EMI testing

Electromagnetic compatibility (EMC) includes electromagnetic interference (EMI) and electromagnetic immunity (EMS). In short, EMI is the interference of electronic equipment to the external electromagnetic environment, and EMS is the ability of electronic equipment to resist the interference of the external electromagnetic environment. Both EMI and EMS include radiation and conduction. EMC certification is a must for any electronic equipment, and EMI is an important part of EMC.

EMI testing includes

1. The EMC certification body conducts certification testing in the EMC laboratory;

2. The enterprise quality inspection department uses an EMI receiver or a high-performance spectrum analyzer to conduct EMI pre-certification testing;

3. Product R&D and commissioning departments use spectrum analyzers to perform EMI diagnosis;

4. The product development and commissioning department uses an oscilloscope to test power supply ripple, clock jitter and other characteristics because they are one of the factors that generate EMI.

EMI Diagnostics

During the design and debugging phase of electronic equipment, EMI diagnosis at any time is an effective and lowest-cost means to ensure that electronic equipment passes EMC certification. If the final product fails EMC certification, the designer needs to re-diagnose EMI to find out the root cause of the EMI problem, but at this time there are not many rectification measures available, and redesign will double the cost. This shows that EMI diagnosis is often performed in daily work, while EMI pre-certification and certification testing are only performed during the finalization phase of electronic products. See the table below for the differences between EMI diagnosis and EMI pre-certification or certification.

Peak detection and quasi-peak detection:

EMI certification agencies usually test according to the quasi-peak detection of the EMI standard, while peak detection is often used in EMI diagnosis. The two do not match. Is EMI diagnosis still meaningful? Since the purpose of EMI diagnosis is to find the root cause of EMI, it does not require absolutely accurate testing, but relatively repeatable testing. Quasi-peak detection is used to detect the peak value (quasi-peak value) of the signal envelope after weighting. It weights the signal according to the duration and repetition rate. The averaging process of quasi-peak detection is time-consuming and the test time is long, which is not conducive to daily diagnosis. Since the amplitude result of quasi-peak detection test is always less than or equal to the test result of peak detection, peak detection can quickly find EMI problems when performing EMI diagnosis.

Engineers’ “pain points” in EMI testing:

1. EMI issues should be considered from the beginning of circuit board design, but due to funding constraints, EMI diagnostic equipment is often not fully equipped.

2. After the electronic product design is finalized, it undergoes EMC certification testing. The certification body issues a failure report, which only points out whether the radiated or conducted EMI fails. Although the interference frequency is given, the specific location or cause of the EMI failure in the electronic equipment is not indicated. EMI diagnosis needs to be performed by the customer themselves, which is time-consuming and costly.

3. Some equipment cannot be shielded due to environmental constraints, and the root cause of EMI needs to be found and solved from the design perspective. Spectrum analyzer is a basic test instrument for discovering EMI problems, but in some cases it is difficult to trace the root cause of EMI.

4. Some EMI problems can be solved by shielding. Although it can pass EMC certification, EMI affects the performance of the device itself and must be solved from the root, or the problem must be found and avoided. In this case, the spectrum analyzer has the same problem as 3.

5. As data rates increase, periodic burst EMI problems are becoming more and more common. The real root cause of the problem must be found through analysis of the EMI cycle, which requires modulation domain analysis.

6. Conducted EMI can be tracked with an oscilloscope and tested with a spectrum analyzer. Both instruments need to be used in combination.

Tektronix's unique mixed domain oscilloscope-based EMI test solution:

Solution: MDO4014-3 + near-field antenna + P6150 + N to SMA

Features:

a) Five-in-one, complete oscilloscope function can not only meet the needs of daily circuit debugging, but also track the root cause of EMI problems; the spectrum analyzer function can diagnose EMI problems at any time.

b) Unique cross-domain analysis function, which facilitates the analysis of the real root cause of EMI

c) High spectrum sensitivity and wide analysis bandwidth, suitable for EMI diagnostic testing

d) Can test the change of RF amplitude over time, which is convenient for analyzing the root cause of periodic EMI problems

e) P6150 probe (optional) can directly connect the circuit board power or ground wire to the RF input of the MDO spectrum analyzer to test EMI problems caused by power ripple or unreasonable ground wire.