Can an oscilloscope be used to troubleshoot EMI? (4)

　　This is part 4 of a series on EMI troubleshooting. This time we will look at some techniques for capturing EMI with an oscilloscope.

　　Just like any other electronic signal, an oscilloscope is useful for triggering on the signal's characteristics to capture and further analyze it - much the same is true when it comes to capturing electromagnetic interference. However, given the characteristics of EMI, its capture techniques are somewhat different and more challenging than those used to capture regular electronic signals.

　　Below are some recommended settings for EMI troubleshooting.

　　Take full advantage of the dynamic range of the oscilloscope's analog-to-digital converter (ADC). Check the vertical scale of the captured time-domain signal, which is usually in the range of 1 to 5 mV/div. Make sure the ADC is fully utilized and avoid signal clipping, as this will be included in the FFT calculation. Improper settings can result in spectral measurement differences of tens of decibels.

　　Set the appropriate resolution bandwidth (RBW) and span before measurement. For example, a RBW of 100kHz to 1MHz is generally used to measure radiated emissions with a span of 30MHz; EMI for power supply-related signals such as switching power supplies (SMPS) usually does not exceed 30MHz. Finding the right RBW and span before measurement will greatly save your EMI troubleshooting time. However, please note that higher RBW settings require more sampling points and longer record lengths.

Figure 1: EMI observed using four detection methods -- envelope (red trace), average (black trace), RMS (white trace) and sample acquisition with grayscale.

　　It is necessary to use low-pass filters on one or both acquisition and trigger paths. Filters are particularly useful when rejecting irrelevant high-frequency noise. The low-pass filter on the trigger path only rejects the high-frequency noise and uses the processed waveform for trigger decision, while the remaining unfiltered signal is captured and measured. This is particularly useful when the desired trigger signal is superimposed with interference, making it difficult to trigger.

　　Change the acquisition detector as appropriate. Choose an envelope, average, or RMS detector? That depends on the type of radiation you are trying to capture. Peak detection, which is equivalent to the maximum hold of RF instruments, allows you to get a quick overview while scanning the device under test (DUT).

Table: Different window functions are recommended for different measurement types.

Figure 2: Different window functions and their main lobe and side lobe attenuation curves.

　　Choose the best window function for the spectrum. Different window functions have different frequency resolution, amplitude resolution, main lobe level and side lobe attenuation, so not all window functions are recommended for EMI measurement. First, avoid using rectangular window functions, because it is easy to cause spectrum leakage, and the use of such window functions may only make sense in very rare cases. Others are OK, but Gaussian is the most suitable for EMI troubleshooting, because real EMI filters also have Gaussian shapes.

　　Capture using triggers and mask violations.

　　With the oscilloscope properly set up, let's now look at some methods for capturing interference for analysis. Using a spectrum analyzer or EMI test receiver, you typically enable maximum hold to observe interference. However, these instruments cannot pinpoint interference because they do not capture the offending signal.

　　The following examples show some useful techniques for capturing this intermittent EMI using an RTO oscilloscope.

　　1. When you want to associate an event captured in the time domain with an FFT result, trigger on the time domain signal. It is very convenient if the time domain waveform has obvious characteristics that can be directly triggered using the time domain method. With today's oscilloscope ADC effective number of bits (ENOB) reaching more than 7 bits and providing improved trigger sensitivity, using time domain events is the easiest way to achieve correlation with events in FFT.

Figure 3: Time domain triggering gives you a wideband interference capture consistent with what you saw above.

　　2. Trigger on protocols and digital channels. Typically, electromagnetic interference is introduced by communication signals such as SPI, I2C, CAN, or LIN. Repeated serial data patterns operating at TTL levels can easily cause EMI to propagate throughout the printed circuit board (PCB). However, such control signals can cause the device under test to operate in different states by turning on or off specific functions or components, which can often induce EMI problems. Using an oscilloscope, you can trigger on the state and logic of specific bus signals to observe repeatable radiation.

　　Figure 4: CAN protocol triggering on identifier 0630ABCDH with DLC set to 4. You can easily see the EMI introduced by this communication bus. You can use a gated FFT to determine if this interference is caused by the communication signal for further analysis.

　　3. Set template violation capture for time domain/frequency domain signals. When you need to capture a specific interference signal for further analysis, you can use the template violation capture technology to achieve it. Whenever any part of the signal enters the user-defined template area, the oscilloscope can be set to stop acquisition or use a beep to warn the user so that engineers can perform corresponding analysis accordingly.

Figure 5: Mask violation capture. Note that you can use mask violations in both the time and frequency domains.

　　The key difference compared to RF instruments is that the oscilloscope stores the time domain waveform of the interfering signal in memory and can be used for post-processing analysis with FFT. The captured signal stored in memory can also use the history mode that is now common in most oscilloscopes, allowing users to review and compare the signal with the waveform acquired in the past.

　　Capturing interference is not limited to the examples discussed. There is always more than one way to solve a problem, and by paying more attention to the proper setup and taking full advantage of the various features of your oscilloscope, you will be off to a good start in interference hunting.

　　Once you know how to isolate an interference event, the next step is to analyze its characteristics and discover the root cause that caused it.