Transversal Signal Interaction: Overview of High-Performance Wideband Bandpass Filters

This article presents an introduction of recently wideband bandpass filters based on transversal signal interaction concepts. Different resonant structures are reported on, including branch-line coupler/ring resonator, interdigital coupled lines, DSPSL 180° phase-shifting structure, Marchand balun, open/shorted coupled lines, T-shaped structures, and open/shorted stubs. Detailed comparisons of out-of-band transmission zeros, effective circuit size, 3-dB bandwidth, upper stopband, and group delay for the wideband/UWB filters discussed in this article are presented. Different bandwidth of wideband bandpass filters can be realized based on transversal signal interaction concepts, branch-line coupler/ring resonator can be easy to realize wide bandwidth with narrow upper stopbands due to their harmonic response. The filter structures using different 180° phase-shifting structures such as DSPSL, shorted coupled lines, and Marchand balun can meet UWB bandwidth/band demand, and the circuit size can be further reduced, while the selectivity and upper stopband should be further improved. The integrated applications of shorted/open coupled lines and shorted/open stubs can increase the numbers of the transmission zeros out-of-band, besides the circuit size reduction, the upper stopband can be also extended to over 4.7 f₀ . Moreover, the transversal signal-interaction concepts have been also extended to the design of differential wideband//UWB balanced bandpass filters with broadband common-mode suppression in our former works.