How to Measure Phase Noise with a Spectrum Analyzer

Spectrum analyzers provide one of the most convenient test instruments that can be sued for making accurate phase noise measurements.

Spectrum Analyzer Tutorial Includes:
What is a spectrum analyzer     Spectrum analyzer types and technologies     Superheterodyne / sweep spectrum analyzer     FFT spectrum analyzer     Realtime spectrum analyzer     USB spectrum analyzer     Spectrum analyzer tracking generator     Specifications     Spectrum analyzer operation     Noise figure measurements     Phase noise measurements     Pulsed signal spectrum analysis    

Today's spectrum analyzers provide a very effective means of testing phase noise easily and accurately, and can be much easier and more accurate than using approaches using other forms of electronics test instruments.

These electronics test instruments are often designed with routines incorporated into the software to make the testing even easier. When compared with other methods using different forms of test equipment, the spectrum analyzer not only provides a more convenient method of gaining the phase noise measurements, but it is normally more accurate.

Phase noise is growing in importance as a parameter on many RF devices because not only can poor phase noise performance result in increased data errors, but it can also create interference to users on other channels.

Accordingly phase noise measurements are needed for a host of different types of electronics equipment during the design stages. It can be applicable to items from mobile phones, to nodes / units used for the Internet of Things, IoT, short range wireless, radio communications equipment and a large number of other items.

As a result of the variety of items that may need phase noise measurements to be made, a convenient way of achieving this is needed, and the spectrum analyzer is an ideal test instrument to meed this need.

Spectrum analyzer showing phase noise plot
Spectrum analyzer showing phase noise plot

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.

Note on the Phase Noise:

Phase noise consists of small random perturbations in the phase of the signal, i.e. phase jitter. These perturbations are effectively phase modulation and as a result, noise sidebands are generated. These spread out either side of the main signal and can be plotted on a spectrum analyzer as single sideband phase noise.

Read more about Phase Noise.

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.

Many free running oscillators are not 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.

In addition to this the phase noise performance of the spectrum analyzer must be better than that of the item under test, otherwise the test will measure the phase noise characteristic of the spectrum analyzer.

Whilst it is not essential, it helps if the spectrum analyser has a built in routine for phase noise measurement. Many modern test instruments have these routines built in and t can be a great help.

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 filter - 10 log 10 ( BW 1 )

      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:
    • Use screened leads:   Use screened leads for all signal connections
    • Keep away from noise sources:   Ensure that the test system including the unit under test is located away from any sources of interference. As the signal levels being measured will be very low for some frequencies even a small amount of pick-up can cause erroneous results
    • Screened room?   If an RF screened room is available, it may be possible to use this to perform the test, ensuring no interference is picked up.
  • Run unit under test from correct power source:   A power supply can considerably alter the noise performance of the RF circuitry. Ensure that the power supply for the equipment is used, or at least one with a performance level the same. Switching power supplies often generate more noise than analogue linear ones so this should be remembered.
  • Ensure analyser performance suitable:   There are two main issues; namely the spectrum analyzer phase noise itself, and the dynamic range performance:
    • Spectrum analyzer phase noise performance :   For signals that have very low levels of phase noise, it is possible that the unit under test may approach the performance of the analyzer. In this case the phase noise of the oscillator within the analyzer will add to that of the signal under test and this will distort the result. To prevent this, ensure that the phase noise performance of the analyzer is at least 10dB better than that of the unit under test.
    • Spectrum analyzer dynamic range:   The dynamic range performance of the spectrum analyser must also be sufficient. 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.

Using these precautions and any others that may be appropriate ensure that it is possible to obtain some very good results when using a spectrum analyzer to measure phase noise.

Spectrum analyzers are ideal test instruments for making phase noise measurements. With many modern high performance analyzers already incorporating routines for undertaking these tests in any RF design or test scenario, the measurements are not only easy to make but also reliable.

In view of the rigours of making phase noise measurements, it is mainly the top end test instruments that can make these measurements and have the routines built in. Nevertheless, it is possible, with care to use other lower end spectrum analyzers to make estimates of the phase noise performance of circuits, modes and systems, provided that there is an understanding of the limitations of the test.

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