Novel microstrip bandpass filters using direct-coupled triangular stepped-impedance resonators for spurious suppression

This paper presents a novel spurious suppressed bandpass filter designed with triangular stepped-impedance resonators. The proposed resonators are folded for a triangular schematic, which creates two transmission zeros. By varying the line width of resonators, one of the zeros is tuned to suppress the first and second spurious responses, and the other is used to improve skirt selectivity. The current study analytically derives explicit design equations and fabricates a three pole Chebyshev bandpass filter centered at 1.5GHz based on the direct coupled structure. The experimental results show spurious suppression improvement more than 20dB up to 5.6f₀.