A Reconfigurable Bandpass Filter Based on a Varactor-Perturbed, T-Shaped Dual-Mode Resonator

A microstrip bandpass filter (BPF) with high center-frequency tunability, bandwidth reconfigurability, and a one-side band edge transmission zero (TZ) is presented. The reconfigurable BPF is developed with a simple T-shaped dual-mode resonator, which is externally coupled and perturbed with varactor diodes. Also, the design of adjustable admittance inverters for tunable resonators external coupling is proposed to mitigate the in-band performance degradation that hurdles the ones with a fixed coupling. It is demonstrated that the 1 dB bandwidth is able to be varied from 55 to 175 MHz for center frequency ranging from 750 to 1240 MHz with a low insertion loss (2.9 dB).