Almost all governments around the world are trying to control the harmful electromagnetic interference (EMI) generated by electronic products produced in their country (see Figure 1). In order to provide users with a certain level of protection and security, the government will formulate very specific rules and regulations concerning the design of electronic products.
Of course this is a good thing. But it also means that in order to minimize their EMI characteristics and pass official EMI certification testing, many companies must spend a lot of manpower and resources on product design and testing. The bad news is that even with good design principles, high quality components, and careful product characterization, EMI failures can still be affected if the test does not go well at all stages when performing compliance testing. Product release schedule.
Usually in order to avoid such a situation, the company will do some "pre-conformance" measurements during the design and prototype establishment phase. A better approach is to identify and fix potential EMI issues before the product is sent out for compliance testing.
Of course, most companies' laboratories do not have the test chamber conditions needed to make absolute EMI measurements. The good news is that it is completely feasible to identify and resolve EMI problems without copying test room conditions. Some of the techniques discussed in this article can help you reduce the risk of a product failing in the final complete EMC conformance assessment in the test room. This article also gives an example of determining signal characteristics and consistency in order to find an EMI source.
Figure 1: The changing voltage and current in the signal produces an electromagnetic field.
Understanding the EMI report
It is necessary to introduce the EMI test report before discussing the troubleshooting techniques. At first glance, the EMI report seems to provide information directly about the failure of a particular frequency point, so the thing seems simple: use the data in the report to determine which component in the design contains the source frequency of the problem, and pay special attention to it. A round of testing. However, while many of the test conditions are clearly stated in the report, some important things to consider may not be obvious. It is helpful to understand how the test room generates such reports when reviewing the design and trying to determine the source of the problem.
Look at the EMI test report shown in Figure 2, which shows a fault at about 90MHz.
Figure 2: This EMI test report shows a fault at approximately 90MHz.
Figure 3 is a corresponding list data report detailing the test frequency, the measured amplitude, the corrected correction factor, and the adjusted field strength. Then compare the adjusted field strength with the indicator in the next column to determine the margin or excess amount, which is displayed in the rightmost column.
In the margin column shown in Figure 3, you can see that there is a peak that exceeds the limit specified by the specification at 88.7291 MHz, which is -2.3 compared to the specification.
Figure 3: This list of data corresponds to Figure 2, which shows that the fault point is at 88.7291 MHz, but there are many factors that make it doubtful whether this is the actual frequency.
You are done, are you? No, not so fast. Don't let all these numbers make you believe that this is the exact frequency of the problem EMI source. In fact, the frequency given in the test report is most likely not the actual source frequency. The International Committee on Radio Interference (CISPR) states that when performing radiation emission tests, different test methods must be used depending on the specific frequency range. Each range requires a filter and detector type for a particular resolution bandwidth, as shown in Table 1. The filter bandwidth determines the ability to resolve the actual frequency of interest; this means that the frequency range will vary in many ways to troubleshoot the source of the problem.
Table 1: CISPR test requirements vary according to different frequency ranges and affect frequency resolution.
It is important to note here that for certain frequency ranges, the CISPR test requires the use of a quasi-peak (QP) detector type that masks the actual frequency. Usually the EMI department or an external laboratory initially uses a simple peak detector to perform a scan to find the problem area. But when the signals found are above or near the specified limits, they also perform quasi-peak measurements. Quasi-peak is a method defined by the EMI measurement standard to detect the weighted peaks of the signal envelope. It weights the signal based on the duration and repetition rate of the signal to apply more weight to signals interpreted as "harassment" from a broadcast perspective. Higher frequency signals will result in higher quasi-peak measurements than non-frequent pulses. In other words, the more frequently the problem signal occurs, the more likely the absolute amplitude of the problem signal is blocked by the quasi-peak measurement.
The good news is that peak and quasi-peak scans are still useful for pre-conformance testing. Figure 4 shows an example of peak and quasi-peak detection. The figure shows a signal with a pulse width of 8 μs and a repetition rate of 10 ms, both in peak detection and quasi-peak detection. As a result, the quasi-peak detection result is 10.1 dB lower than the peak value.
Figure 4: Comparison of peak detection and quasi-peak detection.
A good rule to remember is that the quasi-peak detection value is always less than or equal to the peak detection value and never greater than the peak detection value. So you can use peak detection to perform your EMI troubleshooting and diagnosis. You don't need to achieve the same level of accuracy as the EMI department or laboratory scans because the measurements are relative values. If the quasi-peak value in your lab report indicates that the design exceeds 3dB and the peak detection exceeds 6dB, then you know that the repair work you need is to reduce the signal by 3dB or more.
Scans from the test room for EMI reporting are usually performed under special conditions and may not be replicated by your company's laboratory. For example, the device under test (DUT) may be placed on a turntable to facilitate collecting signals from multiple angles. This azimuth information is useful because it indicates the DUT area where the problem occurs. Or the EMI test room may perform their measurements in a calibrated RF room and report the measurements as a strong field.
Fortunately, you don't need to completely copy the test room conditions to troubleshoot EMI tests. Unlike absolute measurements performed on highly controlled EMI test lines, the information in the test report can be used to gain insight into the measurement techniques used to generate the report and the relative observations around the device under test to isolate the source of the problem and estimate the effectiveness of the correction. To carry out troubleshooting work.
Where do you start to discover EMI radiation?
It is time to focus our attention on harmful EMI sources. When we look at any product from an EMI perspective, the entire design can be seen as a collection of energy sources and antennas. Common (but not the only) sources of EMI problems include:
â—Power filter
â— Ground impedance
â— There is not enough signal to return
â—LCD radiation
â— Component parasitic parameters
â—Bad cable shielding
â—Switching power supply (DC/DC converter)
â—Internal coupling problem
â— Electrostatic discharge in a metal casing
â— Discontinuous return path
In order to determine the energy source on a particular board and the antenna at the center of a particular EMI problem, you need to check the period of the observed signal. What is the RF frequency of the signal? Is it pulsed or continuous? These signal characteristics can be monitored using a basic spectrum analyzer.
You also need to look at coincidence. Which signal on the device under test (DUT) coincides with the EMI event? It is common practice to use an oscilloscope to detect electrical signals on the DUT. Checking the coincidence of EMI and electrical events is undoubtedly the most time-consuming task in EMI troubleshooting. In the past, it has been difficult to correlate information from spectrum analyzers and oscilloscopes in a synchronized manner.
However, the introduction of a mixed-domain oscilloscope (MDO) has improved the situation by providing simultaneous and time-dependent observation and measurement functions. The instrument shown in Figure 5 can make it easy to see which signal coincides with which EMI event, which simplifies the EMI troubleshooting process.
Figure 5: A mixed-domain oscilloscope (MDO) combines a spectrum analyzer, oscilloscope, and logic analyzer into a single meter to produce synchronized, time-correlated measurements from all three instruments. The picture shows the Tektronix MDO4000B.
MDO combines the functionality of a mixed-signal oscilloscope with the capabilities of a spectrum analyzer. With this combination, you can automatically display analog signal characteristics, digital timing, bus transactions, and RF and implement triggering based on this information. Some MDOs can also capture or observe spectrum and time domain trajectories, including RF amplitude versus time, RF phase versus time, and RF frequency versus time. The RF amplitude and time trajectory are shown in Figure 6.
Figure 6: This figure shows the time-dependent observation function provided by MDO. The figure shows the trajectory of RF amplitude versus time.
Near field detection for relative measurement
Although the conformance test procedure is designed to produce an absolutely calibrated measurement, the troubleshooting effort can largely be performed using a relative measurement of the electromagnetic field occurring from the device under test. What's more, you can use MDO's spectrum analyzer function and RF channel to detect the wave resistance behavior in the near field to find the energy source. At the same time, you can probe the signal with a passive probe on one of the analog channels of the oscilloscope to discover the signal associated with the RF.
But first you have to know some background information about the electromagnetic field to be detected. Figure 7 shows the wave resistance behavior in the near and far fields and the transition between the two. As can be seen from the figure, in the near-field region, the field can range from the dominant magnetic field to the dominant electric field. In the near field, non-radiative behavior is dominant, so the wave resistance depends on the nature of the source and the distance from the source. In the far field, the impedance is fixed, and the measurement depends not only on the observed activity in the near field, but also on other factors such as antenna gain and test conditions.
Figure 7: This figure shows the wave resistance behavior in the near and far fields and the transition between the two. Near field measurements can be used for EMI troubleshooting.
Near-field measurements are a measure that can be used for EMI troubleshooting because it does not require the test site to provide special conditions that allow you to identify the energy source. However, conformance testing is done in the far field, not near field. You usually don't use the far field because there are too many variables to make it complicated: the strength of the far-field signal depends not only on the strength of the source, but also on the radiation mechanism and the shielding or filtering measures that may be taken. As you should remember, if you can observe the signal in the far field, you should be able to see the same signal in the near field. (However, it is quite possible to observe the signal in the near field without seeing the same signal in the far field)
The near field probe is actually an antenna designed to pick up a magnetic field (H field) or an electric field (E field). In general, near field probes do not have calibration data, so they are suitable for relative measurements. If you are unfamiliar with the probes used to measure H-field and E-field variations, it is best to know some near-field probe designs and best practices:
The H-field (magnetic field) probe has a unique loop design as shown in Figure 8. What is important is that the direction of the H field probe is such that the loop plane is consistent with the conductor to be tested, such that the loop is arranged such that the flux line passes directly through the loop.
Figure 8: Aligning the H field probe with the current flow direction allows the magnetic field lines to pass directly through the loop.
The loop size determines the sensitivity and measurement area, so care must be taken when using this type of probe to isolate the energy source. Near field probe kits typically contain many different loop sizes so you can use a tapered loop size to reduce the measurement area.
H-field probes are very useful when identifying sources with relatively large currents, such as:
â—Low impedance nodes and circuits
â— Transmission line
â—Power supply
â— Terminating wires and cables
The E-field (electric field) probe is used as a small monopole antenna and responds to changes in electric field or voltage. When using such probes, it is important that you keep the probe perpendicular to the measurement plane, as shown in Figure 9.
Figure 9: Place the E field probe perpendicular to the conductor to observe the electric field.
In practical applications, the E-field probe is best suited for finding very small areas and identifying sources with relatively high voltages and sources without terminations, such as:
â—High impedance nodes and circuits
â— Unterminated PCB traces
â—Cable
At low frequencies, the impedance of the circuit nodes in the system can vary greatly; at this point a certain circuit or experimental knowledge is required to determine if the H field or E field can provide the highest sensitivity. At higher frequencies, these differences can be significant. In all cases, it is important to perform repetitive relative measurements so that you can be sure that the near-field radiation results due to any changes achieved can be accurately reproduced. Most importantly, the layout and aspects of the near-field probes should be consistent for each test change.
Tracking EMI radiation sources
In this example, the EMI scan of the small microcontroller indicates that there is an overrun fault that appears to be from a wideband signal with a center frequency of approximately 144 MHz. With the MDO's spectrum analyzer function, the first step is to connect the H field probe to the RF input and position the energy source with a relative near field measurement.
As mentioned above, it is important that the direction of the H field probe is such that the loop plane is consistent with the conductor to be tested. By moving the H field probe around the PCB, you can position the energy source. By selecting a probe that gradually reduces the aperture, you can position the search in a smaller area.
Once an apparent source of energy is located, the RF amplitude and time traces shown in Figure 10 can show the complete power versus time relationship for all signals in this range. Using this trace, you can clearly see that there is a large pulse in the display. Moving the spectrum time to pass the record length, it is obvious that the EMI event (the wideband signal centered at around 140 MHz) directly corresponds to this large pulse. To stabilize the measurement, turn on the RF power trigger and increase the record length to determine how often this RF pulse occurs. In order to measure the pulse repetition period, the measurement mark is turned on and the period is directly judged.
Figure 10: The RF amplitude and time trace of the MDO (above) shows a significant pulse at 140 MHz. The spectrum pattern (below) shows the frequency content of this pulse.
The next step in clearly determining the EMI source is to take advantage of the oscilloscope capabilities of MDO. With the same settings, turn on the analog channel 1 of the oscilloscope and browse the PCB to find the source that coincides with the EMI event.
After browsing the signal with the oscilloscope probe for a while, you can see the signal shown in Figure 11: in this case, a power filter. As you can clearly see from the display, the signal connected to channel 1 of the oscilloscope is directly related to the EMI event. It is now possible to develop an EMI repair plan to resolve this issue before conducting a certification test.
Figure 11: Using a passive probe on the oscilloscope analog channel to find the signal associated with the RF.
Summary of this article
Failure to pass EMI compliance testing may put product development plans at risk. However, pre-conformance testing can help you eliminate EMI problems before reaching this stage. Unlike absolute measurements in highly controlled EMI test lines, you can use the information in the EMI test report to make relative measurements and use it to isolate the source of the problem and estimate the repair effect.
Efficient EMI troubleshooting generally uses near-field detection methods to find relatively high electromagnetic fields, determine their characteristics, and then use a mixed-domain oscilloscope to correlate field activity with circuit activity to determine EMI sources. The troubleshooting techniques outlined in this article can effectively help you isolate harmful energy sources so that you can fix the problem before submitting it to EMI certification.
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