EP1181740A1

EP1181740A1 - Dielectric microwave filter

Info

Publication number: EP1181740A1
Application number: EP00925070A
Authority: EP
Inventors: Bernd Mayer
Original assignee: Robert Bosch GmbH
Current assignee: Tesat Spacecom GmbH and Co KG
Priority date: 1999-05-12
Filing date: 2000-04-07
Publication date: 2002-02-27
Anticipated expiration: 2020-04-07
Also published as: DE19921926A1; US6717490B1; WO2000070706A1; DE50001449D1; CA2373501C; EP1181740B1; CA2373501A1

Abstract

The invention relates to a dielectric filter (1) for microwave signals. Said filter has a resonator body (3) in the shape of a truncated cone or a truncated pyramid.

Description

Dielectric microwave filter

The invention relates to a dielectric filter with an input and an output for a microwave signal and a rotationally symmetrical dielectric resonator body that can be excited by the microwave signal to produce electromagnetic vibrations. Such a filter is described for example in DE 196 176 98 Cl.

It also relates to a method for producing dielectric filters and a method for setting the mode coupling in dielectric filters.

Filters for high-frequency, especially microwave signals are used in large numbers in satellites. Due to the very high transport costs for satellite payloads, efforts are made to implement a filter function specified by the task to be performed by the satellite using a filter with the lowest possible weight and volume. Due to the necessary, very high quality, cavity resonator filters are often used. As in the publication "Application of Dual TM Mode to Triple and Quadruple-Mode Filters", Rene R. Bonetti and Albert E. Williams, IEEE Transactions on Micro-wave Theory and Techniques, Volume MTT-35, No. 12, December 1987, pages A method to make better use of the volume described in 1143 to 1149 is the use of dual-mode, triple-mode or quadruple-mode filters which, due to the symmetry of their geometric shape, have modes that have been generated a slight deviation of the shape of the filter from the absolute symmetry causes a coupling from the excited mode into a degenerate, orthogonal mode, which can be excited in the case of a dual mode -Filters - tapped at its output as an output signal or - in the case of higher multiple-mode filters - used to excite another degenerate mode corresponds to a series connection of several single-mode filters with a fraction of their volume and weight.

Another way to reduce the space requirement of filters is to use dielectric workpieces. By using them, the linear dimensions of the filter can be reduce to the square root of the relative permittivity. An example of a filter in which both methods are combined is disclosed in US 4,489,293 A. Due to the reduction in size of the structure, however, compared to a cavity resonator, larger wall currents result in the metallic boundaries of the filter housing surrounding the dielectric resonator and thus a reduction in the resonator quality. A compromise is therefore generally necessary. The limiting metal surfaces are placed at a certain distance from the dielectric.

In order to be able to reduce this distance without having to accept a deterioration in the resonator quality, it is necessary to find resonator geometries in which the field of the technically relevant resonator modes emerging from the dielectric resonator body is relatively small, so that these with the only little interaction between surrounding metal surfaces. In the previously cited document DE 196 176 98 Cl it is proposed to use a hemisphere as the resonator body. The flat surface of this hemisphere is attached to a high-temperature super-conductive plate. In one of the publications “High Temperature Super-Conductor Shielded High Power Dielectric Dual-mode Filter for Applications in Satellite Communications, S. Schornstein, IS Ghosh and N. Klein, IEEE MTT-S Digest, pages 1319 to 1322, 1998 known variant also uses a hemispherical resonator body, which is spaced from a metallic shielding surface by a base made of dielectric material.

Because of its low dielectric losses, single-crystal lanthanum aluminate or the like is preferably used as the material for the resonator body. However, the production of a hemispherical dielectric body from this material is not easy for several reasons. Since the material is very hard and brittle, the shape can only be produced by grinding. In order to grind a precisely curved surface, a high-precision, numerically controlled grinding machine must be used. This type of production is very time consuming and very expensive. The resonance frequency of the resonator body is sensitive to its shape and the dielectric constant of its material. Fine tuning of this resonance frequency is only possible within narrow limits on a hemispherical resonator body.

Since the dielectric constant of the raw material is subject to fluctuations, a sample of each raw material delivery must first be made and then the exact geometry of the resonator body to be manufactured is defined if a predetermined resonance frequency is to be realized.

Advantages of the invention

With the present invention, a dielectric microwave filter is created which can be manufactured inexpensively and which can be tuned in a simple manner to a required resonance frequency. These advantages are achieved in a dielectric filter of the type described in the introduction with the aid of a resonator body which has two differently sized base areas perpendicular to its axis of rotational symmetry and side faces connecting the base areas along straight lines. Such a resonator body can be manufactured quickly and inexpensively by simple cylindrical and / or face grinding.

The proportions of the base and side surfaces are expediently chosen so that the resonator body resembles a hemisphere in order to achieve a mode structure of the natural vibrations of the resonator body which resembles that of a hemisphere and has correspondingly small field fractions outside the resonator body. In a simple embodiment, the resonator body can be a truncated cone or a truncated pyramid with, in principle, any number of pages.

The resonator body preferably carries a foot on one of its base surfaces, preferably the large base surface, which serves to fasten the resonator body in a housing with a distance between the base surface supporting the foot and a metal housing change.

The filter according to the invention is preferably a multiple-mode filter. In a conventional manner, a screw can be used as the symmetry element or mode coupler, which screw is fastened in the housing of the filter and engages in an interior space of the filter surrounding the resonator body. However, a disturbance in symmetry can also be created in that one of the base surfaces of the resonator body is at least partially inclined with respect to the other.

It can happen that higher vibrational modes exist in the useful frequency band of a dielectric filter, the fields of which are concentrated near the surface of the resonator body. Such modes are strongly influenced by the surroundings of the resonator body, in particular by the housing, and are therefore poorly suited for filter purposes. In order to suppress such modes or to shift them out of the useful frequency band, locally dielectric material can be applied and / or removed on the resonator body. Such local changes have little influence on modes with a field concentrated in the interior of the resonator body.

A dielectric body with at least one flat base surface, such as the resonator body of the filter according to the invention, is well suited for fine-tuning its resonance frequency by removing material from the base surface. It is therefore possible to manufacture such resonator bodies as blanks in large numbers, with these blanks scattering of the resonance frequency, for example due to differences in the dielectric constant of the starting material used, can be accepted, and each blank subsequently by removing material on the blank Base area can be fine-tuned to a desired resonance frequency.

Further features and advantages of the invention result from the following description of exemplary embodiments.

characters Figure 1 shows an inventive dielectric filter in axial section;

Figure 2 shows the filter in section along the line II-II of Figure 1;

FIG. 3 shows a perspective view of a resonator body according to a second embodiment of the invention;

FIG. 4 shows cross sections of resonator bodies processed to suppress unwanted modes; and

Figure 5 illustrates the tuning of the dielectric filters to a given resonant frequency.

Description of the embodiments

Figures 1 and 2 show sections through an inventive filter 1, here a dual-mode two-pole filter. Lines II-II in Figure 1 and II in Figure 2 indicate the cutting plane of the other figure. The filter 1 has a cylindrical metallic shielding housing 2. A resonator body 3 made of lanthanum aluminate is arranged in the interior of the housing 2 and connected to the bottom 4 thereof by a foot 5, which is formed in one piece with the resonator body 3.

The resonator body 3 has the shape of a truncated cone with a large base area 6, a small base area 7 and a cone-shaped side face 8 which extends in a straight line in the cross section in FIG. 1. The proportions of the areas 6, 7, 8 are selected such that that the modes supported by the resonator body 3 are similar to those of a hemispherical body; the diameter of the small base area 7 and the height of the resonator body 3 are each in the range from 0.4 to 0.6 times the diameter of the large base area 6. This can be, for example, for a working frequency of the filter in the range from 23 to 25 mm lie.

An input 9 and an output 10 for a microwave signal extend through the bottom 4 of the housing 2. They are in the form of coaxial cables with an inner conductor 11 which penetrates the housing 2 and is at a short distance from the large base 6 of the housing Resonator body ends. The resonator body 3 has a rotational axis of symmetry 12. A field with an electrical field vector 13 is induced in the resonator body 3 via the input 9, which field is oriented along the connection between the input 9 and the axis of symmetry 12, as seen in particular in the top view of FIG to recognize the Resonatorkδrper 3 in Figure 2. A screw 14 which penetrates into the interior of the housing through the base 4 represents a mode coupler which interacts with the portion of the mode excited via the input 9, which is located outside the resonator body, and which thus disturbs the symmetry of the filter 1 and prevents one Transition from microwave energy into a mode orthogonal to the excited mode with field vector 15. Microwave energy from this mode is coupled out via the output 10 of the filter.

FIG. 3 shows a perspective view of a variant of a resonator body for a dielectric filter. This resonator body 3 'has the shape of a truncated pyramid with square bases 6', 7 'and four side surfaces 8'. A foot 5 'is also in the form of a - smaller - truncated pyramid. This resonator body also has a rotational axis of symmetry 12, the axis is fourfold, so that the resonator body 3 'supports orthogonal sets of degenerate modes.

In order to achieve a coupling between the sets of modes, the small base area 7 is ground obliquely in a corner 16. The bevel could also extend over the entire small base area 7 '.

A mode coupling by chamfering a base area instead of using a screw is of course also possible with the filter from FIGS. 1 and 2.

The number of side faces of the resonator body 3 'can of course also be greater than 4. The greater the number of side surfaces, the greater the similarity to the variant proposed in FIG. 1.

According to a variant not shown, the resonator body can also consist in one piece of two or more stacked truncated cones or pyramids, so that there is a side surface connecting the base areas along two or more straight lines. This allows a better approximation of the hemisphere shape. As with any other resonator, an infinite number of vibration modes are also possible in the case of the resonator bodies in FIGS. 1, 2 or 3. This becomes problematic when the resonance frequency of a higher mode falls within the useful band. Then this interference mode has to be pushed or suppressed by special measures. Some such measures are outlined in Figure 4. These measures include, for example, grinding a groove 17 into the side surface 8 of the resonator body 3, extending the side surface 8 beyond the large base surface 6 by attaching a ring 18, blunting the pointed edges 19 between the side surface 8 and the large base surface 6 or that Enlarging the transition cross section 20 between the large base area 6 and the foot 5 connected in one piece.

All of these measures affect the useful modes only slightly. Namely, the useful modes selected from the possible vibration modes of the resonator body are those in which the main part of the electromagnetic field is located within the resonator body 3. This property is also decisive for the fact that these modes are only slightly damped by the metallic housing, so that extremely high qualities can be achieved with these modes. In the fault mode, however, is there is also a significant field component at the edge of the dielectric. Therefore, these modes are strongly influenced by the measures outlined.

FIG. 5 illustrates the production of dielectric filters with an exactly predetermined resonance frequency according to the invention. In a first step, the blank shown in FIG. 5a) is ground from a dielectric material such as single-crystalline lanthanum aluminate. This blank has a resonance frequency f ₀ at a relative dielectric constant ε _r . In order to tune this blank to a resonance frequency f _x or f ₂ predefined for a specific application, it is sufficient to grind material on the base surface 7 facing away from the foot 5, which increases the resonance frequency. The grinding is continued until the resonance frequency of the body 3 matches the desired frequency. If, at a later point in time, dielectric material with a slightly different relative permittivity ε _r + δε _{r is} processed, a blank with the dimensions shown in FIG. 5a) can also be produced first. In order to also tune this blank to the predetermined resonance frequency f ₂ , it is sufficient to remove its small base area 7 slightly more than shown in FIG. 5c) (see FIG. 5d)). Combined with After tuning, the beveling of the base area 7 described in connection with FIG. 3 can also be expediently generated for the purpose of mode coupling.

Apart from the base area 7, no other areas of the resonator body have to be machined. The blank can therefore be inexpensively manufactured in large quantities and placed in stock. Depending on the requirements, a filter with a desired resonance frequency can then be produced very flexibly and quickly.

In particular, all filters for a multiplexer can be produced from a raw form. The delivery time of such a multiplexer can thus be significantly reduced since, after the frequency planner has been announced by the customer, the dielectric bodies for all channels can be made available quickly by grinding a surface.

To remove the base 7, the same processing methods known per se, such as belt grinding, honing or lapping, can be used, which are also used in the production of the blank itself.

Claims

claims

1. Dielectric filter with an input and an output for a microwave signal and a rotationally symmetrical dielectric resonator body which can be excited by the microwave signal to produce electromagnetic vibrations, characterized in that the resonator body (3; 3 ') has two differently sized base areas (6, 7; 6' , 7 ') perpendicular to the rotational axis of symmetry (12) and the base surface (6,7; 6', 7 ') along straight lines connecting side surfaces (8; 8').

2. Dielectric filter according to claim 1, characterized in that the resonator body (3) is a truncated cone.

3. Dielectric filter according to claim 1, characterized in that the resonator body (3 ') is a truncated pyramid.

4. Dielectric filter according to one of the preceding claims, characterized in that the dielectric resonator body (3,3 ') on one Base (6,6 ') carries a foot (5,5') for fastening the resonator body in a housing (2).

5. Dielectric filter according to one of the preceding claims, characterized in that it is a multiple mode filter.

6. Dielectric filter according to claim 5, characterized in that it is a dual-mode filter and that one of the base areas (7) with respect to the other (6) is slightly beveled in order to achieve a mode coupling.

7. Dielectric filter according to one of the preceding claims, characterized in that locally dielectric material is applied and / or removed to the resonator body (3) in order to suppress undesired modes or to shift their frequency.

8. A method for producing dielectric filters with an exactly predetermined resonance frequency, comprising the steps of: - producing a resonator body (3) from dielectric material with at least one base area (7), which has a lower than the predetermined resonance frequency, - Removal of material on the base (7) until the resonance frequency of the resonator body (3) matches the predetermined one.

9. Method for setting the mode coupling in a dielectric resonator body (3 ') of a dielectric multiple-mode filter, in which one (7') is chamfered from two opposite base surfaces (6 ', 7') of the resonator body (3 ').