High selectivity wideband two-channel bandpass filter for 5G microwave communication systems

A two-channel high-selectivity, wideband bandpass filter has been presented. This is based on a microstrip mixed-impedance resonator and defected ground structure with the mixed coupling strategy. Two pairs of peripheral stepped-impedance resonator (SIR) structures are used as stubs, achieving TZs at the immediate lower and upper cut-off sides of the passband and control the bandwidth, while the other two transmission zeros on the upper cut-off side are yielded through the defected ground structure, and the coupling between the SIR and uniform-impedance resonators components, respectively. A prototype of the circuit is fabricated and measured for validation. The measured results exhibited insertion and return losses better than 1.1 and 20.5 dB, respectively, and high isolation levels exceeding 19 dB. In total, four transmission zeros are achieved. The designed filtering circuit realized a 3-dB fractional bandwidth of 58.1%. There is a 0.95f₀ stopband at the upper cutoff. The fabrication of the prototype shows good agreement between the simulated and measured performance.