How to consider and design the ESD of the radio frequency module?

“When hardware engineers design products, ESD immunity is an important consideration. Static electricity is a hazard to most Electronic products, and radio frequency modules are more sensitive to static electricity. So how should ESD immunity be considered and designed for RF module products?

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When hardware engineers design products, ESD immunity is an important consideration. Static electricity is a hazard to most electronic products, and radio frequency modules are more sensitive to static electricity. So how should ESD immunity be considered and designed for RF module products?

Regarding ESD immunity levels, different products and industries correspond to different standards. The IEC61000-4-2 standard promulgated by the International Electrotechnical Commission is suitable for electromagnetic compatibility testing of various electrical and electronic equipment. Before product design, the ESD immunity level of the product needs to be specified, either according to the standard or according to the actual needs of the product. In this way, product design and testing can be carried out on a basis.

Regarding the realization method 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 the key circuit nodes of the input or output. ESD protection devices generally use transient voltage suppressors, which are TVS (Transient Voltage Suppressor) diodes often referred to by hardware engineers. The key feature of TVS tubes is that they have high impedance under normal operating voltage, and when the voltage exceeds the normal operating voltage At this time, it becomes low impedance. At this time, the current is directly directed from the sensitive component to the system ground or the ground (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 a bidirectional TVS instead of One-way TVS.

For the IOT industry, wireless products are an important part of it, and wireless modules are also an important part of wireless products. This article mainly explains the ESD design of the wireless module antenna end and radio frequency interface. The wireless module product can be added in the design ESD protection device, and the base plate of the wireless module needs to consider a reliable and effective electrostatic discharge path at the antenna interface when designing, to ensure that the electrostatic discharge path does not pass through sensitive devices and sensitive lines.

When using a wireless module, sometimes it can be seen from the wireless module product manual that the antenna end needs to avoid static electricity from directly hitting the radio frequency interface of the antenna, and even when holding the module by hand, you must wear anti-static gloves. This means that the radio frequency output terminal of the wireless module is sensitive to static electricity, and the wireless module may be damaged if it exceeds a certain level of static electricity. Here is a recommended ESD design circuit structure for the radio frequency output end of the wireless module, as shown in Figure 3.

Figure 3 ESD design of the radio frequency 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 and the receiving sensitivity of the wireless module is reduced. This puts forward requirements for the selection of TVS tubes. Preferably, TVS tubes can avoid this hidden danger. Therefore, the key indicator of harmonic noise should be paid attention to when selecting devices.

For a wireless module with an external antenna version, it is generally necessary to place an antenna interface (such as an SMA interface) on the bottom plate of the application wireless module 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 quickly be discharged through the discharge path we designed To the earth, this can effectively improve the reliability of the product. In general, it is recommended to use the resistance-capacitance network shown in Figure 4 to conduct static electricity. Pay attention to the PCB layout to ensure that the static discharge path is short and does not pass through static sensitive devices, radio frequency circuits and sensitive lines (such as data lines, clock lines, etc.) ), the capacitor here needs to be a high-voltage-resistant device.

Figure 4 Electrostatic discharge resistance-capacitance network

The following is an ESD design to illustrate this idea.

As shown in Figure 5, J12 is the SMA antenna interface of the 4G wireless module and is exposed in the air, so static electricity may be introduced from the SMA to the PCB ground. In order to avoid the introduction of static electricity into the 4G wireless module and damage the module, it is necessary to design an electrostatic discharge resistance-capacitance network on the PCB. The design principle is that the electrostatic discharge path is short and does not pass through the electrostatic sensitive network. If necessary, you can dig a slot to increase isolation, or Holes are punched in the electrostatic discharge path to reduce the resistance so that the electrostatic discharge can be more and faster from the designed path.

The screw holes of the PCB in the picture below are fixed on the shell by screws. The shell is the main carrier for discharging static electricity, so the method here is to add an electrostatic discharge resistance-capacitance network R36 in parallel with C82 between the SMA and the screw hole (ie the shell). , SMA, electrostatic discharge network and screw holes are in a straight line to ensure that the path is short and there is no trace 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.