Wideband varactor‐tuned balanced bandpass filter with stable passband selectivity

Abstract

This article presents a wideband tunable balanced bandpass filter (BPF) with stable passband selectivity using optimized varactor-loaded half-guided wavelength stepped impedance resonator (SIR). By overall investigation, it can be found that the frequency-tuning range of the varactor-loaded SIR is not only controlled by the variation range of the loaded capacitor, but it can be widened by optimizing the SIR. The coupling region between the two coupled SIRs in the proposed BPF is studied for achieving the constant absolute bandwidth (ABW) during the frequency-tuning process. Meanwhile, the frequency-dependent source-to-load coupling is introduced to realize two self-adaptive transmission zeros (TZs) on both sides of the tunable differential-mode (DM) passband so that the proposed filter can keep high selectivity. Benefiting from the constant ABW and two self-adaptive TZs, the stable DM passband selectivity of the balanced tunable filter can be obtained. For demonstration, a balanced tunable BPF based on the optimized varactor-loaded SIRs is designed, fabricated and measured. The simulated and measured results, showing good agreement, exhibit a tuning range from 0.542 GHz to 0.926 GHz with 3-dB constant ABW of around 105 MHz and in-band insertion loss of 1.5-2.56 dB.