At present, electronic equipment is used in various electronic devices and systems, and printed circuit boards are still the main assembly method. Practice has proved that even if the schematic design of the circuit is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the electronic device. For example, if the two thin parallel lines of the printed board are in close proximity, a delay in the signal waveform is formed, and reflected noise is formed at the end of the transmission line. Therefore, when designing a printed circuit board, care should be taken to use the correct method.
First, grounding
Ground Design In electronic equipment, grounding is an important method of controlling interference. If the grounding and shielding are properly combined, most of the interference problems can be solved. The ground wire structure in the electronic device is roughly systematic, chassis ground (shielded ground), digital ground (logically), and analog ground. Pay attention to the following points in the ground line design:
1) Correct selection of single-point grounding and multi-point grounding In the low-frequency circuit, the operating frequency of the signal is less than 1MHz, and the influence of the inductance between the wiring and the device is small, and the circulating current formed by the grounding circuit has a great influence on the interference, so it should be adopted. Ground a little. When the signal operating frequency is greater than 10MHz, the ground line impedance becomes very large. At this time, the ground line impedance should be reduced as much as possible. When the operating frequency is between 1 and 10 MHz, if a grounding is used, the grounding length should not exceed 1/20 of the wavelength. Otherwise, the multi-point grounding method should be used.
2) Separate the digital circuit from the analog circuit. There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as far as possible, and the ground wires of the two should not be mixed and connected to the ground wire of the power supply. Try to increase the grounding area of ​​the linear circuit.
3) Try to thicken the grounding wire. If the grounding wire is very thin, the grounding potential changes with the change of the current, which causes the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Therefore, the ground wire should be as thick as possible so that it can pass the three allowable currents on the printed circuit board. If possible, the width of the grounding wire should be greater than 3mm.
4) When the grounding wire constitutes a closed-loop circuit Designing a grounding system of a printed circuit board composed only of digital circuits, making the grounding wire into a closed-loop circuit can significantly improve the anti-noise capability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are many power-consuming components, due to the limitation of the grounding wire thickness, a large potential difference will be generated on the ground junction, causing the noise resistance to decrease. If the ground structure is looped, the potential difference will be reduced to improve the noise immunity of the electronic device.
Second, electromagnetic compatibility design
Electromagnetic Compatibility Design Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and efficient manner in a variety of electromagnetic environments. The purpose of the electromagnetic compatibility design is to enable the electronic device to suppress various external interferences, enable the electronic device to work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of the electronic device itself to other electronic devices.
1) Choosing a reasonable wire width The impact interference caused by the transient current on the printed line is mainly caused by the inductance component of the printed wire. Therefore, the inductance of the printed wire should be minimized. The inductance of a printed conductor is proportional to its length and inversely proportional to its width, so that short and precise conductors are advantageous for suppressing interference. Signal lines for clock leads, row drivers, or bus drivers often carry large transient currents, and the printed conductors should be as short as possible. For the discrete component circuit, when the width of the printed conductor is about 1.5mm, the requirement can be fully satisfied; for the integrated circuit, the width of the printed conductor can be selected between 0.2 and 1.0mm.
2) Adopting the correct wiring strategy to use equal routing can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is better to use a well-shaped mesh wiring structure. The specific method is that one side of the printed board is horizontal. Wiring, the other side of the longitudinal wiring, and then connected with metallized holes at the intersection holes.
In order to suppress the crosstalk between the printed circuit board wires, the long distance equalization should be avoided when designing the wiring, and the distance between the wires and the wires should be as far as possible. The signal wires should not cross the ground wire and the power cable as much as possible. A grounded trace is placed between some signal lines that are very sensitive to interference, which effectively suppresses crosstalk.
In order to avoid the electromagnetic radiation generated when the high-frequency signal passes through the printed wiring, the following points should also be noted when wiring the printed circuit board:
Minimize the discontinuity of the printed conductor. For example, the width of the wire should not be abrupt, and the corner of the wire should be greater than 90 degrees to prohibit the loop.
The clock signal lead is most likely to cause electromagnetic radiation interference. The wiring should be close to the ground loop, and the driver should be next to the connector.
The bus driver should be close to the bus it is intended to drive. For leads that leave the printed circuit board, the drive should be next to the connector.
The data bus should be routed with a signal ground between every two signal lines. It is best to place the ground loop next to the least important address leads, as the latter often carry high frequency currents.
Arranging the device when arranging high-speed, medium-speed and low-speed logic circuits on the printed board 3. Suppressing reflection interference In order to suppress the reflection interference occurring at the end of the printed line, the length of the printed line should be shortened as much as possible except for special needs. A slow circuit is used. Terminal matching can be added if necessary, that is, a matching resistor of the same resistance value is added to the ground and the power supply end at the end of the transmission line. According to experience, for the TTL circuit with faster speed, the terminal matching method should be adopted when the printed line is longer than 10cm. The resistance of the matching resistor should be determined according to the output drive current of the integrated circuit and the maximum value of the sink current.
Third, decoupling capacitor configuration
In a DC power supply loop, changes in load can cause power supply noise. For example, in a digital circuit, when a circuit transitions from one state to another, a large spike current is generated on the power line to form a transient noise voltage. The configuration of decoupling capacitors can suppress the noise generated by load changes. It is a common practice for the reliability design of printed circuit boards. The configuration principles are as follows:
The power input terminal is connected to a 10~100uF electrolytic capacitor. If the position of the printed circuit board is allowed, the anti-interference effect of the electrolytic capacitor above 100uF is better.
A 0.01 uF ceramic capacitor is provided for each integrated circuit chip. If you encounter a small printed circuit board space and can not fit, you can configure a 1 ~ 10uF tantalum electrolytic capacitor every 4 ~ 10 chips, the high-frequency impedance of this device is particularly small, the impedance is less than 1Ω in the range of 500kHz ~ 20MHz, Moreover, the leakage current is small (0.5 uA or less).
For devices with weak noise capability, large current changes during shutdown, and memory devices such as ROM and RAM, decoupling capacitors should be directly connected between the power supply line (Vcc) and ground (GND) of the chip.
The lead of the decoupling capacitor should not be too long, especially the high frequency bypass capacitor can not be leaded
Fourth, the size of printed circuit boards and device layout
The size of the printed circuit board should be moderate. When the size is too large, the printed lines are long and the impedance is increased. The noise resistance is reduced and the cost is high. If it is too small, the heat dissipation is not good, and it is susceptible to interference from adjacent lines.
In terms of device layout, as with other logic circuits, the related devices should be placed as close as possible to achieve better noise immunity. as shown in picture 2. Clock generators, crystal oscillators, and CPU clock inputs are prone to noise and should be close to each other. Devices that are prone to noise, small current circuits, high-current circuits, etc. should be kept away from logic circuits as much as possible. If possible, it is important to make a separate board.
Fifth, the thermal design is from the perspective of facilitating heat dissipation
The printed version is preferably mounted upright, the distance between the board and the board is generally not less than 2 cm, and the arrangement of the device on the printed board should follow certain rules: for equipment using free convection air cooling, it is best Integrate integrated circuits (or other devices) in a vertically long manner; for devices that use forced air cooling, it is best to arrange the integrated circuits (or other devices) in a horizontally long manner, and the devices on the same printed board should be as According to the heat generation and heat dissipation degree, devices with low heat generation or poor heat resistance (such as small signal transistors, small scale integrated circuits, electrolytic capacitors, etc.) are placed at the highest flow (inlet) of the cooling airflow, and the heat is large. Or heat-resistant devices (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow.
In the horizontal direction, high-power devices are placed as close as possible to the edges of the printed board to shorten the heat transfer path; in the vertical direction, high-power devices are placed as close as possible to the top of the printed board to reduce the effects of these devices on the temperature of other devices. The temperature-sensitive device is preferably placed in the lowest temperature region (such as the bottom of the device). Do not place it directly above the heat-generating device. Multiple devices are preferably staggered on a horizontal plane. The heat dissipation of the printed circuit board in the device mainly depends on the air flow, so the air flow path should be studied during the design, and the device or the printed circuit board should be properly configured. When the air flows, it tends to flow in a place with low resistance. Therefore, when configuring the device on the printed circuit board, avoid leaving a large air space in a certain area. The same problem should be noted in the configuration of multiple printed circuit boards in the whole machine.
Summary
A large number of practical experience shows that the reasonable device arrangement can effectively reduce the temperature rise of the printed circuit, thus significantly reducing the failure rate of devices and devices. The above is only some general principles for the reliability design of printed circuit boards. The reliability of the circuit board is closely related to the specific circuit. In the design, the corresponding circuit is not required to be processed accordingly, so as to ensure the reliability of the printed circuit board to the utmost extent.
What is 5G CPE?
Definition of 5G CPE
CPE stands for Customer Premise Equipment. The so-called front end refers to the equipment in front of the customer's terminal equipment. When we use Wi-Fi, if the distance is far, or there are more rooms, it is easy to appear signal blind spots, resulting in mobile phones or ipads or computers can not receive Wi-Fi signals. The CPE can relay the Wi-Fi signal twice to extend the coverage of Wi-Fi.
What are the benefits of CPE?
Through the following comparison table, it is not difficult to understand the technical advantages of CPE products:
* Currently, the global 5G FWA service is mainly in the Sub-6GHz band, with only the United States and Italy supporting the millimeter wave band.
* 5G CPE integrates the low cost of Wi-Fi and the large bandwidth of 5G, combining the advantages of the two to form a strong complement to traditional fiber broadband.
The relationship between 5G, FWA and CPE
It can be said that FWA (Fixed Wireless Access) will be the most down-to-earth application of 5G technology. FWA business plays a key role in enabling "connecting the unconnected." FWA is a low-cost, easy-to-deploy flexible broadband solution. Compared with wired access technology, FWA has been an ideal choice for deploying broadband in many countries and regions because it does not need to obtain rights of way, dig trenches and bury cables, and drill holes through walls. The development of 5G technology is further promoting the development of FWA.
FWA services (including 4G and 5G) have reached 100 million users. FWA is no longer a niche service; The FWA industry as a whole has been supported by numerous suppliers. Why is that? In the 5G era, 5G CPE receives 5G signals from operator base stations and then converts them into Wi-Fi signals or wired signals to allow more local devices to get online. For operators, the initial user penetration rate of 5G is low, and the investment is difficult to realize quickly; The CPE business can use the idle network to increase revenue for operators, so major operators vigorously promote the development of 5G CPE.
FWA services can be used for both home (To C) and business (To B), and customers have different requirements for CPE devices when using FWA services in different application environments, resulting in consumer grade 5G CPE and industrial grade 5G CPE (similar to home routers and industrial routers).
In 2020, the global market size of 5G CPE will reach 3 million units, and it is expected that in the next five years, the market size of 5G CPE will maintain a compound growth rate of more than 100%, reaching 120 million units in 2025, with a market value of 60 billion yuan. As an important market for 5G CPE, China's 5G CPE market size will reach 1.5 million units in 2020 and is expected to reach 80 million units in 2025, with a market value of 27 billion yuan.
The difference between 5G CPE and other devices
CPE can support a large number of mobile terminals that access the Internet at the same time, and the device can be directly inserted with a SIM card to receive mobile signals. CPE can be widely used in rural areas, cities, hospitals, units, factories, communities and other wireless network access, can save the cost of laying wired networks.
A Router is a hardware device that connects two or more networks, acts as a gateway between networks, and is the main node device of the Internet. Routers use routes to determine the forwarding of data. If it is a home router, it does not support a SIM card slot, and can only receive signals by connecting to optical fiber or cable and then convert it into WI-FI to provide a certain number (several) of terminal devices to surf the Internet.
Industrial 5G CPE is equivalent to 5G industrial routers, and the technology of the two is not very different. On the one hand, the industrial 5G CPE converts 5G network signals into WiFi signals for transmission, and on the other hand, the data received by the WiFi network is converted into 5G network signals for uploading. In addition, industrial 5G CPE generally supports routing functions.
5G CPE trends
According to a research report, after evaluating the products of some mainstream 5G CPE suppliers, many institutions believe that the development of 5G CPE products will continue in two aspects: one is to support mmWave and Sub-6 GHz at the same time; Second, the design will pay attention to humanized operation and installation. The industry development trend will accelerate the demand for 5G in the medical, education and manufacturing industries due to the epidemic, and 5G FWA will promote global 5G CPE shipments.
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