Asymmetric λ/2 open circuited stubs enhancing the selectivity of parallel coupled line BPFs

In this paper, to enhance the selectivity of the conventional parallel coupled line bandpass filters (BPFs), two λ/2 open circuited stubs, symmetrical and asymmetrical, are investigated at the positions of the two open ends of the input and output λ/4 coupled lines. By simply adding the symmetrical and asymmetrical stubs, a transmission zeros and a pair of transmission zeros are generated at both sides of lower and upper transition bands, respectively, and sharper selectivities and additional in-band transmission poles have been obtained without increasing the filter order and recalculating filter parameters. Group delay peaks have been found as the ones shown in the transition bands of elliptical function type filters. The levels of in-band transmission poles are increased by this new technique without additional couple line sections. A 3rd order conventional parallel coupled line BPF with a fractional bandwidth from 0.75 GHz to 1.35 GHz and the identical BPFs with two symmetrical and asymmetric stubs have been fabricated. Measurement results show that the technique using asymmetrical stubs improves the skirt slopes of the lower and upper transition bands 14.9 dB/50 MHz and 7.8 dB/50 MHz, respectively. The BPF with the stubs shows insertion loss lower than 0.5 dB and return loss better than 10 dB in the passband and additional in-band transmission poles.