PCB Tech - High-speed PCB design solves EMI problems

With the decrease of signal rising time and the increase of signal frequency, the EMI problem of electronic products has attracted more and more attention from electronic engineers. Almost 60% of EMI problems can be solved by high-speed PCB. In high-speed PCB design, key high-speed signal lines such as clocks and traces need to be shielded. If there is no shield or only part of it, it will cause EMI leakage. It is recommended that the shielded wire be grounded with a hole per 1000 mil.

Rule 2: High-speed signal routing closed-loop rules

Due to the increasing density of PCB boards, many PCB LAYOUT engineers are prone to a mistake in the process of routing, that is, high-speed signal networks such as clock signals, which produce closed-loop results when routing multi-layer PCBs. As a result of such a closed loop, a loop antenna will be produced, which will increase the radiated intensity of EMI.

Rule 3: High-speed signal routing open loop rules

Rule 2 mentions that the closed loop of high-speed signals will cause EMI radiation, but the open loop will also cause EMI radiation.

High-speed signal networks such as clock signals, once an open-loop result occurs when the multilayer PCB is routed, a linear antenna will be produced, which increases the EMI radiation intensity.

Rule 4: Characteristic impedance continuity rule of high-speed signal

For high-speed signals, the characteristic impedance must be continuity when switching between layers, otherwise it will increase EMI radiation. In other words, the width of the wiring of the same layer must be continuous, and the impedance of the wiring of different layers must be continuous. The wiring between two adjacent layers must follow the principle of vertical wiring, otherwise it will cause crosstalk between the lines and increase EMI radiation. In short, adjacent wiring layers follow the horizontal and vertical wiring directions, and vertical wiring can suppress crosstalk between lines.

Rule 6: Topological structure rules in high-speed PCB design

In high-speed PCB design, the control of the characteristic impedance of the circuit board and the design of the topological structure under multi-load conditions directly determine the success or failure of the product. The figure shows a daisy-chain topology, which is generally beneficial in the case of a few Mhz. In the high-speed PCB design, it is recommended to use a star-shaped symmetrical structure at the back end.

Rule 7: Resonance rule of trace length

Check whether the length of the signal line and the frequency of the signal constitute resonance, that is, when the length of the wiring is an integer multiple of the signal wavelength 1/4, the wiring will resonate, and the resonance will radiate electromagnetic waves and cause interference.

Rule 8: Return path rule

All high-speed signals must have a good return path. As much as possible to ensure that the return path of high-speed signals such as clocks is minimized. Otherwise it will greatly increase the radiation, and the size of the radiation is proportional to the area enclosed by the signal path and the return path.

Rule 9: Placement rules of decoupling capacitors for devices

The placement of the decoupling capacitor is very important. Unreasonable placement will not have the effect of decoupling at all. The principle is: close to the pin of the power supply, and the area enclosed by the power trace and the ground wire of the capacitor is the smallest.

The above is an introduction to the nine rules of high-speed PCB design to solve EMI problems.