EMC Knowledge Classic Q&A

1. Why should we design products for electromagnetic compatibility?

Answer: EMC meets the functional requirements of the product, reduces the debugging time, and makes the product meet the requirements of EMC standards, so that the product will not produce electromagnetic interference to other equipment in the system.

2. How many aspects can EMC design of products be carried out?

Answer: Circuit design (including device selection), software design, circuit board design, shielding structure, signal line/power line filtering, and circuit grounding mode design.

3. In the field of electromagnetic compatibility, why is it always described in decibels (dB)?

Answer: Because the range of amplitude and frequency to be described is very wide, it is easier to use logarithmic coordinates on the graph, and dB is the unit when using logarithmic coordinates.

4. I don't know much about EMC, but now the data transmission rate in circuit design is getting faster and faster. I also encountered some PCB EMC problems when making PCB, but I think it is too shallow. I want to learn in this area, not follow the trend. I will learn what you learn. I really think that EMC is becoming more and more important in the future circuit design. As I said earlier, I don't know how to start. I want to ask what basic knowledge is needed and what basic courses should be learned to do well in EMC. How to learn is a better way. I know that it is not easy to learn any subject well, and I have never thought of getting it through in a short time. I just want to give some suggestions and try to avoid detours.

A: About EMC, we need to first understand the EMC standards, such as EN55022 (GB9254), EN55024, and the simple test principle. In addition, we need to understand the use of EMI components, such as capacitors, magnetic beads, differential mode inductors, common mode inductors, etc. At the PCB level, we need to understand the PCB layout, lamination structure, the impact of high-speed wiring on EMC, and some rules. Another point is that we need to master some analysis and solutions for EMC problems. These are the basic knowledge that a hardware personnel must master in the future!

5. Basic principles of EMC design

Answer: Electronic circuit design criteria Electronic circuit designers often only consider the function of the product without considering the function and electromagnetic compatibility. Therefore, when the product completes its function, it also produces a large number of functional sexual harassment and other harassment.

Moreover, the sensitivity requirements cannot be met. The EMC design of electronic circuits should be considered from the following aspects:

Component selection In most cases, the degree to which the basic components of the circuit meet the electromagnetic characteristics will determine the degree to which the functional units and the final equipment meet the electromagnetic compatibility. The main criteria for selecting appropriate electromagnetic components include out-of-band characteristics and circuit assembly technology.

Because whether EMC can be achieved is often determined by the response characteristics of components far from the fundamental frequency. In many cases, circuit assembly determines the out-of-band response (such as lead length) and the degree of mutual coupling between different circuit components. The specific rules are:

① At high frequency, compared with the lead type capacitor, the through core capacitor or support capacitor with small lead inductance should be preferred for filtering.

② When lead capacitor must be used, the influence of lead inductance on filter efficiency should be considered.

③ Aluminum electrolytic capacitors may have a few microseconds of temporary dielectric breakdown, so solid capacitors should be used in circuits with large ripple or transient voltage.

④ Use resistors with small parasitic inductance and capacitance. Chip resistors can be used in UHF band.

⑤ The parasitic capacitance of large inductance is large. In order to improve the insertion loss of low frequency part, do not use a single-section filter, but use a multi-section filter composed of several small inductors.

⑥ When using the core inductor, pay attention to the saturation characteristics, especially the high level pulse will reduce the inductance of the core inductor and the insertion loss in the filter circuit.

⑦ Use shielded relays as much as possible and ground the shielded housing.

⑧ The input transformer with effective shielding and isolation shall be selected.

⑨ The power transformer used for sensitive circuits should have electrostatic shielding, and the shielding shell and transformer shell should be grounded.

⑩ The interconnection signal lines inside the equipment must use shielded wires to prevent interference coupling between them.

⑪ In order to connect each shield with its own pins, plug sockets with enough pins should be selected.

6. Performance of GPS electromagnetic interference phenomenon: Especially GPS is used in PMP products, which are hand-held vehicle-mounted dual-purpose GPS terminal products with functions of MP4, MP3, FM FM+GPS navigation. There must be a built-in GPS Antenna. In this way, GPS Antenna and MCU, SDROM, crystal oscillator and other components on GPS terminal products are easy to generate EMI/EMC electromagnetic interference, resulting in a lot of decline in GPS Antenna's ability to receive satellites, and almost no normal positioning. What measures can be taken to solve such EMI/EMC electromagnetic interference?

A: ESD Filter can be added to it, which can prevent static electricity and electromagnetic interference. Our mobile phone customers with GPS function use this method. There are many manufacturers that do this, such as Tec (Raychem), Jiabang, Korea ICT, and so on.

7. Recently, I am trying to build a broadband amplifier with a gain of 0 - 150M and a gain of no less than 80 DB,! What should we pay attention to in EMC?

Answer ①: In the design of broadband amplifier, special attention should be paid to the problem of low noise, such as the power supply must be stable enough.

Answer ②: ⑴ Pay attention to the impedance matching between input and output, such as common base input and output

⑵ Decoupling at all levels, including high-frequency and low-frequency ripple

(3) Deep negative feedback, and prevention of self-excited oscillation and loopback self-excitation

⑷ Design of bandpass filter

Answer ③: It's really hard to answer. We can't see the actual design. All the suggestions are still commonplace: pay attention to the three elements of EMC, pay attention to the conduction and radiation path, pay attention to the power distribution and the ground bomb noise.

150MHz is the bandwidth of analog signal. How fast is the rising edge of digital signal? If the turning frequency is also below 150MHz, I personally think that conduction coupling and power plane radiation will be the main factors to consider. First, do a good job of power distribution, division and decoupling circuit.

80dB, if the gain is high enough, do a good job of isolating the front minimum signal and its reference power supply from the ground, and try to reduce the power impedance of this part.

8. Seek advice on EMC methods and matters in the design of small power DC permanent magnet motors. A 90W DC permanent magnet motor (110~120V, speed 2000/min) has been produced with EMC exceeding the standard. After production, 16 slots were changed to 24 slots, and shaft insulation was made, which failed to meet the standard! Now we have to design and produce 125W motor. How to deal with it?

Answer: The EMC problem in the design of DC permanent magnet motor is mainly due to the back electromotive force generated during the rotation of the motor and the spark caused during the commutation. For specific analysis, RMxpert can be used to design and optimize the motor parameters, and Maxwell2D can be used to simulate the actual EMI radiation.

9. It is very troublesome for us to measure PCB electromagnetic radiation now. We use a spectrometer and a self-made near-field probe. Not to mention the problem of accuracy, it is very painful to meet the point of high voltage, for fear that the spectrometer will be damaged. I wonder if it can be solved by simulation.

A: First of all, EMI test includes near-field probe and far-field radiation test. No simulation tool can replace the actual test; Secondly, the PCB board noise and radiation simulation tool SIwave of Ansys and the high-frequency structure simulator HFSS of any three-dimensional structure can simulate the near-field and far-field radiation of the board and the system, as well as the EMI radiation under the limited shielding environment.

The effectiveness of the simulation depends on your consideration of the EMI problem you designed and the corresponding software settings.

For example, differential mode or common mode radiation on a single board, current source or voltage source radiation, etc. According to our practice and experience, the vast majority of EMI problems can be solved through simulation analysis, and the results are very good compared with the actual test.

10. It is said that Ansys' EMC tools generally simulate frequencies above 1GHz. The clock line with the highest frequency on our board is only 133MHz from the main chip to SDRAM, and most of the other frequencies are KHz. We mainly use ADS/Hyperlynx to do the SI/PI design, which is relatively simple to operate, but now the EMC of the whole board still exceeds the standard, affecting the image quality. In addition, does your tool have an interface with Mentor PADS?

Answer: Ansys' tool can simulate signals from DC to frequencies above tens of GHz, but compared with other tools, the lossy transmission line model above 1GHz is more accurate. As far as I know, ADS/HyperLynx is mainly used for the simulation of SI and crosstalk, as well as the EMI radiation analysis of a single signal line. Currently, there is no

PI analysis function. There are many reasons that affect the EMC of a single board. The solution to signal integrity and crosstalk is only one aspect of the solution to EMC. The noise of the power plane, decoupling strategy, shielding method, and current distribution path will affect the EMC indicators.

These can be investigated through simulation in the SIwave tool of Ansys. It is added that Ansys tool has interface with Mentor PADS.

11. The problem of EMI and the problem of signal integrity are interrelated. How to balance them in the process of defining standards?

A: Although the relationship between signal integrity and EMC is very strong, signal integrity has not been issued with special mandatory standards. As for the balance between signal integrity and EMI, it is not a matter of test specifications. If the balance is to be achieved, it is better to reduce the rising and falling edges of the signal while meeting the requirements of signal integrity.

12. How can PCB design meet EMC requirements as much as possible without causing too much cost pressure?

Answer: The increased cost due to EMC on the PCB is usually due to the increase of the number of layers to enhance the shielding effect and the addition of ferrite beam, choke and other high-frequency harmonic suppression devices.

In addition, it is usually necessary to match the shielding structure of other organizations to make the whole system pass the EMC requirements. The following is only about the design skills of PCB board to reduce the electromagnetic radiation effect generated by the circuit.

Several electromagnetic radiation effects produced by reducing circuits

① Try to select devices with slow signal slope rate to reduce the high-frequency components generated by the signal.

② Pay attention to the placement of high-frequency components, and do not be too close to external connectors.

③ Pay attention to the impedance matching of high-speed signals, the routing layer and its return current path to reduce high-frequency reflection and radiation.

④ Place sufficient and appropriate decoupling capacitors on the power pins of each device to mitigate the noise on the power supply layer and stratum. Pay special attention to whether the frequency response and temperature characteristics of the capacitor meet the design requirements.

⑤ The ground near the external connector can be properly separated from the ground and connected to the chassis ground.

⑥ The ground guard/hunt tracks can be properly used next to some special high-speed signals. However, pay attention to the influence of guard/hunt tracks on the characteristic impedance of wiring.

⑦ The power layer is 20H smaller than the formation, and H is the distance between the power layer and the formation.

13. In high-speed PCB design, what aspects should designers consider the rules of EMC and EMI?

Answer: Generally, EMI/EMC design needs to consider both radiation and conduction The former belongs to the higher frequency part (>30MHz) and the latter is the lower frequency part (<30MHz).

Therefore, we should not only pay attention to the high frequency and ignore the low frequency part. A good EMI/EMC design must consider the device location, PCB stack arrangement, important online routing, device selection, etc. at the beginning of the layout. If there is no better arrangement in advance, the solution after the event will double the result and increase the cost.

For example, the position of the clock generator should not be close to the external connector as much as possible. High-speed signals should go through the inner layer as much as possible and pay attention to the continuity of characteristic impedance matching and reference layer to reduce reflection. The slope rate of the signal pushed by the device should be as small as possible to reduce high-frequency components. When selecting the decoupling/bypass capacitor, pay attention to whether its frequency response meets the requirements to reduce power layer noise.

In addition, pay attention to the return path of high-frequency signal current to minimize the loop area (that is, loop impedance) to reduce radiation The range of high-frequency noise can also be controlled by dividing the stratum Finally, properly select the chassis ground between the PCB and the enclosure.

14. How to reduce EMI problems by arranging laminations during PCB design?

A: First of all, EMI should consider the system. PCB alone cannot solve the problem. For EMI, stacking is mainly to provide the shortest signal return path, reduce the coupling area and suppress differential mode interference. In addition, the stratum is tightly coupled with the power supply layer, which is better than the extension of the power supply layer to suppress the common-mode interference.

15. Safety and regulatory issues: What are the specific meanings of FCC and EMC?

A: FCC: Federal Communication Commission; EMC: electromagnetic compatibility. FCC is a standards organization and EMC is a standard. There are corresponding reasons, standards and test methods for the issuance of standards.

16. In the EMC test, it was found that the harmonic of the clock signal exceeded the standard very seriously, and only the decoupling capacitor was connected on the power supply pin. What aspects should be paid attention to in PCB design to suppress electromagnetic radiation?

A: The three elements of EMC are radiation sources, transmission routes and victims. The transmission path is divided into space radiation transmission and cable transmission. So to suppress harmonics, first look at its propagation path. The decoupling of power supply is to solve the transmission of conduction mode. In addition, necessary matching and shielding are also required.

17. In PCB design, the ground wire is usually divided into protective ground and signal ground; Power ground is divided into digital ground and analog ground. Why should we divide the ground wire?

Answer: The purpose of ground division is mainly for EMC's consideration. It is worried that the power supply of the digital part and the noise on the ground will interfere with other signals, especially analog signals, through the conduction path.

The division of signal and protective ground is due to the consideration of ESD static discharge in EMC, which is similar to the role of lightning rod grounding in our life. No matter how it is divided, there is only one final land. It's just that the way of noise emission is different.

18. In PCB design, is it necessary to add ground wire shield on both sides when laying the clock?

Answer: Whether to add shielded ground wire depends on the crosstalk/EMI condition on the board. If the shielded ground wire is not handled properly, it may make the situation worse.

19. I made a TFT LCD display. When others were doing EMC tests, interference signals were transmitted through space, causing the image displayed on the screen to shake with a large amplitude. Who can tell me what to do! It is to add jamming pulse groups on several signal lines. I don't know the specific name. The jamming signal is radiated through the signal line.

A: If it is a separate LCD, the pulse group test in EMC test is almost impossible to pass, especially when using coupling clamp, it will be enough for you. If LCD is used in the instrument, it is not difficult to solve the problem, such as decoupling of signal lines, proper reduction of the impedance of the LCD inlet with conductive paste, and shielding of conductive screen on the screen surface.

20. In the EMC test some time ago, the GSM fixed wireless telephone has radiation stray phenomenon between 100MHz and 300MHz. After that, the company sent me two shielded case phones sprayed with electrostatic paint, and the laboratory was not allowed to change the whole phone. So I changed the case sprayed with electrostatic paint of ferromagnetic material to the phone to be modified and tested. The test results show that the previous spurious phenomenon is gone, but there is a problem with the main frequency. The main frequency of the telephone is 902MHz, but there are several frequencies between 905-910MHz. This is the basic situation. In the process of modification, I only changed the shell, and the circuit board and other hardware were not modified.

A: The types of phones can be understood as wireless phones, cordless phones, etc. It is necessary to clarify the type of the telephone, the operating frequency range of the host and the type of electrostatic coating materials for the case, such as ferromagnetic or non-ferromagnetic conductive materials and conductivity.

21. Recently, I am writing a 2KW vacuum cleaner software. The function is realized, but it can't pass EMC. Please point out which algorithm is used on the software, which can pass EMC! Function description is as follows:

① Soft start and soft speed regulation function. (The so-called soft start means that the motor accelerates slowly without sudden change in speed)

② The speed of the motor can be adjusted.

③ The motor is controlled by thyristor. The control mode is sine wave chopping.

In terms of hardware, the circuit is very simple, and the hardware processing EMC only has a 0.1uF safety capacitance.

A: It may take more effort to communicate with hardware. It is difficult to solve the problem with software alone.

22. Some basic problems of EMC: some EMC problems often encountered in certification.

A: Here are some of the questions that are summarized for electronic products.

The most common problems of general electronic products are: RE -- radiation, CE -- conduction, ESD -- static electricity.

Communication electronic products include not only the above three items: RE, CE, ESD, and Surge -- surge (lightning, thunder)

The most common problems of medical devices are: ESD - static electricity, EFT - transient pulse immunity, CS - conduction immunity, RS - radiation immunity

For the dry areas in the north, the ESD - electrostatic requirements of the products should be very high.

For areas such as Sichuan and some areas with thunderstorms in southwest China, EFT lightning protection requirements are very high.

23. Who did the EMC design of V.35, E1, G.703 (6? K) and relay interface? Can you give me some suggestions?

The following standards should be met:

GB/T 17626.12 (IEC61000-4-12) Electromagnetic Compatibility Testing and Measurement Techniques Oscillating Wave Immunity Test

GB/T17626.2 (IEC61000-4-2) Electromagnetic compatibility test and measurement technology Electrostatic discharge reactance interference test

GB/T 17626.3 (IEC61000-4-3) Electromagnetic Compatibility Testing and Measurement Techniques - Radio Frequency Electromagnetic Field Radiated Immunity Test

GB/T 17626.4 (IEC61000-4-4) Electromagnetic Compatibility Testing and Measurement Techniques Electrical Fast Transient Burst Immunity Test

GB/T 17626.5 (IEC61000-4-5) Electromagnetic Compatibility Testing and Measurement Techniques Surge Impulse Immunity Test GB/T 17626.6 (IEC61000-4-6) Electromagnetic Compatibility Testing and Measurement Techniques Conducted Disturbance Immunity Induced by RF Fields

A: These standards are the basic standards for EMC testing, and specific indicators need to be determined in combination with your products. Your interfaces are communication interfaces, and generally have standard circuits.

When the filter design of the single board schematic diagram and the correct layout and wiring design of the PCB are designed, they can generally pass the test. In other cases, EMC filter and transient suppression devices need to be added, which needs to be combined with specific interface analysis.

24. Measuring the biomagnetic information of the human body is a new medical diagnosis method. This biomagnetic measurement must be carried out in a magnetic field shielding room, which must be able to shield the alternating electromagnetic field from static magnetic field to 1GHz. Please propose the design scheme of this shielding room.

Answer: First of all, we should consider the selection of shielding materials. Because we need to shield the magnetic field with very low frequency, we should use materials with high permeability, such as permalloy. Due to the reduction of permeability of permalloy after processing, heat treatment must be carried out.

Therefore, the shielding chamber shall be made of assembled plates. Each plate shall be processed according to the design in advance, then heat treated, transported to the site, and installed with great care. The joint of each plate shall be overlapped to form a continuous magnetic path.

The shielding chamber thus formed can have better shielding effectiveness against low-frequency magnetic field, but the gap will produce high-frequency leakage. In order to make up for this deficiency, the outer layer of the permalloy shielding chamber is welded with aluminum plates to form a second layer of shielding, which plays a shielding role in high-frequency electromagnetic fields.