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Mixed domain scope for EMI troubleshooting

Posted: 08 Jan 2014     Print Version  Bookmark and Share

Keywords:embedded system  EMI testing  troubleshooting  mixed domain oscilloscope  MDO 

Modern embedded system designs come with challenges for EMI testing and troubleshooting that didn't exist years ago. These challenges include switching power supplies, high speed system clocks and data buses, bursty information transfers, transmission line and termination issues, and spread spectrum clocking, as well as the integration of wireless interfaces and connectivity. Adding to the challenge is that most of these technologies can lead to issues that are transient, load dependent, and highly variable over time.

With budgets and time pressures greater than ever, it's critical that designers working on embedded systems test for potential EMI issues and head them off as early in the design process as possible. Traditional tools often are not adequate to identify the source of EMI problems in today's electronics. Fortunately, the combination of a practical approach to testing and new tools such as the mixed domain oscilloscope (MDO) helps eliminate guesswork. An MDO incorporates a wideband spectrum analyser along with a traditional mixed signal oscilloscope, helping to make troubleshooting transient RF and EMI signals faster and more efficient.

Catch EMI problems early
EMI compliance is a fact of life for virtually any embedded system, with considerable variability across countries and industries. Specifications define levels for unwanted conducted and radiated emissions, as well as susceptibility/immunity standards for the device. Conducted emissions span 9kHz to 30MHz while radiated emissions range from 30MHz to 6GHz.

Typically, compliance measurements are complex and therefore expensive and are conducted at a test house using a stepped EMI receiver in an anechoic chamber. There's no debating the value of only having to going to the test house once, saving time and expense. The way to ensure a single trip – or at least greatly improve your odds of success – is by carefully conducting pre-compliance measurements. Although you will still need to go to the test house, you can catch problems early on throughout the design process. What's more, scanning doesn't have to take a long time.

For pre-compliance scanning, most designers opt to use a general purpose spectrum analyser rather than invest in a specialised EMI receiver. The key is to understand the differences between the them. Factors that should be considered include resolution bandwidth (RBW), the number of trace points, dwell time, support for CISPR detectors, and antenna factors. On the physical set up, biconical and log-periodic antenna are both good choices, along with a tripod and possibly a pre-amp. In the absence of an anechoic chamber, you can often find an RF-quiet location such as a boardroom or underground parking garage. Although it is difficult to completely duplicate EMI lab conditions, it is possible to make an accurate approximation by paying close attention to as much detail as possible.

Figure 1: A swept-tune spectrum analyser offers good sensitivity and dynamic range for EMI scanning, but low-end model aren't able to handle gigahertz of spectrum.

Instrument selection
A common tool for EMI pre-compliance testing is a swept-tune spectrum analyser. This traditional architecture offers good dynamic range and good sensitivity, but is limited to two measurements: frequency vs. amplitude and amplitude vs. time. This is important because when you are performing a peak scan and find a frequency of interest, the swept-tune spectrum analyser can be placed into zero span mode to look at power vs. time of the signal in order to determine periodicity.

Since this measurement is seen through the eyes of the RBW filter you can only go as wide as the RBW allows. As shown in the architecture of a swept-tune spectrum analyser in figure 1, the video bandwidth filter is a post-detection low-pass filter that smoothes out the trace. For some EMI specifications, you'll need to pay attention to frequency range and the number of trace points the instrument provides. Some low-end spectrum analysers provide about 500 trace points – not enough for gigahertz of spectrum. When selecting a swept-tune spectrum analyser for EMI testing, that number ideally should be in the thousands for better frequency resolution.

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