Practical EMI troubleshooting with a mixed domain oscilloscope

Faride Akretch, Tektronix

December 29, 2013

Faride Akretch, TektronixDecember 29, 2013

Modern embedded system designs present 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 like the mixed domain oscilloscope (MDO) help eliminate guesswork. An MDO incorporates a wideband spectrum analyzer 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 9 kHz to 30 MHz while radiated emissions range from 30 MHz to 6 GHz.

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 analyzer rather than invest in a specialized 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.
Instrument selection

A common tool for EMI pre-compliance testing is a swept-tune spectrum analyzer. 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 analyzer 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 analyzer 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 analyzers provide about 500 trace points – not enough for gigahertz of spectrum. When selecting a swept-tune spectrum analyzer for EMI testing, that number ideally should be in the thousands for better frequency resolution.

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

As shown in Figure 2, a real-time spectrum analyzer (RTSA) uses an ADC to create time domain samples, which are digitally down-converted to create I-Q, which then are passed along to a real-time engine. It’s real time because no samples are dropped during processing. An RTSA offers wide capture bandwidth since it’s not RBW limited, along with comparable sensitivity and dynamic range. From a speed perspective, RTSAs are significantly faster than swept-tune spectrum analyzers for narrow resolution bandwidth. Since the signal can be looked at from an I-Q perspective, you can do multi-domain analysis in a correlated manner, including frequency, amplitude, and phase vs. time. For instance, if you put a marker in one domain it automatically tracks in another. This isn’t helpful for EMI scanning, but it is helpful for EMI diagnostics.

Figure 2: A real-time spectrum analyzer offers high-speed spectrum measurements and correlated measurement domains, helpful for EMI diagnostics.

Another instrument for debugging potential EMI issues and EMI pre-compliance is a mixed domain oscilloscope (MDO). An MDO combines a dedicated spectrum analyzer, analog oscilloscope channels, and digital logic analyzer channels with time correlation across at all inputs. Unlike a swept-tune spectrum analyzer that sweeps from left to right and looks at the spectrum through the eyes of the RBW filter, the MDO instantaneously digitizes up to a full 3.75 GHz span. This in turn enables global triggers across all channels with common acquisition control. The ability to trigger on the RF, digital or analog inputs and maintain precise correlation with the other inputs is useful for EMI diagnostics.

Debugging EMI issues
While unwanted emissions can come from any number of places in an embedded system, the first place to look is the power supplies, especially those with switch mode operation, which often leads to ringing. Depending on the amplitude of the ring, this a ripe source of RF energy. From there, the next most likely source of RF energy is the clock and data. If you are using spread spectrum clocking, it’s important to know how well it’s working.

Resonances are another prime source of EMI issues. These could be emanating from the board itself, wiring geometries, cabling and faulty shielding, improper shielding, and mechanical connection issues. Any location where signals are either coming in or going out are potential problem areas.
The go-to tool for isolating sources of energy on a PCB is a near-field probe. An E-field or stub probe is useful for high voltage, low current sources and provides maximum sensitive when held perpendicular to the source. An H-field or loop probe is better suited to low-voltage, high-current sources and provides maximum sensitivity when held parallel to the source. In general, near field probes can’t be calibrated, but this isn’t a problem from a diagnostic perspective because you are more concerned about relative changes. For instance, if you have a spot frequency that you know is a problem and want to mitigate with shielding or a change in the design, you can measure before and after to find out how much you’ve affected the signal.

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