How to consider and design ESD of RF modules?

火辣西米秀

How to consider and design ESD of RF modules? [Copy link]

When hardware engineers design products, ESD immunity is an important consideration. Static electricity is harmful to most electronic products, and RF modules are more sensitive to static electricity. So how should ESD immunity be considered and designed for RF module products? Regarding the
　　ESD immunity level, different products and industries correspond to different standards. The IEC61000-4-2 standard issued by the International Electrotechnical Commission is suitable for electromagnetic compatibility testing of various electrical and electronic equipment. Before designing a product, it is necessary to first define the ESD immunity level of the product, either according to the standard or according to the actual needs of the product. Only in this way can product design and testing be carried out with a basis.
　　　　Regarding the implementation methods of ESD immunity level, there are mainly shell design, hardware design and PCB layout, component selection, software repair, etc. Among them, in terms of hardware design, an important method is to add ESD protection devices at key circuit nodes of input or output. ESD protection devices generally use transient voltage suppressors, which are often referred to as TVS (Transient Voltage Suppressor) diodes by hardware engineers. The key feature of TVS tubes is that they have high impedance under normal working voltage, and become low impedance when the voltage exceeds the normal working voltage. At this time, the current is directly guided from the sensitive components to the system ground or the earth (shell ground) to suppress the reverse transient high voltage between the two poles of the TVS tube. For static electricity, it is suitable to use bidirectional TVS instead of unidirectional TVS.
　　　For the IOT industry, wireless products are an important part of it, and wireless modules are an important part of wireless products. This article mainly describes the ESD design of the antenna end and RF interface of the wireless module. Wireless module products can be designed with ESD protection devices, and the baseboard of the wireless module needs to consider reliable and effective electrostatic discharge paths at the antenna interface when designing, to ensure that the electrostatic discharge path does not pass through sensitive devices and sensitive lines.
　　When using wireless modules, you will sometimes see in the wireless module product manual that the antenna end needs to avoid static electricity directly hitting the antenna's RF interface, and even if you use your hands to pick up the module, it is best to wear anti-static gloves. This means that the RF output end of the wireless module is sensitive to static electricity. If the static electricity level exceeds a certain level, the wireless module may be permanently damaged. Here, an ESD design circuit structure for the RF output end of the wireless module is recommended, as shown in Figure 3.

　　Figure 3 ESD design of the RF output end of the wireless module
　　Although the use of TVS tubes can improve the ESD performance of the antenna end of the wireless module, there is a hidden danger that harmonic noise may be generated, which may reduce the receiving sensitivity of the wireless module. This puts forward requirements for the selection of TVS tubes. The optimal selection of TVS tubes can avoid this hidden danger, so the key indicator of harmonic noise should be paid attention to in the selection of devices.
　　For wireless modules with external antennas, it is generally necessary to place an antenna interface (such as an SMA interface) on the bottom plate of the application wireless module in order to install the required antenna. At this time, it is necessary to fully consider the discharge path of static electricity on the bottom plate to ensure that static electricity does not enter the wireless module product itself through the RF adapter cable, and to ensure that even if static electricity is introduced into the antenna interface, static electricity can be quickly discharged to the ground through the discharge path we designed, which can effectively improve the reliability of the product. In general, it is recommended to use the resistor-capacitor network shown in Figure 4 to conduct static electricity. Pay attention to the shortest static electricity discharge path during PCB layout and do not pass through static electricity sensitive devices, RF circuits and sensitive lines (such as data lines, clock lines, etc.). The capacitor here must be a high-voltage resistant device.

　　Figure 4 Electrostatic discharge resistor-capacitor network
　　The following is an ESD design case to illustrate this idea.
　　As shown in Figure 5, J12 is the SMA antenna interface of the 4G wireless module and is exposed to the air, so static electricity may be introduced from the SMA to the ground of the PCB. In order to avoid static electricity from being introduced into the 4G wireless module and causing damage to the module, it is necessary to design an electrostatic discharge resistor-capacitor network on the PCB. The design principle is that the electrostatic discharge path is the shortest and does not pass through the electrostatic sensitive network. If necessary, grooves can be dug to increase isolation, or holes can be punched on the electrostatic discharge path to reduce resistance so that static electricity can be discharged more and faster from the designed path.
　　In the figure below, the screw holes of the PCB are fixed to the shell by screws. The shell is the main carrier for discharging static electricity, so the approach here is to add an electrostatic discharge resistor-capacitor network R36 in parallel with C82 between the SMA and the screw hole (i.e. the shell). The SMA, the electrostatic discharge network and the screw hole are in a straight line to ensure that the path is the shortest and there is no routing on the path. Some vias are deliberately added to the electrostatic discharge path to reduce impedance. The final prototype also passed the ESD immunity test.