What Is Phase Noise?
In the simplest terms, phase noise describes the stability of an oscillator in the Frequency Domain while jitter describes stability in the Time Domain.
A Simple 5-Step Path to Understanding Phase Noise
Step 1: Spectral Density
Step 2: When plotting each spectral density point at varied frequency intervals of your choosing (In this case every 1Hz), you're left with a graph that looks like this:
Step 2: Signal Power Density
You are now looking at what is known as the Signal Power Density of the noise.
Now focus only on the upper sideband of the graph from fstart to fstop, this is called the "Single Side Band".
Step 3: Noise Power Density
We can now refer to the plotted part of the single side band as noise (Anything above the nominal oscillator frequency (Fosc) and not harmonically related can be considered phase noise). The technical term for this part of our graph is Noise Power Density (Step 3). We measure noise power density in dBW (LOG(Watts)) at this point because of the large range which we are looking.