The core of the normal operation of electronic products is the coordination between the circuit board and the components and parts mounted on it. To reduce the influence of electromagnetic interference to improve the performance indicators of electronic products.

1. PCB Design

Printed circuit board (PCB) is a support for circuit elements and devices in electronic products, and it provides electrical connections between circuit elements and devices. The complicated layout of the wires on the PCB, the numerous parts and components, and the dense installation will inevitably make the interference between them more and more serious. Therefore, the problem of suppressing electromagnetic interference has become the key to the normal operation of an electronic system.
Similarly, the density of PCBs is getting higher and higher, and the quality of the PCB design has a great impact on the interference and anti-interference ability of the circuit. To achieve the best performance of electronic circuits, in addition to the selection of components and circuit design, good PCB design is also a very important factor in electromagnetic compatibility.

1.1 Reasonable PCB Layer Design

According to the complexity of the circuit, a reasonable selection of the PCB layer number can effectively reduce electromagnetic interference, greatly reduce the PCB volume and the length of the current loop and branch traces, and greatly reduce cross-interference between signals. Experiments show that with the same material, the noise of four-layer board is 20dB lower than that of double-layer board.
However, the higher the number of board layers, the more complicated the manufacturing process and the higher the manufacturing cost. In multi-layer board wiring, it is best to use a "well" -shaped network wiring structure between adjacent layers, that is, the directions of the respective layers of the adjacent layers are perpendicular to each other. For example, the upper side of the printed board is wired horizontally, the next side is wired vertically, and then connected through vias.

1.2 Reasonable PCB Size Design

When the size of the PCB is too large, it will lead to the increase of the printed wiring, the increase of the impedance, the reduction of the anti-noise ability, and the increase of the equipment volume and the cost. If the size is too small, heat dissipation is not good and adjacent lines are susceptible to interference. In general, the physical layer is the physical size of the PCB at the mechanical layer, and the keepout layer is prohibited from determining the effective area for layout and routing.
Generally, according to the number of functional units of the circuit, all the components of the circuit are overall, and finally the optimal shape and size of the PCB is determined. Usually rectangular, with an aspect ratio of 3: 2. When the circuit board surface size is greater than 150 mmx200 mm, the mechanical strength of the circuit board should be considered.

1.3. Circuit Design in PCB

In order to achieve the purpose of electromagnetic compatibility in the layout of PCBs using circuit schematic diagrams, necessary measures must be taken, that is, the necessary additional circuits are added to the circuit schematic diagrams to improve the electromagnetic compatibility performance of their products.

2. PCB Layout

The layout and wiring of the components of digital circuits, analog circuits, and power circuits in electronic devices have different characteristics, and the interference they generate and the methods to suppress interference are different. Due to different frequencies of high-frequency and low-frequency circuits, their interference and methods of suppressing interference are also different. Therefore, in the component layout, digital circuits, analog circuits, and power circuits should be placed separately to separate high-frequency circuits from low-frequency circuits. If possible, they should be isolated or made into a circuit board separately. Special attention should be paid to the distribution of strong and weak signals and the direction of signal transmission in the layout.

2.1 PCB Component Layout
The layout of PCB components is the same as that of other logic circuits. The related components should be placed as close as possible, so as to obtain better anti-noise effect. The arrangement of components on the printed circuit board must fully consider the problem of anti-electromagnetic interference. One of the principles is to keep the leads between components as short as possible. In the layout, the three parts of the analog signal part, the high-speed digital circuit part, and the noise source part (such as relays, high-current switches, etc.) must be reasonably separated to minimize the signal coupling between them.
Clock generators, crystal oscillators, and clock inputs of the CPU are prone to noise and should be closer to each other. Devices prone to noise, small current circuits, high current circuits, etc. should be kept as far away from logic circuits as possible. It is important to make a separate circuit board if possible.

2.2 PCB Wiring
PCB consists of a multilayer structure using a series of laminations, traces, and prepregs on a vertical stack. In multi-layer PCBs, signal lines are routed to the outermost layer for easy debugging.
At high frequencies, the traces, vias, resistors, capacitors, and distributed inductance and capacitance of the printed circuit board cannot be ignored. Resistors cause reflection and absorption of high-frequency signals. The distributed capacitance of the trace will also play a role. When the length of the trace is longer than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect occurs, and the noise is emitted outward through the trace.
Most of the wiring connections of printed circuit boards are made through vias. One via can bring about 0.5 pF of distributed capacitance, and reducing the number of vias can significantly increase speed.

An integrated circuit packaging material introduces 2 to 6 pF capacitors. A connector on a circuit board with a distributed inductance of 520 nH. A dual in-line, 24-pin integrated circuit socket with a distributed inductance of 4 to 18 nH.