High-resolution 60-GHz DCOs with reconfigurable distributed metal capacitors in passive resonators

Mm-wave digitally-controlled oscillators (DCOs) with reconfigurable passive resonators are proposed, which achieve wide tuning range (>;10%) and fine frequency resolution (<;1 MHz) simultaneously. Two 60-GHz implementations: a fine-resolution inductor-based DCO (L-DCO) and a transformer-based DCO (T-DCO) are demonstrated in 90-nm CMOS, exploiting metal capacitors only for frequency tuning. Both DCOs obtain >;9.7% linear tuning range and phase noise lower than -90.5 dBc/Hz at 1-MHz offset across the 56-62 GHz range. The T-DCO achieves fine frequency tuning step of 2.5 MHz, whereas that of the L-DCO is better than 160 kHz. The L-DCO and T-DCO consume 10 mA and 12 mA, respectively, from a 1.2-V supply. The core size of each DCO is 0.4×0.4 mm².

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.