“The electromagnetic environment contains radiated and conducted energy. EMC also includes both radiation and susceptibility. Radiation refers to the unnecessary generation of electromagnetic energy by a product. In order to create an electromagnetically compatible environment, it is often necessary to control radiation. Sensitivity is a measure of the ability of an Electronic product to tolerate the radiation of other electromagnetic products, or the effects of conducted electromagnetic energy or other electromagnetic effects. Immunity is the opposite of sensitivity. Devices with high sensitivity have low immunity.
Electromagnetic Radiation Interference Problems
Wireless communication is very complex, and it is divided into baseband and radio frequency. In the design of digital electronic circuits, most engineers are often troubled by the problem of electromagnetic compatibility (EMC).
EMC refers to the ability of an electronic system to maintain good performance in a target electromagnetic environment without introducing significant amounts of electromagnetic interference into that environment.
The electromagnetic environment contains radiated and conducted energy. EMC also includes both radiation and susceptibility. Radiation refers to the unnecessary generation of electromagnetic energy by a product. In order to create an electromagnetically compatible environment, it is often necessary to control radiation. Sensitivity is a measure of the ability of an electronic product to tolerate the radiation of other electromagnetic products, or the effects of conducted electromagnetic energy or other electromagnetic effects. Immunity is the opposite of sensitivity. Devices with high sensitivity have low immunity. Common EMC problems include: electromagnetic radiation emissions exceeding standard requirements; failure phenomena caused by ESD electrostatic discharge problems such as system freezes, system resets, and Display panel errors; product radiation immunity problems lead to changes in product signal output at certain frequencies Huge, there is a communication error, or the system restarts or crashes. For EMC common problems, Excelpoint Shijian engineer Wolfe Yu put forward his views and solutions.
Principle of Electromagnetic Radiation
Using a closed conductor, loaded with a time-varying current across it, produces a fluctuating magnetic field and an electric field around it. When the electric charge accelerates and moves, if the near field and the far field appear at the same time, the near field follows the electric charge to accelerate the movement, and the far field cannot synchronize with the electric charge movement, and a kink (Kink) will appear. If the electric field is too kinked, it will break away from the original field and radiate out.
Fig.1 Electric field kink and radiation principle
Generally speaking, two parallel cables are used, and the length is relatively short compared to the wavelength, and the generated electric field is a near field, which is usually called “differential signal”, and it is difficult to produce a kink. If the cable is opened at a certain angle, the electromagnetic field will be uncoordinated, forming a near field and a far field, resulting in a kink (Kink), forming an antenna, which is also the antenna principle in wireless communication.
Figure 2 The formation principle of the far field
Harmonic components of the transmitted waveform
In digital circuits, most baseband signals resemble square waves, which are the superposition of countless successively decreasing harmonic components. The harmonic components of the current generate fluctuating magnetic fields that are transmitted down through coupling or radiation.
Figure 3 Harmonic components of digital signals
PCB Transmission Line Model
In the PBC Layout, most engineers like to separate the power lines or signal lines from the ground, which is very dangerous, because once the wavelength of the loop and the high-frequency signal are close, a loop antenna will be formed, and the high-frequency harmonic frequency will be The electromagnetic field is coupled in, forming resonance.
Figure 4 Loop antenna created in PCB layout
According to the following two wirings, the two conductors form a capacitor. According to the principle of electromagnetic field radiation, the wiring on the left side, when electrons move at high frequencies, it is easy to form near and far fields, resulting in kinks and emitting electromagnetic waves. It is also easy to receive external electromagnetic waves. In the wiring on the right, between the power supply and the ground, the loop is small enough to form a closed loop, and the electric field radiation is very weak. It is difficult to generate electric field radiation and it is difficult to be interfered by other electromagnetic fields.
Figure 5 Layout and wiring comparison of power supply and ground
Most signals, especially high-speed signals, are designed as differential circuits. In the differential circuit, it is mainly divided into a DC component and an AC component. The DC component will not generate an alternating magnetic field, and the AC component will generate an alternating magnetic field and cause electromagnetic interference.
Figure 6 Differential circuit model
Regarding the layout and wiring of the differential circuit, it is actually to implement a closed circuit strictly according to the symmetrical line of equal length, and it is difficult for the electromagnetic field to penetrate the differential circuit. This is why the transmission characteristics of differential signals are relatively stable. In addition to the need for equal distribution lines for differential signals, engineers also need to perform power matching and grounding for differential signals.
Figure 7 Principle of equal-length symmetry of differential circuit
In fact, in the circuit structure, the power supply and ground of each circuit are the largest differential pair in the entire circuit. Most of the time, engineers like to wire the power supply into the structure shown in Figure 8. In the early single-panel products In layout and routing, this is the best routing method.
Figure 8 Traditional single-panel layout and wiring diagram
As the communication rate requirements increase, the wiring structure in Figure 8 will form an antenna effect and generate electromagnetic wave radiation. Therefore, in order to reduce electromagnetic interference, Wolfe Yu recommends placing the power and ground together as much as possible when wiring.
Figure 9 Suggestion for main chip power ground wiring
Current sink model
Regarding the issue of current continuity, there is a more vivid analogy: each capacitor is a reservoir, and the water stored in the upstream is greater than that in the downstream, so that the river will not dry up due to the interruption of the current. The power supply principle is the same, but improper design will cause strong electromagnetic interference.
Figure 10 Reservoir theory of current sink model
Let’s take a look at the power supply topology. The DCDC power supply is roughly divided into three loops (one said four loops). The high-frequency AC chopper loop separates the two DC input loops and the output loop. The AC loop is mainly used for high-speed PWM chopper. Electromagnetic cutting occurs, and once the wiring is not good, it is easy to generate electromagnetic radiation.
Figure 11 DCDC power supply topology
For the convenience of design, Wolfe recommends that the two DC loops and the AC loop should be laid out separately during the layout, so that the wiring can be separated when wiring.
Figure 12 DCDC power supply layout and wiring recommendations
Fully enclosed circuit based on coaxial line
In transmission lines, it is difficult for engineers to guarantee that the circuit can achieve 100% closure. In order to prevent electromagnetic radiation from being generated on the transmission line, a transmission method based on coaxial line is proposed. Coaxial transmission is to seal the electromagnetic field between the inner and outer conductors, and the radiation loss and external loss are very small.
Figure 13 Closed electromagnetic field of coaxial line
Coaxial transmission solves the problem of electromagnetic radiation for high-speed signal transmission. In addition to the problem of electromagnetic radiation, there is another problem in circuit transmission. That is, when the signal frequency is very high, in addition to resistive loads, there are reflected signals generated by capacitive loads and inductive loads. The reflected signals will be superimposed on the original signal and change the shape of the original signal, which is called “transmission line loss” “.
Figure 14 Equivalent model of transmission line
Figure 14 is an equivalent model of a transmission line. In addition to resistive loads, there are capacitive and inductive loads. According to the theoretical formula, it is easy to calculate the impedance value of the transmission line. To cancel the reflected signal, engineers can insert a passive network between the circuit source and the load to make the load impedance and source impedance conjugate match, which is impedance matching.
Figure 15 Principle of Impedance Matching
Microchip based CoaXpress® turnkey solutions
Microchip, a representative of Excelpoint, introduces a CoaXpress-based®The latest video transmission scheme is a fully enclosed circuit transmission scheme based on coaxial cable.
The EQCO125T40 integrates equalizers, CDRs and cable drivers to transmit/receive signals on a single cable or PCB trace pair, 1.25 Gbps/12.5 Gbps 8b/10b encoded downstream transmission, and 20.833 Mbps/41.666 Mbps 8b/10b encoded Uplink transmission, the maximum transmission distance can reach 40m.
Coaxial cables are inherently shielded by their outer conductors, making them highly resistant to external electromagnetic interference (EMI) present in many operating environments, especially in noisy factory environments. This enables CoaXpress to be applied in a variety of complex industrial environments and is the only solution for HD camera lens transmission.
Figure 16 Based on CoaXpress®video transmission scheme
This chip is based on Microchip PolarFire®On the video platform, customers can easily complete product development and shorten the development process by using the free IP package provided by Microchip.
Figure 17 Microchip PolarFire®video platform
Figure 18 MIC28517 Evaluation Board
At the same time, Excelpoint Shijian provides relevant reference design and technical guidance for customers, and provides a series of PCB wiring guidance for domestic customers who are prone to a series of technical bottlenecks in DCDC design and layout and wiring, and also provides PCB design files for customers’ convenience. Import to help customers easily design and layout electronic circuits.
About Shijian——The Leading Authorized Distributor of Components in Asia Pacific
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