Phase Noise Measurements

- notes and key details about how to measure RF phase noise and phase jitter and the measurement precautions to adopt.

In view of the importance of phase noise for many RF applications, phase noise measurement features highly in the characterisation of many circuits and systems.

Phase noise measurement is not always an easy test to perform. Measurements can easily be inaccurate if the wrong procedures are used.

Fortunately, today, many spectrum analysers are able to offer the level of performance required to enable phase noise measurements to be made satisfactorily for many requirements.

Phase noise measurement methods

There are a number of methods that can be used to measure phase noise, some of which are more widely used than others:

  • Spectrum analyser method:   Spectrum analysers provide a very easy method for phase noise measurement. When making a phase noise measurement, the analyser measures the noise amplitude in the sidebands appearing either side of the main carrier. Knowing the bandwidth of the filter used in the spectrum analyser, it is then easy to perform a mathematical function to normalise the reading to figures expressed in dBc/Hz - the normal method of specifying phase noise.

    The main drawback of this method is that the spectrum analyser measures the noise in the sidebands that extend out either side of the carrier. The measurement is actually a measurement of V^2 (ω) and not actually Φ^2 (ω). In other words it measures the summation of the phase and amplitude noise and not just the phase noise. For most instances this is accurate because the phase noise >> amplitude noise. Additionally the Lorentzian shape of the V^2 (ω) curve will produce an optimistic value for the phase noise measurement.

    Many spectrum analysers, and particularly the high end spectrum analysers will have sophisticated software to correct for issues with inaccuracies and are able to provide very good levels of measurement accuracy. Being relatively simple to operate and to set-up tests, it is possible minimise the measurement inaccuracies. As a result, spectrum analysers often provide the preferred method for taking phase noise measurements.
  • Phase locked loop method:   The phase locked loop can provide some good results for phase noise measurement.

    PLL technique for measuring phase noise

Phase noise measurement precautions

In order that any phase noise measurements are made as accurately as possible, there are some precautions that can be observed.

  • Clean measurement sources:   When measuring phase noise, it is essential that the noise contribution from the oscillator required in the measurement system is significantly better than that being measured. If not, the noise from the measurement system will contribute to that measured. It is generally accepted that the oscillator in the measurement system should have a performance that is an order of magnitude better, i.e. 10dB. Even at this there will be a small influence on the measured readings. Anything less than a 10dB better performance and it is necessary to calculate the performance of the item under test.
  • Reduce external noise:   It is very easy for external noise to be superimposed in the phase noise generated by the system under test. This external noise on the phase noise measurements can arise from a number of areas and be reduced if the correct steps are taken:

    • Reduce power supply noise:   Noise on the power lines for a system, and particularly an oscillator will give rise to phase noise. When measuring phase noise, use the same power supply as will be used with the system , or one that will provide a similar ripple performance.
    • Remember earth loops:   Earth loops can give rise to noise. These should be addressed during development, but when testing, this may be undertaken with the system in a state where access is required. This could introduce more earth loops and hence additional noise.
    • Eliminate stray pick-up:   Stray pick-up can reflect onto any phase noise measurements. It should be eliminated for any tests to be valid. The system may need to be tested in its fuly assembled state to remove the possibility of stray pick-up, or under some circumstances it may need to be undertaken in a screened room.
    • Eliminate pick-up on cables:   Test cables such as coaxial leads can often provide a source for stray pick-up. Even though the leads are screened, pick up is still possible. To ensure accurate tests, ensure all screens, etc are securely connected within connectors; ensure that cable provides sufficient screening - use good quality coax - possibly double screened, etc.

By Ian Poole

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