US3781721A

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Publication number: US3781721A
Application number: US00311007A
Authority: US
Inventors: G Judd; C Stout
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1972-11-30
Filing date: 1972-11-30
Publication date: 1973-12-25
Anticipated expiration: 1990-12-25

Abstract

An acoustic surface wave device is disclosed wherein spurious response levels are considerably reduced. The ends of an elongated substrate capable of propagating acoustic surface waves are rounded to allow acoustic surface waves launched on the front broad face of the substrate to propagate around these ends to the reverse broad face of the substrate. A roughened area is provided on the reverse broad substrate face to dissipate the acoustic surface waves propagated thereto by scattering.

Description

United States Patent Judd et al.

[451 Dec. 25, 1973 ACOUSTIC SURFACE WAVE DEVICE ELIMINATING SPURIOUS END REFLECTIONS Inventors: Gordon W. Judd, Yorba Linda;

Charles R. Stout, Fullerton, both of Calif.

Hughes Aircraft Company, Culver City, Calif.

Filed: Nov. 30, 1972 Appl. No.: 311,007

Assignee:

US. Cl 333/30 R, 333/72, 310/8, 310/9.8

Int. Cl H03h 7/30,H03h 9/32, HOlv 7/00 Field of Search 333/30 R, 92; 310/9.7, 9.8, 8, 8.1

References Cited UNITED STATES PATENTS 10/1972 Gerard 310/9.8

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Marvin Nussbaum Attorney-W. H. MacAllister, Jr. et al.

[5 7 ABSTRACT An acoustic surface wave device is disclosed wherein spurious response levels are considerably reduced. The ends of an elongated substrate capable of propagating acoustic surface waves are rounded to allow acoustic surface waves launched on the front broad face of the substrate to propagate around these ends to the reverse broad face of the substrate. A roughened area is provided on the reverse broad substrate face to dissipate the acoustic surface waves propagated thereto by scattering.

10 Claims, 3 Drawing Figures Utilization Circuit ACOUSTIC SURFACE WAVE DEVICE ELIMINATING SPURIOUS END REFLECTIONS This invention relates generally to ultrasonic acoustics, and more particularly relates to an acoustic surface wave device wherein spurious reflections from the ends of the device are substantially eliminated.

In some electromagnetic applications, such as surveillance radar, delay times as long as several milliseconds are required. An electromagnetic delay line providing a delay time of this magnitude would be prohibitively long. However, the desired delay can be accomplished through the use of microwave acoustic devices. In such devices, rf energy is converted into acoustic waves and the delay is achieved in the acoustic device, after which the acoustic waves are converted back into rf energy. The velocity of electromagnetic waves in free space is of the order of meters per second. On the other hand, acoustic waves do not exist in free space and velocities of these waves depend strongly on the medium in which propagation occurs. The velocity of acoustic waves is of the order of 10 that of electromagnetic waves; hence acoustic delay lines can be made of considerably smaller and more practical physical dimensions.

Basically, an acoustic surface wave circuit comprises a source of rf signals, a smooth slab-like element or substrate of a material capable of propagating acoustic surface waves, and a load or utilization device. Electroacoustic transducers are attached or held in close proximity to the substrate to convert the rf energy to surface waves in the material and vice versa. A transducer used to convert rf energy to surface waves will be referred to as an input transducer, while a transducer performing the function of reconverting the surface wave energy to rf energy will be referred to as an output transducer.

Electro-acoustic transducers of the foregoing type are inherently bidirectional in that they launch two acoustic waves of equal magnitude traveling in opposite directions. The wave traveling in the direction away from the output transducer is reflected upon reaching the end of the substrate near the input transducer, and it then propagates toward the output transducer. Upon reaching the output transducer, this wave gives rise to a large, undesired spurious signal.

Acoustic energy absorbent materials have been attached to end regions of acoustic surface wave substrates in an effort to attenuate the undesired acoustic energy. However, at frequencies commonly of interest, no material has been found that functions as a perfectly matched load for acoustic surface waves, and unwanted spurious responses are still excessive. For example, some applications require more than 40 db suppression of spurious responses. However, the maximum spurious response suppression thathas been achieved for a variety of absorbent materials applied to devices operating at frequencies of around 30 MHz has been found to be about 35 db.

Accordingly, it is an object of the present invention to improve the characteristics of acoustic surface wave devices.

It is a further object of the present invention to substantially eliminate spurious responses in acoustic surface wave devices as a result of the reflection of surface waves from the ends of the device substrate.

Still another object of the present invention is to provide a surface wave acoustic delay line that is relatively easy and economical to fabricate.

An acoustic surface wave device according to the invention includes a substrate of a material capable of propagating acoustic surface wave energy. The ends of the substrate are rounded to allow acoustic surface wave energy launched on one surface of the substrate to propagate to the opposite substrate surface. Means are provided on this opposite substrate surface to dissipate acoustic surface wave energy propagated thereto.

As a result, spurious signals due to the reflection of acoustic surface wave energy from the ends of the substrate are substantially eliminated.

In addition, fabrication of acoustic surface wave devices employing the aforementioned acoustic absorbers involves a very difficult and tedious procedure. It is necessary to apply the absorber to the substrate and at the same time monitor the output of the acoustic device in response to an applied rf signal. Moreover, it is important to prevent any deposition of the acoustic absorber on the electro-acoustic transducer itself, as such deposition will seriously impair the operating properties of the transducer.

The foregoing and other objects and features of the present invention may be more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a simplified pictorial view illustrating an acoustic surface wave delay line in accordance with the present invention;

FIG. 2 is an enlarged side elevational view of a rounded end portion of the delay line substrate of FIG. 2; and

FIG. 3 is an enlarged view of the central portion of the underside of the delay line substrate of FIG. 1.

Referring to FIG. 1 with greater particularity, there is shown an elongated slab, or substrate, 8 having opposing

broad surfaces

9 and 10, respectively.Substrate surface 8 is provided with an input transducer 11 and anoutput transducer 12. A source of rf energy, indicated generally byblock 13, is electrically coupled to input transducer 11 by means of electrical leads l5 and 16, respectively, connected to elongated

conductive pads

17 and 18 of transducer 11. A utilization circuit indicated generally asblock 14 is electrically coupled tooutput transducer 12 by means ofelectrical leads 19 and 20, respectively, connected to elongated

conductive pads

21 and 22 oftransducer 12. The material from whichsubstrate 8 is fabricated is of the type suitable for the propagation of acoustic surface waves. Many suitable piezoelectric materials have been employed for this purposeand their characteristics can be found in the recent technical literature. For example, LiNbO CdS, ZnO, Bi GeO and SiO form an inclusive, though not exhaustive, list of materials which have been so employed.

Generally, thesurface 9 ofsubstrate 8 that carries thetranducers 11 and 12 is ground and polished to an optical finish, that is, surface irregularities have dimensions that do not exceed the order of wavelengths of visible light. Input andoutput transducers 11 and 12 are deposited, bonded or otherwise mechanically attached to thesubstrate surface 9. Transducers l1 and 12 can be formed of any suitable electrically conductive material such as aluminum or gold. The thickness of the conductive material is typically of the order of 500 to 1,000 Angstroms.

The ends of thesubstrate 8 define respective

curved surfaces

23 and 24 extending between the substrate

broad surfaces

9 and 10 and disposed about an axis perpendicular to the length of thesubstrate 8 and parallel to the

surfaces

9 and 10. As shown, each of the

curved surfaces

23 and 24 is of a semicylindrical configuration with a radius R. The radius R is preferably at least about an order of magnitude greater than the wavelength of the acoustic surface waves propagating along thesubstrate 8 to insure that these acoustic surface waves will propagate around the ends of the substrate with essentially no reflections.

In order to dissipate the acoustic surface wave energy propagated around to theback surface 10 of thesubstrate 8, thesurface 10 may be provided with ascattering region 40. Thescattering region 40 preferably consists of a roughened surface wherein surface irregularities have dimensions of the order of the wavelength of the acoustic surface waves propagating along thesubstrate 8.

In operation, rf energy is appliedto transducer 11 fromrf input source 13 by means of leads l and 16. This rf energy is converted into acoustic surface waves propagating along thesurface 9 ofsubstrate 8. Due to the inherent bidirectional nature of transducer 11, two acoustic surface waves of equal magnitude traveling in opposite directions are launched as indicated by the

arrows

60 and 61 in FIG. 1. Thewave 61 traveling in the direction away from theoutput transducer 12 propagates around thecurved end surface 23 to the back surface of thesubstrate 8. After traversing the roughenedregion 40 on the substrate backsurface 10, this acoustic wave is substantially dissipated by scattering. Thus, instead of being reflected from an acoustic discontinuity at the end of the substrate as in the prior art, the undesiredacoustic surface wave 61 is transmitted to a scattering area and dissipated in a device according to the invention. Moreover, after forwardly travelingacoustic surface wave 60 has passed theoutput transducer 12, this wave propagates around thecurved end surface 24 of thesubstrate 8 and is subsequently dissipated in thescattering region 40 on the substrate backsurface 10. Thus, reflections from the end of the substrate adjacent to theoutput transducer 12 are also eliminated.

It is pointed out that the techniques required to grind and polish the

end regions

23 and 24 of the substrate to the desired curvature are easy to implement and no tedious application of absorbent materials to the substrate is required. Thus the present invention provides an acoustic delay line that is relatively simple and economical to fabricate.

Of even more significance, however, is the considerable reduction in the spurious signal level achieved by the invention. Although the roughenedsubstrate region 40 does result in some acoustic surface wave reflection, the magnitude of this reflection is exceedingly small. Spurious responses from this small discontinuity have been measured to be at levels more than 45 db below the desired output signals. Since the best reduction in spurious response levels achieved with the prior art is about 35 db below the signal level, the present invention reduces the delay line spurious response level in an acoustic surface wave delay line by approximately an order of magnitude.

Although the present invention has been shown and described with respect to a preferred embodiment, nevertheless various changes and modifications obvious to one skilled in the art are deemed to lie within the spirit and scope of the invention.

What is claimed is:

1. An acoustic surface wave device comprising in combination:

a substrate of a material capable of propagating acoustic surface wave energy;

at least one electro-acoustic transducer coupled to a first surface of said substrate for launching acoustic surface waves propagating along said first surface in a predetermined direction;

said substrate having an end region defining a curved surface about an axis substantially perpendicular to said predetermined direction; and

means on a second surface of said substrate opposite to said first surface for dissipating acoustic surface wave energy.

2. An acoustic surface wave device according to claim 1 wherein said curved surface is of a substantially semi-cylindrical configuration with a radius at least about an order of magnitude greater than the wavelength of said acoustic surface waves.

3. An acoustic surface wave device comprising in combination:

a substrate of a material capable of propagating acoustic surface wave energy;

said substrate having an end region defining a curved surface about an axis substantially perpendicular to said predetermined direction; and said substrate further having a scattering region defined on a second surface of said substrate opposite to said first surface for dissipatingacoustic surface wave energy, said scattering region including a roughened surface wherein surface irregularities have dimensions of the order of the wavelength of said acoustic surface waves. 4. An acoustic surface wave device according to claim 3 wherein said curved surface is of a substantially semi-cylindrical configuration with a radius atleast about an order of magnitude greater than the wavelength of said acoustic surface waves.

5. An acoustic surface wave device comprising in combination:

an elongated slab of a material capable of propagating acoustic surface wave energy, the ends of said slab being rounded; I

a pair of electro-acoustic transducers disposed adjacent to respective spaced regions along a surface of said slab; and

means on the opposite surface of said slab for dissipating acoustic surface wave energy.

6. An acoustic surface wave device according to claim 5 wherein said means for dissipating acoustic surface wave energy comprises a scattering region including a roughened surface wherein surface irregularities have dimensions of the order of the wavelength of the acoustic surface wave energy propagated along said slab.

7. An acoustic surface wave device comprising in combination:

an elongated slab of a material capable of propagating acoustic surface wave energy, said slab having first and second opposing broad faces;

first and second electro-acoustic transducers disposed adjacent to respective regions of said first broad face near the respective end regions of said slab;

the respective end regions of said slab each defining a curved surface extending between said first and second broad faces about an axis substantially parallel to said first and second broad faces and substantially perpendicular to the length of said slab; and

means on said second broad face of said slab for dissipating acoustic surface wave energy.

8. An acoustic surface wave device according to claim 7 wherein each said curved surface is of a substantially semi-cylindrical configuration with a radius at least about an order of magnitude greater than the wavelength of the acoustic surface wave energy propagated along said slab.

9. An acoustic surface wave device comprising in combination:

said slab further having a scattering region defined on said second broad face for dissipating acoustic surfaces wave energy, said scattering region including a roughened surface wherein surface irregularities have dimensions of the order of the wavelength of the acoustic surface wave energy propagated along said slab.

Claims

1. An acoustic surface wave device comprising in combination: a substrate of a material capable of propagating acoustic surface wave energy; at least one electro-acoustic transducer coupled to a first surface of said substrate for launching acoustic surface waves propagating along said first surface in a predetermined direction; said substrate having an end region defining a curved surface about an axis substantially perpendicular to said predetermined direction; and means on a second surface of said substrate opposite to said first surface for dissipating acoustic surface wave energy.

3. An acoustic surface wave device comprising in combination: a substrate of a material capable of propagating acoustic surface wave energy; at least one electro-acoustic transducer coupled to a first surface of said substrate for launching acoustic surface waves propagating along said first surface in a predetermined direction; said substrate having an end region defining a curved surface about an axis substantially perpendicular to said predetermined direction; and said substrate further having a scattering region defined on a second surface of said substrate opposite to said first surface for dissipating acoustic surface wave energy, said scattering region including a roughened surface wherein surface irregularities have dimensions of the order of the wavelength of said acoustic surface waves.

4. An acoustic surface wave device according to claim 3 wherein said curved surface is of a substantially semi-cylindrical configuration with a radius at least about an order of magnitude greater than the wavelength of said acoustic surface waves.

5. An acoustic surface wave device comprising in combination: an elongated slab of a material capable of propagating acoustic surface wave energy, the ends of said slab being rounded; a pair of electro-acoustic transducers disposed adjacent to respective spaced regions along a surface of said slab; and means on the opposite surface of said slab for dissipating acoustic surface wave energy.

7. An acoustic surface wave device comprising in combination: an elongated slab of a material capable of propagating acoustic surface wave energy, said slab having first and second opposing broad faces; first and second electro-acoustic transducers disposed adjacent to respective regions of said first broad face near the respective end regions of said slab; the respective end regions of said slab each defining a curved surface extending between said first and second broad faces about an axis substantially parallel to said first and second broad faces and substantially perpendicular to the length of said slab; and means on said second broad face of said slab for dissipating acoustic surface wave energy.

9. An acoustic surface wave device comprising in combination: an elongated slab of a material capable of propagating acoustic surface wave energy, said slab having first and second opposing broad faces; first and second electro-acoustic transducers disposed adjacent to respective regions of said first broad face near the respective end regions of said slab; the respective end regions of said slab each defining a curved surface extending between said first and second broad faces about an axis substantially parallel to said first and second broad faces and substantially perpendicular to the length of said slab; and said slab further having a scattering region defined on said second broad face for dissipating acoustic surfaces wave energy, said scattering region including a rouGhened surface wherein surface irregularities have dimensions of the order of the wavelength of the acoustic surface wave energy propagated along said slab.

10. An acoustic surface wave device according to claim 9 wherein each said curved surface is of a substantially semi-cylindrical configuration with a radius at least about an order of magnitude greater than the wavelength of the acoustic surface wave energy propagated along said slab.