Design of Bandpass Tunable Filter for Green Flexible RF for 5G

This paper proposes a compact three-pole planar tunable bandpass filter (BPF) covering the sub-6 GHz spectrum for 5G wireless communications. The microstrip BPF utilizes three open-loop ring resonators with 50 Ω transmission line impedances for input/output terminals. The coupling coefficients between the adjacent resonators and the external quality factors are controlled to resonate the designed filter at 3.5 GHz with third-order bandpass Butterworth characteristics. The varactor diode and biasing circuit are modelled to tune the resonant frequency in the desirable band. The filter is implemented and measured on a Rogers RO3010 substrate with a relative dielectric constant ε_r= 10.2 and a compact size of 17×5×1.27 mm³, and is optimized and simulated using computer simulation technology (CST) tool. Good agreement is achieved between the simulated and measured results.