Design of a six-pole tunable band-pass filter with constant absolute bandwidth

This paper presents the design and implementation of a tunable bandpass filter with constant absolute bandwidth. Two types of coupled microstrip lines in the anti-parallel and pro-parallel formats with varactor-loaded ⋋/2 resonators are investigated to demonstrate its advanced capacity in control of the bandwidth variation. In this design, two adjacent resonators are coupled with each other through a short-circuited transmission line as the K inverter. To verify the proposed approach, a compact six-pole tunable bandpass filter based on alternative J/K inverters and ⋋/2 resonators is designed, and the measured results show a tuning range from 1.9 to 2.3 GHz with a bandwidth of 580 ± 10 MHz.

1. Introduction

Recently, microstrip tunable filters with constant absolute bandwidth (CABW) [1]–[4] or constant fractional bandwidth (CFBW) [5] are becoming an attractive research topic. Based on the individual and/or mixed electric and magnetic coupling schemes [1]–[3], several microstrip tunable filters with CABW/CFBW variation versus central frequency of the desired passband were investigated and designed. In [4], a dual-mode microstrip resonator was studied to develop a class of tunable filters with CABW by controlling the tuning rate of two resonant frequencies. Note that most of the works reported so far in [1]–[5] are restricted to the second-order topology and have not presented a unified method for high-order response.