Compact Multilayered Bandpass Filter With High Selectivity and Four Transmission Zeros Using a Feedback Capacitor

In this article, a compact high selectivity bandpass filter with four transmission zeros (TZs) based on multilayer liquid crystal polymer (LCP) is proposed. A symmetric LC feedback structure is innovatively integrated into the conventional filter topology using the design of experiment (DOE) method, introducing two additional TZs in the stopband to significantly improve frequency selectivity and harmonic suppression. For compact implementation, a three‐dimensional (3D) integration scheme is employed on the LCP substrate, utilizing vertically stacked spiral inductors and interdigital capacitors to achieve a miniaturized dimensions of 7.8 × 7.2 × 0.193 mm 3 . Measured results demonstrate a center frequency of 3.6 GHz, with the insertion loss (IL) of 1.43 dB and the return loss (RL) better than 23 dB in the passband. Moreover, the optimized feedback structure introduces four TZs at 2.18/2.88/4.39/5.21 GHz, ensuring superior out‐of‐band rejection. This improved design provides a promising solution for the miniaturized design of front‐end modules for modern wireless communication systems.