Design of Millimeter-Wave On-Chip Bandpass Filters by Integrating the Lumped and Distributed Parameters

In this paper, the design method of millimeter-wave (mm-wave) on-chip bandpass filters is proposed by integrating the lumped and distributed parameters. As for the lumped part, a quasi-lumped resonator is designed by combining the interdigital capacitor and microstrip section inductor on both sides. Therefore, the employed resonator is inherently compact. As for the distributed part, the mutual coupling is introduced between the quasi-lumped resonators, indicating the proposed bandpass filter is not a simply lumped filter. In particular, the mutual coupling is a mixed electric and magnetic coupling, and two close-to-passband transmission zeroes (TZs) are accordingly generated with the proposed on-chip filter. For demonstration, a 28-GHz second-order BPF is designed and fabricated. The measured results indicate good features of low insertion loss of only 0.98 dB, high selectivity, and wide stopband suppression.