Filter embedded with non-uniform spur-lines for stopband enhancement design

We propose a coupled-line bandpass filter (BPF) embedded with multiple novel non-uniform spur-lines for a stopband bandwidth (BW) enhancement. The embedded spur-line is originated from a uniform slot by replacing the uniformly width slot with a stepped-width one. The frequency response of the proposed spur-line is analytically analyzed to show the size-reduction effect. As many as five spur-lines are embedded in the filter's three microstrip lines to eliminate the latters' first three spurious responses that beyond the fundamental one, achieving a much wider stopband BW. Experiment is conducted to verify the design and the circuit performance.