A reflectionless surface acoustic wave transducer is proposed which has first and second opposed conductive transducer pads on a substrate of piezo-electric material. A plurality of groups of n interdigitated .lambda./4 electrodes extend from the opposed pads to form an elongated transducer. Each group of n interdigitated electrodes has a spacing within the group of .lambda./4 and the spaces between adjacent groups of the interdigitated electrodes are .lambda./2 such that adjacent groups cancel reflections from each other thereby establishing a reflectionless surface acoustic wave transducer.

BACKGROUND OF THE INVENTION 
The present invention relates to a substantially reflectionless transducer 
structure for surface acoustic wave devices and in particular to a single 
level surface acoustic wave transducer having quarter wave length 
electrodes which substantially eliminates electrode reflections and which 
can be easily made unidirectional. 
Surface acoustic wave devices known as SAW devices have many uses in the 
UHF and VHF frequency ranges. SAW devices have been especially useful as 
impedance elements, resonators, and band pass filters in these frequency 
ranges. Typical SAW devices have a substrate with at least a surface layer 
of piezo-electric material and surface acoustic wave transducers in 
interdigitated form disposed on the piezo-electric surface. The 
transducers convert an electrical signal to surface acoustic waves 
propagating on the piezoelectric surface. Several problems are associated 
with prior art .lambda./4 surface acoustic wave transducers. One of the 
problems occurs because the transducer electrodes cause internal 
reflections which distort the transducer output and the shape of the input 
conductance. Another problem occurs when the transducer is used in filter 
applications. Triple transit distortion is caused by regeneration 
reflections in the transducers. 
The first problem, distortion caused by internal reflections, is solved in 
the prior art by the use of structures having 3 or 4 electrodes per 
wavelength to cancel the internal reflections and provide a symmetrical 
input conductance wave shape. However, the increased number of electrodes 
per wavelength limits the frequency of operation of the structure due to 
photolithographic constraints. 
In order to eliminate triple transit distortion, three-phase and 
single-phase devices are used to form unidirectional transducers. Again, 
the size of the electrodes becomes a limiting factor in the construction 
of the device and thereby limits the frequency of operation of the device. 
The present invention proposes a simple single level interdigitated SAW 
transducer having .lambda./4 electrodes in which reflections are canceled 
and therefore allow a structure to be built at twice the frequency of 
those presently available. The device has an undistorted output and a 
symmetric input conductance, can be placed on standard crystal cuts and 
can be made into single-phase unidirectional transducer as a two level 
structure in a simple manner. Further the single-phase transducer can be 
made with a flat input susceptance and symmetric input conductance for use 
as impedance elements. 
Thus all electrodes in the transducer are .lambda./4 and the gaps between 
electrodes in the transducer are .lambda./4 or .lambda./2. The electrodes 
of the transducer are on a fixed grid for relatively easy construction. 
Electrode reflections in the transducer are completely canceled by 
destructive interference on a local basis. Reflections of each electrode 
in the transducer are canceled by the reflections of a neighboring 
electrode a distance of .lambda./2 away or a multiple thereof. The 
transducer utilizes a plurality of groups of interdigitated .lambda./4 
electrodes extending from opposed transducer pads to form an elongated 
transducer. Each group of the interdigitated electrodes has a spacing 
within the group of .lambda./4 and the spaces between the adjacent groups 
of the interdigitated electrodes are .lambda./2. With this construction, 
adjacent groups cancel reflections from each other. The groups can consist 
of any number of electrodes, n, where n is equal or greater than two. Thus 
electrode groups may consist of two, three, four or more electrodes per 
group. 
In addition the structures can be made unidirectional by mass loading of 
alternate groups. Further, if the mass loading of the alternate groups is 
reversed, the sense of the unidirectionality also changes. 
The frequency response of the two electrodes per group device includes 
group type responses which are 40% above and below the pass band. While 
this is a disadvantage, the responses are sufficiently far removed from 
the pass band for most filtering applications. Also, as the number of 
electrodes per group increases, the group type responses move closer to 
the pass band. This allows coupled resonators and filters to be 
constructed with one transducer having a first number of electrodes n per 
grouping and a second transducer to be constructed of an electrode 
grouping having a different number of electrodes thereby allowing good 
communication between the transducers in the pass band but poor 
communication between the transducers out of the pass band. 
Further, by adding an additional interdigitated electrode in alternate ones 
of said groups of n electrodes a weakly unidirectional transducer is 
obtained 
Thus it is an object of the present invention to provide a single level 
surface acoustic wave transducer which eliminates electrode reflections. 
It is also an object of the present invention to provide a surface acoustic 
wave device having .lambda./4 electrodes which eliminate electrode 
reflections and can easily be made unidirectional with a flat input 
susceptance and a symmetrical input conductance. 
It is yet another object of the present invention to provide a surface 
acoustic wave device having .lambda./4 electrodes on a standard crystal 
cut which eliminates electrode reflections, has a symmetric input 
conductance and an undistorted output. 
It is still another objection of the present invention to provide a surface 
acoustic wave transducer having a plurality of groups of n interdigitated 
.lambda./4 electrodes extending from opposed conductive pads with each 
group of the interdigitated electrodes having a spacing within the group 
of .lambda./4 and the spaces between adjacent groups having a spacing of 
.lambda./2 for maximum coupling between the electrodes and the substrate. 
It is still another object of the present invention to provide either an 
impedance element or a single port resonator with a transducer formed of 
.lambda./4 electrodes. 
SUMMARY OF THE INVENTION 
Thus the present invention relates to a substantially reflectionless 
surface acoustic wave transducer comprising a substrate means having at 
least a surface layer of piezo-electric material on which acoustic waves 
may be propagated, defining first and second opposed conductive transducer 
pads on said substrate, a plurality of groups of n interdigitated 
.lambda./4 electrodes extending from the opposed pads to form an 20 
elongated transducer, each group of the interdigitated electrodes having a 
spacing within the group of .lambda./4 and the spaces between adjacent 
groups of the interdigitated electrodes being .lambda./2 such that 
adjacent groups cancel reflections from each other. 
The invention also relates to a method of forming a substantially 
reflectionless surface acoustic wave transducer comprising the steps of 
preparing a substrate having at least a surface layer of piezo-electric 
material on which acoustic waves may be propagated, defining first and 
second opposed conductive transducer pads on the substrate, extending a 
plurality of groups of n interdigitated .lambda./4 electrodes from the 
opposed pads to form an elongated transducer, spacing the electrodes 
within each group of interdigitated electrodes by .lambda./4 and spacing 
adjacent groups of the interdigitated electrodes with a spacing of 
.lambda./2 such that adjacent groups cancel reflections from each other.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates a typical prior art transducer element which can be used 
to form various surface acoustic wave devices. The transducer element 10 
comprises substrate 12 having at least a surface layer of piezo-electric 
material thereon through which acoustic waves may be propagated. Opposed 
conductive transducer pads 14 and 16 are defined on the substrate 12 in a 
well known manner and include a plurality of interdigitated surface 
acoustic wave electrodes 18 and 20 extending from the opposed pads 14 and 
16 respectively and forming an elongated transducer. Leads 24 and 26 are 
connected to conductive transducer pads 14 and 16 respectively as input 
and output terminals. One of the transducer pads, such as conductive pad 
16, may be grounded as at 28. The number of interdigitated electrodes 18 
and 20 may, of course, vary but a large number of them are used under 
normal conditions to form transducer 10. Only a few of the electrodes 18 
and 20 are shown in FIG. 1 for purposes of simplicity of the drawings The 
electrodes 18 and 20 have a width of .lambda./4 and are spaced by 
.lambda./4 and the device has two electrodes per wave length. 
The prior art transducer of FIG. 1 tends to reflect acoustic signals 
between electrodes. These internal reflections distort the response of the 
transducer. 
Because of these internal reflections, the transducer output is distorted 
and the shape of its input conductance is distorted. Thus, the use of the 
device is limited because of these distortions. In order to overcome the 
distortions, the prior art transducers are formed with three or four 
electrodes per wavelengths. By constructing the transducer in this manner, 
the reflections are canceled thus allowing a transducer to be constructed 
with an undistorted output and an undistorted input conductance. However, 
another problem is created since the greater the number of electrodes per 
wavelength, the smaller in size the electrodes and, thus, frequency of 
operation of the device is limited. 
Further, if the structure of FIG. 1 is used in a filter application, triple 
transit distortion occurs and the transducers must be severely mismatched 
to reduce the distortion, resulting in a high insertion loss. To eliminate 
triple transit distortion, the prior art utilizes unidirectional 
transducers formed as single-phase or three-phase devices as is 
well-known. 
Such a prior art single-phase unidirectional transducer can be seen in FIG. 
2. It is a split electrode transducer constructed with a piezoelectric 
substrate 30 on which transducer pads 32 and 34 are disposed and which 
have electrodes 36 and 38 forming an electrode pair extending in 
interdigitated form from each of the conductor pads 32 and 34. Such a 
conventional split finger electrode transducer is commonly considered to 
have no net internal reflections as is well known in the art because the 
reflected waves from one electrode 36 and its nearest neighbor 38 are 
180.degree. out of phase and cancel because of the .lambda./8 width of the 
electrodes and the spaces between them. The finger and gap widths of a 
transducer as shown in FIG. 2 with split finger construction are oneeighth 
of the operating acoustic wave length thus limiting the frequency range of 
the device by photolithographic constraints to a maximum frequency of 
operation of around 600 MHz when compared to a 1200 MHz range for the 
simplest form of SAW transducer shown in FIG. 1 and described earlier. 
Thus the structure with split finger construction does eliminate internal 
reflections but has frequency constraint problems. The device can be made 
unidirectional by mass loading alternate electrodes as set forth in U.S. 
Pat. No. 4,353,046 thereby enabling the device to be used in applications 
such as filters where triple transit distortion must be eliminated. 
FIG. 3 is a diagramatic representation of the novel unweighted surface 
acoustic wave transducer of the present invention which is substantially 
reflectionless and which utilizes quarter wave length construction of the 
electrodes and which can easily be made unidirectional. The transducer 40 
as shown in FIG. 3 comprises a substrate 42 having at least a layer of 
piezo-electric material on which acoustic waves may be propagated. First 
and second opposed conductive transducer pads 44 and 46 are defined on the 
substrate 42. A plurality of groups 52, 54, 56 and 58 of n interdigitated 
.lambda./4 electrodes 48 and 50 extend from the opposed pads 44 and 46 to 
form an elongated transducer 40. For simplicity of the drawings, only four 
groups of electrodes are shown in FIG. 3. In this case, as shown in FIG. 
3, two electrodes, such as 48 and 50, equal the n interdigitated 
.lambda./4 electrodes in each of the groups 52, 54, 56 and 58. It will be 
noted that each of the groups 52, 54, 56 and 58 of interdigitated 
electrodes have a spacing within the group of .lambda./4 and, for an 
unweighted transducer, with maximum coupling, the spaces 60, 62 and 64 
between adjacent groups of said interdigitated electrodes have a spacing 
of .lambda./2. It will also be noted that for an unweighted transducer, 
with maximum coupling, every alternate .lambda./2 space, such as space 62, 
is formed by two adjacent electrodes 66 and 68 extending from a common 
transducer pad such as pad 46. Such construction causes adjacent groups 
such as 52 and 54 or 56 and 58 to cancel reflections from each other thus 
creating a substantially reflectionless transducer 40. 
Thus, summarizing, all of the electrodes in the unweighted transducer are 
.lambda./4 wide and the gaps or spacing within each group are .lambda./4 
while the spaces between adjacent groups of interdigitated electrodes are 
spaced .lambda./2. In addition, in order to maximize the coupling between 
the electrodes to the transducer acoustic wave, every alternate .lambda./2 
space between unweighted groups is formed by two adjacent electrodes 
extending from a common transducer pad. If other electrodes are added to 
change the operation of the transducer, the spacing of the electrodes must 
stay the same but the electrodes will be coupled to one or the other of 
the transducer pads as necessary to achieve maximum coupling. Thus, with 
the transducer shown in FIG. 3, electrode reflections are completely 
canceled by destructive interference on a local basis. Reflections of each 
electrode in the transducer are canceled by the reflections of neighboring 
electrode .lambda./2 (or a multiple distance thereof) away. Thus electrode 
group 52 of FIG. 3 cancels the reflections of the electrodes in group 54 
while the electrodes of group 56 cancel the reflections of the electrodes 
in group 58. The electrodes in other groups (not shown) would operate in a 
similar manner to cancel the reflections from each other. This means that 
the transducer output is undistorted and the shape of the input 
conductance is undistorted. In addition, the device can be constructed to 
operate at a maximum frequency because of the .lambda./4 electrode 
construction. 
Because of the non-uniform sampling by the non-periodic placement of the 
electrodes, the transducer 40 has undesired out-of-band spurious 
"group-type" responses. These are illustrated in FIG. 4 which shows the 
broad band frequency response for a prototype filter formed with structure 
of the type shown in FIG. 3 on a YZ-LiNb0.sub.3 substrate. It will be 
noted that the group type responses 74 and 76 in FIG. 4 are 40% above and 
below the pass band indicated at 72. This is sufficiently far removed from 
the pass band for most filtering applications. 
FIG. 5 illustrates the acoustic reflection coefficient on the transducer 40 
of FIG. 3 under short circuit conditions. As can be seen, the reflection 
coefficient is extremely small across the pass band. The greatest 
reflections occur at points 78 and 80 which are out of the pass band. 
Other sampling configurations can be employed if desired. For instance, in 
FIG. 6, there is a disclosed schematically a representation of an 
unweighted transducer 82 with electrode groupings 90, 92, 94 and 96 of 
three electrodes each. Again, each of the electrodes is constructed with a 
.lambda./4 width and are separated by spaces within the group such as 
space 110 and 112 which are .lambda./4 in width for maximum coupling. It 
will be noted that the spaces 104, 106 and 108, which are the spaces 
between the adjacent groups of the interdigitated electrodes have a 
spacing of .lambda./2. Again, in this embodiment the reflections generated 
by one group of electrodes are canceled by the reflections generated by an 
adjacent group of electrodes. For instance, the reflections generated by 
electrode group 90 are canceled by the reflections of the electrodes in 
group 92. In like manner the reflections in electrode group 94 are 
canceled by the reflections of electrode group 96. These cancellations 
occur because, as stated earlier, of the .lambda./2 spacing, or multiples 
thereof, between electrodes of one group and the electrodes of an adjacent 
group and the .lambda./4 construction of the electrodes and of the 
spacing between adjacent electrodes within a group. 
The frequency response for the structure of FIG. 6 on a substrate of 
LiNb0.sub.3 is shown in FIG. 7. Note that the group type out-of-band 
responses 118 and 120 for this structure are closer to the pass band at 
point 122 than with the preferred implementation shown in FIG. 4. As will 
be explained hereinafter with relation to FIGS. 9 and 10, different 
sampling schemes can be employed with two transducers in a SAW filter 
configuration or a coupled resonator configuration to effectively suppress 
the out-of-band spurious responses at peaks 118 and 120 in FIG. 7 and 
peaks 74 and 76 in FIG. 4. 
FIG. 8 illustrates a filter which can be constructed of the resonators 
shown in FIG. 3. Thus the two transducer structures 40 are placed on a 
substrate 130 in spaced relationship with appropriate gratings 128. Thus a 
filter of conventional construction is utilized with the two novel 
structures 40. Input terminals are shown at 132 and output terminals at 
134. 
As stated earlier, one of the ways in which the out-of-band spurious 
responses can be effectively suppressed is to couple the transducer in 
FIG. 3 having two electrodes per group to a transducer having a different 
number of electrodes per group such as the transducer in FIG. 6 having 
three electrodes per group. Such resonator filter structure is shown in 
FIG. 9. When these two structures 40 and 82 communicate with each other, 
the center peaks 72 and 122, (in FIG. 4 and FIG. 7) the pass band 
frequency and the point of lowest reflections, are at the same frequency 
and thus the signals pass freely between the transducers 40 and 82. 
However, the out-of-band spurious responses 74, 118, 76, and 120 all occur 
at different frequencies and thus it is difficult for the two transducers 
to effectively couple these signals from one to the other in the 
out-of-band region since when the spurious response of one transducer is 
at a peak, for instance 74, the spurious response of the other is at a 
null, such as at peak 136 in FIG. 7. In like manner, when transducer 82 in 
FIG. 6 has a maximum out-of-band spurious response at 118, transducer 40 
in FIG. 3 has a null as indicated approximately at point 138 in FIG. 4. 
Thus the two transducers do not effectively communicate with each other 
except at the pass band frequency which, of course, is what is desired. 
A further advantage of the transducer of the present invention is that, 
with an additional second-level metalization, the structure can be made 
unidirectional. The unidirectional nature of this configuration can be 
understood by reference to the transduced electric field pattern shown 
under the transducer 40 in FIG. 10. It will be noted that the peak of 
transduction for each electrode occurs at the edge of the electrode. For 
instance peak 138 occurs at the right edge of electrode 48. As is well 
known in the art, when the centers of transduction are positioned 
.lambda./8 or 45.degree. from the effective center of reflection the 
introduction of internal reflections will result in unidirectional 
behavior, as explained in U.S. Pat. No. 4,353,046. Thus in this 
configuration shown in FIG. 10, the locations of the electrodes correspond 
precisely to those locations at which the introduction of internal 
reflections will result in unidirectional behavior. In this structure, 
unweighted electrodes of one group cancel the reflections of the adjacent 
group. Thus, in order to introduce the necessary internal reflections for 
unidirectional behavior, additional mass loading is added to every 
alternate pair of electrodes as shown, for example, by electrodes 48 and 
50 and electrodes 68 and 69. The reflections introduced by this mass 
loading will be located at the center of each mass loaded electrode thus 
causing the transducer to be unidirectional for the reasons set forth in 
U.S. Pat. No. 4,353,046. It is also extremely easy with this transducer 
configuration to reverse the sense of unidirectionality. This can be 
accomplished simply by adding the additional mass loading to the other 
alternate pairs of electrodes such as electrodes 70 and 66 and 140 and 142 
instead of 48 and 50 and 68 and 69 as those shown in FIG. 10. 
FIG. 11 shows the frequency response of the transducer of FIG. 10 on the 
forward acoustic port in an embodiment wherein the transducer is fixed on 
quartz. For comparison, the frequency response on the reverse acoustic 
port is shown in FIG. 12. Thus the frequency response on the reverse port 
is approximately 5DB less than that on the forward port. The structure is 
thus seen to be substantially unidirectional. 
Thus, the structures of FIGS. 8 and 9, can be weighted or mass loaded to 
cause the transducers to be unidirectional towards each other by mass 
loading appropriated ones of the alternate groups of electrodes. 
FIG. 13 illustrates another embodiment of the present invention which is 
weakly unidirectional in the configuration shown. Thus electrode pair 144 
cancels the reflections of electrode pair 146 because of the .lambda./2 
spacing 156. In addition, electrode pair 148 cancels the reflections from 
electrode pair 150 because of the .lambda./2 spacing 158. However, added 
electrodes 152 and 154 produce reflections which are not canceled and thus 
the device is weakly unidirectional. 
Thus there has been disclosed a transducer configuration which is an 
important reflectionless transducer which can be easily made into a 
unidirectional transducer structure in its own right. It has important 
advantages over the conventional single phase unidirectional transducer 
configuration. Since all of the electrodes and gaps are a minimum of 
.lambda./4, compared to .lambda./8 in the conventional configuration, 
these devices can be fabricated at higher frequencies. They have a 
symmetric input conductance and an undistorted output. They can be made 
unidirectional by mass loading alternate electrodes. In addition, 
.lambda./4 electrodes have greater reflectivity which is a problem 
encountered in conventional single-phase unidirectional transducers. These 
transducers can be expected to find applications whereafter a low loss 
transducer technology is required. They will be very useful for filtering 
for band widths in the one percent to five percent range. The maximum 
useful band width is limited only by the maximum internal distributed 
reflectivity that can be achieved This will be particularly useful for low 
loss filter applications at high frequencies such as cellular radio which 
is in the 800-900 MHz range. At these frequencies, the available 
alternative low loss technology, such as the threephase transducer or the 
split finger transducer, becomes extremely difficult to fabricate. Further 
these unidirectional transducers can be fabricated on quartz, LiNb0.sub.3 
or on any other type of substrate. Also, the number of electrodes, n per 
group, may be any practical number where n is equal to or greater than 2. 
While the invention has been described in connection with a preferred 
embodiment, it is not intended to limit the scope of the invention to the 
particular form set forth, but, on the contrary, it is intended to cover 
such alternatives, modifications and equivalents as may be included in the 
spirit and scope of the invention as defined by the appended claims.