Reduction of CMOS Inverter Ring Oscillator Close-in Phase Noise by Current Mode Instead of Voltage Mode Supply

A reduction of low frequency flicker noise caused close-in phase noise in single-ended CMOS inverter ring oscillators is observed when a current mode supply is used instead of a voltage mode supply. Measurements show a 4 to 6 dB reduction at the same oscillation frequency and oscillator core power consumption. An analytical explanation of the reduction is derived by calculating the deviation of the oscillation cycle time resulting from a slowly changing perturbation of the drain average peak currant of a particular MOSFET for both cases. It is shown that the perturbed drain current causes all the other MOSFETs' drain currents to deviate contrary to the causing perturbation in current mode, thus reducing the overall cycle time deviation and close-in phase noise. The current mode supply is shown to be realizable by a large gate area MOSFET operating as a current source

I. Introduction

CMOS ring oscillators can offer extremely wide frequency tuning range and maximum operating frequencies in the several GHz range when implemented in modern CMOS-technologies [1]–[3]. This large tuning range would be interesting for applications in coming ultra-wideband (UWB) systems, in reconfigurable multi-frequency systems and in multi-bit-rate transmission systems. But due to the lack of a high quality resonator, ring oscillators usually have much larger phase noise [4] than small tuning range LC-oscillators. In particular, the flicker noise caused 1/Δf³-region of their phase noise spectrum is wide. Thus, some method to reduce the flicker noise caused phase noise of CMOS inverter ring oscillators could expand their field of application.