Planar Transversal-Signal-Interference Triple-Band Bandpass Filter With Multiple Transmission Zeros and Extended Upper Stopband

An original type of planar two-path transversal-signal-interference triple-band bandpass filter (BPF) with abrupt cut-off filtering slopes is reported. Each RF-propagation path is composed of a replica of a triple-band BPF unit without stopband transmission zeros (TZs) and input/output transmission-line-based phase-shifting sections with unequal lengths for each path. In this manner, although the total lengths of the two RF-propagation paths are similar, their admittance parameters are different. Hence, an overall triple-band BPF response with sharp-rejection characteristics can be obtained from the transversal interference of both paths by creating multiple out-of-band TZs. For experimental-validation purposes, a microstrip prototype of triple-band BPF is designed, manufactured, and characterized. By means of an appropriate layout-level optimization, an ultra-wide upper stopband is also realized into it as an added benefit.