Note: Descriptions are shown in the official language in which they were submitted.
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SAW DEVICE I~ITH CONTINUOUS FINGER PATTERN
This in~ention relates to SA~ (surface acous-tic wave)
devices.
It is well known to provide a SAW device for use as a
bandpass filter. Generally, such a SAW device comprises two IDTs
(inter-digital tra~sducers) which are spaced from and aligned with
one ano-ther on the surface of a substrate of a suitable
piezoelectric material, such as lithium niobate (LiNbO3), for the
propagation oF surface acoustic waves therebetween. Typically one
of the IDTs is an apodized IDT, in which the fingers of the IDT
overlap along their lengths by differing amounts at different
positions along the IDT, which is driven with a signal to be
filtered and constitutes an input transducer, and the filtered
signal is derived from the other IDT which constitutes an output
transducer.
Various measures are known or have been proposed for
suppressing undesired acoustic waves in SAW devices and thereby
improving their performance. In particular, it is known to provide
an acoustic absorber between each IDT and the adjacent end of the
substrate in order to suppress acoustic wave reflections from this
end. Este et al. Canadian patent application No. 477,949 filed
March 29, 1985 and entitled "SAW Devices Including Resistive Films"
describes a SAW device in which the acoustic absorber is constituted
by a thin resistive doped silicon film. In addition, Suthers et al.
Canadian patent application No. 477,948 filed March 29, 1985 and
entitled "SAW Devices with Reflection Suppressing Fingers" describes
a SAW device in which tapered dummy fingers are provided at the back
edge of each IDT in order to suppress acoustic wave reflections from
this back edge. The front edge of the acoustic absorber film is
tapered in a complementary manner to avoid reflections from this.
It has also been found that capacitive coupling of the
driven fingers at one or both ends of an apodized IDT to a ground
plane, which is normally directly under the substrate in order to
reduce electromagnetic feedthrough, can detract from the performance
of the SAW device. This problem can be avoided by making the axis
of the apodization pattern have a V-shape, so that there are short
driven fingers at both ends of the apodized I~T, as described in
Suthers et al. Canaclian pa-tent application No. 503,972 filed March
12, 19~6 and entitled "SA~ Device with Apodized IDT''c
Whilst these rneasures considerably enhance the performance
of SAW devices, and enable spurious time domain signals to be
reduced to better than 50dB below the main, desired, signal, at
least one further unwanted signal can be identified. An object of
thls invention, therefore, is to provide an improved SAW device in
which this further unwanted signal is reduced or suppressed.
According to one aspect of this invention there is provided
a SAW (surface acoustic wave) device comprising a substrate having
two IDTs (inter-digital transducers) thereon aligned for propagation
of a surface acoustic wave with a predetermined wavelength lambda
between front edges of the IDTs, each of the IDTs including a
reflection suppressing region at its front edge, each reflection
suppressing region comprising a plurality of spaced electrically
interconnected fingers of successively decreasing length, the
reflection suppressing regions forming at least part of a continuous
pattern of fingers spaced with a pitch of lambda/4 between the front
edges of the IDTs.
The reflection suppressing regions at the front edges of the
IDTs suppress surface acoustic wave reflections at these front
edges, whereby a spurious surface acoustic wave which is otherwise
reflected once at the front edge of each IDT is substantially
eliminated. The continuous pattern of fingers avoids
discontinuities in the path of the surface acoustic wave.
In an embodiment of the invention the continuous pattern
includes a shield region between the reflection suppressing regions
at the front edges of the IDTs, the shield region comprising a
plurality of electrically interconnected fingers for connection to a
ground point. The shield region conveniently includes fingers of
successively increasing length complementary to and adjacent the
fingers of successively decreasing length of the reflection
suppressing region at the front edge of each IDT.
Advantageously each IDT includes a reflection suppressing
region, comprising a plurality of spaced electrically interconnected
fingers of successively decreasing length, at a back edge of the
IDT, the SAW device further comprising two further shield regions,
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each comprising a plurality of electrically interconnected fingers
for connection to a ground point, each adjacent to the reflection
suppressing region at the back edge of a respective one of the IDTs,
each further shield region including fingers of successively
increasing length complementary to and adjacent the fingers of
successively decreasing length of the respective reflection
suppressing region at the back edge of the respectiYe IDT.
According to another aspect of this invention the~e is
provided a SAW (surface acoustic wave) device comprising a substrate
having an IDT (inter-digital transducer) on a surface thereof, the
IDT comprising: an active region of inter-digltal fingers, for
transducing a surface acoustic wave with a predetermined wavelength
for propagation on said surface via a front edge of the IDT; a front
edge reflection suppressing region, comprising a plurality of spaced
electrically interconnected fingers of successively decreasing
length, at the front edge of the IDT; and a back edge reflection
suppressing region, comprising a plurality of spaced electrically
interconnected fingers of successively decreasing length, at a back
edge of the IDT opposite the front edge; the fingers in the
reflection suppressing regions being spaced, in the direction of
propagation of the surface acoustic wave, with a pitch of one
quarter of said predetermined wavelength.
The IDT may be an apodized IDT, and may include a plurality
of dummy fingers between the active region and each reflection
suppressing region, the fingers of the reflection suppressing
regions and the dummy fingers all being electrically interconnected
and spaced with the same pitch.
The lnvention will be further understood from the following
description with reference to the accompanying drawings, in which
similar references are used in the different figures to denote
similar parts and in which:
Fig. 1 schematically illustrates a SAW device in accordance
with principles known in the prior art and the patent applications
already referred to;
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F;g. 2 sche~-t;cally ;llustrates a SAW dev;ce ;n accordance
w;th an embod;ment of th;s ;nvention, and
F;g. 3 ;s an enlarged ;llustrat;on of part of the SAW dev;ce
of Fig. 2.
Referring to Fig. 1, a SA~ device comprises an apodized
inpu~ IDT 10 and an unapodized output IDT 12 which are spaced apart
and a1igned with one another, for propagation of surface acoustic
waves, on a surface of a piezoelectric substrate 14, for example of
lithium niobate. Each transducer comprises a plurality of
overlapping interdigital fingers (not shown individually) which are
connected to conduct-ive rails 16, 18, 20, and 22~ One rail 16, 20
of each IDT is grounded, the other rail 18 of the IDT 10
constituting an input and the other rail 22 of the IDT 12
constitut;ng an output of the SAW device.
In order to suppress surface acoustic wave reflections from
the back edge of each IDT 10, 12, each IDT includes at its back edge
a plurality of dummy fingers of successively decreasing length and
all connected to the grounded conductive rail 16, 20 of the
respective IDT 10, 12, forming a reflection suppressing region 24,
26 respectively in the manner described in Canadian patent
application No. 477,948 already referred to.
In addition, surface acoustic wave reflections from the
adjacent end edges 28, 30 of the substrate 14 are suppressed by
acoustic absorbers 32, 34 respectively extending between these end
edges 28, 30 and the tapering back edges of the reflection
suppressing regions 24, 26 respectively. The absorbers 32, 34 have
correspondingly tapering front edges, and are each constituted by a
thin resistive film of doped silicon having a resistivity of about 1
megohm/square, as described in Canadian patent applications Nos.
477,948 and 477,94g already referred to.
In order to reduce capacitive coupling, to a ground plane or
grounded case directly beneath the substrate 14, of driven fingers,
which are connected to the conductive rail 18, of the apodized IDT
10 especially at the front and back edges of this IDT, as
illustrated schematically the apodization pattern of this IDT has an
axis 36 which has a V-shape as described in Canadian patent
application No. 503,972 already referred to.
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A SAI,J dev;ce as illustrated in Fig. 1 can be constructed so
that spurious time domain signals are more than 50dB below the ma;n
signal. It has been found, however, that these spurious signals
still include an identifiable signal at about s6dB below the main
signal, which can be suppressed as described below to achieve a
particularly high performance SAW device, this being particularly
advantageous for SAW devices to be used in digital radio
transmission systems. From the timing of this identifiable spurious
signal relative to that of the main signal, it can be determined
that this spurious signal is due to surface acoustic waves being
reflected once at the front edge of each IDT and hence having a
triple transit between the IDTs, as shown by an arrow A in Fig. 1.
Referring now to Fig. 2, a SAW device in accordance with an
embodiment of this invention is illustrated with the same elements
10 to 36 referenced as in Fig. 1. In addition, the SAW device of
Fig. 2 includes three shield regions 38, 40, and 42, two additional
reflection suppressing regions 44 and 46, and four dummy finger
regions 48, 50, 52, and 54. These regions are described in detail
below, with additional reference to Fig. 3 which illustrates~ in
greatly simplified form but in greater detail than Fig. 2, parts of
the SAW device of Fig. 2. Broken lines are used in Fig. 2 to
illustrate the extents of the various regions for convenience of
description and to aid in providing a full understanding of the
invention.
The reflection suppressing region 44 is provided at the
front edge of the IDT 10 in the same manner and with the same design
as the reflection suppressing region 24, so that the IDT 10 is
symmetrical about its center. As is illustrated in Fig. 3, the
reflection suppressing region 44 consists of a plurality of fingers
of successively decreasing length, spaced apart with a pitch of one
quarter wavelength and all electrically connected to the conductive
rail 16. The spacing of the fingers results in reflections at
successive fingers being out of phase whereby they substantially
cancel one another.
The reflection suppressing region 46, also shown in Fig. 3,
is similarly provided at the front edge of the IDT 12, in the same
manner as the reflection suppressing region 26 at the back edge of
this IDT, so tha-t the IDT 12 is also symmetrical about its cen-ter.
As both reflection suppressing regions ~4 and 46 have the same
taper, the phase fron-ts of propaga-ted surface acoustic waves remain
planar. As these regions substantially cancel reflections at the
front edges o-f the IDTs, ~he triple transit spurious signal
discussed above is substantially el imi nated.
The shield region 40 comprises a region 56 having a
plurality of fingers connected at both ends to conductive rails 58
which are connected to the case or ground plane of the SAW device.
On each side of the region 56 the shield region 40 comprises a
region 60 or 62 of fingers tapering in a manner complementary to the
taper of the adjacent reflection suppressing region 44 or 46
respectively, these fingers being connected to only one of the rails
58. The other shield regions 38 and 42 are similar, being disposed
between the respective reflection suppressing region 24 or 26 and
the respective acoustic absorber 32 or 34. Fig. 3 illustrates the
manner in which the front edge of the absorber 34 is stepped to
complement the taper of the adjacent fingers.
The conductive rails 58 of all of the shield regions are
connected to the case or ground plane of the SAW device. In order
to reduce electromagnetic feedthrough of signals from the input to
the output of the SAW device, separate signal ground connections are
desirably used for connections to the rails 16 and 20. The grounded
shield regions are, as best illustrated in Fig. 2, arranged
symmetrically at each end of each IDT, so as to facilitate
symmetrical excitation of the IDTs.
The dummy finger regions 48, 50, 52, and 54 each comprise a
plurality of, for example five, dummy fingers which are connected to
the ground rail 16 or 20 of the respective IDT 10 or 12 and are
positioned between the active region of the respective IDT and a
respective reflection suppressing region 24, 44, 26, or 46. These
dummy finger regions ensure that there is no coupling to the
grounded shield regions; such coupling might otherwise occur due to
the signal ground potentials of the rails 16 and 20 being different
from the case ground potential of the SAW device.
As represented in Fig. 3, the fingers in the various regions
of the SAW device all have a constant pitch of one quarter
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wavelength (this being provided in the active regions of the IDTs 10
and 12 by the use in known manner o-f finger pairs in these regions).
Thus a constant propagation medium is provided throughout the SAW
device for the propagation of surface acoustic waves. Furthermore,
reflection suppressing tapers are used at each transistion between
different regions of the SAW device, and hence at each edge of each
shield region 38, 40, or 42, whereby undesired acoustic wave
reflections are substantially completely eliminated.
As can be appreciated from Fig. 3, the complementary and
adjacent fingers of respectively successively increasing and
decreasing lengths extending from the respective cond~ctive rails at
opposite sides of the IDTs form inclined junctions between different
regions of the IDTs and shield regions. Thus, for example, there is
an inclined junction between the reflection suppressing region 44
and the shield region 40, formed by the successively decreasing
lengths of the fingers of the region 44 and the complementary and
adjacent fingers 60 with successively increasing lengths
(considering the drawing from left to right), the fingers extending
from the rails 16 and 58 respectively at opposite sides of the IDT.
Similar inclined junctions, each of which extends across the full
aperture of the IDTs, are illustrated in Fig. 3 on each side of the
IDT 12. At each such junction, the continuous pattern of fingers
with their quarter wavelength pitch, and the inclination of the
junction, substantially eliminate any reflections or discontinuities
in the SAW propagation path.
The constant propagation medium is clearly seen from Fig. 3.
Throughout the surface acoustic wave path from the inpu-t transducer
10 to the output transducer 12 there is a continuing repetition of
fingers with a quarter wavelength pitch, transitions between
different regions occurring only at the tapered, reflection
suppressing, boundaries. As shown clearly in Fig. 3 all of the
fingers are straight and parallel, with only their lengths varying
to provide the desired SAW characteristics, whereby SAW reflections
due to discontinuities within and between the transducers 10 and 12
are substantially eliminated.
Accordingly, an aspect of this invention provides a SAW
(surface acoustic wave) device comprising a substrate having two
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IDTs (inter-digital transducers) thereon aligned for propagation of
a surface acoustic wave having a predetermined wavelength lambda
between front edges of the IDTs, the SAW device further comprising a
continuous pattern of fingers spaced with a pitch of lambda/4
between the fron-t edges of the IDTs, at least one inclined junction
between two different regions of ~he SAW device being formed by
fingers of successively increasing length complementary ko and
adjacent fingers of successively decreasing length extending from
respective conductive rails at opposite sides of the IDTs.
In the embodiment of the invention described above with
reference to Figs. 2 and 3, the two different regions of th~ SAW
device comprise a reflection supprescing region, at an edge of one
of the IDTs, and a shield region.
Numerous modifications, variations, and adaptations may be
made to the embodiment of the invention described above without
departing from the scope of the invention as defined in the claims.
SUPPLEMENTARY DISCLOSURE
For clarity and simplicity, Fig. 3 illustrates only a few of
the fingers in the respective regions of the SAW device. In
practice, in each of the reflection suppressing regions such as 44,
46, and 26 there will be a relatively large number of fingers, for
example 40 to 100 or more, extending over a distance, in the
direction of propagation of surface acoustic waves between the
transducers, of 10 to 25 or more wavelengths. The complementary
parts of the shield regions 40, namely regions 60 and 62, would have
corresponding numbers of fingers, all of the fingers being spaced
with the same pitch of a quarter wavelength.
Fig. 4 schematically illustrates a SAW device in accordance
with an alternative embodiment of the invention.
The alternative embodiment of the invention shown in Fig. 4
is similar to that of Figs. 2 and 3 except that the shield regions
38, 40, and 42 and their rails 58 are dispensed with, the reflection
suppressing tapers being made more gradual whereby the distance
between the transducers 10 and 12 is unchanged. Similar references
to those in the preceding figures are used in Fig. 4.
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In the embodiment of the invention illustrated in Fig. 4,
the reflection suppressing regions 44 and 46 have complementary
tapers, whereby these two regions, with the adjacent dummy finger
reg10ns 50 and 52, provide an uninterrupted pattern of fingers
throughout the area between the transducers 10 and 12. The
transition between the regions 44 and 46 has the same form as that
shown for each of the tapered transitions in Fig. 3. Thus again in
this embodiment of the invention the continuing repetition of
fingers with a quarter wavelength pitch throughout the SAW device
eliminates reflections, such as the triple transit reflection path
shown in Fig. 1, to provide a significant enhancement of the SAW
device characteristics.
Although in the embodiments of the invention described above
the transducers 10 and 12 have been described as being symmetrical
about their centers, this need n~t necessarily be the case. For
example, it should be understood that the advantages of the
invention may similarly be provided with transducers whose front
edges are tapered and arranged as illustrated in Fig. 4 and whose
back edges are tapered and arranged as illustrated in Figs. 2 and 3,
with or without the shield regions 38 and 42.
