Ultra-wideband Choke Inductors Using Transmission Line Coupling Effects

Choke inductors as parts of bias-T circuits are essential in broadband RFIC application. Basically the bias-Ts constrain RFIC bandwidths. However wideband choke inductors in RFICs are difficult to design due to parasitic capacitances and limited wiring layers. In this paper, first, we compare three on-chip inductors with the Sonnet software in a simplified silicon-based process. The self resonant frequencies of the inductors are 64, 71 and 73GHz, while the bandwidths are 60, 68 and 70GHz, respectively. As a result, the simulations show that single layer inductors are more suitable than the others. Thin dielectric layers and high resistant metal layers compromise multi-layered inductor performance. Next, to improve the bandwidths, we propose a new chock inductor design by lowering their quality factors at self resonant frequencies without resistors. Placing an inductor over a transmission line (IoTL) induces a coupling effect on the self resonant frequency. By carefully changing the transmission line width, we are able to find an optimum condition at the little cost of loss, < 0.25dB. To further test the feasibility, we measure four bias-Ts in the 65nm CMOS process and compare their S-parameters. The 16% improvement of usable bandwidth suggests that inductor-over-transmission line design may be powerful to expand bias-T bandwidth.