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Oscilloscope for EMI debugging? Use multiple FFTs (Part 5)

07 Feb 2014  | Alvin Ding

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In the fifth instalment of this article series on EMI debugging, we tackle the actual usage of time correlated FFT gating and multiple FFTs with oscilloscope.

Typically, far field test only concludes whether a DUT passed or failed. It offers little indication when it comes to determining the problematic area. The ability to correlate is a key factor for emitter source identification. With oscilloscope, engineers can debug EMI with up to four input channels simultaneously for better analysis and investigation compared with conventional spectrum analyser or test receiver.

Time correlated FFT gives insight of interference signal behaviour
Locating the emission source can be the most challenging part in EMI failure analysis, or debugging. One particularly useful feature for EMI debugging is joint time-frequency analysis that reveals how the signal spectrum evolves over time. This analysis method is often used to identify the source of emissions in complex systems where multiple broadband sources exist such as switched power supplies with DC-DC converters.

It was a complex technique used in advance EMI failure analysis due to the complicated time synchronised setup of traditional oscilloscope and spectrum analyser together. However that changes with modern implementation of strong FFT capable oscilloscope.

Applying FFT on oscilloscope's acquired waveforms has no synchronisation and de-skew problems that traditionally exist with multiple instruments setup or oscilloscope equipped with RF channel, hence eliminating the need for careful calibration. The ability to apply multiple gated FFTs to the captured waveform makes it possible to easily monitor the radiated RF spectrum simultaneously at different time segment.

Correlation of different time periods of switching events

Figure 1: Observing the FFT of a full waveform as well as a gated FFT in a single display diagram makes it possible to monitor the radiated RF spectrum simultaneously at different time segment during the power supply switching. In the time domain we see the spectral energy of both switching events; one covering a broader bandwidth and the other with its energy concentrated around 160MHz. It is quite useful to understand which switched power supply is causing broadband noise.

Figure 2: The joint time-frequency approach easily distinguishes narrow band and broad band signals. The time domain signal, however, reveals two switching events, i.e., the turning on and off of a MOSFET. The FFT at the different portion of time signals are highlighted with orange, blue and red colours respectively.

Correlation of interference with time, spectrum, protocol and buses

Figure 3: Multiple FFT – Multiple spectrum views with up to 4 FFT, and FFT gating revealing signals' spectrum content in different parts of the same signal. In the figure, when CAN communication signals with ID 0630ABCDH was sent, the emissions caused by the specific communication can be seen. FFT segment outside of the communication activity shows no interference. The nice thing about oscilloscope is that it is able to trigger on these communication signals for consistent analysis.

Overlapping of multiple FFT offers unprecedented debugging possibilities
Oscilloscopes come standard with either two or four channels. With FFT applied to each of these channels, it immediately opens up new perspectives of EMI debugging possibilities. These are eye-opening analysis that cannot be achieved conventionally with separate time or frequency domain analysis, including the ability to correlate emission traces on the same or different assembly boards and find near/far field transformation factors. It is simply equivalent to using four synchronised spectrum analysers for EMI debugging. However, channel to channel isolation needs to be taken into consideration especially when we are dealing with small near field probed signals of mV range.

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