A Miniaturized Fully Tunable Quasi-Elliptic Bandpass Filter

A miniaturized fully tunable quasi-elliptic bandpass filter (BPF) has been developed in this work. The proposed third-order BPF was formed by only one tri-mode stub-loaded resonator (SLR). The inherent property of the tri-mode SLR will generate two transmission zeros (TZs) on the lower and upper sides of the passband. Therefore, the size reduction and high selectivity are obtained simultaneously. In addition, six varactor diodes were employed to electrically control the resonance frequencies of resonator and the external couplings and, thus, flexible frequency and bandwidth control can be achieved. The results evidently demonstrate that the presented BPF exhibits a wide frequency tuning range from 1 to 1.4 GHz and a wide bandwidth tuning range from 150 to 630 MHz, which is ideally suited for application in modern communication systems.