High-Selectivity Bandpass Filter in Glass Substrate RDL for Millimeter-Wave Applications

A highly selective bandpass filter (BPF) utilizing branch-loaded resonators (BLRs) is presented. The filter is implemented using a redistribution layer (RDL) based on a glass substrate. It consists of four unitized BLRs with electric and magnetic couplings between the resonators. By adjusting the size and couplings of BLR, the passband and four transmission zeros (TZs) of BPF can be readily controlled. An equivalent circuit is proposed and discussed to illustrate the working principle. To validate the design, a BPF prototype with a center frequency of 21 GHz is fabricated and tested. The measured results indicate that the fabricated BPF has a 3-dB fractional bandwidth of 15%. A roll-off of 7.17 dB/GHz is achieved in the low sideband, while a roll-off of 3.44 dB/GHz in the high sideband, resulting in sharp-rejection filtering. Over 30 dB of suppression is achieved from 0 to 15.65 GHz, while over 27 dB from 29.4 to 50 GHz. Including the pads, the fabricated BPF has a compact size of 0.185 λg × 0.151 λg.