Reconfigurable cascaded coupled line filter with four distinct bandwidth states

Abstract

This study presents a cascaded coupled line filtering structure for the development of high-order reconfigurable microwave filters. A reconfigurable bandpass filter of this type that is capable of switching between four distinct bandwidth states ranging from around 20 to 50% 3 dB fractional bandwidth centred at 2 GHz, is described. The filter exhibits insertion losses ranging from 0.47 to 5.5 dB. The circuit is designed from a filter building block proposed previously in earlier literature. The publication enhances the understanding of this building block and shows why such a filter topology is used in order to develop higher order filters. This is done through mathematical modelling of the filter building block and theoretical simulation. The modelling shows that the centre frequency is only dependent on the set of even- and odd-mode impedances, which makes tuning the bandwidth easy. It also shows that changing the characteristic impedance of short-circuit stubs changes the bandwidth. The fabricated filter shows good agreement with electromagnetic simulated results.