A HIGH FREQUENCY MULTI-PORT SWITCHING CIRCUIT

Abstract: This invention concerns a multi-port switching circuit which may operate at frequencies up to an even beyond 100 GHz. The invention may be implemented as a GaAs based monolithic microwave integrated circuit. The circuit comprises at least three ports arranged in a star configuration around a central ring. A single switching device is associated with each port. Each switching device is connected to two transmission lines to provide impedance matching and an interconnecting path around the ring. The transmission lines are initially chosen to have a length of a quarter wavelength at the centre of the band of operation of the switch. The matching lines and the lines which form the interconnecting ring are then subject to an optimisation procedure in which each pair of the switching devices is in turn modelled in their ON state while the remainder of the switching devices is modelled in their OFF state. The optimisation procedure aims to achieve low transmission and insertion loss, while providing good isolation.

Description: "A high frequency multi-port switching circuit" Technical Field This invention concerns a multi-port switching circuit. The embodiments of one realisation may operate at frequencies around 60 GHz; but with appropriate devices embodiments may operate at other frequencies including higher frequencies up to and even exceeding 100 GHz. Background Art Switching networks have been developed which operate at frequencies up to and exceeding 40 GHz. The switching elements in such networks use a combination of shunt passive FET devices and quarter-wave transformers, or combinations of series and shunt passive FET devices. Passive FET devices, in one type of switch, require bias to be applied to the gate and not between the source and drain. Broadband switches using a combination of active and passive switching elements have also been demonstrated. Summary of the Invention The invention provides a multi-port switching circuit, comprising at least three ports, interconnected by transmission lines. The transmission lines are arranged with a central ring and outwardly extending arms. The ports are positioned at the ends of respective arms. The term "ring" has been used in a loose descriptive sense and does not necessarily imply circularity. A switching device, such as a FET or HE T, is associated with each port. The switching device is arranged between a first and a second transmission line. Each switching device may be arranged to shunt the main signal path of the circuit with its main current path extending between the junction of the first and the second transmission line, and signal ground. The first transmission line extends between the port and the switching device to provide impedance matching, and the second transmission line also provides impedance matching and a connecting path to the ring. The first and second transmission lines are initially chosen to have lengths of a quarter wavelength at the centre of the band of operation of the switch. The dimensions of the matching lines and the lines which form the connections to the ring are then determined using a procedure to optimise the performance of the circuit. The optimisation procedure involves the selection of two of the ports as the input and output ports of the switching network. The switching devices associated with these ports are modelled by ON state representations. The other port, or ports, are isolated, and their associated switching devices are modelled in the OFF state. Optimisation of the transmission lines lengths and widths then aims to provide desired performance levels such as low transmission loss, good isolation at all other ports, low return loss or high power handling. Other parameters such as gate width and length and substrate thickness may also be optimised, but these parameters are usually predetermined by selection of a particular fabrication process for the switching circuit. The optimisation procedure continues by varying the signal flow in the circuit. That is, in the first step, the signals flow from a first port to a second port, with the other ports isolated; in the second step, signals flow from the second port to a third port with the other ports isolated. This process continues until a set of optimised parameters is established for each signal path configuration. The range of optimised parameters are then examined and a single best set of parameters is used to complete the design. The optimisation process uses conventional techniques and is able to take into account the effects of all the bends and discontinuities in the switch. The optimisation provides similar switching performance between any pair of ports, independent of the chosen input and output. The switching devices may be arranged symmetrically around the ring to simplify the optimisation process. However, symmetry is not a requirement. HEMTs (High Electron Mobility Transistors) may be used to provide operation at high microwave frequencies. The choice of switching device influences, amongst other things, the power-handling capability of the circuit. Any switching device may be chosen. Two terminal devices, such as diodes including PIN-diodes; three terminal devices such as generic field effect devices, for example the FET, MOSFET, MESFET and HEMT; and multi-terminal devices, such as dual gate devices, could all be used. Switching devices, such as HEMTs, may be modelled in their OFF state by a resistor and a capacitor in series, and in the ON state by a resistor and an inductor arranged in series. However, different and more complex models can be chosen. Switching action may be achieved by biasing a pair of HEMTs in their ON state to create the signal path, while biasing all other HEMTs in their OFF state. Bias is applied to the gate terminals of the HEMTs, the drain terminal is connected to the junction between the first and second transmission lines, and the source terminal is grounded. The OFF or low impedance state is achieved by applying a DC voltage of zero volts to the gate terminal. The ON or high impedance state is achieved by applying a DC voltage slightly greater than that required to pinch the device off. A feature of this circuit is that only a single switching device is required at each port as a result of optimising the performance of the network for low losses and high isolation. Thus the switching circuit offers the benefit of providing a multi-port interconnection requiring an equal number of switching devices equal to the number of switched ports. Embodiments of the multi-port switching circuit using HEMTs may operate in a frequency band around 60 GHz, and are able to provide all the usual switching functions, such as multiplexing at millimetre-wave (mm- wave) frequencies. Switching networks embodying the invention may be used in multifunction circuits to allow functionality to be re-configured by altering the control voltages on the switching devices to re-route the signal. A circuit containing an embodiment of the switching network may provide the ability to amplify a signal, up-conversion, down-conversion, or up and down conversion with amplification. Circuits embodying the invention may offer redundancy that enables continued operation after failure of a circuit connected to the switching circuit. For instance, if a switching circuit was arranged to interconnect a number of identical circuits such as transmit channels, or receive channels, failure in any particular channel can be overcome by altering the control voltages on the switching circuit to re-route the signal path. When the switching circuit is used to interconnect non-identical circuits, such as many transmit and receive circuits having different performance characteristics, then the switching circuit can be configured to use the transmit and receive circuits which have the most appropriate characteristics for the current conditions. For instance, if the transmit and receive circuits have performance characteristics which make them suitable for operation in different conditions then the switching circuit may be configured to use the transmit and receive circuits that are appropriate for the current conditions, and can be re-configured as conditions change. Multiple cascades of individual networks can be connected together to create complicated routing networks. The robustness of the multiple port configuration allows for redundancy in the design of interconnections between systems. Brief Description of the Drawings An example of the invention will now be described with reference to the accompanying drawings, in which: figure 1 is a layout of a three port switch embodying the invention; figure 2 is a graph showing the simulated signal response of the switching network of figure 1; figure 3 is a layout of a six port switch embodying the invention; figure 4 is a graph showing the simulated response of the switching network of figure 3; and figure 5(a) is an OFF state model of a HEMT that may be incorporated into a switch embodying the invention, and figure 5(b) is an ON state model corresponding to figure 5(a). Best Modes for Carrying out the Invention Referring to figure 1, three port switch 1 comprises three transistors 2, 3 and 4 each connected to a central ring 5 by means of respective transmission lines 6, 7 and 8. The transistors 2, 3 and 4 are each associated with a respective external port 9, 10 and 11 by means of respective transmission lines 12, 13 and 14. Transistor 2 has its source 15 at signal ground, its drain 16 connected to the transmission lines, and a gate 17. The terminals of transistors 3 and 4 have not been numbered, for the sake of brevity. In normal operation two of the switches are turned ON to select the input and output ports. Figure 2 shows the simulated magnitude responses when the switch is configured with input applied at port 9 and output taken from port 10; the magnitude responses for any two sets of ports is nominally identical. Curve 18 shows the simulated loss from the input port 9 to the output port 10 to be less than 2 dB at the center frequency of 61 GHz, and to remain less than 3 dB between 54 to 66 GHz. Curve 19 which shows the input match to be better than 20 dB at the centre frequency and remains good over a wide bandwidth; that is greater than 10 dB over 8 GHz of bandwidth. Curve 20 shows the isolation between the input port 9 and the isolated OFF port 11 to be better than 16 dB. Referring to figure 3, six port switch 30 comprises six HEMTs 31, 32, 33, 34, 35 and 36 arranged around a central ring 37. Each of the transistors is connected to the ring 37 by respective lengths of transmission line 38, 39, 40, 41, 42 and 43. The external connection ports 44, 45, 46, 47, 48, and 49 are connected to respective HEMTs by transmission lines 50, 51, 52, 53, 54 and 55. The transmission lines provide impedance matching, for both the signal transmission path and the isolated ports. Figure 4 shows the simulated magnitude response when the switch is configured with input applied at port 44 and output from port 47; the magnitude responses for any two sets of ports is nominally identical. Curve 56 shows the simulated loss from the input port 44 to the output port 47 is just over 3 dB at the center frequency of 61 GHz, and remains less than 4 dB between 57 to 66 GHz. Curve 57 shows the input match is better than 15 dB and remains good over a wide bandwidth; that is greater than 10 dB over 8 GHz of bandwidth. Curve 58 shows the isolation between the input port 44 and any of the OFF ports is better than 16 dB. Figure 5 shows the bi-state model of the two finger, fifty micrometer (ie, 2 by 25 μm fingers) wide HEMT used in this embodiment. In the OFF state shown in figure 5(a) the HEMT is biased at zero volts. In this state the HEMT is represented by a 3.2 ohm resistor and a 0.03 picoFarad capacitor arranged in series. In the ON state shown in figure 5(b), the HEMT is biased slightly beyond pinch-off. In this state the HEMT is represented by a 23.4 ohm resistor and a 3 nanoHenry inductance arranged in series. The switch is optimised using the bi-state model for a stated set of performance parameters in order to produce the required performance. Any of the parameters can, of course, be traded against other parameters to achieve different levels of performance that may be required by different applications; for instance input match could be traded against power handling capability. If the circuit were connected to a number of different circuits having different performance characteristics then it could be optimised accordingly. Although the invention has been described with reference to a particular embodiment, it should be appreciated that the invention could be embodied in many other forms. For instance, there is no limit on the number of ports which can form the switching network, symmetry is not a requirement for the operation of the network, and operation is not limited to particular process technologies or geometries for the active devices. Besides GaAs fabrication technology the invention is applicable to Si and InP processes, among others. Although this invention has been described with reference to a switching circuit which operates at about 61 GHz and it is believed to be iseful at much higher frequencies, it should also be understood that the invention may be Liseful in lower frequency switches. 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: CLAIMS: 1. A multi-port switching circuit, comprising: at least three ports interconnected by transmission lines, the transmission lines are arranged with a central ring and outwardly extending arms, and the ports are positioned at the ends of respective arms; a switching device is associated with each port, and each switching device is arranged between the junction of a first and a second transmission line, and the first transmission line extends between the port and the switching device to provide impedance matching, and the second transmission line provides impedance matching and a connecting path to the ring. 2. A multiport switching circuit according to claim 1, wherein there is a single switching device associated with each port. 3. A multiport switching circLiit according to claim 1 or 2, wherein the single switching device associated with each port is arranged to shunt the main signal path of the circuit with the main current path of the switching devices extending between the junction of the first and second transmission lines, and signal ground. 4. A multi-port switching circuit according to any preceding claim, wherein the switching devices are HEMTs. 5. A multi-port switching circuit according to any preceding claim, wherein the switching devices are arranged symmetrically around the ring. 6. A multi-port switching circuit according to any preceding claim, wherein the dimensions of the matching lines and the lines which form the connections to the ring are determined using a procedure to optimise the performance of the circuit, as follows: the first and second transmission lines are chosen initially to have lengths of a quarter wavelength at the centre of the band of operation of the switch; two of the ports are selected as the input and output ports of the switching network; the switching devices associated with these ports are modelled by ON state representations; the other port, or ports, are isolated, and their associated switching devices are modelled in the OFF state; and the transmission lines lengths and widths are then adjusted to achieve selected performance parameters. 7. A multi-port switching circuit according to claim 6, wherein the procedure continues by varying the signal flow in the circuit; in the first step, the signals flow from a first port to a second port, with the other ports isolated; in the second step, signals flow from the second port to a third port with the other ports isolated; and this process continues until a set of optimised parameters are established for each signal path configuration; the range of optimised parameters are then examined and a single best set of parameters are used to complete the design. 8. A multi-port switching circuit according to claim 7, wherein the switching devices are modelled in their OFF state by a resistor and a capacitor in series, and in their ON state by a resistor and an inductor arranged in series. 9. A nmlti-port switching circuit according to claim 4, wherein the switching action is achieved by biasing a pair of HEMTs in their ON state to create the signal path, while biasing all other HEMTs in their OFF state; bias is applied to the gate terminals of the HEMTs, the drain terminal is connected to the junction between the first and second transmission lines, and the source terminal is grounded; the OFF or low impedance state is achieved by applying a DC voltage of zero volts to the gate terminal; and the ON or high impedance state is achieved by applying a DC voltage slightly greater than that required to pinch the device off. 10. A multi-port switching circuit substantially as herein described with reference to the accompanying drawings. 11. A transceiver incorporating a multi-port switching device according to any preceding claim.

Inventors: GIUGNI STEVE, GIUGNI, STEVE
Applicants: COMMW SCIENT IND RES ORG, GIUGNI STEVE, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION, GIUGNI, STEVE
Application year: 1998
Main classification: H01P1
IPC codes: H01P1