Testing Near-Field Emissions for NI PXI Instruments​

Introduction and Challenge

The NI PXI platform provides a robust, compact solution for high-performance, mixed-measurement systems in validation and production test. What makes PXI special is this capability to perform multiple types of measurements in one small box. However, this benefit is not without a challenge: potential interference between modules.

​Electromagnetic interference (EMI) is caused by the magnetic fields emitted by components like inductors and transformers in neighboring PXI modules. Power supplies tend to emit the largest magnetic fields through these components. When PXI was initially introduced in the late 1990s, the interference was minimal. However, as new instruments became more sensitive and switching power supplies increased the currents they carried, the problem grew.

​The goal was to develop a reliable and automated methodology to measure and mitigate EMI between PXI modules, which is crucial for ensuring the performance and reliability of these instruments.

​Previous Solutions

​Previous methods used to measure the magnetic fields were inconsistent. Internal solutions used probes and spectrum analyzers with makeshift setups like cardboard templates to map emissions. These methods were subjective, non-standardized, and tedious, which led to unreliable results.

​Commercial scanners were available that could map the magnetic fields, but they had two significant limitations:

• ​Single-dimension measurement—These scanners only measured the magnetic field in one dimension, which did not capture the full three-dimensional nature of the field.
• ​Inaccurate environment—They did not allow scanning the card in its native environment within the chassis, which is crucial for accurate measurements.

​Solution Development

​NI engineers Ed Loewenstein and Tim Keil developed a custom scanner using three stepper motors, NI PXI instruments, and LabVIEW to automate the measurement of near-field emissions. The scanner was designed to meet the following requirements:

• ​Measure in three dimensions—Unlike commercial scanners, the solution could reorient the probe in three dimensions to capture the full magnetic field.
• ​Operate in the device’s native environment—The scanner could measure emissions of the PXI module while operating in the chassis, ensuring accurate and relevant data.
• ​Automate data collection—The scanner was automated to ensure consistent operation and data acquisition.

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Figure 1: The near-field emissions probe inside a PXI chassis.

​The team used several NI hardware and software tools to build and operate the scanner:

• ​NI PXIe-4139 Power Supply—The PXIe-4139 is a high-precision system source measure unit (SMU). It was used to supply power to the motors and provide an additional benefit of diagnostic information. By monitoring both the current and voltage, they could ensure safe operation to prevent damage.
• ​NI PXIe-5170 Oscilloscope—The PXI-5170R is a high-speed digitizer designed for precise signal acquisition. It was chosen for its ability to acquire the spectrum from the probe accurately and efficiently. Its high bandwidth and real-time processing capabilities were crucial for building detailed heat maps of emissions.
• ​NI LabVIEW—LabVIEW is a graphical programming environment used to control the entire system. LabVIEW was instrumental in automating the entire testing process—controlling the power supply and stepper motors, reading various sensors, acquiring data from the digitizer, and recording and displaying the results.
• ​NI Vision Development Module—The Vision Development Module is used with LabVIEW for developing applications that need image processing. It allowed the team to slice data dynamically and visualize emissions in real-time, significantly speeding up the process and making it easier to interpret the results.

​Figure 2: The LabVIEW user interface of the Near-Field Emissions program.

​Results and Conclusion

This innovative approach established a standardized, automated, and reliable method for assessing near-field emissions. It enabled engineers to rapidly detect and resolve EMI problems, leading to notable enhancements in module performance and compatibility. Moreover, the solution fostered a culture of continuous improvement, encouraging engineers to iteratively test and refine their designs using the scanner’s feedback.

​The development of this near-field emissions testing methodology solved a critical internal challenge but also demonstrates the power and flexibility of our products.

​About The Engineer: Ed Loewenstein

​Ed Loewenstein started his career with NI in 1987 and is currently a Chief Architect focused on long-term innovation projects. Throughout his career, he has played a hand in nearly the entire NI hardware portfolio, including data acquisition, PXI, and RF products. Ed'’s passion for learning and dedication to continuous improvement shines through in his work. He is a kind, collaborative team player who makes NI better.   

​“I like this kind of project because it uses some of the skills that I already have and makes me acquire new ones. I didn’t know anything about motion control or image processing, but I learned it. And with LabVIEW, I’m not a software guy, but I’m able to write a program that does what I need to do in a credible fashion.”​