X-band negative group-delay lossy stub line

This study investigates the negative group delay (NGD) passive circuit constituted by a fully distributed parallel stub line. The proposed S-parameter modelling is developed. The transmission parameter band-pass NGD function identification is described. The NGD characterisation is established by determination of the NGD level and bandwidth in function of the stub line parameters. It was analytically found that with a basic cell consisted of single stub line, the insertion loss is usually degraded to reach significant figure-of-merit which is obtained with the NGD level-bandwidth product. As a solution, it is shown that this insertion loss can be improved with the consideration of matching network. As proof-of-concept (POC), microstrip distributed circuits printed on the flexible Kapton substrate without lumped element were synthesised, designed and fabricated. The theoretical, simulated and measured results are in good correlation. As expected, band-pass NGD behaviours centred beyond 10GHz were observed. The POC generates NGD level of about -1.5ns over the NGD bandwidth 0.65GHz. In the future, thanks to the theoretical approach simplicity and the topology integrability, the NGD principle can be potentially useful for the improvement of RF/microwave circuits.