A constant absolute bandwidth filter with dual independent tunable passbands

This paper presents a novel procedure to the design of independent tunable dual-band bandpass filters with constant absolute 3-dB bandwidths. Only two bias voltages are required to control the locations of the passbands, respectively. Two kinds of dual-mode resonators are proposed to be selected while designing the passbands. Based on different parameters of even- and odd-modes, there are four types of dual-band selectivity to choose. Three transmission zeros are introduced at the adjacent of passbands, resulting sharp skirt. To demonstrate the design method, a dual-band tunable filter with constant absolute bandwidths was fabricated and measured. From the experimentally results, it was found that the proposed filter exhibited the first passband center frequency tunable range from 1.70 to 2.27 GHz with a constant 3-dB absolute bandwidth 130 ± 10 MHz and the second passband center frequency tunable range from 2.25 to 2.77 GHz with a constant 3-dB absolute bandwidth 80 ± 10 MHz.

I. Introduction

Due to their potential to significantly reduce the overall size and complexity of communications multiband systems, RF tunable filters are becoming an active research topic [1]–[2].