PCB Design Skills Questions and Answers on PCB Design Skills 1. Ground connection problem? A system is often divided into several PCBs, with power supplies, interfaces, motherboards, etc., and the ground wires between the boards are often interconnected, resulting in the formation of many loops, resulting in problems such as 2. Low-frequency loop noise? Each When the signal or power supply between the interconnection of the PCB boards is in action, for example, there is a power supply on the A board or the signal is sent to the B board, there must be an equal amount of current flowing from the ground layer back to the A board (this is the Kirchoff current law). 3. Can the estimation of impedance characteristics provide some empirical data, formulas and methods to estimate the impedance of the wiring. When the impedance matching requirements cannot be met, is it better to add a parallel matching resistor at the end of the signal line or add it to the signal line? The series matching resistance is good. Can I add a ground wire in the middle of the differential signal line. 4. Why is my Protel software design in a DOS version computer not the full picture? There are many old electronic engineers who have encountered this problem when they first started to draw PCB circuit diagrams with computers. Is it because my computer has insufficient memory? My computer has 2GB of RAM! But why do the graphics on the screen still lack arms and legs? Yes, there is a problem with the memory configuration. You only need to add the following lines to your CONFIG.SYS file (this file is in the C:\ root directory, if not, create one), save and exit and restart the computer. can. 5. Why should the PCB file be converted into a GERBER file and drilling data and then submitted to the PCB factory? Most engineers are accustomed to designing the PCB files and sending them directly to the PCB factory for processing. The internationally popular method is to convert the PCB files into GERBER files and drilling data and submit them to the PCB factory. Because electronic engineers and PCB engineers have different understanding of PCB, the GERBER file converted by the PCB factory may not be what you want. Commonly used CAD software can generate these two format files. How to check the correctness of the generated GERBER? You only need to import these GERBER files and D code files in the free software Viewmate V6.3, and then you can see them on the screen or print them out through the printer. Drilling data can also be generated by various CAD software, the general format is Excellon, which can also be displayed in Viewmate. Of course, no PCB can be made without drilling data. 6How should we choose PCB board? The choice of PCB board must strike a balance between meeting design requirements and mass production and cost. The design requirements include both electrical and mechanical parts. This material issue is usually more important when designing very high-speed PCB boards (frequency greater than GHz). . As far as electricity is concerned, pay attention to whether the dielectric constant and dielectric loss are suitable for the designed frequency. 7. How should we avoid high frequency interference? The basic idea to avoid high-frequency interference is to minimize the electromagnetic field interference of high-frequency signals, which is called crosstalk. Can increase the distance between high-speed signal and analog signal, or add ground guard/shunt traces beside the analog signal. Also pay attention to the noise interference from the digital ground to the analog ground. 8. How can the signal integrity problem be solved in high-speed design? Signal integrity is basically a problem of impedance matching. The factors that affect impedance matching include the structure and output impedance of the signal source, the characteristic impedance of the trace, the characteristics of the load end, and the topology of the trace. The solution is to rely on the topology of termination and adjustment of the wiring. 9. How can the differential wiring method be realized? There are two points to pay attention to in the layout of the differential pair. One is that the length of the two wires should be as long as possible, and the other is that the distance between the two wires (the distance is determined by the differential impedance) must be kept constant, that is, to keep parallel. There are two parallel ways, one is that the two wires run on the same side-by-side, and the other is that the two wires run on two adjacent layers above and below (over-under). Generally, the former has more side-by-side implementations. 10. For the clock signal line with only one output terminal, how do we implement differential wiring? To use differential wiring, it makes sense that both the signal source and the receiving end are differential signals. So if a clock signal has only one receiving end, then we cannot use differential wiring.
