A SWITCHABLE LOW-PASS FILTER

Abstract: A low-pass or band-rejection filter for microwave frequencies has a substantially planar structure and is constructed of a transmission line having inductor portions (10) and wider capacitor portions (11). The inductor portions are designed as linear microstrip elements having widths being varied by making areas (13) at the sides of the linear elements (10) superconducting. In changing the widths of the transmission line also the inductances thereof are changed accordingly. The areas (13) at the sides of the microstrip elements comprise rather narrow areas located directly at the central, normal metal conductor (9). These narrow areas (13) have in the non-superconducting state some electrical conductivity which can be small but still not quite insignificant in relation to that of the metal conductor. However, owing to the fact that they contact the normal metal conductor (9) only at very narrow edges instead of contacting it at a large surface they do not significantly affect the transmission characteristics of the transmission line in the normal state of the areas which can be made superconducting.

Description: A SWITCHABLE LOW-PASS FILTER TECHNICAL FIELD The present invention relates to a microwave filter to be used in microwave integrated circuits, in particular a band rejection or low-pass filter, BACKGROUND OF THE INVENTION AND STATE OF THE ART In transmission paths in microwave integrated circuits there is of course a need for filtering elements like in other electronic fields. In particular there may be a need for filters the characteristics of which can be varied, such as a filter having a filtering effect only for a specific state of a control signal. Very compact microwave filters can be built using high- temperature cuprate superconductors using e.g. planar stripline structures. Such filters are used in high-performance radio communication systems, e.g. as microwave receiving filters for radio base stations, in which filters having very sharp shirts and low insertion losses as well as small sizes and small weights are important. In the Japanese patent application JP 2/101801 a microwave band-rejection filter is disclosed having transmission lines designed as linear microstrip, metal elements placed on top of an area of a layer of the superconducting material. The superconducting material area has a pattern substantially agreeing with that of the metal conductor except in some regions where the width of the superconducting area is larger than that of the metal conductor. When the superconducting material is made to pass into a non-superconducting state, most of the electric current passes through the common metal material of the metal conductor whereas, in the superconducting state, the electrical current passes only through the superconducting underlying material. The elements thereby obtain a variable filtering effect. However, a disadvantage of this design resides in providing a region having some, though it may be low, electrical conductivity placed under the normal conductor, since this region causes losses in the transmission line. The conductivity of materials, which are superconducting at a low temperature and are suitable for microwave integrated circuits, have in their normal state an electrical conductivity corresponding to some 10 "3 to 10 "2 of the electrical conductivity of the material of the always normal metal conductor. SUMMARY It is an object of the invention to provide a switchable filter based on a microstrip transmission line for microwaves, the filter exhibiting low losses. Thus, a low-pass or band-rejection filter for e.g. microwave frequencies is designed as substantially planar structure and is constructed of transmission lines designed as linear microstrip elements which have widths which are varied by making areas at the sides of the linear elements superconducting. In changing the widths of the transmission lines also the inductances thereof are changed accordingly. The areas at the sides of the microstrip elements comprise rather narrow areas located directly at the central, normal metal conductor and are thus electrically connected thereto along at least portions of the sides or of the edges of the central microstrip elements. These narrow areas have in the non-superconducting state some electrical conductivity which can be small but still not quite insignificant in relation to that of the metal conductor. However, owing to the fact that they contact the central, always normal metal conductor only at very low or thin edges thereof instead of contacting it at a large surface they do not significantly affect the transmission characteristics of the transmission 5 path in the normal state of those areas which can be made superconducting. The transmission lines also comprise capacitance areas which contribute to their capacitance. The capacitance areas project laterally from central stem elements of the transmission lines and are portions of the central, normal metal conductor and are thus made from a normal electrically conducting material which can not be made superconducting at the considered temperatures. o BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of a non-limiting embodiment with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a planar, switchable microwave filter structure, Fig. 2 is a cross-sectional view of the structure of Fig. 1, and s Fig. 3 is a diagram of the insertion loss of a filter structure according to Figs. 1 and 2 as a function of the microwave frequency. DETAILED DESCRIPTION In the planar microstrip line element illustrated in Figs. 1 and 2 a dielectric substrate 1 is used having an electrically conducting ground layer 3, such as a metal layer of e.g. Cu, Ag 0 or Au, on its bottom surface, the ground layer covering substantially all of the bottom surface as a contiguous layer. On the top surface there is a patterned electrically conducting layer 5 suitably made of metal, e.g. of the same metal as the bottom layer, i.e. of copper, silver or gold. The patterned 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 an 5 outline comprising both a central stem path 9 having a uniform, rather narrow shape of width W 0 defining the propagation directions and further having lateral extensions 11 of length b, all having the same rectangular shape, extending laterally from the central stem, one extension being located opposite an identical one to form a larger rectangle having width W c . The lateral extensions are thus located symmetrically in relation to the axis of the central stem 0 and they are furthermore arranged with a uniform spacing along the stem, so that there is a gap length of 1 between the extensions 11, this gap length then being the length of the stem portions 10 between the extensions. This structure defines a cut-off frequency f cn of a microwave propagating along the filter. The cut-off frequency appears from the diagram of Fig. 3 illustrating the insertion loss s of the microstrip element of Figs. 1 and 2 as a function of the frequency of a microwave passing through the microstrip structure. The respective different portions of the structure mainly contribute to either the inductance L or the capacitance C thereof and thereby define the cut-off frequency f cn , since it generally is proportional to (LC) " ' Λ . Thus, the size of the lateral extensions 11 primarily defines the capacitance of the filter element and the narrow stem portions 10 of the central stem 9 between the extensions 11, in particular their width, primarily define the inductance L. The inductance L of the filter element is changed by adding electrically conducting areas or regions 13 directly at the side or sides of the normal conductor pattern 5 at selected places. These regions 13 are made of a superconducting material, preferably a high temperature superconducting material. The regions 13 are preferably located at both sides of the central stem portions 10. All of the electrical current will then pass, when these lateral superconducting areas 13 are in a superconducting state, only in these areas according to the Meissner effect which will reduce the inductance of the transmission path in the filter structure. In the normal state of the superconducting material of the lateral areas 13 these areas do not too much disturb the current distribution in the always normal central stem portions since in the normal state of the areas 13 they have, for typical high temperature superconductivity materials, an electrical conductivity σ n of about 5- 10 5 S/m to be compared to the electrical conductivity σ c of the material of metal areas 10, 11 comprising about 10 8 S/m. For a suitable choice of the resulting width W of stem portions 10 together with the superconducting regions 13 the inductance L of the filter element can be considerably reduced resulting in a higher cut-off frequency f cs , see Fig. 3. A switching between the superconducting state and the normal state of the regions 13 can be achieved in any conventional way, such as by varying the temperature, the magnetic field or a direct current level as to what is required or desired. This switching is symbolized by the control unit 15 shown in Fig. 1. A preferred way may be to have a control making an electrical current higher than the critical current of the superconducting material pass or not pass through the microstrip line. By letting always a fixed bias current, thus a direct current, pass through the line, the fixed bias current having an intensity slightly slower than that of the critical current, and adding or not adding thereto a small control current such as a current pulse, the reversible switching between the superconducting state and the normal state can be made extremely fast. Numerical simulation has indicated that the inductance L of a microstrip line can easily be reduced to half its value for a suitable width of the superconducting value. The corresponding relative shift of the cut-off frequency ((f cs - f cn )/f cn ) will then have an estimated value of about 40% .

Claims: CLAIMS 1. A filter structure for microwaves, characterized by a central microstrip line made of an electrically conducting material exhibiting no superconducting properties above a considered temperature and regions made of a material exhibiting superconducting properties above the considered temperature, the regions being located at sides of the central microstrip line and in the same plane as the central microstrip line. 2. A filter structure according to claim 1, characterized in that the regions have shapes of strips of uniform widths. 3. A filter structure according to claim 2, characterized in that all regions have a same width. 4. A filter structure according to any of claims 1 - 2, characterized in that the central microstrip line has lateral extensions extending from a central stem. 5. A filter structure according to claim 4, characterized in that the central stem has a substantially uniform width. 6. A filter structure according to any of claims 4 - 5, characterized in that all lateral extensions have substantially the same shape. 7. A filter structure according to any of claims 4 - 6, characterized in that the lateral extensions have substantially rectangular shapes. 8. A filter structure according to any of claims 4 - 7, characterized in that the lateral extensions are uniformly distributed along the central stem. 9. A filter structure according to any of claims 4 - 8, characterized in that the regions are placed at sides of portions of the central stem between the lateral extensions. 10. A filter structure according to any of claims 1 - 9, characterized in that the central microstrip line and the regions have such a shape that the filter structure is substantially symmetric about a longitudinal axis of the central microstrip line. 11. A filter structure according to any of claims 1 - 10, characterized by control means for making electrical currents flow through the regions, thereby bringing, when the filter structure is above the considered temperature and the regions are in a superconducting state, the regions to change to a non-superconducting state.

Inventors: ZHOU SHU-ANG, ZHOU, SHU-ANG
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1999
Main classification: H01P1
IPC codes: H01P1