Electromagnetic compatibility analysis and design of high-speed circuits

The primary task of designing a circuit board is to work properly, but now the number of layers of PCB boards is getting higher and higher, the thickness is getting thinner, the line width and line spacing are getting smaller and smaller, and electromagnetic interference affects the quality of the printed board. factors emerge in endlessly. Based on this, we mainly discuss the factors that cause electromagnetic compatibility, the factors that cause electromagnetic compatibility, and conduct a detailed analysis and discussion of the corresponding methods to solve the problem.

1 Preface

Electromagnetic compatibility means that when electrical and electronic systems and equipment operate at a set level within specified safety limits in a specific electromagnetic environment, they will not cause damage or cause irreparable performance deterioration due to external electromagnetic interference. At the same time, the electromagnetic radiation they generate is not greater than the verification limit level 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 systems and between systems and working together reliably.

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2 Factors causing electromagnetic compatibility

Theoretical and practical research proves that no matter whether it is a complex system or a simple device, the occurrence of any electromagnetic interference must meet three basic conditions: the existence of a certain interference source, the complete coupling channel of interference, and the response of the interfered object.

2.1 Sources of electromagnetic interference

The source of electromagnetic interference refers to any component, device, equipment, system or natural phenomenon that produces 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 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: When a signal is transmitted on a transmission channel, it has undesirable effects on adjacent transmission lines due to electromagnetic coupling. The interfered signal appears as a certain coupling voltage and coupling current injected into it. Excessive crosstalk may cause false triggering and timing delays of the circuit, causing the system to fail to work properly.

(b) Return loss: When high-frequency signals are transmitted in cables and communication equipment, when they encounter uneven wave impedance points, the signal will be reflected. This reflection not only increases the transmission loss of the signal, but also causes Distorting the transmission signal has a great impact on the transmission performance.

(3)Power supply noise interference

 Power supply noise in PCB is mainly composed of noise generated by the power supply itself or induced by disturbance. It mainly manifests 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;

⑤Power line coupling.

(4)Ground noise interference

Due to the resistance and impedance on the ground wire, when the current passes 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 the ground wire mainly includes ground wire 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 occur on the connecting cable between devices. Due to the unbalanced electrical characteristics between the various circuits, the current on each wire will be different, thus generating differential mode voltage, which will affect the circuit. In addition, external electromagnetic fields may induce currents 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, mutual modulation will occur 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 in a high-frequency working state, the ground impedance is often large, and the common impedance coupling interference is particularly obvious at this time.

There are two ways to eliminate common impedance coupling: one is to reduce the impedance of the common ground line, so that the voltage on the common ground line also decreases, thereby controlling the common impedance coupling. Another method is to use appropriate grounding methods to prevent circuits that are prone to mutual interference from sharing ground wires. Generally, it is necessary to avoid sharing ground wires for strong current circuits and weak current circuits, and for digital circuits and analog circuits to share ground wires. As mentioned before, the core problem of reducing the ground wire impedance is to reduce the inductance of the ground wire. This involves using a flat conductor as the ground wire and multiple, widely spaced, parallel conductors as the ground wire. For printed circuit boards, laying out a ground wire grid on a double-layer board can effectively reduce the ground wire impedance. Although using a dedicated layer for ground wires in a multi-layer board has a very small impedance, it will increase the cost of the circuit board. . The grounding method that avoids common impedance through proper grounding is parallel single-point grounding. The disadvantage of parallel grounding is that there are too many ground wires. Therefore, in practice, it is not necessary for all circuits to be grounded in parallel at a single point. For circuits with less mutual interference, single-point grounding in series can be used. For example, circuits can be classified according to strong signals, weak signals, analog signals, digital signals, etc., and then series single-point grounding is used within similar circuits, and parallel single-point grounding is used for different types of circuits.

2.2 Suppress coupling channels

The main coupling channels of electromagnetic interference in high-speed circuits include radiation coupling, conduction coupling, capacitive coupling, inductive coupling, power supply coupling, and ground 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 routing. The wires used at the input and output ends should be kept away from adjacent parallel lines as much as possible to avoid signal feedback or crosstalk. A ground wire can be added between the two parallel wires for isolation. For external signal lines, the input leads should be shortened as much as possible and the input impedance should be increased. 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 damped oscillation. By adding a damping resistor in series (the resistance value is usually 22 to 2 200 hm, with a typical value of 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 inductors and capacitors, the following two aspects can be used to suppress it: on the one hand, it is to select appropriate components. For inductors and capacitors, they should be selected according to the frequency characteristics of different components. For other components, they should be selected according to the frequency characteristics of different components. Choose devices with smaller parasitic inductance and capacitance. On the other hand, layout and wiring should be carried out rationally. Long-distance parallel wiring should be avoided as much as possible, and the wiring between electrical interconnection points in the circuit should be kept as short as possible. The corners of signal lines (especially high-frequency signals) should be designed in a 45-degree direction or in a circular or arc shape, and should not be drawn at an angle less than or equal to 90 degrees. Conductors on adjacent wiring surfaces should be routed perpendicularly, obliquely or in a curved manner to reduce the parasitic capacitance and inductance of vias. The shorter the leads between vias and pins, the better, and you can consider drilling multiple vias in parallel. or micro vias to reduce equivalent inductance. When selecting component packaging, standard packaging should be selected to reduce lead impedance and parasitic inductance caused by packaging mismatch.

For power coupling and ground coupling, attention should first be paid to reducing the impedance of the power line and ground line, and necessary measures must 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 the switching current. Design the power and ground wires into a grid shape instead of a comb shape because the grid shape can significantly shorten the line loop, reduce line impedance, and reduce interference. When there are multiple integrated circuits installed on the printed circuit board, and some components consume large amounts of power, and a large potential difference occurs in the ground wire, forming a common impedance interference, it is advisable to design the ground wire into a closed loop, which has no potential. Poor, with higher noise margin. 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 spike pulse generated by the printed wire. Make the direction of the ground wire and power wire consistent with the direction of data transmission to improve the noise margin of the circuit board. Try to use multi-layer printed circuit boards to reduce ground potential differences and reduce crosstalk between power line impedance and signal lines. When there is no multi-layer board and double-sided boards have to be used, the ground wire must be widened as much as possible. Usually the ground wire should be thickened enough to pass 3 times the actual current flowing through the wire, or a small bus bar should be used to Public power lines and ground wires should be distributed on the edges of both sides of the printed board as much as possible. Connect a 1μF to 10μF tantalum capacitor to the power bus plug for decoupling, and connect a 0.01μF to 0.1μF high-frequency ceramic capacitor in parallel with the decoupling capacitor.

2.3 Protect sensitive objects

The protection of sensitive objects mainly focuses on two aspects. On the one hand, it 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 equipment is a double-edged sword. On the one hand, users hope that electronic devices have high sensitivity to improve their ability to accept 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 on a case-by-case basis.

For analog electronic equipment, the usual method is to use optimized 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 if the working indicators permit. For example, the DC noise tolerance of CMOS digital circuits is much higher than that of TTL digital circuits; when working If the indicators permit, try to use digital circuits with low switching speeds, because the higher the switching speed, the faster the voltage or current changes caused by it, making it easier to cause coupling interference between circuits; within the acceptable range of the circuit Under the premise, increase the threshold voltage as much as possible, and use the method of setting a voltage divider or voltage regulator tube in front of the circuit to increase the threshold voltage; adopt load impedance matching measures, even if the load impedance is equal to the wave impedance of the signal line, eliminate the digital signal in the transmission Distortion caused by refraction and reflection during the process. Usually, 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 carried out in practice according to the actual situation to achieve the best protection effect.

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3 Summary

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 an electronic system can realize functions and meet design specifications. As the complexity of electronic systems increases and the operating frequency increases, electromagnetic compatibility design will play an increasingly prominent role in electronic design. The more important it is.

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