1. Field of the Invention
The present invention relates generally to strip-line-type circuits, more particularly to loop transmission lines as shunt capacitors used in such circuits.
2. Description of the Related Art
Capacitors are one of the basic building blocks for electronic and microwave circuits. In microwave engineering, strip-line-type circuits, including microstrip, strip-line, and multi-layer circuits, can use large metal conductor patches to approximate shunt capacitors. Such patch capacitors can be found, for example, in bias networks of amplifiers, microstrip low pass filters, and matching networks.
As with parallel plate capacitors, the capacitance realized from conductive patches on a microstrip circuit is directly proportional to the area of the patches and the dielectric constant of the substrate. Examples of microstrip patch capacitors are shown in FIG. 1. These patch capacitors can occupy a significant amount of surface area, depending on the amount of capacitance required and the type of the substrate used. Use of conductive patch shunt capacitors thus places significant limitations on the layout flexibility and minimum sizes of circuits.
It is thus desirable to construct shunt capacitors that offer more layout options or potential for more compact circuit design or both. The present invention is directed to achieve one or more of these goals.
In accordance with the principles of the invention, a strip-line-type circuit includes a shunt capacitor that includes a closed conductive loop. The circuit may further include a transmission line connected to the closed conductive loop. The transmission line may be connected to the closed conductive loop at two nodes, in which case the closed conductive loop is divided into two segments, connected in parallel at the two nodes. The impedance of one of the two segments may be substantially larger than the impedance of the other segment, as in the case, for example, where one segment is substantially longer than the other.
The closed conductive loop may be a layer of conductive thin-film pattern formed on a layer of dielectric material, including a loop made of a superconductor such as YBa2Cu3O7-d (YBCO) formed on a magnesium oxide, sapphire or lanthanum aluminate substrate.
The circuit may be a multi-layer circuit in which the closed conductive loop extends to multiple layers of conductive patterns.
The closed loop may take on a variety of shapes, including circular, rectangular and swirl shapes.
More particularly, the circuit may be a filter that includes an inductor with each of its ends connected to a closed conductive loop that acts as a shunt capacitor. The filter may include multiple inductors connected in series, with the junctions between the inductors connected to shunt capacitors realized by closed conductive loops.
The filter may be constructed from a variety of materials, including the above-listed examples. For example, the filter may a band-stop filter having five or more poles constructed from YBCO film on a magnesium oxide substrate no larger than about 50 mm in any dimension.