Analysis and design of wideband bandpass filters with improved upper stopband

Two high-order wideband bandpass filters with improved upper stopband based on transversal signal-interference techniques are analyzed and designed in this paper. By choosing different open stubs and open/shorted coupled lines, two wideband bandpass filters with several transmission zeros from 0 GHz to 2f₀ (f₀ is center frequency of the stopband) can be achieved respectively, due to the superposition of signals of the two transmission paths. The bandwidths of the two wideband bandpass filters can be easily adjusted by changing the characteristic impedance of the resonant structures of the two transmission paths. One wideband bandpass filter prototype with 3-dB fractional bandwidth of 80% (0.6-1.8 GHz) is designed and fabricated for demonstration. The theoretical and measured results are in good agreement and show good in-band filtering performance and high selectivity.