U.S. patent application number 09/844832 was filed with the patent office on 2002-05-02 for voltage tuned dielectric varactors with bottom electrodes.
Invention is credited to Sengupta, Louise C., Zhu, Yongfei.
Application Number | 20020051334 09/844832 |
Document ID | / |
Family ID | 22745478 |
Filed Date | 2002-05-02 |
United States Patent
Application |
20020051334 |
Kind Code |
A1 |
Zhu, Yongfei ; et
al. |
May 2, 2002 |
VOLTAGE TUNED DIELECTRIC VARACTORS WITH BOTTOM ELECTRODES
Abstract
A voltage tunable dielectric varactor includes a substrate
having a first dielectric constant and having generally a planar
surface, first and second electrodes positioned on the generally
planar surface of the substrate, the first and second electrodes
being separated to form a first gap therebetween; a tunable
dielectric layer positioned on the first and second electrodes and
in the first gap, the tunable dielectric layer having a second
dielectric constant greater than the first dielectric constant; and
third and fourth electrodes positioned on a surface of the tunable
dielectric layer opposite the first and second electrodes, the
third and fourth electrodes being separated to form a second gap
therebetween.
Inventors: |
Zhu, Yongfei; (Columbia,
MD) ; Sengupta, Louise C.; (Ellicott City,
MD) |
Correspondence
Address: |
PIETRAGALLO, BOSICK & GORDON
ONE OXFORD CENTRE, 38TH FLOOR
301 GRANT STREET
PITTSBURGH
PA
15219-6404
US
|
Family ID: |
22745478 |
Appl. No.: |
09/844832 |
Filed: |
April 27, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60201349 |
May 2, 2000 |
|
|
|
Current U.S.
Class: |
361/277 ;
361/311 |
Current CPC
Class: |
H01G 7/06 20130101; H01P
1/181 20130101 |
Class at
Publication: |
361/277 ;
361/311 |
International
Class: |
H01G 005/00; H01G
004/06 |
Claims
What is claimed is:
1. A voltage tunable dielectric varactor comprising: a substrate
having a first dielectric constant and having a generally planar
surface; first and second electrodes positioned on the generally
planar surface of the substrate said first and second electrodes
being separated to form a first gap therebetween; a tunable
dielectric layer positioned on the first and second electrodes and
in said first gap, the tunable dielectric layer having a second
dielectric constant greater than said first dielectric constant;
and third and fourth electrodes positioned on a surface of the
tunable dielectric layer opposite the first and second electrodes,
said third and fourth electrodes being separated to form a second
gap therebetween.
2. A voltage tunable dielectric varactor as recited in claim 1,
wherein: said first gap is narrower than said second gap.
3. A voltage tunable dielectric varactor as recited in claim 1,
wherein the tunable dielectric layer has a dielectric constant
ranging from 2 to 1000, tuning of greater than 5%, and a loss
tangent of less than 0.02.
4. A voltage tunable dielectric varactor as recited in claim 1,
wherein the substrate has a permittivity of less than about 30.
5. A voltage tunable dielectric varactor as recited in claim 1,
wherein first, second, third and fourth electrodes each comprise
one of the group of: gold, silver, copper, platinum,
platinum-rhodium, and ruthenium oxide.
6. A voltage tunable dielectric varactor as recited in claim 1,
wherein the substrate comprises one of the group of: MgO, Alumina,
LaAlO.sub.3, sapphire, quartz, silicon, and gallium arsenide.
7. A voltage tunable dielectric varactor as recited in claim 1,
wherein the tunable dielectric layer comprises one of: a tunable
ferroelectric thick film; a tunable ferroelectric bulk ceramic; and
a tunable ferroelectric thin film.
8. A voltage tunable dielectric varactor as recited in claim 1,
further including: an RF input and an RF output for passing an RF
signal through the tunable dielectric layer in a first direction,
and wherein the first and second gaps lie parallel to a second
direction substantially perpendicular to the first direction.
9. A tunable phase shifter according to claim 1, wherein the
tunable dielectric layer comprises a material selected from the
group of: barium strontium titanate, barium calcium titanate, lead
zirconium titanate, lead lanthanum zirconium titanate, lead
titanate, barium calcium zirconium titanate, sodium nitrate,
KNbO.sub.3, LiNbO.sub.3, LiTaO.sub.3, PbNb.sub.2O.sub.6,
PbTa.sub.2O.sub.6, KSr(NbO.sub.3), NaBa.sub.2(NbO.sub.3).sub.5,
KH.sub.2PO.sub.4, and combinations thereof.
10. A tunable phase shifter according to claim 1, wherein the
tunable dielectric layer comprises a barium strontium titanate
(BSTO) composite selected from the group of: BSTO-MgO,
BSTO-MgAl.sub.2O.sub.4, BSTO-CaTiO.sub.3, BSTO-MgTiO.sub.3,
BSTO-MgSrZrTiO.sub.6, and combinations thereof.
11. A tunable phase shifter according to claim 1, wherein the
tunable dielectric layer comprises a material selected from the
group of: Mg.sub.2SiO.sub.4, CaSiO.sub.3, BaSiO.sub.3, SrSiO.sub.3,
Na.sub.2SiO.sub.3, NaSiO.sub.3-5H.sub.2O, LiAlSiO.sub.4,
Li.sub.2SiO.sub.3, Li.sub.4SiO.sub.4, A1.sub.2Si.sub.2O.sub.7,
ZrSiO.sub.4, KAlSi.sub.3O.sub.8, NaAlSi.sub.3O.sub.8,
CaAl.sub.2Si.sub.2O.sub.8, CaMgSi.sub.2O.sub.6, BaTiSi.sub.3O.sub.9
and Zn.sub.2SiO.sub.4.
12. A tunable phase shifter according to claim 1, wherein the
tunable dielectric layer comprises: an electronically tunable phase
and at least two metal oxide phases.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of the filing date of
provisional application Serial No. 60/201,349, filed May 2,
2000.
FIELD OF THE INVENTION
[0002] This invention relates to tunable radio frequency devices,
and more particularly, to voltage-tuned dielectric varactors.
[0003] Varactors are voltage tunable capacitors in which the
capacitance can be changed by applying an electric field to the
device. This property has wide applications in electrically tuned
radio frequency circuits, such as tunable filters, phase shifters,
delay lines, voltage controlled oscillators, etc. The most commonly
used varactor is a semiconductor diode varactor, which generally
has a low quality factor, Q, especially at high frequencies, low
power handling capacity, low third intermodulation product (IP3),
and a limited capacitance range. Another type of voltage tunable
varactor uses ferroelectric materials.
[0004] Tunable ferroelectric materials are materials whose
permittivity (more commonly called dielectric constant) can be
varied by varying the strength of an electric field to which the
materials are subjected. Even though these materials work in their
paraelectric phase above the Curie temperature, they are
conveniently called "ferroelectric" because they exhibit
spontaneous polarization at temperatures below the Curie
temperature. Tunable ferroelectric materials including
barium-strontium titanate (BST) or BST composites have been the
subject of several patents.
[0005] Dielectric materials including barium strontium titanate are
disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled
"Ceramic Ferroelectric Material"; U.S. Pat. No. 5,427,988 to
Sengupta, et al. entitled "Ceramic Ferroelectric Composite
Material-BSTO-MgO"; U.S. Pat. No. 5,486,491 to Sengupta, et al.
entitled "Ceramic Ferroelectric Composite Material-BSTO-ZrO.sub.2";
U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled "Ceramic
Ferroelectric Composite Material-BSTO-Magnesium Based Compound";
U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled "Multilayered
Ferroelectric Composite Waveguides"; U.S. Pat. No. 5,846,893 to
Sengupta, et al. entitled "Thin Film Ferroelectric Composites and
Method of Making"; U.S. Pat No. 5,766,697 to Sengupta, et al.
entitled "Method of Making Thin Film Composites"; U.S. Pat. No.
5,693,429 to Sengupta, et al. entitled "Electronically Graded
Multilayer Ferroelectric Composites"; and U.S. Pat. No. 5,635,433
to Sengupta, entitled "Ceramic Ferroelectric Composite
Material-BSTO-ZnO". These patents are hereby incorporated by
reference. In addition, copending, commonly assigned U.S. patent
applications Ser. No. 09/594,837, filed Jun. 15, 2000, and Ser. No.
09/768,690, filed Jan. 24, 2001, disclose additional tunable
dielectric materials and are also incorporated by reference. The
materials shown in these patents, especially BSTO-MgO composites,
show low dielectric loss and high tunability. Tunability is defined
as the fractional change in the dielectric constant with applied
voltage.
[0006] One tunable dielectric varactor is shown in U.S. Pat. No.
5,640,042. That patent shows a planar ferroelectric varactor,
including a carrier substrate layer, a high temperature
superconducting metallic layer deposited on the substrate for
lattice matching, a thin film ferroelectric deposited on the
metallic layer, and metallic conductors for connecting the varactor
to radio frequency transmission lines. Other tunable dielectric
varactors are shown in PCT patent applications PCT/US99/24161 and
PCT/US99/26113, and U.S. patent application Ser. No. 09/660,309,
which is hereby incorporated by reference. In some varactor
applications, is it desirable to provide a tunable varactor that
has a relatively low capacitance.
[0007] It would be desirable to have a tunable dielectric varactor
that does not require a superconducting layer, and can operate at
room temperature, has low dielectric losses and can be constructed
to obtain relatively low capacitances (typically<2 pF).
SUMMARY OF THE INVENTION
[0008] Voltage tunable dielectric varactors constructed in
accordance with this invention include a substrate having a first
dielectric constant and having generally a planar surface, first
and second electrodes positioned on the generally planar surface of
the substrate, the first and second electrodes being separated to
form a first gap therebetween; a tunable dielectric layer
positioned on the first and second electrodes and in the first gap,
the tunable dielectric layer having a second dielectric constant
greater than the first dielectric constant; and third and fourth
electrodes positioned on a surface of the tunable dielectric layer
opposite the first and second electrodes, the third and fourth
electrodes being separated to form a second gap therebetween.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a top plan view of a tunable dielectric varactor
constructed in accordance with this invention;
[0010] FIG. 2 is a cross-sectional view of the varactor of FIG. 1,
taken along line 2-2;
[0011] FIG. 3 is a top plan view of another tunable dielectric
varactor constructed in accordance with this invention;
[0012] FIG. 4 is a cross-sectional view of the varactor of FIG. 3,
taken along line 4-4;
[0013] FIGS. 5 and 6 are schematic diagrams of an equivalent
circuit of the varactors of this invention; and
[0014] FIG. 7 is a graph showing the voltage on a capacitor in the
equivalent circuit of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] Referring to the drawings, FIG. 1 is a top plan view of a
tunable dielectric varactor constructed in accordance with a first
embodiment of this invention, and FIG. 2 is a cross-sectional view
of the varactor of FIG. 1, taken along line 2-2. The varactor 10 of
FIGS. 1 and 2 includes a carrier substrate layer 12, a first bottom
electrode 14 and a second bottom electrode 16. The bottom
electrodes are positioned on a generally planar surface 18 of the
substrate and separated to form a gap 20. A tunable dielectric film
22 covers the bottom electrodes and fills the gap. Top electrodes
24 and 26 are positioned on a top surface 28 of the tunable
dielectric layer. The top electrodes are separated by a second gap
30. Input connection 32 and output connection 34 are provided to
connect the varactor to an external circuit. A variable DC voltage
source 36 is connected to the top electrodes to provide an electric
field that is used to control the dielectric constant of the
tunable dielectric material.
[0016] FIG. 3 is a top plan view of a tunable dielectric varactor
constructed in accordance with another embodiment of this
invention, and FIG. 4 is a cross-sectional view of the varactor of
FIG. 3, taken along line 4-4. The varactor 40 of FIGS. 3 and 4
includes a carrier substrate layer 42, a first bottom electrode 44
and a second bottom electrode 46 positioned on a generally planar
surface 48 of the substrate. The bottom electrodes are separated to
form a gap 50. A tunable dielectric film 52 covers the bottom
electrodes and fills the gap. Top electrodes 54 and 56 are
positioned on a top surface 58 of the tunable dielectric layer. The
top electrodes are separated by a second gap 60. Input connection
62 and output connection 64 are provided to connect the varactor to
an external circuit. A variable DC voltage source 66 is connected
to the top electrodes to provide an electric field that is used to
control the dielectric constant of the tunable dielectric material.
The varactor of FIGS. 3 and 4 is similar to that of FIGS. 1 and 2,
except that each of the top electrodes include portions 68 and 70,
respectively, each extending along a side of the tunable dielectric
material.
[0017] A simple equivalent circuit of varactors constructed in
accordance with this invention is shown in FIG. 5. The equivalent
capacitance, C, of the varactor can be expressed as:
C=C1+(C2C/2)/(C2+C3/2)
[0018] Where C1 is the capacitance contributed by the top
electrodes and the top gap; C2 is the capacitance contributed by
the bottom electrodes and the bottom gap; and C3 is the capacitance
contributed by one top electrode and one bottom electrode on one
side of the device. If C3>>C2>>C1, then C.apprxeq.C2.
Typically, C<2 pF.
[0019] It can be seen that C2>C1 even in case of the top gap
being the same width as the bottom gap, because more tunable
material (with high dielectric constant) is involved in the bottom
electrode gap than in the top electrode gap. In order to achieve
C2>>C1, for example C2/Cl>20, the top gap should be much
bigger than the bottom gap. A typical width of the bottom gap is 1
.mu.m to 3 .mu.m, while the width of the top gap is greater than 60
.mu.m.
[0020] It is easy to make C3>>C2, since the thickness of the
tunable material is thinner than the width of the bottom gap, more
tunable material is involved in C3 than in C2, or in other words,
the electrode area in C3 is much bigger than that of C2. A typical
thickness of the tunable film is about 0.2 .mu.m to 1 .mu.m. The
ratio of C3/C2 is typically greater than 50.
[0021] FIG. 6 is a schematic circuit showing the DC bias voltage
distribution among the various capacitances. It can be seen that
more than 95% of the applied voltage drops across C2, when C3/C2 is
more than 40.
[0022] Tuning of the varactor can be expressed as:
(C.sup.0-C.sup.v)/C.sup.0=1-[(2+C3/C2.sup.0)/(2+C3/C2.sup.v)]
(C.sup.0-C.sup.v)/C.sup.0.apprxeq.1-(C2v/C2.sup.0)
(C.sup.0-C.sup.v)/C.sup.0=(C2.sup.0-C2.sup.v)/C2.sup.0
[0023] where C.sup.0, C.sup.v are the capacitance of C at 0 volts
and a bias voltage, v, respectively, and C2.sup.0, C2.sup.v are the
capacitance of C2 at 0 volts and a bias voltage, v,
respectively.
[0024] These equation show that the tuning of the varactor is
dominated by tuning of C2. Typical tuning of the varactor is about
20% to 70%.
[0025] The varactor is fabricated by the steps of: (a) deposition
of the bottom electrodes on a substrate; (b) deposition of the
tunable film to cover the bottom electrodes and the whole substrate
surface or a strip line; (c) deposition of the top electrodes.
Proper etching and/or mask processing is needed to achieve the
specific patterns of the bottom and top electrodes, and the pattern
of the tunable film in the case where a strip line of tunable film
is used.
[0026] The substrate may be MgO, alumina (AL2O3), LaAlO3, sapphire,
quartz, silicon, gallium arsenide, or other materials compatible
with tunable films and their processing. The bottom electrodes may
be platinum, platinum-rhodium, ruthenium oxide or other conducting
materials that are compatible with tunable films and their
processing. It is important to use a low loss and high tunability
film in the varactor. These tunable dielectric materials have
dielectric constants ranging from 2 to 1000, and tuning of greater
than 5%, with loss tangents of better that 0.02. The thin films or
thick films of these materials may be deposited on substrates by
technologies of metal-organic solution deposition (MOSD or simply
MOD), metal-organic chemical vapor deposition (MOCVD), pulse laser
deposition (PLD), sputtering, screen printing, and so on. The top
electrodes may be gold, silver, copper, platinum, ruthenium oxide,
or other conducting material compatible with tunable films.
[0027] In the preferred embodiments, the tunable dielectric
material can be deposited on the entire surface of the substrate,
or only in the center area on the bottom electrodes and in the
bottom gap, to reduce the dielectric loss of the varactor.
[0028] This invention provides tunable dielectric varactors that
operate at room temperature, and in which the capacitance is tuned
by varying the dielectric constant through the application of a
bias voltage.
[0029] The tunable dielectric material can comprise at least one
electronically tunable dielectric phase, such as barium strontium
titanate, in combination with other compounds. Barium strontium
titanate of the formula Ba.sub.xSr.sub.1-xTiO.sub.3 is a preferred
electronically tunable dielectric material due to its favorable
tuning characteristics, low curie temperatures and low microwave
loss properties. In the formula Ba.sub.xSr.sub.1-xTiO.sub.3, x can
be any value from 0 to 1, preferably from about 0.15 to about 0.6.
More preferably, x is from 0.3 to 0.6.
[0030] Other electronically tunable dielectric materials may be
used partially or entirely in place of barium strontium titanate.
An example is Ba.sub.xCa.sub.1-xTiO.sub.3, where x can vary from
about 0.2 to about 0.8, preferably from about 0.4 to about 0.6.
Additional electronically tunable ferroelectrics include
Pb.sub.xZr.sub.1-xTiO.sub.3 (PZT) where x ranges from about 0.05 to
about 0.4, lead lanthanum zirconium titanate (PLZT), lead titanate
(PbTiO.sub.3), barium calcium zirconium titanate (BaCaZrTiO.sub.3),
sodium nitrate (NaNO.sub.3), KNbO.sub.3, LiNbO.sub.3, LiTaO.sub.3,
PbNb.sub.2O.sub.6, PbTa.sub.2O.sub.6, KSr(NbO.sub.3) and
NaBa.sub.2(NbO.sub.3)5 KH.sub.2PO.sub.4.
[0031] The varactor can also include electronically tunable
materials having at least one metal silicate phase. The metal
silicates may include metals from Group 2A of the Periodic Table,
i.e., Be, Mg, Ca, Sr, Ba and Ra, preferably Mg, Ca, Sr and Ba.
Preferred metal silicates include Mg.sub.2SiO.sub.4, CaSiO.sub.3,
BaSiO.sub.3 and SrSiO.sub.3. In addition to Group 2A metals, the
present metal silicates may include metals from Group 1A, i.e., Li,
Na, K, Rb, Cs and Fr, preferably Li, Na and K. For example, such
metal silicates may include sodium silicates such as
Na.sub.2SiO.sub.3 and NaSiO.sub.3-5H.sub.2O, and lithium-containing
silicates such as LiAlSiO.sub.4, Li.sub.2SiO.sub.3 and
Li.sub.4SiO.sub.4. Metals from Groups 3A, 4A and some transition
metals of the Periodic Table may also be suitable constituents of
the metal silicate phase. Additional metal silicates may include
Al.sub.2Si.sub.2O.sub.7, ZrSiO.sub.4, KAlSi.sub.3O.sub.8,
NaAlSi.sub.3O.sub.8, CaAl.sub.2Si.sub.2O.sub.8,
CaMgSi.sub.2O.sub.6, BaTiSi.sub.3O.sub.9 and Zn.sub.2SiO.sub.4.
Tunable dielectric materials identified as Parascan.TM. materials,
are available from Paratek Microwave, Inc. The above tunable
materials can be tuned at room temperature by controlling an
electric field that is applied across the materials.
[0032] In addition to the electronically tunable dielectric phase,
the electronically tunable materials can include at least two
additional metal oxide phases. The additional metal oxides may
include metals from Group 2A of the Periodic Table, i.e., Mg, Ca,
Sr, Ba, Be and Ra, preferably Mg, Ca, Sr and Ba. The additional
metal oxides may also include metals from Group 1A, i.e., Li, Na,
K, Rb, Cs and Fr, preferably Li, Na and K. Metals from other Groups
of the Periodic Table may also be suitable constituents of the
metal oxide phases. For example, refractory metals such as Ti, V,
Cr, Mn, Zr, Nb, Mo, Hf, Ta and W may be used. Furthermore, metals
such as Al, Si, Sn, Pb and Bi may be used. In addition, the metal
oxide phases may comprise rare earth metals such as Sc, Y, La, Ce,
Pr, Nd and the like.
[0033] The additional metal oxides may include, for example,
zirconnates, silicates, titanates, aluminates, stannates, niobates,
tantalates and rare earth oxides. Preferred additional metal oxides
include Mg.sub.2SiO.sub.4, MgO, CaTiO.sub.3, MgZrSrTiO.sub.6,
MgTiO.sub.3, MgAl.sub.2O.sub.4, WO.sub.3, SnTiO.sub.4, ZrTiO.sub.4,
CaSiO.sub.3, CaSnO.sub.3, CaWO.sub.4, CaZrO.sub.3,
MgTa.sub.2O.sub.6, MgZrO.sub.3, MnO.sub.2, PbO, Bi.sub.2O.sub.3 and
La.sub.2O.sub.3. Particularly preferred additional metal oxides
include Mg.sub.2SiO.sub.4, MgO, CaTiO.sub.3, MgZrSrTiO.sub.6,
MgTiO.sub.3, MgAl.sub.2O.sub.4, MgTa.sub.2O.sub.6 and
MgZrO.sub.3.
[0034] The additional metal oxide phases are typically present in
total amounts of from about 1 to about 80 weight percent of the
material, preferably from about 3 to about 65 weight percent, and
more preferably from about 5 to about 60 weight percent. In a
particularly preferred embodiment, the additional metal oxides
comprise from about 10 to about 50 total weight percent of the
material. The individual amount of each additional metal oxide may
be adjusted to provide the desired properties. Where two additional
metal oxides are used, their weight ratios may vary, for example,
from about 1:100 to about 100:1, typically from about 1:10 to about
10:1 or from about 1:5 to about 5:1. Although metal oxides in total
amounts of from 1 to 80 weight percent are typically used, smaller
additive amounts of from 0.01 to 1 weight percent may be used for
some applications.
[0035] In one embodiment, the additional metal oxide phases may
include at least two Mg-containing compounds. In addition to the
multiple Mg-containing compounds, the material may optionally
include Mg-free compounds, for example, oxides of metals selected
from Si, Ca, Zr, Ti, Al and/or rare earths. In another embodiment,
the additional metal oxide phases may include a single
Mg-containing compound and at least one Mg-free compound, for
example, oxides of metals selected from Si, Ca, Zr, Ti, Al and/or
rare earths.
[0036] The tunability may be defined as the dielectric constant of
the material with an applied voltage divided by the dielectric
constant of the material with no applied voltage. Thus, the voltage
tunability percentage may be defined by the formula:
T=((X-Y)/X).multidot.100;
[0037] where X is the dielectric constant with no voltage and Y is
the dielectric constant with a specific applied voltage. High
tunability is desirable for many applications. Voltage tunable
dielectric materials preferably exhibit a tunability of at least
about 20 percent at 8V/micron, or more preferably at least about 25
percent at 8V/micron. For example, the voltage tunable dielectric
material may exhibit a tunability of from about 30 percent to about
75 percent or higher at 8V/micron.
[0038] The combination of tunable dielectric materials such as BSTO
with additional metal oxides allows the materials to have high
tunability, low insertion losses and tailorable dielectric
properties, such that they can be used in microwave frequency
applications. The materials demonstrate improved properties such as
increased tuning, reduced loss tangents, reasonable dielectric
constants for many microwave applications, stable voltage fatigue
properties, higher breakdown levels than previous state of the art
materials, and improved sintering characteristics. A particular
advantage of the above materials is that tuning is dramatically
increased compared with conventional low loss tunable dielectrics.
The tunability and stability achieved with the present materials
enables new RF applications not previously possible. A further
advantage is that the materials may be used at room temperature.
The electronically tunable materials may be provided in several
manufacturable forms such as bulk ceramics, thick film dielectrics
and thin film dielectrics.
[0039] This invention provides tunable dielectric varactors having
high Q, high tuning ability, high IP3 values, wide capacitance
ranges, high power handling capacity, and low cost. These varactors
are suitable for use in tunable device applications such as tunable
filters, phase shifters, voltage controlled oscillators, etc., in
the VHF, UHF, microwave and millimeter wave frequency ranges.
[0040] While the present invention has been described in terms of
what are at present believed to be its preferred embodiments, it
will be apparent to those skilled in the art that various changes
may be made to the preferred embodiments without departing from the
scope of the invention as defined by the following claims.
* * * * *