Phase noise

Phase noise measured by signal source analyzer (SSA). The SSA shows the positive part of the phase noise. In this picture there is a phase noise of the main carrier, 3 other signals and "noise hill".
A weak signal disappears in the phase noise of the stronger signal

In signal processing, phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity (jitter). Generally speaking, radio-frequency engineers speak of the phase noise of an oscillator, whereas digital-system engineers work with the jitter of a clock.