Phase Noise is a measure of the spectral purity of a signal in an oscillator system. It quantifies the short-term random variation of the frequency of the signal, and is a product of thermal noise and low frequency flicker noise injected into the oscillator. Most RF systems require an overall integrated phase noise specification to be met, as phase noise can corrupt both the upconverted and downconverted signal paths. For digital systems, the integrated phase noise can be converted and expressed as phase jitter.