| CPC H01P 1/20 (2013.01) [H01P 1/203 (2013.01); H01P 1/20381 (2013.01); H01P 7/08 (2013.01); H05K 1/0353 (2013.01); H05K 1/115 (2013.01); H05K 2201/0141 (2013.01)] | 15 Claims |

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1. A three-dimensional (3D) filter, comprising:
a circuit board comprising a first circuit layer, a second circuit layer and a third circuit layer, wherein the second circuit layer is located on the first circuit layer, and the third circuit layer is located between the first circuit layer and the second circuit layer;
a first ring resonator disposed in the first circuit layer of the circuit board, wherein the first ring resonator has a first ring surrounded area corresponding to a first cut-off frequency band;
a second ring resonator disposed in the second circuit layer of the circuit board, wherein the second ring resonator has a second ring surrounded area corresponding to a second cut-off frequency band; and
a via structure passing through the first circuit layer, the second circuit layer and the third circuit layer, and electrically connected to the first ring resonator and the second ring resonator;
wherein the first ring surrounded area of the first ring resonator is different in size from the second ring surrounded area of the second ring resonator, and the first cut-off frequency band is different in frequency from the second cut-off frequency band; the first ring resonator corresponds to one of a higher cut-off frequency band and a lower cut-off frequency band, and the second ring resonator corresponds to the other one of the higher cut-off frequency band and the lower cut-off frequency band.
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