Question: The signal frequency of high-speed PCBs is getting higher and higher, and the location of electromagnetic interference sources is becoming more and more difficult. Answer: EMI problems in high-speed PCBs (usually PCBs with a signal frequency of more than 100MHz or a signal rising edge of less than 1ns) are indeed more difficult to troubleshoot because high-frequency signals have stronger radiation capabilities and are prone to transmission line effects, crosstalk, and other problems. However, as long as you master the three techniques of "focusing on high-speed nodes, using simulation assistance, and combining near-field scanning", you can efficiently locate the source of electromagnetic interference in high-speed PCBs.
First, focus on high-speed nodes to narrow down the scope of troubleshooting. The interference sources of high-speed PCBs are mainly concentrated in devices and traces related to high-speed signals, such as clock generators, DDR memory, PCIe interfaces, and high-speed serial buses (e.g., SATA, USB3.0). These high-speed nodes have steep signal rise edges and rich harmonics, making them the main sources of radiation and coupling interference. When locating, troubleshoot these high-speed nodes first to quickly narrow the range. For example, DDR memory has high frequency and long traces, which can easily cause crosstalk and radiation, which is the focus of high-speed PCB troubleshooting. When processing orders for high-speed products such as server PCBs and industrial control PCBs, Jiepei prioritizes checking the layout and traces of these high-speed nodes, often finding interference sources quickly.
Secondly, use simulation tools to predict interference sources in advance. Many interference sources of high-speed PCBs already exist in the design stage and are only exposed during the testing phase. Therefore, using simulation tools to simulate EMI during the design stage can predict interference sources in advance and avoid troubles in later troubleshooting. Commonly used simulation tools include Cadence Allegro, Mentor Graphics, HyperLynx, etc., which can simulate signal radiation, crosstalk, impedance matching, etc., and visually show which devices or traces will cause EMI problems. For example, through simulation, it can be found that the impedance of a certain high-speed trace does not match, resulting in reflections and radiation, thereby optimizing the trace in advance. For novices, although the learning cost of simulation tools is high, once mastered, it can greatly improve design efficiency and reduce the troubleshooting time of EMI problems in the later stage.
Finally, combine near-field scanning to accurately locate the source of interference. For high-speed PCBs that have already been manufactured, near-field scanning is the most accurate localization method. Near-field scanning instruments can move the probe across the PCB surface to generate a 2D or 3D image of the radiation intensity, visually displaying the location and radiation intensity of the interference source. For example, if the near-field scanning image shows that the radiation intensity around a crystal oscillator is significantly higher than that of other areas, and the frequency is consistent with the crystal frequency, it can be determined that the crystal oscillator is the source of interference. In addition, for crosstalk issues in high-speed traces, near-field scanning can also show the coupling between traces, helping to locate the coupling interference source. Jiepei recommends conducting near-field scanning tests during the high-speed PCB proofing stage to detect and resolve interference issues in advance to avoid losses during mass production.
It is important to note that the location of electromagnetic interference sources in high-speed PCBs requires a combination of simulation during the design phase and actual measurements during the testing stage to achieve efficiency and accuracy. At the same time, the protection of high-speed PCBs also needs to start from the design stage, such as adopting measures such as impedance matching, differential routing, and ground shielding to reduce EMI issues at the source.
It is important to note that the location of the source of electromagnetic interference in high-speed PCBs requires a combination of simulation during the design phase and actual measurements during the testing phase to be efficient and accurate. At the same time, the protection of high-speed PCBs also needs to start from the design stage, such as impedance matching, differential routing, ground shielding, etc., to reduce EMI issues at the source.
