Basic Methods of Reliability Design

System reliability design

　　1. Simplify the design

　　During the design process, the system will keep the circuit as simple as possible while meeting the performance indicators. The system design fully draws on the design experience of 2G repeater stations and adopts highly reliable, modular standard RF modules to improve the integration of the system. The monitoring panel is directly borrowed from the 2G repeater station monitoring panel, and the monitoring program is redesigned according to the 3G communication protocol. The power supply adopts the company's mature modular power supply solution to improve the reliability of the product.

　　2. Module and component selection and control

　　Give priority to the components in the company's component outline, give priority to the components provided by certified qualified suppliers, reduce the variety and specifications of components as much as possible, and strictly control the selection of components with non-standard specifications;

　　For some RF modules that need to be purchased from outside, on the one hand, strict access certification is carried out on the suppliers, and on the other hand, strict performance inspection is carried out on the purchased modules entering the warehouse to ensure the quality of the purchased modules. Before installation, 100% of the purchased modules and components will be subjected to environmental stress screening test (ESS) to ensure that the early performance defects of the components have been eliminated before installation.

　　3. Thermal design considerations

　　When designing the structure of the repeater station, we conduct thermal analysis and prediction on the product, design and control the maximum temperature rise inside the product, use high-power heat sinks, and reserve sufficient margins. At the same time, when installing the power amplifier module with high heat generation, we coat the bottom with thermal conductive silicone grease to ensure that the temperature rise on the surface of the power amplifier does not exceed 25°C.

　　After the overall structural design is completed, the thermal field distribution of electronic equipment is analyzed and studied in accordance with the heat generation mechanism and propagation mode of electronic equipment, and a reasonable thermal design method is adopted to ensure that the electronic equipment works within the allowable temperature range. Through CAE-assisted analysis software, the thermal effect analysis of the repeater product is carried out in three aspects: model establishment, model solution and result interpretation, and the parameter position of the key components and parts of the whole equipment is optimized; and the forced convection and natural convection cold structure design scheme of the electronic system is optimized. After the simulation scheme meets the design requirements, the equipment structure design scheme is finally evaluated through the environmental temperature rise test to ensure the thermal design reliability of the repeater equipment.

　　4. Derating design

　　Reducing the stress (generally referring to temperature stress and electrical stress) that components are subjected to in the circuit can improve the reliability of components. The operating temperature range of components is required to be greater than the operating temperature range of the entire machine. The withstand voltage of components such as resistors and capacitors should be greater than twice the rated operating voltage. The actual power consumption of the power module should not exceed 70% of the rated power.

　　5. Communication reliability

　　Since the whole machine adopts a modular design, it involves the communication between each module and the main monitoring module. In order to ensure the stability of the communication, we use the RS485 bus communication method. This method uses a high-voltage differential digital signal bus communication, which can greatly improve the signal's anti-interference ability. At the same time, each of our modules uniformly adopts negative pressure protection, overcurrent protection and other anti-online interference designs to ensure the stability of internal module communication.

　　6. FMEA analysis

　　FMEA is an important means of reliability analysis. As the whole repeater adopts mature modular design technology, according to the design experience of 2G repeaters, the fault or failure of the power amplifier module has a greater impact on the function of the whole machine. When the power amplifier module fails or parameter drift occurs, the impact on the whole machine is that the whole machine has no output or the output power is out of control, which may cause network paralysis in severe cases. Therefore, the power amplifier module is identified as the key part of the whole machine and must be focused on during the research and development and production process. The power amplifier module must be strictly tested and screened before installation, and the surface temperature rise of the power amplifier module during use must be strictly controlled not to exceed 25°C.

　　7. Environmental protection design

　　The structural design of the whole repeater station will fully consider the impact of vibration and impact. Components that are too high in the monitoring panel will be fastened by special measures such as mechanical fixation or adhesive fixation. All wiring in the box will be classified and bundled firmly. Copper pipes that are too long will be fixed by special measures to prevent failures in a vibration environment. Products should be subjected to vibration and impact tests according to relevant standards before conversion.

　　By selecting modules and components with a wider operating temperature range, we can ensure that the products meet the high and low temperature and damp heat test requirements specified in relevant standards. High and low temperature and damp heat tests must be carried out during routine product testing.

　　8. Electrostatic protection and EMC design

　　On the premise of meeting the functional requirements, the monitoring board and RF module shall give priority to using components with high anti-static discharge capability (i.e. high damage threshold). All IO pins derived from the monitoring board are equipped with protection devices such as electrostatic protection circuits.

　　The power module of the whole machine is designed with over-current protection, voltage protection and short-circuit protection circuits, which can immediately cut off the power supply under abnormal circumstances.

　　The considerations for electromagnetic compatibility design are: modular design, step-by-step refinement, starting from the power supply module, first ensuring the electromagnetic compatibility indicators of the power supply module, multi-stage filtering, to ensure the rationality of the voltage range output from the power supply module. In each active module, for the power supply design part, high-power and high-voltage capacitors and inductors are used to filter the input DC voltage to ensure the reliability of the system RF indicators in the presence of surges, power ripples, etc.

　　In the design of the repeater, it is necessary to filter the power supply and the power supply circuits of each internal module. According to the requirements of 3GPP TS25.113 "Base Station (BS) and repeater ElectroMagnetic Compatibility (EMC)", the equipment must pass electromagnetic compatibility tests, including conducted disturbances, radiated spurious emissions (applicable to wireless and optical far ends), radiated continuous disturbances (applicable to optical near ends), harmonic currents, voltage fluctuations and flicker, electrostatic discharge immunity, radiated immunity, electrical fast transient pulse groups, surge (impact) immunity, conducted immunity, voltage sags and short interruptions (AC power) test items, to ensure the reliability of the electromagnetic compatibility design of the equipment.

　　Since 3G repeater products draw on the design experience and design patterns of mature G-site products in terms of overall structure design, overall assembly and key modules, and the test requirements for EMC radiation and stray interference of 3G repeaters in 3GPP TS25.113 "Base Station (BS) and repeater ElectroMagnetic Compatibility (EMC)" are exactly the same as those of relevant standards for 2G repeaters; considering the comparability of similar products, the EMC test report of the GZF900 series products can basically illustrate the EMC performance of the 3G repeater.

　　9. RoHS considerations

　　Regarding the EU's ROHS directive, the company's existing welding equipment fully supports lead-free soldering processes. The company is also starting with component procurement, trying to use lead-free soldering devices as much as possible, and the company has passed the ISO14001 environmental system certification.

　　10. Packaging reliability considerations

　　The company improves the packaging method and packaging materials of the products to ensure the reliability of product transportation and ensure that the products reach customers safely. Before the product is finalized, it is required to carry out transportation vibration test to better ensure the mechanical strength of the product, the reliability of the installation process and the reliability of product packaging and transportation.