VCO phase noise

The unavoidable trembling of an oscillator's frequency. An ideal local oscillator would be a single, infinitely thin spike at one frequency — a perfectly steady tick. A real VCO's tick jitters slightly in timing, smearing that spike into a narrow skirt of energy around the carrier. Phase noise is the measure of how fat that skirt is, quoted as power relative to the carrier in a 1 Hz band, at some offset — e.g. −110 dBc/Hz at 1 MHz offset.

Why care? In a receiver, a noisy LO 'reciprocally mixes' a strong neighbouring channel down on top of your weak wanted signal, deafening you. In a transmitter, phase noise spills power into adjacent channels and corrupts the constellation of high-order QAM. Leeson's model shows phase noise improves with higher resonator Q and more signal power, which is exactly why on-chip LC tanks with the best achievable inductor Q are prized in RFIC VCO design.