Stopband-Extended and Size-Miniaturized Low-Pass Filter Based on Interdigital Capacitor Loaded Hairpin Resonator with Four Transmission Zeros

Abstract In this paper, a compact low-pass filter (LPF) with wide stopband is proposed based on interdigital capacitor loaded hairpin resonator. The structure composed of an upper high-impedance transmission line, a middle interdigital capacitor, and a pair of inter-coupled symmetrical stepped-impedance stubs. Detailed investigation into this structure based on even-odd mode approach reveals that up to four transmission zeros can be generated and reallocated by choosing the proper circuit parameters. And owing to the aid of transmission zeros, the fabricated quasi-elliptic LPFs experimentally demonstrate a wide 20dB stopband from 1.4 fc to 5.1 fc using a compact size of only 0.005 λ g 2 .