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Using RF Monitoring Probes to Troubleshoot Transient Failures

Our previous column (see “Using a Near-Field Probe to Troubleshoot Transient Failures,” In Compliance Magazine, February 2024) introduced a valuable technique for troubleshooting electric fast transient (EFT) failures at the PCB level. However, in large systems, such as big cabinets housing numerous electronic components, employing the near-field probe method can be time-consuming and, depending on the voltage level, potentially unsafe (for instance, when dealing with high-voltage circuits requiring isolation). In such scenarios, an alternative approach is necessary. This “Troubleshooting EMI Like a Pro” column presents a technique suitable for use in such situations.

Large systems typically consist of interconnected PCBs or modules linked by cables. Placing RF current probes at various wire connection points enables the tracing of current flow during transient events. These RF current probes must be terminated to the 50-ohm impedance of an oscilloscope. Transient events vary in type, with their frequency response indicated by rise time. For instance, electrostatic discharge (ESD) exhibits the fastest rise time (sub-1 nanosecond), while electric fast transients typically have a rise time of about 5 nanoseconds. Both types suggest unpredictable RF current flow, especially in low-impedance systems. However, engineers often have a reasonable estimation of the route. Therefore, multiple RF probes can be utilized to trace the current flow.

Ideally, a pair of matched probes should be used, as they possess nearly identical transfer impedance. However, obtaining such a pair is often challenging and may require engaging with probe manufacturers and incurring a premium cost [1]. Nonetheless, two probes of the same model can suffice, provided their transfer impedance is reasonably close.

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More crucial is ensuring the correct placement of the probes so that their markings align with the current’s polarity. RF current probes exhibit directional properties, and the polarity of the probe output is determined by the current flowing through them, with reversed probes showing a mirrored output. Since the goal is to trace transient current accurately, proper probe orientation is essential.

The bandwidth of the current probe should be sufficient to capture the transient event, often requiring a 1 GHz probe. Consequently, a 1 GHz oscilloscope is preferred, although a 500 MHz scope is usually adequate for troubleshooting. Given the typically large transient currents, probe sensitivity is not a priority; a small transfer impedance, such as 0dBΩ, is ideal as it provides true current readings on the oscilloscope without requiring mathematical conversion.

In large and complex systems, the need for additional probes may arise. However, it’s impractical for manufacturers to have more than two probes. As an alternative, one can place one probe at the transient noise entry point and then relocate the second probe to various key locations. After capturing results at each position, engineers can compare them. This allows for a comprehensive understanding of the current path.

In our first example, a unit experienced susceptibility issues during an EFT/burst test. When the transient noise was injected into the unit via its mains lead, the connected laptop via a USB link froze, rendering it unresponsive. In this case, it was logical to place the first RF probe at the mains entry point and the second probe on the USB link. Currents during the transient event were captured, revealing overshoot and ringing on both cables. Introducing high-impedance components such as ferrite cores in a multi-turn configuration reduced the overshoot and ringing, ensuring normal communication between the computer and the unit.

Figure 1: Case study 1

In our second case, an electric vehicle was in charging mode when an EFT generator applied a common-mode transient to all wires (as part of the immunity test in charging mode). Consequently, the vehicle’s horn started blaring, indicating susceptibility failure. Given the size of the truck, three probes were placed at locations where the transient signal was likely to penetrate. Tests were repeated with probes placed in different positions. Upon observing a significant level of current consistent with the waveform at the mains entry, it was evident that the path required attenuation.

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Figure 2: Case study 2

During transient events, secondary effects may occur due to the complexity of the unit [2]. It is also worth mentioning that the ringing observed during the test often presents the most significant issue as it indicates resonance—a common cause of emission and immunity failure.

References

  1. Ken Wyatt, “Matched Set of Current Probes Arrived!” 
  2. Doug Smith, “ESD and EFT Internally Regenerated by Power Supplies.” 

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