Apparatus and Method for Phase-Encoded Surface Wave Devices

1. Field of the Invention

This invention relates generally to acoustic surface wave devices and more particularly to techniques for synthesizing desired device frequency characteristics. The desired frequency characteristics are provided in the present invention by appropriate phase-encoding of the transducer elements as contrasted with the amplitude-encoding of the transducer elements of the prior art.

2. Description of the Prior Art

In acoustic surface wave devices, varying electric signals, applied to properly-positioned transducer electrodes deposited on a piezoelectric substrate, generate acoustic waves which propagate along the substrate surface. Correspondingly, the passage of acoustic waves on the surface of a piezoelectric substrate through a region in which properly-positioned transducer electrodes have been deposited, will generate an electric signal at the output of the transducer. These phenomena can be used to provide a group of electronic devices having predetermined transfer-function characteristics.

To provide specified frequency characteristics in an acoustic surface wave device, the desired characteristics, which are typically specified in the frequency domain, are transformed into a time domain representation. The time domain representation is chosen because of the correspondence to the physical situation in which an acoustic wave propagates with a velocity characteristic of the device substrate and interacts with the electro-acoustic elements of a transducer in an acoustic surface wave device. The propagation of the acoustic surface wave in the device corresponds to propagation along the time axis in the time domain representation.

The characteristics of the transducer elements, with which the propagating acoustic surface wave interacts, are chosen to produce an electrical analog of the function displayed in the time domain representation. In particular, one set of transducer elements, assigned an arbitrary polarity, is associated with the local time domain maxima, while a second set of transducer elements, arbitrarily assigned the opposite polarity, is associated with the local time domain minima. The magnitude of local maxima and minima of the time domain representation is reproduced in the acoustic surface wave device by appropriate amplitude of overlap between adjacent transducer elements of the two transducer elements sets, resulting in an amplitude-encoding of the transducer elements.

In reproducing the characteristics specified in the time domain representation, the interactive region of the acoustic surface wave transducer (i.e., the region of element overlap) typically increases from zero, reaches a maximum and decreases to zero. Thus a transducer, when used to generate acoustic surface waves, provides a relatively diffuse wave pattern. The generation of acoustic surface waves is therefore generally restricted in the prior art to a series of uniformly overlapping transducer elements to provide a relatively narrow region of propagation of the surface waves about a center frequency and is not suitable for imposition of specified characteristics. Such characteristics can typically be imposed only on the output transducer of an acoustic surface wave device.

It is therefore an object of the present invention to provide an improved acoustic surface wave device.

It is another object of the present invention to provide a method of modulating an acoustic surface wave device.

It is further object of the present invention to provide a method of achieving specific frequency characteristics for a device transfer function.

It is a still further object of the present invention to provide a method of phase-encoding of transducer elements in an acoustic surface wave device.

It is yet another object of the present invention in which desired frequency characteristics can be specified for both an input and an output transducer of the acoustic surface wave device.

It is still another object of the present invention to provide a method of generating acoustic surface waves having specified frequency characteristics in which the direction of propagation of the surface waves is approximately perpendicular to the transducer elements.

It is a more particular object of the present invention to provide acoustic surface wave transducers which produce signal components, generated by an associated transducer element group, having specified characteristics.

It is yet another particular object of the present invention to provide an acoustic surface wave device in which a phase-encoded electro-acoustic transducer is functionally equivalent to an amplitude-encoded electro-acoustic transducer.

It is still another object of the present invention to control the phase of a signal associated with an element of an electro-acoustic transducer by altering the element position.

It is yet a further object of the present invention to alter the phases of signals associated with elements of an electro-acoustic transducer, wherein a combination of the phase-altered signals produces a signal of preselected phase and amplitude.

The aforementioned and other objects of the present invention are accomplished by providing an electro-acoustic transducer comprised of a conducting material deposited on a piezoelectric material. The transducer includes groups of interleaved elements or groups of portions of interleaved elements which are generally displaced from a position indicated for an amplitude-encoded transducer by a time domain representation of desired device characteristics. The vector combination of the signals from the element or element sections of a transducer element pair provides, by appropriate location relative to an amplitude-encoded transducer element, a resulting signal of the phase and amplitude indicated in the time domain representation.

The elements or element sections of an electro-acoustic transducer are disposed so as to be generally parallel to adjacent elements or element sections along the entire element length, thereby increasing the directionality of generated acoustic waves. Directionality of wave generation is compromised, in the prior art, by variable overlap of adjacent elements typically required for producing a specified frequency response.

A first electro-acoustic transducer, exchanging signals with a second electro-acoustic transducer, can be comprised of a plurality of transducer sections. The location of the transducer section elements relative to the second electro-acoustic transducer elements can contribute to producing the specified frequency response.

These and other features of the invention will be understood upon the reading of the following description in conjunction with the drawings.

FIG. 1 is a schematic diagram of the transformation between the frequency domain and the time domain.

FIG. 2 is a schematic representation of the implementation characteristics specified in the time domain by prior art amplitude-encoded transducer elements in an acoustic surface wave device.

FIG. 3 is a schematic representation of the implementation of characteristics specified in the time domain by phase-encoded transducer elements in an acoustic surface wave device according to one embodiment of the present invention.

FIG. 4 is a schematic representation of the implementation of characteristics specified in the time domain by separated phase-encoded transducer sections in an acoustic surface wave device according to a second embodiment of the present invention.

Referring first to FIG. 1, a schematic representation of a transformation of a function in the frequency domain, i.e. a relationship between amplitude and frequency, into the time domain, i.e. a relationship between amplitude and time, is demonstrated. This transformation is typically effected by means of the techniques of Fourier analysis. The significance of this transformation is based on the fact that the desired characteristics for a device are generally more conveniently specified in the frequency domain. The device design, on the other hand, is frequently more easily synthesized from characteristics specified in the time domain.

Referring next to FIG. 2, the use of characteristics, specified in the time domain, to design the transducer element configuration according to the prior art is shown. The first step is to relate the time domain characteristics to characteristics associated with the acoustic surface wave device. This relationship, shown on the axis of the time domain representation, is accomplished by multiplying the coordinates along the time axis by v.sub.o, the propagation velocity of acoustic surface wave in the device.

Transducer electrodes are typically associated in the prior art with local maxima and minima of the desired device characteristic. One set of transducer elements deposited on Substrate 5 and coupled to a common Conducting Region 30 are associated with local maxima. Another set of electrodes, coupled to a common Conducting Region 20, are associated with local minima. The Conducting Regions are also assigned polarities which are arbitrary as will be clear to one skilled in the art.

Two adjacent electrodes, e.g. Electrode 31 coupled to Conducting Region 30, and Electrode 21 coupled to Conducting Region 20, respond to an acoustic wave pulse located momentarily between Electrode 31 and Electrode 21 by generating an electrical signal between Terminal 60, coupled to Conducting Region 30, and Terminal 70, coupled to Conducting Region 20. In the preferred embodiment, the direction of propagation of an acoustic wave traversing the Transducer Region 15 generally perpendicular to the electrodes, while the wave front has a generally uniform amplitude. Thus the acoustic wave pulse located between Electrode 21 and Electrode 31 produced a signal between Terminal 60 and Terminal 70 substantially proportional to the amplitude of the overlapping regions of the two adjacent electrodes. In designing the surface device therefore, the overlap amplitude of Electrode 31 and Electrode 21 provide a quantity proportional to the amplitude of the curve in the time domain between local maxima a and local minima b.

When the propagating acoustic wave pulse is momentarily located between Electrode 21 and Electrode 33 an electric signal of polarity opposite to that generated when the surface wave pulse is located between Electrode 31 and Electrode 21, is generated across Terminal 60 and Terminal 70. In this case, in order that the amplitude of the signal generated by Electrode 33 and Electrode 21 has a proper amplitude for reconstruction of the time domain function, the amplitude of the overlap between Electrode 33 and Electrode 21 has the same ratio to the amplitude of the overlap between Electrode 21 and Electrode 31 as the amplitude between maxima d and minima b has to the amplitude between minima b and maxima a.

The remaining Electrodes 22, 23, 24, 25, 26, 32, 34, and 35 are specified in a similar manner. The result in designing the Electrodes according to this procedure is that a reproduction of the function in the time domain, and consequently of the frequency domain, is obtained for the transducer. That is, the response to acoustic surface wave by the transducer or the generation of surface waves by the transducer will have the response specified in the frequency domain.

Referring next to FIG. 3, the implementation of specified time domain characteristics by a transducer according to one embodiment of the present invention is shown. In this embodiment of the present invention, Terminal 10 and Terminal 11 provide electrical coupling to one electro-acoustic transducer, shown schematically as being comprised of Electrode Elements 12, 13, and 14, while Terminal 70 and Terminal 60 provide electrical coupling to a second electro-acoustic transducer. Conducting Region 20 is coupled to Terminal 70 and arbitrarily assigned a first polarity while Conducting Region 30 is coupled to Terminal 60 and arbitrarily assigned a second polarity. An Electrode 51, coupled to Conducting Region 30, is associated with the position of the largest of a local maxima a. Referring next to minima b of the time domain, an Electrode 42 is positioned at the minima and in the preferred embodiment extends nearly to the Conducting Region 30. The electric signal generated by an acoustic wave in the region between Electrode 42 and Electrode 51 is proportional to the amplitude between local maxima a and local minima b.

The Electrode adjacent to Electrode 42 and corresponding to local maxima d is divided into Electrode Section 52 and a substantially equal Electrode Section 53. In the preferred embodiment of this configuration, these two Electrode Sections are coupled together and are symmetrically displaced about the position specified by maxima d in the time domain. However, other arrangements will be clear to those skilled in the art. The Electrode Sections extend substantially the entire length of adjacent Electrode 42. Therefore the amplitude of the overlap between adjacent Electrodes is substantially equal.

In addition, the Electrode corresponding to the symmetrical local maxima and symmetrical local minima of the time domain representation both have Electrode Sections displaced by an equal amount and in the same direction. As the Electrodes are farther removed from the center of the transducer, the displacement of the Electrode sections becomes increasingly larger.

Referring next to FIG. 4, a second embodiment of the present invention is illustrated. In this Figure, Terminals 10 and 11 provide electrical coupling to two substantially identical transducers comprised of Electrode Elements 12, 13, and 14. As illustrated in FIG. 4, these two transducers represent Portions of an Output Transducer. However, it will be clear to one skilled in the art that these transducers can represent portions of an input transducer.

Conducting Region 20 is coupled to Terminal 70 while Conducting Region 30 is coupled to Terminal 60. Electrode Sections 91, 76, 90, 74, and 78 are coupled to Conducting Region 30 while Electrode Sections 88, 84, 81, 80, 82, and 86 are coupled to Conducting Region 20. In the preferred embodiment, the set of Electrode Sections coupled to Conducting Region 20 and to Conducting Region 30 extend approximately half the distance between Conducting Region 30 and Conducting Region 20. Furthermore, the positions of these Electrode Sections are substantially the positions indicated by the local maxima and local minima of the time domain diagram. Each of the Electrode Sections has another Electrode Section coupled to it. For example, Section 87 is coupled to Section 86, but displaced from Section 86. Similarly, Section 89 is coupled to Section 88. Furthermore, Section 89 is displaced from Section 88 by the same amount and in the same direction as Section 87 is displaced from Section 86. Similarly, each Electrode Section coupled to a Conducting Region is coupled to another Electrode Section. When the second Electrode Section coupled to the first Electrode Section is displaced therefrom, then another second Electrode Section is displaced for another first Electrode Section, the first Electrode Section symmetrically located with respect to portions of the Output Transducer, by an equal amount and in the same direction. The portions of the Output Transducer, similarly, are located symmetrically with respect to the undisplaced Sections of the Input Transducer. The second Electrode Section extends for substantially the distance remaining from the first Electrode Section to the opposite Conducting Region. Except for a small region coupling the first and second Electrode Sections, the Electrodes coupled to the conducting regions extend substantially the entire distance between the Conducting Regions and are substantially parallel to the other Electrodes.

The operation of the preferred embodiment shown in FIG. 3 can be understood by considering the result of application of an electrical signal of the design center frequency between Terminals 60 and 70. The acoustic surface waves, launched toward Transducer Electrode Elements 12, 13, and 14, will have two components, one for each Electrode Section. Where the displacement from the position typically indicated by the local maxima or minima of the time domain representation is labeled d, then the acoustic surface wave generated by one Electrode Section will be retarded by time d/v.sub.o relative to an acoustic surface wave generated from the electrode of the prior art, while an acoustic surface wave generated by the second Electrode Section will be generated in advance of the acoustic surface wave generated by the electrode of the prior art, by a time d/v.sub.o. Both the advanced and the retarded wave will be combined by the Electrode Elements 12, 13, and 14. The amplitude, because of the vectorial signal addition, will be diminished, while the symmetry of the displacement will produce a resulting signal at Terminal 10 and 11 which is in phase with a signal generated at the average Electrode Section position, i.e. at the position designated by the local maxima or minima of the time domain representation.

It will be clear to those skilled in the art that neither the symmetry of Electrode Section length nor the symmetry of Electrode Section displacement from the average position is essential. The essential feature is that the vector addition of acoustic surface waves generated by the electrode section results in the generation of an electrical signal which is of the proper phase and amplitude.

Referring to FIG. 4, an embodiment of phase-encoded surface wave devices is shown in which a coupled pair of transducers exchange surface waves with a first transducer. Electrode Section 89 and Electrode Section 87 are positioned equidistant from a first and a second of the coupled transducers respectively. In addition, Section 89 and Section 87 are equidistant from the second and the first of the coupled portion of the Output Transducers respectively. Coupled to Section 87, Electrode Section 86 is displaced toward the first and away from the second coupled transducer. Similary, Electrode Section 88, coupled to Section 89 is displaced toward the first and away from the second transducer. Because of the difference in the length of the propagation region, the acoustic surface waves, generated by Section 87 and 89 arrive at each of the coupled portions of the Output Transducers in-phase, while the acoustic surface wave generated at Section 86 arrives at a first portion of the coupled Output Transducer in advance of the in-phase signal and the acoustic surface wave generated at Section 88 arrives at a second portion of the coupled Output Transducer retarded from the in-phase signals.

Because the magnitude of the displacement of Section 86 from Section 87 is equal to the displacement of Section 88 from Section 89, and because Section 88 and 86 are substantially the same length, the signal generated by the coupled transducer will provide an in-phase signal of diminished amplitude. The amplitude can be arranged to provide an amplitude determined by the corresponding local maxima of the time domain representation for electrical signals of the design center frequency. Similarly, the entire time domain representation can be provided by appropriate displacement of the Electrodes Sections not located at the local maxima or minima indicated by the local maxima and minima of the time domain representation.

It will be apparent to one skilled in the art that Electrode Sections 86 and 88 can extend substantially the entire distance between Conducting Region 20 and Conducting Region 30. In such an embodiment, the displacement would have to be varied appropriately to provide the correct amplitude for the vector combination signal.

In either embodiment of the phase-encoding of the surface wave device, the Electrode elements are substantially parallel, except possibly for a small Electrode Section coupling region and extend substantially the entire distance between the Conducting Regions. This arrangement provides generated surface waves with greater directionality than can be achieved with the amplitudeencoded transducers.

It will be apparent to those skilled in the art that the phase-encoding of the transducer elements to provide predetermined characteristics without determining the amplitude-encoded transducer associated with the predetermined characteristics. The amplitude-encoded transducer has been used to clarify the operation of the instant invention.

The above description is included to illustrate the operation of the preferred embodiment and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the invention.