RF Phase Noise and Phase Jitter Tutorial

- an overview or tutorial about 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.

Noise in signal sources

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 specification

It is often necessary to specify the levels of phase noise. In this way the performance of an oscillator or signal source can be quantified and compared against the required specification. The phase noise of phase jitter may be specified in a number of ways, but the one that is most commonly used is to look at the noise level, i.e. the single sideband phase noise at a given point.

When looking at the noise level there are three elements to the specification:

• The phase noise level itself:   this is expressed in dB relative to the carrier, i.e. dBc
• The offset from the carrier:   as the noise level varies according to the frequency offset from the carrier, the frequency offset must be given.
• The measurement bandwidth:   The noise power is proportional toth e bandwidth and therefore it is necessary to state the bandwidth that ahs been used. Normally a 1Hz bandwidth is the standard. Usually spectrum analyzers are unable to measure in a 1 Hz bandwidth and therefore they measure the signal in a slightly wider bandwidth and mathematically adjust the level to that of a 1 Hz bandwidth.

Thus a typical phase noise specification for a signal generator or other oscillator may be -100 dBc / Hz at a 100 kHz offset.

Phase jitter specification

Apart from the more usual method of quoting phase noise as the level of the single sideband phase noise at a given offset, it is also possible to look at the phase jitter in the phase domain. Here the phase jitter is expressed in a number of radians RMS phase jitter within a band of frequencies. This form of definition is less often seen, but has the advantage that it allows the overall phase jitter to be specified. As it is the phase jitter that can cause bit errors within phase modulated data transmissions, this form of specification is often more useful.

It is possible to convert from the phase noise in terms of jitter measured in RMS radians to phase noise as a power level in a given bandwidth by integrating the noise over the bandwidth.

Summary

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.