EMI Failure Analysis Techniques - IEEE EMC Society

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Amplitude (dBm)0–20–40–60–80Broadband Spectrum of a Clock SignalAmplitude (dBm)–40–45–50–55–60–65–70–75–80–85Narrowband Spectrum–90143.975 143.98143. 98514 3.99143.9 95 144 144.005 144.01 144.015 144.02 144.025Frequency (MHz)sidebands. We will observe the spectrumshown in Figure 6.If these two signals radiate fromdifferent antennas, the two antennaslikely have different radiationpatterns (Figure 7). In such a case,the sideband-to-carrier ratio will bea function of the antenna rotation.In Figure 8, the carrier signal is reducedby about 5 dB if the turntableis rotated from 86° to 191° but thesidebands remain at about -70 dB.Two sources, one with sidebands andone without, are emitted from differentantenna structures.–100–1200 100 200 300 400 500 600 700 800 900 1,000Frequency (MHz)Fig. 4. Super narrowband scan of the far-field signal from an EUT.Zero Span MeasurementTwo signals may have similar sidebandmagnitudes, but differentmodulations. Amplitude modulat ion(AM) cannot be distinguisheddirectly from small-angle frequencyemission! As an example, the EMI antenna receives the vectorsum of the signals. If two sources of similar magnitude reachthe antenna, they can constructively or destructively interfere.If they interfere destructively and one signal is shielded, thetotal signal will increase.When multiple sources transmit at the same frequency butdiffer in spectral details, the source can still be identified by carefullyanalyzing the narrowband spectrum of the far-field andcomparing it with that of locally measured signals. The case illustratedin Figure 4 shows how two potential EMI sources can bedifferentiated from sideband spectra. If the far-field is dominatedby a signal with sidebands, a modification will change both thecarrier and the sidebands. This is the case at 72 MHz (Figure 5).But at 252 MHz the situation is quite different. The signalreceived by the log-periodic antenna is a superposition of bothsources, and the modification affects only the signal withoutMagnitude (dBm)–55–60–65–70–75–80–85Example of Two Kinds of EmissionSource Show Constructive Combination EffectBefore ModificationAfter Modification–90251.97 251.98 251.99 252 252.01 252.02Frequency (MHz)–40BeforeAfter ModificationFig. 6. Far-field before and after a modification at 252MHz. Only the signal having no sidebands is reduced by themodification.Magnitude (dBm)–50–60–70–80Radiation Pattern ofSignal ATableRadiation Pattern ofSignal BInterfrenceRegion–9071.98 71.99 72 72.01 72.02Frequency (MHz)Fig. 5. Far-field before and after modification at 72 MHz.Main signal and sidebands change by the same amount.The far-field is dominated by the signal with sidebands.Fig. 7. Two sources distinguished by the sidebands patternhave different antenna structures, thus different radiationpatterns.68 ©2009 IEEE
(dB)–60–65–70–75–80–85–9586°191°287.99 288 288.01Frequency (MHz)Fig. 8. Far-field spectrum around 288 MHz for twoturn-table positions.Amplitude (dBm)Amplitude (dBm)–55RBW: 5 MHz; VBW: 5 MHz,Center:667.6 MHz, SPAN: 0 Hz–57.5–60–62.5–650 5 10 15 20 25 30 35 40 45 50Time (ms)–50–60–70–80–900 10 20 30 40 50 60 70 80 90 100Time (µs)Fig. 10. Zero span measurement data at 667.6 MHz with50 ms and 100 µs sweep time.E-Field (dBµV/m)2520151050–5RBW: 2 KHz; VBW: 2 KHz,Center: 667.6 MHz, SPAN: 10 MHz663 664 665 666 667 668 669 670 671 672modulation (FM). IQ demodulation or a time domain viewhelps. IQ demodulation is not implemented in many spectrumanalyzers, but zero span can reveal the amplitude demodulatedsignal, provided that the modulation frequency is less than halfof the largest RBW of the spectrum analyzer.The center frequency of the spectrum analyzer is set to thefrequency of interest using 0 Hz span, so that the horizontalaxis shows time instead of frequency. The RBW must be largerthan the spectrum occupied by the modulation, and the sweeptime needs to be adjusted to see the AM modulation.Typically, switched power supplies have switching frequenciesbetween 30 kHz and 3 MHz. Data stream AM modulationcan have a much broader range, whereas periodic jitter of PLLshas no amplitude modulation.Zero span measurement reveals how the amplitude changeswith time for any modulated RF signal within a specified bandwidth.Zero span measurement is quite useful to understandwhich switched power supply is causing a broadband noise. Inthe following case, an electronic device has a narrowband radiatedsignal centered at 667.6 MHz (Figure 9). The sidebandsof the signal are complex and unsymmetrical due to multiplesources and modulations. An ordinary spectrum measurementor time domain measurement cannot unravel the mixed signals.But zero span measurement reveals several switching activitiesof the complex signal under different sweep time settings. Figure10 shows two amplitude modulation signals: one with aperiodicity of 16.7 ms (from 60 Hz AC power supply) withdown-going pulses, the other with a periodicity of about 15 µs(from the 66.7 kHz switching frequency of one synchronousbuck converter). The next step would be using near field probingto locate the buck converter that switches at 66.7 kHz, andcompare or correlate the near field signal to the far field fordetermining the EMI source signal.Fig. 9. Spectrum of the 667.6 MHz radiated signal has verycomplex sidebands, together with some clock signals.ConclusionThis is the first article of a series covering different techniquesfor EMI failure analysis. It presented an overview of the EMIanalysis techniques, and started with the basic measurementsthat can be done with a spectrum analyzer to obtain more informationrelative to the standard settings. The next article willcover time domain methods and the time varying spectral contentof signals.References[1] http://www.fcc.gov/oet/info/rules, FCC Rules & Regulations, May. 2009.[2] Application Notes, “Spectrum Analysis Basics: From 1997 Back to BasicsSeminar,” Agilent Technologies, 1997.[3] Werner Schaefer, “EMC Narrowband and Broadband Discrimination witha Spectrum Analyzer or EMI Receiver,” Conformity, Dec. 2007.[4] Zhe Li and Pommerenke, D., “EMI-debugging of complex systems usingdifferent time, modulation, STFFT and frequency domain signal analysistechniques,” IEEE International Symposium on Electromagnetic Compatibility,2005. vol. 2.[5] Parhami, P., Marino, M., Watkins, S., Nakauchi, E., “Innovative pre-compliancetest methodology using ambient cancellation and coherence detectiontechniques,” IEEE International Symposium on ElectromagneticCompatibility, 1999, vol. 2.[6] Gang Feng, Wei Wu, Pommerenke, D., Jun Fan, Beetner, D.G., “Timesynchronized near-field and far-field for EMI source identification,”©2009 IEEE69
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