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.