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PCB Tech

Three special routing techniques for PCB LAYOUT
2021-09-26
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Author:Frank

Three special routing techniques for PCB LAYOUT
The following describes the PCB layout routing from three aspects: right-angle routing, differential routing, and serpentine routing:
1. Right-angle routing (three aspects)
    The influence of right-angle wiring on the signal is mainly reflected in three aspects: one is that the corner can be equivalent to a capacitive load on the transmission line, which slows down the rise time; the other is that impedance discontinuity will cause signal reflection; the third is that the right-angle tip is generated In the field of RF design above 10GHz, these small right angles may become the focus of high-speed problems.
2. Differential wiring ("equal length, equidistant, reference plane")
    What is a differential signal? In layman's terms, the driving end sends two equal and inverted signals, and the receiving end judges the logical state "0" or "1" by comparing the difference between the two voltages. The pair of traces carrying differential signals is called differential traces. Compared with ordinary single-ended signal traces, differential signals have the most obvious advantages in the following three aspects:
1. Strong anti-interference ability, because the coupling between the two differential traces is very good. When there is noise interference from the outside, they are almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals. Therefore, the external common mode noise can be completely canceled.
2. It can effectively suppress EMI. For the same reason, due to the opposite polarity of the two signals, the electromagnetic fields radiated by them can cancel each other out. The tighter the coupling, the less electromagnetic energy vented to the outside world.
3. The timing positioning is accurate. Because the switch change of the differential signal is located at the intersection of the two signals, unlike ordinary single-ended signals, which rely on the high and low threshold voltages to determine, it is less affected by the process and temperature, and can reduce the error in the timing. , But also more suitable for low-amplitude signal circuits. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signaling technology.
Three, serpentine line (adjust the delay)

pcb board

    Snake line is a type of routing method often used in Layout. Its main purpose is to adjust the delay to meet the system timing design requirements. The two most critical parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when the signal is transmitted on the serpentine trace, the parallel line segments will be coupled in a differential mode, S The smaller the value, the larger the Lp, and the greater the degree of coupling. It may cause the transmission delay to be reduced, and the quality of the signal is greatly reduced due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk. The following are some suggestions for Layout engineers when dealing with serpentine lines:
1. Try to increase the distance (S) of parallel line segments, at least greater than 3H, H refers to the distance from the signal trace to the reference plane. In layman's terms, it is to go around a big bend. As long as S is large enough, the mutual coupling effect can be almost completely avoided.
2. Reduce the coupling length Lp. When the double Lp delay is close to or exceeds the signal rise time, the crosstalk generated will reach saturation.
3. The signal transmission delay caused by the serpentine line of the Strip-Line or Embedded Micro-strip is less than that of the Micro-strip. In theory, the stripline will not affect the transmission rate due to differential mode crosstalk.
4. For signal lines with high-speed and strict timing requirements, try not to take serpentine lines, especially not winding lines in a small area.
5. It is possible to often use serpentine traces at any angle, which can effectively reduce mutual coupling.
6. In the design of high-speed pcb, the serpentine line has no so-called filtering or anti-interference ability, and can only reduce the signal quality, so it is only used for timing matching and has no other purpose.