Phase noise is an important parameter to characterise the frequency stability of oscillators and synthesised signal generators. Accurate measurement of phase noise is required for various applications in radar, communication and navigation systems. A single-channel phase-detector based phase noise measurement system is described. The system’s measurement errors and uncertainties have been analysed in details. The expanded uncertainty is about 2.7dB for calibrating phase noise of a signal generator at 0.001–1.6GHz for frequency offsets from 1Hz to 100kHz. The uncertainty budget for measuring a signal generator’s phase noise at 640MHz is also presented.