Title:
MICROWAVE ULTRASONIC DELAY LINE
United States Patent 3582834


Abstract:
A low loss microwave ultrasonic delay line wherein the area of the transducer over which the electric field is normal to the surface of its electrodes is substantially smaller than the cross-sectional area of the delay rod. The end of the rod opposite to the end where the transducer is mounted is shaped to focus the acoustic energy to return to the aforementioned area.



Inventors:
EVANS GARY E
Application Number:
04/720291
Publication Date:
06/01/1971
Filing Date:
04/10/1968
Assignee:
WESTINGHOUSE ELECTRIC CORP.
Primary Class:
Other Classes:
310/335, 330/5
International Classes:
H03H9/125; H03H9/36; (IPC1-7): H03H9/30
Field of Search:
330/30,72,5.5,35 308
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Primary Examiner:
Saalbach, Herman Karl
Assistant Examiner:
Baraff C.
Claims:
I claim as my invention

1. A microwave ultrasonic delay line comprising, in combination; an electroacoustical transducer including at least two electrodes; a delay medium mounted at one of its ends in transfer relationship with said transducer; thin films each in the order of 0.1 acoustical wavelengths of the energy propagating through the delay medium disposed on said other end of said medium to provide zoned acoustical lenses for narrow bands of frequencies to refocus the acoustical wave propagating through said said delay medium to said area of the transducer over which the electric field is normal to the surface of said electrodes; said area being substantially smaller than the cross-sectional area of said delay medium.

2. A microwave ultrasonic delay line comprising, in combination; a delay medium; an electroacoustical transducer including at least two electrodes; one electrode being of substantially the same cross-sectional area as said delay medium, the other electrode being substantially smaller in cross-sectional area to provide an active area of said transducer over which the electric field is normal to the surface of the transducer; said delay medium mounted at one of its ends in transfer relationship with said one electrode, the other end of said medium being shaped to focus the acoustic energy to return to said area of said transducer over which the electric field is normal, said area being substantially smaller than the cross-sectional area of said delay medium whereby the resistive component of the load impedance presented by said transducer is increased in magnitude inversely proportional to the reduction in size of the area of said transducer over which the electric field is normal; and thin film each in the order of one-tenth of the acoustical wavelength of the energy propagating through the delay medium disposed on said other end of said medium for reflecting and focusing the acoustical wave propagating through said delay medium to said active area.

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to delay lines and more particularly relates to a low loss microwave ultrasonic delay line.

2. Description of the Prior Art

The most usual microwave ultrasonic delay line is a one-sixteenth to one-half inch diameter crystalline rod with a transducer on one end used for input and output of microwave energy fed to it through a matching network from a microwave circulator or coupler. The other end of the rod is polished flat for reflection of the acoustic energy back to the aforementioned transducer mounted at the opposite end. The incoming energy and outgoing energy are separated in the microwave circulator or coupler.

The conventional arrangement results in large losses because of inefficient transducers. The transducer usually consists of a microwave circuit matching into a piezoelectric disc. The loss is usually due to the very low value of resistance presented by the acoustic load compared to the contact or microwave circuit resistances. If the area of the transducer is made small enough to reduce the aforementioned resistance losses, then the beam spreading of the acoustic waves becomes great thereby providing another limitation to low loss operation.

An object of the present invention is to provide a microwave ultrasonic delay line wherein the diffraction and resistance losses are substantially less than combinations of the prior art.

Another object of the present invention is to provide a microwave ultrasonic delay line wherein the impedance presented by the transducer is in the range to match the connecting transmission line.

SUMMARY OF THE INVENTION

Briefly, the present invention accomplishes the above cited objects by reducing the area of the transducer over which the electric field is normal to the surface of its electrodes to a magnitude substantially less than the cross-sectional area of the delay medium, thereby reducing resistance losses. The opposite end of the delay medium is formed to focus acoustical energy to the aforementioned area to substantially reduce diffraction losses. The combination provides a microwave ultrasonic delay line having low total loss.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will be readily apparent from the following detailed description taken in conjunction with the drawing, in which:

FIG. 1 is a schematic diagram illustrating an application of the present invention in a typical microwave circuit for accomplishing a delay;

FIG. 2 is a schematic diagram of an illustrative embodiment of the present invention;

FIGS. 3 and 4 are a cross-sectional end view and amplified side view of a component utilized in the illustrative embodiment of FIG. 2;

FIGS. 5 and 6 are alternate illustrations of yet another component utilized in the illustrative embodiment of FIG. 2;

FIG. 7 is a schematic diagram of an alternative illustrative embodiment of the present invention; and

FIG. 8 is a diagrammatic showing yet another component in alternate form for utilization in the illustrative embodiments shown.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A typical microwave application of an ultrasonic delay line is illustrated in FIG. 1. Microwave energy to be delayed is connected into the arrangement by a circulator 2 through a matching network 4 to an ultrasonic delay line 6. An electroacoustical transducer 8 is connected to receive the microwave energy by its electrodes 10 and 12. The resultant vibrational energy or ultrasonic energy is transmitted through a delay medium or rod 14 from where it is returned by a reflecting end 16 to the transducer 8 for retransmission through the matching network 4 and circulator 2 to its output port.

Microwave ultrasonic delay lines are usually lost due to the very low value of resistance presented by the acoustic load compared to the contact or microwave circuit resistances to which it is to be matched. The resistive component of the load impedance presented by the transducer is inversely proportional to the area A of the electrode face. That is, the load impedance of the transducer can be represented by

The larger the resistive component of the transducer can be made the more efficiently it can be matched to the transmission line. The impedance of the transmission line may be typically in the order of 50 ohms while the resistive component of the transducer is of very low value, typically much less than 1 ohm.

The foregoing equation demonstrates that making the active area of the transducer smaller will result in the resistive component being larger and hence easier to match to the transmission line. However, when the area of the transducer contact is made smaller the acoustic signal is diffracted by spreading of the signal as it propagates through the delay medium. Thus, for a given wavelength λ and path length L, the diameter of the transducer is usually made large to prevent diffraction of the acoustic waves between input and output to the transducer. Conventionally, the transducer area is of a magnitude such that the square of the diameter is considerably greater than the product of wavelength λ and path length L to prevent such beam spreading, i.e. d2 >>λ L.

The present invention provides means for overcoming this limitation and is illustrated in FIG. 2. The usual delay line is a one-sixteenth to one-half inch diameter crystalline rod with a transducer at one end used for input and output and the far end polished flat for reflection. The input and output are commonly separated in a microwave circulator. In accordance with the present invention, the area of the transducer over which the electric field is normal to the surface of its electrodes is greatly reduced thereby substantially increasing the resistive component thereof for more efficient matching. At the same time, in order to overcome the diffraction resulting from beam spreading, the reflecting end 26 is ground spherical about the transducer 18. The spreading of the beam is illustrated by the dotted line defining an envelope 19. The radius of the spherical surface 26 is selected to be equal to the rod length and measured from the transducer 18.

An amplified view of a realizable transducer 18 is illustrated in FIGS. 3 and 4, although not to scale. The rod 24 may be in the order of an inch in length while the thickness of the transducer 18 is in the order of 1 micron. The transducer 18 includes its two electrodes 20 and 22 separated by a piezoelectric crystal. An insulating film 23 separates the electrode 20 from the crystal 21. An isolating film 25, transforming film 27 and bonding film 29 forms the electrode 22 disposed on the opposite side of the piezoelectric crystal 21. The active area, or areas of the transducer determinative of the resistive component is that area of the transducer 18 over which the electric field is normal to the surface of its electrodes 20 and 22, and confined to the piezoelectric crystal 21 between them. The electric field is indicated as extending between the two electrodes.

As an alternate to the grinding of the reflective end 26, focusing can be conveniently accomplished with thin films as illustrated in FIG. 5. The films are vacuum deposited by techniques well known to those skilled in the art. A zoned reflector 36 is built up of a plurality of thin films each in the order of 0.1 λ where λ is the acoustic wavelength of the energy propagating through the delay medium. The thicknesses are exaggerated for purposes of illustration. The zoned reflector 36 may be many wavelengths thick and is fashioned, for example, as an acoustical arrangement of the dielectric lenses illustrated, for example, by Samuel Silver, page 397, of "Microwave Antenna Theory and Design", Boston Technical Publishers, Inc., 1964 .

FIG. 6 illustrates an alternate zoned reflector 46 for narrow bands wherein wavelengths are skipped, accomplishing fine phase control with but a few depositions.

Regardless of which embodiment of the zoned reflector is used, the surface must be contoured to within a small fraction of a wavelength, but the area over which this is required is relatively small. For example, the total deviation from flatness required for a 1 millimeter surface on a 1 inch radius sphere is only 5 microns.

An alternative illustrative embodiment is as shown in FIG. 7 wherein a transducer 78 is mounted on one end of the rod 74 with that end being ground flat and reflective. The opposite end of the rod 74 is ground parabolic. Hence, the acoustical energy propagates through the rod 74 4 times, following the path illustrated by the arrows. The acoustic energy is reflected by the parabolic surface 76 to the flat reflective surface 77 from where it is returned to the parabolic surface 76 for redirection to the transducer 78. Thus, the delay is increased while using the same length delay medium.

The previous illustrative embodiments utilize pulsed microwave energy wherein the transducer translates microwave energy to acoustic energy which in turn is propagated through the delay medium and received once more by the same transducer for retransmittal to the microwave transmission line. An alternate embodiment is illustrated in FIG. 8 wherein separate transducers 81 and 82 are disposed at one end of a delay medium 83 with the first transducer 81 acting as the transmitter and the second transducer 82 performing the receive function. Since the transducers are selected to have an effective area substantially less than the cross-sectional area of the delay medium 83, two such transducers can readily be formed across the end of the delay medium.

While the present invention has been described with a degree of particularity, for the purposes of illustration, it is to be understood that all modifications, substitutions and alterations within the spirit and scope of the present invention are herein meant to be included.