ESD immunity is an important consideration for hardware engineers when designing products. 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 promulgated by the International Electrotechnical Commission is suitable for electromagnetic compatibility testing of various electrical and electronic equipment. Before product design, it is necessary to specify 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 the product design and test be based on the basis.
Regarding the realization method of ESD immunity level, there are mainly shell design, hardware design and PCB layout, component selection, software repair, etc. 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, that is, TVS (Transient Voltage Suppressor) diodes that hardware engineers often say. At this time, it becomes a low impedance. At this time, the current is directly guided from the sensitive components to the system ground or the earth (case 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 One-way 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 wireless modules. Wireless module products can be added to the design. ESD protection devices, and the design of the base plate of the wireless module needs to consider a reliable and effective electrostatic discharge path at the antenna interface to ensure that the electrostatic discharge path does not pass through sensitive devices and sensitive lines.
When using the wireless module, it is sometimes seen from the product manual of the wireless module that the antenna end needs to avoid static electricity directly hitting the RF interface of the antenna, and even when handling the module by hand, it is best to wear anti-static gloves. This means that the radio frequency output end of the wireless module is sensitive to static electricity, and exceeding a certain level of static electricity may cause permanent damage to the wireless module. Here, an ESD design circuit structure of the radio frequency output end of a wireless module is recommended, as shown in Figure 3.
Figure 3 ESD design of radio frequency output end of 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 reduces the receiving sensitivity of the wireless module. This puts forward requirements for the selection of TVS tubes. TVS tubes can be selected to avoid this hidden danger. Therefore, the key indicator of harmonic noise should be paid attention to in device selection.
For the wireless module of the external antenna version, it is generally necessary to place an antenna interface (such as an SMA interface) on the base plate where the wireless module is applied 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, the static electricity can be quickly discharged through the discharge path we designed. to the earth, which can effectively improve the reliability of the product. In general, it is recommended to use the RC network as shown in Figure 4 to conduct static electricity. Pay attention to ensure that the electrostatic discharge path is the shortest during PCB layout and does not pass through electrostatic sensitive devices, radio frequency circuits and sensitive lines (such as data lines, clock lines, etc. ), the capacitor here needs to be a high-voltage device.
Figure 4 ESD RC network
The following is an example of 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 to the air, so static electricity may be introduced from the SMA to the ground of the PCB. In order to prevent static electricity from being introduced 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 the shortest and does not pass through the static-sensitive network. If necessary, a slot can be cut to increase isolation, or Vias are drilled on the static discharge path to reduce the resistance so that the static electricity can be discharged more and faster from the designed path.
In the figure below, the screw holes of the PCB are fixed on the casing by screws. The casing is the main carrier for discharging static electricity, so the method here is to add an electrostatic discharge resistance-capacitance network R36 between the SMA and the screw holes (ie the casing) in parallel with C82. , SMA, electrostatic discharge network and screw holes are in a straight line to ensure that the path is the shortest, and there is no trace on the path, and some vias are deliberately added to the electrostatic discharge path to reduce impedance. The final prototype also passed the ESD immunity test.
Figure 5 ESD design example of external antenna interface