Miniaturization of Negative Group Delay Bandstop Filter Using Gradient Slow-Wave Structures

This paper presents a miniaturized negative group delay bandstop filter (NGD-BSF) using the gradient slow-wave (SW) structures. The electrical lengths of the SW structures with different gradient lengths are calculated by the equivalent circuit. By replacing the $\lambda_{g}/4$ microstrip stub of the NGD-BSF with a gradient SW structure, a size-reduction factor of 27.71% is achieved. The simulated results show that the group delay and insertion loss are -6.5 ns and 18.8 dB at the operating frequency of 2 GHz, and the worst return loss is 13.4 dB within the NGD band from 1.962 GHz to 2.046 GHz.