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What is the role of the PCB board serpentine?

2019-05-23 10:15:41

Any trace on the PCB will cause a time delay to the signal when passing through a high-frequency signal. The main function of the serpentine trace is to compensate for the lesser delays in the "same group of related" signal lines. Usually there is no or less logic than other signals; the most typical is the clock line, usually it does not need to go through any other logic processing, so its delay will be less than other related signals.

The isochronal length of the high-speed digital PCB board is to keep the delay difference of each signal within a range, and to ensure the validity of the data read by the system in the same period (when the delay difference exceeds one clock cycle, the next cycle will be misread. Data), generally requires that the delay difference does not exceed 1/4 clock cycle, the line delay difference per unit length is also fixed, the delay is related to the line width, the line length, the copper thickness, and the plate layer structure, but the line too long 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 act as an inductor.

Conversely, the inductance will cause the higher harmonic phase shift in the rising element of the signal. As the signal quality deteriorates, the minimum spacing of the serpentine line is required to be twice the line width. The smaller the rise time of the signal, the more susceptible it is to the 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, and the serpentine trace in the computer motherboard.

Mainly used in some clock signals, such as CIClk, AGPClk, it has two functions:

1, impedance matching

2. Filter inductor.

For some important signals, such as HUBLink in the INTEL HUB architecture, a total of 13 runs 233MHz, the requirements must be strictly equal length to eliminate the hidden danger caused by time lag, winding is the only solution. In general, the line spacing of serpentine traces is >= 2 times the line width.

The snake line on the PCI board is designed to meet the line length requirements of the PCI 33MHz Clock. In the general ordinary PCB board, it is a LC filter with distributed parameters, and can also be used as an inductive coil of a radio antenna. A short and narrow serpentine trace can be used as a fuse.