Integrated millimetre-wave transceiver

Abstract: An integated millimetre-wave transceiver comprising a substrate on which is provided an annular slot antenna (D) and a balanced mixer (F). Transmitter power is applied to one point of the antenna and the balanced mixer (F) is coupled to a second point of the antenna, the second point being orthogonal to the one point in the plane of the antenna. The coupling of the balanced mixer to the second point is by way of a coplanar waveguide (C). A short circuit (H) in the coplanar waveguide (G) at a quarter wavelength from the mixer prevents an even (asymmetric) mode on the coplanar waveguide from being propagated to the antenna. An IF signal is derived using an RF band stop filter (I).

Description: The invention relates to a planar circuit for a millimetre-wave continuous wave (CW) transceiver especially for use in radar. With pulsed radars, the transmitter sends out periodic pulses and during the interpulse period the transmitter is switched-off and a receiver is switched-on to receive energy reflected by objects in the path of the transmitted beam. In the case of continuous wave radar there is simultaneous transmission and reception of energy by way of one and the same antenna. In order to separate the signals a magnetic circulator is provided having an input port connected to an RF source, an output/input port coupled to an antenna and an output port for the received signal. The received signal is applied to a mixer in which it is mixed with a local oscillator signal derived by coupling-out a portion of the signal from the RF source. A disadvantage of this known arrangement is that the circuit, particularly the magnetic circulator, cannot be fabricated in monolithic technology. An object of the present invention is to be able to make a monolithic CW transceiver. According to the present invention there is provided an integrated millimetre wave transceiver comprising a substrate on one surface of which a conductive pattern forms first and second coplanar waveguides integrated with a coplanar annular slot antenna, the first coplanar waveguide extending to a first point on the periphery of the annular slot antenna for feeding r.f. power to the antenna for transmission, the second coplanar waveguide extending from a second point on the periphery of the annular slot antenna for coupling-out a signal from the antenna corresponding to received r.f. radiation, said second point being orthogonal to said first point in the plane of the antenna whereby the received signal is separated from the transmitted signal by a difference in polarisation, and a balanced mixer which is coupled to said second point by the second coplanar waveguide. Such a circuit is suitable for GaAs monolithic microwave integrated circuit (MMIC) implementation and contains components to allow the simultaneous transmission and reception of signals. This is achieved by feeding the antenna in a cross-polarised manner and using the cross polarisation as a means to separate the transmitted and received signals. Annular slot antennas are known from US Patent 3,665,480 (FASSETT) but the antennas described therein all comprise stripline or microstrip structures which require at least a pair of conductors spaced from each other in a different plane to that of the antenna. A coplanar structure is used in accordance with the invention for both the annular slot antenna and its waveguides. A coplanar configuration is an important structure for millimetre-wave MMIC work. Its truly planar construction results in simpler GaAs slice processing with good circuit yields since a ground plane is not required on the substrate's second surface and the substrate itself can be made thick. These features also benefit the RF performance. Many of the problems that are associated with microstrip such as high circuit losses and surface wave effects are less serious. Furthermore, coplanar waveguides are able to support two modes of propagation (one of which has a zero cut-off frequency) and this gives considerable circuit design scope. If desired the balanced mixer may be coupled to the second point by an odd mode coplanar waveguide. A short circuit may be provided in the coplanar waveguide coupling the antenna to the balanced mixer at a point a quarter of a wavelength from the mixer. The short circuit prevents an even mode excited by a local oscillator from propagating to the antenna. The balanced mixer may comprise a pair of coplanar Schottky barrier diodes which can be integrated. The IF signal from the balanced mixer may be derived using an R.F. stop band filter. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which :- Figure 1 is a plan view of a planar circuit which comprises a transceiver made in accordance with the invention, and Figure 2 is an end view of the substrate as viewed from the lower part of Figure 1. The transceiver shown in the drawings comprises an insulating substrate S having a conductive layer 10 forming a ground plane provided on one surface thereof. In the conductive layer an annular slot antenna is formed as a square coplanar patch antenna D. The patch antenna D comprises opposite pairs of slots D1, D2 and D3, D4, which form a structure having more equal E and H plane polar diagrams than a single slot. R.F. power is coupled to a mid-point of the slot D1 and is radiated normal to the coplanar patch, that is the substrate, by the slots D1, D2 with a polarisation which is in line with the feed (vertical). Horizontally polarised received signals are conveyed from a mid-point of the slot D4 to a balanced mixer F. RF power from a source 12 is conveyed along a slot A to a power splitter formed by a transition T. The transmitter power is coupled to the odd (symmetric) mode of coplanar waveguide C which feeds the slots D1, D2 of the patch antenna D. The remainder of the R.F. power constituting a local oscillator signal is conveyed in slot line E to a balanced mixer F. The balanced mixer F comprises a pair of mixer diodes 14, 16, for example coplanar Schottky barrier diodes. The local oscillator signal excites an even (assymetric) mode on the coplanar waveguide G. This is prevented from propagating to the antenna D by a short circuit at H which is spaced a quarter of a wavelength from the balanced mixer F. The short circuit at H and those at B1 and B2 ensure that only the odd mode is allowed to propagate along their respective coplanar waveguides and that ground plane continuity is preserved around the edge of the patch. Since the odd mode cannot be supported on slot line E, the signal goes into the diodes 14, 16. The IF signal goes out through the centre conductor of the coplanar line G. The IF will be in the range from a few kilohertz up to a few megahertz. The mixer is sensitive to signals that are received in a horizontal sense, i.e. in line with the coplanar line G feed to the mixer (F) and cross-polarised to the transmitter. This provides isolation between transmitted and received signals. The IF frequency is extracted from the coplanar waveguide C by an RF stop-band filter I which in the illustrated embodiment comprises three sections 18, 20, 22 each having a length of a quarter of a wavelength of the RF frequency. Sections 18 and 22 constitute low impedances and the intermediate section constitutes a high impedance. The advantage of this circuit is that it is suitable for monolithic integration onto a single GaAs chip whose substrate is shown at S or as a hybrid circuit. It contains nearly all the RF components for a CW radar transceiver to give a good performance at millimetre wave frequencies, for example 94 GHz. The chip could be positioned at the feed of a parabolic dish or focus of a lens to make a compact system. A circular polariser could be positioned between the circuit's antenna and the dish or in conjunction with the lens so as to allow crossed circular transmit and receive polarisation. It has been stated above that the drawings illustrate an embodiment of the invention and, in order to avoid any misunderstanding, it is hereby further stated that, in the following claims, where technical features mentioned in any claim are followed by reference signs relating to features in the drawings and placed between parentheses, these reference signs have been included in accordance with Rule 29(7) EPC for the sole purpose of facilitating comprehension of the claim, by reference to the example.

Claims: An integrated millimetre wave transceiver comprising a substrate (S) on one surface of which a conductive pattern (10, 18, 20, 22, etc) forms first and second coplanar waveguides (C & G) integrated with a coplanar annular slot antenna (D; D1, D2, D3, D4), the first coplanar waveguide (C) extending to a first point on the periphery of the annular slot antenna (D; D1, D2, D3, D4) for feeding r.f. power to the antenna (D) for transmission, the second coplanar waveguide (G) extending from a second point on the periphery of the annular slot antenna (D; D1, D2, D3, D4) for coupling-out a signal from the antenna (D) corresponding to received r.f. radiation, said second point being orthogonal to said first point in the plane of the antenna whereby the received signal is separated from the transmitted signal by a difference in polarisation, and a balanced mixer (F; 14, 16) which is coupled to said second point by the second coplanar waveguide (G). A transceiver as claimed in Claim 1 for a continuous wave radar, characterised in that the balanced mixer (F) is also coupled to the first coplanar waveguide (C) for mixing portions of the received signal and the transmitted signal to provide an IF signal for the radar. A transceiver as claimed in Claim 1 or Claim 2, wherein the coplanar waveguides (C & G) comprise odd mode coplanar lines. A transceiver as claimed in Claim 1, 2 or 3, in which a short circuit (H) is provided in the second coplanar waveguide (G) at a point a quarter of a wavelength from the balanced mixer (F). A transceiver as claimed in any one of Claims 1 to 4, in which the balanced mixer (F) comprises a pair of coplanar Schottky barrier diodes (14 and 16). A transceiver as claimed in any one of Claims 1 to 5, in which the balanced mixer (F) is coupled to the first coplanar waveguide (C) by a slotline (E) in an area of the conductive pattern which forms ground planes (10) of the coplanar annular slot antenna (D), first coplanar waveguide (C) and second coplanar waveguide (G). A transceiver as claimed in any one of Claims 1 to 6, in which an R.F. stop band filter (I; 18, 20, 22) is coupled to the balanced mixer (F) for deriving an IF signal. A transceiver as claimed in any one of the Claims 1 to 7, in which the substrate (S) of the coplanar antenna (D) and first and second waveguides (C and G) is a GaAs chip on which the balanced mixer (F) is integrated.

Inventors: EVANS DAVID HILLARY C O PHILIP, EVANS, DAVID HILLARY C/O PHILIPS COMPONENTS
Applicants: PHILIPS ELECTRONICS UK LTD, PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN, PHILIPS ELECTRONICS UK LIMITED
Application year: 1988
Main classification: H01Q25
IPC codes: H01Q1, G01S7, H04B1, H01Q25, H01Q23