Tunable Bandpass Filter Using a Switched Tunable Diplexer Technique

Abstract

In this paper, a new technique using two p-i-n diodes to switch between two channels of a tunable diplexer with constant return loss is proposed to design a tunable bandpass filter (BPF) with wide frequency tuning range (FTR). The used third-order tunable diplexer with two independently tuned channels consists of one common varactor-tuned lumped-element dual-resonance resonator and two varactor-tuned LC resonators. The ports of its two channels are connected by a single-pole double-throw switch, so that one port of the tunable BPF is constituted. And its common port serves as the other port of the tunable BPF. In the switched tunable diplexer design, its direct current (dc) bias voltages are also appropriately selected to improve the in-band and out-of-band performances of the tunable BPF. As examples, a third-order tunable diplexer with constant return loss of 15 dB is first designed. The measured results show that the 3 dB cutoff frequency of its lower channel can be tuned from 0.5 to 1.02 GHz, while the 3 dB cutoff frequency of its upper channel can be tuned from 1.01 to 1.78 GHz. Then, a tunable BPF is designed, and its 3 dB cutoff frequency covers from 0.485 to 1.82 GHz. The proposed tunable BPF exhibits good return loss, wide FTR, and compact size.