Analysis and design of high-order wideband bandstop filters with sharp rejection

High-order wideband bandstop filters (BSFs) with sharp rejection based on transversal signal-interference techniques are analysed and designed. By choosing different open stubs and open/shorted coupled lines, third/fifth/seventh-order wideband BSFs with six transmission zeros from 0 GHz to 2f(0) (f(0) is the centre frequency of the stopband) can be achieved, respectively, due to the superposition of signals from the two transmission paths. The bandwidths of the three high selectivity wideband BSFs can be easily adjusted by changing the characteristic impedance of the resonant structures of the two transmission paths. Three wideband BSF prototypes with 3 dB fractional bandwidths of 113% (1.74-6.26 GHz), 115% (1.85-6.45 GHz) and 112.3% (1.76-6.25 GHz) are designed and fabricated for demonstration. The theoretical and measured results are in good agreement and show good in-band filtering performance and high selectivity.