Wideband Tunable Differential Bandstop Filter Based on Double-Sided Parallel-Strip Line

Abstract

This paper presents a wideband tunable differential bandstop filter (BSF) using optimized varactor-loaded stepped-impedance resonators (SIRs) based on the double-sided parallel-strip line (DSPSL). A detailed investigation on the relationship between the frequency tunability and the properties of the SIR is presented. It can be found that there are two extra factors for determining frequency-tuning range of the varactor-loaded SIR, i.e., the impedance and length ratios, in addition to the variation range of the varactor’s capacitance. Thanks to the balanced properties of the DSPSL, the common mode shows all-stop response. For demonstration, a second-order wideband tunable differential BSF using two varactor-loaded DSPSL SIRs is designed, and the design procedures are given in detail. The simulation and measured results, showing good accordance, exhibit a tuning range from 0.58 to 1.07 GHz, corresponding to a fractional tuning range of 59.4%.