FEED STRUCTURE FOR ELECTROMAGNETIC WAVEGUIDES

Abstract: The present invention relates to feed structures for electromagnetic waveguides, cavities, antennas and other structures. A "feed structure" is an arrangement for electrical conductors used to transfer RF energy between any transmission lines and a waveguide. There are problems associated with designing and manufacturing conventional feeds. The present invention overcomes these problems by directly coupling an integrated circuit incorporating an RF circuit element to a waveguide, cavity, antenna or other structure. Coupling is achieved by an aperture formed in a ground plane of the integrated circuit and connected to a transmission line linked to the RF circuit element.

Description: FEED STRUCTURE FOR ELECTROMAGNETIC WAVEGUIDES Field of the Invention The present invention relates generally to feed structures for electromagnetic waveguides, cavities and antennas, and, particularly, but not exclusively, to a feed structure for feeding RF radiation from integrated circuits to RF waveguides, cavities and antennas, to an integrated circuit incorporating an RF feed structure, to an assembly comprising a waveguide, cavity, antenna or mounting flange and an integrated circuit comprising a RF feed structure, and to a method of constructing a monolithic integrated circuit . Background of Invention The term "feed structure" is usually used to refer to the arrangement of electrical conductors which are used to transfer RF energy between adjoining transmission lines (such as icrostrips, striplines, coplanar transmission lines etc.) and an RF guiding structure, such as a waveguide, cavity or antenna, preferably in such a way that the energy is efficiently coupled from one medium to the other and propagates freely within the waveguide, cavity or free space. Other common terms used for such structures are "transitions" and/or "launches". Much of the following description will be given in relation to coupling to waveguides. It will be appreciated, however, that the principles of the present invention can be equally applied to RF guiding propagation structures such as waveguides, radiating propagation structures (antennas) and cavities. Waveguides are typically used to convey microwave and millimeter wave signals because of their relatively low loss and controlled propagation characteristics at high RF frequencies. Since waveguides need to be interfaced to electronic circuitry of some form, it is necessary to employ structures which efficiently couple RF energy from the waveguide to other transmission line structures which in turn interface to electronic circuitry. For example, a Microwave Monolithic Integrated Circuit (MMIC) might typically be mounted on a planar substrate (eg a circuit board) so that signals propagate from the MMIC to the substrate via bond wires and then along substrate transmission lines, through an aperture and into the waveguide by means of a portion of the substrate which acts as the radiating element (a monopole feed, for example) . There are many variants of this arrangement in the known prior art. Typically, these prior art feed arrangements involve three separate components: (1) electronic devices, (2) interconnecting substrates incorporating transmission lines and (3) waveguide radiating elements . There are many problems associated with designing and manufacturing conventional planar waveguide feeds. These problems include: (i) Component Cost: Expensive substrate materials (for the interconnecting substrate) such as quartz must be used to ensure low RF loss and to allow high precision conductor patterning which is typically needed for the radiating element . (ii) Manufacturing Cost: Expensive, high-precision, time-consuming, manufacturing techniques need to be employed to assemble the feed structure. (iii)RF Performance: The characteristic impedance of the overall feed assembly is difficult to predict and control during manufacture which results in poor RF reproducibility . Each component of the assembly introduces a degree of variability in circuit impedances, particularly any MMIC- to-substrate bond wires used, (iv) Heatsinking: A dielectric substrate is typically used to allow transmission lines and radiating elements to be patterned on the surface. This often means that the thermal conductivity of the substrate is relatively poor and does not provide adequate heatsinking for MMICs . Summary of the Invention The present invention provides an integrated circuit comprising: - an RF circuit element, and an RF coupling means for coupling RF energy between the integrated circuit and an RF guiding structure, the coupling means including a ground plane referenced transmission line structure which traverses a region of the integrated circuit where a portion of the ground plane reference is absent. The RF circuit element may be an RF transmitter or receiver or transceiver. Preferably, the RF circuit element is arranged to transmit or receive high RF frequencies, and may be a microwave circuit element or millimeter wave circuit element. Preferably, the RF coupling means directly couples the RF energy between the integrated circuit and the RF guiding structure. This means that the coupling means couples the RF signal directly to the RF guiding structure preferably in the absence of any interconnecting substrate between the integrated circuit and the RF guiding structure such as the interconnecting substrates incorporating transmission lines which are utilised in prior art arrangements. The term RF guiding structure includes, but is not limited to, a waveguide, cavity, antenna or mounting flange. The ground plane reference is preferably formed by electrically conductive material and the absent portion is formed by an aperture in the electrically conductive material, which acts as a radiating element, for radiating coupled RF energy. The aperture is preferably formed by removing a portion of the electrically conductive material of the integrated circuits ground plane to form a re-entrant feature in the outline of the ground plane. The re-entrant feature may be formed by a polygon of more than four sides. The radiating element may be formed by removing a portion of the electrically conductive material of the or antenna aperture (where the RF guiding structure is a waveguide, cavity, antenna or mounting flange) . Where the ground plane is formed by a metallic surface, the aperture acts as the radiator. Preferably, the aperture is a slot formed in the ground plane. Preferably, it extends only part way across the ground plane . Preferably, the integrated circuit is mounted in direct contact with a wall of the RF guiding structure (the wall containing the feed aperture) , so that an entire surface of the integrated circuit is in contact with the wall . The present invention further provides an assembly comprising: - an RF guiding structure, and an integrated circuit for transmitting or receiving RF signals in accordance with any one of the preceding claims, the integrated circuit being mounted such that it couples RF signals between the waveguide, cavity, antenna or mounting flange and integrated circuit through an aperture in a surface or wall of the RF guiding structure. The present invention yet further provides a feed structure wherein the ground plane reference is formed by electrically conductive material and the absent portion is formed by an aperture in the electrically conductive material which acts as a radiating element, for radiating coupled RF energy. The present invention yet further provides a method of constructing a monolithic integrated circuit, comprising the steps of forming a feed structure in the monolithic integrated circuit, feeding RF signals directly to an RF guiding structure, for transmission or reception of RF energy from the RF guiding structure, the feed structure comprising a ground plane referenced transmission line structure which traverses a region of the integrated circuit where a portion of the ground plane reference is absent . The present invention yet further provides an RF coupling arrangement comprising an RF circuit element, and an RF coupling means for directly coupling RF energy between the integrated circuit and an RF guiding structure, the coupling means including a ground plane reference transmission line structure which traverses a region of the integrated circuit where a portion of the ground plane reference is absent. Brief Description of Figures Features and advantages of the present invention will become apparent from the following description of an embodiment thereof, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic of a perspective view of a prior art assembly illustrating a prior art RF feed structure; Fig. 2 is a schematic of an exploded perspective view of an assembly in accordance with an embodiment of the present invention pertaining to waveguides and/or resonant cavities, Fig. 3 is a schematic of a partial cross-section view of the assembly of Fig. 2, with the addition of a back- cover cavity cap, Fig. 4 is a schematic of an exploded perspective view of an assembly in accordance with an embodiment of the present invention pertaining to antennas, and Fig. 5 is a schematic of an exploded perspective view of an assembly in accordance with an embodiment of the present invention pertaining to mounting flanges. Description of Preferred Embodiment Fig. 1 illustrates a prior art feed structure for coupling RF energy produced by a monolithic integrated circuit 1 to a waveguide or resonant cavity 2. As illustrated, the integrated circuit 1 is mounted on a planar substrate 3 (e.g. a circuit board), which may be of an expensive substrate material such as quartz, so that signals propagate from the integrated circuit 1 to the substrate via bond wires 4 and then along substrate transmission lines, in this case a microstrip 5 having a microstrip ground plane 6, through an aperture (not shown) and into the waveguide 2 by means of a portion of the substrate 3 which acts as a radiating element, in this case being a monopole antennae 7. This arrangement suffers from the disadvantages which are discussed above, i.e. expensive substrate materials required, expensive, high precision, manufacturing techniques required; difficulties in controlling characteristic impedance, and inadequate thermal conductivity of the substrate material. Fig. 2 illustrates an assembly in accordance with an embodiment of the present invention, which includes a waveguide or resonant cavity 10 and an integrated circuit 11. The integrated circuit 11 includes a coupling means 12 which is arranged to couple directly to corresponding coupling means 13 in the waveguide or resonant cavity 10 for coupling RF radiation to or from a RF circuit element 14 formed in the integrated circuit 11. As can be seen, the integrated circuit is mounted directly to the waveguide 10, with no interconnecting substrate. The RF signal is instead coupled directly from the integrated circuit 11 to the waveguide 10. In more detail, the integrated circuit incorporates a ground plane 15 on its under side (i.e. the opposite side to that containing the active circuit elements such as transistors) . The ground plane 15 usually provides a ground plane reference for circuit elements formed in the integrated circuit. The ground plane 15 is usually a continuous metallic layer. In accordance with this embodiment of the present invention, the coupling means 12 includes an aperture 12, in this case in the form of a slot extending partially along the ground plane 15. An RF transmission line 16, in this case a microstrip-like structure, extends from the RF circuit element 14 (which may be a microwave transmitter, receiver or transceiver) , across the region where the aperture 12 is located but spaced from the aperture within the integrated circuit, (in this case the transmission line 16 is formed on the opposite side of the integrated circuit to the ground plane) to a "via hole" 17. The via hole 17 provides the return connection to the ground plane 12. The positioning of the aperture 12 is such that return currents of the microstrip transmission line 16 are diverted around the aperture 12 , so that RF energy radiates outwardly from the aperture 12 in a direction perpendicular to the integrated circuit 14 surface. As indicated by arrow 18, the integrated circuit is positioned against the waveguide 10 such that the aperture 12 is positioned against corresponding aperture 19 in the waveguide 10. RF energy thus can be made to couple directly to or from waveguide cavity 20 formed by the waveguide 10. The shape of the aperture 19 in the waveguide 10 may be designed to optimise coupling into the waveguide and may incorporate various three dimensional angular features to achieve this . An ideal shape may be established by the skilled person. As illustrated in Fig. 3, in this embodiment cap 21 is mounted to the waveguide 24/integrated circuit 14 assembly to enclose the integrated circuit 11 (thereby providing mechanical and environmental protection) and to form a cavity which reflects RF energy propagating away from the waveguide or resonant cavity 20 back towards the waveguide aperture 19, as illustrated by RF propagation lines 22. The cap may include a means 23 for tuning the cavity 24, the means may be a screw 23 which protrudes into the cavity 24. The width of the slot 12 can be made relatively small (e.g. 0.2 x 2mm) and occupies only a small fraction of the total integrated circuit area (which might be of the order approximately 3 by 3mm) . As monolithic integrated circuits incorporating RF circuit elements are manufactured from quite expensive materials, this is advantageous because it means there is only a small increase in the cost of the integrated circuit in providing this feature. Note that the integrated circuit may be a microwave monolithic integrated circuit. Note that although other feed structures on the integrated circuit would be encompassed by the present invention, conventional feed structures such as patches or monopoles would lead to a dramatic increase in size of the integrated circuit and are not preferred. Fig. 4 illustrates an assembly in accordance with an embodiment of the present invention, which includes horn antenna 30 and an integrated circuit 11. The integrated circuit 11 includes a coupling means 12 which is arranged to couple directly to corresponding coupling means 13 in the antenna 30 for coupling RF radiation to or from a RF circuit element 14 formed in the integrated circuit 11. As can be seen, the integrated circuit is mounted directly to the antenna 20, with no interconnecting substrate. The RF signal is instead coupled directly from the integrated circuit 11 to the antenna 30. Fig. 5 illustrates an assembly where the integrated circuit 11 is mounted on a mounting flange 40. As described in figures 1 to 4, the integrated circuit contains coupling means 12 which couples RF radiation through the flange aperture 43 and into whatever structure the flange is mounted against (not shown) . The flange is mechanically fixed to this structure by means such as screws through mounting holes 44. The integrated circuit may be secured to the waveguide resonant cavity, antenna or mounting flange by gluing, soldering or mechanically fitting the circuit to the outer surface, without the need for specialised RF bonding techniques. Assembly processes are therefore very simple. Further, there is no need for expensive substrates. The integrated circuit is attached directly to the metallic surface of the structure, in the preferred embodiment. Further, preferably there are minimal interconnections between the integrated circuit and the structure so that RF impedances are easy to calculate and control in production. RF insertion losses are also minimised. The waveguide, antenna or mounting flange may contain structures such as filters which modify the spectral composition of signals coupled from the integrated circuit. These filter structures can be used to increase the efficiency of which the integrated circuit aperture radiates into the waveguide, cavity, antenna or mounting flange. Finally, in the preferred embodiment, the structure (which is usually metallic or metallic matrix) provides excellent heat sinking for the integrated circuit. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive .

Claims: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. An integrated circuit comprising: an RF circuit element, and - an RF coupling means for coupling RF energy between the integrated circuit and an RF guiding structure, the coupling means including a ground plane referenced transmission line structure which traverses a region of the integrated circuit where a portion of the ground plane reference is absent. 2. An integrated circuit in accordance with claim 1 wherein the ground plane reference is formed by electrically conductive material and the absent portion is formed by an aperture in the electrically conductive material, which acts as a radiating element, for radiating coupled RF energy. 3. An integrated circuit in accordance with claim 2 wherein the aperture is formed by removing a portion of the electrically conductive material of the integrated circuit's ground plane to form a re-entrant feature in the outline of the ground plane. 4. An integrated circuit in accordance with claim 2 wherein the aperture is formed by removing a portion of the electrically conductive material of the integrated circuit's ground plane to form a polygon of more than four sides. 5. An integrated circuit in accordance with claim 2 wherein the radiating element is formed by removing a portion of the electrically conductive material of the integrated circuit's ground plane such that the resulting ground plane is separated into two or more distinct areas. 6. An integrated circuit in accordance with any preceding claim, wherein the transmission line structure includes a microstrip or co-planar waveguide. 7. An assembly comprising: an RF guiding structure, and an integrated circuit for transmitting or receiving RF signals in accordance with any one of the preceding claims, the integrated circuit being mounted such that it couples RF signals between the waveguide, cavity, antenna or mounting flange and integrated circuit through an aperture in a surface or wall of the RF guiding structure . 8. An assembly in accordance with claim 7, including a cap mounted to the RF guiding structure and enclosing the integrated circuit, the cap forming a cavity which is arranged to reflect RF energy propagating away from the RF guiding structure back towards it. 9. An assembly in accordance with claim 7 or claim 8, wherein the integrated circuit is mounted such that it directly abuts the surface or wall of the RF guiding structure. 10. A feed structure for feeding microwave signals to or from an RF guiding structure, the feed structure being formed in an integrated circuit and including a ground plane reference transmission line structure which traverses a region of the integrated circuit where a portion of the ground plane reference is absent. 11. A feed structure in accordance with claim 10, wherein the ground plane reference is formed by electrically conductive material and the absent portion is formed by an aperture in the electrically conductive material which acts as a radiating element, for radiating coupled RF energy. 12. A method of constructing a monolithic integrated circuit, comprising the steps of forming a feed structure in the monolithic integrated circuit, feeding RF signals directly to an RF guiding structure, for transmission or reception of RF energy from the RF guiding structure, the feed structure comprising a ground plane referenced transmission line structure which traverses a region of the integrated circuit where a portion of the ground plane reference is absent. 13. An RF coupling arrangement, comprising an RF circuit element, and an RF coupling means for directly coupling RF energy between the integrated circuit and an RF guiding structure, the coupling means including a ground plane reference transmission line structure which traverses a region of the integrated circuit where a portion of the ground plane reference is absent . 14. An RF coupling arrangement in accordance with claim 13 , wherein the ground plane reference is formed by electrically conductive material of the RF guiding structure, and the absent portion is formed by an aperture in the electrically conductive material, which acts as a radiating element, for radiating coupled RF energy.

Inventors: CUNNINGHAM SHAUN, JOHNSON MARK, CUNNINGHAM, SHAUN, JOHNSON, MARK
Applicants: COMMW SCIENT IND RES ORG, CUNNINGHAM SHAUN, JOHNSON MARK, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION, CUNNINGHAM, SHAUN, JOHNSON, MARK
Application year: 2000
Main classification: H01P5
IPC codes: H01P5