How to solve mutual interference problems in high speed PCB design
At present, most hardware engineers only design PCBs based on experience. During the debugging process, many signal lines or chip pins that need to be observed are buried in the middle layer of the PCB, and cannot be detected by tools such as an oscilloscope. If the product fails the functional test, They also have no effective means to find the cause of the problem. In order to verify the EMC characteristics of the product, only the product is taken to the standard EMC measurement room for measurement. Since this measurement can only measure the external radiation of the product, even if it fails, it can not provide useful information for solving the problem, so the engineer can only Modify the PCB by experience and repeat the test. This test method is very expensive and may delay the time to market of the product.
Of course, there are a lot of high-speed PCB analysis and simulation design tools that can help engineers solve some problems. However, there are still many limitations on the device model. For example, the IBIS model that can solve signal integrity (SI) simulation has many devices without models. Or the model is not accurate. To accurately simulate the EMC problem, the SPICE model must be used, but almost all ASICs currently cannot provide the SPICE model. Without the SPICE model, EMC simulation cannot take into account the radiation of the device itself. The radiation is much larger). In addition, simulation tools often have to compromise on accuracy and simulation time. The relatively high precision requires a long calculation time, while the fast simulation tool has low accuracy. Therefore, simulation with these tools cannot completely solve the mutual interference problem in high-speed PCB design.
We know that the return path of high-frequency signals in a multilayer PCB should be on the reference ground plane (power plane or ground plane) adjacent to the signal line layer. Such reflow and impedance are minimal, but there will be splits in the actual formation or power plane. And hollowing out, thereby changing the return path, resulting in a large reflow area, causing electromagnetic radiation and ground bounce noise. If the engineer can understand the current path, it can avoid large return paths and effectively control the electromagnetic radiation. However, the signal return path is determined by various factors such as signal line wiring, PCB power supply and ground distribution structure, power supply point, decoupling capacitor and device placement position and quantity. Therefore, it is very difficult to theoretically determine the return path of a complex system. .
Therefore, it is very important to eliminate the radiation noise problem in the design stage. We can use the oscilloscope to see the waveform of the signal, which can help solve the signal integrity problem. Is there any device that can see the "graphics" of the radiation and the reflow on the board?
Electromagnetic field high-speed scanning measurement technology
Among various electromagnetic radiation measurement methods, a near-field scanning measurement method can solve this problem, and the method is based on the principle that electromagnetic radiation is formed by a high-frequency current loop on a device under test (DUT). For example, Emscan, the electromagnetic radiation scanning system of EMSCAN of Canada, is based on this principle. It uses an H-field array probe (with 32×40=1280 probes) to detect the current on the DUT. During the measurement, the DUT is placed directly on the scan. Above the device. These probes detect changes in the electromagnetic field due to changes in high frequency currents, and the system provides a visual image of the spatial distribution of RF current on the PCB.
The Emscan electromagnetic compatibility scanning system has been widely used in industries such as communications, automotive, office appliances, and consumer electronics. Through the current density map provided by the system, engineers can find areas with EMI problems before testing for electromagnetic compatibility standards. Take appropriate measures.