Class-D CMOS Oscillators

This paper presents class-D CMOS oscillators capable of an excellent phase noise performance from a very low power supply voltage. Starting from the recognition of the time-variant nature of the class-D LC tank, accurate expressions of the oscillation frequency, oscillation amplitude, current consumption, phase noise, and figure-of-merit (FoM) have been derived. Compared with the commonly used class-B/C architectures, the optimal class-D oscillator produces less phase noise for the same power consumption, at the expense of a higher power supply pushing. A prototype of a class-D voltage-controlled oscillator (VCO) targeted for mobile applications, implemented in a standard 65-nm CMOS process, covers a 46% tuning range between 3.0 and 4.8 GHz; drawing 10 mA from 0.4 V, the phase noise at 10-MHz offset from 4.8 GHz is -143.5 dBc/Hz, for an FoM of 191 dBc/Hz with less than 1-dB variation across the tuning range. A version of the same VCO with a resonant tail filter displays a lower 1/f³ phase-noise corner and improves the FoM by 1 dB.