of strips 21a, 
21b . . . and 22a, 22b . . . are the same. That is, those portions of the 
strips of electrode 21, which extend along the corresponding portions of 
the adjacent strips of electrode 22, have the same length. 
Owing to the convolution of its interdigitated pattern, the transducer of 
FIG. 4 has the impulse response characteristic RE with respect to the 
electrical reflection components, which is identical with that shown in 
FIG. 2. The impulse response characteristic RA with respect to acoustic 
reflection components produced by the portions B of strips 21a, 21b . . . 
and 22a, 22b . . . is also identical with that of FIG. 2. Hence, the 
second embodiment (FIG. 4) has the same spurious response characteristic 
as shown in FIG. 2. 
FIG. 5 illustrates another surface acoustic wave transducer, a third 
embodiment of the invention. This transducer comprises a first comb-shaped 
electrode 31 and a second comb-shaped electrode 32. Electrode 31 has 
parallel strips 31a, 31b, 31c . . . which have the same width of 
.lambda./8. Electrode 32 has parallel strips 32a, 32b . . . which have the 
same width of 5.lambda./8. The strips of electrode 31 are interdigitated 
with those of electrode 32. The strips of electrodes 31, 32 are spaced 
apart from each other for a distance of .lambda./8. The strips 32a, 32b . 
. . have each four slits having a width of .lambda./8. More specifically, 
strip 32a has slits 32a-1, 32a-2, 32a-3, and 32a-4 vertically aligned 
(FIG. 5); strip 32b has slits 32b-1, 32b-2, 32b-3 and 32b-4 also 
vertically aligned (FIG. 5), and so forth. 
In the strip 32a, slits 32a-1 to 32a-4 define an electrode 32a-11 having a 
width of .lambda./8 and another electrode 32a-12 having a width of 
3.lambda./8. Those portions B1 of electrodes 32a-11 and 32a-12, which are 
horizontally aligned with slit 32a-1 (FIG. 5), produce both acoustic and 
electrical reflection components. That portion A1 of strip 32a, where no 
slit is formed, generates no acoustic reflection components. Those 
portions B2, B3 and B4 of electrodes 32a-11 and 32a-12, which are 
horizontally aligned with slits 32a-2, 32a-3 and 32a-4 (FIG. 5), produce 
both acoustic and electrical reflection components. Those portions A2 and 
A3 of electrodes 32a, which are positioned among the portions B2, B3 and 
B4, generate no acoustic reflection components. Of that portion of strip 
32b which extends along strip 31a, the total length of slits 32a-1 to 
32a-4 corresponds to the length of the slit 22b-1 of strip 22b shown in 
FIG. 4. 
Similarly, in the strip 32b, slits 32b-1 to 32b-4 define an electrode 
32b-11 having a width of .lambda./8 and another electrode 32b-12 having a 
width of 3.lambda./8. The features of the strip 32b are similar to those 
of strip 32a. The total length of slits 32b-1 to 32b-4 is longer than that 
of the slits 32a-1 to 32a-4 of strip 32a. The ratio of the former to the 
latter is equal to the ratio of the length of slit 22b-1 (FIG. 4) to the 
length of slit 22c-1 (FIG. 4). 
The transducer of FIG. 5, in which the portions A1-A3 and B1-B4 are aligned 
along a line at right angles to the direction in which surface acoustic 
waves propagate, achieve an effect that an amplitude of the response RA in 
the electrode-interdigitating direction can be made to have effectively a 
uniform value. As a result, it is also possible to have an effect of 
mutual cancellation of the responses RA and RE in the SAW beam width 
direction, thus increasing a tolerance of selecting a type of interdigital 
transducer which is used in combination with the transducer of this 
invention when it is desired to form a filter. 
In the first and second embodiments shown in FIGS. 1 and 4, respectively, 
the envelope of the region B is shaped like a diamond. Instead, the 
interdigitated pattern may be so designed that the envelope may change 
along a curve, not a straight line, so that various secondary effects can 
be compensated for. 
In all the embodiments described above, the strips of either comb-shaped 
electrode have the same length, and the strips of both comb-shaped 
electrodes are interdigitated for the same distance. This invention is not 
limited to these embodiments. It may be applied to a surface acoustic wave 
transducer, where the interdigitated length varies in the direction of 
wave propagation. 
FIG. 6 shows a fourth embodiment of the invention, a surface acoustic wave 
transducer in which the interdigitated length changes in the direction of 
wave propagation. This transducer comprises two comb-shaped electrodes 41 
and 42. Electrode 41 has strips 41-1, 41-2, 41-3 . . . having the same 
width of .lambda./8. Electrode 42 also has strips 42-1, 42-2, 42-3 . . . 
having the same width of .lambda./8. Strips 41-3, 41-4, 41-7, 41-8 . . . 
are interdigitated with strips 42-3, 42-6, 42-7, 42-10 . . . , so that the 
surface acoustic wave may have such an envelope as indicated by curves W1 
(FIG. 6) Similarly, the other specified strips of electrode 41 are 
interdigitated with the other specified strips of electrode 42 so that the 
surface acoustic wave may have such an envelope as represented by curves 
W2. 
One or more metal layers such as chromium layers are formed on some of the 
strips of the transducer (FIG. 6) which are mutually interdigitated. More 
precisely, two chromium layers 43-1 and 43-2 are formed on strip 41-3; one 
chromium layer 43-3 is placed on strip 42-4; two chromium layers 43-4 and 
43-5 are laid on strip 42-7; three chromium layers 43-6, 43-7 and 43-8 are 
arranged on strip 41-8 at predetermined intervals; and one chromium layer 
43-9 is formed on strip 42-9. 
In the transducer of FIG. 6, all the strips have the same width of 
.lambda./8 and are spaced apart from each other for a distance of 
.lambda./8. Hence they produce no acoustic reflection components, except 
for those portions on which the chromium strips 43-1, 43-2, 43-3 . . . are 
formed. Hence, these portions both acoustically and electrically reflect 
surface acoustic waves. The number of chromium strips formed on each strip 
varies with the direction of wave propagation. The transducer of FIG. 6 
can attain the same effects as those of the embodiments shown in FIGS. 1 
and 4. 
As stated earlier, in the embodiment of FIG. 1, the surface acoustic waves 
emitted from the assembly 12 to the assembly 13 contain acoustic and 
electrical reflection components (FIG. 2). The acoustic reflection 
components cancel out the electrical reflection components in the 
transducer electrode assembly 13. For this reason, two or more assemblies 
12 cannot be used to achieve better transmission and reception of surface 
acoustic waves. This problem is solved by the embodiment of FIG. 5. 
This problem is also solved by another embodiment of the invention, which 
is illustrated in FIG. 7. The surface acoustic wave transducer of FIG. 7 
comprises two comb-shaped electrodes 51 and 52. Electrode 51 has strips 
51a, 51b, 51c . . . having the same width of .lambda./8. Similarly, 
electrode 52 has strips 52a, 52b, 52c . . . having the same width of 
.lambda./8. The strips of both electrodes 51 and 52 only electrically 
reflect surface acoustic waves; they do not acoustically reflect the 
waves. The strips of electrode 51 are interdigitated with those of 
electrode 52 in the fashion shown in FIG. 7. Between strips 51b and 52c, 
four reflecting or floating electrodes 53a, 53b, 53c and 53d having the 
same width of .lambda./8 are aligned along a line at right angles to the 
direction of wave propagation. The floating electrodes are usually 
provided at a position shifted by .lambda./8 from the center of the split 
electrodes 51b and 51c. Further, between strips 51d and 52e, three columns 
of reflecting electrodes are arranged. The first column is comprised of 
five reflecting electrodes 53e-53i; the second column consists of four 
reflecting electrodes 53j-53m; and the third column is made up of three 
reflecting strips 53n, 53o and 53p. The reflecting electrodes 53e-53p have 
the same width of .lambda./4. 
The reflecting strips 53a-53d reflect the surface acoustic waves reaching 
them. Other waves pass through the gaps between strips 53a-53d. Since the 
strips 53a to 53d are distributed in the width direction, the reflection 
component due to the response RA is made to have a similar amplitude in 
the interdigital direction and is undergone by the diffraction of the 
passed waves. Therefore, cancellation of the responses RA and RE can also 
be realized in the beam width direction. After all, the surface acoustic 
waves propagating from this transducer contain no spurious components. Two 
or more transducer electrode assemblies of the type shown in FIG. 7 may be 
arranged on a piezoelectric substrate, thereby forming a surface acoustic 
wave transducer of a high efficiency.