Phase Noise Measurement of RF Signals by Photonic Time Delay and Digital Phase Demodulation

A simple and high-sensitivity frequency-discriminator-based phase noise measurement system using photonic time delay and digital phase demodulation is comprehensively investigated and experimentally demonstrated. By applying a low-loss optical fiber to provide a large amount of time delay, high phase noise measurement sensitivity and large operation bandwidth are guaranteed. Meanwhile, digital phase demodulation is employed to avoid sophisticated feedback control and complex calibration required in conventional frequency-discriminator-based phase noise measurement scheme, which not only eliminates the phase noise measurement error introduced by the feedback loop but also suppresses the influence of the amplitude noise in the signal under test. In addition, a solution for the in-phase and quadrature mismatch problem in the digital phase demodulation is proposed. An experiment is performed. Accurate phase noise measurement is achieved in a large bandwidth from 2 to 35 GHz. With a 2-km single-mode fiber serving as the delay line, phase noise measurement sensitivity as low as -134 dBc/Hz at 10 kHz is achieved at a RF frequency of 10 GHz. The phase noise sensitivity can be further improved by applying a longer fiber. The proposed system is simple, accurate, and stable, and can be applied for analysis of high-frequency and ultralow phase noise microwave signal sources.