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Oscilloscope for EMI debugging? Really? (Part 1)

31 May 2013  | Alvin Ding, Markus Herdin

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With the availability of high-performance oscilloscopes providing powerful FFT analysis and excellent noise performance, a new tool exists for debugging EMI problems. Based on the results from compliance testing, the oscilloscope proves to be a valuable tool to quickly understand unwanted emissions and identify their root cause. Having access to both time-domain and frequency-domain, in the same instrument, allows for faster analysis of unwanted emissions. Since the oscilloscope is usually available at the desk of the design engineer, it enables debugging of EMI problems in R&D and allows tests before going to the EMC lab, thereby significantly increasing the likelihood of a successful compliance test.

This is the first in a series that discusses various use cases and highlights limitations of high-performance oscilloscopes with powerful FFT implementations for EMI applications.

New digital oscilloscopes, such as the Rohde & Schwarz RTO, which we will use in this use-case analysis, bring EMI debugging capabilities directly onto the desk of the design engineer. This model offers front-end sensitivity that compares with spectrum analysers and allows simultaneous access to both the time domain and the frequency domain.

Digital oscilloscopes cannot replace EMI receivers because they lack pre-selection filters, preamplifiers, EMI weighting detectors, EMI bandwidths and dynamic range. They prove efficient, however, when identification of unwanted EMI emissions and analysis of the root-cause is what you need.

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Figure 1: This example shows spectral emissions captured with a near-field probe. With a resolution bandwidth of 100kHz, the measured noise floor of the RTO oscilloscope at 605MHz centre frequency is about -105dBm. This enables analysis of weak signals.

Spectral analysis is key
The key element for EMI debugging is the spectral analysis function in the oscilloscope. Conventional FFT implementations in oscilloscopes are rather inflexible, since the spectral parameters are usually controlled by the time-domain setting; this makes it difficult to navigate in the frequency domain, and therefore difficult to use in this application. Furthermore, without an intelligent way of combining the spectral analysis function with the time-domain, a key advantage of oscilloscopes for EMI debugging is lost—the possibility to analyse unwanted emissions in both domains in order to more quickly identify the root-cause of the problem.

We went for a more intuitive approach. The paradigm is a spectrum analyser use-model that gives the user direct control of typical spectrum analyser parameters like start and stop frequency, resolution bandwidth or the detector type. The oscilloscope takes care of time-domain settings by intelligently making use of features like deep memory, powerful signal processing and advanced algorithms like overlapped-FFT. This allows for setting time and frequency parameters partly independently of each other, giving a lot of flexibility when analysing both time and frequency domains of the captured signal.

The big advantage of this concept in the field of EMI debugging is that an oscilloscope captures the whole bandwidth with a single acquisition. This allows for efficiently analysing the whole spectrum of interest. Moreover, the flexible configuration options of the oscilloscope's FFT makes it possible to look at different parts of the spectrum at the same time.

Overlapped-FFT and intensity grading allow visualizing the temporal characteristics of the spectrum and gives insight into the structure of the signal

Overlapped FFT
A key advantage of the overlap FFT algorithm is the high sensitivity to spurious signals. The captured signal is split into many parts just before FFT processing, such that, for each overlapping part of the time signal, a new spectrum is calculated. This ensures that sporadic signals, carrying a small amount of energy, show up in the corresponding spectral line; intensity grading then allows the underlying characteristics of the underlying signal structure to be revealed.

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