Electromagnetic compatibility standards strictly specify measuring devices and measurement layouts

EMC certification has begun in China. The State Administration for Exit Inspection and Quarantine has taken the lead in conducting mandatory electromagnetic compatibility testing on six imported commodities in the imported commodity catalog that implement the safety and quality licensing system, including personal computers, monitors, printers, switching power supplies, televisions, and audio equipment. The pilot work for domestic electromagnetic compatibility certification has been completed. However, due to the late start of electromagnetic compatibility experiments in China, the measurement equipment and equipment required to establish electromagnetic compatibility laboratories are expensive, and electromagnetic compatibility measurement is not popular in China's electronic product research and production enterprises. There is still a large gap between electromagnetic compatibility measurement equipment and technology and foreign countries.

Due to various complex factors, the consistency of EMC measurement results is very poor compared to EMC standard measurements, and it is difficult for different laboratories to fully agree, including frequency points and measured values. Inaccurate measurement results can incorrectly guide product development and may lead to unnecessary economic losses. Inconsistent measurement results can lead to confusion about EMC for enterprises. If the accuracy and consistency of measurement results cannot be guaranteed, it will affect the smooth development of EMC certification.

The accuracy and consistency of EMC measurement results mainly depend on the accuracy of measurement instruments and the management level of the laboratory. The accuracy of measurement instruments must be ensured through measurements by professional manufacturers, as our measurement departments have not fully met the requirements for GB/T electromagnetic compatibility measurement instruments to measure in accordance with the provisions of 6113-1995. We have also ignored the measurement of the measurement environment, such as the electromagnetic environment, power supply environment, grounding, etc. at the measurement site. Therefore, relying on instrument measurements to ensure the accuracy and consistency of EMC measurement results is not reliable, and only improving and maintaining the management level of EMC laboratories is the key. The following is how to ensure the accuracy and consistency of EMC measurements from the perspective of laboratory management.

1.Verification of instruments, equipment and accessories

In order to make up for deficiencies in instrument measurement, the laboratory must also regularly inspect the instrument to confirm that its performance is consistent with the previous one. The operation manual for electromagnetic compatibility measuring instruments generally contains maintenance content. Select regular inspections that are closely related to daily measurement items. The following is the maintenance content of the measurement receiver for reference.

Due to the wide frequency range of electromagnetic compatibility measurement, the value of disturbance measurement is small, and the value of immunity measurement is large. The connection accessories and transmission path of the instrument and equipment also have a significant impact on the measurement results. The laboratory must also regularly check the shielding effect of instrument connection accessories and shielded cables. The operation method can refer to the shielding effect measurement method of immunity measurement. The shielding effect of the cable should be greater than 40dB. The above verification results should be recorded and archived.

2. Standardize test conditions and procedures

The measurement conditions include the placement of the equipment under test, useful signals from standard operating conditions, and stable power supply voltages. The laboratory should take effective measures to ensure standardization of measurement conditions, such as the working state of the equipment under test, which cannot ensure the consistency of measurement results.

Place the equipment under test at a fixed point, use a regulated power supply, and calibrate useful signals with a spectrum analyzer. If a useful signal is provided through a radiation field, the distance and angle between the receiving antenna and the transmitting antenna will have a significant impact on the quality of the useful signal and the reception effect of the device under test. Therefore, the receiving antenna and the transmitting antenna should be fixed in parallel on an 80 cm non-metallic bracket to ensure the consistency of useful signals. At the same time, the working state of the measuring equipment must also be set in accordance with standard requirements, such as measuring TV power supply disturbances, graphic signals, brightness, and volume that can affect the power load current, changing the oscillation frequency or duty ratio of the TV switching power supply with the power load current, and injecting the TV into the public power network.

The measurement steps include the setup of the instrument, the connection between the instrument and the equipment under test, the connection between the measuring instruments, and the operation process. In the GB 13837-1997 standard, the setting of the instrument is just a piece of paper. GB6113-1995 is too complex and more suitable for eye group management. In fact, the correctness of instrument settings directly affects the accuracy of measurement results. The laboratory must clearly understand the settings and steps related to continuous listening and instrument use based on the requirements of GB6113-1995, and take effective measures to ensure that there are no errors and leaks, such as using a computer for program control settings. The connection and placement of instruments and sympathy equipment are strictly regulated in measurement standards, and laboratories require spoons to take effective measures to ensure that these requirements are met.

For example, when measuring the disturbance voltage injected into the power supply terminal, "the power line between the equipment under test and the ground artificial power network should be as short as possible, and the 0.8-meter portion of the power line should be folded parallel to the power line into a bundle of 03-0.4 meters.". The outlet of the power cord of the device under test is vertically arranged with the power socket of the artificial power network. The short path is relatively easy to fix. It's not easy to manually fold the binding cord and attach a monster to each piece of ice. The laboratory should produce a non-metallic clamp for folding wires, so that each folding harness can be fixed at a length of 0.3 meters, which is not easy to bind. Through careful work, using forced means to fix each step may be inconsistent.

3. Verification test system

Generally speaking, many instruments have their own self checking and calibration functions, and must be regularly measured. However, most electromagnetic compatibility measuring instruments need to form a special measurement system with other instruments or accessories. Any link or matching problem in the system can affect the performance of the entire measurement system. Currently, there is no way to measure a complete measurement system, so only the measurement of the instrument itself can effectively ensure the accuracy and consistency of EMC measurement results. The laboratory must develop a set of self inspection methods for measuring systems that fully comply with actual operating conditions.

The ideal method is to have a standard disturbance source and sensitive device used as a measurement device to validate the measurement system. However, making such harassment sources and sensitive devices is very difficult, and it is difficult to meet unified standards. Even in the future, ordinary laboratories cannot afford it. The laboratory can select one or several representative and stable prototypes to seal as special prototypes for system verification. During the periodic verification process, the prototype is measured and then compared with previous measurement results to identify the cause of the inconsistency until it is consistent with previous measurement results. If the measurement instrument is validated at the beginning of installation and its accuracy has been promised by the equipment supplier, as long as the system validation work can be continued in the future, the accuracy and consistency of the electromagnetic compatibility measurement system can be maintained.

The electromagnetic compatibility standard strictly specifies the measurement device and measurement layout, and specifies the same measurement conditions and methods. The limits given in the standard are based on measurement conditions that fully comply with the standard. Only in this way can the results be meaningful and comparable. Strictly speaking, the measurement of electromagnetic compatibility should be referred to as an experiment. It uses the devices, conditions, and methods strictly specified in the standard to evaluate the electromagnetic compatibility performance of the prototype. The above cheap and practical methods can effectively ensure that the testing work fully meets the requirements of the standards, and most EMC laboratories can do so. As long as meticulous laboratory management is achieved, the accuracy and consistency of electromagnetic compatibility measurements can be effectively guaranteed.