Friends engaged in the power industry know that the so-called EMC is actually the ability to work normally in its electromagnetic environment, and does not constitute an unbearable electromagnetic harassment in this environment. Usually, the test includes testing the electromagnetic harassment intensity to the outside, and performs sensitivity testing under electromagnetic environment conditions specified in the electromagnetic harassment strength, the former is to confirm whether it meets the limit value requirements specified in the standard; It is confirmed whether to comply with the refraction requirements for the relevant standards. Mastering a certain EMC test technology is necessary, this article will share those EMC test points and their troubleshooting schemes.
Single chip microcomputer system EMC test
Test environment
In order to ensure the accuracy and reliability of the test results, the electromagnetic compatibility measurement has high requirements for the test environment, and the measurement venue has outdoor open space, shielding chamber or electric wave dark room.
2. Test equipment
Electromagnetic compatible measuring equipment is divided into two categories: one type is an electromagnetic interference measuring device. The device can be measured by electromagnetic interference; the other is the electromagnetic sensitivity measurement, the equipment simulates different interference sources, by appropriate Coupling / decoupling network, sensors or antenna, is applied to various types of measured devices for sensitivity or interference measurement.
3. Measurement method
Electromagnetic compatibility testing is based on standards, there are many measurement methods, but it can be divided into 4 categories; conductive launch test, radiation launch test, transmission sensitivity (immunity) test and radiation sensitivity (immunity) test.
4. Test diagnostic steps
5. Test preparation
● Test site conditions: EMC test laboratory is a semi-dark room and shield chamber. The former is used for radiation emission and radiation sensitivity testing, which is used to conduct emission and conductivity sensitivity testing.
● Environmental level requirements: The electromagnetic environment level of the conduction and radiation is preferably much lower than the standard specified limit value, generally enables at least less below the limit value of 6db.
● Test table.
● Isolation of the device and the device under test.
● Sensitivity discrimination criteria: Generally provided by the items, and to monitor and discriminate, determine the degree of performance reduction in the measurement and observation.
● Place of measured devices: To ensure the repetition of the experiment, the placement of the measured equipment is usually specified.
Figure 1 Electromagnetic compatibility test diagnostic steps
6. Test type
Conduct emission test, radiation transmission test, conduction immunity test, radiation immunity test.
7. Common measuring instrument
Electromagnetic Interference (EMI) and electromagnetic sensitivity (EMS) testing requires many electronic instruments, such as spectrum analyzers, electromagnetic field interference measuring instruments, signal sources, functional amplifiers, oscilloscopes, and the like. Since the EMC test frequency is wide (20 Hz ~ 40GHz), the amplitude is large (μV to KW), the mode is much large (fm, AM, etc.), the gesture is much (flat, oblique, etc.), so properly use the electronic instrument very important. Suitable instruments for measuring electromagnetic interference are spectrum analyzers. The spectrum analyzer is an instrument that disseminated the voltage amplitude with the regular changes in the frequency, and the waveform it shows is called a spectrum. The spectrum analyzer overcomes the shortcomings of the oscilloscope in the measurement of electromagnetic interference, and can accurately measure the interference strength on each frequency, and the spectrum analyzer can directly display the respective spectral components of the signal.
When solving electromagnetic interference issues, the most important problem is to judge interference. Only when the interference source is accurately positioned, the measures to solve the interference can be proposed. Determining the frequency of the signal is the easiest way to determine the frequency of the signal, since the frequency characteristics are most stable in all the features of the signal, and the circuit designer tends to be very clear about the signal frequency of each portion in the circuit. Therefore, as long as the frequency of the interference signal is known, it is possible to speculate which portion of the interference is generated. For electromagnetic interference signals, this measurement is very simple to do this measurement with a spectrum analyzer due to its amplitude is often less than the normal working signal. Since the intermediate frequency bandwidth of the spectrum analyzer is narrower, the signal filtering with the frequency of the interference signal can be filtered off, and the interference signal frequency is accurately measured, thereby judgeting the circuit generating the interference signal.
Electromagnetic compatibility troubleshooting technology
1. Solution to the conductive problem
● Reduce EMI current by a high impedance in series.
● Short the EMI current to the ground or lead to other loop conductors by parallel a low impedance.
● The EMI current is cut through the current isolation device.
● Use its own effect to suppress EMI current.
2. Electromagnetic compatible capacitive solution
A common phenomenon is to be connected to a separate impedance from a separate impedance without regarding a separated impedance. Typically, when a length of an input and output line reaches or exceeds 1/4 wavelength, the transmission line becomes "long". It actually uses the following formula to represent this change:
L ≥ 55 / f
Where: the L unit is M, F unit is MHz. This formula considers the average propagation speed, which is 0.75 times the free space theory.
Dielectric material and tolerance: most of the capacitance used in electromagnetic interference filtering is a polar capacitor
Difference mode (line to line) filter capacitive capacitor
Common model (line to ground / casing) filter capacitor
A common mode (CM) decoupling usually uses small capacitance (10 to 100 nF). Small capacitors can short-circuited to the casing before or when they enter the sensitive circuit, or when it is farther away from the noise circuit. In order to obtain a good high frequency attenuation circuit, it is critical to reduce or eliminate parasitic inductance. Therefore, it is necessary to use ultrashine wires, especially the use of non-lead components.
3. Sensibility, series loss electromagnetic compatibility solutions
For the capacitance, if Zs and Z1 are not a pure resistance, use their actual values when calculating frequencies. When the capacitor is connected in series in the power supply or signal circuit, it must be satisfied:
● The flowing current flowing should not cause an inductance overheating or too much, and there is impression;
● The current flowing does not cause the inductance magnetic saturation, especially for high-conducting materials.
Solutions have the following: magnetic core materials; ferrite and loading of ferrite; inductance, differential mode and common mode; ground choke; combined inductor capacitive element.
4. Solution of radiation type problem
In many cases, radiation electromagnetic interference problems may be generated and excluded in the conduction phase, and some solutions are to suppress the interference device on the radiation transmission channel, as working like a field shield. According to the mask theory, the effect of this shield mainly depends on the frequency of the electromagnetic interference source, the distance between the shielding device and the characteristics of the electromagnetic interference field - electric field, magnetic field or plane wave.
● Conductor belt. Use copper or aluminum strips to create a direct shield and low-resistance connectivity or bus. They are very convenient for temporary solutions and relatively permanent solutions. The thickness is between 0.035 to 0.1 mm, and the back is a conductive binder to mount. If the copper conductive strip is used, it passes the electrical resistance of about 20mΩ / cm2. Application occasion: electrical shielding hood; failure point positioning; as an emergency solution, turn the plastic connector into a metal, shield ordinary flat cable, etc. .
● Net shield strips and zipper jackets. Tin's steel mesh belt: Mainly used to install a well-equipped electric toll jacket as an easy-to-mounted bandage type shield. In order to reduce the magnetic field radiation or sensitive problem of electricity bills, steel mesh is an effective solution.
Zipper shielding jacket: When there is obvious indication that the electricity bill is mainly used for the cause of EMI coupling.
● EMI seal. Applications: When the conditions are present, and the true SE is required, the EMI seal is the most commonly used method of solving radiation problems, sensitive problems, ESD, electromagnetic pulses, and Tempest issues.
The chassis leaks have been confirmed as the main radiation path.
The meshing surface is not smooth enough, flat or not hard enough, itself cannot provide a good connection contact.
● EMI shielding of windows and ventilation: Suitable for shielding of the aperture.
The probably the plan of the plane is: SE≈104 (-20-LGL) -20LGF
In the formula, the SE unit is DB; L is the size of the mesh or mesh, the unit is mm; F unit is MHz. Of course, as the frequency is lowered, the upper limit of the shielding efficiency SE of the mesh is limited to the metal itself. In the nearland, the shield of the H field, its shielding power She is not affected by the frequency, can be approximately drawn by the following formula:
SEH ≈10LG (πr / L)
EMC test
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