A new technique for making broadband and variable passive delay elements is described. By introducing a variable inductance structure and using it along with available varactors, synthesized transmission lines are implemented with variable delay while maintaining a constant Zo over the line bandwidth. Inductance tuning is realized through the effect of mutual inductance. As a demonstration prototype, a single unit cell and two cascaded unit cells were implemented in 90 nm digital CMOS process. Delay values ranging from 14 ps - 40 ps were obtained from DC to 8 GHz while maintaining matched condition over the bandwidth with delay variations of less than plusmn%5. These delay cells could be used in broadband impulse-based beamforming systems to provide variable delays in each RF path.
