Probing Problems Based On EMI

April 6, 2012

Electromagnetic interference (EMI) has long been a concern for design engineers. EMI stems from unintentional radio-frequency transmitters or emitters, which can be the result of unexpected signal leakage, unplanned-for harmonic frequencies, and even high-level spurious signal products. The consequence is usually interference with a receiver that is operating within the frequency band of the EMI leakage. EMI levels in electronic products are limited by regulatory agencies, including the American National Standards Institute (ANSI) and Comite International Special des Perturbations (CISPR), which require that well-established measurements be used to certify that those products comply with standardized limits for EMI. Fortunately, a 12-page application note from Tektronix, “Real-Time Spectrum Analysis for EMI Diagnostics,” explains how EMI measurements can be made with a real-time spectrum analyzer (RSA), including the types of filters that must be applied for accurate results.

The note explains that many of the EMI standards are based on how interference affects analog electronic communications systems, and thus were not written for the needs of modern digital communications systems. For systems employing digital modulation, even a short burst of interference can result in a loss of data. Fortunately, modern test instruments, such as Tektronix’s lines of RSAs, can view wide spans of frequency spectrum instantaneously, making it possible to detect even transient interferers. General-purpose spectrum analyzers are often used early in the design stages of a product, ensuring proper electromagnetic compatibility (EMC) of different circuits within a design. However, once all those circuits have been integrated (with the potential to interfere with each other), the measurement power of an RSA can help not only to find EMI problems, but to troubleshoot the overall performance of the design.

The application note addresses the flexibility offered by an RSA in terms of changing receiver filters to comply with different EMI measurement systems. In addition to the filter bandwidths, the shapes of the filters are also defined by different standards, including MIL-STD-461E for military applications. In addition, the note reviews how detectors are used in EMI measurements, so as to find a single point that represents a signal at any instant in time. Detection methods can reveal positive or negative voltage peaks, average or root-mean-square (RMS) values of voltage, or even quasi-peak values of voltage. Quasi-peak detectors read the weighted peak value of a signal envelope. These detectors, which are available in an RSA, have fast attack times with slow decay times, and read higher levels for signals that are more frequent than for those that are less frequent. The 12-page application note provides an excellent overview of EMI measurements and how an RSA serves as a versatile tool for making these measurements. A free PDF is available for download from the Tektronix website.

Tektronix, Inc.
14150 SW Karl Braun Dr.
P.O. Box 500, Beaverton, OR 97077
(800) 833.9200

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