10 Tips to Make Great Phase Noise Measurements with X-Series Signal Analyzers

The N9068EM0E Phase Noise Measurement Application for X-Series Signal Analyzers makes it simple to measure the phase noise of a Device Under Test (DUT). The measurement app allows for simple Log Plot and Spot Frequency phase noise measurements. Although the measurements are automated, there are additional advanced functions and considerations to be aware of in order to perform great phase noise measurements. Even experienced users of the Phase Noise Application may have overlooked some useful features. As new technologies emerge, new challenges and needs will arise for testing phase noise performance. For example, clock sources for 6G and AI devices have ever more constricted requirements and needs for testing phase noise well above 50 GHz.

This application note presents a series of practical tips and techniques to help users maximize the effectiveness their X-Series Signal Analyzers for phase noise measurements. Topics include optimizing attenuation levels, using noise floor minimizing features like Overdrive and Noise Floor Extension (NFE), improving sensitivity at far offsets, using various forms of phase noise cancellation and rejection, measuring spurs, and extending measurements above 50 GHz. Even for demanding measurements of low phase-noise DUTs, pulsed RF signals, or drifty carriers, these insights are designed to enhance user measurement accuracy and efficiency.

A signal analyzer is often considered the Swiss army knife of RF Test and Microwave measurement devices. Signal analyzers can be used for swept-tuned spectrum measurements (hence the legacy designation of “spectrum analyzer”), as well as wideband vector signal measurements. Two of the most common measurement applications that are purchased for X-Series Analyzers are the general-purpose applications for measuring Phase Noise (N9068EM0E) and Noise Figure (N9069EM0E). Signal analyzers generally have excellent phase noise performance, which makes them well suited to measure the phase noise performance of various DUTs. For swept tuned measurements in the spectrum analyzer mode of a signal analyzer, the result trace generally includes both the AM and PM contributions from the DUT. For this reason, the published specifications for signal analyzers generally have a section for “noise sidebands” rather than for phase noise, since phase noise is not the only component displayed in the resulting trace. When measuring the phase noise of a DUT with a signal analyzer, the measured results are a combination of the DUT’s and analyzer’s phase noise performance. Therefore, it is helpful to refer to the analyzer’s noise sidebands performance to determine how the analyzer’s performance at the various offset frequencies from the carrier compare to the expected DUT performance. Generally, the analyzer should have performance at least as good as the device being measured; otherwise, the measurement results will be dominated by the analyzer’s performance and not the DUT’s. A good rule is to have a margin of at least 3 dB better phase noise performance than the DUT, and up to 10 dB if possible.

Understanding the advanced features in the Phase Noise Measurement Application (N9068EM0E) for X-Series Signal Analyzers can yield improved results for standard, far-offset, high performance (low phase-noise), drifty carrier, E/V/W band, and pulsed RF phase noise measurements by reducing the impact of the analyzer’s residual phase noise floor.