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

03 Oct 2013  | Alvin Ding

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In the first two articles of the series, we discussed the capability oscilloscopes can deliver in EMI debugging. Let's now look at some guidelines on the general approach to detecting EMI with near-field probes.

Far-field measurements performed at the final compliance test do not help in locating the source of the problem, which makes it difficult for engineers to apply counter-measures and remedies. The ability to locate and determine the emissions source is therefore top priority in debugging any EMI problems.

Near-field probes, better known as sniffer probes, have been widely used for the identification of the emission source. They act as an antenna for the detection of near-field electronic or magnetic emission from the device under test (DUT). This helps the engineers reduce the scope of the measurements and focus as close to the emission source as possible. In such scenarios, the accuracy of or close correlation with the far-field result is not critical.

Sniffer probe at work

Figure 1: By scanning around the circuit, you can easily observe an emission peak whenever the sniffer probe comes near the source.

The near-field test shows what type of field is dominant and the strength of emission from the source, albeit only relative results. In general, the closer the source, the stronger it will register in the field measurements, which gives a good indication of where to start debugging EMI problems. Let's now discuss the general approach to detect EMI with near field probes.

Utilising the analogue front end
It is important to have a high-sensitivity and high-dynamic range oscilloscope as the emissions are normally measured in the ranges of mV. Always remember to adjust the vertical scale to fit the screen as much as possible such that the scope utilises the full scale ADC on the captured signal while avoiding signal clipping.

A general guideline of vertical range setting is to start low at somewhere from 1mV/div to 4mV/div to be able to detect weak signals and adjust to a higher vertical range as required. It is also important to ensure that equipment is still able to capture at the bandwidth needed at such high sensitivity.

Near-field broadband scan
A brief scan across all the circuit components, traces, connectors in wideband is crucial to gain a quick overview of the radiated emissions in the span of interest. In conventional receivers where emission is measured at one frequency, it limits the observation range because the source may be generating emissions or resonance at a higher frequency. For instance, embedded boards may have RF modules that can interfere with other components at frequencies in the gigahertz range.

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