Constant absolute bandwidth tunable asymmetric order dual-band BPF with reconfigurable bandwidth using mode control technique

Abstract

Herein, a dual-band bandpass filter with bandwidth tuning and constant bandwidth centre frequency tuning is presented, where the first and second bands are, respectively, of two and three modes. Based on λ/2 resonator modal analysis, a varactor-ended stub is centrally loaded to a varactor-terminated resonator to control even modes, and two varactor-ended stubs are also loaded off-centred symmetrically to tune, mostly, odd modes. Using these varactors, the first passband is formed by pairing the first even and odd modes while the second passband using the next pair of even and odd modes. To make the second band bandwidth also constant, the first mode of another resonator kept parallel to the first resonator has been used making the second passband of three poles. S-matrix has been derived to analyse and coupling matrices have been synthesised to design the filter. A filter has been fabricated where the first band is tunable from 0.9 to 1.24 GHz (31.7%) with absolute bandwidth of 220 MHz while the second passband is tunable from 1.8 to 2.45 GHz (30.5%) with absolute bandwidth of 280 MHz. The first and second bands' bandwidth can be tuned from 110 to 360 MHz and from 230 to 700 MHz.