New miniaturized ring resonator bandpass filter with wide upper stopband

A new configuration for ring resonator is proposed for bandpass filter design with size reduction and stopband extension. The planar self-coupled ring is twisted at its middle and a short CPW section is implemented in the ground plane for the three-dimensional interconnection. The resonator has two resonance modes and the filter possesses one transmission zero on the high side of the passband. With a pair of open low-impedance stubs attached to the ring, the circuit has an area of only 9.65% of that of the conventional ring resonator filter, and the upper stopband is extended up to higher than five times the center frequency. Measured results of a fabricated prototype circuit show good agreement with the simulation.