Simple Coupled-Line Tunable Bandpass Filter With Wide Tuning Range

In this paper, a tunable bandpass filter using coupled line with a wide tuning range is proposed. The proposed filter has a simple structure which is composed of a coupled-line section and two pairs of varactors. Meanwhile, simultaneously tuning on bandwidth and center frequency are introduced by varactors. The tunability is mainly realized by varactors connected to one end of the coupled line and varactors between ports and coupled line provide an impedance matching. A theoretical analysis is presented to illustrate the tunability by using even- and odd-mode theory. Additionally, the detailed effect of the two types of varactors is researched, respectively. To verify the concept, a prototype is designed, fabricated, and measured. The transmission zero exhibited in measurement is introduced by bias circuits. The measured results show that the tunable bandpass filter achieves a tunable center frequency of 0.494-1.257 GHz (87.15%) and a 3-dB bandwidth of 0.282-0.943 GHz (334%).