RF Phase Noise & Phase Jitter Tutorial

- an overview or tutorial about the basics of RF phase noise or phase jitter, its theory, causes, nature and effects.

Phase noise or phase jitter is a key element in many RF and radio communications systems as it can significantly affect the performance of systems. While it is possible in an ideal world to look at perfect signals with no phase noise, that are a single frequency, this is not the case. Instead, all signals have some phase noise or phase jitter in them. In many cases this may not have a significant effect, but for others it is particularly important and needs to be considered.

For radio receivers, phase noise on the local oscillators within the system can affect specifications such as reciprocal mixing and the noise floor. For transmitters it can affect the wideband noise levels that are transmitted. Additionally it can affect the bit error rate on systems using phase modulation as the phase sitter may just cause individual bits of data represented by the phase at the time to be misread.

Phase noise is also important for many other systems including RF signal generators, where very clean signals are required to enable the generator to be used as a reference source.

Spectrum analyser showing phase noise plot
Spectrum analyser showing phase noise plot

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.

Noise in signal sources

The term phase noise is used to describe the phase variations that arise as a result of random frequency variations of the signal. The noise arises from general noise in the circuit that manifests itself as frequency variations.

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 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 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.

A typical phase noise spectrum of a signal source
Typical phase noise spectrum or profile of a signal source

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.

While phase noise is an unwanted addition to all signals, its presence must be accounted for in many applications. Phase noise is an important aspect of frequency synthesizer and signal generator design, and levels of phase noise must be considered at the earliest stages of design of these items.

By Ian Poole

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