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

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