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Why should there be serpentine traces on the PCB

2020/05/13 16:53
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Any trace on the PCB will cause a time delay to the signal when it passes through the high-frequency signal. The main function of the serpentine trace is to compensate for the part of the "same set of related" signal lines that have a small delay. These parts Usually there is no or less than other signals through other logic processing; the most typical is the clock line, usually it does not need to undergo any other logic processing, so its delay will be less than other related signals.
The equal-line length of the high-speed digital PCB board is to keep the delay difference of each signal within a range and ensure the validity of the data read by the system in the same cycle (when the delay difference exceeds one clock cycle, it will misread the next cycle. Data), generally require a delay difference of no more than 1/4 clock cycle, the line delay difference per unit length is also fixed, the delay is related to the line width, line length, copper thickness, layer structure, but too long the line will increase the distributed capacitance And distributed inductance, so that the signal quality, so the clock IC pin is generally connected to the RC termination, but the serpentine trace does not function as an inductance, on the contrary, the inductance will make the higher harmonics in the rising element of the signal phase shift , Causing the signal quality to deteriorate, so the pitch of the serpentine line is required to be at least twice the width of the line. The smaller the rise time of the signal, the more susceptible it is to the influence of distributed capacitance and distributed inductance.
Because the application has different functions, if the serpentine trace appears in the computer board, it mainly acts as a filter inductor to improve the anti-interference ability of the circuit.
Is mainly used in some clock signals, such as CIClk and AGPClk, and it has two functions:
1. Impedance matching.
2. Filter inductance.
For some important signals, such as HUBLink in the INTEL HUB architecture. There are 13 in total, running at 233MHz, and the requirements must be strictly equal in length to eliminate the hidden danger caused by time lag. Winding is the only solution. Generally speaking, the pitch of the serpentine is> = 2 times the line width. The meander line on the PCI board is to meet the line length requirement of PCI 33MHz Clock. If it is a general-purpose PCB board, it is a distributed parameter LC filter, which can also be used as an inductance coil of a radio antenna. Short and narrow serpentine traces can be used as fuses and so on.