Electronic component operating with acoustic waves

Electronic components which operate with acoustic waves are provided as interdigital structures and/or reflective arrangements. An oscillating displacement of the significant plurality of sub-groups is provided parallel to the principal axis of propagation in a weighted region, each sub-group having at least two successive real finger edges and, under certain conditions, virtual finger edges. A plurality of principal groups is provided and in a number which is equal to or greater than the time/bandwidth product. Each principal group has at least two real sub-groups and the respective degree of displacement of the finger edges of the individual sub-group corresponds to an additional oscillating phase modulation for the sequence of the sub-groups of the weighted region with the oscillation being such that the number of oscillations within a principal group is a whole number which is not greater than half the number of the sub-groups therein.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electronic component which operates 
with acoustic waves as an interdigital structure and/or "reflective array" 
arrangement having an input and an output transducer and with, under 
certain conditions, at least one reflector structure. At least one of the 
structures has at least one weighted region of the fingers of the 
structure. 
2. Description of the Prior Art 
Numerous embodiments of the electronic components are known from the prior 
art which operate with acoustic waves and which are employed as electrical 
frequency filters, signal generators, oscillators and the like. As a rule, 
these components have at least one input transducer and at least one 
output transducer which, depending upon the embodiment, can also coincide 
operationally to form one transducer. Such transducers can have the format 
of an interdigital structure including meshing electrode fingers. 
Of significance are embodiments of such an electronic components which, in 
addition to the input transducer and the output transducer, also have one 
or more reflector structures which likewise comprise a respective 
multitude of fingers or, respectively, digit strips or, respectively, 
configurations corresponding thereto. There are "in-line" reflector 
structures (normal incidence) wherein 180.degree. reflection at the 
fingers occurs in the reflector structure. Other such reflector 
arrangements have two or more reflector structures disposed next to one 
another whereby the fingers of one reflector structure are disposed at an 
angle of approximately 90.degree. relative to the fingers of another 
reflector structure (oblique incidence). 
An electronic component of the types set forth above can be dimensioned 
such that it executes a signal processing corresponding to a prescribed 
transfer function and emits the output signal corresponding to that 
function. Thereby dimensioned are the finger spacing and the weighting of 
the individual fingers, whereby a changing finger spacing is provided for 
filters having a dispersive property. As known, the mathematical 
determination of the required dimensioning is carried out by forming the 
Fourier transform of the transfer function. The Fourier transform is also 
referred to as the filter pulse response. It can be represented as a 
complex function 
EQU s(t)=a(t).multidot.e.sup.j[.omega. o.sup.t+.phi.(t)]. 
For a(t).noteq. constant, this function requires at least one weighted 
digital structure in the component. The weighting of a digit structure or, 
respectively, of the individual fingers of the structure is a 
designational reduction of the mechanical or, respectively, 
electro-mechanical efficiency of the fingers of the structure. Known in 
this regard for an interdigital structure is to have the mutually-adjacent 
electrode fingers situated at mutually-different potentials overlap one 
another to different degrees. Given high weighting, a topical overlap 
which is now only very slight occurs, this leading to disadvantageous wave 
diffractions. 
In addition to the usually very disadvantageous measure of a more or less 
pronounced shortening of a respective finger, the measure also exists for 
reflector structures of replacing a finger (unweighted) designed as a 
through strip with a series of individual points (referred to as "dots") 
corresponding to the strip. The density and/or size of the "dots" 
dimensioned larger or smaller corresponds to a more or less shortened and, 
therefore, weighted finger (and such a respective structure replacing a 
finger is also referred to herein as a finger or digit strip). 
Disadvantageous in such an execution is that the "dots" cause noise 
signals due to the undesired reflection and scatter behavior (particularly 
given a higher weighting, i.e. a lower density of the "dots"). The 
original embodiment of weighted fingers in a reflector structure consists 
of realizing such fingers as grooves in the surface of the substrate and 
to make such a groove deeper or shallower corresponding to the weighting. 
This technology, in turn, has the disadvantage that it is extraordinarily 
expensive and is difficult to control. 
In connection with the above, one may refer to the 1979 Ultrasonic 
Symposium Proceedings IEEE, pp. 696-700 of Chapman and of Kitano at pp. 
585-589 of the same publication and 1976 Ultrasonic Symposium Proceedings 
IEEE, pp. 406-410 of Godfrey, all of which are fully incorporated herein 
by this reference. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a technological execution 
of the weighting of the fingers of a digit structure, particularly 
employed for a reflector structure, which is less involved and can be more 
easily controlled (for example, in comparison to weighting by way of 
grooves with varying depths) and/or leads at most to minimum disruptions 
(for instance, in comparison to a weighting with "dots"), or, 
respectively, to provide such for an electrode finger structure which 
exhibits no defraction effect induced thereby, even given higher 
weightings. 
The above object is achieved, according to the present invention for an 
electronic component of the type generally set forth above in that an 
oscillating displacement .DELTA.Z of a significant plurality of sub-groups 
is providedparallel to the principle axis Z of the wave propagation in a 
weighted region, in comparison to that case in which the region would be 
unweighted. In this structure each individual sub-group has at least two 
successive, real finger edges and, under certain conditions, further 
virtual finger edges. A plurality of n principle groups is present, where 
n is equal to or greater than T.multidot.B, where T.multidot.B is the 
time/bandwidth product and where T is the prescribed duration of the 
filter pulse response and B is the prescribed bandwidth of the transfer 
function. Each individual principle group has at least two real 
sub-groups. The respective degree of the displacement of the finger edges 
of the respective individual sub-group corresponds to an additional 
oscillating phase modulation b(t) for the sequence of the sub-groups of 
the weighted region and the phase modulation b(t) oscillates such that the 
plurality of oscillations within a principle group is an integer which is 
not greater than half the plurality of the real sub-groups contained in 
the principle group. With this structure, the relationship 
EQU e.sup.jb(t) .apprxeq.a(t)/r(t) 
applies, wherein e.sup.jb(t) is the average of the e function over a 
principle group approximately equal to the ratio a(t)/r(t), where a(t) is 
the prescribed amplitude modulation of the filter pulse response, r(t) is 
equal to or greater than a(t) and is valid for all values of t. The degree 
of the displacement .DELTA.Z.sub.i of the finger edges of a respective 
i.sup.th sub-group at its location Z.sub.i of its center is 
EQU .DELTA.Z.sub.i =.lambda..sub.i /2.pi..multidot.b(t.sub.i), 
wherein t.sub.i =Z.sub.i /V, where Z is the coordinate of the principle 
direction of the wave propagation with the velocity v and .lambda..sub.i 
is the wavelength at the location of the i.sup.th sub-group. 
The present invention is based on the following considerations and 
perceptions. 
Over the frequency entered on the abscissa 2, FIG. 1 illustrates the 
prescribed amplitude curve 1 of a transfer function (the amplitude being 
entered on the ordinate 3). With a broken-line curve 4 (to which the 
envelope delay time is entered on the ordinate), FIG. 1 additionally 
illustrates the prescribed curve of the envelope delay time .tau.(f) of 
the overall required transfer function of the component of the present 
invention operating with acoustic wave and appertaining to the prescribed 
amplitude curve 1. The illustrated transfer function is an example as 
typically occurs in chirp modulation filters. This transfer function 
comprises the two curves 1 and 4 and corresponds to a specific Fourier 
transform which is the time-dependent filter pulse response: 
EQU s(t)=a(t).multidot.e.sup.j[.omega. o.sup.t+.phi.(t)]. (I) 
For the purpose of dimensioning the fingers according to the present 
invention (for simplification the filter response is essentially realized 
by only one finger structure), this expression is converted into the 
following form in which the function s(t) occurs from the convolution 
integral with the function s(t): 
##EQU1## 
In this representation, the amplitude function a(t) has been transformed 
into the phase function b(t) which is correspondingly decisive for the 
dimensioning, according to the present invention, of the shift of finger 
edges for the purpose of weighting the appertaining structure. 
The weighting a(t) to be realized in accordance with the invention occurs 
from the transfer function prescribed for the appertaining component, 
deriving according to the amplitude 1 and the envelope delay time 4 
thereof. 
This mathematical transformation of s(t) into s(t) is accompanied by the 
appearance of the expression r(t) and of the convolution function g(t). 
The function r(t) is the envelope of the digital structure not equipped 
with the additional phase weighting of the present invention. For the sake 
of completeness, it should be pointed out that the function r(t) can also 
contain an additional amplitude modulation which would have to be 
realized, given a component according to one of the known methods of 
finger weighting, for example, one of the methods described above. The 
convolution function g(t) essentially represents the filter pulse response 
of an input and/or output transducer given an interdigital structure or, 
respectively, given an "in-line" reflector structure, or it corresponds to 
the convolution-like integration (filtering) inherent in the aperture of a 
reflector structure having obliquely-located reflector fingers, i.e. 
having double 90.degree. reflection. The function g(t) with the modified 
function of the present invention, namely the function s(t) as in equation 
II. This operation mathematically referred to as convolution corresponds, 
in the frequency range (of the illustrated FIG. 1) to side bands being 
filtered out. Such side bands occurring in conjunction with the present 
invention are illustrated in FIG. 1 with a dotted curve and a referenced 
5. 
The side bands 5 are filtered out in conjunction with the present 
invention, in particular, predominantly by way of corresponding 
dimensioning of the input transducer and/or of the output transducer. As 
likewise already stated above in different terms, this can be achieved for 
the reflector structure having oblique fingers by selecting a 
correspondingly large aperture value, i.e. correspondingly long fingers of 
the structure. Only for the sake of completeness, it should also be 
pointed out that this filtering can also occur outside of a component 
constructed in accordance with the present invention, for example, in 
further modules of an overall device having a filtering effect. Because of 
the very simple possibility of filtering such side bands 5 offered by a 
component constructed in accordance with the present invention, however, 
the latter possibility is seldom employed. 
In order to realize the modified function s(t) according to equation II, 
finger edges of a digital structure constructed in accordance with the 
present invention are oscillatingly displaced (with a minimum frequency), 
in particular, relative to a topical position (in the structure) which 
would derive for a corresponding digital structure if this had no 
weighting at the appertaining location of the respective finger edge, i.e. 
if the fingers were unweighted at that point. In an unweighted, 
non-dispersive digital structure, for example, the fingers and, therefore, 
their finger edges as well, are disposed at equi-distant intervals, for 
example, in .lambda..sub.o /2 intervals given interdigital arrangements 
(without "split fingers") and in "in-line" reflector structures and at 
intervals .lambda..sub.o given a reflector structure having oblique 
fingers and double 90.degree. reflection, in particular, measured parallel 
to the principle axis or, respectively, normals of the wave front of the 
acoustic wave. 
The displacement of the finger edges provided by the present invention 
occurs in accordance with the function b(t) which will be discussed below, 
in particular, group-wise for the finger edges. A respective plurality of 
finger edges is combined into individual sub-groups, i.e. the same degree 
of displacement in terms of amount and direction applies to all finger 
edges of a respective sub-group. A multitude of sub-groups is present in a 
weighted region of a digital structure constructed in accordance with the 
invention. Each individual sub-group has at least two real finger edges, 
but can also comprise a greater plurality of finger edges. These further 
finger edges can be real and virtual finger edges. A real finger edge is 
an edge of a finger or, respectively, of a digit strip such as is actually 
present in the structure, for example, as a metallization strip. A virtual 
finger edge, however, in the sense of the present invention, is a finger 
edge of a finger or digit strip which is omitted in the structure. In 
particular, it is not necessary for a digital structure that it be 
provided with a maximally-possible plurality of fingers or, respectively, 
digit strips in accordance with the wave length of the center frequency. 
As known, a digital structure also has the corresponding effect when a 
larger or smaller plurality of fingers is omitted (thinned digital 
structure). 
The definition of a sub-group set forth above contains, among other things, 
the possibility that it consists of two real finger edges, whereby two 
real finger edges belong to one and the same finger (fingers). A further 
possibility is that it again consists of two real finger edges whereby, 
however, these two finger edges are the mutually-neighboring finger edges 
of two adjacent fingers. The boundary between two adjacent sub-groups, 
accordingly, extends through a finger. A sub-group can, for example, also 
comprise three real finger edges, such sub-group encompassing the complete 
finger and half of a neighboring finger whose other half belongs to the 
next sub-group. When, for example, every second finger is omitted in an 
appertaining digital structure, then a smallest sub-group comprises four 
finger edges, in particular, two real edges and two virtual edges, i.e. of 
a complete finger or of two halves of respective fingers with an 
intervening omitted finger. 
According to the invention, a plurality of sub-groups is combined into a 
respective principle group for the dimensioning of the finger edge 
displacement. Each individual principle group contains at least two real 
sub-groups, i.e. sub-groups actually represented in the structure by 
fingers. The rule applies to these two sub-groups of a respective 
principle group that the finger edges of the one sub-group are displaced 
opposite (towards or away from) the finger edges of the other sub-group. 
Other possibilities exist for a principle group having more than two 
sub-groups; therefore, for example, given three sub-groups the center 
sub-group remains undisplaced and only the finger edges of the one outer 
sub-group are displaced relative to those of the other. In a principal 
group comprising four sub-groups, for example, one or two sub-groups can 
remain displaced. The amounts of displacement of the real sub-groups of a 
principal group (having more than two real sub-groups) need not be 
identical. However, the sum of the displacements of all sub-groups of such 
a principal group must be equal to zero, leaving out of consideration 
that, given different finger intervals, the dimension of the finger 
interval distances (as a very slight correction value) enters due to a 
prescribed dispersion or, respectively, non-constant group delay time 
and/or leaving out of consideration that fingers of a respective sub-group 
have differing electro-mechanical (interdigital transducer) or, 
respectively, mechanical reflector) efficiencies and/or that their 
plurality differs. 
A rule concerning the minimum plurality n of principal groups also applies 
for practicing the invention, in particular, n must be at least equal to 
or greater than the time/bandwidth product (T.multidot.B), wherein T is 
the duration of the filter pulse response and B is the bandwidth of the 
prescribed transfer function (v. FIG. 1.). 
As was already mentioned above, further specifics concerning the function 
of the additional phase modulation b(t) are to be provided. The 
relationship 
EQU e.sup.jb(t) .congruent.a(t)/r(t) III 
holds true wherein e.sup.jb(t) is the average of the function e over a 
principal group. 
An envelope a(t) which differs from the constant, causes a weighting of the 
amplitude 1 of the transfer function, in particular, for a realization of 
the structure having fingers or, respectively, digital strips. 
It is advantageous when the necessarily occurring side bands overlap, at 
most insignificantly with the required principal band. This is achieved, 
in particular, by the selection of the additional phase modulation b(t), 
so that the plurality of oscillations contained in the respective 
principal group is equal to half the real sub-groups contained in the 
principal group.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The transfer function has already been discussed above with respect to FIG. 
1 in terms of magnitude 1 and envelope delay time 4. 
Referring to FIG. 2, a component 11, constructed in accordance with the 
present invention, is an interdigital structure comprising an input 
transducer 12 and an output transducer 13. The input transducer 12 has a 
conventional structure. In particular, the spacings of the individual 
fingers of the transducer 12 are mutually equidistant and =.lambda..sub.o 
/2. The transducer 13, on the other hand, is designed in accordance with 
the present invention. The finger weighti ng of the transducer 13, which 
would be executed in a conventional manner by different lengths of the 
individual fingers, is realized here by a displacement .DELTA.Z, according 
to the invention, of the individual fingers of a respective sub-group, 
whereby all fingers have the same length in general. An additional 
weighting could be provided wth a slighter or greater difference in finger 
length, this weighting, however, having nothing at all to do with the 
present invention. By way of a correspondingly large displacement .DELTA.Z 
of the fingers and, therefore, of the finger edges of the individual 
sub-groups, one can also realize a very high weighting without defraction 
effects occurring, these, in particular, occurring given finger length 
weighting when the overlap of adjacent fingers remains only very small in 
accordance with high weighting. 
The terminals of the transducers 12 and 13, the substrate on which the 
transducers are located, the end edges of the substrate body slanted in 
the direction .+-.Z and further details not affecting the invention are of 
conventional design and it is therefore not necessary to discuss the same 
in further detail herein. 
The "in-line" reflector illustrated in FIG. 3 comprises a transducer 12 
which serves as an input transducer and an output transducer. In 
practically all structural details, it can correspond to the transducer 12 
of FIG. 2. Even given the embodiment according to FIG. 3, the transducer 
12 can be so broadband that it does not influence the overall required 
transfer function 1. A reflector design in accordance with the present 
invention is illustrated at 23 and comprises individual digit strips which 
are usually of equal length. In accordance with conventional methods, one 
would realize the weighting of such a reflector by way of correspondingly 
different depths of the etched grooves or by way of a greater or lesser 
density of "dots" (namely with the disadvantages set forth above). Given 
the present invention, however, the digit strips of the structure 23, i.e. 
the finger edges of the digit strips to be more precise, are displaced by 
the respective dimension .DELTA.Z. For a non-dispersive digital structure 
23, the displacement .DELTA.Z occurs as a displacement relative to an 
equidistant finger center-to-center spacing. When the digital structure 23 
has a dispersion, then it already has non-equidistant finger 
center-to-center spacings even in the unweighted condition on which the 
same modulated respective displacement .DELTA.Z of the individual finger 
edges is then superimposed. 
Whereas an embodiment of the invention is illustrated for the transducer 13 
according to FIG. 2 wherein the fingers or, respectively, digit strips 
continue to retain mutually identical widths and only have different 
spacings from one another (whereby individual fingers are also omitted as 
virtual fingers), the illustration of FIG. 3 shows fingers or, 
respectively, digit strips which exhibit different widths. These different 
widths derive from the displacement of the finger edges whereby, for 
individual fingers, the two edges of the individual finger are displaced 
towards one another (narrower fingers) and, given other individual 
fingers, the two edges of such a finger are displaced away from one 
another (broader fingers). 
FIG. 4 illustrates what is meant to be understood by a 90.degree. 
reflector. Such a component according to the present invention also has an 
input transducer 12 and an output transducer 12 which can be identically 
designed. An example of a wave path is indicated with a broken line 31. A 
respective reflection occurs in the reflector 33 and in the reflector 33' 
which both have oblique disposed reflector fingers. The principal wave 
direction or, respectively, abscissa of the displacement .DELTA.Z of the 
fingers of the structures 33 and 33' is again indicated with the arrow Z. 
The two structures 33, 33' have virtual fingers, i.e. corresponding 
fingers are omitted at gaps. Given these structures according to FIG. 4, 
the weighting of the structures 33, 33' also comprises finger edge 
displacement .DELTA.Z. As a rule, the fingers of the structures 33, 33' 
are of equal length for each of the structures and, in particular, also 
for the structures among one another. 
The aforementioned aperture of such a structure is the projection of the 
actual finger length onto the normal and the abscissa Z, i.e. is equal to 
the width of the individual structures 33, 33'. 
For the sake of completeness, it should be pointed out that the 
displacement .DELTA.Z offered by the present invention is measured 
parallel to the abscissa Z, i.e., is measured in a direction oblique 
relative to the finger direction. The explanations provided concerning the 
following figures relate to fingers or, respectively, finger edges aligned 
perpendicular to the abscissa Z. These explanations below accordingly 
apply by analogy for obliquely-placed fingers or, respectively, finger 
edges as well, as occur in the structures 33, 33'. 
On a substrate shown broken off at both ends along the direction Z, FIG. 5 
schematically illustrates a portion of a digital structure 52 designed in 
accordance with the present invention, i.e. a weighted digital structure 
52 according to the invention. A plurality of real and virtual sub-groups 
follow one another from the left towards the right, i.e. in the direction 
Z. The first sub-group as viewed from the left is referenced 53 and the 
next sub-group is referenced 54. In accordance with the arrows 55, the 
finger edges and, therefore, the fingers of these two sub-groups 53 and 54 
are displaced towards one another. The degree of the displacement is 
indicated by the mathematical expressions below the sub-groups 53 and 54. 
The displacement .DELTA.Z.sub.i applies to each finger edge of the 
sub-group 53, is directed towards the right, and, moreover, is also a 
function of the locus coordinate Z.sub.i (the center of the sub-group 53). 
The analogous case applies to the sub-group 54. The digit strips 
illustrated with solid lines are real fingers having real finger edges. 
Shown with broken lines and referenced 56 are two finger locations, i.e. 
virtual fingers being indicated, which are in fact not present on the 
structure. These two fingers are advantageously omitted because, as can be 
seen from the drawing, their spacing from one another is greatly reduced 
by the mutually opposing displacement .DELTA.Z of the two sub-groups 53 
and 54. Such a spacing between two fingers which has become very small, in 
particular, creates substantial problems when such a structure is an 
interdigital structure wherein, as illustrated in FIG. 2, 
mutually-adjacent fingers lie at mutually-different electrical potential 
and causes a modulation of the electric field between and/or electrical 
arcing becomes very probable. Three further sub-groups following one 
another towards the right are referenced 57, 58 and 59. The spacing 
between the sub-group 54 and the sub-group 57 provides an indication of at 
least one virtual sub-group, such that (k-2) real sub-groups are lacking 
at that location, i.e. the corresponding plurality of virtual sub-groups 
is to be assumed. 
As illustrated, the finger edges of the fingers of the sub-group 57 are 
displaced towards the left, those of the sub-group 59 are displaced toward 
the right and those of the sub-group 58 are not displaced. 
Together, the sub-groups 53 and 54 form a principal group 152. In this 
principal group, the overall displacement +.DELTA.Z.sub.i and 
-.DELTA.Z.sub.i+l is essentially equal to zero. The analogous case applies 
to the principal group 152' which is formed by the sub-groups 57, 58 and 
59. 
The individual principal groups correspond to the required minimum 
plurality n of principal groups and follow one another along the abscissa 
Z. The sub-groups which form the individual principal groups form a 
corresponding sequence of groups. 
FIG. 6 illustrates a section of a particular embodiment of the invention. 
The boundaries of neighboring sub-groups are identified with broken 
straight lines 61. As can be seen, each sub-group has two finger edges 62 
which, however, belong to different fingers. The boundaries 61, therefore, 
extend through the individual fingers so that narrow fingers 63 and broad 
fingers 64 follow in alternation. The displacement .DELTA.Z of the 
individual finger edges 62 corresponds, in terms of amount, to its topical 
displacement in the direction Z relative to that position of the finger 
edges of a comparible structure having fingers of mutually-identical width 
which would have no weighting corresponding to the finger edge 
displacement. 
FIG. 7 illustrates a device wherein the boundaries 61 of neighboring 
sub-groups lie between neighboring (real or virtual) fingers. In the 
embodiment of FIG. 7 as well, each individual sub-group has only two 
finger edges which (in contrast to FIG. 6), however, belong to one and the 
same finger 71. In contrast to the unweighted case, the finger width here 
remains constant. As can be seen, however, the adjacent fingers are 
alternately displaced towards one another or away from one another. The 
respective degree of approach or distancing of the fingers 71 from one 
another, just as with the broadening and narrowing of the fingers 
according to FIG. 6, depends on the weighting required at the respective 
location of the digital structure. 
FIG. 8 indicates an embodiment of the invention wherein the two illustrated 
sub-groups between the boundary 61 comprise six respective finger edges. 
As seen in FIG. 6, the boundary 61 extends through individual fingers. The 
illustration shows how broader fingers 64, very narrow fingers 63 and 
fingers 66 with unaltered width thereby occur. Should the realization of a 
finger 63 as illustrated in FIG. 8 present technological difficulties, 
particularly because it has become extremely narrow, such a finger can 
also be omitted without causing any disadvantage, i.e. can be a virtual 
finger. 
Although I have described my invention by reference to particular 
illustrative embodiments thereof, many changes and modifications of the 
invention may become apparent to those skilled in the art without 
departing from the spirit and scope of the invention. I therefore intend 
to include within the patent warranted hereon all such changes and 
modifications as may reasonably and properly be included within the scope 
of my contribution to the art.