Synthesis Method for Millimeter-Wave High-Selectivity On-Chip Filter in IPD Process Supported by Circuit Transformations

This article presents a synthesis method for on-chip fourth-order bandpass filters (BPFs) for 5G millimeter-wave (mmWave) applications. The method transforms the conventional $K$ -filter prototype into microstrip schematics comprising capacitively loaded microstrip (CLM) resonators and semi-lumped vias acting as impedance inverters. The approach enables rapid calculation of design parameters under varying bandwidth, center frequency, and harmonic suppression requirements. Two BPFs are designed and fabricated using the integrated passive device (IPD) process to validate the method. The first operates at 24 GHz with an insertion loss of 3.98 dB, a 3-dB fractional bandwidth (FBW) of 13.5% and 53-dB stopband attenuation. The second employs a cross-coupled structure at 26 GHz, achieving an insertion loss of 3.92 dB, a FBW of 12.5%, and transmission zeros (TZs) at 22.8 and 28.4 GHz, corresponding to a rectangular coefficient (20-dB BW/3-dB BW) of 1.53. The results validate the theoretical counterparts and demonstrate that the designed filters exhibit good passband selectivity and stopband suppression.