Phase Noise Analysis Overview
Phase noise is one of the most important figures of merit of a signal-generating device and can well be a limiting factor in a mission-critical application in aerospace and defense, as well as in communications.
The basic concept of phase noise involves frequency stability, which is defined as the degree to which an oscillating source produces the same frequency throughout a specific period of time. Frequency stability consists of two components: long term and short term.
Long-term stability describes the frequency variations that occur over hours, days, months, or even years. By contrast, short-term frequency stability is about changes in the nominal carrier frequency of less than a few seconds duration. The focus of the phase noise analysis feature is on short-term frequency stability.
While there are many technical terms to quantify phase noise, one of the most commonly adopted measures is the "single-sideband (SSB) phase noise", ℒ(f). Mathematically, the US National Institute of Standard and Technology (NIST) defines ℒ(f) as the ratio of the power density at an offset frequency from the carrier to the total power of the carrier signal.
For a more complete explanation of what phase noise is, see "Reference Clocks and Phase Noise" in the application note 
Using Clock Jitter Analysis to Reduce BER in Serial Data Applications.