“In order to reduce development costs, reduce product development risks, and shorten the time to market of new products, embedded core boards have been widely used in the fields of industry, transportation, medical, instrumentation, energy, and the Internet of Things. What should I do when the whole product with embedded core board has electromagnetic compatibility problems? This article teaches you how to analyze positioning and solve problems effectively.

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Abstract: In order to reduce development costs, reduce product development risks, and shorten the time to market of new products, embedded core boards have been widely used in the fields of industry, transportation, medical care, instruments, energy, and the Internet of Things. What should I do when the whole product with embedded core board has electromagnetic compatibility problems? This article teaches you how to analyze positioning and solve problems effectively.

1. Electromagnetic compatibility

According to the definition of GJB 72A-2002 “Electromagnetic Interference and Electromagnetic Compatibility Terminology”, Electromagnetic Compatibility (Electro Magnetic Compatibility, EMC) refers to “the coexistence state of equipment, subsystems, and systems that can perform their respective functions together in a common electromagnetic environment. .Including the following two aspects:

1. When the equipment, subsystems and systems operate in a predetermined electromagnetic environment, the designed working performance can be achieved according to the specified safety margin without damage or unacceptable degradation due to electromagnetic interference;

2. The equipment, subsystems, and systems work normally in the predetermined electromagnetic environment and will not bring unacceptable electromagnetic interference to the environment (or other equipment).

To sum up, EMC usually includes the following two requirements:

1. Electromagnetic Interference (EMI) refers to any conducted or radiated electromagnetic energy that may interrupt, hinder, or even reduce or limit the performance of radio communications or other electrical and Electronic equipment. According to the transmission path of electromagnetic energy, it can be divided into radiated interference and conducted interference. Common EMI test items are as follows:

Table 1 Common EMI test items

EMI test project	Corresponding test standard
Conducted disturbance (CE) of power lines and signal lines	CISPR 16, GB/T 6113
Radiated Disturbance (RE)
Harmonic current (Harmonic)	IEC61000-3-2, GB 17625.1
Fluctuation and Flicker	IEC61000-3-3, GB 17625.2

2. Electromagnetic susceptibility (Electro Magnetic Susceptibility, EMS) refers to the characteristics of equipment, devices or systems that may degrade working performance due to electromagnetic interference. Common EMS test items are as follows:

Table 2 Common EMS test items

EMS test items	Corresponding test standard
Electrostatic Discharge Immunity (ESD)	IEC61000-4-2, GB/T17626.2
Radio frequency electromagnetic field radiation immunity test (RS)	IEC61000-4-3, GB/T17626.3
Electrical Fast Transient Burst Immunity (EFT/B)	IEC61000-4-4, GB/T17626.4
Lightning (Surge) Immunity (SURGE)	IEC61000-4-5, GB/T17626.5
Conducted Immunity (CS) for RF Induced Fields	IEC61000-4-6, GB/T17626.6
Power frequency magnetic field immunity (PFM)	IEC61000-4-8, GB/T17626.8
Dips and Interrupts (DIP)	IEC61000-4-11, GB/T17626.11

2. Common Electromagnetic Compatibility Problems

For products with embedded core boards, common electromagnetic compatibility problems are as follows:

Table 3 Common EMC problems of embedded core board products

Electromagnetic Compatibility Test Project	Common EMC Problems
Conducted disturbance (CE) of power lines and signal lines	Conducted disturbance of power line or signal line exceeds the standard
Radiated Disturbance (RE)	The radiation disturbance of the whole machine exceeds the standard
Electrostatic Discharge Immunity (ESD)	The system crashes, resets, the liquid crystal Display flickers or the screen is blurred, the communication data is wrong or interrupted, the measurement data jumps or is wrong, and the buttons and LED indicators malfunction.
Electrical Fast Transient Burst Immunity (EFT/B)
Conducted Immunity (CS) for RF Induced Fields
Lightning (Surge) Immunity (SURGE)	Damaged power interface Short circuit, communication data error, interruption or communication interface damage

3. Solving the problem of electromagnetic compatibility

For the electromagnetic compatibility problem, in terms of the mechanism of occurrence, it is inseparable from the three aspects of the interference source, the propagation path and the sensitive source. Whether it is the EMI electromagnetic interference problem or the EMS electromagnetic immunity problem, we should focus on these three aspects for analysis, and we will learn about it. , and then supplemented by targeted and effective measures to cure the disease. The diagnosis and rectification of electromagnetic compatibility problems can be divided into three steps: problem phenomenon and description, analysis and root cause location, rectification and effect verification.

The following is a case where the EMI radiation of a certain type of embedded core board product exceeds the standard, and the above three steps are explained. The diagnosis and rectification of the EMS electromagnetic immunity problem can also be bypassed by analogy.

1. Problem phenomenon and description (step 1)

For the problem of EMI electromagnetic interference, focus on 4 aspects to describe:

l The test standard based on;

l Excessive frequency points or frequency bands;

l How much exceeds the standard limit;

l The working state and configuration of the product during the test.

For the EMS electromagnetic immunity problem, grasp the following four aspects to describe:

l The test standard based on;

l Test level when the product is abnormal;

l Abnormal phenomenon of the product and its corresponding interference position;

l The working state and configuration of the product during the test.

Whether it is an EMI electromagnetic interference or an EMS electromagnetic immunity problem, special emphasis is placed here on the exact description of the working state of the product and the configuration of the field test. All these descriptions must be objective and true. The clearer and more detailed the descriptions are, the more helpful it will be for subsequent problem analysis and root cause positioning. The working status of the product, such as whether the power output is fully loaded, what communication is in the specific product, the internal block diagram of the system, etc.; on-site test configuration, such as whether to use DC or AC power supply, whether to connect an external adapter, whether to connect external communication cables, cables length, grounding conditions, system block diagram during product testing, etc.

2. Analysis and root cause localization (step 2)

For EMC problem analysis and root cause positioning, it is similar to investigating a case. The following are some methods for problem analysis and root cause location of EMI and EMS respectively.

For the positioning of the EMI electromagnetic interference problem, the general idea is: break the whole into zero and approach it gradually. There are usually two methods:

1) Exclusion method: If the product is a complex system, it can be divided into parts and approached gradually, such as closing or removing all parts of the product one by one (such as unplugging the LCD screen and the cable, removing the external Unplug the communication cable, use the original switching power adapter to use linear regulated power supply or battery power supply, shield the output of some signal sources with software, etc.), only keep the minimum system work, first check whether the minimum system can meet the EMI standard limit requirements. If there is no problem with the EMI of the smallest system, we will open or restore the removed parts one by one, and compare the difference between the test results before and after, in order to locate the main source or path of EMI.

The analysis and location of EMI problems of complex products or systems are usually suitable for anechoic chambers (either built by enterprises or rented by other enterprises or third-party organizations).

2) Near-field scanning method: If the enterprise does not have a self-built anechoic chamber, and it is not convenient to rent an anechoic chamber, the near-field scanning method can be used, which requires a spectrum analyzer (or an oscilloscope with spectrum analysis function) and a near-field probe, etc. Appendix.

By performing near-field scanning of the radiated electromagnetic field on the whole or part of the product, it is possible to preliminarily determine areas or locations with high risks, and even directly locate the main source of EMI interference. After implementing targeted rectification, perform near-field scanning again, and compare the results before and after rectification. If there is obvious improvement after rectification (for example, the amplitude of over-standard frequency points has dropped a lot or almost eliminated), we can go to the anechoic chamber. Do further measurement verification.

Here are a few things to make clear about near-field scanning:

——Near-field scanning is a qualitative measurement. By monitoring the frequency of product radiation and the strength of the near-field electromagnetic field, the radiation disturbance level of the product can be preliminarily judged.

——The measurement area of ​​product radiation disturbance by near-field scanning is much smaller than that of anechoic chambers. To make a metaphor, near-field scanning is “peeping the leopard in the tube”, while the anechoic chamber is “looking at the whole situation and seeing the details clearly”.

——In short, the results of the near-field scanning are for reference only, and the measurement results of the anechoic chamber are the objective facts, which cannot be compared, let alone confuse the two.

As mentioned above, the exclusion method is suitable for the location of radiated disturbance and conducted disturbance, while the near-field scanning method is usually only applicable to the location of radiated disturbance. In the analysis and location of actual EMI problems, you can also use the two methods in combination.

Common EMI interference sources for embedded core board products are shown in the following 4.

Table 4 Common EMI sources of embedded core board products

EMI interference source	abnormal performance
DC-DC power supply circuit	The differential mode interference exceeds the standard at its switching frequency, or the common mode interference in the high frequency band exceeds the standard
LCD driver circuit	The fundamental frequency and frequency multiplication radiation disturbance of liquid crystal clock and data exceeds the standard
Ethernet communication circuit	The fundamental frequency and frequency multiplication radiation disturbance of Ethernet communication clock and other signals exceeds the standard
Active crystal oscillator circuit	The fundamental frequency and frequency multiplication of the clock signal frequency have radiation disturbance exceeding the standard
High-speed digital signal circuit	The fundamental frequency and frequency multiplication of high-speed communication data frequency have excessive radiation disturbance

For the positioning of the EMS electromagnetic immunity problem, it is mainly to find the components or circuits that are more sensitive to electromagnetic interference in the product or system.

When locating the sensitive source of electromagnetic interference, we can also use the elimination method to divide the whole into parts, from the whole machine to the components, from the outside to the inside, and then from the components to the circuit, gradually approach, and trace the source until the final sensitive source is found. Location.

Common EMS sensitive sources in embedded core board products are shown in Table 5 below.

Table 5 Common EMS sensitive sources of embedded core board products

EMS sensitive source	abnormal performance
reset circuit	System or functional circuit reset phenomenon
Clock circuit	The system crashes or the functional circuit stops working
liquid crystal Display circuit	The LCD display is flickering, white or blurry
AD sampling circuit	Error in measurement sampling data or interruption of sampling
interface communication circuit	Communication data has errors, dropped frames, or is interrupted
input signal circuit	Malfunction of system or functional circuit

In addition to the above-mentioned “interference sources” and “sensitive sources”, we also need to pay special attention to the influence of “propagation paths” when analyzing EMC problems. The “propagation path” of electromagnetic interference is usually divided into two types: radiation and conduction, and “cable” (including interconnection interface cables, power cables, and power supply and signal traces on PCBs, etc.) is the physical transmission path of electromagnetic interference. According to relevant statistics, nearly 90% of EMC problems are caused by cables. Cables are efficient electromagnetic wave receiving and transmitting antennas, and are also a good channel for electromagnetic interference conduction.

3. Correction and effect verification (step 3)

The rectification of product EMC problems usually includes three methods: “shielding”, “filtering”, and “grounding”. I believe that most users have already had a preliminary understanding, and they can also consult relevant books or online literature.

Here we mainly introduce some points that are easily overlooked in the process of product EMC rectification.

“Shielding” is the main method for rectifying radiated electromagnetic interference, generally shielding the cables, structures and holes of the product. It should be noted that shielding measures must be used in conjunction with grounding measures, otherwise the effect will be halved or useless; in addition, 360-degree closed lap joints should be achieved when implementing shielding measures, leaving no holes or equivalent rods. antenna.

“Filtering” is the main method for rectification of conducted electromagnetic interference, which usually requires additional filter components, such as common mode inductors, differential mode inductors, capacitors, magnetic beads and resistors, on the product and its circuit. Generally, there are two types of filtering methods: dredging type and consumption type. The dredging type “filtering” measures (such as capacitor filtering) need to be used in conjunction with “grounding”. It should be noted that the filtering measures should be placed as close as possible to the interference source or sensitive source, so as not to greatly reduce the effect of the “filtering” measures.

“Grounding” is an effective path for dredging electromagnetic interference in the two major measures of “shielding” and “filtering”. There are many grounding methods, such as single-point grounding, multi-point grounding, capacitor grounding, resistance-capacitance grounding, inductance grounding, magnetic bead grounding, resistance grounding, etc. In the actual product or system, which grounding method is suitable for use depends on the Which type of ground (such as earth, safety ground, digital ground, analog ground, power ground, etc.) belongs to, one grounding method, one ground type, two problems must be figured out in order to avoid the situation of connecting the wrong “ground”, summary It means “the grounding method is determined by the grounding purpose”.

The above-mentioned “shielding” and “filtering” are rectified from the hardware aspect. From a broader perspective, “shielding” and “filtering” can also be implemented by software methods. Here are some rectification methods of “shielding” and “filtering” on the software for your reference. In actual use, you can draw inferences from one case.

Table 6 Software EMC rectification methods

Software Corrective Actions	method and purpose
Set unused I/O ports as output ports	Set the I/O direction register and shield the unused I/O ports to avoid coupling to the EMS interference input, which will cause the system to malfunction
Software filtering for key input signals or high and low level signals	Perform anti-shake processing on the input signal (ie, secondary level judgment), eliminate transient interference signals, and prevent the system from malfunctioning
Perform software filtering processing on AD sampling data	Digital filtering (such as average value filtering, median filtering, limiting filtering, etc.) on AD sampled data to reduce the influence of interference on measurement results
Perform software verification processing on communication data	Parity check, CRC check and other methods are used for communication data to eliminate the adverse effect of interference on communication data and avoid communication errors

Fourth, the application of the core board matters needing attention

The above are the rectification methods and general ideas after the product has EMC problems. So how to avoid EMC problems when using the core board for product development? Next, let’s take a look at the caveats.

1. EMC design of power circuit

The power interface is one of the interfaces that must be done in the product EMC test, including EMI electromagnetic interference and EMS electromagnetic disturbance. When designing the power interface of products with embedded core boards, pay attention to the following aspects:

1) Add TVS diode protection to the power supply circuit of the product backplane near the power interface to suppress surge interference. It is necessary to select a TVS diode of the appropriate size in combination with the supply voltage.

2) If DC/DC (usually placed on the backplane) is used as the DC power supply for the core board, it is recommended to add an EMI filter circuit to the DC/DC input to reduce the probability of conducting and radiating disturbances exceeding the standard.

3) If the core board has multiple power supply pins, add filter capacitors near the core board for these power supply signals on the bottom board. SMD capacitors with different capacitance values ​​such as 10uF, 0.1uF, 1nF, etc. are used in parallel for broadband filtering.

2. EMC design of interface circuit

The interface circuit of the core board is extended to the backplane through the board-to-board connector (or stamp hole), and some even become the input and output interfaces of the whole product, such as buttons, LED indicators, SPI, I²C. Serial port, LCD, Ethernet, etc. If these interface circuits are not designed for EMC or the EMC design is unreasonable, on the one hand, the external electromagnetic interference is transmitted to the core board through the interface and interface circuit, which will cause misoperation at light, and damage the core board at worst. On the other hand, the electromagnetic interference of the core board and the bottom plate will emit electromagnetic interference in the form of conduction or radiation, resulting in the EMI test of the whole product exceeding the standard.

When designing the interface circuit of products with embedded core boards, the following precautions should be taken:

1) If the input signal of the product is a high and low level signal (such as button, reset, switch, etc.), and the signal traces on the backplane are long, RC filtering should be added on the backplane near the I/O input of the core board;

2) If the input signal of the product is a high-speed digital signal (such as clock, SPI, I²C, AD sampling signals, etc.), should be considered more from the PCB layout and wiring, to ensure that the high-speed signal traces on the backplane are as short as possible, with as few vias as possible, and ESD and other protection devices can be appropriately added to the signal traces, but It is not advisable to have too large parasitic capacitance or filter capacitance, so as not to affect the quality of high-speed signals;

3) For the high and low level output signals extended from the core board to the bottom board, more from the perspective of anti-electromagnetic interference, the signal traces on the bottom board should be as short as possible, and ESD and other protective devices can also be appropriately added;

4) For the output of high-speed digital signals (such as SPI, I2C, serial port, Ethernet and LCD interface) extended from the core board to the bottom board, it is more considered from the perspective of EMI, and matching resistance can be appropriately added to these high-speed digital signals. Or RC filter circuit, and the traces of high-speed signals on the backplane should be as short as possible and with fewer vias.

5) For the output interface with many data lines and clock lines (such as LCD interface), the resistance can be serially inserted in the signal line, and the resistance can be placed on the backplane close to the core board to reduce the EMI emission energy from the source. , At the same time, a multi-layer board design can also be used. The high-speed signal and clock traces are routed in the inner layer of the PCB as much as possible, and the LCD clock line is grounded to reduce the signal loop area and further reduce EMI interference.

3. Layout and assembly of the core board on the backplane

In addition to the optimized design of power supply and interface circuits, the unreasonable layout or assembly method of the core board on the backplane can easily cause EMC problems. The core board has the following precautions in the layout and assembly of the bottom plate:

1) The core board should be arranged in the central area of ​​the bottom board as much as possible, and avoid placing it on the edge of the bottom board or the peripheral interface near the bottom board, which can reduce the adverse effect of external electromagnetic interference on the core board;

2) If the core board has positioning holes (connected to GND) and board-to-board connectors, we recommend to reserve positioning holes corresponding to the core board (also connected to GND) on the bottom plate, and assemble the core board to the bottom board through the connector At the same time, install copper pillars in the positioning holes of the core board and the bottom plate, and lock them with screws, which can not only absorb shock, but also ensure that the entire core board has a good grounding and maintains a low grounding impedance.

3) When designing or installing the internal structure of the product, the interface cables cannot be placed directly above the core board, or pass through the gap between the core board and the bottom plate, or be close to the core board, so as to avoid the interference of the core board as much as possible. It is coupled to the interface cable, or external interference is coupled to the core board through the interface cable.

Due to space limitations, the above is only a brief introduction to the EMC problem analysis and solution of the whole product with embedded core board for users’ reference and reference. We can also provide users with technical services for EMC design or rectification to help users achieve the predetermined electromagnetic compatibility performance indicators as soon as possible.