Optimisation-based design of transversal signal-interference microwave bandpass and lowpass filters with extended stopband

Abstract

Generalised bi-path stepped-impedance-line signal-interference bandpass and lowpass transversal filtering sections (TFSs) with increased stopband bandwidth above the main transmission band are presented. They are realised by partitioning the two in-parallel transmission-line paths in their related classic spectrally periodic TFS approach into a plurality of line sub-segments, whose characteristic-impedance and electrical-length values are derived through optimisation. It is demonstrated that the stopband range above the transmission band in these generalised non-frequency-periodic TFSs can be remarkably broadened with regard to those intrinsic to their conventional bandpass and lowpass TFS counterparts. Furthermore, as added benefits, this is achieved for shorter transmission-line paths and a sharper cut-off slope between the transmission band and the extended stopband. For experimental-validation purposes, three proof-of-concept microstrip prototypes are manufactured and characterised. They correspond to a 1-GHz bandpass TFS, a 1-GHz two-TFS-in-series-cascade-based bandpass filter (BPF) with augmented selectivity and out-of-band power-rejection levels, and one lowpass TFS with a designed 3-dB cut-off frequency of 1.25 GHz.