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Oscilloscope for EMI debugging? Still not convinced? (Part 2)

16 Aug 2013  | Alvin Ding

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In the previous part, we explored oscilloscopes potential capability in EMI debugging by extending its FFT functions. New and powerful features like multiple & overlapping FFT processing, gating FFT to post process acquired data with different spectrum variables and capability in mask violation detection; all these are very new concepts that you can implement into your debug work.

To utilise these concepts in practical applications, we review some of the most common challenges engineers face in debugging EMI problems, and discuss oscilloscope's capabilities for testing.

1. Emissions are low in the mV range: Electromagnetic interferences typically have very low amplitude of around mV (1mV = 60dBuV) to be captured and analysed. Traditional oscilloscopes have neither the full bandwidth nor capability to display 1mV/div (some leverage on software magnification). Therefore it is important for the oscilloscope to be able to capture these very low amplitude signals with a high sensitivity and a high measurement dynamic.

Typical spectral emission

Figure 1: A typical spectral emission captured with a near-field probe. Notice the 1mV/div setting on the enlarged left picture. An oscilloscope with a true 1mV/div is required to capture such emissions with near-field probe.

2. Understanding the behaviour of the unwanted emissions: An overview of the radiated emission from the DUT in the entire frequency range of interest is useful as it helps understanding the nature of emissions—whether it is caused by magnetic-field or electric-field, broad band or narrow band. The key aspect of the oscilloscope is to have both wide capture bandwidth of up to the GHz range for the broadband overview, and the flexibility in changing the FFT span for the differentiation of broad or narrow band emission.

Typical spectral emission

Figure 2: Top: spectrum from 0MHz to 1GHz; bottom: spectrum from 125MHz to 175MHz. A good practice for EMI debugging starts from a broadband overview, especially when emissions failure conditions are unknown. When abnormal emissions are observed, change the FFT span within the same single acquisition for better analysis.

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