App note from SiTime about phase noise, its fundamental and going to actual measurements. Link here
Phase noise is one of the fundamental metrics for oscillators. An experienced engineer can tell a lot about the quality of an oscillator and whether it fits the application by looking at the phase noise plot. RF engineers focus on the phase noise levels at certain carrier offset frequencies to make sure that the required modulation scheme can be supported. Professionals designing high speed serial links like 40GbE will apply a band pass filter to phase noise of a reference clock, integrate it, and convert it to phase jitter to predict the bit error rate of a system.