A Simple Frequency-Agile Bandpass Filter With Predefined Bandwidth and Stopband Using Synchronously Tuned Dual-Mode Resonator

This letter proposes a novel and simple synchronously tuned dual-mode resonator (STDR) to achieve a frequency-agile bandpass filter (FA-BPF) with predefined bandwidth (BW) location, BW variation, and stopband characteristics. This solution overcomes the difficulty with predefining the BW of previous tunable filter designs. The proposed STDR is controlled by two varactors and a single bias. The two resonant frequencies’ separation of the resonator can be adjusted by changing the varactor loading position and the shorted stub length. Meanwhile, the transmission zero location can be predefined by specifying the corresponding dimensions. Based on the predefined STDR, the FA-BPF can be achieved easily. An FA-BPF with a constant BW is realized to demonstrate such a unique feature. The measurement results of the FA-BPF verify the theory, and also exhibit high selectivity, wide frequency tuning range, and simple design/ control procedure.