Phase noise and phase jitter are two quantities that describe the same parameter. Both phase noise and phase jitter look at the effect of small phase deviations on a signal.
Phase noise is the easier concept to grasp as it can be seen on a spectrum analyzer. Phase jitter is less easy to visualise, and often phase jitter, measured in radians RMS is seldom mentioned and yet it is just as valid a method of specifying the phase perturbations on a signal.
Both formats: phase noise and phase jitter have their uses and are equally valid but they tend to be used in different applications.
Phase noise & phase jitter
Phase noise and phase jitter are effectively different methods of visualising and specifying the phase perturbations that occur on a signal.
Phase noise describes the performance in the frequency domain, whereas the phase jitter performance in the time domain.
The choice of which domain to use depends upon the application and the way the figures are best expressed.
Typically for RF applications such as signal sources and systems used for transmission and reception of digital signals (including satellite systems) phase noise, i.e. frequency domain is the normal method used.
However for applications such as TDM (time division multiplexing) systems will be interested in phase jitter far more as poor jitter performance can result in data clashes and an increased number of data re-sends.
However it is not unusual for the phase jitter performance of an RF system to be quoted in terms of phase jitter. Under these circumstances it is necessary to understand the phase noise / phase jitter relationship, and to be able to apply a phase noise to phase jitter conversion.
Phase noise and phase jitter units
Phase noise and phase jitter are measured using different units as they are parameters using different domains.
- Phase noise: Phase noise is typically measured as a given level relative to the carrier. A 1 Hz bandwidth is used as standard, and as the level varies with the offset from the carrier, this offset must be mentioned. The specification is in the form -nnn dBc/Hz mmm kHz or MHz from the carrier, e.g. -110dBc/Hz at 10kHz.
For a free running oscillator the level typically falls with increasing frequency. For PLL frequency synthesizers this may not always be the case. The level fall and also rise with increasing frequency, especially closer to the carrier where the phase locked loop bandwidth modifies the noise profile.
- Phase jitter: Phase jitter is measured in radians rms. This indicates the angular jitter from the steady carrier.
Imagine a steady state carrier represented by a vector rotating around a circle. For a steady carrier it should rotate around the central point in a perfectly steady fashion. When phase noise is present it will not move at a perfectly steady rate. Instead sometimes it will move very slightly faster and other times slow. These variations can be considered as vibrations on the rotating vector.
It is possible to convert between the two parameters.