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Radiation disturbance field test

Release time:

2022-09-20 00:00

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　　Shielded rooms and anechoic chambers are necessary test sites for EMC tests. Due to factors such as the size, structure complexity, and operating conditions of the equipment under test, or the weight exceeds the load-bearing capacity of the shielded room and anechoic chamber, or requires water, electricity, wind, etc. Driven by external forces, it is impossible to conduct normal tests in a closed laboratory. At this time, the EMC performance of the equipment under test can only be evaluated by on-site measurement.

　　The so-called on-site test, also known as On-Site test, refers to moving the EMC test instrument to the site where the product is running for testing. Generally speaking, due to the mutual coupling between the electrical structure in the field environment (which more or less affects the electromagnetic field of the environment), the measurement antenna, and the equipment under test, the field measurement results of the same type of EUT in different places are different from those measured in the standard test field. The results are different, so field measurements are not used for type tests.

　　The following is a brief introduction to the on-site test-radiation disturbance.

　　1 Applicability type

　　On-site testing is generally applicable to industrial, scientific and medical equipment that are not used in households and are not directly connected to residential low-voltage power supply grid facilities, and that do not intentionally generate or use radio frequency energy.

　　2 Test environment

　　On-site measurement should pay attention to the influence of site environment to ensure the normal function of EUT. The essential parameters of the physical environment, such as temperature and humidity, should be specified.

　　The field radiation disturbance test must consider the electromagnetic environment to ensure the accuracy of the radiation disturbance measurement. When the EUT is not powered on, the electromagnetic environment noise level is at least 6dB lower than the specified limit for ease of measurement. However, this requirement is not always achievable on all frequencies. As long as the measured ambient level plus the radio noise emission of the EUT still does not exceed the limit, it means that the EUT complies with the limit.

　　If the requirement that the ambient level is 6dB lower than the limit cannot be met, the following alternative methods can be used:

　　1) The antenna can be placed closer to the device under test, but not less than 3m for measurement. The results are then extrapolated to the specified limit distance.

　　2) When the broadcasting station is off the air and the environmental level of industrial equipment is low, the measurement is carried out in the frequency band where the original environmental level exceeds the specified frequency band.

　　3) Analyze the direction of the strong environmental signal, and distinguish the direction of the receiving antenna from the environmental signal as much as possible.

　　4) For EUT narrowband disturbances that occur near radio frequency signals, when both fall within the standard frequency band, a narrower bandwidth may be used for measurement.

　　3 Arrangement of EUT

　　The arrangement of the equipment under test shall meet the following two conditions:

　　1) The EUT and the components connected to the EUT and the connecting cables shall be arranged according to the typical application;

　　2) The equipment (system) under test shall be arranged in the way that produces the greatest disturbance.

　　Interface cables shall be connected to each port of the EUT. The extra-long part of the cable is tied into an S shape with a 30cm~40cm line segment. If the cables are thick or stiff, or cannot be handled as such, this shall be stated in the test report.

　　The grounding of the EUT should be handled according to the actual working conditions of the EUT. If there is a grounding terminal, the grounding wire or grounding point should be connected to the grounding plane (or as a facility for the earth). If the EUT is not grounded during operation, it is not grounded during the test.

　　4 Test site

　　The field test is similar to the open field but without any metal ground plane. The test should be performed around the EUT at approximately the same measurement distance to determine the direction in which the maximum disturbance field strength is generated. As many measurement points as possible should be selected in the actual situation, at least should be measured in four orthogonal directions, the maximum disturbance field strength will be generated by different frequencies in different directions, so the test should be carried out separately.

　　The antenna measures the maximum disturbance field strength under horizontal polarization and vertical polarization respectively.

　　5 Measuring distance

　　The 10m distance is preferred for on-site testing, because the estimated disturbance level at this distance will be much higher than the general environmental level that allows the test. Usually do not use less than 3m or greater than 30m distance. The measurement distance is the distance between the point of the EUT closest to the antenna and the projection of the midpoint of the antenna on the ground. Or it is measured from the radiation center of the antenna to the EUT. Both methods can be used when the measuring distance is 10m.

　　If it is necessary to use measuring distances other than those specified, the measurement results shall be extrapolated using the methods specified in the product EMC standard.

　　6 Antenna height changes

　　For radiation disturbance measurement, the height of the antenna from the ground should be changed within the specified range, so as to obtain the maximum reading when the direct wave and the reflected wave are in the same phase. As a general rule, for measurement distances less than and equal to 10m, it is best to vary the antenna height between 1m and 4m.

　　In order to obtain the maximum reading, the minimum height of the antenna above the ground may be lowered to about 1m. These height scans are suitable for horizontal polarization and vertical polarization, but for vertical polarization, the minimum height should be adjusted to the lowest point of the antenna. At least 25cm away from the ground of the venue.

　　7 Disturbance measurements in the presence of ambient emissions

　　If the ambient emission is high, the following measurement procedures can be referred to when measuring on site.

　　1) Measurement method of EUT in narrowband environment emission:

　　- the spectrum whose analysis bandwidth is narrower than the detector bandwidth of the CISPR measurement receiver;

　　- determine the appropriate measurement bandwidth to select band disturbances emitted close to the environment;

　　- use of peak detectors (if the disturbance is amplitude-modulated or pulse-modulated) or mean-value detectors;

　　- Assuming that the narrowband disturbance is emitted in a relatively wide-band environment, and a narrower measurement bandwidth is used at this time, the ratio of the EUT disturbance to the environmental emission increases;

　　- If the EUT disturbance and environmental emission cannot be separated, they should be treated as overlapping.

　　2) Measurement method of EUT disturbance in broadband environment emission:

　　- Spectrum with an analysis bandwidth equal to the detector bandwidth of the CISPR measurement receiver;

　　- use narrow bandwidth for measurement (assuming EUT narrowband disturbance, narrow bandwidth application will increase the ratio of EUT disturbance to environmental emission);

　　- measurement of EUT narrow-band disturbances with an average value detector;

　　- If the EUT disturbance cannot be separated from the environmental emission, it should be treated as spectrum overlap.

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