GB2263363A

GB2263363A - Electrical filter.

Info

Publication number: GB2263363A
Application number: GB9200228A
Authority: GB
Inventors: Robert Charles Hunt
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1992-01-07
Filing date: 1992-01-07
Publication date: 1993-07-21
Anticipated expiration: 2012-01-07
Also published as: GB9200228D0; GB2263363B

Abstract

A filter, e.g. for use with microwave signals in radio transmitting and receiving equipment, comprises two or more coaxial resonators 1a-1c made from a high dielectric constant ceramic material coated with an electrically conductive material. The resonators are mounted on a multilayer circuit board 2 which provides both a ground plane for the filter and includes electrically conductive tracks arranged to provide electrical coupling elements between the resonators. Tuning screws 30 are provided to adjust the effective length of the resonators. <IMAGE>

Description

Electrical Filter The invention relates to an electrical filter, and in particular to an R.F., e.g. microwave, signal filter for use in radio transmitting and receiving equipment.

It is an object of the invention to provide such a filter which can be made at relatively low cost and which is of relatively small size.

In one aspect the invention provides a filter comprising two or more coaxial resonators made from a high dielectric constant ceramic material coated with an electrically conductive material, each resonator being mounted on a multilayer circuit board which provides both a ground plane for the filter and includes electrically conductive tracks arranged to provide electrical coupling elements between the resonators.

Suitable resonators, comprising an open-ended tube of ceramic material coated continuously with electrically conductive material on the inside, outside and one endface are available for use in making an electrical oscillator, but have not been used before in the construction of a filter. Because of their high dielectric constant, the resonators can be of small size, typically having a length and diameter of the order of six millimeters. Of course, this will vary according to the dielectric constant of the material and the frequencies of the signals which they are required to process. Ceramic materials also tend to have more stable temperature characteristics than metals which have been used in the past, and by selection of the chemicals in the mixture can be manufactured with a pre-determined and desired temperature coefficient.

Suitable multilayer circuit boards can be manufactured very cheaply and the fact that the conductive tracks define electrical coupling elements means that, e.g., no separate capacitors or inductors are required.

Preferably the multilayer circuit board includes ground planes on opposed outer most faces thereof, and electrically conductive tracks sandwiched between the ground planes which are connected to the resonators and which define capacitive coupling elements. The ground planes are preferably connected together about the edges of the board so as to provide a grounded shield about the electrically conductive innermost tracks.

Also preferably each resonator is provided with. tuning means to adjust the resonant frequency or effective length of the respective resonator. One possible tuning means comprises a screw located within an internally threaded nut mounted upon the multilayer circuit board and which projects into the resonator.

In order that the invention may be well understood, an embodiment thereof will now be described with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a perspective view of a filter according to the invention; Figure 2 is a circuit diagram of the filter shown in Figure 1; Figure 3 is a exploded cross-sectional view along lines A-A of Figure 1; Figure 4 is a plan view of one of the plates forming the multi layer circuit board showing the electrically conductive tracks which define the coupling elements; and Figure 5 is a plan view of another one of the plates.

As shown in Figure 1, a band pass filter comprises three coaxial resonators la, lb, lc mounted upon a multilayer circuit board 2. Each coaxial resonator, as best seen in Figure 3, comprises an open-ended cylindrical tube 3 made from a ceramic material having a high dielectric constant, typically within the range of 20 to 80. Suitable ceramic compounds include, for example, materials such as barium oxide, titanium oxide and zirconium oxide. The ceramic tube 3 includes a through passageway 4. The outside 5, one endface 6 and the inside wall 7 of the passageway 4 are continuously coated with an electrically conductive material such as silver. The other end face 8, the uppermost as shown, has no silver coating so that the resonator has an open circuit configuration.

The multilayer circuit board 2 comprises three plates 10, 11, 12 sandwiched together. Each plate comprises a substrate 10a, lla, 12a formed, e.g., from fibreglass or other suitable dielectric material. The outer most faces of the upper and lower plates 10, 12 respectively are coated with a layer of copper, or like electrically conductive material, 10b, 12b, respectively. The middle plate 11, best shown in Figure 4, includes electrically conductive tracks llb which define coupling or impedance elements such as capacitors C1-6. The electrically conductive tracks include portions 20, 21 which are spaced apart from and extend in parallel to each other. The amount of coupling can be selected by adjusting the width track geometry, such as the spacing or the amount of overlap.Aligned holes 25 are located along each of the longer edges of each respective plate 10, 11 and 12. Each hole 25 is through-plated so as to electrically connect together the conductive layers 10b and 12b. In this way a grounded shield can be provided about the innermost coupling elements C1-6. Further holes 27, electrically insulated from the grounded layers 10b, 12b, are provided at each of the shorter edges of the plate 2 for providing an input and output to the filter circuit.

Further holes 28 are aligned with each of the respective coaxial resonators la-c. The walls 10c of the hole 28 in the lower most plate 10 are through plated to conduct electrical signals between the tracks llb on the middle plate 11 and the silver coating 7 on the innermost walls of the through passageway 4. An annulus 10d, free of electrically conductive material, surrounds the hole 28 in the lower most board 10 to electrically insulate the layer 10b from the plated hole 10c. The diameter of the annulus 10d is selected so that, as best seen in Figure 3, when the coaxial resonator lc is attached to the face 10b, e.g. by soldering, the outermost 5 is electrically connected to the earthed face 10b. Thus connected, each coaxial resonator behaves like a coaxial transmission line which is short circuited at the end face 6.The length of the resonator is that which is selected to have the desired resonance characteristics, but is typically substantially equal to a quarter of a wavelength of the signal within the dielectric material, or an odd multiple thereof.

A tuning screw 30 is engaged with a nut 31 secured to the earthed layer 12b above each resonator. The screw 30 extends in use through the passageway 28 and into the passageway 4 of the respective coaxial resonator to provide a variable capacitance VC1-3 for tuning the effective length of the resonator.

The invention is not limited to the embodiment shown.

For example the input and output arrangements may be of any one of the known types. Similarly there may be, of course, a greater, or fewer, number of coaxial resonators than that shown. Adding extra resonators will steepen the filter characteristics. The two ground planes 12b and 10b may be connected together in a different manner to that shown, e.g.

by means of edge plating. The variable capacitances VC1-3 may be omitted, if the coaxial resonators are manufactured to a high accuracy. Other alternatives to the variable capacitances include the use of interleaved tracks at single and earth potentials respectively, and which are trimmed, e.g., by a laser trimmer to a size which gives the required amount of electrical coupling. The resonators need not be of circular cross-section, but could be of other shapes, such as square.

Claims

1. A filter comprising two or more coaxial resonators made from a high dielectric constant ceramic material coated with an electrically conductive material, each resonator being mounted on a multilayer circuit board which provides both a ground plane for the filter and includes electrically conductive tracks arranged to provide electrical coupling elements between the resonators.

2. A filter, according to claim 1, in which each resonator comprises an open ended tube of ceramic material coated continuously with electrically conductive material on the inside, outside and one endface.

3. A filter, according to claim 1 or claim 2, in which the multilayer circuit board includes ground planes on opposed faces thereof and electrically conductive tracks sandwiched between the ground planes which are connected to the resonators.

4. A filter, according to any preceding claim, in which each resonator is provided with tuning means to adjust the effective length of the respective resonator.

5. A filter, according to claim 4, in which the tuning means comprises a screw located within an internally threaded nut mounted upon the circuit board and which projects into the coaxial resonator.

6. A filter, according to any preceding claim, in which the electrically conductive tracks include portions which are spaced from one another so as to provide capacitance between the two portions.

7. A electrical filter substantially as shown in any one of the drawings.