A filtering technique to lower LC oscillator phase noise

Based on a physical understanding of phase-noise mechanisms, a passive LC filter is found to lower the phase-noise factor in a differential oscillator to its fundamental minimum. Three fully integrated LC voltage-controlled oscillators (VCOs) serve as a proof of concept. Two 1.1-GHz VCOs achieve -153 dBc/Hz at 3 MHz offset, biased at 3.7 mA from 2.5 V. A 2.1-GHz VCO achieves -148 dBc/Hz at 15 MHz offset, taking 4 mA from a 2.7-V supply. All oscillators use fully integrated resonators, and the first two exceed discrete transistor modules in figure of merit. Practical aspects and repercussions of the technique are discussed.