Experimental Evaluation of Coupled-Line Tunable Inductors with Switchable Mutual Coupling

This paper investigates and characterizes a tunable inductor structure based on coupled-line configurations, referred to as a coupled-line tunable inductor (CLTI). By integrating switches along the coupled-line paths, the mutual inductance can be selectively enabled or disabled, providing a means for active inductance modulation. Spiral inductors with one-turn and two-turn cores were used in conjunction with inner-coupled-line placements to explore different coupling configurations. The test structures were implemented using printed circuit board (PCB) technology, and their performance was analyzed through electromagnetic simulations and vector network analyzer (VNA) measurements. The results confirm that switch-controlled coupled lines enable effective inductance tuning, with a measurable reduction in inductance when the coupled-line path is activated. In the switch-OFF state, only minimal performance degradation was observed due to parasitic effects. These findings provide useful insights into the practical behavior of coupled-line tunable inductors and suggest their applicability in RF circuits and adaptive analog systems, particularly where integration and compact tunability are desired.