Three special wiring techniques for PCB

Today, we will introduce three special wiring techniques for PCB. The following will explain the wiring of PCB LAYOUT from three aspects: right-angle wiring, differential wiring, and serpentine wiring:

1.Right-angle wiring (three aspects)

The impact of right-angle wiring on signals is mainly reflected in three aspects: first, the corner can be equivalent to the capacitive load on the transmission line, slowing down the rise time; second, the impedance discontinuity will cause signal reflection; third, the EMI generated by the right-angle tip, in the RF design field above 10GHz, these small right angles may become the focus of high-speed problems.

2.Differential wiring (“equal length, equal distance, reference plane”)

What is a differential signal? In layman’s terms, the driver sends two equal and opposite signals, and the receiver determines the logic state “0” or “1” by comparing the difference between the two voltages. The pair of wiring that carries the differential signal is called differential wiring. Compared with ordinary single-ended signal routing, differential signals have obvious advantages in the following three aspects:

3.Strong anti-interference ability, because the coupling between the two differential routings is very good.

When there is noise interference from the outside, it is almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals, so the common mode noise from the outside can be completely offset.

4.It can effectively suppress EMI.

In the same way, because the polarity of the two signals is opposite, the electromagnetic fields they radiate to the outside can offset each other. The tighter the coupling, the less electromagnetic energy is released to the outside.

5.Accurate timing positioning.

Because the switching change of the differential signal is located at the intersection of the two signals, unlike the ordinary single-ended signal, which relies on the high and low threshold voltages for judgment, it is less affected by the process and temperature, which can reduce the timing error. At the same time, it is also more suitable for circuits with low amplitude signals. The currently popular LVDS (low voltage differential signaling) refers to this small amplitude differential signal technology.

6.Snake line (adjusting delay)

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 key parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when the signal is transmitted on the serpentine line, coupling will occur between the parallel segments in the form of differential mode. The smaller S and the larger Lp, the greater the degree of coupling. It may cause the transmission delay to decrease and the signal quality to be greatly reduced due to crosstalk. The mechanism can refer to the analysis of common mode and differential mode crosstalk.

Here are some suggestions for layout engineers when dealing with serpentine lines:

1. Try to increase the distance (S) of parallel segments to at least 3H, where H refers to the distance from the signal line to the reference plane. In layman’s terms, it is to route 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 delay of twice the Lp approaches or exceeds the signal rise time, the generated crosstalk will reach saturation.
3. The signal transmission delay caused by the serpentine line of strip-line or embedded microstrip is smaller than that of microstrip. In theory, the transmission rate of strip-line will not be affected by differential mode crosstalk.
4. For high-speed signal lines and signal lines with strict timing requirements, try not to use serpentine lines, especially not to meander in a small range.
5. Serpentine lines at any angle can be often used to effectively reduce mutual coupling.
6. At high speed, serpentine lines have no so-called filtering or anti-interference capabilities, and can only reduce signal quality, so they are only used for timing matching and have no other purpose.
7. Sometimes you can consider winding in the form of spiral routing. Simulation shows that its effect is better than normal serpentine routing.