mars 24, 2021
1859
How to improve my PCB wiring skills? A very important method is to see if you can ask and answer questions. Or, we can also grow from other people’s questions and answers.
1. Recently I have studied PCB design. For high-speed multilayer PCBs, what are the appropriate line width settings for the power line, ground line and signal line? What are the common settings? Can you give an example? For example, how to set the working frequency at 300Mhz?
Answer: For 300MHz signals, impedance simulation must be done to calculate the line width and the distance between the line and the ground; the power line needs to determine the line width according to the size of the current; when the ground is in a mixed-signal PCB, the "line" is generally not used, but the entire plane is used , So as to ensure the minimum loop resistance and a complete plane under the signal line.
2. For high-speed PCBs, how to deal with the avoidance of vias in the wiring process? What are some good suggestions?
Answer: For high-speed PCBs, it is best to drill fewer vias, and increase the signal layer to solve the need to increase vias.
3. What problems should be paid attention to when high-frequency signal wiring?
Answer: 1) Impedance matching of the signal line; 2) Spatial isolation from other signal lines; 3) For digital high-frequency signals, the effect of differential lines will be better.
4. Differential lines generally need equal length. If it is difficult to realize in LAYOUT, is there any other remedial measures?
Answer: The problem of equal length can be solved by taking the serpentine line. Now most PCB software can automatically take the equal length, which is very convenient.
5. When should the equal length of the line be considered? If you want to consider the use of equal-length cables, what is the maximum difference between the lengths of the two signal lines? How to calculate?
Answer: Differential line calculation idea: If you transmit a sinusoidal signal, your length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees. At this time, the two signals are completely canceled. Therefore, the length difference at this time is the maximum. By analogy, the signal line difference must be less than this value.
6. Which kind of situation is suitable for serpentine routing in high speed? Are there any disadvantages, for example, for differential wiring, the two sets of signals are required to be orthogonal?
Answer: Serpentine routing has different functions due to different applications:
(1) If the serpentine trace appears in the computer board, it mainly acts as a filter inductance and impedance matching to improve the anti-interference ability of the circuit. The serpentine traces in the computer motherboard are mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM and other signal lines.
(2) In addition to the function of filtering inductance, it can also be used as the inductance coil of the radio antenna and so on in the general ordinary PCB board. For example, it is used as an inductor in 2.4G walkie-talkies.
(3) The wiring length requirements for some signals must be strictly equal. The equal line length of high-speed digital PCB boards is to keep the delay difference of each signal within a range to ensure the validity of the data read by the system in the same cycle ( When the delay difference exceeds one clock cycle, the data of the next cycle will be read incorrectly).
(4) The serpentine trace acts as a distributed parameter LC filter in some special circuits.
7. What is the maximum wiring length when there are high-speed logic devices in the circuit?
Answer: The wiring is not afraid of long, it is afraid of asymmetrical or relatively large difference, so that it is easy to cause wrong logic due to delay.
8. How to avoid noise introduced during wiring?
Answer: The digital ground and the analog ground should be grounded at a single point, otherwise the digital ground return will flow through the analog ground and cause interference to the analog circuit.
