Compact tunable microstrip filter with wide-stopband restriction and wide tuning range for 4G and 5G applications

This paper presents a compact planar tunable filter covering the 2.5 to 3.8 GHz spectrum for 4G and 5G wireless communications using a new hybrid technique. The microstrip filter uses five coupled line resonators with λ/4 open-circuited stubs. The coupling between the resonators is adjusted to tune the centre frequency with Butterworth characteristics. The proposed bandpass filter (BPF) is designed on a Rogers RO3010 substrate with a relative dielectric constant of 10.2 and a compact size of 13×8×0.81 mm 3 . The coupling coefficients between the adjacent resonators, external quality factors, varactor diodes and biasing circuit are designed to resonate the tunable filter at 3.5 GHz. The bandwidth is adjustable between 90 and 110 MHz with return losses between 15 to 25 dB and insertion loss around 0.8 dB. Computer simulation technology (CST) software is used to simulate and optimize the designed tunable filter, with hybrid co-simulation between CST MWS and CST DS is used to implement the structure, taking into account the SPICE model for the varactor diodes and the effect of the biasing circuit