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Locate near-field EMI faults with sniffer probes (Part 3)

03 Oct 2013  | Alvin Ding

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Wideband scanning

Figure 2: It is useful to first have a quick view of emission in the whole span of interest before homing in on a narrow band for further analysis and debugging.

H-field and E-field

Figure 3: The magnetic field and electric field generated from the signal plane.

So it is a good practice to perform such wideband scanning with a wider span to get an understanding of the overall emission. High-performance oscilloscopes, such as the R&S RTO with its acquisition bandwidth in the gigahertz range, give insight into the wide span emission from components in the DUT.

Start with the larger loop probes
Loop probes offer varying sensitivities to H-Field emissions. The larger the diameter of the loop, the higher is the sensitivity of the probe. Start with a larger loop probe to get a general idea location of radiation, followed by a smaller loop with sufficient sensitivity. The smaller loop will provide better spatial resolution to locate sources of radiation within the required frequency band.

H-field and E-field probes

Figure 4: The signal capture behaviours of an H-field probe (left) and an E-field probe (right).

Use H-field & E-field probes
In any circuit board, there is seldom a purely electric field or a purely magnetic field emission. It is important to use both probes to understand the nature of emission and with that, identify what specifically is causing it.

H-field is generated by current in PCB traces, cables, etcetera, whereas E-field is generated by voltage difference between adjacent traces and signal planes. Different routing design will cause one field to dominate the other in different area and therefore ability to look at both fields often brings more flexibility in measurements.

Handles for near-field scanning
Small movements or slight repositioning of the near-field probe may alter field strength readings, thus rendering difficult repeatability of the measurements especially when the emissions were only a few dB from the noise floor. A probe stand or probe fixture will provide a fixed or at least a steady position during a measurement. It is very important to keep the probe distance and orientation constant to minimise variation in measurements.

Observing random & spurious signal
When observing near-field signals, chances are EMI emission is random. When moving a probe across the DUT, it may pick up such signals for very short intervals and maybe be able to reproduce it consistently. The max-hold function is proven useful for observing such difficult scenarios.

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