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

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 f
_z1
, f
_z2
. The possibility of moving f
_z1
, f
_z2
with respect to centre frequency f
₀
and the problem of obtaining the minimum interval between f
_z1
, f
_z2
and f
₀
are studied. The measured and simulated responses of microstrip fourth-order BPFs are presented.