WO1997019512A1

WO1997019512A1 - Dielectric resonator

Info

Publication number: WO1997019512A1
Application number: PCT/GB1996/002806
Authority: WO
Inventors: John Konstandelos
Original assignee: Pyronix Limited
Priority date: 1995-11-17
Filing date: 1996-11-18
Publication date: 1997-05-29
Also published as: GB2307355A; AU7581496A; GB9523550D0

Abstract

A circuit comprising a dielectric resonator wherein the dielectric resonator is positioned within a chamber, the chamber being formed by a housing with a wall having selectable or predeterminable electromagnetic characteristics, and wherein the position of the dielectric resonator relative to the wall or walls, and the dimensions of the chamber, providing a controlled, and preferably uniform electrical and magnetic fied environment for the dielectric resonator.

Description

DIELECTRIC RESONATOR

This invention relates to dielectric resonators and more particularly to a dielectric resonator employed, for example, in an oscillator circuit.

It is known to employ a dielectric resonator as a frequency determining element in an oscillator, either as the stabilising element in a free-running transistor oscillator, or in the feed-back network of a series or parallel feed-back oscillator. The dielectric resonator, hereinafter occasionally referred to as a puck, is usually mounted on a substrate, for example, a printed circuit board, closely adjacent to, or overlapping, one or more strip lines. When coupled to a single stripline, the puck open-circuits the line at its resonant frequency. When coupled to two or more striplines, the puck can be operated in transmission mode at its resonant frequency.

The resonant frequency of the puck is a function of the dielectric constant of the material from which it is made, its physical dimensions, and the boundary conditions set by any surrounding cavity or components. Frequency tuning can be accomplished by mechanical means, for example, by varying the distance between the puck and a component such as a metal member, which can, for example, be located directly above it. Frequency tuning can also be accomplished by electrical means, for example, by coupling the puck to a second resonator circuit, such as a varactor and stripline combination, and varying the varactor capacitance.

The commonly used resonant mode for most dielectric resonator applications is the TE_01β mode, in which the magnetic fields lie in the axial plane of the puck and the electric fields lie in concentric circles around the puck axis. The puck can support an infinite number of other TE modes as well as TM and TEM modes and therefore care must be taken in selecting the desired mode.

The puck and its associated circuitry are normally housed in a cavity, preferably formed by a housing provided with an electrically conducting wall, firstly to prevent radiated emissions, and secondly to prevent radiation losses from reducing the Q value of the puck.

In conventional microwave circuitry design it is generally recommended that the separation between the puck and the walls of the housing or components forming the cavity, is equivalent to at least one puck diameter in any direction, and preferably significantly more, in order to prevent the proximity of the walls of the housing or components exerting undue influence on the puck characteristics. In many microwave devices this requirement is in conflict with the need to keep the circuit dimensions to a minimum. The need to reduce the circuit size often results in the puck being mounted asymmetrically within the cavity so that the distances from the puck to each of the walls of the housing or components providing the cavity are different. This has the effect of distorting the fields around the puck, and, depending on the geometry, can suppress the desired TE_01β mode or excite other, unwanted modes.

EP 0 026 086 discloses a microwave filter having a tunable bandwidth of 870 MHz to 890 Mhz, that is approximately 2%. The filter cavity has an elliptical cross-section. The filter can be connected to other microwave elements using co-axial cable as is well known within the art. Due to the complex elliptical cross- section, the filter disclosed in EP 0 026 086 will be difficult to manufacture in volume and therefore expensive.

It is an object of the present invention to mitigate the problems associated with the prior art.

It is an object of the present invention to provide an oscillator circuit comprising a dielectric resonator in which the puck is positioned within a controlled, and preferably uhiform, field environment without any abrupt changes, such that improved consistency and improved resonator performance can be obtained. According to a first aspect, the present invention provides an oscillator circuit comprising a dielectric resonator wherein the dielectric resonator is positioned within a chamber, the chamber being formed by a housing and having a wall with selectable or predeterminable electromagnetic properties, and wherein the position of the dielectric resonator relative to the wall or walls, and the dimensions of the chamber, providing a controlled, and preferably uniform, electrical and magnetic field environment for the dielectric resonator.

A further embodiment provides a circuit wherein the chamber further houses at least the excitation means.

A still further embodiment provides a circuit wherein the chamber also houses a transistor coupled to the excitation means.

Yet another embodiment of the present invention provides a circuit wherein the excitation means and the dielectric resonator are mutually disposed so as to provide substantially constant coupling therebetween across a predeterminable range of frequencies.

Preferably, an embodiment an oscillator circuit is provided wherein the separation between the dielectric resonator and the housing wall is equal to or less than the diameter of the dielectric resonator. In a further aspect the invention provides an oscillator circuit wherein the dielectric resonator is positioned symmetrically within a chamber.

The oscillator of the invention can be a free- running transistor oscillator, or a series or parallel feed back oscillator.

The dielectric resonator or puck can be of any suitable size and shape, but will usually be cylindrical or disc-shaped.

Preferably the dielectric resonator or puck is disposed symmetrically, and more preferably, centrally with respect to the chamber. Preferably the chamber and housing are specifically dedicated to the dielectric resonator, that is to say, the chamber is preferably free of other components which would affect the uniformity of the field environment. In a preferred embodiment the housing providing the chamber exclusively surrounds the dielectric resonator on all sides, apart from its mounting surface, and no other circuit components are present within the chamber.

The chamber is preferably cylindrically shaped, although other geometrically curved shapes, for example, conical chambers and elliptical chambers, are not excluded, provided that they can provide a controlled, and preferably uniform, electrical and magnetic field environment, without abrupt changes, for the dielectric resonator.

It is found that, by using the present invention, the dimensions of the chamber can be made very small, and, for example, it has been found that the separation between the puck and the housing wall can be reduced to substantially less than one puck diameter.

It will be appreciated that placing any object within close proximity to the puck will change the puck resonant frequency and the housing of the present invention is no exception to this. This change in frequency can be compensated for, however, by changing the dimensions of the puck, or by providing mechanical or electrical frequency tuning means as previously described. Preferably the housing is provided with mechanical frequency tuning means which can comprise, for example, a turn screw positioned in a threaded hole in the wall of the housing.

Where the dielectric resonator is part of a circuit of a microwave device, the housing may be moulded integrally with the casing for the device, or affixed to the casing as a separate component. Alternatively, where the dielectric resonator is mounted on a printed circuit board the housing can be affixed thereto. In a preferred configuration, the housing is moulded integrally with, or affixed to, the casing, and extends from the casing to the printed circuit board or to a position adjacent to the surface thereof.

Suitable materials for the wall or walls of the housing are as follows:

(i) electrically conducting materials; for example, metals (brass, aluminium etc) or metal plated plastics materials.

(ii) partially conducting/absorbing materials; for example, metal loaded plastic materials, (iii) microwave absorbing materials; for example lossy foams or magnetically loaded rubber materials.

(iv) microwave transparent or partially transparent (lossy) materials; for example, engineering plastics materials, such as ABS and polycarbonate plastics.

In preferred embodiments of the invention, wherein the puck operates within a substantially uniform electrical and magnetic field environment, it is found that many of the conditions that can suppress the desired TE_01β mode, or excite unwanted modes, can be substantially reduced or eliminated. The use of a cylindrical or conical chamber, in accordance with a preferred aspect of the invention, can enable puck performance in terms of tuning range and circuit Q to be improved, and can also provide additional screening to help reduce radiated emissions. The use of a cylindrical or conical chamber also permits the overall size to be reduced substantially.

Whilst the invention has been described in terms of a dielectric resonator oscillator it will be appreciated that it is not limited thereto, and could be applied to other circuits such as, for example, dielectric resonator filters.

An embodiment of a dielectric resonator oscillator in accordance with the invention will now be described, by way of example only, with reference to the accompanying Drawings in which:

Figures 1(a) and (b) show, in perspective view, two arrangements for mounting a puck on a printed circuit board;

Figure 2 shows, in sectional side elevation, a conventional housing provided with mechanical frequency tuning means;

Figure 3 shows, in diagrammatic form, a dielectric resonator provided with electrical frequency tuning means; Figure 4 illustrates diagrammatically the magnetic and electrical field distribution for a puck operating in the TE_01β mode;

Figure 5 shows in sectional side elevation a dielectric resonator situated within a cylindrical chamber in accordance with the invention;

Figure 6 shows the arrangement of Figure 5 in plan view;

Figure 7 shows a microwave device incorporating a dielectric resonator oscillator according to the invention in plan view;

Figure 8 shows a side elevation of the device of Figure 7, partly in section;

Figure 9 shows an end elevation of the device of Figure 7, partly in section; and

Figure 10 shows a perspective view of the device of

Figure 7.

Figure 11 illustrates an arrangement of the circuit boards; Figure 12 shows a circuit board layout suitable for use in a detector unit in accordance with an embodiment;

Figure 13 shows features of the circuit boards which provide coupling between the microwave circuit and the antenna circuit;

Referring firstly to Figures 1(a) and (b), there is shown a dielectric resonator or puck 1 mounted on a printed circuit board 2. The puck is of generally cylindrical or disc-shaped construction, and is mounted in coupling relationship to a strip line 3, in Figure 1(a), and in coupling relationship to a pair of strip lines 3 and 4, in Figure 1(b). In Figure 1(a) the puck open-circuits the strip line 3 at its resonant frequency, and in Figure 1(b) the puck operates in transmission mode between the strip lines 3 and 4 at its resonant frequency.

Referring to Figure 2, there is shown a typical prior art arrangement. The puck 5 is shown mounted on a printed circuit board 6 and coupled to a strip line 7. The puck and the stripline are surrounded by a rectangular housing 8 and the puck is mounted asymmetrically with respect to the housing. A screw or plunger 9 set in a hole 10 in the top surface 11 of the housing 8 is situated immediately above the puck 5. The screw or plunger 9 can be moved towards or away from the puck 5 in order to alter the resonant frequency of the puck.

Referring now to Figure 3, there is shown a further prior art arrangement. The puck 12 is positioned between strip lines 13 and 14 on a printed circuit board. Strip line 14 is connected to a varactor 15 and made to resonate around the puck frequency. This resonant circuit is electromagnetically coupled to the puck thereby forming a pair of mutually coupled resonant circuits. By varying the varactor capacitance the overall circuit can be tuned to the desired resonant frequency.

The electric and magnetic fields of an isolated puck 19 operated in TE_01β mode are depicted in Figure 4.

Referring now to Figure 5, there is shown, in diagrammatic form, a puck provided with a cylindrical cavity in accordance with the invention. The cylindrical puck 20 is mounted on a printed circuit board (pcb) 21 and is coupled to a stripline 22. The puck 20 is mounted co-axially within a cylindrical housing 23 which is integrally moulded with a casing 24. A threaded hole 25 in the casing 24 is provided with a turn screw 26, the arrangement being such that the screw is immediately above the puck 20. Integrally manufacturing the casing and housing enables a motion detector realised using the present invention to be made smaller than prior art devices.

The diameter of the cylindrical cavity is approximately 15 mm.

The cylindrical housing 23 is formed from a compound of stainless steel fibres and plastics material. The housing 23 extends from the casing 24 to a point just above the pcb substrate 21. Utilising a cylindrical cavity obviates the distortions in the fields generated which invariably result from using a rectangular cavity.

By mounting the puck 20 at the centre of the cylindrical cavity formed by the housing 23 it is found that the puck is subjected to a highly uniform field environment. Frequency tuning of the puck can be accomplished by varying the distance between the puck 20 and the turn screw 26. The turn screw 26 is provided with a slot 27 (see figure 6) whereby its height can be adjusted by means of a screwdriver.

Preferably, the puck has a diameter of 5 mm thereby enabling a relatively small resonator or oscillator to be realised. The frequency of operation of the oscillator is, in preferred embodiments, between 10.4 GHz and 11 Ghz, that is to say the oscillator has a 6% bandwidth.

In Figures 7 to 10 there is shown a microwave motion detection device comprising a dielectric resonator oscillator in accordance with an embodiment. The circuit of the microwave motion detection device is described in detail in our co-pending UK patent application number GB9513251.0 filed 29 June 1995, the entire disclosure of which is incorporated herein by reference for all purposes.

Referring firstly to Figure 7, the device, illustrated generally at 30, is provided with a printed circuit board 31 upon which various components are mounted. Referring to Figure 8 the dielectric resonator or puck is shown at 32, mounted on the printed circuit board 31. The device 30 is provided with a casing 33 having a raised conical portion 34 which is co-axial with the puck 32. The raised conical portion 34 of the casing

33 contains a conical housing 35 which extends from the raised portion 34 downwardly until it almost touches the printed circuit board 31. The housing forms a cylindrical chamber 36 which is co-axial with the puck

32. The raised portion 34 has a chamfered recess 37 within which there is situated a turn screw 38 in a threaded hole 39. The screw 38 is co-axial with the puck 32, and by turning the screw the distance between its lower end 40 and the upper surface 41 of the puck 32 can be varied for frequency tuning purposes.

As will be apparent from the drawings, the walls of the chamber 36, which are made of a compound of stainless steel fibres and plastics material are circularly symmetric about an axis through the puck 32.

Referring to Figure 11, the unit comprises two circuit boards 111, 112 mounted adjacent and parallel to one another, in a stacked, back-to-back configuration. Circuit board 111 would normally be housed in a screened enclosure (not shown). Board 111 accommodates microwave circuitry on its inwardly-directed face (not shown), and on its exposed face has a ground plane layer 113. Circuit board 112 has an antenna consisting of a pair of microstrip patches 114 which are coupled together and joined to a feed line 115. The antenna both transmits and receives the microwave signals. Circuit board 111 has a resonant slot 116, the purpose of which will be explained .in more detail hereinafter.

Figure 12 shows the microwave circuit board 111 in more detail as viewed from the component side of the board. The circuit comprises an oscillator 120, a ring hybrid coupler circuit 121 and a single diode mixer 122.

Power is supplied to the oscillator 120 by a supply line 123. The oscillator is a mechanically tunable dielectric resonator oscillator 124.

The output of the oscillator is provided to a first port 125 of the ring hybrid 121 where it is split into two components, a clockwise component directed to the antenna port 126 and an anticlockwise component directed to the local oscillator port 127 of the mixer 122. The second mixer port 130 is isolated from the oscillator port 125 as explained previously. The clockwise component of the oscillator output travels from the port 126 along the feed line 128 to the slot coupler 129 from whence it is coupled to the antenna feed line 115, as will be more particularly described with reference to Figure 13. The power to the antenna is radiated over the coverage area.

Reflected signals are received by the antenna and coupled to the ring hybrid via the same microstrip lines and resonant slot. Received signals from the antenna patches 114 thus enter the feed line 115, are coupled via the slot coupler 129 to the feed line 128, and enter the ring hybrid at the antenna port 126. Here the received signal is divided into a clockwise component which passes to the RF port 130 of the mixer 122, and an anticlockwise component which is directed to the oscillator port and is dissipated. Although some of the received power is lost in this way, it is found in practice that more than enough power is available for comparative purposes, and the substantial advantages of the use of the ring hybrid outweigh this disadvantage. The LO port 127 is isolated from the antenna port 126 as previously explained.

The total length of the ring hybrid track is one and one half wavelength and the track distances between ports 125 and 126, ports 126 and 130, and ports 130 and 127 are respectively each one quarter wavelength. The result is that the sum of the received signal and the LO signal when mixed in the single diode mixer 122 produces an IF Doppler signal which could, for example, be of the order of 100Hz at the IF port which can be extracted, filtered and processed in known manner.

The puck is simultaneously coupled to two strip lines 131 and 133 and, together with the strip lines, forms the feedback element of the transistor. Strip-line 131 is arranged to follow the arcuate profile of the dielectric puck 124 thereby ensuring that the coupling point 132 is a substantially constant distance from the puck. This results in increased predictability and stability of the electrical characteristics of the oscillator and is a further independent aspect of the invention. Using an arcuate stripline enables the coupling between the strip line and the puck to remain substantially constant thereby ensuring constant power transfer therebetween. An alternative embodiment of the invention can be realised in which the feedback strip line 133 is arranged to follow the arcuate profile of the puck.

The curved strip lines of the above embodiments of the invention enable smaller circuits to be realised than the prior art arrangements which utilise straight strip lines.

In a still further embodiment, there walls of the chamber are preferably cylindrical. The cylindrical walls in conjunction with the arcuate strip-line 131 advantageously ensures that the excitation field is always coincident with a radius of the puck. Therefore, the coupling between the stripline and the puck is improved. Further, the combination of an arcuate stripline and a cylindrical cavity improves the Q factor of the resonator.

Referring now to Figure 13, there is illustrated the coupling means between the microwave circuit and the transmitter/receiver antenna circuit. The circuit boards 111 and 112 are superimposed such that the slot 116 in the ground plane 113 lies orthogonally between the feed line 128 and the antenna feed line 115. The feed lines 115 and 128 extend beyond the slot and are bent through 90 degrees before terminating in open circuits. In yet a further embodiment, the transistor shown in figure 3 is also housed within the chamber. This arrangement advantageously allows an oscillator circuit to be realised which is smaller than prior art oscillators.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. An oscillator circuit comprising a dielectric resonator and excitation means for exciting a predeterminable mode or frequency of operation of said resonator, the dielectric resonator being positioned within a chamber, the chamber being formed by a housing and having a wall with selectable electromagnetic properties, and wherein the position of the dielectric resonator relative to the wall or walls, and the dimensions of the chamber, providing a controlled, and preferably uniform, electrical and magnetic field environment for the dielectric resonator.

2. A circuit according to claim 1, wherein the chamber further houses at least the excitation means.

3. A circuit according to any preceding claim, wherein the chamber also houses a transistor coupled to the excitation means.

4. A circuit according to any preceding claim, wherein the excitation means and the dielectric resonator are mutually disposed so as to provide substantially constant coupling therebetween across a predeterminable range of frequencies.

5. A circuit according to any preceding claim, wherein the dielectric resonator has an arcuate profile and said excitation means is arranged to substantially follow said arcuate profile.

6. A circuit according to any preceding claim, wherein the oscillator is operable in a plurality of modes or frequencies having associated coupling points on the excitation means, and the excitation means is arranged such that the coupling points are substantially a constant distance from the dielectric resonator.

7. A circuit according to any preceding claims, which comprises a free-running transistor oscillator, or a series or parallel feedback oscillator.

8. A circuit according to any preceding claim, wherein the separation between the dielectric resonator and the housing wall is equal to or less than the diameter of the dielectric resonator.

9. A circuit according to any preceding claim, which is part of a microwave device having a casing, and in which the housing is moulded integrally with the casing or affixed to the casing as a separate component.

10. A circuit according to any preceding claim, wherein the chamber is symmetrical.

11. A circuit according to any preceding claim, in which the dielectric resonator is positioned symmetrically with respect to the chamber.

12. A circuit according to any preceding claims, wherein the dielectric resonator is positioned centrally with respect to the chamber.

13. A circuit according to any preceding claims, in which the dielectric resonator is cylindrical or disc-shaped.

14. A circuit according to claim 13, wherein the diameter of the chamber is approximately 15 mm.

15. A circuit according to any preceding claim, wherein the diameter of the dielectric resonator is approximately 5 mm.

16. A circuit according to any preceding claim, in which the dielectric resonator is mounted on a printed circuit board and the housing extends from the casing to the printed circuit board or to a position adjacent to the surface thereof.

17. A circuit according to any preceding claim, wherein the material of the wall of the housing comprises:

(i) a metal or a metal plated plastics material; (ii) an artificial dielectric, such as a metal loaded plastics material; (iii) a lossy foam material or a magnetically loaded rubber material; or (iv) an engineering plastics material.

18. A circuit according to any of the preceding claims, in which the housing is provided with mechanical tuning means for the dielectric resonator.

19. A circuit according to any of the preceding claims, substantially as hereinbefore described with reference to figures 5 to 13 of the accompanying Drawings.

20. A circuit comprising a dielectric resonator and a chamber substantially as hereinbefore described.

21. An oscillator substantially as hereinbefore described with reference to and/or as illustrated in figures 7 to 13 of the accompanying Drawings.

22. A circuit according to any preceding claim, wherein said excitation means comprises a strip-line.

23. A circuit as claimed in any preceding claims, wherein said selectable or predeterminable electromagnetic properties include at least one of either the degree of conductivity, absorption, reflectivity or transparency of the wall of the chamber.

24. A circuit as claimed in any preceding claim, wherein the wall is made of one or more of (i) a electrically conducting material;

(ii) a partially electrically conducting/absorbing material;

(iii) an absorbing material; or

(iv) a transparent or partially transparent (lossy) material.