Reconfigurable Non-Foster Elements and Squint-Free Beamforming Networks Using Active Transversal Filter-Based Negative Group Delay Circuit

A new type of reconfigurable negative group delay (NGD) circuits based on active transversal filter topology is introduced in this work. By incorporating varactors with the NGD circuit at the input and output ports of active transversal filter based on distributed amplifiers (DAs), one can notably enhance the tunability for both amplitude and phase responses, thereby enabling NGD characteristic synthesis with a wider reconfigurable range. In this article, the proposed reconfigurable NGD circuits are applied to create both tunable non-Foster elements and tunable squint-free beamforming networks, which are verified by experiment. Furthermore, a general N-stage transfer function for the proposed reconfigurable NGD circuit is derived, in which a two-stage particular case is given as an illustrative proof-of-concept example. Based on the reconfigurable NGD circuits, a prototype of a tunable negative capacitor is fabricated and measured, of which the capacitance can be tuned from -1.0 to -4.0 pF over a bandwidth of 100 MHz at 1 GHz or around 10% relative bandwidth. In addition, such an NGD circuit is utilized to create a low-dispersion superluminal transmission line, which enables a reconfigurable series antenna feedline for squint-free beam steering. Experimental results show that the main beam of the NGD antenna array has a tuning range from -15° to 15° within a squint-free bandwidth of 100 MHz at 2 GHz or within a relative bandwidth of around 5%.