Radial stub based negative group delay circuit theory

This study addresses a negative group delay (NGD) circuit theory regarding a distributed passive topology consisted of an open-ended radial stub. The equivalent circuit composed of the lumped R, L and C elements is proposed. Then, the S-parameter model of the passive topology is established. The NGD analysis is described from the S-parameter model. The synthesis method enabling to determine the stub parameters as a function of the targeted NGD level, frequency centre and reflection coefficient is formulated. The radial stub topology limit is explained with an analytical condition from the synthesis relation. The developed NGD theory feasibility is validated with two cases of proof-of-concept (POC). First, ideal POC circuit with NGD value varied from -2 to -1 ns around the centre frequency 1.3 GHz is confirmed with ideal optimised circuit presenting figure-of-merit of approximately -0.27. Then, the theoretical prediction is verified numerically and experimentally by designing and fabricating an actual microstrip radial stub circuit. The calculated model, simulation and measurements are in very good correlation. The prototype test results confirm that the NGD circuit exhibits an NGD of approximately -1.2 ns level at approximately 0.85 GHz centre frequency.