WO1997043888A1

WO1997043888A1 - Microwave circuit device

Info

Publication number: WO1997043888A1
Application number: PCT/GB1997/001349
Authority: WO
Inventors: Paul Martin Kenny
Original assignee: Pyronix Limited
Priority date: 1996-05-16
Filing date: 1997-05-16
Publication date: 1997-11-20
Also published as: GB9610241D0; GB2313239B; GB2313239A; AU2905097A

Abstract

A microwave circuit device comprising: a microwave circuit board comprising a microwave circuit having components and/or discontinuities capable of radiating unwanted emissions; an antenna circuit board comprising at least one antenna; a ground plane layer, the circuit boards being superimposed with the ground plane layer intervening; an enclosure for the microwave circuit board having a peripheral edge surrounding the microwave circuit; and a conductive border for the microwave circuit and/or antenna circuit; the conductive border surrounding the microwave circuit and/or antenna circuit and being disposed adjacent to the junction between the peripheral edge of the enclosure and the microwave circuit board, the conductive border and the ground plane layer being electrically connected together by an electrically conductive layer passing transversely through the microwave circuit board and/or the antenna circuit board, whereby unwanted emissions from the microwave circuit components and/or discontinuities are substantially contained within the enclosure.

Description

MICROWAVE CIRCUIT DEVICE

This invention relates to microwave circuit devices, and more particularly to the suppression of spurious unwanted harmonic emissions from microwave circuit components and discontinuities.

Microwave circuits are used in a variety of applications, for example, they are commonly employed in motion detection units for detecting a moving person or object by means of a Doppler frequency shift.

In WO92/09905 there is described a motion detector unit for detecting motion by means of Doppler frequency shift which comprises a microwave circuit board, an antenna circuit board and a ground plane layer, the two boards being superimposed with the ground plane layer intervening, the microwave circuit board comprising an oscillator and a mixer, and the antenna circuit board comprising transmit and receive antennae, each antenna having a respective feed stripline which substantially overlies an associated stripline on the microwave circuit board, the ground plane layer having, for each antenna, a respective slot to provide coupling between the microwave circuit and the antenna, the slot being resonant at the fundamental frequency of the oscillator, with said feed stripline and said associated stripline orthogonal to the resonant dimension of the slot and extending beyond the slot.

In UK patent application no GB9513251.0 there is described a motion detection unit for detecting motion by means of Doppler frequency shift which comprises:

a microwave circuit board comprising an oscillator, a hybrid coupler circuit element and a mixer;

an antenna board comprising at least one transmit and receive antenna; and

a ground plane layer,

the two boards being superimposed with the ground plane layer intervening,

wherein, in operation, the hybrid coupler circuit element divides power from the oscillator into power for the antenna transmission signal and local oscillator (LO) power to the mixer, and directs a receive signal from the antenna to the mixer, the local oscillator (LO) power and the receive signal being combined in the mixer to produce a Doppler signal output.

In UK patent application no GB9523550.3 there is described a microwave motion detection device provided with a circuit comprising a dielectric resonator wherein the dielectric resonator is positioned within a chamber, the chamber being formed by a housing having a wall comprising one or more of:

(i) a reflective material;

(ii) a partially reflective/absorbent material;

(iii) an absorbent material; or

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

The entire disclosures of all the above patent applications are incorporated herein by reference for all purposes.

In order to conform to current EMC standards, any spurious emissions outside the allocated frequency band from microwave circuit components and discontinuities must be suppressed. In particular, devices such as oscillators and dielectric resonators, which are used in motion detection units, can generate significant levels of harmonic emission. These emissions can leak out through the mechanical joints between, for example, the enclosure and the circuit board of the microwave motion detection unit. High frequency signals are often very difficult to suppress as they can leak out through very small gaps.

Microwave absorbing material can be used to line the enclosure to absorb some of the radiated emissions, but this in itself may not be enough to suppress unwanted emissions sufficiently as a significant amount may be contained within the enclosure and board substrates.

Another method of preventing leakage is to seal the gaps between the circuit board and the enclosure with a gasket. This gasket is usually custom-made and can be manufactured from a variety of materials. For example, all-metal gaskets such as beryllium copper spring gaskets, wire mesh gaskets, or conductive elastomer gaskets, such as metal or carbon loaded elastomers, can be used. Alternatively, microwave absorbing gaskets can be used to attenuate emissions. Instead of gaskets, a conductive adhesive can be used to seal the gap and hold the microwave circuit board in place.

However, the cost of the gasket or adhesive can be quite significant.

In the present invention, an improved method of suppressing unwanted radiated emissions from a microwave circuit and/or antenna circuit is provided wherein a conductive border is disposed around the microwave circuit and/or antenna circuit, which border is conductively connected to a ground plane.

In a first aspect, therefore, the present invention provides a microwave circuit device comprising:

a microwave circuit board comprising a microwave circuit having components and/or discontinuities capable of radiating unwanted emissions;

an antenna circuit board comprising at least one antenna;

a ground plane layer, the two circuit boards being superimposed with the ground plane layer intervening;

an enclosure for the microwave circuit board having a peripheral edge surrounding the microwave circuit; and

a conductive border for the microwave circuit and/or antenna circuit, the conductive border surrounding the microwave circuit and/or antenna circuit and being disposed adjacent to the junction between the peripheral edge of the enclosure and the microwave circuit board, the conductive border and the ground plane layer being electrically connected together by an electrically conductive layer passing transversely through the microwave circuit board and/or the antenna board, whereby unwanted emissions from the microwave circuit components and/or discontinuities are substantially contained within the enclosure.

In another aspect, the invention provides a microwave circuit device comprising:

an antenna circuit board comprising at least one antenna;

a ground plane layer, the two boards being superimposed with the ground plane layer intervening; and

a conductive border for the microwave circuit and/or antenna circuit, the conductive border surrounding the microwave circuit and/or antenna circuit and being disposed adjacent to the peripheral edge of the microwave circuit board and/or antenna circuit board, the conductive border and the ground plane layer being electrically connected together by an electrically conductive layer extending transversely of the microwave circuit board and/or the antenna circuit board, whereby unwanted radiated emissions from the microwave circuit components and/or discontinuities are controlled. In still further aspects, the invention provides a novel circuit board assembly and circuit board as hereinafter described.

Preferably the microwave circuit device is a microwave motion detection unit for detecting motion by means of Doppler frequency shift, and the invention will henceforth be more particularly described with reference to such a device. It is to be understood, however, that although the invention is particularly useful in microwave motion detection units it is broadly applicable to microwave circuit devices in general and is not limited thereto.

Preferably the microwave circuit comprises one or more components printed or etched on the microwave circuit board.

Preferably the microwave circuit and the antenna circuit are superimposed and preferably share a common ground plane, although they could each have their own individual ground planes. The microwave circuit and antenna circuit could also comprise a single board with a microwave circuit and antenna etched on one face and the ground plane on the opposite face.

The antenna circuit board can comprise only a single transmit and receive microstrip patch if desired, and for many applications this will be the preferred configuration. Arrays of two or more transmit and receive patches coupled together can, however, be used, where greater directivity is required. Elements other than rectangular microstrip patches can also be used. For example, printed dipoles or slots, or any other printed element appropriate to the circuit topology and application.

In the motion detection device, the two boards are superimposed, preferably in a stacked back-to-back configuration, with a ground plane layer intervening. In a preferred arrangement, the antenna circuit board has a conductive layer on one face only, and the microwave circuit board has a printed circuit on one face and a ground plane layer on its opposite face.

Preferably power is transferred between the microwave circuit board and the antenna circuit board by means of aperture coupling, although other coupling techniques could be used if desired. In the aperture coupling technique, the microwave energy from a first feed line is coupled to a second feed line through a common aperture, which may take the form either of a small circular hole or a thin slot between the feed lines. Preferably power is transferred between the microwave circuit and the antenna circuit via a slot etched into the ground plane layer. The slot is preferably orthogonal to the - antenna and microwave circuit feed lines which overlap and terminate in open circuits. To reduce circuit area, the feed lines can be bent through an angle, such as 90°, after crossing the slot. Alternatively, the aperture coupling may comprise first and/or second feed striplines terminating in a T- section comprising a cross-bar strip parallel with the slot as described in W092/09905. In a still further embodiment, the microwave circuit feed stripline may be coupled directly to the antenna patch by arranging the slot between the stripline and patch and omitting the antenna feed stripline.

Microwave circuit components capable of radiating unwanted emissions including, for example, oscillators, dielectric resonators and like components.

Discontinuities in the printed or etched microwave circuit can also give rise to unwanted emissions.

The enclosure for the microwave circuit board is preferably conductive and can be constructed in metal, such as brass or aluminium, or be of a metal coated plastic. The enclosure could comprise a microwave absorbing material. A plastic material loaded with metal filings or granules could be used.

The enclosure can comprise a moulded cover, for example, of a metal loaded plastics material, the cover having an edge region conforming substantially to the edge of the microwave circuit board and being a close fit therewith. It is important to ensure that any gaps between the peripheral edge of the microwave circuit board and the peripheral edge of the enclosure are minimised.

Preferably the conductive border is arranged at the peripheral edge of the microwave circuit board and/or antenna circuit board and is preferably disposed such that it is interposed between the microwave circuit board and the peripheral edge of the enclosure. In most instances, the conductive border is preferably continuous, since any gaps could allow microwave emissions to escape. The conductive border can be separately applied or deposited, but preferably it is formed during the deposition or etching of the microwave circuit. The border can be of the same general thickness as the deposited or etched microwave circuit.

In a preferred embodiment according to the invention, both the microwave circuit and the antenna circuit are provided with a conductive border, which borders are each electrically connected to the ground plane layer.

In a particularly preferred embodiment, the or each conductive border is connected to the ground plane layer by one or more apertures having-electrically conductive walls, extending transversely through the respective circuit board. Preferably a plurality of such apertures with electrically conductive walls are provided, preferably spaced at intervals around the conductive border or borders. The apertures are preferably spaced equidistant from each other, the size and number of apertures and the spacing being dependent upon the microwave frequency or frequencies which it is desired to control.

Embodiments of a microwave circuit device, circuit board assembly and circuit board according to the invention will now be more particularly described, by way of example only, with reference to the accompanying

Drawings, in which:

Figure 1(a) shows a sectional side elevation through a prior art microwave circuit device;

Figure 1(b) shows a portion of the edge region of the device in Figure 1 in greater detail;

Figure 1(c) shows a perspective view of the underside of the microwave circuit board of the device of Figure 1(a); Figure 2(a) shows a sectional side elevation through a prior art microwave device provided with gasket suppression means;

Figure 2(b) shows a portion of the edge region of the device of Figure 2(a) in greater detail;

Figure 3(a) shows a plan view of a microwave circuit board according to the invention provided with a conductive border;

Figure 3(b) shows a section along the line A-A of Figure 3(a);

Figure 3(c) shows a microwave circuit device according to the invention comprising a circuit board assembly with an installed enclosure; and

Figure 3(d) shows a portion of the edge region of the device of Figure 3(c) showing the assembly in greater detail.

Referring firstly to Figure 1, there is shown a prior art microwave motion detection unit indicated generally at 1 which comprises circuit boards 2, 3 mounted adjacent and parallel to one another in a stacked, back-to-back configuration. Microwave circuit board 3 is housed in a screened enclosure 4 and accommodates microwave circuitry'on its inwardly directed face 5 (as shown in Figure 1(c)). On the upper face of microwave circuit board 3 there is a ground plane layer 6 (as shown more clearly in Figure 1(b)). Antenna circuit board 2 has an antenna consisting of a pair of microstrip patches 7 which are coupled together and joined to a feed line (not shown). The antenna both transmits and receives the microwave signals.

Figure 1(c) shows the microwave circuit board 3 in more detail as viewed from the component side of the board. By way of example, the circuit is shown as comprising an oscillator 8 which in this case is a mechanically tunable dielectric resonator oscillator. Unwanted emissions from the oscillator and other components are indicated by the arrows 9.

Referring to Figure 1(b) there is shown a section through an edge region of the device 1 where the peripheral edge 10 of the enclosure 4 is clamped to the edges of the microwave circuit board 3 and the antenna circuit board 2 by an angle frame clamp 11. The arrows 9 show unwanted harmonic emissions escaping through the substrates 12 and 13 of the circuit boards 3 and 2 respectively, through gaps 14 between the edge 10 of the enclosure 4 and the circuit boards 3 and 2, and around the ground plane 6. Referring now to Figures 2(a) and 2(b), there is shown a further prior art microwave circuit device provided with an emission suppressing gasket 20. Other numerals refer to similar features as shown in Figures 1(a), (b) and (c).

The gasket 20 is placed between the ground plane 6, the peripheral edge 10 of the enclosure 4 and the angle clamping frame 11. The presence of the gasket 20 provides an improvement in that it can prevent unwanted emissions escaping via the gap between the ground plane 6 and the enclosure 4, but a further difficulty is introduced in that some method of fixing the antenna circuit board 2 to the common ground plane 6 is now required. The presence of the gasket therefore leads to a more complicated and more expensive construction.

In Figures 3(a), (b), (c) and (d) there are shown an embodiments of a circuit board, circuit board assembly and microwave circuit device according to the invention. The device in this case is a microwave motion detector unit. Referring firstly to Figures 3(a) and (b), there is shown a circuit board 30, which can be either a microwave circuit board or an antenna circuit board, provided with a deposited or etched, firmly adherent, conductive border 31, and a ground plane 32. The conductive border 31 can comprise, for example, a deposited metallic film layer. The conductive border 31 and the ground plane 32 are electrically connected by a series of plated through holes 33 in the board 30 which are arranged at intervals along the border 31. The conductive border 31 can be of any width but would preferably be of a width exceeding the hole diameters. The size, number and spacing of the plated through holes 33 will be dependent upon the operating frequency of the microwave circuit device. For a circuit operating at around 10 GHz, for example, these holes can be approximately 0.6mm in diameter and spaced at about 2.5mm intervals.

The conductive border can be, for example, formed at the same time as the microwave circuit is etched upon the circuit board, and the holes can be drilled either before or after the etching process. After the holes have been formed, their walls can be plated by known techniques. Alternatively, the border and the plated holes can be formed simultaneously by electrodeposition.

By a similar technique, an antenna circuit board 35 with a conductive border 34 can be produced, having plated through holes 36. In this case, however, the ground plane may optionally be omitted.

As shown in Figures 3(c) and (d), in the microwave circuit device the circuit boards 30 and 35 are mounted adjacent and parallel to one another, in a stacked, back- to-back assembly. Circuit board 30 is housed in a screened enclosure 37. The circuit boards are clamped to the peripheral edge 38 of the enclosure 37 by means of an angle frame clamp 39.

The improvement obtained with the microwave circuit device of the invention can clearly be seen from Figure 3(d). The combination of the conductive borders 31 and 34, and the plated through holes 33 and 36, provides an improved barrier to emissions escaping around the edges of the circuit boards 30 and 35. The plated through holes 33 and 36 also act as a barrier to emissions escaping through the substrates of the circuit boards 30 and 35.

Whilst the invention may be used in combination with a gasket if desired, in low power applications the combination of the conductive border and plated through holes may be sufficient to satisfy EMC requirements without the need for a gasket.

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. A microwave circuit device comprising: a microwave circuit board comprising a microwave circuit having components and/or discontinuities capable of radiating unwanted emissions; an antenna circuit board comprising at least one antenna; a ground plane layer, the circuit boards being superimposed with the ground plane layer intervening; an enclosure for the microwave circuit board having a peripheral edge surrounding the microwave circuit; and a conductive border for the microwave circuit and/or antenna circuit, the conductive border surrounding the microwave circuit and/or antenna circuit and being disposed adjacent to the junction between the peripheral edge of the enclosure and the microwave circuit board, the conductive border and the ground plane layer being electrically connected together by an electrically conductive layer passing transversely through the microwave circuit board and/or the antenna circuit board, whereby unwanted emissions from the microwave circuit components and/or discontinuities are substantially contained within the enclosure.

2. A device according to Claim 1, in which the microwave circuit is a microwave motion detection unit for detecting motion by means of Doppler frequency shift.

3. A device according to Claim 1 or 2, in which the microwave circuits and antenna circuits are superimposed and share a common ground plane.

4. A device according to any of the preceding Claims, in which the antenna circuit board comprises a transmit and receive microwave antenna.

5. A device according to any of the preceding Claims, in which the antenna circuit board has a conductive layer on one face only, and the microwave circuit board has a printed circuit on one face and a ground plane layer on its opposite face.

6. A device according to any of the preceding Claims, in which power is transferred between the microwave circuit board and the antenna circuit board by means of aperture coupling.

7. A device according to any of the preceding Claims, in which the enclosure comprises a microwave absorbent material and/or a conductive material.

8. A device according to any of the preceding Claims, in which the conductive border is arranged at the peripheral edge of the microwave circuit board and/or antenna circuit board.

9. A device according to any of the preceding Claims, in which the conductive border is interposed between the microwave circuit board and the peripheral edge of the enclosure.

10. A device according to any of the preceding Claims, in which both the microwave circuit and antenna circuit are each provided with a conductive border which is electrically connected to the ground plane layer.

11. A device according to any of the preceding Claims, in which the or each conductive border is electrically connected to the ground plane layer by one or more apertures having electrically conductive walls the apertures extending transversely through the respective board or boards.

12. A device according to Claim 11, in which a plurality of apertures with electrically conductive walls are provided, spaced at intervals around the conductive border or borders.

13. A device according to Claim 11 or 12, in which the apertures are spaced equidistant from each other, the size and number of apertures and the spacing being dependent upon the microwave frequency or frequencies which it is desired to control.

14. A device according to any of the preceding Claims, in which the or each conductive border has a width greater than the aperture diameter.

15. A microwave circuit device comprising: a microwave circuit board comprising a microwave circuit having components and/or discontinuities capable of radiating unwanted emissions; an antenna circuit board comprising at least one antenna; a ground plane layer, the two boards being superimposed with the ground plane layer intervening; and a conductive border for a microwave circuit and/or antenna circuit, the conductive border surrounding the microwave circuit and/or antenna circuit and being disposed adjacent to the peripheral edge of the microwave circuit board and/or antenna circuit board, the conductive border and the ground plane layer being electrically connected together by an electrically conductive layer extending transversely of the microwave circuit board and/or the antenna circuit board, whereby unwanted radiated emissions from the microwave circuit components and/or discontinuities are controlled.

16. A microwave circuit device substantially as hereinbefore described with reference to and as illustrated in Figure 3 of the accompanying Drawings.

17. A microwave circuit device substantially as hereinbefore described.

18. A circuit board assembly for a microwave circuit device comprising: a microwave circuit board comprising a microwave circuit; an antenna circuit board comprising at least one antenna; a ground plane layer; and a conductive border for the microwave circuit and/or antenna circuit, the conductive border surrounding the microwave circuit and/or antenna circuit and being disposed adjacent to the peripheral edge of the microwave circuit board and/or antenna circuit board, the conductive border and the ground plane layer being electrically connected together; whereby radiated emissions from the microwave circuit are controlled.

19. A microwave circuit board having a microwave circuit and a conductive border on one side and a ground plane layer on the other side, the conductive border and the ground plane layer being connected by plated through holes in the circuit board.

20. A microwave circuit board substantially as hereinbefore described.