GB2276276A - Coaxial resonator and multi-layer circuit board arrangement for a band stop filter - Google Patents

Abstract

A band stop filter for use with microwave signals in radio transmitting and receiving equipment, comprises at least one coaxial resonator made from a high dielectric constant ceramic material (3) coated with an electrically conductive material (5 - 7). The resonator is 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 into and out of the resonator. The arrangement may include one or more resonators with a tuning screw. Portions of the electrical tracks may provide reactive elements including capacitance. <IMAGE>

Description

ELECTRICAL BAND STOP FILTER The invention relates to an electrical band stop filter, and in particular to an R.F., e.g. UHF and microwave, band stop signal filter for use in radio transmitting and receiving equipment.

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

The invention provides a filter comprising at least one coaxial resonator made from an open ended tube of high dielectric constant ceramic material coated continuously with an electrically conductive material on the inside, outside and one endface, 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, and have been used to produce band pass filters, as shown in our co-pending U.K. patent application No. 9200228.6.

However such filters have not been used before in the construction of a band stop filter.

Although band pass filters as described in U.K.

patent application No. 9200228.6 have proved satisfactory, they share the drawbacks of all band pass filters that significant losses occur in the pass band and that as the pass band is made narrower and the degree of rejection outside the pass band is made greater these losses become greater. As a result band stop filters can be preferable in some applications. 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 outermost 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 in order to correct for errors due to manufacturing tolerances. 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 band stop 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 multilayer circuit board used in the filter of Figure 1 showing the electrically conductive tracks which define the coupling elements; Figure 5 is a plan view of another one of the plates; Figure 6 is a circuit diagram of an alternative design of band stop filter according to the invention; Figure 7 is a plan view of one of the plates forming the multilayer circuit board used in the filter of Figure 6 showing the electrically conductive tracks which define the coupling elements; Figure 8A shows the characteristics of the band stop filter of Figure 2; and Figure 8B shows the characteristics of the band stop filter of Figure 6, equivalent parts having the same reference numerals throughout.

As shown in Figure 1, a band stop filter comprises three coaxial resonators la, ib, ic 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, gila, 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 lib which define coupling or impedance elements such as capacitors C1-4 and inductances L1-L3. The electrically conductive tracks include portions 20, 21 which are spaced apart from and extend in parallel to each other. These form the capacitive coupling elements shown as C2 and C3 in Figure 4. The amount of capacitive coupling can be selected by adjusting the width and geometry of the tracks, such as the spacing or the amount of overlap. The electrically conductive tracks also include portions 19 which form capacitances C1 and C4 and portions 32 which form inductances L1-L3. The amount of reactance of the portions 19 and 32 can be selected by adjusting the geometry, width and length of the tracks. 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 elements C1-4 and L1-L3. 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 for 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 lowermost plate 10 are through plated to conduct electrical signals between the tracks lib 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 lowermost 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 ic 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, but does not make electrical contact with the silver coating 7.

Referring to Figure 6 an alternative form of band stop filter is shown in which the coaxial resonators 1 are coupled capacitively using capacitors C5-C7 which replace the inductors L1-L3 used in the band stop filter of Figures 1-5. The necessary changes in the electrically conductive tracks on the plate 11 which form the coupling elements are shown in Figure 7 which corresponds to Figure 4 in Figures 1-6. As can be seen the continuous conductive tracks which form inductors L1-L3 have been replaced by conductive tracks including spaced apart parallel portions 33, 34 forming capacitors C5-C7.

Referring to Figures 8A and 8B the differences in the performance of the inductively coupled filter of Figures 1-5 and of the capacitively coupled filter of Figure 6 and 7 graphs of attenuation against frequency.

As can be seen in Figure 8A the lower edge of the stop band of the inductively coupled filter is steeper than the upper edge of the stop band, whereas in Figure 8B the upper edge of the stop band of the capacitively coupled filter is steeper than the lower edge of the stop band.

Depending on the intended use of the filter either one of these characteristics could be the more desirable.

The invention is not limited to the embodiments 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.

To give greater control of the attenuation characteristics of the overall filter the resonant frequencies of the separate coaxial resonators can have different staggered values instead of all being equal.

Instead of the three layer circuit board arrangement shown in the examples a two layer circuit board with one layer having conductive tracks on one face and a conductive layer on the opposite face could be used.

Claims (9)

1. A band stop filter comprising at least one coaxial resonator made from an open ended tube of high dielectric constant ceramic material coated continuously with an electrically conductive material on the inside, outside and one endface, the 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 into and out of the resonator.

2. A band stop filter according to claim 1 in which there are a plurality of coaxial resonators, each resonator being mounted on the 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.

3. A filter, according to 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 the or 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 arranged to form reactive elements.

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

8. An electrical filter substantially as shown in or as described with reference to any one of Figures 1 to 5 of the drawings.

9. An electrical filter substantially as shown in or as described with reference to any one of Figures 6 and 7 of the drawings.