Some small details you have to pay attention to in EMC analysis and design

Many people probably don’t know EMC, so let’s explain EMC. Everyone is familiar with EMC, which is often referred to as electromagnetic compatibility. This time, as long as it is an explanation of the EMC analysis and design level, these small details can be avoided. Take detours. Electromagnetic compatibility includes two aspects of requirements: on the one hand, electromagnetic interference; on the other hand, electromagnetic sensitivity.

First share your views on ground current:

A. Once a current flows through a certain impedance, a certain voltage drop will occur; using Ohm's law on an electronic circuit board, there is no OV potential. The unit of voltage or current may be in the range of microvolts or microamps. A smaller finite value.

B. Current always returns to its source; a loop may have many different paths, and the current amplitude on each path is different, which is related to the impedance of the path. We do not want certain currents to flow on one of the paths, so we need to take suppression measures on that path!

Therefore, when designing products, it is the most economical way to first consider grounding. Failure to consider this during the design phase means the system may fail in terms of EMC. A well-designed grounding system; including PCB design from electronic circuits, is the key design to prevent EMI and EMS problems from a system perspective.

Use the following PCB flooring and clock grounding design cases to share key designs: The design of PCB wiring flooring in electronic products is a method used by many design engineers for current low-cost Internet of Things and smart products; due to the product The system requires MCU/CPU driven by crystal oscillators to work at high frequencies; PCB floor design can bring advantages to system EMC design!

Theory and analysis of double-sided PCB system flooring; what impact does copper flooring have on EMI?

Good aspects of flooring:

Note that if the signal line is laid on the edge of the PCB board, the external signal coupling of this signal line will be reduced; the PCB copper laying ground must be at the edge of the PCB board; the copper laying ground should be laid between the signal lines. It can reduce the crosstalk between signals. The EMI radiation in crosstalk is indirectly related to it! If both can achieve or achieve the EMI effect, the effect will be better! The floor is not counted according to the area of ​​each place. Only when you lay it out with a goal will it have a good effect!

My point of view: PCB has the conditions for laying copper, and this copper laying is effective. If it can improve the EMI performance, it is recommended to adopt the rules of copper laying on PCB!

be careful:

When paving the ground in this design, it is necessary to ensure that the impedance between the signal ground plane and the reference ground potential of the system is small enough; otherwise, the ground paving will have an impact on radiation interference or anti-radiation interference!

It is very common for a double-panel system to have a clock signal; if the system crystal oscillator clock frequency is high, the interference harmonic energy will be strong; in addition to being conducted from its input and output layout wiring, the interference harmonics will also be radiated from the space. If the layout Unreasonable wiring can easily cause strong peak noise radiation problems. Therefore, the layout of the crystal oscillator and CLK signal lines is very important when laying out the PCB board. High-frequency clock signals need to be able to mirror and reflow, and the design of complete ground copper is very critical. of.

The layout and routing of the clock source is briefly explained as follows:

A. For the active crystal oscillator: the coupling capacitor should be as close as possible to the power pin of the crystal oscillator. The order of placement is: according to the direction of power inflow, and in order from large to small capacitance. The capacitor with the smallest capacitance is closest to the pin.

B. The shell of the crystal oscillator must be grounded. The crystal oscillator signal source can be designed to be grounded to prevent external radiation and shield the crystal oscillator from interference from external signals.

C. Do not lay other traces under the crystal oscillator to ensure complete grounding. At the same time, do not lay other traces within 300 mil of the crystal oscillator to prevent the crystal oscillator noise source from coupling to other wiring, devices, etc. and affecting performance.

D. The traces of the clock signal should be as short as possible, and the crystal oscillator should not be placed on the edge of the PCB board!

If there are reset, data & address signal lines, etc. on the edge of the circuit board, the edges of the PCB board need to be designed with ground copper; ensure the mirror symmetry design of the ground return path of the key signal lines.

Therefore, the copper flooring for PCB double-sided panels is not laid according to area in every place. The flooring must be laid with a target to have a good effect. Incorrect flooring design will worsen the EMC performance of the system. Summary: Through learning, I can further deepen my impression of EMC design. The above are some small details of EMC, I hope they can help you.