About EMI test

2018-09-21 13:43:17 26

Electromagnetic compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment and not produce unbearable electromagnetic interference on any equipment in its environment. EMC includes EMI electromagnetic interference and EMS electromagnetic interference. EMI is the interference of electronic devices to the external electromagnetic environment. EMS is the ability of electronic devices to resist the interference of the external electromagnetic environment. Both EMI and EMS include two parts: radiation and conduction. All electronic devices must comply with EMC certification, and the key part of EMC is EMI.

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There are four major points in the EMI test:

1. EMC certification bodies conduct certification tests in EMC laboratories.

2. the enterprise quality inspection department uses EMI receiver or high index spectrum analyzer to conduct EMI pre qualification test.

3. the product development and testing departments use EMI to diagnose.

4. product development and testing departments use oscilloscopes to test the characteristics of power ripple and clock jitter, because they are one of the factors that produce EMI.

EMI diagnosis

The effective and lowest cost means to ensure that electronic devices pass EMC certification is to make EMI diagnosis at any time during the design and commissioning phase of electronic equipment. If the EMC certification of the final product is not qualified, the design engineer needs to re enter the EMI diagnosis to find the root of the EMI problem, but in this case, the remedial measures can be used very little, and the design will be redesigned, and the cost will be multiplied. To sum up, EMI diagnosis is often done in daily work, and EMI pre certification and certification test are only carried out in the electronic product stereotype stage.

Peak detection and quasi peak detection

EMI certification bodies are usually tested in accordance with the quasi peak detection of the EMI standard. Peak detection is often used in the EMI diagnosis. They do not match. Is there any significance in the EMI diagnosis? Because finding out the root of EMI is the fundamental purpose of EMI diagnosis, what we need is the relative repeatability of the test, rather than the absolute accuracy of the test. The quasi peak detection is used to detect the peak value (quasi peak value) after the envelope is weighted, and it is weighted by the time and repetition rate. The average process of quasi peak detection takes time and the test time is long, which is not conducive to routine diagnosis. Since the results of the quasi peak detection range are always less than or equal to the test results of the peak detection, the EMI problem can be quickly detected by peak detection in the EMI diagnosis.

The "pain point" of the Engineer in the EMI test

1. from the beginning of the circuit board design, we need to consider the EMI problem. However, due to the limitation of project funds, the EMI diagnostic equipment can not be equipped in place.

2. after the design of electronic products, the EMC certification test is carried out. The certification body gives out the unqualified report. It only points out that radiation or conduction EMI is not qualified. Although the interference frequency is given, it does not point out the specific location or reason of EMI disqualification in the electronic equipment. It needs to carry out EMI diagnosis by itself, and spend time and funds.

3. some equipment can not be shielded mainly by environmental constraints, we need to identify the root causes of EMI and solve problems from design. The basic test instrument for finding EMI is spectrum analyzer, but in some special cases, it is difficult to trace the root of EMI.

4. can solve some EMI problems through the shielding method, although it can be authenticated by EMC, but for EMI, it is necessary to solve the device's own performance, or to find out the problem, and avoid it. In this case, the spectrum analyzer has the same problem as the above third points.

5. as the speed of data is accelerated, the EMI problem of periodic burst is increasing, and the real root of the problem must be found through the analysis of the EMI cycle, which requires the analysis of modulation domain.

6. conduction type EMI can be traced by an oscilloscope. At the same time, it needs to be tested with a spectrum analyzer. Two kinds of instruments need to be used together.

EMI test scheme based on unique mixed domain oscilloscope

Solution: hybrid domain oscilloscope + near field antenna + low capacitance probe + SMA-N type adapter (N head to SMA head)

Characteristic:

1. five in one, the full scope of the oscilloscope function can not only meet the needs of the routine circuit adjustment, but also trace the root of the EMI problem; the function of the spectrometer can be used to diagnose the EMI problem at any time.

2. a cross domain analysis function with characteristics to facilitate the analysis of the real root of EMI.

3. high sensitivity and wide bandwidth, suitable for EMI diagnostic test.

4. it is possible to test the variation of RF amplitude over time, so as to facilitate the analysis of the root causes of periodic EMI problems.

The 5. capacitor probe can directly connect the circuit board power or ground wire to the RF input of the MDO spectrum analyzer, and test the EMI problem caused by the power ripple or the unreasonable ground wire.