A Low Phase Noise Oscillator Principled on Transformer-Coupled Hard Limiting

Reduced phase noise conversion in a monolithic oscillator suitable for basestation applications is realized by hard limiting and subsequently restoring the resonating waveform. Overdriven transistors hard limit the drain voltage swing and it is shown analytically that this desensitizes the oscillation phase to circuit noise. A pair of tuned, 1:2 step-up transformers in the feedback path restore the fundamental frequency component with sufficient gain to overdrive the transistors forming the oscillator core, with greater selectivity than an equivalent LC tank. The 8 GHz, 65 nm CMOS oscillator prototype targeting the GSM-900 base-station specification consumes 32 mA from 1.5 V. Normalized to 915 MHz, the phase noise measured at 1 MHz offset is -147.7 dBc/Hz, validating predictions from theory and simulation. The measured frequency pushing is less than 16 MHz/V.

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