Synthesis design of high-selectivity wideband balanced bandpass filter based on parallel coupled lines

Abstract

In this paper, a high-selectivity equal-ripple wideband balanced bandpass filter (BPF) is proposed. Comprising two parallel coupled lines (PCLs) linking the 50-Ohm input and output feedlines, a pair of branch lines connecting the respective differential input and output ports, and a transverse transmission line interconnecting the left and right branch lines within the symmetrical plane SS', this design configuration yields a bandpass characteristic under differential-mode (DM) excitation, while also inherently achieving bandstop performance under common-mode (CM) excitation. The optimization objective function employs the ideal Chebyshev transfer function to attain equal-ripple DM filtering responses. Additionally, to bolster selectivity, a PCL structure replaces the left branch line, generating additional transmission zeros (TZs) positioned on both sides of the DM passband. For validation, simulation and measurement of an equal-ripple wideband balanced BPF operating at 3.7 GHz demonstrate close agreement between the simulated, measured, and synthesized results.