A Self-Equalized Linear-Phase Absorptive BPF Using Negative-Group-Delay Admittance Inverters

In this letter, a microstrip negative-group-delay (NGD) admittance inverter (J-inverter) is proposed to implement a self-equalized linear-phase absorptive bandpass filter (ABPF). It is revealed that this NGD J-inverter, being composed of a coupled-line loaded with a lumped resistor and an open-circuited stub, has an NGD pit in its frequency response. The superposition of the NGD pits associated with a specific number of the NGD J-inverters is utilized to counterbalance the group delay (GD) peaks at the lower and upper edge of the ABPF passband. A three-order ABPF prototype with a passband of 1.5–2.5 GHz is designed using six NGD J-inverters. Measured GD fluctuation within the 3-dB bandwidth is reduced to 0.18 from 0.59 ns compared with the ABPF using the conventional $\lambda $ /4 J-inverter. The sharpness of the out-band roll-off has been retained. The measured input–output return loss is better than 12 dB over 0.6–3.5 GHz.