U.S. patent number 3,569,750 [Application Number 04/779,909] was granted by the patent office on 1971-03-09 for monolithic multifrequency resonator.
This patent grant is currently assigned to Collins Radio Company. Invention is credited to William D. Beaver.
United States Patent |
3,569,750 |
Beaver |
March 9, 1971 |
MONOLITHIC MULTIFREQUENCY RESONATOR
Abstract
A multifrequency resonating device comprising a single,
piezoelectric quartz crystal plate having a plurality of different
thickness plateaus formed thereon and separated, one from the
other, by sections of beveled surface areas which create acoustic
impedance mismatches between the plateaus. Each plateau has coated
upon both sides thereof conductive electrodes and suitable terminal
leads extending therefrom. Each plateau is resonant at a different
frequency without the necessity of separate crystal blank holders
or separate temperature compensation circuits.
Inventors: |
Beaver; William D. (Costa Mesa,
CA) |
Assignee: |
Collins Radio Company (Dallas,
TX)
|
Family
ID: |
25117963 |
Appl.
No.: |
04/779,909 |
Filed: |
November 29, 1968 |
Current U.S.
Class: |
310/320; 257/416;
310/312; 333/191 |
Current CPC
Class: |
H03H
9/56 (20130101) |
Current International
Class: |
H03H
9/19 (20060101); H03H 9/00 (20060101); H03H
9/56 (20060101); H01v 007/00 () |
Field of
Search: |
;310/8.2,8.3,8.5,8.6,9.5--9.8 ;333/72,30 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hirshfield; Milton O.
Assistant Examiner: Budd; Mark O.
Claims
I claim:
1. A mechanical resonating device for generating a plurality of
resonant frequencies and comprising:
a monolithic crystal plate having length, width, and thickness, and
divided into at least two elongated regions;
each of said elongated regions being further divided into sections,
with each successive adjacent section in each region having a
thickness less than the preceding section in said region, and with
the thickness of each section being defined by the major surfaces
thereof, which are parallel to each other; and
opposing conductive electrode means formed on said sections; each
of said sections being resonant at a given frequency different from
the resonant frequencies of other sections.
2. A mechanical resonating device in accordance with claim 1 in
which said sections of said monolithic crystal plate are separated
one from the other by segments of said crystal plate positioned
therebetween with their exposed surfaces providing a beveled
transition between the different thicknesses of adjacent
sections.
3. A mechanical resonating device in accordance with claim 1 in
which each section of each region, except the thickest and the
thinnest portions of the crystal plate, have a thickness greater
than one and less than the other of two adjacent sections in the
same region and in the adjacent region.
4. A mechanical resonating device in accordance with claim 3 in
which said sections of said monolithic crystal plate are separated
one from the other by segments of said crystal blank positioned
therebetween with their exposed surfaces providing a beveled
transition between the different thicknesses of adjacent portions.
Description
This invention relates generally to multifrequency resonators, and
more particularly to a multifrequency resonator employing a bank of
resonators formed on a common or piezoelectric quartz crystal
blank.
In the prior art, banks of crystals have long been employed to
generate a multiplicity of frequencies. However, in these prior art
devices, a separate crystal resonator is usually employed for each
given frequency, or a harmonic thereof. The use of separate crystal
resonator for each frequency involves a separate holder for each
crystal as well as a separate temperature coefficient compensating
circuit for each crystal, when temperature compensation is
employed. Both the crystal holders and the temperature compensating
circuits add appreciably to the expense of the resultant
multifrequency resonator bank.
In other types of prior art devices several plated areas have been
formed on the same monolithic crystal plate. Such use of the
monolithic crystal has been confined largely to either filter-type
circuits where the energy transfer between coated areas is either
electric or mechanical, or sometimes a combination of the two
energy transfer means.
A characteristic of prior art devices using monolithic crystal
filters is that the difference in frequency of the various coated
areas is determined by the size of the coated area as well as the
thickness of the coating. The foregoing is due to the fact that the
crystal thickness is substantially uniform, thus limiting the
frequency difference of the various resonating areas to that
obtainable by changing the size and thickness of the conductive
coating.
An object of the present invention is a relatively simple and
inexpensive wide range multifrequency resonator employing a single
quartz crystal plate.
A second purpose of the invention is a multifrequency resonator
employing a single monolithic crystal plate.
A third object of the invention is a multifrequency resonator
employing a monolithic crystal means and having a substantially
uniform temperature coefficient.
A fourth purpose of the invention is the improvement of
multifrequency crystal resonators generally.
In accordance with one form of the invention, there is provided a
quartz plate having a multiplicity of sections, or plateaus, of
various thickness. Each of these sections or plateaus have their
two major surfaces lying in parallel planes, which define the
thickness of the section. The two major surfaces of each section
have a portion thereof coated with a conductive material. Further,
the sections of different thicknesses are separated, one from the
other, by areas of changing thickness which are small in comparison
with the areas of the plateaus, but yet large enough to provide
substantial mechanical isolation between the plateaus. Thus, each
plateau constitutes a relatively isolated resonating area of the
monolithic crystal.
A feature of the invention lies in the fact that the temperature
coefficient for all of the resonating areas is substantially the
same since a common crystal plate having a given crystallographic
orientation is employed.
A second feature of the invention arises from the fact that the
single monolithic crystal needs only a single container or holder
whereas in prior art devices each of several crystals require a
separate holder.
The above mentioned and other objects and features of the invention
will be more fully understood from the following detailed
description thereof when read in conjunction with the drawings in
which:
FIG. 1 shows a cross section of a plated area of a crystal to
facilitate the understanding of the relationship between the
various parameters of the structure and its resonating
frequency;
FIG. 2 is a perspective view of the invention showing a monolithic
crystal having five plateaus;
FIG. 3 is a side view of the structure of FIG. 2;
FIG. 4 is a perspective view of a monolithic quartz crystal having
ten plateaus, all of different thicknesses;
FIG. 5 is a side view of the structure of FIG. 4; and
FIG. 6 is an end view of the structure of FIG. 4.
Referring now to FIG. 1, the quartz crystal 10 has a pair of
electrodes 11 and 12 coated thereon. The FIG. is included in the
specification primarily as a basis of a brief discussion of the
factors determining crystal resonant frequency. Said relationship
is given approximately by the following expression:
where C is the effective elastic constant in the direction normal
to the plane of the crystal plate 10, t is the thickness of plate
10, t' is the thickness of the applied conductive electrodes 11 and
12, .rho. is the density of the quartz, and .rho.' is the density
of electrodes 11 and 12. From the foregoing expression, it is
apparent that the frequency of the resonator can be changed by
changing the thickness of either the plate 10 or the electrode 11,
or by changing the density of the electrode 11. However, compared
to the crystalline quartz, the metal electrodes having high
internal mechanical friction which results in lowered mechanical 0.
There is an optimum thickness for the metal electrodes, which
optimum thickness is determined primarily by mechanical
characteristics such as the nature of the bonding to the quartz,
and also by internal electrical characteristics of the coating,
such as the internal resistance thereof. However, any thickness of
electrode 11 beyond this optimum thickness results in a decreased 0
of the overall structure with resultant damping of the resonance,
thus seriously limiting frequency variation obtainable by this
technique. Consequently, any appreciable variation frequency is
best obtained by changing the thickness of the quartz plate rather
than by changing the thickness of the electrode coating.
In FIG. 2 there is shown a perspective view of one form of the
invention wherein crystal plate 13 has five plateaus formed
thereon, and designated by reference characters 14, 15, 16, 17, and
18. The thickness of each of the five plateaus 14 through 18 is
shown in FIG. 3 and designated by characters t.sub.1 through
t.sub.5 respectively.
It can readily be seen from FIGS. 2 and 3 that each of the plateaus
14 through 18 consist of two major surfaces upon each of which has
been deposited an electrode of conductive material. For example, on
plateau 14, electrodes 19 and 20 have been deposited, with input
leads 21 and 22 extending respectively from the associated
electrode to the edge of the plateau. The leads 21 and 22 are
formed by depositing a conductive material on the quartz in a
well-known manner.
Each of the remaining four plateaus also have a pair of electrodes
and input terminals coated thereon. More specifically, electrodes
23 and 24 are coated upon plateau 15; electrodes 25 and 26 upon
plateau 16; electrodes 27 and 28 upon plateau 17; and electrodes 29
and 30 upon plateau 18.
Each of the five plateaus has a different thickness, as shown in
FIG. 1 and, consequently, each plateau in combination with the
electrodes coated thereon will resonate at a different
frequency.
Since frequency is inversely proportional to the thickness of the
quartz plate, then the thinnest plateau 18 will have the highest
frequency, and the thickest plateau 14 will have the lowest
frequency with intermediate frequencies being generated
therebetween by plateaus 17, 16, and 15.
It is to be noted that in FIG. 2 and FIG. 3 the relative
thicknesses of the plateaus are greatly exaggerated for purposes of
illustration. In actual practice, the difference in thickness
between adjacent plateaus will be more on the order of micrometers,
with the exact difference being determined by the frequency
difference desired. With a plateau having a major surface of
cross-sectional area equal to 20 sq. mm. a difference in thickness
of 0.0043 millimeters will result, for example, in a difference of
600 kHz. at a center frequency of 15 mHz.
Separating each of the plateaus is a beveled surface such as the
beveled surface 31 between plateaus 14 and 15, and beveled surface
32 between plateaus 15 and 16. These beveled surfaces define a
slice of the quartz crystal which separates adjacent plateaus and
isolates them, one from the other, both mechanically and
electrically. The mechanics by which such isolation occurs is well
known in the crystal art and will not be described in detail herein
other than to state generally that it is primarily a matter of
acoustic impedance differences. More specifically, the energy
generated at a given frequency plateau 14 for example, cannot pass
freely through beveled section 31 into the plateau 15 because the
acoustic wave will not propagate in the adjacent region. In
essence, the beveled area 31 acts as a bidirectional energy trap,
i.e., for both the energy generated in the resonator defined by
plateau 15, and the energy generated in the resonator defined by
plateau 14.
The multiplateau crystal structure shown in FIGS. 2 and 3 can be
manufactured in at least two ways, one of which includes the
etching of the plate in either hydrofluoric acid or an ammonium
bifluoride solution and the second of which involves a sand
blasting operation for different periods of time in order to create
plateaus of different thicknesses.
More specifically, in the etching technique the plateaus are
created by holding the plate at one end and inserting a given
length of it into the solution for a given amount of time to
produce a desired thickness change. For example, the crystal plate
of FIG. 2 is inserted into the etching solution so that portion of
the plate which includes plateaus 15, 16, 17, and 18 are all
immersed in the etching solution for a length of time necessary to
create the proper thickness for plateau 15. The crystal plate is
then pulled out of the etching solution a short distance so that
only that length of the plate including plateaus 16, 17, and 18 is
immersed in the liquid. Such immersion continues until the plateau
16 obtains its desired thickness.
In a similar manner plateaus 17 and 18 are formed. The beveled
surfaces between plateaus can then be perfected to the degree
desired by mechanical abrasion means.
In the sand blasting technique, the quartz plates are secured onto
a large glass plate by beeswax or other suitable means. The glass
plate is then secured onto the periphery of a large wheel which
carries the quartz crystal carrying glass plate under the sand
blast. At each pass of the wheel, the quartz surface is abraded a
small but measurable amount. The region which is not to be abraded
is masked off with either a metal shield or a silicone rubber mask
which deflects the abrasive. It has been found that the amount of
material removed from the quart crystal with each pass under the
abrasive means can be quite accurately predetermined so that the
total amount of material removed from the crystal plate is
measurable in terms of number of wheel revolutions.
Referring now to FIG. 4, there is shown a perspective view of a
monolithic quartz crystal having ten plateaus of different
thicknesses which are identified by reference characters 40 through
49. Each of these plateaus is separated from the adjacent plateaus
by beveled surfaces such as beveled surface 50. That portion of the
crystal defined by a given plateau, resonates, in cooperation with
the electrodes thereon, at a frequency in accordance with the
thickness of that portion of the crystal.
THE crystal plate configuration shown in FIG. 4 can be formed in a
manner similar to that described in connection with FIG. 2 with the
following additional steps. The crystal plate of FIG. 4, at some
stage in its operation, must be dipped into the etching solution
sideways such that the plateaus 45 through 49 define crystal
thicknesses less than that defined by the corresponding plateaus 40
through 44.
In FIG. 5, there is shown a side view of the structure of FIG. 4
illustrating the different thicknesses defined by the c various
plateaus. It is to be understood that each portion of the crystal
defined by a plateau must have electrodes of conductive material
formed thereon, as in the case of the structure of FIG. 2, in order
to complete the resonator.
One such pair of electrodes is identified by reference characters
51 and 52 of FIG. 4 and 51' and 52' in FIG. 5, and reference
characters 51" and 52" of FIG. 6.
* * * * *