U.S. patent number 3,613,034 [Application Number 04/870,903] was granted by the patent office on 1971-10-12 for waveguide structure with pseudocavity region for constraining pump and idler energies.
This patent grant is currently assigned to North American Rockwell Corporation. Invention is credited to James N. Linnd, William E. Meyer.
United States Patent |
3,613,034 |
Linnd , et al. |
October 12, 1971 |
WAVEGUIDE STRUCTURE WITH PSEUDOCAVITY REGION FOR CONSTRAINING PUMP
AND IDLER ENERGIES
Abstract
A noel waveguide structure usable in a broadband microwave
parametric amplifier is described which comprises a rectangular
waveguide with a pair of substantially U-shaped channels extending
oppositely from the top and bottom of the waveguide to a depth of
1/4 guide wavelength at the lower idler frequency. The channels
define a pseudocavity region in the waveguide, within which region
the idler and pump energy is constrained and parametric interaction
occurs. The waveguide is of sufficient width to allow propagation
of a signal in the lowest order transverse electric mode and
contains a varactor diode as a nonlinear reactance to couple pump
energy at a high frequency to a signal at a much lower
frequency.
Inventors: |
Linnd; James N. (Costa Mesa,
CA), Meyer; William E. (Buena Park, CA) |
Assignee: |
North American Rockwell
Corporation (El Segundo, CA)
|
Family
ID: |
27087976 |
Appl.
No.: |
04/870,903 |
Filed: |
August 14, 1969 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
617231 |
Feb 20, 1967 |
3501706 |
Mar 17, 1970 |
|
|
Current U.S.
Class: |
333/208;
333/248 |
Current CPC
Class: |
H01P
7/06 (20130101); H03F 7/04 (20130101) |
Current International
Class: |
H01P
7/06 (20060101); H01P 7/00 (20060101); H03F
7/00 (20060101); H03F 7/04 (20060101); H01p
007/06 () |
Field of
Search: |
;333/73W,83,95,98 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Saalbach; Herman Karl
Assistant Examiner: Gensler; Paul L.
Parent Case Text
CROSS REFERENCES TO RELATED APPLICATIONS
This application is a division of a prior application by the same
applicants, Ser. No. 617,231 (now U.S. Pat. No. 3,501,706), filed
in Group 252 on Feb. 20, 1967.
Claims
We claim:
1. A rectangular waveguide having means for constraining energy at
a given frequency to within a region of said waveguide,
said means comprising at least one channel extending orthogonally
from a wider wall of said waveguide to a depth of essentially
one-quarter guide wavelength of said frequency,
said channel being substantially U-shaped and comprising an end
section and first and second side grooves, said channel forming a
pseudocavity capable of supporting a mode of electrical signals at
a frequency different from that of at least another mode
supportable in said waveguide.
2. The structure as defined in claim 1 wherein a second end section
is included at the open end of said grooves of said channel to form
a substantially rectangular-shaped channel which defines a
rectangular pseudocavity region.
3. In a waveguide having a lowest propagation frequency, means for
constraining energy having a second frequency higher than said
lowest propagation frequency to within a pseudocavity region of
said waveguide, said means comprising,
two channels extending oppositely one from the other from the top
and bottom of said waveguide to a depth of essentially one-quarter
guide wavelength of said second frequency,
said channels being substantially U-shaped and comprising an end
section and first and second side grooves wherein the thickness of
said end section and said side grooves measured in the plane of
attachment to said waveguide is no greater than one-half the depth
of said channels.
4. A structure as defined in claim 3 wherein the width of said
pseudocavity region is an integral number of one-half guide
wavelengths at said second frequency, and the length of said
pseudocavity region is an integral number of one-half guide
wavelengths plus one-quarter guide wavelengths at said second
frequency, said width being the distance between said side grooves
of said U-shaped channels and said length being the distance from
the open end of a side groove to the remote side of the end
section.
5. A structure as defined in claim 3 wherein one end of said
waveguide is short circuit terminated, wherein said channels are
substantially U-shaped, and wherein said terminated end of said
waveguide provides one boundary of said pseudocavity region.
6. A structure as defined in claim 5 further comprising means for
introducing energy into said pseudocavity region, said means
comprising a second waveguide extending from said terminated end of
said waveguide and having a cutoff frequency higher than said
lowest propagation frequency.
7. A structure as defined in claim 5 wherein the thickness of said
channels is approximately one-tenth guide wavelength at said second
frequency.
8. A structure as defined in claim 7 wherein said pseudocavity
region has a width of one guide wavelength at said second
frequency, and a length of one and one-quarter guide wavelength at
said second frequency.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a novel rectangular waveguide structure
having substantially U-shaped channels, extending from its top and
bottom, forming a pseudocavity region in which pump and idler
energies may be constrained. The structure facilitates parametric
amplification with extremely large percentage bandwidths of
microwave signals introduced into the waveguide.
2. Description of the Prior Art
The principle factor limiting the sensitivity of microwave signal
receivers is the noise inherent in the device used to amplify such
signals prior to their detection. While low noise amplification may
be achieved by the use of masers, these devices are restricted in
their operation to natural frequencies of the maser material and
usually must be operated in a cryogenic environment. A more
acceptable approach is to use a parametric amplifier in which
energy from an intense pump wave is coupled to the signal by a
component exhibiting nonlinear reactance. Parametric amplification
provides extremely low noise operation without the need for
refrigeration.
Typical prior art microwave parametric amplifiers utilize
structures having two adjacent waveguide cavities, one tuned to the
signal frequency and a second tuned to the pump frequency. A
back-biased diode having nonlinear capacitance, mounted in an
aperture in a common wall between the two cavities, reactively
couples energy between the pump and signal cavities.
Another typical parametric amplifier structure includes a
rectangular waveguide to contain the pump energy. A varactor diode,
mounted coaxially in a circular waveguide which intersects one wall
of the rectangular guide, serves as the requisite nonlinear
reactance. The signal may be introduced into the circular
waveguide, and energy may be extracted either at the signal
frequency via the circular waveguide or at the idler frequency via
the rectangular waveguide.
While the parametric amplifiers described hereinabove may be
tunable over a limited frequency range, they are essentially
narrowband devices. Prior art broadband parametric amplification
has required use of complex mechanical structures. Typical of such
devices are the "Coupled-Cavity Travelling-Wave Parametric
Amplifiers" described by K. P. Grabowski and R. D. Weglein in the
Proceedings of the IRE, Volume 48, No. 12, beginning at page 1973.
These devices utilize a series of inductively coupled microwave
cavities each containing individual signal, idler, and pump
resonant chambers and each containing a varactor diode. The
cavities are cascaded and pump energy is applied to the diodes in
an appropriate phase relationship to allow travelling wave
parametric operation. While broadband performance may be achieved,
the system is cumbersome mechanically, requires a plurality of
cavities, and demands considerable care to maintain the proper pump
phase at each varactor diode.
This application utilizes a novel waveguide structure having a
pseudocavity region which permits pump and idler energy to be
constrained to a small portion of a waveguide. This structure
permits efficient parametric interaction between a signal present
in the waveguide at a relatively low frequency and a pump having a
considerably higher frequency. The structure is mechanically
simple, and when used in a parametric amplifier requires only one
reactive element and permits amplification with large percentage
bandwidths.
SUMMARY OF THE INVENTION
The inventive waveguide structure has a pseudocavity region within
which the pump and idler energy may be constrained. The structure
comprises a rectangular waveguide having a width sufficiently large
to support signal energy in the lowest order transverse electric
mode. The rectangular waveguide includes substantially U-shaped
channels extending from its top and bottom, and terminates in a
second waveguide beyond cutoff. The channels, which preferably
exhibit a depth of one-quarter guide wavelength of the lowest idler
frequency, define the pseudocavity region. The length and width of
the region are selected to ensure that both pump and idler energies
are constrained and that a maximum electric field occurs at the
location of a varactor diode disposed within the region. This
varactor diode couples energy from the pump to the signal. The
amplified signal may be extracted from the rectangular waveguide
utilizing a microwave circulator. Nondegenerate broadband
parametric amplification is achieved using a pump frequency much
higher than that of the signal.
It is thus an object of this invention to provide a waveguide
structure having a pseudocavity region for constraining pump and
idler energy to a portion only of a waveguide.
It is a further object of this invention to provide a microwave
structure including a shorted waveguide containing substantially
U-shaped channels extending from the top and bottom of the
waveguide to form a pseudocavity region capable of containing
therein signals substantially above the cutoff frequency of the
waveguide.
Further objects and features of the invention will become apparent
from the following description and drawings which are utilized for
illustrative purposes only.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded elevation view of the inventive microwave
waveguide structure including channels defining a pseudocavity
region in the waveguide.
FIG. 2 is an elevation view showing the interior surface of the
waveguide structure illustrated in FIG. 1.
FIG. 3 is a diagram of a possible electric field distribution
within the channels of the pseudocavity as viewed in a plane
generally along the line 3--3 of FIG. 2.
FIG. 4 is a diagram of a possible electric field distribution
within a portion of the pseudocavity as viewed in a plane generally
along the line 4--4 of FIG. 1.
FIG. 5 is a graph showing the operational mode spectrum of the
inventive broadband parametric amplifier.
FIG. 6 is a block diagram of a broadband parametric amplifier
utilizing the inventive waveguide structure illustrated in FIGS. 1
and 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The novel waveguide structure which forms the subject matter of
this application is illustrated in FIG. 1. The waveguide structure
comprises four major sections 10a, 10b, 10c, and 10d, which may be
assembled into a unitary structure. In a preferred embodiment each
of the sections 10a, 10b, 10c, and 10d may be milled from a solid
block of a metal such as copper having high conductivity. Dowel
pins (not shown in the figures) may be used to ensure accurate
alignment of the assembled sections. Alternately, the structure may
be constructed as a single block for example, by casting. The
interior surface of the assembled waveguide structure is
illustrated in FIG. 2.
As illustrated by FIGS. 1 and 2, the waveguide structure comprises
a waveguide section 20 which extends through section 10a, 10b, and
10c, and which terminates at shorting plane or wall 24 of section
10d. A second waveguide 40 having width a' and height b' also
extends from wall 24 and terminates in port 45. The end of
waveguide section 20 opposite wall 24 is open and forms port 25.
Waveguide section 20 has a width a and height b, the latter
measured between top 21 and bottom 22 of waveguide section 20. As
is well known to those skilled in the art, the lowest frequency
signal which can be propagated in waveguide section 20 is one whose
free space wavelength is equal to 2a. Waveguide section 40, whose
width a' is considerably narrower than a, thus has a lowest
propagation frequency considerably higher than that of waveguide
section 20. Thus wall 24 and waveguide 40 appear as a short circuit
termination to a signal propagating in waveguide section 20 in the
lowest mode.
Extending respectively from top 21 and bottom 22 of waveguide
section 20 are substantially U-shaped channels 30a and 30b. These
channels, which may terminate against wall 24, define pseudocavity
region 35 of waveguide section 20. Channels 30a and 30b
respectively comprise end sections 31a and 31b, first side grooves
32a and 32b, and second side grooves 33a and 33b. End sections 31a
and 31b may be constructed by milling a rectangular opening 31 in
section 10b as illustrated in FIG. 1. Grooves 32(a and b) and 33(a
and b) may be constructed by milling appropriate slots through
section 10c. In a preferred embodiment, shown most clearly in FIG.
1, the thickness of channels 30a and 30b is t; that is, end
sections 31a and 31b, and side grooves 32 and 33, each have the
same thickness t. The length and width of pseudocavity region 35
are given by 1 and w respectively, as shown in FIG. 1. Channels 30a
and 30b each extend to a depth d above and below respective
waveguide top 21 and bottom 22. Criteria for the selection of the
various dimensions t, l, w and d are described in detail
hereinbelow in conjunction with the operational description of
pseudocavity region 35.
Varactor diode 12 is situated in pseudocavity region 35 and may be
supplied with a DC bias via a wire introduced through hole 16b in
block 10c (see FIG. 1). A second contact to varactor diode 12 is
provided by point contact 14 (not shown in FIG. 2) which extends
from bottom 22 of waveguide section 20. Point contact 14 may be
fashioned at the end of a metal rod and inserted into hole 16a in
section 10c.
Operation of the pseudo cavity region 35 defined by substantially
U-shaped channel members 30a and 30b best may be understood by
reference to FIGS. 3 and 4, which are viewed respectively along the
lines 3--3 of FIG. 2 and 4--4 of FIG. 1. In particular FIGS. 3 and
4 show typical electric field patterns which may be induced in
channels 30a and 30b when these channels are of the preferred
dimensions.
In a preferred embodiment, the depth d of channels 30a and 30b
essentially is equal to one-quarter guide wavelength of the energy
desired to be constrained within pseudocavity region 35. For energy
introduced in the appropriate mode, this ensures that the electric
field (represented by arrows 15 in FIGS. 3 and 4) is a maximum in
the planes of top and bottom surfaces 21 and 22 of waveguide
section 20. Thus, a wave introduced into pseudocavity region 35
(e.g., via waveguide section 40) will see a very high impedance at
the periphery of pseudocavity region 35 defined by channels 30.
Note that the energy will be constrained to pseudocavity region 35
and will not extend out into waveguide section 20 beyond the region
defined by substantially U-shaped channels 30a and 30b. In a
preferred embodiment width w of pseudocavity region 35 essentially
is equal to an integral number of one-half guide wavelengths of the
signal being constrained. Similarly the preferred length l of
pseudocavity region 35 is equal to an integral number of one-half
guide wavelengths plus one-quarter guide wavelength. These
preferred values are illustrated in FIGS. 3 and 4, which show the
electric field distribution for energy introduced into pseudocavity
region 35 in the TE.sub.303 mode.
Using the configuration of FIGS. 3 and 4, varactor diode 12 may be
placed three-quarters of a guide wavelength from wall 24 and midway
between grooves 32a and 33a. As illustrated, this will ensure an
electric field maxima at the location of varactor diode 12 for
energy introduced into pseudocavity region in the TE.sub.303
mode.
In a preferred embodiment, the groove thickness t of channels 30a
and 30b should be less than one-half the depth d of grooves 32 and
33. An optimum value for thickness t is in the order of one-tenth
guide wavelength. A greater thickness t may result in the
excitation of spurious modes within channels 30a and 30b, resulting
in a significant decrease in the ability to constrain energy to
within pseudocavity region 35.
Since the thickness t is considerably less than the width a of
waveguide section 20, a relatively low-frequency signal introduced
into waveguide section 20 will see only a very small inductive
perturbation in its field due to the existence of channels 30a and
30b. Thus a signal of a first relatively low frequency (introduced
via port 25) may be present throughout waveguide section 20 at the
same time that a wave at a second much higher frequency (introduced
by way of port 45 and waveguide 40) may be present only in the
pseudocavity region 35 of waveguide section 20.
Signals having a one-quarter guide wavelength (.lambda.g/4)
slightly greater or less than d also will be constrained somewhat
by channels 30a and 30b to pseudocavity region 35. However, the
degree of containment will be slightly reduced from the optimum
value obtained when the channels depth d is exactly .lambda.g/4.
Thus it is possible to use pseudocavity region 35 to constrain
energy at several closely related frequencies.
In another embodiment of the inventive waveguide structure (not
illustrated), a second pair of channels may extend from the top and
bottom of waveguide 20 to form a second pseudocavity region
surrounding the first pseudocavity region. This second region then
may function to constrain any residual energy not completely
contained by the first, smaller pseudocavity. In yet another
embodiment, two adjacent pseudocavity regions may be formed in the
same waveguide. If appropriate care is taken to prevent excitation
of spurious modes, the side grooves of one pseudocavity
simultaneously may be used as the side grooves of the adjacent
channel, even though the two adjacent regions are of different
size.
Further, it will be understood that various other modifications may
be made to waveguide structure 20 within the spirit and scope of
this invention. For example, although FIGS. 1 and 2 illustrate
channels 30a and 30b as terminating against wall 24, this is not
required. Rather channel sections 30a and 30b may be separated some
distance from wall 24 and include yet another groove to form
substantially rectangular shaped channels. These channels then will
define a rectangular pseudocavity region within the main waveguide
20, into which region energy may be introduced, e.g., by way of a
coaxial waveguide introduced through top 21 and bottom 22.
Alternately, it may be possible to form pseudocavity region 35
using only one channel, extending either from top 21 or bottom 22.
Such a configuration would provide somewhat less energy containment
than the opposing channel embodiments illustrated, but may be
advantageous in certain applications where it is impractical to
prepare grooves in one waveguide wall.
The characteristics described hereinabove indicate that the
waveguide structure illustrated by FIGS. 1 and 2 is well suited for
application in a microwave parametric amplifier capable of
broadband operation in the nondegenerate operational mode.
The theory of parametric amplification has been described widely in
the literature, as for example, in the textbook entitled "Coupled
Mode and Parametric Electronics" by William H. Louisell, published
in 1960 by John Wiley and Sons, New York. Basically, parametric
amplification involves the mixing of a signal at frequency
.omega..sub.s =2.pi.f.sub.s with an intense energy source called a
pump having a frequency .omega..sub.p. (In the following
discussion, the term "frequency" will be used to denote the
circular frequency .omega.=2.pi.f,where f is the frequency of
interest). When combined in the presence of a nonlinear reactance
(e.g., a varactor diode) energy is coupled between the pump and the
signal. The interaction also gives rise to energy at two additional
frequencies .omega..sub.i1 =.omega..sub.p -.omega..sub.s and
.omega..sub.i2 =.omega..sub.p +.omega..sub.s ; these are called
idlers. Nondegenerate parametric operation occurs when
.omega..sub.p is not equal to 2.omega..sub.s.
Fig. 5 is a graph illustrating a parametric operational mode
spectrum; the wideband parametric amplifier may be operated in the
corresponding mode. As indicated in FIG. 5, solid vertical line 51
represents the power present at signal frequency .omega..sub.s, the
center frequency of the amplifier passband. Similarly, line 52
represents the power present in the pump wave at frequency
.omega..sub.p. Lines 53 and 54 respectively represent the power
which will be present at the idler frequencies .omega..sub.i1 and
.omega..sub.i2 when parametric interaction occurs.
For optimum wideband parametric amplification with nearly constant
gain across the band of interest, it is desirable to use an input
band-pass filter to define the bandwidth of the signal to be
amplified. Further, it is desirable to have extreme separation
between frequencies .omega..sub.s and .omega..sub.p . The
theoretical considerations on which these factors are based are
described in the article by George Matthaei, entitled "A Study of
the Optimum Design of Wideband Parametric Amplifiers and
Up-converters," published in the IRE Transactions on Microwave
Theory and Techniques, Volume MIT-9, No. 1, Jan. 1961, pages 23-
38.
As shown superimposed on the operational mode spectrum of FIG. 5,
dashed curve 56 illustrates the preferred attenuation
characteristics of an input band-pass filter which may be used as a
component of the wideband parametric amplifier. Note that the
passband region is centered around frequency .omega..sub.s and that
the width of the passband defines the bandwidth of the amplifier.
Note also in FIG. 5 that frequency .omega..sub.p is widely
separated from .omega..sub.s. For example, if .omega..sub.s is
selected to be 9 GHz. and the passband of attenuation curve 56 is
selected to extend from 8 GHz. to 10 GHz., then .omega..sub.p may
be selected to be a frequency of 94 GHz. In this example, the idler
frequencies will be .omega..sub.i1 =85 GHz. .+-.1 GHz. and
.omega..sub.i2 =103 GHz. .+-.1 GHz. (Of course, it is to be
understood that these frequencies are cited by way of example only,
and that the parametric amplifier described herein may be operated
at other frequencies as well.)
A block diagram of the broadband parametric amplifier 60 utilizing
the inventive waveguide structure is shown in FIG. 6; parametric
interaction occurs in pseudocavity region 35, in which region
varactor diode 12 is located. The characteristics of the waveguide
structure have been described hereinabove in conjunction with FIGS.
1 and 2. In a preferred embodiment, the depth d of channels 30a and
30b is selected to equal one-quarter guide wavelength at the lower
idler frequency .omega..sub.i1 . Since the pump and upper idler
frequencies (.omega..sub.p and .omega..sub.i2 respectively) are not
far removed from lower idler frequency .omega..sub.il, energy at
these frequencies also will be constrained to within pseudocavity
region 35, but to a slightly lesser degree. This characteristic is
illustrated by curve 57 in FIG. 5 which represents the "passband"
of pseudocavity region 35. Note that at frequencies .omega..sub.p
and .omega..sub.i2, pseudocavity region 35 will present a slightly
reactive load as viewed from waveguide 40.
The signal to be amplified is introduced into parametric amplifier
60 by way of signal input port 61 in microwave circulator 60.
Microwave circulator 62 is of a type well known to those skilled in
the art, and functions to allow a signal introduced through input
port 61 to exit via common port 63 while insuring that a signal
which enters via port 63 will leave circulator 62 via output port
64.
The signal to be amplified enters signal band-pass filter 65 from
circulator common port 63. Signal band-pass filter 65, well known
to those skilled in the art, may be of the type described, e.g., in
chapter 9 of the book entitled "Microwave Filters, Impedance
Matching Networks, and Coupling Structures" by George Matthaei, et
al., McGraw-Hill Book Company, 1964. In a preferred embodiment,
signal band-pass filter 65 will exhibit the attenuation
characteristics described generally by curve 56 in FIG. 5. That is,
filter 65 will have minimum attenuation within the desired
passband, and high attenuation at all other frequencies.
The signal from band-pass filter 65 next passes through signal
impedance matching network 66, which to match the input signal to
the small inductive perturbation which it will see in waveguide 20
as a result of the presence of pseudocavity region 35. Impedance
matching network 66 also serves to introduce the signal into
waveguide 20 in a particular mode, e.g., in the TE.sub.10 mode, the
lowest mode which can be supported by waveguide structure 20. The
design of impedance matching network 66 is well known to those
skilled in the art, and is described for example in Chapter 6 of
the book "Microwave Filters, Impedance-Matching Networks, and
Coupling Structures," referenced above. In a preferred embodiment,
waveguide structure 20 will have a width a (see FIG. 1) which is
one-half free space wavelength at the lowest frequency of the
signal passband. This will allow the signal to be introduced into
waveguide structure 20 in the TE.sub.10 mode.
Pump energy may be supplied from pump source 70 which may comprise
a reflex klystron oscillator and a microwave isolator, both
operating at the pump frequency .omega..sub.p . These components
are well known to those skilled in the art. Energy from pump source
70 passes through pump impedance matching network 72, upper idler
termination 74, and waveguide 40 into pseudocavity region 35. The
function of impedance matching network 72 is to match the pump wave
to the reactance exhibited by pseudocavity region 35 at the pump
frequency. Network 72 performs the further function of ensuring
that the pump energy is introduced into pseudocavity region 35 in
an appropriate mode e.g., the TE.sub.303 mode illustrated in FIGS.
3 and 4) to ensure that the pump electric field will be a maxima at
the location of varactor diode 12.
In a preferred embodiment, the width a' of waveguide 40 is selected
to provide a cutoff frequency between lower idler frequency
.omega..sub.i1 and pump frequency .omega..sub.p. Further, waveguide
40 should be sufficiently long so that pump impedance matching
network 72 does not interact with energy at lower idler frequency
.omega..sub.i1 . In a typical embodiment, a waveguide 40 length
which will ensure 20 db. attenuation at idler frequency
.omega..sub.i1 is sufficient for satisfactory operation. These
considerations will allow pump energy from pump source 70 to be
introduced into pseudocavity region 35 via waveguide 40, but will
prevent energy at the lower idler frequency .omega..sub.i1 from
exiting pseudocavity region 35 via waveguide 40. In effect, this
will completely constrain energy at the lower idler frequency
.omega..sub.i1 to within pseudocavity region 35. Upper idler
termination 74 is a reactive trap which functions to reflect energy
at the upper idler frequency .omega..sub.i2 (which energy will
propagate through waveguide 40) back into pseudocavity region
35.
With the parametric amplifier illustrated in FIG. 6, when a signal
and a pump are introduced into waveguide structure 10, parametric
amplification will occur and the amplified signal may be extracted
via impedance matching networks 66, signal band-pass filter 65,
circulator common port 63, circulator 62, and signal output port
64. The bandwidth of the amplifier will be defined by the passband
of signal band-pass filter 65 (see curve 56 of FIG. 5).
It will be apparent that the inventive waveguide structure
described in conjunction with FIGS. 1 and 2 is not limited to use
in a broadband parametric amplifier, and may be employed in various
other microwave devices. Moreover, while the invention has been
described and illustrated in detail, it is to be clearly understood
that the same is by way of illustration and example only, and is
not to be taken by way of limitation, the spirit and scope of the
invention being limited only by the terms of the appended
claims.
* * * * *