# How to Measure Phase Noise with a Spectrum Analyzer

### Spectrum analyzers provide one of the most convenient ways of making accurate phase noise measurements.

When looking at how to make a phase noise measurement, one of the most popular options is that of using a spectrum analyzer.

Spectrum analyzers are able to make good phase noise measurements looking at the single sideband phase noise profile of a signal.

## What is phase noise

Phase noise results from the short term phase fluctuations that exist on any signal. This is known as phase jitter and is measured directly in radians.

The phase jitter manifests itself on a signal as sidebands that spread out either side of the main signal. This is known as single sideband phase noise, and when looked at in this manner it is easier to visualise and also measure.

Phase noise is important for a number of reasons:

• Degrades performance of data transmissions:   Most data transmissions like those used for cellular communications, Wi-Fi, and many other applications use forms of modulation that use phase as part or all of the modulation technique. Any phase noise will reduce the margin between the different states and will impact signal margins and resulting bit error rates. This means it is important to have a good phase noise performance for any local oscillators.
• Adjacent channel interference:   The phase noise spreads out either side of the main signal and can fall into nearby channels causing interference to other users. As a result, spurious emissions, including phase noise must be kept below certain limits to ensure interference is not a problem.

The phase noise is measured as the noise power in a given bandwidth. The standard is a 1Hz bandwidth. Although the measurement may be made in a wider bandwidth, it can be easily converted to the value for a 1Hz bandwidth.

In addition to this, the value of the noise is related to the carrier level. A given number of decibels down on the carrier. The standard abbreviation indicating this is dBc.

Finally the offset from the carrier must be stated because the noise level varies as the offset from the carrier is changed.

This a typical specification is quoted in terms of decibels down on the carrier in a 1Hz bandwidth at a given frequency offset, i.e. dBc / Hz at xx kHz offset.

## 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.

## How to measure phase noise with a spectrum analyzer

Although there are many ways of measuring phase noise, the most straightforward is to use a spectrum analyzer.

Essentially the analyzer is connected to the output of the unit under test via any suitable attenuator needed to reduce the power into the analyzer (if the output power from the unit under test is high).

In some instances it may be necessary to lock the oscillator standards of the analyzer and unit under test together. In this way there will be no signal drift which could be an issue for close in measurements.

The analyzer is then set to measure the signal level out from the carrier - often this may from the carrier out to a frequency of 1 MHz or possibly more. Ideally to a point where the noise has reached the noise floor.

The bandwidth of analyzer must be set so that a good balance is achieved between the resolution of the scan and the time taken for the scan to be taken. The noise level can then be converted to that found in a 1Hz bandwidth.

## Analyzer filter & detector characteristics

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

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

It is possible to use a simple formula to make an adjustment for the filter bandwidth:

${L}_{1Hz}={L}_{\mathrm{filter}}-10{\mathrm{log}}_{10}\left(\frac{\mathrm{BW}}{1}\right)$

Where:
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. Typically a known factor for the filter in use needs to be incorporated to ensure a correct transformation.

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.

## Phase noise measurement precautions

There are a few important precautions to remember when measuring phase noise with a spectrum analyzer

• Ensure no external noise can be picked up:   The spectrum analyzer measures the single sideband phase noise and therefore any amplitude noise that is present will add to this, degrading the result. Ensure that no external noise can be picked up by the analyzer: