SAW Device With Continuous Finger Pattern

The invention 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;

FIG. 2 schematically illustrates a SAW device in accordance with an embodiment of this invention;

FIG. 3 is an enlarged illustration of part of the SAW device of FIG. 2; and

FIG. 4 schematically illustrates a SAW device in accordance with another embodiment of the invention.

Referring to FIG. 1, a SAW device comprises an apodized input IDT 10 and an unapodized output IDT 12 which are spaced apart and aligned 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 conductive 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 constituting 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, for example 40 to 100, 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 U.S. Pat. No. 4,642,507 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 U.S. Pat. No. 4,684,841 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 U.S. patent application Ser. No. 839,657 already referred to.

A SAW device as illustrated in FIG. 1 can be constructed so that spurious time domain signals are more than 50 dB below the main signal. It has been found, however, that these spurious signals still include an identifiable signal at about 56 dB 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, in this example, 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 grounded 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 that the IDT 12 is also symmetrical about its center. As both reflection suppressing regions 44 and 46 have the same taper, the phase fronts of propagated surface acoustic waves remain planar. As these regions substantially cancel reflections at the front edges of the IDTs, the triple transit spurious signal discussed above is substantially eliminated.

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.

For clarity and simplicity, FIG. 3 illustrates only a few of the fingers in the respective regions. 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.

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 wavelength (this being provided in the active regions of the IDTs 10 and 12 by the use in known manner of 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 conductive 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 input 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.

FIG. 4 illustrates an alternative embodiment of the invention, which 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.

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 regions 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 not 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.

Numerous other modifications, variations, and adaptations may be made to the embodiments of the invention described above without departing from the scope of the invention as defined in the claims.