The layout of PCB Layout design is described from three aspects: right-angle layout, differential layout and serpentine:
I. Layout design right-angle routing (three aspects)
The influence 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, slowing down the rise time; Second, impedance discontinuity will cause signal reflection; Third, the EMI generated by the right-angle tip, even in the RF design field above 10GHz, these small right angles may become the key objects of high-speed problems.
II. Layout design differential wiring (“equal length, equal distance, reference plane”)
What is a DifferentialSignal? Generally speaking, the driver sends two signals of equal value and opposite phase, and the receiver judges whether the logic state is “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 routing, the most obvious advantages of differential signal are as follows:
1. Strong anti-interference ability, because the coupling between the two differential traces is very good. When there is noise interference from the outside, it is almost coupled to the two lines at the same time, while the receiver only cares about the difference between the two signals, so the external common-mode noise can be completely cancelled.
2. It can effectively suppress EMI. By the same token, because the polarities of the two signals are opposite, the electromagnetic fields radiated by them can cancel each other out. The closer the coupling is, the less electromagnetic energy is released to the outside world.
Third, Layout design serpentine (adjust the delay)
Serpentine is a kind of routing method often used in Layout. Its main purpose is to adjust the delay and meet the requirements of system timing design. The two most critical parameters are the parallel coupling length (Lp) and the coupling distance (S). Obviously, when a signal is transmitted on a serpentine line, the parallel line segments will be coupled in the form of differential mode. The smaller S is, the larger Lp is, and the greater the coupling degree is. It may lead to the reduction of transmission delay and greatly reduce the signal quality 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 the parallel line segment, at least greater than 3H, where h refers to the distance from the signal trace to the reference plane. In layman’s terms, it is to take the line around a big bend. As long as S is large enough, it can almost completely avoid the mutual coupling effect.
2. Reduce the coupling length Lp. When the double Lp delay approaches or exceeds the signal rise time, the crosstalk will reach saturation.
