Interdigital transducer with finger-width weighting for surface wave arrangements

An interdigital transducer is weighted by means of finger-width weighting for surface wave arrangements. The weighting is produced by differing finger widths in conjunction with constant widths of finger gaps. Constant finger overlap length is preferably provided.

BACKGROUND OF THE INVENTION 
The present invention relates to an interdigital transducer for surface 
wave arrangements having busbars, transducer fingers alternately connected 
to the busbars, and finger gaps between the transducer fingers. 
Surface wave arrangements operating by means of acoustic waves which 
propagate in the surface of a substrate have been known for approximately 
two decades. Such arrangements are used as electronic filters instead of 
arrangements comprising inductors and capacitors. The interdigital 
transducers to be used as output or input transducers comprise a number of 
finger-shaped electrode strips which are arranged on the substrate surface 
in an interdigital assignment relative to one another. To be more precise, 
these fingers are (electrically) connected in an alternating fashion to 
one or the other busbar. 
In order to have a filter with a prescribed filter curve, it has been known 
just as long to select the overlap lengths of the interdigital fingers of 
the transducer in a manner matched to the filter curve. This is thus the 
so-called overlap weighting. 
Examples of overlap weighting are described in EP-A-0,188,263 and 
DE-A-2,546,193. Both examples have, moreover, a design such that the 
transducers are reflection-free. The finger widths and the finger gaps are 
positioned in a fundamentally lambda-periodic fashion. Individual offsets 
are required for the freedom from reflection. 
A surface wave structure which can also be overlap-weighted is described in 
DE-A-4,010,310, which was not published prior to the date of filing the 
present application. In this structure, final fingers are provided which, 
with respect to their finger width and their finger spacing, have 
different dimensions by comparison with the other fingers of the 
structure. These divergently dimensioned fingers are not weighted and 
serve merely to suppress reflections which otherwise emanate from final 
fingers. 
Also known is finger-omission weighting, the desired weighting of the 
transducer being realized by purposive omission of individual fingers of 
the transducer. Omitted fingers can be replaced in this case by 
non-alternating electrically connected fingers. Moreover, finger-offset 
weighting is known, in which individual prescribed fingers of a transducer 
are arranged offset with respect to the prescribed periodicity of the 
transducer. These fingers are therefore no longer exactly in phase with 
the transducer periodicity of the comparatively un-weighted interdigital 
transducer. 
An example of a finger-omission weighting is described in EP-A-0,369,835. 
FIG. 7 of this printed publication shows an interdigital finger structure 
having fingers whose centerline spacings are non-periodic. The finger 
centerline spacing is 4.5 lambda at the ends of the transducer and 1 
lambda at the center of the transducer, and spacings of other fingers have 
values between these multiples of the wavelength lambda. There are no 
special instructions for the finger gaps. 
These types of weighting which have been mentioned above have been 
adequately described before, together with their advantages and 
disadvantages, in the prior art. Overlap weighting leads to problems in 
adhering to the aperture of a transducer. Many of the characteristics of 
the overlap-weighted transducer can be described only in two dimensions. 
In particular, the influence of manufacturing tolerances becomes strongly 
noticeable in the case of only small overlap lengths. This results 
frequently in problems in designing filters and in the reproducibility at 
critical points. Furthermore, it is only with reluctance that two 
overlap-weighted transducers are used directly opposite one another in a 
filter. To be precise, this is attended by the difficulty of having to 
correlate each individual excitation in the transducer operating as input 
transducer with each detecting overlap of the transducer operating as 
output transducer. It is known, for example in television band pass 
filters, to weight only one transducer and to provide the second 
transducer with a time signal which corresponds to a simple rectangle. 
The method of finger-omission weighting, which is more rarely applied, in 
any case, has the disadvantage that the desired time signal cannot be 
effectively scanned, because all that is available for each overlap is to 
assign "ones" and "zeros", that is to say finger present and finger 
omitted, so that the dynamics of this weighting method are mostly 
unsatisfactory. 
SUMMARY OF THE INVENTION 
The aim of the invention which is to be described below is to eliminate the 
shortcomings of the prior art. It is thus the object of the present 
invention to specify a weighting method for interdigital transducers for 
surface wave arrangements by means of which the prescribed filter function 
to be realized can be better apportioned between input transducer and 
output transducer of the filter, and/or by means of which substantially 
weaker side lobes (secondary lobes) are to be achieved and/or by means of 
which better filter characteristics are to be obtained with the same 
substrate length without it being necessary to set up more stringent 
requirements with respect to manufacturing tolerances. 
This object or its key problems respectively arising in the individual case 
is/are achieved in accordance with the present invention by means of an 
interdigital transducer which are periodically positioned with a constant 
width of the finger gaps at least in the region of the prescribed 
weighting of the transducer, wherein, starting from a mean dimension for 
which all transducer fingers are of equal width, in accordance with the 
prescribed weighting it is the case that in a weighting region the width 
of the transducer fingers of one busbar are equal to or greater than, and 
the width of the transducer fingers of the other busbar are equal to or 
narrower than this mean dimension. Finger gaps of equal width are provided 
which are positioned with their centers offset by comparison with the 
periodicity of the arrangement of the transducer fingers in accordance 
with the prescribed weighting, and the transducer fingers are essentially 
periodically positioned with reference to their respective center. 
It remains to refer to an only apparently relevant prior art, that is, U.S. 
Pat. Nos. 3,936,774 and 4,162,465. Both U.S. Patents exhibit nonweighted 
structures. U.S. Pat. No. 3,936,774 relates to a dispersive structure 
having finger spacings differing in accordance with the band width. U.S. 
Pat. No. 4,162,465 relates to a transducer in which the differing finger 
positioning and differing finger width are periodic over the transducer. 
That is to say, no weighting is present in this regard. This design of 
transducer serves to generate in the transducer a specific reflected wave 
component which interferes with a signal component which is based on the 
electrical signal reflection at the load impedance. 
A new weighting method, which consists in a width variation of the 
interdigital fingers provided on the transducer, has been found for the 
present invention. This new finger weighting is only apparently along the 
lines of the known finger-offset weighting. In fact, this new method of 
finger weighting has different physical foundations. In the present 
invention, the strip-shaped gaps between two adjacent transducer fingers 
uniformly have an identical width in principle (in the direction of the 
principal direction of wave propagation). The individual fingers of the 
transducer are, by contrast, of differing widths in accordance with the 
measure of the local weighting in the transducer. The wavelength, 
resulting in the material of the substrate, of the acoustic wave occurring 
for the center frequency of the filter in the transducer determines the 
periodicity p (=lambda) of the wave in the principal direction of wave 
propagation (orthogonal to the alignment of the fingers). The transducer 
fingers of an interdigital transducer weighted according to the invention 
are (at least in one region of the transducer) positioned such that the 
center points of the transducer fingers, seen in the direction of the 
width of the fingers, that is to say seen in the principal direction of 
wave propagation, correspond to this periodicity 2p. That is to say, the 
periodicity measure p/2 is always adhered to from the center of one finger 
to the center of the respectively adjacent finger. This is precisely not 
the case in the known finger-offset weighting. 
In a transducer according to the invention, low activity is present 
wherever particularly wide fingers of one busbar, that is to say of one 
polarity, alternate with particularly narrow fingers of the other busbar. 
In a finger-length-weighted transducer this would be the region of small 
overlap lengths having a large diffraction effect. This disadvantageous 
diffraction effect is ruled out in the invention. 
Compared to transducers having known finger weighting, it is frequently the 
case that transducers weighted according to the invention have a width of 
the gap between adjacent fingers which is smaller than corresponds to 
known transducers with dimensions for which the finger width is equal to 
the width of the finger gap, that is to say respectively lambda/4. 
For example, it is provided in the invention to select the width of the 
finger gaps, which remains of large dimension, to the width of the widest 
finger width occurring in the weighted transducer. Given a present, 
technically dictated limit of 0.5 .mu.m to the optical resolution in the 
production of finger structures, this means that transducers weighted 
according to the invention can easily have center frequencies into the 
range of 500 MHz. In particular, the entire range of television 
intermediate-frequency filters is easily covered. 
A transducer according to the invention generally has the same finger 
length throughout. Despite low transducer weighting at the start and at 
the end of such a transducer, aperture thereof is not changed. 
In the case of the invention, however, the centers of the finger gaps are 
not positioned in accordance with the periodicity p. 
In the case of the invention, the finger-width weighting according to the 
invention can also further have a certain measure of finger-overlap 
weighting superimposed, by means of which a larger weighting margin is to 
be obtained. The overlap measure then provided is, however, kept very 
restricted, that is to say a minimum overlap length is always adhered to, 
in order to continue to rule out undesired diffraction effects at the ends 
of the transducer, or to restrict them in a prescribed fashion. 
Additional features and advantages of the present invention are described, 
and will be apparent from, the detailed description of the presently 
preferred embodiments and from the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Further explanations of the invention emerge from the following description 
relating to the figures, in which embodiments are specified. 
FIG. 1 shows a diagram of the principle of a transducer weighted according 
to the invention with low weighting at each end of the transducer. With 
regard to weighting alone, that is to say apart from the side effects in 
transducers weighted in a known fashion, the transmission curve or the 
time signal of this transducer of FIG. 1 is equivalent to the meandering 
line, specified in FIG. 2, of a finger-length weighted interdigital 
transducer. 
In FIG. 1, 1 designates the interdigital transducer, and 2 and 3 the two 
mutually opposite busbars. 4 designates the fingers connected to the 
busbar 2, and 5 the fingers connected to the busbar 3.6 designates the 
known dummy fingers. 7 refers to the gaps, mentioned frequently above, 
between the adjacent fingers 4 and 5. The undulating line designated by 8 
and plotted in FIG. 1 is the periodicity, thus revealed in the direction 
of the principal direction of wave propagation H, with p=lambda of the 
acoustic wavelength of the wave which is emitted by the transducer 1 on or 
in the surface of the substrate. Clearly, the centers, revealed by large 
points, of the widths of the fingers 4 and 5 coincide essentially with the 
"zero crossings" of the undulating line 8. This is characteristic of the 
invention and distinguishes it essentially from the prior art. 
The meandering curve of the finger-length weighting of the transducer 
according to FIG. 1 is plotted in FIG. 2, which has already been 
mentioned. The diagram of FIG. 2 does not, of course, differ from the 
prior art, since apart from undesired side effects, the invention must, 
after all, perform according to the prior art with respect to the filter 
curve achieved by weighting. FIG. 3 shows the transfer function of a 
transducer according to FIG. 1, as seen with respect to a single overlap 
of the second associated transducer of the filter. The transfer function 
shown in FIG. 3 is essentially the Fourier transform of the time signal 
which is set up (as a result of the charge distribution present during 
operation in a transducer according to FIG. 1) along the principal 
direction of wave propagation H of the acoustic wave in the transducer. 
As is clear from FIG. 1, the fingers 4 of one busbar 2 are equally wide or 
wider than a mean dimension of the finger width. The fingers 4, 5 of the 
two busbars 2, 3 are equally wide in the center of the transducer (where 
the highest weighting prevails). This mean dimension is equal to one-half 
the period length p/2, decreased by the width of a finger gap 7. Low 
weighting prevails where the finger widths differ particularly strongly 
from this mean dimension (at the transducer ends). 
The assignment of wider fingers (for example finger 4) to one busbar (2), 
and of narrower fingers (for example finger 5) to the other busbar (2) can 
also be partially interchanged inside a transducer (narrower fingers on 
one busbar and then wider fingers on the other busbar). "Essentially" 
periodically positioned is to be understood such that given a finger width 
varies from finger to finger of a busbar. The exact geometrical center M 
of a finger is offset somewhat in the direction in which the widths of the 
fingers increase. 
It should be understood that various changes and modifications to the 
presently preferred embodiments described herein will be apparent to those 
skilled in the art. Such changes and modifications may be made without 
departing from the spirit and scope of the present invention and without 
diminishing its attendant advantages. It is, therefore, intended that such 
changes and modifications be covered by the appended claims.