Electronic Design - EMC analysis and design of high-speed circuits

Electromagnetic compatibility means that when electrical and electronic systems and equipment operate at a set level within the specified safety limits in a specific electromagnetic environment, they will not be damaged or deteriorated to irreparable performance due to external electromagnetic interference. At the same time, the electromagnetic radiation generated by them is not greater than the limit level of verification, and does not affect the normal operation of other electronic equipment or systems, so as to achieve the purpose of non-interference between equipment and equipment, system and system, and work together reliably.

1 Factors of electromagnetic compatibility

(1) Frequency characteristics of resistance. In a digital circuit, the main function of a resistor is to limit current and determine a fixed level. In a high-frequency circuit, the high-frequency parasitic capacitance existing at both ends of the resistor will cause damage to the normal circuit characteristics. The pin inductance of the same resistance has a great influence on the EMC of the circuit.

(2) The frequency characteristic of the capacitor. Capacitors are generally used in the power bus. They provide decoupling, bypass and maintain a fixed DC voltage and current. However, in high-frequency circuits, when the operating frequency of the circuit exceeds the self-resonant frequency of the capacitor, its parasitic inductance will make the capacitor behave as an inductive characteristic, thereby losing its original function and affecting the performance of the circuit.

(3) Frequency characteristics of inductance. The inductor is used to control EMI in the PCB. When the operating frequency of the circuit increases, the equivalent impedance of the inductor will increase with the increase in frequency. When the operating frequency of the circuit exceeds the upper limit of the operating frequency of the inductor, the inductance will affect the normal operation of the circuit.

(4) Frequency characteristics of wire. The traces on the PCB and the lead wires of the components have parasitic inductances and capacitances. These parasitic inductances and capacitances will affect the frequency characteristics of the wires, which may cause resonance between the components and the wires, causing the wires to become electromagnetic interference The important transmitting antenna. Generally, the wire exhibits resistance characteristics in the low frequency band and inductance characteristics in the high frequency band. Therefore, on the PCB, the length of the wire is generally required to be less than one twentieth of the wavelength of the operating frequency to prevent the wire from becoming a source of electromagnetic interference.

(5) Static electricity. The problem of electrostatic discharge has become a major public hazard to electronic products, which may cause permanent damage to the product. Therefore, in the product design, corresponding electrostatic protection measures must be taken. Commonly used anti-static measures include choosing anti-static materials, adopting electrical isolation measures, improving the insulation strength of products, and setting up good electrostatic shielding layers and discharge channels.

(6) Power supply. With the widespread application of high-frequency switching power supplies and the continuous increase of power system loads, the problem of power supply interference to products has gradually become an important factor affecting the EMC characteristics of products. Therefore, some sensitive equipment that is susceptible to interference has not directly used AC power supply but switched to DC power supply. Although this has increased the complexity and cost of the system, it has effectively improved the stability of the system.

(7) Thunder and lightning. Lightning is essentially a strong electrostatic discharge process that neutralizes positive and negative charges. The resulting strong electromagnetic pulses are the main cause of damage to various electronic devices. The impact of lightning on electronic equipment includes direct lightning and induced lightning. Nowadays, various indoor electronic devices are generally not susceptible to direct lightning, but they are still susceptible to damage from induced lightning. In order to ensure the safe operation of electronic equipment, electronic equipment must be protected against lightning strikes. Commonly used lightning protection measures include the installation of lightning rods, the installation of lightning arresters, and lightning protection cables.

2 Elements of electromagnetic compatibility

Theoretical and practical studies have proved that, regardless of a complex system or a simple device, any electromagnetic interference must meet three basic conditions: the existence of a certain interference source, a complete coupling channel with interference, and the response of the interfered object.

2.1 Sources of electromagnetic interference

Electromagnetic interference source refers to any element, device, equipment, system or natural phenomenon that produces electromagnetic interference. High-frequency circuits are particularly sensitive to electromagnetic interference, so a variety of measures need to be taken to suppress electromagnetic interference. Through theoretical and experimental analysis, it is known that in high-frequency circuits, electromagnetic interference mainly comes from the following aspects:

(1) Noise interference from device operation

(A) Electromagnetic interference occurs when digital circuits are working.

(B) Electromagnetic field interference caused by changes in signal voltage and current.

(2) High-frequency signal noise interference

(A) Crosstalk: It means that when a signal is transmitted on a transmission channel, it has an undesirable effect on the adjacent transmission line due to electromagnetic coupling. The interfered signal appears to be injected with a certain coupling voltage and coupling current. Excessive crosstalk may cause false triggering of the circuit, time delay, and cause the system to fail to work normally.

(b) Return loss: When a high-frequency signal is transmitted in cables and communication equipment, it will reflect the signal when it encounters uneven wave impedance. This reflection will not only increase the transmission loss of the signal, but also Distorting the transmission signal has a great impact on the transmission performance.

(3) Power supply noise interference

The power supply noise in the PCB is mainly composed of the noise generated by the power supply itself or the noise induced by the disturbance, which is mainly manifested as: 1. distributed noise caused by the inherent impedance of the power supply itself; 2. common mode field interference; 3. differential mode field interference; 4. line-to-line Interference; 5. Power line coupling.

(4) Ground noise interference

Because of the resistance and impedance on the ground wire, when the current passes through the ground wire, a voltage drop will be generated. When the current is large enough or the operating frequency is high enough, the voltage drop will be large enough to cause interference to the circuit. The noise interference caused by the ground wire mainly includes ground loop interference and common impedance coupling interference.

(A) Ground loop interference: When multiple functional units are connected to the ground wire, if the current in the ground wire is large enough, a voltage drop will be generated on the connecting cables between the devices. Due to the unbalanced electrical characteristics between the various circuits, the current on each wire will be different, so a differential mode voltage will be generated, which will affect the circuit. In addition, the external electromagnetic field may also induce current in the ground loop, causing interference.

(b) Common impedance coupling interference; when multiple functional units share the same ground wire, due to the existence of the ground wire impedance, the ground potential of each unit will be modulated mutually, which will cause interference between the signals of each unit. In a high-frequency circuit, the circuit is in a high-frequency working state, and the ground impedance is often large. At this time, the common impedance coupling interference is particularly obvious.

There are two ways to eliminate the common impedance coupling: one is to reduce the impedance of the common ground part, so that the voltage on the common ground is also reduced, thereby controlling the common impedance coupling. Another method is to avoid the common grounding of circuits that are easy to interfere with each other through proper grounding. Generally, avoid the common grounding of high-current circuits and weak-current circuits, and the common grounding of digital circuits and analog circuits. As mentioned earlier, the core problem of reducing the impedance of the ground wire is to reduce the inductance of the ground wire. This includes using a flat conductor as a ground wire, and using multiple parallel conductors that are far apart as a ground wire. For printed circuit boards, laying a ground wire grid on a double-layer board can effectively reduce the ground wire impedance. In a multilayer board, a special layer of ground wire has a small impedance, but it will increase the cost of the circuit board. . The grounding method to avoid common impedance through proper grounding is parallel single-point grounding. The disadvantage of parallel grounding is that there are too many grounded wires. Therefore, in practice, it is not necessary for all circuits to be connected in parallel with single-point grounding. For circuits with less mutual interference, single-point grounding in series can be used. For example, circuits can be classified according to strong signal, weak signal, analog signal, digital signal, etc., and then use single-point grounding in series within similar circuits, and single-point grounding in parallel for circuits of different types.

2.2 Inhibit coupling channel

The main coupling channels of electromagnetic interference in high-speed circuits include radiation coupling, conduction coupling, capacitive coupling, inductive coupling, power coupling, and ground coupling.

For radiation coupling, the main suppression method is to use electromagnetic shielding to effectively isolate the interference source from the sensitive object.

For conductive coupling, the main method is to reasonably arrange the direction of high-speed signal lines during signal wiring. The wires used for the input and output terminals should be avoided as far as possible to avoid signal feedback or crosstalk. A ground wire can be added between the two parallel wires to isolate them. For external connection signal lines, the input lead should be shortened as much as possible and the input end impedance should be increased. It is best to shield the analog signal input line. When the impedance of the signal wire on the board is not matched, it will cause signal reflection. When the printed wire is long, the circuit inductance will cause damping and oscillation. By connecting a damping resistor in series (the resistance value is usually 22～2 200 hm, and the typical value is 470 hm), the oscillation can be effectively suppressed, the anti-interference ability can be enhanced, and the waveform can be improved.

For the coupling interference of inductance and capacitance, the following two aspects can be used to suppress: one is to select suitable components, for inductance and capacitance, it should be selected according to the frequency characteristics of different components, and for other components, it should be selected according to the frequency characteristics of different components. Choose a device with small parasitic inductance and capacitance. On the other hand, the layout and wiring should be carried out reasonably, and long-distance parallel wiring should be avoided as much as possible. The wiring between electrical interconnection points in the circuit should be the shortest. The corners of the signal (especially high-frequency signal) lines should be designed in a 45-degree direction, or circular or arc shape, and should not be drawn in an angle less than or equal to 90 degrees. Adjacent wiring surface wires take the form of perpendicular, oblique or curved traces to reduce the parasitic capacitance and inductance of the vias. The shorter the lead between the vias and the pins, the better, and multiple vias can be considered in parallel. Or miniature vias to reduce the equivalent inductance. When selecting component packages, standard packages should be selected to reduce lead impedance and parasitic inductance caused by package mismatch.

For power coupling and ground coupling, first attention should be paid to reducing the power line and ground line impedance, and necessary measures must be taken for waveform distortion and oscillation caused by common impedance, crosstalk, and reflection. Connect bypass capacitors between the power and ground wires of each integrated circuit to shorten the flow path of the switching current. The power line and ground line are designed into a grid shape instead of a comb shape. This is because the grid shape can significantly shorten the circuit loop, reduce the line impedance, and reduce the interference. When multiple integrated circuits are installed on the printed circuit board, and some components consume large amounts of power, and the ground wire has a large potential difference, forming a common impedance interference, it is advisable to design the ground wire as a closed loop, which has no potential Poor, with higher noise tolerance. The leads should be shortened as much as possible, and the ground of each integrated circuit should be connected to the entrance ground of the circuit board at the shortest distance to reduce the spikes generated by the printed wires. Keep the ground wire and power wire in the same direction as the data transmission direction to improve the noise tolerance of the circuit board. Try to use multi-layer printed circuit boards to reduce the ground potential difference and reduce the power line impedance and crosstalk between signal lines. When there is no multilayer board and double-sided boards have to be used, the ground wire must be widened as much as possible. Generally, the ground wire should be thicker to pass 3 times the actual current flowing through the wire. The common power line and ground line are distributed as far as possible on the edges of both sides of the printed board. Connect a tantalum capacitor of 1μF～10μF to the power bus plug for decoupling, and connect a high frequency ceramic capacitor of 0.01μF～0.1μF in parallel with the decoupling capacitor.

2.3 Protect sensitive objects

The protection of sensitive objects is mainly concentrated in two aspects. On the one hand, the channel between sensitive objects and electromagnetic interference is cut off. The other is to reduce the sensitivity of sensitive objects.

The sensitivity of electronic devices is a double-edged sword. On the one hand, users want high sensitivity of electronic devices to improve the ability to receive signals; on the other hand, high sensitivity also means that they are more likely to be affected by noise. Therefore, the sensitivity of electronic equipment should be determined according to specific conditions.

For analog electronic equipment, the usually adopted method is to use preferred circuits, such as designing low-noise circuits, reducing bandwidth, suppressing interference transmission, balancing input, suppressing interference, and selecting high-quality power supplies. Through these methods, the sensitivity of electronic equipment to electromagnetic interference can be effectively reduced, and the anti-interference ability of the equipment can be improved.

For digital electronic equipment, digital circuits with high DC noise tolerance should be used when the working index permits. For example, the DC noise tolerance of CMOS digital circuits is much higher than that of TTL digital circuits; If the index permits, try to use a digital circuit with a low switching speed, because the higher the switching speed, the faster the voltage or current changes caused by it, and the easier it is to produce coupling interference between circuits; acceptable in the circuit Under the premise, the threshold voltage should be increased as much as possible, and the threshold voltage should be increased by setting a voltage divider or a voltage regulator tube in front of the circuit; the load impedance matching method is adopted to eliminate the digital signal transmission even if the load impedance is equal to the wave impedance of the signal line. Distortion due to refraction and reflection in the process. Under normal circumstances, the protection of sensitive objects needs to be used in combination with the shielding of interference sources and the suppression of coupling channels, and repeated experiments are required in practice according to the actual situation to achieve the best protection effect.

Summarize

The electromagnetic compatibility analysis and design of high-speed circuit boards is a very systematic work and requires a lot of work experience accumulation. Electromagnetic compatibility design is one of the keys to whether electronic systems can achieve functions and meet design indicators. As the complexity of electronic systems increases and operating frequencies increase, the position of electromagnetic compatibility design in electronic design will become more and more prominent. The more important.