Three special wiring skills for multilayer pcb
I. Right-angle wiring skills (three aspects)
The effect of right-angle routing on the signal is mainly reflected in three aspects. First, the corner can be equivalent to the capacitive load on the transmission line, and the rise time can be reduced.The other is the impedance discontinuity will cause the signal reflection;Third, the EMI produced by the right Angle tip, and the RF design field above 10GHz, these small right angles may become the focus of high-speed problems.
II. Differential wiring skills (” equal length, equal distance, reference plane “)
What is Differential Signal?Generally speaking, the driving end sends two equivalent and inverting signals, and the receiving end judges whether the logical state is “0” or “1” by comparing the difference between the two voltages.The pair of lines carrying differential signals are called differential lines.Compared with ordinary single-end signal routing, difference signal has the following three advantages:
1. Strong anti-interference ability. Because the coupling between two difference lines is good, when there is noise interference from outside, it is almost coupled to two lines at the same time.
2. It can effectively restrain EMI. Similarly, since the two signals have opposite polarity, their external electromagnetic fields can offset each other.
3. Accurate timing positioning. As the switching change of difference signal is located at the intersection of two signals, instead of the common single-end signal relying on high and low two threshold voltages, it is less affected by process and temperature, which can reduce the error in the timing sequence and is more suitable for circuits with low amplitude signals.The current popular LVDS, low voltage differential signaling, is a small amplitude differential signal.
III. Serpentine wiring skills (adjusting delay)
Serpentine is a common type of line passing in Layout.Its main purpose is to adjust the delay and meet the design requirements of the system timing.The two most critical parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when the signal is transmitted in a serpentine travel line, coupling will occur between parallel line segments, in the form of differential mode. The smaller the S, the larger the Lp, the larger the coupling degree.This may lead to a reduction in transmission delay and a significant reduction in signal quality due to crosstalk.Here are some Suggestions for the Layout engineer to work with the serpentine:
1. Increase the distance (S) of the parallel line segment as much as possible, at least greater than 3H. H refers to the distance from the signal line to the reference plane.Generally speaking, if the S is large enough, the coupling effect can be avoided almost completely.
2. Reduce the coupling length of Lp; when twice the delay of Lp approaches or exceeds the signal rise time, the crosstalk generated will reach saturation.
3. Signal transmission delay caused by strip-line or Embedded micro-strip serpentine.
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