U.S. patent number 3,737,816 [Application Number 05/169,417] was granted by the patent office on 1973-06-05 for rectangular cavity resonator and microwave filters built from such resonators.
This patent grant is currently assigned to International Standard Electric Corporation. Invention is credited to Helmut Honicke.
United States Patent |
3,737,816 |
Honicke |
June 5, 1973 |
RECTANGULAR CAVITY RESONATOR AND MICROWAVE FILTERS BUILT FROM SUCH
RESONATORS
Abstract
A tunable capacity-loaded rectangular cavity resonator having an
inner conductor and a tuning plunger capacitively coupled to and
disposed coaxially of the inner conductor is disclosed. The
resonant frequency and impedance of the resonator are essentially
determined by the inner conductor dimensions. A plurality of these
resonators forming a microwave filter are coupled together by means
of inductive diaphragms. The slope of the coupling admittance in
the tuning range can be chosen by height and width variations of
the aperture in the diaphragm with respect to the length of the
tuning plunger. Coupling apertures can be provided in all four
sides.
Inventors: |
Honicke; Helmut (7531
Dietlingen, DT) |
Assignee: |
International Standard Electric
Corporation (New York, NY)
|
Family
ID: |
5782473 |
Appl.
No.: |
05/169,417 |
Filed: |
August 5, 1971 |
Foreign Application Priority Data
|
|
|
|
|
Sep 15, 1970 [DT] |
|
|
P 20 45 560.1 |
|
Current U.S.
Class: |
333/212;
333/209 |
Current CPC
Class: |
H01P
1/219 (20130101); H01P 7/06 (20130101) |
Current International
Class: |
H01P
7/00 (20060101); H01P 7/06 (20060101); H01P
1/219 (20060101); H01P 1/20 (20060101); H01p
001/20 (); H01p 007/06 () |
Field of
Search: |
;333/73W,82B,83R,82R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gensler; Paul L.
Claims
I claim:
1. A rectangular cavity resonator comprising:
a rectangular cavity having its width, length and height freely
chosen within limits so that the resonant frequency of said
rectangular cavity is above a desired operating frequency;
inductive coupling diaphragms disposed in selected walls of said
rectangular cavity and having a given configuration to provide a
desired coupling factor for coupling energy into and out of said
rectangular cavity;
a capacitive inner conductor disposed to extend through a first
wall of said rectangular cavity into said rectangular cavity toward
a second wall thereof opposite said first wall;
said inner conductor including
a first portion directly connected to said first wall having a
first given diameter;
a second portion directly connected to the end of said first
portion adjacent said second wall having a second given diameter
greater than said first given diameter, and
a coaxial bore concentric with the longitudinal axis of said inner
conductor having a third given diameter less than said first given
diameter; and
a cylindrical tuning plunger disposed within said coaxial bore
concentric with said longitudinal axis of said inner conductor
having fourth given diameter less than said third given
diameter;
said tuning plunger including being composed of a low-loss
insulating material and capable of adjustment relative to said
second wall by a screw thread, a given length of said tuning
plunger adjacent said second wall being covered with a conductive
material to provide a capacitive coupling between said second wall
and said second portion;
said first, second and third given diameters of said inner
conductor, said fourth diameter of said tuning plunger and the
length of said tuning plunger extending from said second portion
being selected to cooperate in providing said desired operating
resonant frequency for said rectangular cavity.
2. A resonator according to claim 1, wherein
said conductive material is silver.
3. A resonator according to claim 1, wherein
said tuning plunger travels a given distance to tune said
rectangular cavity through the tuning range thereof, and
the height of said coupling aperture is approximately equal to said
given distance.
4. A resonator according to claim 1, wherein
a plurality of said resonators are disposed in an arrangement to
provide a microwave filter having a meander-shaped path for energy
traveling therethrough.
5. A resonator according to claim 1, wherein
a plurality of said resonators are disposed in an arrangement to
provide a microwave filter having U-shaped path for energy
traveling therethrough.
6. A resonator according to claim 5, wherein
said plurality of said resonators disposed in said arrangement
include additional ones of said inductive diagrams to provide
different length U-shaped paths for said energy to provide a
microwave filter having a constant envelope delay and an optimally
flat attenuation characteristic.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a rectangular cavity resonator and
to microwave filters formed from such resonators in a ladder
network.
The German Patent 1,120,530 - 21a4-73 describes a method of
manufacturing rectangular cavity resonators from flat sheet metal
parts, of the sheets being provided with tongues fitting into
corresponding recesses of the other sheets, so that the rectangular
waveguide can be formed by plugging together the sheet metal parts,
fixing the tongues and then be finished by brazing.
From articles entitled "Maximally-Flat Filters In Waveguide", W. W.
Mumford, Bell System Technical Journal, Vol. 27 (1948), p.p. 648 to
714, and "Direct-Coupled Resonator Filters", S.B. Cohn, Proc. IRE,
February 1957, p.p. 187 to 196, as well as "Direct-coupled Cavity
Filters For Wide and Narrow Bandwidth" L. Young, IEEE Transactions
on Microwave Theory and Techniques, May 1963, p.p. 162 to 178, it
is known that microwave filters for the TE.sub.10 mode may be
composed of rectangular resonators with inductive diaphragms. In
this case, the resonant frequencies of the resonators must always
lie above a definite fundamental frequency which, for the TE.sub.10
mode, depends on the waveguide width. It is also necessary to
maintain the lengths of the resonators within less than 1 percent
otherwise the resonant frequencies of the resonators coupled
directly via inductive diaphragms would differ widely from each
other.
It is also known that the cut-off wavelength of a waveguide section
can be increased by means of rectangular projections (so-called
"ridged waveguide"). The manufacture of such known waveguides is
too expensive, so that their use for the manufacture of resonators
for filters is out of the question.
From G. Craven's articles "Waveguide Band Pass Filters Using
Evanescent Modes", Electronic Letters, Vol. 2, No. 7 July 1966),
and "Tuning Techniques For Multisection Waveguide Bandpass Filters
Using Evanescent Modes", Electronic Letters, Vol. 2, No. 11
(November 1966), microwave filters have become known which are
operated considerably above the cut-off wavelength of the
waveguide. These filters are not composed of individual coupled
resonators but consist of a single waveguide in which capacitive
screws are arranged at a certain spaced relation. These spacings
must be maintained with high accuracy.
SUMMARY OF THE INVENTION
In accordance with an object of the present invention the
resonators and the microwave filters composed thereof use none of
the above-mentioned conventional methods other than the method of
manufacture disclosed in the above-cited German Patent. For their
realization, a different approach is adopted.
Another object of the present invention is to employ the
manufacturing method of the above-cited German Patent to provide
resonators which, independent of the respective resonant frequency
required for a filter, have identical outer dimensions, and which,
in order to form a ladder network must not have a straight line
path for energy propagating through the filter. Rather the
resonators must permit achieving a U-shaped or meander-shaped path
for energy propagating through the filter by arranging the
resonators side by side and in series even if the direction of
energy propagation is reversed.
Therefore, in order to be able to provide such a filter
arrangement, a rectangular resonator is required which not only
permits an arbitrary joining together of a plurality of resonators
thanks to identical outer dimensions but in which, in addition, the
input and the output inductive diaphragms can be arranged in each
of the four side walls.
In order to attain the above-mentioned objects, a rectangular
cavity resonator which is composed of individual sheet metal parts
which, mechanically fixed against each other, are interconnected by
brazing, is used for building up microwave filters in which the
coupling of the individual resonators is effected by means of
inductive diaphragms. The invention is characterized in that width,
length and height of the individual resonators can be freely chosen
within limits such that the resonant frequency of the resonators,
dependent thereon, still lies above the operating frequency and
that a capacitive inner conductor is provided in each resonator,
that the inner conductor is designed so that the inner conductor
solely determines any desired resonant frequency of the resonator,
and that the coupling factor can be chosen by suitably designing
the inductive diaphragms.
A feature of the present invention is the provision of a
rectangular cavity resonator comprising a rectangular cavity having
its width, length and height freely chosen within limits so that
the resonant frequency of the rectangular cavity is above a desired
operating frequency; inductive coupling diaphragms disposed in
selected side walls of the rectangular cavity and having a given
design to provide a desired coupling factor for coupling energy
into and out of the rectangular cavity; a capacitive inner
conductor disposed in one wall of the rectangular cavity and
extending into the rectangular cavity toward a wall opposite the
one wall, the inner conductor being designed so that only the inner
conductor can provide the desired operating frequency for the
rectangular cavity.
Further features of the invention include the design of the inner
conductor as well as the construction of filters employing the
resonators of the invention.
BRIEF DESCRIPTION OF THE DRAWING
Above-mentioned and other features and objects of this invention
will become more apparent by reference to the following description
taken in conjunction with the accompanying drawing in which:
FIG. 1 illustrates the rectangular cavity resonator in accordance
with the principles of the present invention;
FIG. 2 illustrates the resonator of FIG. 1 with another embodiment
of the inner conductor thereof;
FIG. 3 is a diagram showing the tuning possibilities for a
resonator for 2.1 GHz and 4 GHz;
FIG. 4 is a diagram showing the deviations from a linear dependence
between the frequency and the depth of insertion of the tuning
plunger;
FIGS. 5a and 5b illustrate the influence of the inductive diaphragm
on the resonator of the present invention;
FIG. 6 illustrates influence of FIG. 5 in a diagram;
FIG. 7 shows a few of the possibilities for the position of the
coupling diaphragms;
FIG. 8 shows a microwave filter with constant envelope delay for
impedance transformation composed of the resonators according to
the invention;
FIG. 9 shows a microwave filter built up by means of the resonators
of this invention having a U-shaped path for the energy traversing
the filter;
FIG. 10 shows a (meander-shaped) microwave filter built up by means
of the resonators of this invention having a meander-shaped path
for the energy transversing the filter;
FIG. 11 is a cross-section through such filters; and
FIG. 12 is a top view of the filter of FIG. 11.
DESCRIPTION OF The PREFERRED EMBODIMENTS
FIG. 1 shows a rectangular cavity resonator 4 capacitively loaded
by an inner conductor 3. The wavelength of this resonator may be
greater than the cut-off wavelength .lambda..sub.c = 2a. The
lengths c of such resonators formed into a filter by means of
inductive diaphragm 2 may be chosen largely independent of the
resonant frequencies and the loaded Q's of the individual
resonators under other aspects such as necessary resonator quality,
space saving, etc. To realize the resonance condition, it is
sufficient to properly dimension the inner conductor 3.
If the desired resonant wavelength of the resonator 4 is greater
than the cut -off wavelength, i.e. if a TE.sub.10 mode is
non-existent, a capacitively loaded resonator may be regarded and
calculated as a coaxial resonator. The resonance conditions for
such coaxial resonators can be determined from the equation
.omega.CZ = ctg 2.pi.(l/.lambda.), (1)
where
.omega. = resonant frequency of the resonator,
.lambda. = resonant wavelength,
l = length of the inner conductor 3,
C = capacitance of the inner conductor 3 with respect to the bottom
plate of the resonator, and
Z = characteristic impedance of the resonator.
But otherwise, too, the same conditions can be expected as far as
quality is concerned. For reasons of design, it is frequently
necessary to make the resonator lengths equal; for space-saving
reasons, they should be as short as possible. Then, according to
equation (1), in order the realize the desired resonant
frequencies, the length of the inner conductor and, consequently,
the capacitance C would have to be correspondingly increased,
whereby the distance to the opposite bottom plate 10 would be
reduced.
In the case of highly capacitively loaded resonators, however, the
result of this is that little play is left for tuning the resonator
4 by means of a capacitively coupled plunger 5. This renders the
tuning difficult, and the manufacturing tolerances must be kept
small. If a linear tuning characteristic of a single resonator by
means of the plunger 5 and identical tuning characteristics of a
plurality of such resonators are required, as is necessary for
continuously tunable filters which are tuned by means of a
micrometer drive or a toothed-gear drive according to a counting
dial, this can be achieved only -- leaving out any expensive
compensating measures such as those described in the German Patent
1,266,412 -- if the inner conductor 3 is sufficiently spaced from
the opposite bottom plate.
To insure that the tuning plunger 5 has a sufficiently great
mechanical range of variation and also in the case of a small
waveguide height b, the inner conductor 3 is provided, at its lower
end with a collar 7, as shown in FIG. 2, which forms a relatively
great capacitance with respect to the opposite bottom plate 10
while a sufficiently high characteristic impedance is obtained by
means of a reduction 8 of the shank of the inner conductor 3. The
diaphragms 2 have coupling apertures 9 whose mode of action will be
described later.
In order to meet the requirements placed on the tunableness of the
resonator 4 and, consequently, of a microwave filter formed of such
resonators A . . . F such as shown in FIG. 8, within a desired
frequency range, the relation between the diameter D3 of the bore 6
of the inner conductor 3 and the diameter D4 of the tuning plunger
5 must be properly chosen. As can be seen from the curves of FIG. 3
designated A and B, the diameter ratio D3/D4 can be used to
influenced both the increment and the slope (linearity) of the
frequency variation during tuning. The tuning plunger 5 is to be
made of a low-loss dielectric material, such as quartz glass. If
this plunger is provided, at its end moving into the space between
collar 7 and opposite bottom plate, with a cap-shaped metallic
coating, e.g. of silver, a considerably greater increment of the
frequency variation-by the factor 3, for example-is obtained, as
shown by the curves of FIG. 3 designated B. The highest achievable
resonant frequency of the resonator if the plunger is completely
retracted is designated f.sub.gr.
FIG. 4 shows the deviations .DELTA. t of the depth of insertion of
the tuning plunger 5 from a completely linear shape for the case of
the shape for D3/D4 = 2 of a tuning plunger 5 with silver-coated
cap, shown in FIG. 3, t itself being the depth of insertion of the
tuning plunger 5.
Now, the influence of the coupling apertures 9 in the diaphragms 2
on the characteristics of the resonator 4 or a microwave filter
composed of such resonators A . . . F will be described.
If the coupling apertures 9 are made as full-height slot
diaphragms, the normalized susceptance y.sub.k of the k-th
diaphragm of a filter is obtained from the loaded Q's of the
(k-1)th circuit Q.sub.K.sub.-1 and the k-th circuit Q.sub.k
according to the relation
The loaded Q's Q.sub.k.sub.-1 and Q.sub.k of the filter are
calculated from the circuit parameters according to the data the
filter must meet. They determine the overall width of the filter.
The following equation holds:
Q.sub.k = f.sub.m /B.sub. k, (3)
where B.sub.k = bandwidth of the loaded k-th circuit and f.sub.m =
resonant frequency. If equation (3) is introduced into equation
(2), then
Furthermore, according to the equivalent circuit (FIG. 5a) of the
inductive diaphragm of FIG. 5a the following equation holds:
y.sub.k = (Z.sub.o)/(2.pi. f.sub.m L.sub.k) (5)
Introducing equation (5) into (4) and solving for B.sub.k.sub.-1 .
B.sub.k, then (Q.sub.k, Q.sub.k.sub.-1 >> 1)
B.sub.k.sub.-1 .sup.. B.sub.k .about. [ 8 .sup.. L.sub.k /Z.sub. o
.sup.. f.sub.m.sup.2 ] .sup.2 (6)
According to equation (6), the bandwidth of the loaded filter with
the inductive slot diaphragms shown in FIG. 5a increases as the
square of the frequency. Other factors not mentioned here are even
more frequency-dependent.
To compensate for this frequency dependence, the inductance L.sub.k
in equation (6) must have a frequency response which counteracts
any extension of the bandwidth as the frequency increases. This can
be achieved by means of a diaphragm as shown in FIG. 5b, which has
a coupling aperture whose height h approximately corresponds to the
stroke of the tuning plunger 5, the distance plunger 5 moves to
tune the resonator through its frequency range. As the tuning
plunger 5 approaches the bottom plate during tuning to lower
frequencies, the magnetic field lines forming in the region of the
coupling apertures 9 increase in density and cause a tighter
coupling of the resonators. Thus, the equivalent circuit of FIG. 5b
has a coupling inductance whose value changes with 1/f.sup.2.
For optimum compensation, the height h and the width d of the
coupling apertures 9, the length of the inner conductor 3 and the
spacings of the inner conductor 3 from the diaphragms must be
chosen so that, in the tuning range, the coupling admittance
y.sub.k according to equation (4) decreases linearly in first-order
approximation as the frequency decreases. Here, it must be taken
into account that in multi-section Butterworth or Chebishev
parameter microwave filters, the coupling admittances between the
individual resonators and the distances of the inner conductors 3
from the diaphragms 2 differ from each other. Hence, it follows
that the resonator lengths, i.e. the spacings between the
diaphragms 2 slightly differ from each other, too.
In FIG. 6, curve a clearly shows the strong dependence of the
bandwidth B on the frequency adjustment of a four-section filter
with resonators according to FIG. 2 having a slot diaphragm
according to FIG. 5a. If the filter is provided with diaphragms 2
and coupling apertures as shown in FIG. 5b and the other measures
to compensate for the coupling admittance are also taken, the curve
b is obtained which shows only little change in bandwidth.
Now, two embodiments of the resonators according to the invention
will be described.
For a rectangular resonator with the internal dimensions a = 58 mm,
b = 29 mm, and c = 50 mm, a diameter D1 of the collar 7 of the
inner conductor 3 of 20 mm was chosen for a resonant frequency of
2.3 GHz. The length l of the inner conductor was 17 mm. By the
reduction 8 of the inner conductor 3 to a diameter D2, which was
about 15 percent smaller than D1, it was possible to shorten the
length l of the inner conductor by about 1.5 mm, i.e. by 8
percent.
At a diameter ratio D3/D4 of silver-coated caps on the plunger 5 of
2 and 11 mm in length, a practically linear tuning characteristic
could be achieved (FIG. 4).
The saving in volume with a filter composed of such resonators as
compared to a filter for the TE.sub.10 mode approximately amounts
to the factor 4.5. In this case, an unloaded Q of the resonator of
5000 could be realized.
In the second embodiment, it is assumed that the rectangular cavity
resonator has the internal dimensions a = 46 mm, b = 29 mm, and c =
34 mm. For a resonant frequency of 4.2 GHz, the diameter of the
collar of the inner conductor is D1 = 15 mm. The length of the
inner conductor is 8 mm, and the smaller diameter D2 of the
reduction 8 is 13 mm. Although that resonator is operated below the
cut-off wavelength .lambda.c = 92 mm, i.e., in which the TE.sub.10
mode is existent, length of the inner conductor and resonator
length c could also be reduced by means of the diameter ratio
D1/D2. On principle, due to the dispersion factor, the length c of
the resonator 4 without inner conductor 3 would have to be about
.lambda.g/2, i.e. about 57 mm, for the above-mentioned frequency of
4.2 GHz. Thus, a considerable shortening of the resonator length c
is also achieved for those resonators whose resonant frequencies
are above the cut-off frequencies.
FIG. 7 illustrates a further advantage which the resonator
according to the invention has for the design of multi-section
microwave filters. Since the electric field lines extend parallel
to the axis of the inner conductor while the magnetic field lines
extend around the middle conductor, coupling-out can be effected at
each of the three other side walls of the resonator. One inductive
coupling-in or coupling-out aperture 9 may be associated with a
plurality of inductive coupling-out or coupling-in apertures.
For example, by means of additional transverse diaphragms k6, k7
between the resonators B and E as well as A and F, as shown in FIG.
8, it is possible to realize microwave filters with constant
envelope delay and optimally flat attenuation characteristic in the
passband such as are described by J. D. Rhodes in an article
entitled "The Design And Synthesis Of A Class of Microwave Bandpass
Linear Phase Filters" , IEEE Transactions on Microwave Theory and
Techniques, Vol. 17 (1969), No. 4, p.p. 189 to 204. Since, however,
the stop band attenuation of such filters is very poor, they are
particularly suitable for wide-band impedance transformation
because stop band attenuation is of no importance there.
FIGS. 9 and 10 show a microwave filter with bandpass behavior,
which consists of a ladder network of 6 resonators A . . . F. While
in the case of the filter shown in FIG. 9 the resonators are
disposed in an arrangement to provide a U-shaped energy path, the
arrangement of the resonators of FIG. 10 provides a meander-shaped
energy path.
It is self-evident that the arrangements illustrated here are
extendible by increasing the number of resonators, e.g. three rows,
with the flow of energy being turned at the end of each row.
FIG. 11 shows a longitudinal section through a filter as shown in
FIGS. 8 and 9 but without the additional diaphragms k.sub.6 and
k.sub.7 of FIG. 8 while FIG. 12 is a top view of a filter as shown
in FIGS. 8 and 9.
The, thusly, realized microwave filters, e.g. a filter according to
FIGS. 9, 11 and 12 for the range 3.8 . . . 4.2 GHz, exactly
corresponded, in their electrical behavior, to a microwave filter
consisting of resonators arranged in a straight line.
The resonators according to the invention have a number of
advantages regarding the construction of microwave filters:
Length, width and height can be freely chosen within certain
limits. In so doing, parameters such as the unloaded Q of the
resonator, space saving, etc., can be taken into account.
The lengths c of all resonators of a filter can be the same.
The resonant frequency is determined only by the inner conductor.
By the special design of the inner conductors, a wide and
practically linear tuning range is obtained.
The resonators of a filter may be disposed to provide a U- or
meander-shaped configuration of the energy path transmitted
therethrough, so that compact designs are obtained which are
adapted to the space available.
As to the arrangement of the coupling points, there is a large
scope.
Compared with other, conventional designs, relatively low
requirements are placed on the maintenance of tolerances which are
difficult to hold during the manufacture.
While I have described above the principles of my invention in
connection with specific apparatus it is to be more clearly
understood that this description is made only by way of example and
not as a limitation to the scope of my invention as set forth in
the objects thereof and in the accompanying claims.
* * * * *