Wideband Tunable Bandpass Filter Using Optimized Varactor-Loaded SIRs

This letter presents a detailed investigation on the relationship between the frequency tunability and the properties of the stepped-impedance resonator (SIR). It can be seen that the frequency tuning range of the varactor-loaded SIR relies on the impedance and length ratios of the SIR when the capacitance of the varactor is within a fixed varying range. There exists an optimum length ratio of the SIR to achieve the largest frequency tuning range. For demonstration, a wideband tunable bandpass filter based on the optimized varactor-loaded SIR is designed, fabricated, and measured. The coupling coefficient between two coupled SIRs can be adjusted for meeting the requirement that the fractional bandwidth (FBW) of the passband remain constant in the frequency tuning process. The simulation and measured results, showing good accordance, exhibit a tuning range from 0.6 to 1.015 GHz with a 3-dB constant FBW of about 15.5% and an in-band insertion loss of 1.1-2.8 dB.