A SWITCHABLE MICROWAVE DEVICE

Abstract: A planar microwave device such as a transmission line comprises a patterned electrically conducting layer located on a dielectric layer (1) of uniform thickness. A central microstrip line (9) is enclosed by lateral (11) strips which are located on each side of the central strip and are connected to the central strip by bridge portions (15) to form windows (13). Each lateral strip has an interrupt region (17) located between the bridge regions. The interrupt regions can all be switched between an electrically conducting state and an electrically non-conducting or high resistance state. The state changes will affect the characteristic impedance of the transmission line in order to make it work e.g. in a transistor-like fashion or a switchable filter or inductor. The electrically conducting material can be a superconductor and the interrupt regions then contain Josephson junctions or superconducting weak links. Furthermore, the electrically conducting material can instead be a normal conductor and the interrupt regions can then comprise a photoconductive material.

Description: A SWITCHABLE MICROWAVE DEVICE TECHNICAL FIELD The present invention relates to a switchable microwave device to be used in microwave integrated circuits, in particular for the control of the propagation of 5 microwaves in a stripline or similar transmission line. BACKGROUND When building microwave integrated circuits the propagating microwaves have to be controlled in different respects. Thus, U.S. patent 5,770,546 discloses superconducting bandpass filters, the o characteristics of which are modified by exerting a mechanical force or stress or varying a magnetic field. U.S. patent 5,585,330 discloses a method of varying the resistance of a superconducting stripline by controlling the superconducting characteristics thereof. In the published International patent application WO 00/04603 an inductor for primarily microwave frequencies is disclosed which comprises a transmission line s designed as a linear microstrip element made of a central line comprising normal electrically conducting material, such as a suitable metal. The microstrip element has a width which is varied by making areas at the sides of the central line superconducting. In changing the effective width of the microstrip the inductance thereof is changed accordingly. The areas at the sides of the microstrip element are located directly at the o central, normal metal conductor. In the published International patent application WO 00/04602 a low-pass or band-rejection filter for e.g. microwave frequencies is disclosed which is constructed in a way similar to that of the inductor disclosed in the cited International patent application WO 00/04603. SUMMARY 5 It is an object of the invention to provide a microwave device such as a transmission line having a characteristic impedance which can be switched between different values depending on a control signal. The problem to be solved by the invention is how to construct a microwave device, in particular a planar microwave device built in basically the same wavy as planar circuits 0 for ordinary electrical signal processing, which device has a characteristic impedance that can be changed between different values, the difference between the values in particular being so large that a modulation of a microwave can be produced or even a switching function can be obtained. Thus generally, a microwave device such as a transmission line comprises a micro- 5 strip line having an impedance which can be controlled to have a value which can be one of at least two different values. Thereby the device will have transistor/amplifier characteristics. The impedance can be controlled in different ways by changing the physical geometry of the stripline, by changing it in a mechanical or other way and in particular by making electrical interrupts in the outer regions of the stripline. The transmission line is thus planar type and it has a central microstrip line. Lateral strips are located on each side of the central strip and the different strips are all made of an electrically conducting material. In a preferred geometrical configuration the lateral strips are separated from the central microstrip line by windows which in the longitudinal direction of the central microstrip line are closed by bridge regions. Each lateral strip has an interrupt region located between the bridge regions and the interrupt regions can all be switched between an electrically conducting state and an electrically non-conducting or high resistance state. In a first case the electrically conducting material is a superconducting material and the interrupt regions then contain Josephson junctions or superconducting weak links. In a second case the electrically conducting material is a normal conductor and the interrupt regions comprise a photoconductive material. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of non-limiting embodiments with reference to the accompanying drawings, in which: - Fig. 1 is a diagram showing graphs of the voltage-current characteristics of a transmission line for varying values of the characteristic impedance, - Fig. 2 is a perspective view of a planar, switchable microwave transmission line having an impedance that can be changed between two different values, - Fig. 3 is a side view of another embodiment of a planar, switchable microwave transmission line, and - Fig. 4 is a perspective view of a planar, switchable microwave transmission line having an impedance that can be changed between three different values. DESCRIPTION OF PREFERRED EMBODIMENTS A piece of microstrip line or of other types of striplines has a characteristic impedance Z 0 given by R + iωL Z ft = = Z, G + iωC OR iZ 01 (1) where R, G, L and C are the line resistance, conductance, inductance and capacitance respectively of the stripline. It is observed that for low frequencies the impedance Z 0 mainly depends on the resistive properties of the line whereas for high frequencies such as microwave frequencies the impedance mainly depends on the inductive and capacitive characteristics of the line, i.e. ^ = - (2) C Thus, by controlling the inductance and/or capacitance of a stripline, the characteristic impedance Z 0 can be changed, resulting in a variation of voltage-current characteristic curves as is illustrated in Fig. 1. A specific change of the absolute value of the characteristic impedance can be obtained by properly designing the device. For example, in the cited International patent applications a change of the inductance of the order of 1 :2 is accompanied by some small change of the capacitance resulting in a change of the impedance of about \ β « 1.4 which can be insufficient for many 5 applications. However, in the particular structure of a microstrip device comprising windows to be described hereinafter, by a suitable design relatively large changes of the characteristic impedance can be obtained. In the planar microstrip line element illustrated in Fig. 2 a dielectric substrate 1 is used having an electrically conducting ground layer 3, such as a metal layer of e.g. Cu, 10 Ag or Au, on its bottom surface, the ground layer covering substantially all of the bottom surface as a continuous layer. On the top surface there is a patterned electrically conducting layer 5. In a first case the patterned electrically conductive layer is a superconductor and in a second case it is made of a normal electrically conductive material, e.g. of a metal such as the same metal as the bottom layer, i.e. of copper, silver or gold. The pattern- 15 ed layer 5 forms a transmission or propagation path intended for microwaves travelling e.g. in the direction of the arrows 7. The patterned layer 5 has a central stem path 9 having a uniform width W j defining the propagation direction. Furthermore, it has lateral strips 11 , both having uniform widths and extending in parallel to the central stem, one lateral strip 11 being located on one side of the central stem path and the other lateral 20 strip being located on the opposite side of the central stem 9. The lateral strips 11 are preferably located symmetrically in relation to the axis of the central stem 9. The lateral strips 11 are separated from the central stem by windows 13 in the patterned layer which thus also are stripshaped and have a uniform width. The distance from an outer edge of a lateral strip to the outer edge of the opposite lateral strip is W 0 which is also the effective 25 width of the waveguide when the lateral strips are operative to guide an electromagnetic wave. The windows 13 have the length b and are at their ends closed by transverse bridge portions 15 extending from the central stem path 9 to the lateral strips 11. The lateral strips have at their central portions, located centrally in relation to the bridge portions 15, 30 fields or gap regions 17 made in a special way or made from another, specially selected material. The special regions 17 are in the first case configured to form Josephson junctions or superconducting weak links and comprise in the second case a photoconducting material. In the first case, a localized magnetic field can be generated at the special regions 35 17 by conductmg an electric current at the regions to form a superconducting current loop, as symbolized by the device 19 in Fig. 2. The current can be generated by a voltage supply 21 connected to the device 19 through a resistor R and a switch 23 controlled by some supervising or signal control circuit, not shown. The Josephson junctions or superconducting weak links in the special regions 17 are controlled to take a normal or a superconducting state by this current and thereby by the supervising or control circuit. Instead of the control by means of a superconducting current loop the temperature of the special regions can be controlled, the device 19 then being e.g. a resistive heating element. In the second case a light source 25 such as a suitable semiconductor laser is arranged to emit light onto the special regions 17 which are then made of a photoconductive material, see Fig. 3. The light source 25 is connected to and controlled by some supervising or signal control circuit, not shown. When the light source 25 is energized, it will electrically connect the ends of the respective lateral strip 11 to each other and make the strip a contiguous or continuous electrical path whereas when it is not energized the lateral strip 11 will have an electrical interruption at the special region 17. An electromagnetic wave or microwave may propagate along the transmission line structure as shown in Figs. 2 and 3. When the special regions 17 act as electrical interrupts, the AC current in the structure due to the microwave will be confined in the cen- tral strip 9 of width W j . When the special regions act as electrical connections, most of the AC current derived from the microwave will instead flow in the lateral strips 11 owing to the skin effect, the outer edges of the lateral strips having a distance of W 0 . The inductance per unit length of a microstrip line is mainly determined by the total width W of the line, e.g. being approximately inversely proportional to the width W, i.e. approxi- mately proportional to 1/W, provided that the height of the microstrip line 5 to its ground plane 3 is fixed. Thus, by changing the states of the gap regions 17 to make the lateral strips either continuous or interrupted, the inductance L of the microstrip line will also be changed. Besides, the capacitance C per unit length of the transmission line will be changed, the capacitance being approximately proportional to the width W of the trans- mission line, provided that the height of the transmission line 5 to its ground plane 3 is fixed for a given dielectric material of the substrate 1. Thus, the characteristic impedance of the transmission line can be changed by changing the effective width W of the line such as by producing electrical interrupts in the special regions 17 as described above. Since the power of a microwave propagates along the planar structure as described above, the structure can be used to modulate the microwave by varying the characteristic impedance which is in turn given the active width of the structure as controlled by a supervising or signal controlling circuit. Furthermore, since the power of the control signals required to change the state of for example Josephson junctions or superconducting weak links in the special regions 17 can be much smaller than the electric power of a microwave propagating along the structure, the structure will have an amplifying function similar to that of a transistor. The structure illustrated in Fig. 2 can generally have a plurality of lateral strips symmetrically located at each side of the central strip as illustrated in Fig. 4. Each lateral strip 11 will then have a special or interrupt region 17, the special regions 17 of a pair of two symmetrically placed lateral strips being controlled to simultaneously adopt a conducting or a non-conducting or high resistance state.

Claims: 6 CLAIMS 1. A device comprising a planar transmission line for microwaves having variable transmission characteristics, characterized by a central microstrip line and lateral strips made of an electrically conducting material, the lateral strips being separated from the central microstrip line by windows closed in the longitudinal direction of the central microstrip line by bridge regions electrically connecting the central microstrip line and the lateral strips, each lateral strip having an interrupt region located between the bridge regions, the interrupt regions being capable of being switched between an electrically conducting state and an electrically substantially non-conducting or high resistance state. 2. A device to claim 1, characterized in that the electrically conducting material is a superconducting material and that the interrupt regions comprise a Josephson junction or a superconducting weak link. 3. A device according to claim 1, characterized in that the interrupt regions comprise a photoconductive material. 4. A device according to claim 1, characterized in that the interrupt regions are through control devices connected to control circuits to have an amplifying or modulating function.

Inventors: SHOU SHU-ANG, SHOU, SHU-ANG
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 2000
Main classification: H01P3
IPC codes: H01P1, H01P3