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

摘要

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 λ/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 λ/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.