Series-block and line-width weighted saw filter device

Surface acoustic wave (SAW) filter performance is enhanced using series-block weighting and line-width weighting combined for providing a transducer having a finely adjusted uniform weighting across the beam profile. Such a transducer permits practical transducer pairing within a surface acoustic wave filter, as well as use with transducers requiring uniform weighting, such as transducers employing apodized weighting. A three electrode per wavelength transducer electrode finger geometry includes line-width weighting for providing a fine weighting control to complement the inherently coarse weighting control of the series-block weighted transducer. Selected transducer geometries including three electrode per wavelength structures are employed for use in wide band tapered electrode finger SAW devices.

BACKGROUND OF INVENTION 
1. Field of Invention 
The invention relates generally to a surface acoustic wave (SAW) filter and 
more particularly to weighting of both input and output transducers for 
improved filtering for use in wideband and narrowband SAW devices. 
2. Background Art 
An electrical filter transmits signals having frequencies within certain 
designated ranges or passbands, and suppresses signals having other 
frequencies outside the passband or within attenuation bands. An ideal 
filter would transmit the signal within the passband without attenuation 
and completely suppress signals within the attenuation bands. Practical 
filters do attenuate the passband signal due to absorption, reflection, or 
radiation, which results in a loss of desired signal power. Further, such 
filters do not completely suppress signals within the attenuation bands. 
The use of surface acoustic wave (SAW) devices as filters or resonators is 
well known for having the advantages of high Q, low series resistance, 
small size, and good frequency temperature stability when compared to 
other frequency control methods such as LC circuits, coaxial delay lines, 
or metal cavity resonators. As is well known in the art, a SAW device 
typically contains a substrate of piezoelectric material such as quartz, 
lithium niobate, or zinc oxide. Input and output transducers are formed 
upon the substrate. The transducers convert input electrical signals to 
surface acoustic waves propagating upon the surface of the substrate and 
then reconvert the acoustic energy to an electric output signal. The input 
and output transducers are configured as interdigital electrode fingers 
which extend from pairs of transducer pads. Interdigital transducers may 
be formed by depositing and patterning a thin film of electrically 
conductive material upon the piezoelectric substrate. 
Alternating electrical potential coupled to the input interdigital 
transducer induces mechanical stresses in the substrate. The resulting 
strains propagate away from the input transducer along the surface of the 
substrate in the form of surface acoustic waves. These propagating surface 
waves arrive at the output interdigital transducer where they are 
converted to electrical signals. Typical transducers will have a sin x/x 
passband shape which is not a preferred filter shape because its 
transition bandwidth is equal to the filter bandwidth, and more 
importantly, the first sidelobe is typically only 13 dB below the main 
response. To synthesize arbitrary passbands, transducer weighting is 
employed. Filtering is thus accomplished in the process of generating the 
surface acoustic wave by the input transducer and in the inverse process 
of detecting the wave by the output transducer. The most effective 
filtering will therefore be accomplished if both input and output 
transducers are weighted and thus participate in the filtering process. 
Common transducer weighting techniques include apodization and withdrawal 
weighting. Apodization is typically used for wideband filters and either 
apodization or withdrawal weighting typically used for narrowband filters. 
Apodization varies the length of the electrodes to achieve an electrode 
weighting. With apodization, Fourier transform techniques can be readily 
applied for computing a filter impulse response when defining a spacial 
geometric pattern for the interdigital transducer fingers. It is well 
known that it is not practical to have an input apodized transducer 
launching a wave directly into an output apodized transducer because an 
apodized transducer launches a wave which has a non uniform beam profile, 
and as a receiving transducer, it must see a uniform beam profile. If a 
surface wave incident upon an apodized transducer is not uniform over the 
entire width of the beam, the frequency response will change dramatically. 
For this reason, apodized input and output transducers can not be used to 
form a filter unless an added structure such as a multi-strip coupler is 
used. The multistrip coupler positioned between the apodized input and 
output transducers transfers energy from a non uniform beam into an 
adjacent track in which a surface acoustic wave is launched as a uniform 
beam, and thus compatible with an apodized transducer receiving the 
uniform beam. However, using an apodized input transducer for generating a 
surface acoustic wave and transmitting the wave through a multistrip 
coupler to an apodized output transducer widens the filter device thus 
requiring increased space within electronic systems seeking to be ever 
more miniaturized. Further, apodized transducer to apodized transducer 
through a multistrip coupler is only useful on high coupling substrates 
such as lithium niobate, whereas it is not practical on quartz. An 
unweighted transducer will also be used with an apodized transducer if 
less side lobe rejection can be tolerated for the system. 
By way of example, a SAW filter design may require performance that an 
apodized transducer can provide yet places constraints on filter size or 
piezoelectric substrate type such that the use of a multistrip coupler is 
precluded. An alternative approach will often include the use of 
withdrawal weighting of one transducer with apodization of the other. A 
transducer having withdrawal weighting launches a uniform wave across the 
beam profile and thus is compatible with an apodized transducer. In 
withdrawal weighting techniques, electrode fingers are selectively 
removed, or withdrawn, from a uniform interdigital transducer having 
constant finger overlap in order to attain a desired transducer response. 
Since the remaining electrodes all have constant overlap, the withdrawal 
weighted interdigital transducer can be used with the apodized transducer, 
amplitude weighted, without the need for the multistrip coupler. Good 
sidelobe suppression can be obtained using this combination of overlap and 
withdrawal weighting. It is thus attractive for SAW transducers used on 
low-coupling piezoelectric substrates such as quartz, where the use of 
multi-strip couplers is normally impractical. However, since a filter 
approximation will deteriorate if too many electrodes are withdrawn, this 
technique is limited to narrowband filter applications where the number of 
interdigital transducer electrodes is large. Further, although the 
withdrawal weighted transducer satisfies the uniform wave condition, 
withdrawal weighting is a coarse weighting technique, and as a result 
produces generally poor far out sidelobe response with less than desirable 
noise rejection. 
A weighting technique providing fine adjustments and the desirable uniform 
beam profile is needed. 
SUMMARY OF INVENTION 
In view of the foregoing background, it is therefore an object of the 
present invention to provide a transducer useful with narrow and wide 
passband SAW devices and which provides an improved performance over 
conventional filters. It is further an object of the present invention to 
provide a transducer that is useful in synthesizing passbands with fine 
tap weighting that has a uniform beam profile. Such a transducer can then 
be used in a SAW filter with an apodized transducer or with itself as an 
input and an output transducer, thus providing weighting in both input and 
output transducers of the filter. It is yet another object of the 
invention to provide a transducer capable of both coarse and fine 
weighting control that is uniform across the beam width. 
These and other objects, features, and advantages of the invention are 
provided by combining series block weighting and line width weighting in a 
transducer that is used in a SAW filter wherein both input and output 
transducers are weighted for providing effective filtering. In one 
embodiment of the invention, a transducer is provided for a surface 
acoustic wave device which includes a piezoelectric substrate for 
propagating surface acoustic waves. The surface acoustic waves have a 
longitudinal axis of propagation across the device. The transducer is 
adapted for coupling to an electrical load and/or source. The transducer 
comprises one or several strings of subtransducers placed adjacent to each 
other in the same acoustic channel having substantially the same acoustic 
aperture. The subtransducer string is electrically connected in series and 
acoustically cascaded. Each subtransducer comprises corresponding bus bars 
and includes a plurality of interdigitized electrode fingers extending 
transversely to the propagation axis and inwardly from each bus bar for 
providing a relative weighting for the transducer. Additionally, at least 
some electrode fingers of the subtransducer have differing width 
dimensions for providing fine weighting control for the transducer.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENT 
The present invention will now be described more fully hereinafter with 
reference to the accompanying drawings, in which preferred embodiments of 
the invention are shown. As illustrated, by way of example, with reference 
to the above related application, this invention may be embodied in many 
different forms and should not be construed as limited to the embodiments 
set forth herein. Rather, these embodiments are provided so that this 
disclosure will be thorough and complete, and will fully convey the scope 
of the invention to those skilled in the art. Like numbers refer to like 
elements throughout. 
Referring now initially to FIG. 1, a surface acoustic wave filter 10 
includes an input transducer 12 and output transducer 14 deposited on a 
piezoelectric substrate 13. Each transducer 12, 14 comprises one or 
several strings of subtransducers 110, 112, 114, as illustrated by way of 
example with reference to FIG. 2. The string of subtransducers 110, 112, 
114 includes a number of taps or electrode fingers 20, 21 within a 
wavelength in subtransducers 110, 112, 114 formed by the combination of 
outer bus bar fingers 20 and inner bus bar fingers 21 and their 
corresponding bus bars 16, 17, 18, 19, and finger electrode sets 20, 21, 
22, 24, illustrated again with reference to FIGS. 1 and 2. The number of 
electrode fingers in each subtransducer vary so that the voltage applied 
to the string of subtransducers 110, 112, 114 is divided in such a manner 
as to weight the taps with respect to each other. The relative tap weight 
is proportional to the relative voltage applied to the tap. 
As a point of interest, note that series subtransducers are described in 
U.S. Pat. Nos. 4,635,008 and 4,908,542 to Solie as a means of transforming 
the impedance of tapered transducers using strings of series connected 
transducers (called subtransducers). By way of the illustrated example, in 
this disclosure, each subtransducer had the same number of electrodes, so 
the voltage is divided equally between the subtransducers, and the voltage 
at every tap is the same. Therefore, there is no tap weighting, a 
distinguishing feature of the present invention. 
With reference to FIG. 2, first note that the three subtransducers 110, 
112, 114 are acoustically cascaded (share the same acoustic track or 
device aperture 116) and are electrically in series. If the impedance is 
dominated by capacitance, which is generally the case, the impedance of 
each subtransducer is inversely proportional to the number of the taps 
within that subtransducer. An equivalent circuit of this string of 
subtransducers 110, 112, 114 is illustrated with reference to FIG. 3, 
where capacitors 118, 120, 122, respectively, have values z.sub.1, z.sub.2 
and z.sub.3. Again with reference to FIG. 2, it can be seen that there are 
two taps in the first subtransducer 110, five and one-half in the second 
112, and three in the third 114 An index "n" is the number of taps or 
wavelengths. Note that for this example, there are two electrode fingers 
20, 21 per tap. There are two gaps per wavelength. If two adjacent fingers 
or electrodes are connected to the same bus bar, there is no voltage 
across that gap. Therefore, it is not counted as a gap for the purpose of 
counting taps per subtransducer. It follows, therefore that z.sub.1 
=z.sub.0 /2, z.sub.2 =z.sub.0 /5.5 and z.sub.3 =z.sub.0 /3, where z.sub.0 
is a tap impedance specified over one wavelength. The outer bus bars 16, 
18 are herein also referred to as major bus bars. By way of example, if we 
define the voltage or tap weight across the major bus bars 16, 18 as 1.0, 
then the tap weights in the three subtransducers 110, 112, 114 are herein 
calculated by simple voltage division as t.sub.1, t.sub.2, and t.sub.3, 
where: 
##EQU1## 
Assuming all the tap electrodes or fingers are identical, the taps within a 
subtransducer all have the same weight and are a block of taps of the same 
strength. For one preferred embodiment, there are several strings in a 
transducer and often they are constructed symmetrically about the center 
of a transducer. A string may consist of one subtransducer, in which case 
there is no voltage division and all taps in this string have a tap weight 
of unity. 
An advantage of block weighting is the uniformity of weighting across the 
SAW aperture and that it can be achieved with a single layer metalization. 
The disadvantages are that the taps cannot be individually weighted, and 
the possible tap weight values are somewhat limited (i) by the fact that 
the sum of the tap weights across the subtransducers of a string must be 
one, and (ii) by the constraints of setting the impedances by the number 
of taps. Nonetheless, block weighting provides a useful technique for use 
in the present invention. It can therefore be said that block-weighting 
provides coarse or quantized tap weights. 
In preferred embodiments of the present invention, weighting the taps 
includes varying the width of the electrode fingers 20, 21, as earlier 
described in the reference application and as will be herein again 
described. This is referred to as line-width weighting. Within a range of 
line-widths, as a selected electrode finger width 84, w.sub.2, as 
illustrated with reference to FIG. 4, is increased, the tap weight or 
transduction strength of the tap is increased. This is valid for the range 
where w/.lambda. (finger width to wavelength) is .ltoreq.0.4, and for most 
practical examples where w/.lambda..ltoreq.0.25. The lower range of 
w/.lambda. is limited by the line width that can be fabricated both on the 
device and on the mask. For lower frequency devices, this range of 
w/.lambda. can be large enough to change the relative tap weight strength 
from 1.0 down to around 0.5, whereas at higher frequencies, the tap weight 
range will be much more limited. In general, it can be said that the range 
can not significantly approach zero and can only decrease somewhat from 
unity (relative tap weight). This line-width weighting, however, is useful 
when combined with series-block weighting. Unlike line-width weighting 
techniques used in the past, limited because of limited tap weight range, 
when combined with series-block weighting, the achievable tap weight range 
is much broader. The two tap weighting techniques are complimentary when 
used together because block weighting is coarse weighting with a large 
range of tap weights and line width weighting is a fine weighting with a 
limited range (just enough to bridge a gap between the relatively large 
steps of the block weights). 
By way of example and with reference to FIG. 5, line-width weighting is 
used with series-block weighting to implement a Hamming weighted function. 
Again with reference to FIG. 5, consider the transducer 124 consisting of 
a set of taps (n.sub.1) or main subtransducer 126 in the center that are 
directly connected to the primary (hot and ground) bus bars 128, 129 and a 
string of three subtransducers connected in a series symmetrically on each 
side 130L, 132L, 134L, 130R, 132R, 134R of the main subtransducer 126. 
Electrically the circuit 136 of the subtransducers (representing a 
transducer by a capacitor) is as shown in FIG. 6. The three subtransducers 
(130L-134R) in series on each side of the center subtransducer 126 will 
divide the voltage between them by normal voltage division. If the number 
of electrodes or fingers in the subtransducer is n.sub.1, n.sub.2, n.sub.3 
and n.sub.4, as illustrated with reference to FIG. 5, the relative voltage 
across the center subtransducer is a.sub.1 =1.0 (full strength), and on #2 
is: 
##EQU2## 
and on subtransducers #3 and #4 is 
##EQU3## 
A plot 140 of the tap weight that can be realized by block weighting is 
illustrated with reference to FIG. 7. This is an approximation to a 
Hamming function. Using line-width weighting as herein described, we can 
reduce the tap weight of each tap within a block or subtransducer by a 
factor of m.sub.1 where 1.gtoreq.m.sub.1 .gtoreq.0.7 (the value of 0.7 is 
by way of example only). As a result, the combined series-block weighting 
and line-width weighting combination 142 is as illustrated with reference 
to FIG. 8. As can be seen, combined series-block and line-width weighting 
142 provides an improved approximation to a desired tap weight function. 
As a result, frequency sidelobes will be correspondingly lower. 
With reference to FIG. 4, in one embodiment of the present invention, 
electrode fingers of a subtransducer include a finger 84 of narrow width 
adjacent a finger 88 of relatively larger width. The finger width 84, 88, 
illustrated here by way of example, and gap 92 are selected as a function 
of impedance of the source 30 and the load 28, as illustrated with 
reference to FIG. 1. Such a selection is made to produce mechanically 
loaded reflections for canceling regenerated waves. With reference again 
to FIG. 4, there are three electrodes per tap or wavelength (designated by 
.lambda. in FIG. 4). The number of taps in a subtransducer determines the 
relative coarse weight of the transducer. Transduction tap strength and 
reflection tap strength are then further adjusted by varying the electrode 
finger widths 84, and 88 as shown by way of example again with reference 
to FIG. 4 for providing a fine line-width weighting mechanism for the 
subtransducer. 
Series block weighting and line-width weighting as herein described is also 
applied to a tapered transducer 80 as illustrated with reference to FIG. 
12. Tapered transducers are typically used in pairs, as illustrated with 
reference to FIGS. 9, 9A, and 10. An input transducer 54 and output 
transducer 56 in a linear phase filter applications, by way of example, 
will include each transducer having opposing bus bars 52, 53. Surface wave 
propagation, as illustrated, by way of example, with reference to SAW 
device 50 of FIG. 10, is from left to right as shown by the arrow 58. The 
transverse dimension 60 is here defined as the X direction, and it can be 
seen that the period of the electrodes or fingers 62 (which defines the 
wavelength) becomes smaller as X increases. Consequently, the frequency 
increases with X. As illustrated with reference to FIG. 10, transducer 
tapered fingers 62 in one embodiment have the fingers 62 tapered along 
lines 64 which emanate from a single focal point 66 as is the embodiment 
of FIG. 11. In another embodiment of the present invention, and as 
illustrated with reference to FIG. 9A, the tapered fingers 220 follow 
hyperbolically curved lines 65. The high frequencies are detected in the 
upper portion 68 of the saw aperture, and the lower frequencies in the 
lower portion 70 of the SAW aperture shown in FIG. 10. There can be two 
electrodes per wavelength, as earlier described. A variety of other 
electrode structures may be used. A major constraint imposed on this 
structure is that, except for taper, every horizontal spacial interval or 
channel 72 of the transducers 54 should be essentially the same as all 
other channels 72. In other words, all frequencies within the range of the 
device, though shifted up or down as the transducer 54 operates, will be 
excited (or detected) by the same electrode structure. 
The frequency response of the tapered transducer is derived from this 
non-tapered (narrow channel) response by "sliding" the non-tapered 
response over frequency channel by channel. A consequence of this process 
is that the better the selectivity of the narrow channel region, the 
better the selectivity of the tapered transducer. The series-block 
weighting permits a coarse weighting over a broad range of tap weight 
values, while the line-width weighting provides a more precise weighting 
over a continuum weighting, combining for a near zero weight to a maximum 
normalized weight (e.g. 0-1). 
As earlier described in the related application, a tapered SPUDT 
transducer, by way of example, is configured with four electrodes per 
wavelength, as illustrated with reference to FIGS. 9 and 9A, and improved 
further with series-block weighting, as described and illustrated with 
reference to FIG. 5. Another embodiment includes a tapered three electrode 
per wavelength geometry, as described and illustrated with reference to 
FIG. 4, which as described is coarsely weighted using series-block 
weighting techniques and finely weighted using line-width weighting 
techniques in combination with the block weighting. 
To fully support the claims of the present invention, the following 
detailed description, supported by the reference application will be 
provided to present a thorough and complete disclosure. 
One embodiment of the present invention, a tapered SPUDT SAW filter device 
210 is illustrated and described with reference to FIGS. 9 and 9A. The 
device 210 comprises input and output transducers 212, 214 with opposing 
bus bars 216, 218 each having a plurality of interdigitized, continuously 
tapered electrode fingers 220 configured in finger pairs 222 with each 
pair having a finger of narrow width 224 adjacent to a finger with a 
larger or wider width 226, by way of example in the embodiment 
illustrated. The finger widths 224, 226 are selected as a function of the 
impedance of a load 28 or source 30 so as to produce mechanical electrical 
loaded reflections in a substrate 232 upon which the transducers 212, 214 
are placed. 
Providing an unbalanced split electrode or finger geometry is known in 
narrow bandwidth SAW filters to successfully cancel reflected waves 
inherently generated by SAW devices. Such reflected waves lead to triple 
transit interference. Tapering the electrode fingers in wide bandwidth SAW 
devices is known to permit the transduction of a wide range of surface 
acoustic wavelengths. The combination as described herein has the 
unexpected result of significantly reducing insertion loss and enhancing 
triple transit suppression, both very much desirable in wide bandwidth SAW 
filter devices. 
In yet another embodiment of a tapered SPUDT transducer, the transducer can 
be configured with three electrodes per wavelength as illustrated with 
reference to FIG. 12, and improved further with series block weighting as 
described and illustrated in a non-tapered version in FIG. 5. The smaller 
width fingers are illustrated with numeral 123 and the larger width 
fingers are illustrated with numeral 125, by way of the series block 
weighted and line width transducer example of FIGS. 2 and 5. As further 
described herein and as illustrated, again with reference to FIGS. 2 and 
5, a plurality of interdigitized electrode fingers 20, 21 extend 
transversely into the transducer aperture 116 from at least one of the 
outer 16, 18 and inner 17, 19 bus bars, each electrode finger extending 
into the aperture and generally across the axis of propagation, as earlier 
described with reference to numeral 58 of FIG. 10, from its corresponding 
bus bar, wherein a selected set of fingers and their corresponding bus 
bars form one subtransducer, 130, 132, 134 by way of example, within a 
string of subtransducers. Each subtransducer 134L to 134R in FIG. 5, 
within the string is positioned adjacent each other for providing a 
transducer acoustic channel 117, and thus an acoustically cascaded series 
of subtransducers, each subtransducer electrically connected within the 
series for providing a series-block weighting control to the transducer, 
and wherein at least some of the electrode fingers have differing width 
dimensions 123, 125, by way of example, for providing an additional 
line-width weighting control to the transducer. 
As described with reference to FIG. 1, and supporting illustrations, series 
block weighting is successfully used for weighting an input transducer 12 
used with a similar weighted output transducer 14, configured as a mirror 
image to each other. Further, because of the uniform weighting across the 
beam profile, the block weighting transducer 12 having the additional line 
width weighting, a preferred embodiment of the present invention, is 
successfully used with an apodized transducer 15, as illustrated with 
reference to the SAW device 101 of FIG. 1A, typically requiring a 
receiving transducer having a uniform sampling of the beam across the 
aperture. 
While specific embodiments of the invention have been described in detail 
herein above, it is to be understood that various modifications may be 
made from the specific details described herein without departing from the 
spirit and scope of the invention as set forth in the appended claims. 
Having now described the invention, the construction, the operation and use 
of preferred embodiments thereof, and the advantageous new and useful 
results obtained thereby, the new and useful constructions, methods of use 
and reasonable mechanical equivalents thereof obvious to those skilled in 
the art, are set forth in the appended claims.