Compact High-Q Bandpass Filter Using 3-D Stacked Stripline

This article presents a novel compact high-Q bandpass filter (BPF) utilizing a 3-D stacked stripline configuration. T-shaped stepped impedance resonators (SIRs) are employed to achieve miniaturization. By folding the filter geometry from an inline arrangement into a U-shape along the broadside direction, both broadside and edge coupling structures are realized, enabling various cross-coupling schemes for flexible placement of transmission zeros (TZs). A comprehensive analysis of both electric and magnetic coupling structures is conducted to support the overall filter design. To validate the concept, a tenth-order general Chebyshev BPF prototype centered at 3.485 GHz with a 1 dB bandwidth of 380 MHz is designed, fabricated, and measured. The filter is constructed by vertically soldering two patterned sheet metal layers together with three stacked cavities. Despite having an electrical size of only 0.58 × 0.23 × 0.19 λ_g³, the filter exhibits a high unloaded Q-factor (Q_u) of 1200, along with up to six TZs and a spurious-free frequency range extending to 12 GHz. Measured results show an insertion loss of 0.58 dB at the center frequency and a return loss of better than 20 dB within the passband, demonstrating favorable agreement with simulations. Featuring solid electrical performance, the proposed filter is ideally suited for 5G and 5G-Advanced (5G-A) communication base stations.