Tunable Bandpass Filter With Independently Controllable Dual Passbands

This paper presents a two-pole dual-band tunable bandpass filter (BPF) with independently controllable dual passbands based on a novel tunable dual-mode resonator. This resonator principally comprises a λ/2 resonator and two varactor diodes. One varactor is placed at the center of the resonator to determine the dominant even-mode resonant frequency; the other is installed between two ends of the resonator to control the dominant odd-mode resonant frequency. These two distinct odd- and even-mode resonances can be independently generated, and they are used to realize the two separated passbands as desired. Detailed discussion is carried on to provide a set of closed-form design equations for determination of all of the elements involved in this tunable filter, inclusive of capacitively loaded quarter-wavelength or λ/2 resonators, external quality factor, and coupling coefficient. Finally, a prototype tunable dual-band filter is fabricated and measured. Measured and simulated results are found in good agreement with each other. The results show that the first passband can be tuned in a frequency range from 0.77 to 1.00 GHz with the 3-dB fractional-bandwidth of 20.3%-24.7%, whereas the second passband varies from 1.57 to 2.00 GHz with the 3-dB absolute-bandwidth of 120 ± 8 MHz.

I. Introduction

Electronically tunable/reconfigurable filters are one of the most essential microwave components for multiband communication systems due to their attractive features, i.e., miniaturizing the overall system size, relaxing the system complexity, and reducing the cost in fabrication. Microwave tunable filters have been studied for several decades, and, in general, they can be classified into three categories: 1) yittrium–iron–garnet (YIG) filters [1]; 2) varactor diode filters [2]–[4]; and 3) radio-frequency microelectromechanical system (RF-MEMS) filters [5], [6]. Although the YIG filters have large tuning range and small in-band insertion loss, their high power consumption, slow tuning rate, large size, and large weight block them from many applications in modern highly integrated communication systems. RF-MEMS filters utilize RF-MEMS capacitors with the advantages of high -factor at RF and low distortion levels. Varactor-diode filters are widely studied due to their small size, nanosecond tuning speed, and low cost.