Abstract
This paper presents the design of a highly-tunable single-ended and balanced filter with improved upper stopband performance. The proposed filters are designed using a spurline and a U-shaped stepped impedance resonator (SIR) with a varactor diode connected to the open-circuited end. A spurline is introduced in both designs to achieve wide stopband performance by suppressing the higher-order modes. The proposed filters have high tunable ranges (single-ended: 2.35-3.99 GHz; balanced: 2.39-4 GHz), low insertion losses (IL) (single-ended: 0.07-0.2 dB; balanced: 0.21 to 1.1 dB) and wide upper stopband performance (single-ended: 3.79f(1) with a rejection level higher than 20 dB; balanced: 3.79f(1) with a rejection level higher than 20 dB). Performance of both filters are experimentally validated by comparing the experimental results with the simulated ones. Both filters can cover most of the fifth-generation (5G) wireless communication bands with different biasing voltages (capacitance). When both filters are compared with state-of-the-art filters, the proposed filters are found to achieve high tunable ranges with low IL and wide upper stopband performance.