Microstrip Bandpass Filter with mixed Couplings and Resonators Located at Different Levels

Abstract

This article discusses the new principle of placing second-order bandpass filters (BPFs) with mixed couplings when they are connected in cascade. This principle consists in arranging second-order BPFs at different levels, which lids in multilevel coupling structure. This placement of second-order BPFs minimizes the parasitic interaction between non-adjacent resonators in microstrip filters, which reduces distortion of frequency responses. This placement principle is used to create of two microstrip fourth-order BPFs, obtained by cascading two second-order BPFs with stepped-impedance resonators (SIRs) and mixed couplings. Two mixed couplings lead to two transmission zeros fz1, fz2. The possibility of moving fz1, fz2 with respect to centre frequency f0 and the problem of obtaining the minimum interval between fz1, fz2 and f0 are studied. The measured and simulated responses of microstrip fourth-order BPFs are presented.