Note: Descriptions are shown in the official language in which they were submitted.
SPECIFICATION
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Su~nary of the Invention
This invention pertains in general to surface wave devices.
These are a class of well-known devices con~rising a medium capable
of prcpagating surface waves, an input OT launching transducer for
launching surface waves across a surface of the substrate~ and an
output or receiving transducer for receiving the propagated surface
waves. Such devices have been developed to an advanced state and
are now in commercial use.
This invention pertains particularly to novel surface wave
devices having the property that the mechanical component of spu~ious
waves reflected from a transducer boundary or internal irregularity
are suppressed. ~ ~;
It has been known that an electrode array composed of a
pair of interdigitated co~bs of conductive ~eeth at unlike potentials,
if coupled ~o a piezoelectric medium, produces acous~ic surface waves
on the medium. In a simplifi~d embodiment of a piezoelectric ceramic
poled perpendicularly to the pTopagating surface, the waves traYel at
right angles to the teeth. The suTface wa~res are conveTted back into
electrical signals by a similar array of conductive teeth coupled to
the piezoelectric medium and spaceh from ~he input electrode array. In
principle, the tooth pa~tern is analogous to an anten~na array. Consequentl~,
similar signal selectivity is possible, thereby eliminating the need for ~
the critical or much larger and more cunbeTsome components normally ~ -
aL~sociated with frequency-selective eiTcuitry. Thus, such a device, Wit}
its small size, is particularly useful in conjunction with solid-state
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functional integrated circuitry where signai selectivity is desired.
The usual surface wave device has a finite distance bet~een its
input and output transducers. Hence, a finite time is required for an
acoustic surface wave to travel along the path from the input transducer
to the output transducer. At the output transducer, part of the acoustic-
wave energy is converted to electrical energy and delivered to a load.
~nother part of the acoustic-wave energy is transmitted past the output
transducer where it may be terminated or dissipated. A still -further part
of the arriving acoustic-wave energy is reflected bac~ along the original
path toward the input transducer. This re:Elected surface wave, which is
smaller in magnitude than the original surface wave, intercepts the input
transducer from which a portion of ~he wave again is similarly reflected
back along the same path to the output transducer where it appears as a
diminished replica of the original surface wave. Because of the additional
distance of travel, the smaller version of the original surface wave arrives
at the output transducer later than that original wave. The time delay
is equal to twice the time required for a surface wave to transverse the
path from the input transducer to the output transducer. When such a
surface wave device is used, for example, as a signal-selective device in
a television intermediate-frequency amplifier, the triple-transit
reflected signal components appear as a ghost in the picture and makes
it highly undesirable, if not completely unacceptable, for normal viewing.
The non-aimed end of a surface wave transducer is another source
of spurious reflections. Intelligence-bearing surface waves launched by a
transducer in the opposite direction from the receiving transducer reflect
internally from the non-aimed end, propagate internally across the
transducer, and emerge from the aimed end out of phase~with the main waves.
The internally reflected spurious boundary wave will appear as a ghost when
received by the output transducer.
Still another source of spurious reflections are irregularities
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within the transducer tooth pattern in the form of radical changes in the
width or spacing of the comb teeth.
Kncwn methods for approaching the spurious wave problem have included
optimizing the signal-transducing characteristics of one or both of the
input and output transducers, depositing an attenuating material between
the input and output transducers, and utilizing an additional transducer,
spaced from the input and output transducers, responsive to a portion of the
original surface wave for generating a still additional acoustic surface
wave that at least partially counteracts the`undesired acoustic wave
originally reflected back from the output transducer.
An improvement in the latter respect is disclosed and claimed
in a patent to Adrian DeVries, No. 3,737,155, assigned to the same
assignee as the present application. As there taught, reflection components,
arising by reason of mechanical loading of the substrate by a transducer,
and local electric field shorting caused by ~he transducer electrodes, are
at least reduced by subdividing each "tooth" of the interdigitated combs
into a pair of conductively connected ribbons spaced apart by one-fourth
the acoustic wavelength. For best results, the DeVries transducer is
connected to a source or load impedance significantly smaller than
the impedance of the transducer itself. It is recognized in U.S. Patent No.
3,723~419 by Robert Adler and assigned to the same assignee as the
present invention, that at least under certain load conditions an additional
contributor to the production of reflection components in a transducer of the
DeVries type may be electrical loading of the substrate which occurs as a
result of electrical shorts created by conductive bars that interconnect
different ones of the ribbons. The approach described and claimed in the
Adler application seeks to overcome such additional electrical loading by
individually connecting the respective ribbons of each adjacent "tooth"-
forming pair to correspondingly separate electrical loads. Howe~er, the
necessity of associating the surface wave device with such plural loads and
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the attendant isolation schemes result in substantial comple~ity.
Yet another approach to eliminating triple-transit
reElections is taught in U.S. Patent No. 3J662,293-DeVries, assigned to the
assignee o the present application. It is there taught that in order to
5 inhibit the development of spurious reflections, a plurality of surface
discontinuities such as grooves are formed in the wave propagating surface
alongside the output transducer. These grooves re-flect surface waves and
are spaced from the input transducer by such a distance that surface waves
reflected from the grooves reach the input transducer in a predetermined
10 time and phase relationship with respect to acoustic surface waves
reflected by the output transducer. At least partial cancellation of the
spurious reflections results.
Still another approach is taught in U.S. Patent No. 3,748,603-
Wojcik, also assigned to the assignee of the present application. Wojcik
15 discloses a surface wave device having input and output transducers disposed
on a surface of a wave-propagative medium. ~ne or both of the transducers
includes a pair of interdigitated combs of conductive material disposed along
the propagaticn path. Adjacent teeth of the combs are spaced apart by a
center-to-center distance of one-half the acoustic center wavelength.
20 Electrically isolated conductive ribbons are disposed individually be~ween
the teeth in respective different pairs of adjacent teeth. The center-to- t-
center spacing between each of the ribbons and the ones of the teeth
adjacent thereto is one-fourth the acoustic center wavelength. The Wojcik
teaching results in at least partial cancellation of wa~es reflected internally
25 from the regular pattern of teeth which constitutes the body of the trcansducer.
It is kno~ that in a surface wave resonator9 surface waves reflected L
from a periodic array of electrically isolated conductive teeth or ribbons
will be 180 out of phase with waves reflected from a periodic array of
ribbons or teeth which are bussed together. See "Relations for ~nalysis
and Design of Surface Wave Resonators" Matthaei et al, IEEE Transactions on
f,
~25398
Sonics, Vol. SU-L3, No. 2, ~arch, l76. See also "Reflective Arrays for
Resonators", P.S. Cross, 1975 Ultrasonics Sym~osium Proc~edinos, T~E_
Ca-t. No. 75 CHO 994-4SU, "Properties o:E Reflective Arrays for Surface
Acoustic Resonators", P.S. Cross, IEEE Transactions on Sonics, Vol. SU-23,
S No. 4, July~ '76, and "Reflections on Surface ~aves from Periodic
Discontinuities", C. Dunnrowicz et al, 1976 Ultrasonics S~y~posium
Proc ~ Cat. No. 76 CHII20-5SU.
It is very significant that all oL the approaches to reflected
wave cancellation which rely on quarter wave spacing of reflecting boundaries,
e.g., 3,662,293-DeVries or 3,748,603-~Yojcik, effect theoretically perfect
cancellation at the synchronous (center) frequency only.
Other U.S. patents which pertain to surface wave devices and in
particular to cancellation of spurious reflection~s in surface wave devices
are: 3,757,256-Whitehouse et al; 3,573,673-DeVries et al; 3,582,838-DeVries;
15 - 3,596,211-Dias et al; 3,559,115-DeVries; and 3,582,580-Adler et al.
Objects of the Invention
It is a general object of the present-invention to provide a new
improved surface wave device in which the mechanical components of spurious
surface waves which are reflected from a surface wave transducer boundary,
or an internal irregular;ty within a transducer, are suppressed.
It is another important object o~ this invention to provide a
surface wave device in which such suppression is not limited to the center
frequency only, but is produced in a broad band of frequencies in the
reflected wave.
It is yet another object of the present invention to provide such
a surface wave device in which the reflection suppression feature adds no
cost to the device.
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In a surface wave transmitting device which
has a piezoelectric surface wave propagative medium and
an interdigitated comb surface wave transducer on a
surface of the medium for launching or receiving on the
surface of the medium surface waves of predetermined
waveleng-th, the improvement, according -to the presen-t
invention is characterized by the transducer having
at a transducer boundary o.r at an internal irregularity
in the transducer tooth pattern a first tooth on a first
comb connected to its associated bus bar, and, on an
opposed second comb a second too-th collinear with the
first tooth bu-t electrically isolated from its associated
bus bar.
~rief Description of the Drawings
The features o the present invention which are
believed to be novel are set for-th with particularity
in the appended claims. The invention, toge-ther with
further objec-ts and advantages thereof, may best be understood
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by reference to the following description takén in conjunction with the
accompanying drawings in which:
Figure l is a partly schematic plan view of a surface wave
device embodying the teachings of the present inventioni and
Figure 2 is a transducer pattern which may be used in the
Figure 1 device and represents an alte m ative embodiment of the invention.
Description of the Preferred Embodiments
. . .
In figure 1, an input signal source 10 is connected across
an electrode array which is mechanically coupled to a piezoelectric
acoustic-wave-propagating medium or substrate 13 to constitute therewith
an input transducer 12. An output electrode array is also mechanically
coupled to substrate 13 to constitute therewith an output transducer 14.
Transducers 12 and 14 are each constructed of two interleaved or inter-
digitated comb-type electrodes of a conductive material, such as gold or
lS aluminum, which may be vacuum deposited on a smoothly-lapped and polished
planar upper surface of the substrate 13. The piezoel0ctric material is one
such as PZT or lithium niobate which propagates acoustic surface waves.
In operation, direct piezoelectric surface-wave-transduction is
accomplished by input transducer 12. Periodic electric fields are
produced across the comb array when a signal from source 10 is applied to
the electrodes. These fields cause perturbations or deformations of the
surface of substrate 13 by piezoelectric action. Efficient generation of
surface waves occurs when the strain components produced by the electric
fields in the piezoelectric substrate substantially match the strain components
associated with the surface-wave mode. These mechanical perturbations
travel along the surface of substrate 13 as generalized surface waves
representative of the input signal.
Source 10 might, for example, be the radio-frequency portion of a
television receiver tuner that produces a range of signal frequencies. However,
due to the selective na~ure of transducers 12 and l~, only a particular
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frequency and its intelligence-carrying sidebands are converted to
surface waves. Those surface waves are transmitted along the substrate
to output transducer 14 where they `are converted to an electrical
signal for transmission to a load 15 connected across the two interdigitated
combs in output transducer 1~. In this example, load 15 represents a
subsequent radio-frequency input stage of the tuner such as the heteroclyne
converter which downshifts the signal frequency to an intermediate frequency.
The potential developed between any given pair of successive
teeth in electrode array 12 produces two waves~traveling along the surface
10 of substrate 13 in opposing directions, perpendicular to the teeth for the ~,
illustrative case of a piezoelectric ceramic substrate which is poled
perpendicularly to ~he surface. When the center-to-center distance between
the teeth is one-half of the acoustic wavelength of the wave at the desired
input signal frequency, the so-called center of synchronous frequency,
relative maxima of the output waves are produced by pie~oelectric transduction
in transducer 12.
To improve the frequency response of transducer 12, it is
preferably of the "apodized" type as shc~n? wherein the combs of
the transducer have a predetermined gradation in the length of overlap
between the interactive comb teeth. Apodized surface ~ave transducers are
well-known in the art. Specifically, examples of active teeth in the
transducer 12 are those shown at 16, 17, 18 and 19. An active tooth is a
tooth having a section overlapping a tooth connected to the bus bar of an
opposed comb. ~he interactive regions of these active teeth can be seen in
figure 12 to define a "manta" configuration symmetric with the transducer
axis.
In order to present a uniform acoustiçal impedance and propagation
velocity to surface waves propagating through the transducer 12 (and thus to
prevent perturbations in the wavefronts), on the opposite side of the
transducer from each of the active ~eeth, and collinear therewith, is an
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inactive tooth. The inactive teeth associated with each of the active
teeth 16, 17, 18 and 19 are designated 16a, 17a, 18a and l9a, respectively.
Since a metalized surface area induces a lower acoustical velocity to surface
waves than a non-metalized surface area, the area filling with inactive
teeth evens out the acoustical velocity throughout the transduc.er.
To prevent internal tooth-from-tooth reflections within ~he
transducer, the transducer 12 is shown as being of the split tooth type
as taught by DeVries in the above discussed Patent No. 3,737,155. Each of the
teeth 16, 17, 18 and 19, for example, is separated, nominally, by a half-
wavelength at the center frequency. Ihis causes the wave contributions fromeach interactive tooth edge to be in phase. ~owever, to prevent internal
reflections from the teeth which make up the tran~sducer pattern, each of the
teeth is split into nominally one-eighth center wavelength ribbons separated
by nominally one-eighth center wavelength spacing. Ihis means that successive
ribbons have their leading edges spaced by one quarter center wavelength
~nominal), resulting in cancellation at the center freq~ency of internally
reflected spurious mechanical wave components.
Ihe ribbons constituting each tooth in the body of the
transducer are interconnected by a shorting bar, one of which is shown at
20 in the figure. The shorting bars are provided to insure that should a
discontinuity in one of the ribbons which constitute a tooth be open
circuited during fabrication of the transducer, the entire tooth would never-
theless be excited from one of the comb bus bars 22, 2~.
Surface wave devices generate a variety of spurious reflected waves.
Some are electrical in nature and others are mechanical. ~e are here concerned
with the mechanical components of reflected surface waves.
Spurious mechanical wave components derive from a number of sources.
As explained above, mechanical wave reflections occur whenever a traveling
surface wave meets a boundary of a surface presenting a different acoustical
impedance to the wave. It can be seen that such boundaries occur at the
leading and trailing edges of each tooth or ribbon and at the entry into
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and exit from the transducer body. By using techniques such as taught
in DeVries-3,737,155, reflections from the teeth which constitute
the body of a transducer are suppressed. Ilowever, reflections of the
waves at the boundaries of the transclucer, either upon entering into
or exiting therefrom, create boundary-reflccted spurious waves which are
not compensated by the DeVries teaching.
As noted, spurious reflections arc also caused by irregularities
within the transducer tooth pattern. These may take the form of radical
changes in the width or spacing of the coml) teeth. Apodized transducers
are commonly of computer generated design ~Id may have tooth width and
spacing variations. Typically these variations, if any, are gradual
and cause no reflection problems. However, if the changes are radical,
the resulting reflections may be of such n~;lgllitude as to be objectionable.
The Figure 1 embodiment below is ;~ddressed to the suppression of
spurious boundary reflections; the figure ~ embodiment shows, in addition,
that the present invention also suppresses reflections f~rom irregularities
within a transducer tooth pattern.
Ihe invention is particularly ad~ltageously applied to transducers
of the apodized t~pe or other types wherein the terminal teeth are not
widely disparate in length, for example, not greater than an order of
magnitude different in length. The reasoll for this limitation will become
evident hereinafter.
As shown in the figure, transducel~ 12 is terminated at its end
nearest the output transducer 14 by an active terminal tooth 26 and an
inactive terminal tooth 28. In the illus-tlated preferred embodiment, the
active and inactive teeth 26, 2~ comprise ~lle ribbon o-f a split tooth DeVries-type transducer element. A single ribbon is used in each of the terminal
teeth 26, 28 to prevent the interactive field strength at the transducer
boundary from becoming disproportionately large. As with the other ribbons
constituting the teeth in the transducer ~`, the terminal teeth 26, 28 are
5~3~38
each nominally one-eighth center wavelength wide and are spaced
nominally one-eighth wavelength from their neighboring transducer teeth.
Without more, the transclucer 12 would have the undesirable property
of reflecting waves entering the transducer or leaving it~ producing
spurious triple-transit and other reflection components when received
at the output transducer 14.
In accordance with this invention, by a very small change in
the design of the metalized transducer pattern, and at absolutely no cost
to the manufacturer of the device, a significant improvement in the
transient response of the device results. Ihis modification or change
is no less than to merely electrically break from its bus bar the inactive
terminal tooth 28 such that it is electrically isolated or "floating". ~his
modification produces a very dramatic change in the nature of the boundary-
reflected surface waves.
Ihis invention takes advantage of the property of surface-reflected
waves that waves reflected from a tooth or ribbon which is joined to a bus
bar are 180out of phase with boundary-reflected waves ~rom an electrically
isolated tooth. Referring to the figure, it will be understood then that
traveling surface waves exiting or entering the transducer 12 will produce
boundary-reflected waves from ~he active terminal tooth 26 which are 180
out of phase with the boundary-reflected waves off inactive terminal ~ooth 28.
Although such is not believed to be a critical limitation,
in the illustrated preferred embodim~nt, for reasons to be explained, the
active terminal tooth 26 and the inactive terminal tooth 28 are arranged
to be collinear.
Before proceeding further, it should be explained thai the property
of surface waves of reflecting at 180-displaced phase relationship off a
bussed tooth and an electrically isolated tooth is a property which takes
place at substantially all surface wave frequencies. In other words, unlike
all spurious wave cancelling mechanisms which employ quarter wave-length or
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other spacings to achieve cancellation of reflected waves, this property
is substantially wavelength independent.
Thrning again to the figure, there is sho~n in broken lines at
30 a simulation of waves boundary-reflected from active .terminal tooth 26.
Similarly, broken lines 32 depict surface waves boundary-reflected off
inactive term m al tooth 28. As explained and as shown in the figure, the
spurious reflected waves 30 and 32 are 180 out of phase. By arranging
the terminal teeth 26, 28 along the transducer axis such that spurious
boundary-reflected waves from one of the tee`th 26, 28 are in cancelling
phase relationship with waves reflected from the other tooth, and with the
provision that the output transducer 14 is of sufficient width on the medium
surface to intercept and electrically destructively combine the out-of-
phase spurious waves, the output transducer 14 will develop output
signals in which spurious signal components due to the aforesaid mechanical
boundary-reflected waves are suppressed.
Because the energy in the reflected waves 30, ~2 is, all other
things being equal, in direct proportion ~o the length of the teeth 26, 28,
it follows that the spurious wave suppression which is achieved in the
output signals developed from the output transducer 14 is in inverse
relation to the magnitude of the difference in the lengths of the terminal
teeth 26, 28. In the ideal case for perfect cancellation, the lengths of
the active and inactive terminal teeth 26, 28 would be nearly equal, with
adjustment for differences in coefficients of reflection from teeth 26 and
28. Ihis implies a zero or node.in the apodization pattern of the transducer
teeth at the transducer boundary.
In the illustrated figure 1 embodiment, it is seen that the
active and inactive terminal tooth lengths are generally the same, but
are not exactly equal. As mentioned, for significant reflected wave cancellation
to occur, the active and inactive teeth should not differ in length by more
than an order of magnitude. Any difference greater than that will result in
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3L~Z53~
relatively insignificant spurious wave cancellation.
Thus it can be seen that by the simple e~pedient, according to
this invention, of floating the inactive terminal teetll, ver~ substantial
suppression of boundary-reflected waves can be achieved in a broad band
of wavelengths centered on the center wavelength for which the surface
wave device is designed.
As shown in the figure, the invention is preferably applied at both
ends of transducer 12. At the opposed non-aimed end of the transducer
the inactive tooth 34 is electrically isolated from its associated bus bar.
The active tooth 36 on the other side of the transducer is electrically
connected to its bus bar. Thus surface waves boundary-reflected internally
back through the transducer toward the output transducer 14 will have
components reflected from the inactive tooth 34 which are 180 out of phase
from wave components reflected from the active tooth 36. Upon reaching the
output transducer 1~ the reflected wave energy is cancelled in inverse
relation to the magnitude of the difference in the lengths of the terminal
teeth 34, 36.
For the reasons given above, the output transducer may be of -
standard construction; it should be of sufficient width to in~ercept boundary-
reflected spurious wave components from both the active and inactive
terminal teeth.
The figure 1 embodiment illustrates the present invention as applied
to suppress spurious wave reflections-off boundaries of a surface wave
transducer (external or on internal openings). The invention has broader
application. The present invention may be applied to transducers of the
type having first and second con~s defining a pattern of interdigitated teeth
and having at a transducer irregularity a first tooth on the first comb
connected to its associated bus bar and on the opposed second comb a second
tooth electrically isolated from its associated bus bar. As described above,
the internal irregularity may be in the form of a radical deparkure in width
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of a tooth (either broader or narrower~, or a radical departure in
tooth spacing, of such abruptness a~s to cause objectionable spurious
wave reflections.
Figure 2 depicts a surface wave transclucer metalization pattern 38
representing a second embodiment of the invention. The figure 2 pattern
may be ~ubstituted for the pattern of transducer 12 in the figure 1
embodiment. The figure 2 transducer pattern is also of the apodized type,
the pattern being sho~n in full rather than in the somewhat abbreviated
form of the figure 1 transducer 12.
l~he figure 2 embodiment shows the invention applied at terminal
teeth 40, 42 at one end of the transducer and terminal teeth 44, 46 at the
opposed end. In this connection, the above description with respect to the
figure 1 embodiment is applicable to the figure 2 transducer pattern.
The figure 2 embodiment is depicted in order to show application
of the invention to s~yress spurious waves reflected from irregularities
in the tooth pattern within the body of a transducer.
In the figure 2 embodiment there is disclosed a tooth made up of
tooth elements 48, 50 connected by a shorting bar 51. The tooth element
48 is characterized by its having a radical departure in tooth element
width and in overall tooth width from the non~nal tooth element width and
tooth width of the majority of teeth in the transducer pattern. Such a
radical departure, which may, for example9 be outside the range of from
2/3 to 1 1/3 times the nominal tooth or tooth element width, will cause
internal reflections within the transducer body ~both directions). Opposite
Z5 the tooth elements 48, 50 which comprise the aforesaid wide tooth, and
collinear therewith is disposed a second too-th similarly constructed,
co~prising tooth elements 52, 54. In order to suppress reflections off these
collinearly arranged tooth elements 48, 52, in accord~nce with this invention,
the tooth element 52 is isolated from its associated bus bar 56. The manner
in which spurious wave cancellation upon reflection off the tooth elements 4S
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and 52 occurs is as described above with respect to the terminal teeth
26, 28 and 34, 36.
Cpposite the tooth constituted by elements 58 and 60 is a
similarly constituted tooth comprising elemènts 62 and 64. Tooth element
62 is narrower than nominal and is of the same width as tooth 58.
In order to compensate for spurious waves reflecting internally
off the narrower-than nominal tooth elements 60, 62, in accordance with the
teachings of this invention the tooth element 60 is electrically isolated
from its associated bus bar 66. Ihus in accordance with the invention~
spurious surface waves reflected from the tooth elements 48, 52 and from
the tooth elements 60, 62 will be in canceling phase relationship due to
the fact that in each of the pairs, one tooth element is electrically
connected to its associated comb bus bar while the other in the pàir is not.
The figure 2 embodiment also shows that the invention is not
lS limited in its application to tooth pairs one of which is active and the
other inactive. In the figure 2 embodiment the teeth constituted by tooth
elements 48, 50 and 52, 54 are positioned at a node in the apodization
pattern. Thus neither of the teeth can be considered to be active. It is
important to understand that the present invention, for its effective
performance, requires only that one of the associated pairs of teeth
(or tooth elements) be electrically isolated from its associated bus bar
while the other is electrically connected to its bus bar.
In applications to apodized transducers where the bounda~y or
irregularity does not occur at a node, it is preferred to isolate the
inactive, rather than the active~ tooth or tooth element only so as not to
lose the programmed signal generation capacity of the active tooth.
The figure 2 application of the invention to an internal
irregularity in a transducer pattern located at a node in an apodization
pattern represents an ideal application of the invention in terms of
obtaining perfect cancellation. As explained above with respect to figure 1,
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suppression occurs in inverse relation to the magnitude of the difference
in length of the connected and disconnected teeth or tooth elements.
Where the diEference is substantially zero, perfect cancellation may occur,
all other considerations aside.
Whereas in the figure 1 and 2 embodiments the apodized transducer
with spurious wave suppression according to the invention have been described
as being the input transducer. It should be understood that the said
transducer may be as well be the output transducer. If such is the case,
waves launched by the input transducer ~which may be as shown at 14 in
figure 1) will not be reflected off boundaries or irregularities in the
output transducer since it is adapted with the present invention. Triple
transit reflections are thus suppressed.
Ihe invention may also be applied to surface wave devices of
other types and constructions. For example, the invention is applicable
in surface wave systems including a surface wave multi-strip coupler element
which picks up and integrates out the reflected wave co~ponents. A multi-
strip coupler does not develop output signals, but rather acts to transfer
surface acoustic waves from one transducer to another.
While particular embodiments of the invention have been shown
and described, it will be obvious to one skilled in the art that changes
and modifications may be made without departing from the invention in its
broader aspects, and, therefore, the aim in the appended claims is to cover
all such changes and modifications as fall within the true spirit and scope
of the invention.
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