Two new compact planar triple-mode resonators embedded with inductive or capacitive cross coupling are proposed for bandpass filter design. Both resonators consist of a shorted half-wave ring. In the first resonator, two shorted half-wave sections with inductive coupling are connected to the shorted via of the ring. In the second, two quarter-wave sections with capacitive coupling are connected. The inductive coupling is realized by a short grounded high-impedance segment, and the capacitive coupling is implemented by an interdigital capacitor. When both the inductive and capacitive coupling coefficients are properly realized, a transmission zero can be created at designated position in either upper or lower stopband. The shorted ring is accommodated inside the area of the other two resonance sections so that a compact area can be achieved. As compared with a conventional dual-mode ring resonator filter, the proposed resonators use only 46.7% and 22.5% in area. Two triple-mode resonator bandpass filters are fabricated and measured to validate the ideas. Measurement results have good agreement with the simulation responses.
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