U.S. patent number 3,714,609 [Application Number 05/060,151] was granted by the patent office on 1973-01-30 for microwave ultrasonic delay line.
Invention is credited to Terry F. Newkirk, David J. Whitney.
United States Patent |
3,714,609 |
Whitney , et al. |
January 30, 1973 |
MICROWAVE ULTRASONIC DELAY LINE
Abstract
An ultrasonic delay line for use at microwave frequencies is
provided in a thin epitaxial strip of ultrasonic wave conducting
material laid down on a substrate body. An input transducer
launches ultrasonic waves into the epitaxial strip at one end and
an output transducer detects the ultrasonic waves at the other end
of the strip. The major surfaces of the epitaxial strip are
preferably adjacent a medium such that the interface therewith is
highly reflective to the ultrasonic waves and the thickness of the
epitaxial strip is precisely determined when it is formed so that
the thickness is of the same order of magnitude as the inflection
thickness for the material at an ultrasonic frequency in the
microwave range.
Inventors: |
Whitney; David J. (Amherst,
NH), Newkirk; Terry F. (Lynnfield, MA) |
Family
ID: |
22027696 |
Appl.
No.: |
05/060,151 |
Filed: |
July 31, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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656273 |
Jul 26, 1967 |
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Current U.S.
Class: |
333/145;
330/5.5 |
Current CPC
Class: |
H01P
9/02 (20130101); H01P 9/04 (20130101); H03H
9/40 (20130101) |
Current International
Class: |
H03H
9/40 (20060101); H03H 9/00 (20060101); H03h
007/30 (); H03h 009/30 () |
Field of
Search: |
;333/30,71 ;330/5,5.5,31
;310/8.2,9.4 ;317/235 ;331/107 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gensler; Paul L.
Parent Case Text
This application is a continuation-in-part of our co-pending
application Ser. No. 656,273 now abandoned.
Claims
What is claimed is:
1. A dispersive ultrasonic delay line comprising
a substrate body;
an epitaxial layer attached to said substrate body, comprising a
single unitary crystalline material having a center of inversion
symmetry and in which microwave ultrasonic material waves
experience tolerable attenuation per unit length of the material,
having a thickness of the same order of magnitude as the inflection
thickness for said material at a preselected microwave frequency,
and having major and minor surfaces,
means for forming substantially reflective interfaces with
substantial portions of said major surfaces of said epitaxial
layer,
means for launching longitudinal ultrasonic material waves into
said epitaxial layer at a first portion thereof, and
means for detecting said ultrasonic material waves in said
epitaxial layer at a second portion thereof remote from said first
portion.
2. Apparatus as recited in claim 1 wherein
said substantially reflective interfaces define the boundaries of a
path along the longitudinal axis of said epitaxial layer along
which said ultrasonic material waves are conducted from said
launching means to said detecting means.
3. Apparatus as recited in claim 1 further including
an acoustic wave absorbing means abutting at least one minor
surface of said epitaxial layer.
4. Apparatus as recited in claim 1 wherein
said epitaxial layer is formed of a material selected from the
group of single unitary crystalline materials having a center of
inversion symmetry consisting of
sapphire
rutile
spinel
zirconium silicate, and
zirconate.
5. Apparatus as recited in claim 1 wherein
said substantially reflective interface forming means comprises a
plurality of contiguous layers of alternately high and low acoustic
impedance disposed adjacent and substantially parallel to said
epitaxial layer, each contiguous layer being of a thickness equal
to one quarter wavelength at said preselected microwave
frequency.
6. Apparatus as recited in claim 1 wherein
said epitaxial layer is in the form of an epitaxial membrance and
the major surfaces of said membrane are adjacent a gas.
7. Apparatus as recited in claim 1 wherein
said epitaxial layer is in the form of an epitaxial membrane and
the major surfaces of said membrane are adjacent a vacuum.
8. Apparatus as recited in claim 1 wherein
said substrate body has a substantially cylindrical configuration,
and
said epitaxial layer is formed in a helical path about said
cylindrical substrate body.
9. Apparatus as recited in claim 1 wherein
said epitaxial layer is formed in a planar spiral path for
conducting said ultrasonic material waves.
Description
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to ultrasonic delay lines and more
particularly to strip type dispersive delay lines wherein
ultrasonic material waves are launched into one part of the strip
and are detected at another part a predetermined delay interval
later.
Heretofore, ultrasonic delay lines have been formed of wires and of
strips. The ultrasonic waves are launched into one end of the wire
or strip by means of a transducer and are detected at another part
by another transducer. The delay is produced by the relatively slow
velocity of the ultrasonic wave through the medium of the strip as
compared with the velocity of electromagnetic waves in a conductor.
In the strip type dispersive delay lines, the geometry of the
strip, the material of the strip, and the mode of the ultrasonic
wave launched into the strip by the transducer are such that the
time delay varies with the frequency. In non-dispersive strips, on
the other hand, the time delay is substantially constant over the
frequency band of use.
In the frequency range 1 to 10 Mhz, dispersive strip delay lines
made of aluminum alloy have been used. For example, a strip 56
inches long, 11/2 inches wide and 0.086 inches thick produces a 633
microsecond delay at the center frequency of one Megahertz and a
200 microsecond delay change over a 100 KH.sub.z bandwidth.
Dispersion is approximately linear about the inflection point on
the dispersion curve (frequency versus delay) over the bandwidth.
Efforts have been made to provide dispersive ultrasonic delay lines
for operation at higher frequencies and with wider bandwidths. For
example, fused quartz strip lines on the order of about 0.005
inches thick have been used at 30 Megahertz but mechanical
difficulties of fabricating the quartz strip have not been
overcome.
OBJECTS AND SUMMARY OF THE INVENTION
It is one object of the present invention to provide an ultrasonic
delay line for use at frequencies higher than practicable with
ultrasonic delay lines in the past.
It is another object of the present invention to provide an
ultrasonic delay line having substantially linear dispersion over a
wider bandwidth than obtained heretofore.
It is another object of the present invention to provide an
ultrasonic delay line having tolerable insertion losses for
operation in the microwave frequency range.
It is another object to provide a solid state ultrasonic delay
line.
In accordance with principle features of embodiments of the present
invention, a dispersive strip type ultrasonic delay line is
provided in an epitaxial strip of ultrasonic wave conducting
material on a substrate body. The strip is laid down on the
substrate employing well-known epitaxial layer growth techniques
and is of uniform thickness and a few microns thick. The thickness
is predetermined so that at the selected operating frequency, for
example, 1,000 Mhz, operation is around the lower inflection point
in the delay versus frequency curve of the first longitudinal mode.
For a specific material, this inflection point will occur at a
fixed frequency times thickness product. For example, by
extrapolation at 1,000 Mhz the inflection thickness for a strip of
5052H32 aluminum would be about 2 microns. Hence, a strip 2 microns
thick by 1.40 inches long would produce a delay change of 5
microseconds over a 100 Mhz bandwidth. If the delay linearity
desired is 0.5 percent and the bandwidth is 10 percent, then this 2
micron aluminum strip would serve in a system having compression
ratio of 500 to 1.
The major surfaces of the epitaxial strip for conducting the
ultrasonic waves must form interfaces with the surrounding medium
which is highly reflective to the ultrasonic waves, whereas
reflections from the minor surfaces of the epitaxial strip are to
be avoided. Optimum reflection is obtained when the surrounding
medium adjacent the major surfaces is a vacuum and the medium
adjacent the minor surfaces along the edge of the epitaxial strip
is highly absorbent and reflects substantially none of the
ultrasonic waves. If it is inconvenient to provide a vacuum
adjacent the major surfaces of the epitaxial strip, then a gaseous
environment may serve just as well, because most gases have an
extremely low acoustic characteristic impedance and will reflect
close to 100 percent of the ultrasonic acoustic energy conducted
within the strip which is incident upon the interface at the major
surface.
In the event that it is inconvenient to employ either a vacuum or a
gas adjacent one or both of the major surfaces of the epitaxial
strip, then solid materials can be employed consisting of a
plurality of thin film layers one upon another immediately adjacent
either one or both of the major surfaces of the epitaxial strip.
The plurality of layers of thin film are of selected thickness and
selected acoustic characteristic impedance so that they provide in
effect a highly reflective surface at the interface with the
epitaxial delay strip. For example, if the acoustic wave path
across each thin film is an integral number of half wavelengths of
the ultrasonic wave and if the thin films are alternately of
relatively high and relatively low acoustic impedance, then an
effective interface will be produced at the major surface of the
epitaxial strip delay line adjacent these thin films which is
substantially highly reflective to the ultrasonic waves. A number
of embodiments of the present invention illustrate the use of a
plurality of such thin films adjacent one major surface of the
epitaxial strip delay line and with vacuum or gas environment
adjacent the other major surface to provide a compact rugged
structure for use in the microwave frequency range.
Embodiments of the present invention described herein include such
an epitaxial strip delay line supported by a substrate body and
having a piezoelectric type transducer at each end thereof, one for
launching ultrasonic waves into the epitaxial strip and the other
for converting the ultrasonic waves into a high frequency
electrical signal. The transducers consist of, for example,
semiconductors which exhibit significant piezoelectric effects such
as CdS, and which preferably match the acoustic characteristic
impedance of the epitaxial strip so that the ultrasonic waves are
launched from the transducer into the strip with negligible
reflection at the interface therebetween. The transducer material
is preferably selected so that it has about the same acoustic
characteristic impedance as the epitaxial strip and if this is
inconvenient then the transducer may be comprised of a plurality of
layers of materials of selected acoustic characteristic impedance
between the transducer piezoelectric material and the epitaxial
strip. These layers serve to match the characteristic impedances
between the two, and thereby, produce minimum reflection.
Multi-layer transducers of this sort are described in an article
entitled "Multi-Layer Thin Film Piezoelectric Transducers" by John
DeKlerk on page 99 of IEEE Transactions on Sonics and Ultrasonics,
Volume SU13, No. 3, August 1966.
Other objects and features of the present invention will be
apparent from the following specific description taken in
conjunction with the appended figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a three-quarter view of the epitaxial strip delay line
supported by a substrate body and with transducers at each end for
coupling to microwave transmission lines;
FIGS. 2 and 3 are sectional views showing the epitaxial strip delay
line and substrate to illustrate steps in making the device;
FIGS. 4 and 5 are longitudinal and transverse sectional views of
the epitaxial strip delay line and substrate showing a plurality of
layers of thin film adjacent one major surface of the strip for
reflecting ultrasonic waves transmitted through the major
surface;
FIG 6 is a model illustrating reflections from a number of thin
layers into the delay line strip in phase coincidence with waves
therein;
FIG. 7 is a plot of dimensionless parameters representing delay and
frequency for a typical dispersive ultrasonic strip delay line;
FIG. 8 is a plot of delay versus frequency for the same typical
dispersive ultrasonic strip delay line;
FIGS. 9, 10 and 11 are sectional views taken longitudinally through
the epitaxial strip delay line and substrate to illustrate various
transducer configurations for converting high frequency electrical
waves into ultrasonic acoustic waves and for converting the
ultrasonic acoustic waves into high frequency electric waves;
FIG. 12 is a three-quarter view of the epitaxial strip delay line
and transducers supported on a rod shaped substrate so that the
strip defines a helix along which the ultrasonic waves are
conducted; and
FIG. 13 illustrates the epitaxial strip delay line and transducers
on a substantially flat substrate body, the strip defining a spiral
path along which the ultrasonic waves are conducted.
DESCRIPTION OF PREFERRED EMBODIMENT
FIG. 1 illustrates fundamental features of the present invention
and the principle parts thereof. These include an elongated
substrate body 1 which supports an epitaxial layer 2 of selected
single unitary crystalline material forming the strip delay line
and which couples with the input transducer 3 at one end and the
output transducer 4 at the other end. These transducers may each be
equipped with coaxial transmission line couplings such as 5 and 6
which connect with an input transmission line and an output
transmission line, respectively. The major surfaces 7 and 8 of the
epitaxial strip delay line are preferably immediately adjacent an
environment which forms an interface with these surfaces such that
the ultrasonic waves conducted by the strip reflect from the
interface and a negligible amount of ultrasonic wave energy is
transmitted through the interface and absorbed by the environment.
Along at least one of the minor surfaces 9 and 10 of the strip, the
substrate material 1 may abut the strip so that acoustic energy
transmitted through these minor surfaces is absorbed by the
substrate material. It is preferred that such absorption occur
along a substantial portion of the minor surface of the strip to
avoid interference phenomena due to multiple reflections between
the minor surfaces. It is sufficient if such absorption occurs
along at least one of the minor surfaces, preferably along the
entire length of the strip and for this reason the minor surface 9,
concealed from view in FIG. 1, preferably abuts the substrate
material 1 along its entire length, whereas the minor surface 10
may form an interface with the non-reflective environment which
surrounds the major surfaces.
The most highly reflective interface that can be obtained at the
surface of an acoustic wave conducting material is an interface
with a vacuum. In order to provide such a vacuum interface at each
of the major surfaces of the epitaxial strip in FIG. 1, the
epitaxial strip is first formed on the substrate body 1 employing
well-known vapor or liquid phase epitaxial growth techniques to lay
down the epitaxial strip 2 a few microns thick on the substantially
thicker, sturdier substrate body 1. This is shown in the
longitudinal cross section of the strip and substrate in FIG. 2.
Next, a portion of the substrate body immediately adjacent the
lower major surface 8 of the strip is removed leaving a hollow 11
in the substrate which the strip in the form of an epitaxial
membrane bridges. This can be done employing well-known photo-etch
techniques, whereby the substrate material is dissolved to a
predetermined depth. Or, a portion on the surface of the substrate
upon which the epitaxial strip is formed may be of a material of
substantially lower melting temperature than the substrate body
proper or the epitaxial strip and so the composite may be heated to
melt and drain off this material leaving the epitaxial strip or
membrance with two major surfaces 7 and 8 exposed along substantial
portions thereof to an environment which may be a vacuum
environment or a gaseous environment.
As mentioned above, at least one of the minor surfaces 9 or 10 of
the strip preferably abuts an acoustic wave absorbing material for
attenuating undesirable acoustic wave modes in the strip. This can
be accomplished by growing the epitaxial strip 2 on the substrate
body 1 as shown in FIG. 2 and then performing the photo-etch
process along only one minor surface 10 of the strip so that the
substrate material is removed along the lower major surface 8 up to
and just short of the other minor surface 9 leaving minor surface 9
in contact with the substrate material.
The high reflection at a solid to gas interface can also be
obtained at an interface between solid materials even though the
characteristic acoustic impedance of the solid materials are of the
same order of magnitude. A plurality of layers of thin films of
solid material of predetermined thickness and of material of
predetermined acoustic characteristic impedance adjacent solid wave
conducting material, will produce a high reflection to those waves
at the interface. The phenomena is somewhat analogous to the
well-known optical dielectric mirror. The optical dielectric mirror
is formed by laying down a plurality of dielectric films on a
transparent light conducting medium. These optical films are of
alternately high and low refractive index and preferably
one-quarter optical wavelength thickness and can be designed to
reflect over 90% of the optical radiation incident thereon. An
analogous acoustic wave reflection structure is described in U.S.
Pat. No. 3,422,371 entitled "Thin Film Piezoelectric Oscillator" by
Poirier and Newkirk. An epitaxial strip delay line structure
employing a plurality of thin films adjacent one major surface of
the strip, all carried on the substrate body is illustrated in the
longitudinal and transverse section views shown in FIGS. 4 and 5,
respectively.
As shown in FIG. 4, a plurality of layers 12 of thin films of
selected material are laid down on a portion of the upper surface
of the substrate body 1. The epitaxial strip delay line 2 is formed
on these thin film layers so that a substantial portion of the
lower major surface 8 of the strip forms an interface with the
uppermost thin film layer 13. The thin films and the epitaxial
strip are preferably formed in a cavity 14 in the substrate body 1
so that the minor surfaces 9 and 10 of the strip abut the substrate
body while the other major surface 7 forms an interface with the
surrounding environment which may be a vacuum or gas. The sides and
bottom of the substrate body 1 are preferably clad in an acoustic
wave absorbing material 15 so that acoustic energy transmitted
through the minor surfaces of the delay line strip or through the
layers of thin film are absorbed. For this purpose, absorbing
material 15 is provided along the sides and bottom of the substrate
body 1.
The reflections of the high frequency acoustic waves from the
layers of thin film 12 are illustrated in the model diagram of FIG.
6. This diagram shows the thin film layers 12 in cross section
between the epitaxial strip delay line 2 and the substrate body 1.
Three such thin film layers 13, 16 and 17 are shown. In operation,
the incident high frequency acoustic waves represented by the
positive and negative phase wave front positions 21 to 24 shown as
heavy solid lines are incident upon the thin film 13 immediately
adjacent the epitaxial strip. The angle of incidence of these waves
is denoted .alpha. and the angle of reflection of these same waves
is also .alpha. . The corresponding reflective wave front positions
at this interface are represented by the heavy broken line. These
incident and reflected wave front positions are all denoted
positive or negative phase and represent conditions at a given
instant of time. The distance between the successive incident
positive wave fronts is a wavelength .lambda. of the high frequency
acoustic energy in the strip 2 and the angle .alpha. is the angle
of incidence of the dominant longitudinal mode of such waves
conducted in the strip 2.
The first thin film layer 13 adjacent the epitaxial strip 2 is
composed of a material of substantially higher acoustic
characteristic impedance than the material composing in the strip
and so the reflected acoustic wave front at positions 25 to 28 are
in the same phase as the associated incident wave fronts. A portion
of the incident wave front energy is transmitted through the layer
13 along the path represented by the light solid line 18 and this
portion reflects from the interface between the thin film layer 13
and layer 16 immediately below as indicated by the light broken
line 19. The layer 16 is composed of a material of relatively low
acoustic characteristic impedance and so the wave incident at the
interface between films 13 and 16, upon reflection, experiences a
phase reversal. If the thickness of the layer 13 is such that the
path of the incident wave through the layer to the reflecting
interface between the layers 13 and 16, and from that interface
back to the interface 8, is an even number of one-half wavelengths
of the acoustic wave to a point along said interface which the same
incident wave front strikes an even number of half cycles from the
original point of incidence, then the portion which reflects from
the interface between films 13 and 16 will re-enter the epitaxial
strip in phase coincidence with a reflected wavefront at 26. Thus,
it will reinforce the wavefront at 26.
By a similar action, a portion of the wavefront incident on the
interface between films 13 and 16 will be transmitted through film
16 to the interface between that film and the next film 17. Since
film 17 is composed of material of relatively high acoustic
characteristic impedance, the reflection at the interface between
films 16 and 17 will not experience a phase reversal and will be
returned to the epitaxial strip 2 to reinforce the reflected wave
front at position 27.
The interface between film 17 and the substrate also produces a
reflection and since the substrate is composed on a material of
relatively lower acoustic characteristic impedance, this reflection
will experience a phase reversal and the reflection will return to
the epitaxial strip to reinforce the reflected wave front position
28 therein. Thus, the plurality of thin films 12 produce a
plurality of reflecting surface for the incident high frequency
acoustical wave energy such that reflected waves return to the
epitaxial strip delay line 2 to reinforce reflected waves at the
interface of the strip with the layers of thin films and so this
interface behaves just as a near perfect reflector of the incident
acoustic waves of a given wavelength .lambda. at a given angle of
incidence .alpha. .
The model as shown in FIG. 6 is simplified for the sake of brevity
and presumes the velocity of the acoustic wave in all the materials
shown to be the same. Obviously, the number of layers of thin film
can be far greater than shown and the number of layers required
will depend upon the relative magnitudes of the acoustic
characteristic impedance of the materials. In this model the thin
film layer 13 is composed of a material of substantially higher
acoustic characteristic impedance than the material composing the
epitaxial strip 2, and so there is no phase reversal of reflected
wavefronts at this interface. Other models can be envisioned
wherein the first layer 13 is of lower acoustic characteristic
impedance than the strip 2 and the subsequent layers 16 and 17 are
of relatively high and low impedance respectively. In that case,
the rule is the same as given above. The incident and reflected
wave path through each film is preferably an even number of half
wavelengths of the acoustic wave and each path returns to the
interface at surface 8 at a point along said surface which is an
even number of half cycles from the original point of incidence. It
should be quite clear that the model shown in FIG. 6 is but one
combination of thin film layers adjacent a major surface of the
epitaxial strip delay line for producing substantial reflection at
the major surface while at the same time providing solid support
for the strip. The material selected for the films 13, 16 and 17
and the substrate 1 must be such that the model structure as shown
in FIG. 6 can be constructed and must be such that the individual
films can be formed of very precise thickness in terms of fractions
of a wavelength of the high frequency acoustic waves. In addition,
the film 13 adjacent the epitaxial strip must be such that the
epitaxial layer which comprises the strip can be grown thereon.
Various combinations of materials that can be employed to form the
epitaxial strip delay line 2, the thin films 13, 16 and 17 and the
substrate body 1 such as:
Epitaxial Strip 2 Al.sub.2 O.sub.3 Thin Film 13 W Thin Film 16 Al
Thin Film 17 Au Substrate Body 1 Si
The performance of the epitaxial strip delay line is illustrated by
the curves plotted in FIGS. 7 and 8. FIG. 7 is a plot of the
dimensionless delay parameter DVs/L versus the dimensionless
frequency parameter fh/Vs for the first longitudinal acoustic wave
mode in a selected epitaxial strip material. The dimensionless
delay parameter plotted on the ordinate in FIG. 7 is the product of
the free space sheer velocity V.sub.s of the acoustic wave in the
selected material times the delay D per unit length L of the wave
in the material. The dimensionless frequency parameter plotted
along the abscissa in FIG. 7 is the product of frequency, f, times
the strip thickness, h, divided by the free space sheer wave
velocity V.sub.s. On the curve in FIG. 7, the inflection point of
the first longitudinal mode is denoted as point 31 and is the
preferred point of operation, because the dispersion on each side
of this point is substantially linear over the greatest range of
frequency and delays. The degree of this linearity is illustrated
by the plot in FIG. 8 of delay D versus frequency f about the
inflection point 31. The broken line in FIG. 8 represents the
linear dispersion desired and the solid line represents the actual
dispersion characteristics of the line about the inflection point.
If, for example, the epitaxial strip 2 is of aluminum and deviation
of the actual curve from the desired linear dispersion
characteristic is limited to 0.5 percent and the frequency band of
operation is centered at 1 Gigahertz, then the bandwidth of
operation can be 10 percent and the compression ratio will be over
500 to 1 for a strip 2 microns thick by about one-half inch wide by
1.40 inches long.
The insertion loss in the aluminum strip delay line two microns
thick is prohibitively high and so for many applications it is
preferred to employ material which has inherently lower insertion
loss than the aluminum. Materials which have utility in the
practice of the present invention must possess a center of
inversion symmetry which excludes any piezoelectric properties and
must have a crystallographic plane which exactly matches that of
the available transducers. Materials which have been found to have
application in the invention include sapphire (Al.sub.2 O.sub.3),
rutile (TiO.sub.2), spinel (MgAl.sub.2 O.sub.4), zirconium silicate
(ZrSiO.sub.4) and zirconate (ZrO.sub.2). Sapphire, for example, in
an epitaxial strip of 2 microns thickness exhibits performance
characteristics substantially similar to aluminum but has far less
insertion loss to the high frequency acoustic waves.
Various forms of input and output transducers and transducers for
tapping signals along the epitaxial strip delay line are shown in
FIGS. 9 to 11. These figures show the embodiment wherein one major
surface of the epitaxial strip is contiguous with a stack of thin
films of selected materials and thicknesses so that acoustic energy
reflects internally within the strip at the interface of this major
surface with the thin films just as already described. The figures
each show a cross section taken longitudinally through the center
of the epitaxial strip and substrate. The cross section in FIGS. 9
and 10 also cuts through the center of the transducer which may be
as wide as the strip.
In FIG. 9, the piezoelectric transducer 32 is a thin film
transducer and is preferably acoustically matched to the epitaxial
strip delay line 2 against which it abuts. The transducer 32 may be
a single layer type or multi-layer type, whichever is required to
provide an acoustic impedance match, and consists of a film 33 of
piezoelectric semiconductor material such as CdS sandwiched between
two thin film layers 34 and 35 of electrically conductive material
such as gold. The lower conductive layer 34 is contiguous with the
epitaxial strip delay line 2. This lower film 34 connects with
additional film 36 partially encasing a dielectric support block 37
onto which is mounted a coaxial transmission line connector 38. The
center conductor 39 of the transmission line connector connects to
the upper conductive film 35 on the piezoelectric semiconductor
material. The coaxial connector may include a load 40 for absorbing
high frequency electrical energy to reduce reflections from the
connector to the transmission line to which it connects.
In operation, the transducer in FIG. 9 is energized by a connection
to a coaxial transmission line and high frequency electrical energy
at, for example, one gigahertz is fed to the transducer. The high
frequency electric field produced between the thin conductive films
34 and 35 excites the piezoelectric semiconductor material 33 and
by virtue of the piezoelectric effect, a substantial portion of the
electrical energy is converted to acoustic energy of the same
frequency. This acoustic energy is transmitted through the
conductive film 34 and into the epitaxial strip delay line 2 by
virtue of the contiguous contact therebetween and produces in the
strip an acoustic wave of the same frequency which travels through
the length of the strip to an output transducer at the other end
thereof. The output transducer may be identical to the input
transducer shown in FIG. 9 and operates in a reciprocal fashion to
convert the acoustic wave energy incident upon the piezoelectric
film thereof into high frequency electrical waves.
The transducer 41 as shown in FIG. 10 includes two thin films 42
and 43 of, for example, gold sandwiching the piezoelectric
semiconductor material 44. One film 42 connects to the active
element of a high frequency transmission line and the other 43 is
contiguous with an extension 45 of the epitaxial strip delay line 2
and may be electrically grounded as shown. The exciting high
frequency electric field bounded by the conductive films 42 and 43
produces, by virtue of the piezoelectric effect, high frequency
acoustic waves in the semiconductor material 44 which flow parallel
to the exciting field and so these high frequency acoustic waves
are launched into the extension 45 in a direction substantially
parallel to the longitude or length of the epitaxial strip delay
line 2. For this purpose, the surface 46 of the extension 45 may be
contoured or tapered so that the high frequency acoustic waves at
one end thereof will be channeled into the epitaxial strip delay
line 2 and will propagate through the delay line in the
longitudinal mode.
In FIG. 11, the transducer 51 consists of a thin epitaxial layer 52
of piezoelectric semiconductor material of substantially the same
thickness as epitaxial strip delay line 2 and is disposed between
two electrically conductive layers 53 and 54 of the same thickness
and which may be of gold. The gold layer 53 is preferably grounded
and connected to the base 55 of a coaxial connector 56. The center
conductor 57 of the coaxial connector couples to the conductive
film 54. These conductive films 53 and 54 serve to bound the
excitation high frequency electric field imposed on the
piezoelectric semiconductor film 52 and which gives rise to the
high frequency acoustic waves in the semiconductor. The conductive
film 53 may be of material selected to provide an acoustic
impedance match between the semiconductor material 52 and the
epitaxial strip delay line 2. In the event, these are already
reasonably matched acoustically and the strip is composed of an
electrically conductive material, then the abutting end of the
strip may serve as the exciting electric field boundary. In either
event, it is preferred that the strip 2 and the conductive film 53
adjacent the strip be grounded and connected to the outer conductor
of the coaxial connector and it is preferred that the center
conductor of the coaxial connector connect to the other conductive
film 54.
In operation, the high frequency electric field produced in the
piezoelectric semiconductor film 52 is parallel to the longitude of
the strip delay line 2 and so the acoustic waves generated in the
piezoelectric strip 52 are incident upon the end of the delay line
2 normal thereto. Normal incidence is preferred so that the
incident waves are conducted by the strip in the first longitudinal
mode.
Taps may be provided along the epitaxial strip delay line 2 and may
consist of piezoelectric semiconductor transducers which function
similarly to the input transducer 51. Tow such taps denoted A and B
are shown along the delay line 2 in FIG. 11. Tap A consists of a
strip 59 of piezoelectric material running transverse to the delay
line 2. This piezoelectric strip 59 may be of the same thickness as
the delay line or it may be thicker. However, the strip 59 is
preferably only a few microns wide, or as wide as necessary to
produce adequate high frequency electric signal levels in
electrically conductive transverse strips 60 and 61 disposed in
relationship thereto so as to bound a dc electric field applied to
the strip 59 in a direction substantially parallel to the length of
the delay line 2. For this purpose, the electrically conductive
strips 60 and 61 may be formed in the thin film layers 12 and also
run transverse to the delay line 2 contiguous with the transverse
edges of the piezoelectric strip 59.
The circuit 62 for tap A connected to the conductive strips 60 and
61 could include a dc source 63 in series with a transformer 64. In
operation, the dc electric field bounded by the strips 60 and 61
has a fringing field which encompasses the piezoelectric strip 59
and is directed parallel to the length of the delay line 2. High
frequency acoustic waves conducted along the delay line through the
strip 59 transverse thereto generate high frequency electric
signals in the conductive strips 60 and 61 and these are coupled to
an output for tap A via the transformer 64. A similar transducer
for feeding the output tap B is located at another position along
the epitaxial strip delay line 2. Thus, a plurality of taps at
different delay positions along the delay line may be provided to
form a multi-tap delay line for use in, for example, a pulse
compression radar system.
FIGS. 12 and 13 are isometric views of two convenient conformations
for providing a relatively long acoustic delay path contained in a
space of substantially smaller dimensions than the length of the
path. In FIG. 12, the epitaxial strip delay line 71 is formed
employing any of the techniques and structures already described,
on a cylindrically shaped substrate body 72 in such a manner that
the epitaxial strip delay line forms a helix thereon. At one end of
this helix is mounted the input transducer 73 and a coupling 74 for
connection to a transmission line and at the other end is mounted
the output transducer 75 with a coupling 76 for a transmission
line.
In FIG. 13, the epitaxial strip delay line 77 is formed on a
relatively flat surface of a substrate body 78 and defines a
spiral. The input transducer 79 may be located at the center of the
spiral and output transducer 80 may be located at the edge of the
spiral.
This completes descriptions of a number of embodiments of the
present invention of an ultrasonic delay line consisting of an
epitaxial layer of unitary crystalline material having a center of
inversion symmetry, deposited on a suitable substrate so that high
frequency acoustic waves launched into one end of the epitaxial
strip are conducted therethrough and experience a high degree of
reflection from the major surfaces thereof, and are converted at
another end of the epitaxial layer into high frequency electric
waves such that the delay therebetween is predetermined and
significant for a useful purpose. The embodiments described herein
illustrate but a few applications of principle features of the
invention and are not intended to limit the spirit and scope of the
invention as set forth in the accompanying claims.
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