When the hardware engineer is designed, the ESD immunity is an important consideration. Static electricity is harmful to most electronics, and the RF module is more sensitive to static electricity. So how do ESD antisenics should be considered and designed for RF module products?
Regarding the ESD immunity level, different industries of different products correspond to different standards, and IEC61000-4-2 standards issued by the International Electrotechnical Commission are suitable for electromagnetic compatibility of various electrical and electronic equipment. You need to specify the level of ESD immunity to products before product design, or define it according to standards or according to the actual needs of the product. This can be based on product design and testing.
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About the implementation of the ESD immunity level, mainly shell design, hardware design and PCB layout, component selection, software repair, etc. Among them, in terms of hardware design, an important way is to add an ESD protection device at the key circuit node of the input or output. The ESD protection device generally uses transient voltage suppressors, which is also the TVS (Transient Voltage Suppressor) diode, and the key feature of the TVS tube is that it has high impedance under normal operating voltages, while the voltage exceeds the normal operating voltage. The time is low impedance. At this time, the current is directed directly from the sensitive component to the system or earth (housing), suppressing the reverse transient high voltage between the two poles between the TVS tube, for static electricity, suitable for selecting two-way TVS, rather than One-way TVS.
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For the IoT industry, wireless products are an important part, and the wireless module is an important part of the wireless product. This paper mainly describes the ESD design of the wireless module antenna terminal and the radio frequency interface, and the wireless module product can be added. ESD protection devices, while the backplane of the wireless module needs to consider a reliable and effective electrostatic bleed path at an antenna interface to ensure that the electrostatic bleed path is not sensitive to sensitive devices and sensitive lines.
When using the wireless module, from the wireless module product manual sometimes see the antenna side needs to avoid electrostatically hit the radio frequency interface of the antenna, even if you get the module, it is best to bring an anti-static glove. This shows that the RF output of the wireless module is sensitive to static, more than a certain static level may cause the wireless module to be permanently damaged. A wireless module radio frequency output is recommended here, as shown in Figure 3.
Although using the TVS tube can improve the ESD performance of the wireless module antenna, there is a homogeneous noise to generate harmonic noise, so that the receiving sensitivity of the wireless module is lowered. This requires the selection of the TVS tube, and it is preferred that the TVS tube can avoid this hidden danger, so the key indicator of the harmonic noise in the device selection.
For an external antenna version of the wireless module, it is generally necessary to place an antenna interface (such as a SMA interface) on the bottom plate of the wireless module to install the desired antenna. At this time, you need to fully consider the bleed path of the static electricity on the bottom plate, to ensure that the electrostatic should not enter the wireless module product itself through the RF terminal, ensuring that even if there is electrostatic import antenna interface, the static electricity can quickly pass the bleed path we designed. To the earth, this can effectively improve the reliability of the product. In general, it is recommended to use the sealing network shown in FIG. 4 to conduct static electricity. Note that the electrostatic bleed path is guaranteed when the PCB layout is shorter and the electrostatic sensitive device, radio frequency circuit, and sensitive lines (such as data cable, clock line, etc.) ), The capacitance here is a high pressure resistant device.
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Below, this idea is described by an ESD design case.
As shown in FIG. 5, J12 is a SMA antenna interface of 4G wireless modules, and is exposed in air, so static electricity may be introduced into the PCB from SMA. In order to avoid electrostatic import 4G wireless modules, the module is damaged, it is necessary to design electrostatic bleed-resistant networks on the PCB. The design principle is that the electrostatic bleed path is shorter and does not pass the electrostatic sensitive network. If necessary, the slot can be added to increase isolation, or The holes on the electrostatic bleed path are tied to reduce the resistance so that the static electricity can be more, faster from the design path.
The screw hole of the PCB is fixed to the outer casing in the figure below, and the outer casing is the main carrier of the discharge of static electricity, so the practice is to increase the electrostatic discharge blocking network R36 between the SMA and the screw hole (ie casing). C82 , SMA, electrostatic discharge network and screw holes are on a straight line, guaranteed the shortest path, and there is no trace on the path to reduce the impedance in the electrostatic bleed path. The final sample made also passed the ESD immunity experiment.
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