Tunable True-Time-Delay Unit Based on Bridged T-Coil

This paper presents the design of tunable true-time-delay (TTD) units with both discrete and continuous time delay states. Tuning is achieved by varying the capacitances of a bridged T-coil (BTC) circuit. The effectiveness of the design approach is demonstrated with both switched capacitor banks and varactors. All tunable TTD units have been designed in a Tower Semiconductor 0.18 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{m} \text{SiGe}$</tex> BiCMOS process. Measurements of a fabricated two-state delay unit (20 ps and 25 ps delay) with capacitance tuning implemented with nfet switches show a tunable range of 25% over an 8 GHz bandwidth with < 1.8 dB insertion loss and > 15.8 dB return loss. Full-wave simulation results for a varactor-tuned design demonstrate a tuning range of 16 ps - 25 ps <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\pm 25\%)$</tex> over an 8 GHz bandwidth.