What is Phase Noise: phase jitter

Phase noise / phase jitter is a key parameter on RF signals which affects the performance of radio systems in various ways.

Electronic & RF Noise Includes:
What is phase noise     Phase noise / jitter specification     Phase noise / phase jitter conversion    

RF noise topics:   Noise basics     Avalanche noise     Burst noise     Flicker noise     Shot noise     Thermal noise    

Phase noise is a key parameter for many oscillators and signal sources as it governs many aspects of the overall performance.

Phase noise can also be looked at as phase jitter they are two methods of looking at the same parameter: phase noise looks at the signal spectrum, i.e. in the frequency domain, whereas phase jitter looks at the variations of phase upon the signal.

The presence of phase noise on local oscillator or other signals affects radio systems by affecting specifications like reciprocal mixing, reducing the distance between points on PSK and QAM constellation diagrams and much more.

In view of the way in which it affects the performance of many aspects of radio communications systems, it is a key parameter in many specifications.

Phase noise as seen on a spectrum analyzer
Phase noise as seen on a spectrum analyzer

Phase noise in signal sources

The term phase noise is used to describe the noise spectrum resulting from phase jitter that arise as a result of random phase variations of the signal. The noise arises from general noise in the circuit that manifests itself as phase jitter. As phase and frequency are inextricably linked this can also be seen as random frequency variations as well.

Noise in signal sources can be considered in many ways as jitter and variations can occur over different timescales. As a result, stability can be considered in two main forms:

  • Long term stability   The long term stability of a signal addresses how the signal varies over a long term, typically hours, days and longer. This addresses subjects such as long term drift etc. It is normally specified in terms of a frequency change in parts per million, etc over a given period of time.
  • Short term stability   the short term stability of a signal source focuses on the variations that take place over a much shorter period - typically over periods of less than a second. These variations may be totally random, or they may be periodic. The periodic variations may be what are termed spurious signals, and the random ones appear as noise.

Phase noise vs amplitude noise

Phase noise can be introduced into circuits in a variety of ways, especially when frequency synthesisers are used. However for oscillators, the source of phase noise results from "thermal" and "flicker" or 1/f noise.

As most oscillators operate in saturation, this limits the amplitude components of the noise which are generally around 20dB lower than the phase noise components. This means that phase noise predominates and therefore amplitude noise is often ignored.

This assumption is true for most applications, but the amplitude components should not be forgotten as they may need to be considered in some applications.

Phase noise basics

Phase noise is the noise spectrum that is seen spreading out either side of a signal as a result of the phase jitter that exists.

Phase noise is of particular importance to RF designers. Phase jitter manifests itself as phase noise that spreads out either side of the main wanted carrier. In most cases it reduces in level the further the offset from the carrier.

Typical phase noise spectrum of free running oscillator
Typical phase noise spectrum of a free running oscillator

In view of the way frequency synthesizers operate, the phase noise spectrum or profile varies within the loop bandwidth, although ultimately it falls away in level with increasing offset from the carrier.

Importance of phase noise in communications systems

Phase noise or phase jitter is of particular importance, because it reduces the signal quality and hence increases the error rate of the communications link.

In practice, spurious phase modulation is technically more important that amplitude modulation. This is partly because the majority of radio links these days use angle modulation which is affected more by phase noise. It is also as a result of the fact that in complex signal sources, the amplitude noise content is much lower in level than the phase noise content.

Basic phase noise definitions

There is a variety of terms associated with the basic concept of phase noise. One of the key aspects of understanding phase noise is to understand the various definitions associated with it.

  • Phase noise:   Phase noise is defined as the noise arising from the short term phase fluctuations that occur in a signal. The fluctuations manifest themselves as sidebands which appear as a noise spectrum spreading out either side of the signal.
  • Phase jitter:   Phase jitter is the term used for looking at the phase fluctuations themselves, i.e. the deviations in the position of the phase against what would be expected from a pure signal at any given time. Accordingly phase jitter is measured in radians.
  • Spectrum:   The spectrum of the phase noise refers to the plot that would be obtained from a spectrum analyser. The spectrum of the signal would show the centre wanted signal with the noise sidebands extending either side of the main carrier.
  • Spectral density:   The spectral density describes the RMS phase distributions as a continuous function, expressed in units of RMS phase for a given unit bandwidth.
  • Single sideband phase noise:   Single-sideband phase noise or SSB phase noise is the noise that spreads out from the carrier as a sideband. The single sideband phase noise is specified in dBc/Hz at a given frequency offset from the carrier.

These are some of the main terms associated with phase noise and phase jitter.

Phase noise is present on all signals to a greater or lesser degree. In some instances the level of phase noise is not particularly important, but in others it can be crucial to the overall operation of whatever system the oscillator or oscillators where they are used. With ever increasing requirements for radio systems of all sorts, phase noise is a parameter that is of increasing importance.

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