Coupling structure, resonator excitation structure and filter for coplanar-waveguide circuit

#The present invention generally relates to a coupling structure, a resonator excitation structure and a filter mainly used for microwave or millimeter-wave band coplanar-waveguide circuits.

#In the prior art, two kinds of couplings are known as a resonator excitation structure at input/output of coplanar-waveguide circuits such as filters. One is capacitive coupling where an open end of an exciting-line is close to a resonator. The other is inductive coupling where an exciting line is directly connected to a resonator.

#FIG. 1 is a plan view of an excitation structure employing a conventional capacitive coupling (See Non-patent Document #1). A coplanar-waveguide circuit 1 includes an exciting line 4 longitudinally running at the center thereof. An end of the exciting line 4 is extended laterally like a T-shape. The T-shape portion of the exciting line 4 faces a T-shape portion of a resonator 6 via a gap to form an excitation portion 5. The sides of the coplanar plane circuit 1 are covered with corresponding ground conductors 2, 3.

#FIG. 1 is a plan view of an excitation structure employing a conventional capacitive coupling;

#FIG. 2 is a plan view of an excitation structure employing a conventional inductive coupling;

#FIG. 3 is a plan view of an excitation structure employing a conventional inductive coupling;

#The following is a description of embodiments of the present invention, with reference to the accompanying drawings.

#Throughout all the figures, members and parts having the same or similar functions are assigned the same or similar reference signs, and redundant explanations are omitted.

#FIG. 6 shows plan views of an excitation structure according to a first embodiment of the present invention. A coplanar-waveguide circuit 1 shown in FIG. 6(a) has ground conductors 2, 3 at corresponding sides. An exciting line 4 as a signal input/output line is provided at the central area of the coplanar plane circuit 1 in order not to generate undesired transmission modes or propagation modes in a shielded waveguide housing the circuit substrate. In this embodiment, a circuit to which the exciting line 4 is connected is a quarter-wavelength spiral resonator 6. An end of the exciting line 4 is folded L-shape like and short-circuited to the ground conductor 2 at non-short-circuit side of the resonator 6. This short-circuit line faces a charge concentrated portion of the resonator 6 via a gap having a width α, to form an excitation portion 5 using inductive coupling. A strength of the external coupling is determined by factors such as the gap width α, a length β of the short-circuit line of the exciting line 4, and a distance s between the short-circuit line of the exciting line 4 and the ground conductor 2.