Title:
ACOUSTIC SURFACE WAVE DEVICE ELIMINATING SPURIOUS END REFLECTIONS
United States Patent 3781721


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.



Inventors:
Judd, Gordon W. (Yorba Linda, CA)
Stout, Charles R. (Fullerton, CA)
Application Number:
05/311007
Publication Date:
12/25/1973
Filing Date:
11/30/1972
Assignee:
HUGHES AIRCRAFT CO,US
Primary Class:
Other Classes:
310/313B, 310/313R
International Classes:
H03H9/02; (IPC1-7): H03H7/30; H01V7/00; H03H9/32
Field of Search:
333/3R,72 310
View Patent Images:
US Patent References:



Primary Examiner:
Rolinec, Rudolph V.
Assistant Examiner:
Nussbaum, Marvin
Claims:
What is claimed is

1. An acoustic surface wave device comprising in combination:

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:

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:

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:

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:

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.

Description:
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 108 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 10-5 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. Electro-acoustic 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 that has 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 an output transducer 12. A source of rf energy, indicated generally by block 13, is electrically coupled to input transducer 11 by means of electrical leads 15 and 16, respectively, connected to elongated conductive pads 17 and 18 of transducer 11. A utilization circuit indicated generally as block 14 is electrically coupled to output transducer 12 by means of electrical leads 19 and 20, respectively, connected to elongated conductive pads 21 and 22 of transducer 12. The material from which substrate 8 is fabricated is of the type suitable for the propagation of acoustic surface waves. Many suitable piezoelectric materials have been employed for this purpose and their characteristics can be found in the recent technical literature. For example, LiNbO3, CdS, ZnO, Bi12 GeO20 and SiO2 form an inclusive, though not exhaustive, list of materials which have been so employed.

Generally, the surface 9 of substrate 8 that carries the tranducers 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 and output transducers 11 and 12 are deposited, bonded or otherwise mechanically attached to the substrate surface 9. Transducers 11 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 the substrate 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 the substrate 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 the substrate 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 the back surface 10 of the substrate 8, the surface 10 may be provided with a scattering region 40. The scattering 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 the substrate 8.

In operation, rf energy is applied to transducer 11 from rf input source 13 by means of leads 15 and 16. This rf energy is converted into acoustic surface waves propagating along the surface 9 of substrate 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. The wave 61 traveling in the direction away from the output transducer 12 propagates around the curved end surface 23 to the back surface 10 of the substrate 8. After traversing the roughened region 40 on the substrate back surface 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 undesired acoustic surface wave 61 is transmitted to a scattering area and dissipated in a device according to the invention. Moreover, after forwardly traveling acoustic surface wave 60 has passed the output transducer 12, this wave propagates around the curved end surface 24 of the substrate 8 and is subsequently dissipated in the scattering region 40 on the substrate back surface 10. Thus, reflections from the end of the substrate adjacent to the output 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 roughened substrate 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.