PCB Technical - Several methods of high-speed PCB wiring

1. Right-angle routing

Right-angle wiring is generally a situation that needs to be avoided in high-speed PCB wiring as much as possible, and it has almost become one of the standards for measuring the quality of wiring. Right-angle wiring will change the line width of the transmission line and cause discontinuity in impedance. In fact, not only right-angle routing, but also corners and acute-angle routing may cause impedance changes.

The influence of right-angle routing on the signal is mainly reflected in three aspects:

One is that the corner can be equivalent to the capacitive load on the transmission line, which slows down the rise time;

Second, discontinuous impedance will cause signal reflection;

The third is the EMI generated by the right-angle tip.

2. Differential routing

Differential signals are used more and more widely in high-speed PCB wiring. The most critical signals in circuits are often designed with a differential structure. Differential signals are two equivalent and inverted signals sent from the driving end, and the receiving end compares this The difference between the two voltages is used to determine whether the logic state is "0" or "1". The pair of traces carrying differential signals are called differential traces. Compared with ordinary single-ended signal routing, the most obvious advantage of differential signal is its strong anti-interference ability, effective suppression of EMI, and accurate timing positioning.

For PCB engineers, the most concern is how to ensure that these advantages of differential wiring can be fully utilized in actual wiring. Perhaps anyone who has been in touch with Layout will understand the general requirements of differential wiring, that is, "equal length and equal distance". The equal length is to ensure that the two differential signals maintain opposite polarities at all times and reduce the common mode component; the equal distance is mainly to ensure that the differential impedances of the two are consistent and reduce reflections. "As close as possible" is sometimes one of the requirements of differential wiring. But all these rules are not used to mechanically apply, and many engineers seem to still not understand the essence of high-speed differential signal transmission. The following focuses on several common misunderstandings in PCB Design differential signal design.

Misunderstanding 1: It is believed that the differential signal does not need a ground plane as a return path, or that the differential traces provide a return path for each other.

Misunderstanding 2: It is believed that keeping equal spacing is more important than matching line length. The most important rule in the design of PCB differential traces is the matching line length. Other rules can be flexibly handled according to design requirements and actual applications.

Misunderstanding 3: Think that the differential wiring must be very close. Keeping the differential traces close is nothing more than to enhance their coupling, which can not only improve immunity to noise, but also make full use of the opposite polarity of the magnetic field to offset electromagnetic interference to the outside world. If we can ensure that they are fully shielded from external interference, then we no longer need to achieve the purpose of anti-interference and suppress EMI through strong coupling with each other. Increasing the distance from other signal traces is one of the most basic ways.

high-speed PCB wiring

3. Serpentine line

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 designer must first have this understanding: the serpentine line will destroy the signal quality, change the transmission delay, and try to avoid using it when wiring. However, in actual design, in order to ensure that the signal has enough hold time, or to reduce the time offset between the same group of signals, it is often necessary to deliberately wind the wire. When a signal is transmitted on a serpentine trace, the parallel line segments will be coupled in a differential mode. The smaller the S and the greater the Lp, the greater the degree of coupling. This may result in a reduction in transmission delay and a significant reduction in signal quality due to 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 approaches 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. Theoretically, the stripline will not affect the transmission rate due to differential mode crosstalk. 4. For high-speed signal lines and those with strict timing requirements, try not to use serpentine lines, especially not winding lines in a small area.

5. You can often use serpentine traces at any angle, such as the C structure in Figure 1-8-20, which can effectively reduce mutual coupling.

6. In high-speed PCB wiring, 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.

7. Sometimes you can consider the spiral routing method for winding.