Measuring Phase Noise with a Spectrum Analyzer

- phase noise can be measured very easily using a spectrum analyser, but there are a few key facts to understand.

Phase noise is a key parameter for many systems, and measuring it accurately is of great importance.

One of the easiest methods to measure phase noise is to use a spectrum analyser using what is termed a direct measurement technique.

Using a spectrum analyser to measure phase noise can provide excellent results provided that the measurement technique is understood and precautions are adopted to ensure the most accurate results.

Phase noise

A certain level of phase noise exists on all signals and extends out either side of the wanted signal or carrier. The shape of the phase noise plot will depend upon whether it is a free running oscillator, or locked within a phase locked loop, as this will alter the noise profile.

Phase noise display for a typical free running oscillator when displayed on a spectrum analyzer
Phase noise of a free running oscillator

Note on Phase Noise:

Phase noise consists of small random perturbations in the phase of the signal, i.e. phase jitter. An ideal signal source would be able to generate a signal in which the phase advanced at a constant rate. This would produce a single spectral line on a perfect spectrum analyzer. Unfortunately all signal sources produce some phase noise or phase jitter, and these perturbations manifest themselves by broadening the bandwidth of the signal.

Click on the link for a Phase Noise tutorial

Pre-requisites for measuring phase noise

The main requirement for any phase noise measurement using a spectrum analyser is that it must have a low level of drift compared to the sweep rate. If the level of oscillator drift is too high, then it would invalidate the measurement results.

This means that this technique is ideal for measuring the phase noise levels of frequency synthesizers as they are locked to a stable reference and drift levels are very low.

Free running oscillators are not normally sufficiently stable to use this technique. Often they would need to be locked to a reference in some way, and this would alter the phase noise characteristics of at least part of the spectrum.

Phase noise measurement basics

The basic concept of using a spectrum analyser to measure the phase noise levels of a signal source involve measuring the carrier level and then the level of the phase noise as it spreads out either side of the main carrier.

Typically the measurement is made of the noise spreading out on one side of the carrier, as the noise profile is normally a mirror image of the other and there is no reason to measure both sides. As a result the term single sideband phase noise is often heard.

As the level of phase noise is proportional to the bandwidth of the filter used, most phase noise measurements are related to the carrier level and within a bandwidth of 1 Hz. The spectrum analyser uses a suitable filter bandwidth for the measurement and then adjusts the level for the required bandwidth.

Diagram showing the typical spectrum analyzer display when shwoing phase noise

Typically phase noise measurements are specified as dBc/Hz, i.e. level relative to the carrier expressed in decibels and within a 1 Hz bandwidth.

Filter and detector characteristics

The filter and detector characteristics have an impact on the spectrum analyser phase noise measurement results.

One of the key issues is the bandwidth of the filter used within the analyser. Analysers do not possess 1 Hz filters, and even if they did measurements with a 1 Hz bandwidth filter would take too long to make. Accordingly, wider filters are used and the noise level adjusted to the levels that would be found if a 1 Hz bandwidth filter had been used.

Equation for normalising phase noise to 1Hz - often used for spectrum analyzer applications

      L1Hz = level in 1 Hz bandwidth, i.e. normalised to 1 Hz, typically in dBm
      LFilt = level in the filter bandwidth, typically in dBm
      BW = bandwidth of the measurement filter in Hz

As the filter shape is not a completely rectangular shape and has a finite roll-off, this has an effect on the transformation to give the noise in a 1Hz bandwidth.

The type of detector also has an impact. If a sampling detector is used instead of an RMS detector and the trace is averaged over a narrow bandwidth or several measurements, then it is found that the noise will be under-weighted.

Adjustments for these and any other factors are normally accommodated within the spectrum analyser, and often a special phase noise measurement set-up is incorporated within the software capabilities.

Spectrum analyser requirements

When measuring phase noise with a spectrum analyser, there are some minimum requirements for this type of measurement.

  • Spectrum analyser phase noise:   In order to be able to measure the phase noise of a signal source using a spectrum analyser, the specification of the analyser should be checked to ensure it is sufficiently better than the expected results for the source.

    The reason for this is that if a perfectly good signal source was being measured, the phase noise characteristic of the local oscillator in the spectrum analyser would be seen as a result of reciprocal mixing.

    As a rough guide, the phase noise response of the analyser should be 10dB better than that of the signal source under test.
  • Dynamic range:   The dynamic range of the spectrum analyser is also an issue. The analyser must be able to accommodate the carrier level as well as the very low noise levels that exist further out from the carrier.

    It is easy to check whether the thermal noise is an issue. The trace of the phase noise of the signal source can be taken and stored. Using exactly the same settings, but with no signal, the measurement can be repeated. If at the offset of interest there is a clear difference between the two, then the measurement will not be unduly affected by the analyser thermal noise.

Phase noise test precautions

When measuring phase noise with a spectrum analyser, there are a few precautions that can be taken to ensure that the test results are as accurate as possible.

  • Minimised extraneous received noise:   During a spectrum analyser phase noise measurement, some of the levels that are measured are very low. It is therefore necessary to ensure that levels of extraneous received noise are minimised. The unit under test should be enclosed to ensure that no noise is picked up within the circuit. This is particularly true of any oscillator circuit itself such as a synthesizer voltage controlled oscillator.

    The use of double screened coax cable between the test item and the analyser may be considered. A screened room could also be used. In this way
  • Use representative power supply:   The power supply used to supply the item under test can have a major impact on its performance. Issues such as switching spikes on switch mode regulators can have a major impact on performance. Accordingly a power supply that is representative should be chosen to power the signal source being measured.
  • Analyser set-up:   Care should be taken to ensure that the spectrum analyser is correctly set up to measure phase noise. Often in-built phase noise measurement settings will be available for these measurements and they can be used as a starting point.

Measuring phase noise with a spectrum analyser is one of the easiest and accurate methods that can be employed. High end analysers are designed with this as a regular measurement that will need to be made, and issues with extraneous noise and many other problems are minimised. Although other methods can be adopted to measure phase noise, special systems often need to be developed, and in view of the very low levels of phase noise involved, these systems may not always be as accurate. A unit that has been developed and optimised with these measurements in mind is bound to have solved the majority of problems and provide a far more time and cost effective method of making these measurements.

By Ian Poole

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