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