I. Introduction
Frequency-modulated continuous-wave (FMCW) radar sensors operating in the millimeter-wave (mm-wave) regime offer fine-resolution object detection and identification at short range. A linear frequency ramp with a minimum 6-GHz bandwidth is required to achieve a range resolution under 5 cm at 60 GHz [1]. To synthesize an FMCW carrier, the all-digital phase-locked loop (ADPLL) shown in Fig. 1 is proposed, which allows reconfigurability and permits the highest degree of integration of the radio front-end with the digital base-band [2]. MOS varactors are commonly used to tune DCOs in the low-GHz range. However, their quality factor is as low as for 50 fF at 60 GHz and the varactor capacitance ratio, , of 2–3 limits the DCO frequency tuning range. Moreover, a frequency tuning step lower than 1 MHz also cannot be achieved using varactor tuning. Thus, it is no surprise that only two DCOs operating above 50 GHz [3] [4] are reported, and none reported to date have achieved 10% tuning range and sub-MHz tuning steps simultaneously.