Balanced dual-band BPF with intrinsic common-mode suppression on double-layer substrate

A balanced dual-band bandpass filter (BPF) with enhanced common-mode (CM) suppression is proposed. The dual-band differential-mode (DM) bandpass response is obtained using a double-layer substrate. The stub-loaded stepped-impedance resonators are used both in the top and the bottom layers to constitute a dual-band BPF with two controllable passbands. The slotline portion in the middle layer is excited under DM operation. Moreover, the even-symmetrical field along the slotline is highly attenuated under CM excitation, thus achieving excellent CM suppression over the desired DM passbands. To verify the proposed concept, a balanced dual-band BPF, operating at 1.9 and 2.8 GHz, is designed and fabricated, and the simulated and measured results are in good agreement.