Designing a Low-phase-noise, Low-power-dissipation CMOS-Ring VCO according to a New Figure of Merit

Abstract

Since the conventional figure of merit (FOM) neglects the flicker-noise effect in a voltage-controlled oscillator (VCO), we devised a new FOM that considers the flicker-noise effect and, thus, allows phase noise (which is dominated by flicker noise) of different VCOs to be compared. Using the proposed FOM, we analyze the phase noise and power dissipation in an inverter-based ring oscillator, since it has a simple topology and few noise sources. From this analysis, we draw three conclusions: first, varying load capacitance changes oscillation frequency only without changing proposed FOM; second, to improve the proposed FOM, large transistor length should be chosen, third, comparing FOM of a VCO and FOM calculated from target specifications of phase noise and power dissipation shows that the designed VCO meets target specifications. This second and third conclusion is useful for easily designing an actual inverter-based quadrature-ring VCO. A prototype inverter-based quadrature-ring VCO was fabricated by using 0.18-μm CMOS technology with an active die area of 0.018 mm2. It consumes 2.61 mW from a 1.8-V power supply. Moreover, it has an oscillation frequency of 338.5 MHz, phase noise of -110 dBc/Hz at 1-MHz offset frequency, and proposed FOM of 160.4 dB (at 0.1-MHz offset frequency).