EMC analysis and design of high-speed circuits

Electromagnetic compatibility refers to the ability of electrical and electronic systems and equipment to operate within specified safety limits and at a set level in a specific electromagnetic environment without causing damage or performance degradation to an irreparable extent due to external electromagnetic interference, while the electromagnetic radiation generated by them is not greater than the verified limit level, and does not affect the normal operation of other electronic equipment or systems, in order to achieve equipment and equipment The purpose of mutual non-interference and common and reliable operation between systems.

Factors causing electromagnetic compatibility

(1) The frequency characteristic of the resistor. In digital circuits, the main role of resistors is to limit current and determine fixed levels. In high-frequency circuits, high-frequency parasitic capacitors that exist at both ends of the resistors can damage normal circuit characteristics. The pin inductance of the same resistor has a significant impact on the EMC of the circuit.

(2) Frequency characteristics of capacitors. Capacitors are commonly used in power buses to provide decoupling, bypass, and maintain fixed DC voltages and currents. 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 working performance of the circuit.

(3) Frequency characteristics of the inductor. Inductors are used to control EMI within a PCB. When the operating frequency of a circuit increases, the equivalent impedance of the inductor increases 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) The frequency characteristics of the wire. The wiring on a PCB and the pin wires of components have parasitic inductances and capacitances that can affect the frequency characteristics of the wires, potentially creating resonance between components and wires, making the wires an important transmitter of electromagnetic interference. Generally, wires exhibit resistance characteristics in the low frequency band and inductance characteristics in the high frequency band. Therefore, on a PCB, the length of the wire is generally required to be less than one twentieth of the wavelength of the operating frequency to avoid the wire becoming a source of electromagnetic interference.

(5) Static electricity. Electrostatic discharge (ESD) has become a major public hazard in electronic products, which may cause permanent damage to products. Therefore, corresponding electrostatic protection measures must be taken in product design. Common antistatic measures include selecting antistatic 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 wide application of high-frequency switching power supplies and the continuous increase in power system load, the interference of power supplies to products has gradually become an important factor affecting the EMC characteristics of products. Therefore, some sensitive devices that are susceptible to interference have switched from AC power supply to DC power supply, which increases the complexity and cost of the system, but effectively improves the stability of the system.

(7) Lightning. Lightning is essentially a strong electrostatic discharge process in which positive and negative charges are neutralized, resulting in strong electromagnetic pulses that are the main cause of damage to various electronic devices. The impact of lightning on electronic equipment includes direct lightning and inductive lightning. Currently, various indoor electronic equipment are generally not susceptible to direct lightning, but they are still vulnerable to inductive lightning damage. In order to ensure the safe operation of electronic equipment, lightning protection must be provided for electronic equipment. Common lightning protection measures include setting lightning rods, installing lightning arresters, and lightning conductors.

2.Elements of Electromagnetic Compatibility

Theoretical and practical research has proven that no matter whether 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 Electromagnetic interference sources

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

(1) Noise interference caused by device operation

(a) Electromagnetic interference occurs when digital circuits operate.

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

(2) High frequency signal noise interference

(a) Crosstalk: When a signal is transmitted through a transmission channel, it has unexpected effects on adjacent transmission lines due to electromagnetic coupling, and the interfered signal appears to be injected with a certain coupling voltage and current. Excessive crosstalk may cause false triggering and timing delays in the circuit, resulting in the system not working properly.

(b) Return loss: When high-frequency signals are transmitted in cables and communication equipment, they will reflect on the signal when they encounter uneven points in wave impedance. This reflection not only leads to increased transmission loss of the signal, but also causes distortion of the transmission signal, which has a significant impact on transmission performance.

(3) Power supply noise interference

Power supply noise in PCBs mainly consists of noise generated by the power supply itself or induced by interference, 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 wire noise interference

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

(a) Ground wire 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 occur on the connecting cables between devices. Due to the imbalance in electrical characteristics between various circuits, the current on each wire will be different, resulting in a differential mode voltage, which affects the circuit. In addition, external electromagnetic fields may also induce current in the ground loop, resulting in interference.

(b) Common impedance coupling interference; When multiple functional units share the same section of ground wire, due to the existence of ground wire impedance, mutual modulation occurs between the ground potentials of each unit, resulting in mutual coupling between the signals of each unit and interference. In high-frequency circuits, when the circuit is operating at high frequencies, the ground wire impedance is often large, and the common impedance coupling interference is particularly significant.

There are two ways to eliminate common impedance coupling: one is to reduce the impedance of the common ground wire, so that the voltage on the common ground wire also decreases, thereby controlling common impedance coupling. Another method is to avoid sharing ground wires with circuits that are prone to mutual interference through appropriate grounding methods. Generally, it is necessary to avoid sharing ground wires with strong and weak current circuits, and sharing ground wires with digital and analog circuits. As mentioned earlier, the core issue of reducing ground wire impedance is reducing the inductance of the ground wire. This includes using flat conductors as ground wires, and using multiple parallel conductors that are farther apart as ground wires. For printed circuit boards, arranging a ground wire grid on a double layer board can effectively reduce the ground wire impedance. Although using a single layer of ground wire in a multilayer board has a small impedance, it will increase the cost of the circuit board. The grounding method for avoiding common impedance through appropriate grounding methods is parallel single point grounding. The disadvantage of parallel grounding is that there are too many grounded conductors. Therefore, in practice, it is not necessary for all circuits to be connected to a single point ground in parallel. For circuits with less mutual interference, a series single point ground can be used. For example, circuits can be classified into strong signals, weak signals, analog signals, and digital signals, and then connected to a single point in series within the same type of circuit. Different types of circuits can be connected to a single point in parallel.

2.2 Suppress coupling channels

The main coupling channels of electromagnetic interference in high-speed circuits include radiation coupling, conduction coupling, capacitance coupling, inductance coupling, power source coupling, and ground wire coupling.

For radiation coupling, the main suppression method is to adopt electromagnetic shielding to effectively isolate interference sources from sensitive objects.

For conductive coupling, the main method is to reasonably arrange the direction of high-speed signal lines during signal wiring. The wires used at the input and output terminals should be kept as parallel as possible to avoid signal feedback or crosstalk. A ground wire can be added between two parallel wires to isolate them. For external signal lines, the input lead should be shortened as much as possible to improve the input impedance. It is best to shield the analog signal input lines. When the impedance of the signal wires on the board does not match, it will cause signal reflection. When the printed wires are long, the line inductance will cause reduced amplitude oscillation. By serially inserting a damping resistor (typically 22 to 2200 hm, with a typical value of 470 hm), oscillation can be effectively suppressed, anti-interference ability enhanced, and waveform improved.

For the coupling interference of inductance and capacitance, the following two aspects can be used to suppress it: on the one hand, select the appropriate components. For inductance and capacitance, the selection should be based on the frequency characteristics of different components. For other components, select devices with smaller parasitic inductance and capacitance. On the other hand, layout and wiring should be carried out reasonably, avoiding long-distance parallel wiring as much as possible, and striving to minimize the wiring between electrical interconnection points in a circuit. The corners of signal (especially high-frequency signal) lines should be designed to run at 45 degrees or be circular or circular in shape. It is strictly prohibited to draw them at an angle less than or equal to 90 degrees. The wires on adjacent wiring surfaces take the form of perpendicular, oblique, or curved routing to reduce parasitic capacitance and inductance of vias. The shorter the lead between the vias and pins, the better. It is also possible to consider drilling multiple vias or micro vias in parallel to reduce equivalent inductance. When selecting component packaging, standard packaging should be selected to reduce lead impedance and parasitic inductance caused by package mismatch.

For power and ground coupling, first of all, attention should be paid to reducing the impedance of the power and ground wires. Necessary measures should be taken to prevent waveform distortion and oscillation caused by common impedance, crosstalk, and reflection. Bypass capacitors are connected between the power supply and ground wires of each integrated circuit to shorten the flow path of switching current. Design the power and ground wires in a grid shape instead of a comb shape because the grid shape can significantly shorten the circuit loop, reduce line impedance, and reduce interference. When multiple integrated circuits are installed on a printed circuit board, and some components have a high power consumption, resulting in a large potential difference in the ground wire, resulting in common impedance interference, it is advisable to design the ground wire as a closed loop, which has no potential difference and higher noise tolerance. The lead wire should be shortened as much as possible, and the ground of each integrated circuit should be connected to the entrance ground wire of the circuit board at the shortest distance to reduce the spike pulse generated by the printed wire. Align the direction of the ground and power lines with the direction of data transmission to improve the noise tolerance of the circuit board. Try to use multilayer printed circuit boards to reduce ground potential differences, power line impedance, and signal line crosstalk. When there is no multilayer board and it is necessary to use a double-sided board, the ground wire line must be widened as much as possible. Generally, the ground wire should be thickened to pass 3 times the actual amount of current flowing through the wire, or a small bus should be used to distribute the common power line and ground wire as far as possible on the edges of both sides of the printed board. Connect 1 at the power bus plug μ F～10 μ F's tantalum capacitor is decoupled and a 0.01 is connected in parallel to the decoupling capacitor μ F～0．1 μ F's high-frequency ceramic capacitor.

2.3 Protecting Sensitive Objects

The protection of sensitive objects mainly focuses on two aspects, one is to cut off the channel between sensitive objects and electromagnetic interference. The other aspect is to reduce the sensitivity of sensitive objects.

The sensitivity of electronic devices is a double-edged sword. On the one hand, users hope that the sensitivity of electronic devices is high to improve their ability to accept signals; On the other hand, high sensitivity also means that the likelihood of being affected by noise is greater. Therefore, the sensitivity of electronic devices should be determined based on specific circumstances.

For analog electronic devices, the usual approach is to use preferred circuits, such as designing low noise circuits, reducing bandwidth, suppressing interference transmission, balancing inputs, suppressing interference, and selecting high-quality power supplies. These methods can effectively reduce the sensitivity of electronic equipment to electromagnetic interference and improve the anti-interference ability of the equipment.

For digital electronic devices, digital circuits with high DC noise tolerance should be used when the operating indicators permit. For example, the DC noise tolerance of CMOS digital circuits is much higher than that of TTL digital circuits; If the operating indicators permit, try to use digital circuits with low switching speeds, as the higher the switching speed, the faster the voltage or current changes caused by it, and the more likely it is to generate coupling interference between circuits; Increase the threshold voltage as much as possible on the premise that the circuit is acceptable, and use the method of setting a voltage divider or regulator in front of the circuit to increase the threshold voltage; Using load impedance matching measures, even if the load impedance is equal to the wave impedance of the signal line, eliminates the distortion caused by refraction and reflection during the transmission of digital signals. Generally, the protection of sensitive objects needs to be combined with the shielding of interference sources and the suppression of coupling channels, and repeated experiments need to be conducted in practice based on the actual situation to achieve the best protection effect.

3 Summary

The electromagnetic compatibility analysis and design of high-speed circuit boards is a highly systematic task that requires a lot of work experience accumulation. Electromagnetic compatibility design is one of the key factors that affect whether electronic systems can achieve their functions and meet design requirements. With the increasing complexity and operating frequency of electronic systems, the position of electromagnetic compatibility design in electronic design will become increasingly prominent and important.