How to Measure RF Phase Noise

Measuring phase noise is essential to optimizing narrow channel communications in RF transmissions. The instability of the oscillator causes phase noise, which appears as jitter in the time domain. Phase noise impacts the signal quality and increases error rates in communications links. Unlike other signal noise, phase noise is best understood in the spectral domain.

There are three common techniques for measuring phase noise: direct spectrum, phase detector, and two-channel cross-correlation. The direct spectrum technique measures the spectral energy of the broadband signal using an instrument like a spectrum / signal analyzer, but there are limitations due to the amplitude noise mimicking phase noise. Advanced applications use a dedicated phase noise instrument that employs a phase detector and two-channel cross-correlation techniques. Both techniques operate on the baseband signal and require a downconverter.