A 33.6-to-46.2GHz 32nm CMOS VCO with 177.5dBc/Hz minimum noise FOM using inductor splitting for tuning extension

Abstract

Signal processing in ultra-wide bandwidths is one of the key challenges in the design of multi-Gb/s wireless transceivers at mm-Waves, where channels covering 57GHz to 66GHz are specified. Further considering spreads due to process variations and the stringent reference phase noise to ensure signal integrity calls for an ultra-wide tuning range and low-noise on-chip oscillator. Meeting this target is even more challenging when adopting an ultra-scaled CMOS technology node where key passive components suffer from a reduced quality factor (Q) [1]. In a 32nm node the thickness of metals closer to the substrate is half that in a 65nm process leading, for example, to MOM capacitors with roughly half Q. The penalty is only marginally compensated by the higher transistor ft, improved only by ~20%. Various techniques exploiting alternative tuning implementations have been published recently. Magnetic tuning methods where the equivalent tank inductance is varied through reflection of the secondary coil impedance of a transformer demonstrate outstanding tuning ranges but at the cost of a severe trade-off with tank Q and poor noise FOMs [2,3]. A bank of capacitors switched in and out in an LC tank is the most popular tuning approach [4-6]. However the quality factor is severely degraded, when large ranges are involved. In this work, the switched-capacitor tank of the VCO shown in Fig. 20.3.1 is centered around two different resonance frequencies by splitting the inductor through the switch Msw. In particular, an up-shift is produced when the switch is off due to its parasitic capacitance. The frequency range is significantly increased without compromising tank Q leading to large tuning range and high FOM simultaneously. Prototypes of the VCO have been realized in 32nm CMOS showing the following performances: 31.6% frequency tuning range, minimum phase noise of -118dBc/Hz at 10MHz offset from 40GHz with 9.8mW power dissipation. Despite being realized in an ultra-scaled 32nm standard digital CMOS process without RF thick metal options, the oscillator shows state-of-the-art performances.