0 Preface
Electromagnetic compatibility means that electrical and electronic systems and equipment in a specific electromagnetic environment, when operating at a set level within specified safety limits, will not cause damage due to external electromagnetic interference or cause performance degradation to irreparable The degree, at the same time, the electromagnetic radiation generated by themselves is not greater than the limit level of the verification, and does not affect the normal operation of other electronic equipment or systems, so as to achieve the purpose of non-interference between the equipment and the equipment, the system and the system, and work together reliably.
1 Factors arising from electromagnetic compatibility
(1) Frequency characteristics of the resistor. In digital circuits, the main function of the resistor is to limit current and determine a fixed level. In high-frequency circuits, high-frequency parasitic capacitances present across the resistor can cause damage to normal circuit characteristics. The pin inductance of the same resistor has a large effect on the EMC of the circuit.
(2) The frequency characteristics of the capacitor. Capacitors are typically used on power buses that provide decoupling, bypassing, and maintaining 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 cause the capacitor to exhibit an inductive characteristic, thereby losing its original function and affecting the performance of the circuit.
(3) Frequency characteristics of the inductor. The inductor is used to control EMI within the PCB. When the operating frequency of the circuit increases, the equivalent impedance of the inductor increases with increasing 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 the wire. The traces on the PCB and the lead wires of the components have parasitic inductance and capacitance. These parasitic inductances and capacitances 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 a resistance characteristic in a low frequency band and an inductance characteristic in a high frequency band. Therefore, on a PCB, the length of the wire generally needs 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 of electronic products, which may cause permanent damage to the product. Therefore, in the product design, corresponding electrostatic protection measures must be taken. Common anti-static measures include the choice of anti-static materials, electrical isolation measures, improved product insulation strength and a good set of electrostatic shielding and venting channels.
(6) Power supply. With the wide application of high-frequency switching power supplies and the increasing load of power systems, the interference problem 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 not directly used AC power supply instead of 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, and the resulting strong electromagnetic pulse causes a major cause of damage to various electronic devices. The impact of lightning on electronic equipment includes direct lightning and inductive lightning. Nowadays, various indoor electronic devices are generally not susceptible to direct lightning, but they are still vulnerable to inductive lightning. In order to ensure the safe operation of electronic equipment, lightning protection must be applied to electronic equipment. Common lightning protection measures include setting lightning rods, installing lightning arresters and lightning protection lines.
2 Elements of electromagnetic compatibility
Theoretical and practical research proves that regardless of complex systems or simple devices, any electromagnetic interference must have three basic conditions: there are certain interference sources, full coupling channels with interference, and responses of disturbed objects.
2.1 Electromagnetic interference source
Electromagnetic interference sources are any component, device, device, system, or natural phenomenon that produces electromagnetic interference. High-frequency circuits are particularly sensitive to electromagnetic interference, and many measures are needed to suppress electromagnetic interference. After theoretical and experimental analysis, it is known that in high frequency circuits, electromagnetic interference mainly comes from the following aspects:
(1) Noise interference of device operation
(a) Electromagnetic interference occurs when digital circuits operate. Â Â
(b) Electromagnetic field interference caused by signal voltage and current changes.
(2) High frequency signal noise interference
(a) Crosstalk: When a signal is transmitted on a transmission channel, it has an undesired 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 can cause false triggering and timing delays in the circuit, causing the system to malfunction.
(b) Return loss: When high-frequency signals are transmitted in cables and communication equipment, when the wave impedance is uneven, the signal will be reflected. This reflection will not only increase the transmission loss of the signal, but also Distorting the transmitted signal has a great influence 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 by the disturbance. The main performances are: 1 distributed noise caused by the inherent impedance of the power supply; 2 common mode field interference; 3 differential mode field interference; Interference; 5 power line coupling.
(4) Ground noise interference
Since there is resistance and impedance on the ground, when the current passes through the ground, a voltage drop occurs. When the current is large enough or the operating frequency is high enough, the voltage drop is large enough to cause interference to the circuit. The noise interference caused by the ground line mainly includes the ground loop interference and the common impedance coupling interference.
(a) Ground loop interference: When multiple functional units are connected to the ground, if the current in the ground is large enough, a voltage drop will occur across the connecting cables between the devices. Due to the unbalanced electrical characteristics between the various circuits, the current on each of the wires will be different, resulting in a differential mode voltage that affects the circuit. In addition, external electromagnetic fields may also induce current in the ground loop, causing interference.
(b) Common impedance coupling interference; when a plurality of functional units share the same ground line, due to the presence of the ground line impedance, mutual modulation occurs between the ground potentials of the respective units, thereby causing interference between the respective unit signals to cause interference. In the high-frequency circuit, the circuit is in a high-frequency working state, and the ground line impedance tends to be large, and the common impedance coupling interference at this time is particularly obvious.
There are two ways to eliminate the common impedance coupling: one is to reduce the impedance of the common ground portion, so that the voltage on the common ground line is also reduced, thereby controlling the common impedance coupling. Another method is to avoid the common grounding of the circuit which is easy to interfere with each other by an appropriate grounding method. Generally, the grounding circuit of the strong electric circuit and the weak electric circuit is avoided, and the digital circuit and the analog circuit share the ground line. As mentioned earlier, the core problem of reducing the ground impedance is to reduce the inductance of the ground. This includes the use of flat conductors for grounding and the use of multiple parallel conductors that are far apart. For printed circuit boards, the ground wire grid on the double-layer board can effectively reduce the ground line impedance. In the multi-layer board, the ground wire is specially used for one layer, although it has a small impedance, but this will increase the cost of the circuit board. . The method of grounding to avoid common impedance by proper grounding 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 to connect all circuits in parallel at a single point. For circuits with less mutual interference, single-point grounding in series can be used. For example, the circuit can be classified according to strong signals, weak signals, analog signals, digital signals, etc., and then grounded in series with a single point in the same type of circuit, and different types of circuits are connected in parallel with a single point.
2.2 Suppressing the coupling channel
The main coupling channels for electromagnetic interference in high-speed circuits include radiative coupling, conductive coupling, capacitive coupling, inductive coupling, power coupling, and ground-line coupling.
For radiative coupling, the main suppression method is to use electromagnetic shielding to effectively isolate the interference source from sensitive objects.
For conductive coupling, the main method is to arrange the direction of the high-speed signal line reasonably when the signal is routed. 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 lines to isolate them. For external signal lines, the input leads should be shortened as much as possible to improve the input impedance. It is best to shield the analog signal input line. When the impedance of the signal conductor on the board is not matched, it will cause signal reflection. When the printed conductor is long, the line inductance will cause amplitude oscillation. By stringing in the damping resistor (resistance is usually 22 ~ 2 200 hm, typically 470 hm), it can effectively suppress the oscillation, enhance the anti-interference ability and improve the waveform.
For the coupling interference of inductors and capacitors, the following two aspects can be used for suppression: on the one hand, the selection of suitable components, and for inductors and capacitors, it should be selected according to the frequency characteristics of different components. For other components, Select a device with a small parasitic inductance and a small capacitance. On the other hand, it is reasonable to carry out layout and wiring. It is necessary to avoid long-distance parallel wiring as much as possible, and the wiring between electrical interconnection points in the circuit is the shortest. The corners of the signal (especially the high-frequency signal) should be designed to be 45-degree or circular or circular, and should not be drawn to an angle of less than or equal to 90 degrees. Adjacent wiring surface conductors are in the form of mutually 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 it is possible to consider multiple vias in parallel. Or micro vias to reduce the equivalent inductance. When selecting component packages, standard packages should be chosen to reduce lead impedance and parasitic inductance due to package mismatch.
For power coupling and ground coupling, first of all, attention should be paid to reducing the power line and ground impedance, and necessary measures must be taken for waveform distortion and oscillation caused by common impedance, crosstalk and reflection. Bypass capacitors are respectively connected between the power supply and the ground of each integrated circuit to shorten the flow path of the switching current. The power line and the ground line are designed in a lattice shape instead of the comb shape, because the grid shape can significantly shorten the line loop, reduce the line impedance, and reduce interference. When a plurality of integrated circuits are mounted on the printed circuit board, and some components consume a large amount of power, and a large potential difference occurs on the ground line to form a common impedance interference, the ground line should be designed as a closed loop, and the loop 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 entry ground of the board with the shortest distance to reduce the spikes generated by the printed leads. Let the ground and power lines go in the same direction as the data transmission to improve the noise margin of the board. Try to use a multi-layer printed circuit board to reduce the ground potential difference and reduce the power line impedance and crosstalk between signal lines. When there is no multi-layer board and you have to use the double-panel, you must widen the ground line as much as possible. Usually, the ground line should be thickened to the amount of current that can flow through 3 times the actual flow of the wire, or a small busbar method. The common power and ground wires are distributed as far as possible on the edges of both sides of the printed board. Connect a 1μF to 10μF tantalum capacitor to the power busbar 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 is mainly concentrated in two aspects, one is to cut off the channel between sensitive objects and electromagnetic interference. Another aspect is to reduce the sensitivity of sensitive objects.
The sensitivity of electronic equipment is a double-edged sword. On the one hand, users want high sensitivity of electronic devices to improve the ability to accept signals. On the other hand, high sensitivity also means that the possibility of being affected by noise is greater. Therefore, the sensitivity of the electronic device should be determined according to the specific situation.
For analog electronic devices, the usual method 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. Through these methods, the sensitivity of the electronic device to electromagnetic interference can be effectively reduced, and the anti-interference ability of the device can be improved.
For digital electronic equipment, digital circuits with high DC noise tolerance should be used with the permission of working indicators. For example, the DC noise margin of CMOS digital circuits is much higher than the DC noise tolerance of TTL digital circuits. When the indicator is allowed, try to use a digital circuit with low switching speed. Because the higher the switching speed, the faster the voltage or current caused by it changes, so the more easily the coupling interference between circuits is generated. Under the premise, increase the threshold voltage as much as possible, and use the method of setting the voltage divider or voltage regulator before the circuit to increase the threshold voltage; using load impedance matching, even if the load impedance is equal to the wave impedance of the signal line, eliminate the digital signal transmission. Distortion due to the effects of refraction and reflection during the process. In general, the protection of sensitive objects needs to be combined with the shielding of interference sources and the suppression of coupling channels, and it is necessary to carry out repeated experiments according to actual conditions in practice to achieve the best protection effect.
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. Electromagnetic compatibility design is one of the keys to whether an electronic system can achieve functions and meet design specifications. As the complexity of electronic systems increases and the operating frequency increases, the position of electromagnetic compatibility design in electronic design will become more and more prominent. The more important.
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