A bandstop filter with flat negative group delay (NGD) is proposed. The flatter NGD and reduced insertion loss have been achieved in the passband of the bandstop filter. This paper provides the group delay and insertion loss equations for each component of bandstop filter, then analyzes the effects of different unit values on bandwidth, flatness, and minimum NGD. The experimental results show that the flatness is 5.06% and the NGD is −0.498 ns. The insertion loss is 6.72 dB, and the relative NGD bandwidth is 85.2%. The simulation and measurement results are basically consistent with the theoretical results. The feasibility of the design is proved.
