EMC test of single-chip microcomputer system and troubleshooting of electromagnetic compatibility

The so-called EMC is: the ability of a device or system to work normally in its electromagnetic environment without causing unbearable electromagnetic disturbance to anything in the environment. The EMC test includes two major aspects: testing the intensity of electromagnetic disturbance sent to the outside world to confirm whether it meets the limit value requirements specified in the relevant standards; testing its sensitivity under the electromagnetic environment conditions that specify the intensity of electromagnetic disturbance, so as to ensure Confirm whether the immunity requirements specified in the relevant standards are met.

introduction

The so-called EMC is: the ability of a device or system to work normally in its electromagnetic environment without causing unbearable electromagnetic disturbance to anything in the environment. The EMC test includes two major aspects: testing the intensity of electromagnetic disturbance sent to the outside world to confirm whether it meets the limit value requirements specified in the relevant standards; testing its sensitivity under the electromagnetic environment conditions that specify the intensity of electromagnetic disturbance, so as to ensure Confirm whether the immunity requirements specified in the relevant standards are met. For the engineering and technical personnel engaged in the design of the single-chip application system, it is very necessary to master a certain EMC testing technology. EMC is the abbreviation of Electromagnetic Compatibility (Electro-Magnetic Compatibility), which includes electromagnetic interference () and electromagnetic susceptibility (EMS). Because electrical products have electromagnetic interference to other electrical appliances or receive electromagnetic interference from other electrical appliances when they are in use, it is not only related to the reliability and safety of product work, but may also affect the normal operation of other electrical appliances, and even lead to safety hazards.

Single chip system EMC test

(1) Test environment

In order to ensure the accuracy and reliability of the test results, the electromagnetic compatibility measurement has high requirements on the test environment. The measurement site includes an outdoor open field, a shielding room or an anechoic chamber.

(2) Test equipment

Electromagnetic compatibility measurement equipment is divided into two categories: one is electromagnetic interference measurement equipment, the equipment can be connected to appropriate sensors, and electromagnetic interference can be measured; the other is electromagnetic sensitivity measurement, the equipment simulates different interference sources, Coupling/decoupling networks, sensors or antennas, applied to various types of equipment under test for sensitivity or interference measurements.

(3) Measurement method

Electromagnetic compatibility test is based on different standards, there are many kinds of measurement methods, but can be divided into 4 categories; conducted emission test, radiated emission test, conducted sensitivity (immunity) test and radiation sensitivity (immunity) test.

(4) Test and diagnosis steps

Figure 1 shows the EMI emission and fault analysis steps of a device or system. Following this step can improve the efficiency of test diagnosis.

(5) Test preparation

①Test site conditions: The EMC test laboratory is a semi-anechoic chamber and a shielding room. The former is used for radiated emission and radiation susceptibility testing, and the latter is used for conducted emission and conducted susceptibility testing.
②Environmental level requirements: The electromagnetic environment level of conduction and radiation is very good and far below the limit value specified by the standard. Generally, the environmental level is at least 6dB lower than the limit value.
③ Test table.
④Isolation of measuring equipment and equipment under test.
⑤ Sensitivity judgment criterion: generally provided by the tested party, and truthfully monitored and judged, to determine the degree of performance degradation by means of measurement and observation.
⑥ Placement of the device under test: In order to ensure the repeatability of the experiment, there are usually specific regulations on the placement of the device under test.

(6) Types of tests

Conducted emission test, radiated emission test, conducted immunity test, radiated immunity test.

(7) Commonly used measuring instruments

Electromagnetic interference (EMI) and electromagnetic susceptibility (EMS) testing requires the use of many Electronic instruments, such as spectrum analyzers, electromagnetic field interference measuring instruments, signal sources, functional amplifiers, oscilloscopes, etc. Because the EMC test frequency is very wide (20Hz~40GHz), the amplitude is very large (μV level to kW level), there are many modes (FM, AM, etc.), and there are many attitudes (flat, inclined, etc.), so it is very important to use electronic instruments correctly. important. A suitable instrument for measuring EMI is a spectrum analyzer. A spectrum analyzer is an instrument that displays the law of voltage amplitude changing with frequency. The waveform it displays is called spectrum. The spectrum analyzer overcomes the shortcomings of the oscilloscope in measuring electromagnetic interference, and can accurately measure the interference intensity at each frequency. The spectrum analyzer can directly Display each spectral component of the signal.

When solving the problem of electromagnetic interference, it is very important to determine the source of the interference. Only after the source of interference is accurately located, can measures to solve the interference be proposed. It is a very simple method to determine the source of interference according to the frequency of the signal, because in all the characteristics of the signal, the frequency characteristics are very stable, and people often know the frequency of the signal in each part of the circuit very clearly. Therefore, as long as the frequency of the interference signal is known, it is possible to infer where the interference is generated. For electromagnetic interference signals, since their amplitude is often much smaller than that of normal operating signals, it is very simple to do this measurement with a spectrum analyzer. Because the IF bandwidth of the spectrum analyzer is narrow, it can filter out signals with different frequencies from the interference signal, and accurately measure the frequency of the interference signal, thereby judging the circuit that generates the interference signal.

Electromagnetic Compatibility Troubleshooting Technology

(1) Solving the conduction problem

① Reduce the current by connecting a high impedance in series.
② Short-circuit the EMI current to ground or lead to other return conductors by connecting a low impedance in parallel.
③ Cut off the EMI current through the galvanic isolation device.
④ Suppresses EMI current by its own action.

(2) Capacitive solution for electromagnetic compatibility

A common phenomenon is not to see one side of the filter capacitor as directly connected to a separate impedance, but as connected to the transmission line. Typically, when the length of an input and output line reaches or exceeds 1/4 wavelength, the transmission line becomes “long”. This change can actually be approximated by the following equation:

In the formula: the unit of l is m, and the unit of f is MHz. This formula takes into account the average speed of propagation, which is 0.75 times that of free space theory.

Dielectric Materials and Tolerances

Most capacitors used in EMI filtering are non-polar capacitors.

Differential Mode (Line-to-Line) Filtering Capacitive Capacitors.

Common Mode (Line-to-Ground/Case) Filter Capacitors

Common mode (CM) decoupling usually uses small capacitors (10 to 100nF). Small capacitors can short undesired high frequency currents to the chassis before they enter sensitive circuits or when they are far from noisy circuits. In order to obtain a good high frequency attenuation circuit, reducing or eliminating parasitic inductance is the key. Therefore, it is necessary to use ultra-short wires, and it is especially desirable to use leadless components.

(3) Inductive and series loss electromagnetic compatibility solutions

As far as capacitance is concerned, Zs and Z1, if not purely resistive, use their actual values ​​when calculating frequency. When a capacitor is connected in series with a power supply or signal circuit, it must meet:

① The working current flowing through should not cause the inductance to overheat or be too large, and it will be worse than that;

② The current flowing through can not cause magnetic saturation of the Inductor, especially for materials with high magnetic permeability.

Solutions are as follows:

core material;
Ferrites and ferrite-loaded cables;
Inductance, differential mode and common mode;
grounding choke;
Combined inductor and capacitor components.

(4) Solutions to Radiation Problems

In many cases, radiated EMI problems can arise and be eliminated during the conduction phase, and some solutions can be to suppress the interference device on the radiated transmission channel, acting like a field shield. According to the shielding theory, the effect of this shielding mainly depends on the frequency of the electromagnetic interference source, the distance from the shielding device and the characteristics of the electromagnetic interference field – electric field, magnetic field or plane wave.

①Conductor tape. Use copper or aluminum tape to quickly and easily create a direct shielded and low-impedance connection or bus. They are convenient for temporary solutions and relatively long-term solutions. The thickness is between 0.035 ~ 0.1mm, and the backside has conductive adhesive for installation. If copper conductive tape is used, its pass resistance is about 20mΩ/cm2. Applications: Electrical shielding; locating leak points in the event of a fault; as an emergency solution, turning plastic into metal, shielding ordinary flat cables, etc.

②Mesh shielding tape and zipper jacket. Tin-coated steel mesh tape: Primarily used as an easy-to-install bandage-type shield over an already assembled utility jacket. In order to reduce the magnetic field radiation or sensitivity of electricity bills, steel mesh belts are an effective solution.

Zip-Up Shield Jacket: Use when there is a clear indication that electricity bills are the primary cause of EMI coupling.

③EMI gasket. Application: When the following conditions exist and true SE is required, EMI gaskets are a common solution to radiated, sensitive, ESD, EMP, and TEMPEST issues.

Chassis leaks have been identified as the main radiation path.

The mating surfaces are not smooth, flat or hard enough to provide a good bonding contact by themselves.

④ EMI shielding of windows and ventilation panels: suitable for shielding apertures.

The approximate model of a plane wave is:

≈104 (-20-lgl)

In the formula, SE is in dB; l is the size of the grid or mesh, in mm; f is in MHz. Of course, as the frequency decreases, the upper limit of the shielding efficiency SE of the mesh is limited by the metal itself. In the near field, for the shielding of the H field, the shielding power SHE is not affected by the frequency, which can be approximated by the following formula:

≈10lg (πr/l)

Among them, r is the distance from the source to the shield, l is the mesh size, and both units are mm.

⑤Conductive coating: It is used to establish EMI shielding cover in the plastic casing of the system, improve the shielding effectiveness SE of the existing ordinary or deteriorated conductive surface, prevent ESD or electrostatic accumulation phenomenon, and increase the contact area of ​​the joint surface or sealing gasket.

⑥Conductive foil: Aluminum is a good conductor and has no absorption loss below 10MHz, but it has better reflection loss for any frequency of the electric field. Please refer to the relevant information for applications.

⑦Conductive cloth: It can be used in any stereo shielding occasion where the frequency range from 100kHz to GHz needs to reach 30~30dB attenuation.

Application of Electromagnetic Compatibility New Devices and New Materials

Power Line Filter

The power line filter is installed between the power line and the electronic equipment, which is used to simulate the parasitic electromagnetic interference in the power transmission, and plays an important role in improving the reliability of the equipment. Filters allow some frequencies to pass, while simulating components at other frequencies. According to the characteristics of the interference source, frequency range, voltage and impedance and other parameters and the requirements of the load characteristics, the filter should be properly selected.

Signal blocking transformer

Pulse type (digital or thyristor gate drive) or analog isolation transformers and isolation transformers used in AC power The principle is the same as the isolation transformer used in AC power, but the transmission frequency band is completely different, useful signal processing Some performance requirements of the transformer (eg distortion, 3dB bandwidth, loss, symmetry, impedance, pulse delay, etc.) are very strict. This kind of transformer is a broadband device, and the ratio fMAX/fMIN of very high frequency to very low frequency is dozens of times. By cutting off the common-mode ground loop at the transmitter or receiver, the isolation transformer simulates common-mode noise without changing the differential-mode signal. Since the common mode voltage is applied to one side of the transformer and both sides of the secondary side, this isolator must have a high breakdown voltage: typically 1.5kV, and in some cases up to 10kV.

The main advantages of a signal transformer are its simplicity, durability, longevity and linearity, all at an affordable price. When the frequency increases, its electromagnetic compatibility performance decreases.

Application:

When loop isolation is required, its frequency ranges from DC to tens of MHz;

When transmitting analog small signals (≤10mV) under low noise and low distortion conditions, there may be a common mode voltage of several V to several kV on the signal line;

In the thyristor application circuit, isolate the flip-flop from the common-mode voltage;

As a field solution, it can be used to cut a ground loop and build a balanced or unbalanced transmission line.

Power Isolation Transformers, Power Regulators and Uninterruptible Power Supplies

(1) Power isolation transformer

Ordinary isolation transformers can break ground loops on mains power lines in the low frequency range. As the frequency increases, the electrical isolation decreases due to the presence of a side-to-side registered capacitor C1-2. In order to reduce the influence of the parasitic capacitance, one and two windings of the dropped, helical and discrete type can be used, which can reduce the parasitic capacitance to 1/3~/10 of the original value.

(2) Faraday shielding transformer

Wrap a layer of aluminum foil or copper foil between the primary and secondary coils and keep them out of contact with the coils to avoid short circuits. Faraday shield or electrostatic shield to ground. The scope of application is as follows:

It is applied to the indoor power supply or power distribution box as a simple 1:1 isolation transformer to isolate the 50/60Hz ground loop;

Regenerate neutral AC power to ground in a certain part of the same system, and maintain electrical isolation from the main power distribution point;

It is used to prevent the ground fault detector in the system from being frequently triggered by the transition when there is a large earth leakage current in the system;

Can be used in combination with power line filters whose attenuation characteristics only start above tens or hundreds of kHz.

Transient suppressor

Varistors and solid-state varistors (transzorbs) are components with nonlinear VI characteristic curves that can be used as voltage regulators. When the voltage is passed through the device, it is clamped at a voltage equal to or greater than the breakdown voltage VBR. The device has a fast response speed but is limited in the amount of energy it can handle.

Bonding, Ground Continuity and RF Impedance Reduction Devices

①The grounding braid or metal width and flat wire has a smaller inductance than the same cross-sectional round wire. As a preferred selection reference, you can use:

flat metal strip;

Flat braid with flat ground terminal;

Patch cord for round, stranded wire.

②Printed (PCB) ground pads. To build a more direct low-impedance EMI current receiver, a ground pad is required. Usually there is a spring clip in the middle of the resin-type gasket to provide strong and reliable pressure on the OV copper plate on one side and the mounting chassis on the PCB on the other side. Since the spring is made of copper-tin material, the electrical contact performance is good, and the contact resistance is in the order of mΩ.

③ Metal electric charge trunking and its fertilized metal braided layer. The role of metal cable trays, common conductors and metal braids is to transmit part of the ground loop between several interconnected devices. Think of it as a common mode short circuit between different chassis or grounds, but in reality this method cannot be used for longer distances other than DC or AC 50/60Hz; can be used in computer rooms, factory floors or other Large venues with many unshielded bills, it is impossible or difficult to swap them out for shielded bills or into plumbing.

④ The ground impedance is reduced, and the raised metal bottom plate is grounded. In order to reduce the input of conducted transient interference and the influence of the surrounding RF field on the system, it can be improved by setting up an indoor reference ground plane or a ground network. With this approach, a 20dB improvement can easily be achieved at frequencies up to a few hundred MHz, and the ground potential deviation between different devices in the same room can also be reduced.

Another technique: Indoors, it is recommended to install a raised metal floor (RMF), and use the ribs of the floor tiles as the ground reference grid; replace the plastic shock-absorbing gasket with a conductive shock-absorbing gasket, and you can build a good long-lasting electrical connection.

②Temporary grounding plate. This latter solution was originally used by IBM’s installation planning engineers to install a piece of copper or plated steel. For those occasions where there is no “actual ground”, due to the large capacitance (300-1000pF) between the temporary grounding plate and the special structure of the building, this provides the Faraday shielding layer of electromagnetic interference filters, transient protectors and isolation transformers. There is a better absorption device. At high frequencies, this virtual ground is more effective than a long, green, or green-yellow ground wire.

Epilogue

In the actual EMC test application, in addition to passing the qualification test of the standard qualification laboratory, there are two feasible methods recognized by the industry: TCF (Technical Construction File) and Self Certification (self-test certificate). The anti-interference ability test is a very practical test item. A good way to achieve electromagnetic compatibility is to treat all digital and analog circuits as circuits that respond to high-frequency signals, and use high-frequency design methods to handle electricity bill shielding, PCB routing, and common mode filtering. It is also important to use the entire ground and power planes, as is the case for analog circuits, to limit high-frequency common-mode loops. Most transient disturbances are high frequency and generate strong radiated energy.

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