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.

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.

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.