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Why is it necessary to measure bottom noise before formal testing of radiation emission and conduction emission?
Author.
LCS
Source:
Post time:
2025-07-21
In electromagnetic compatibility testing, measuring background noise is a key step in ensuring the accuracy of test results. Elaborate on the necessity of background noise testing from both radiation emission and conduction emission perspectives.
The necessity of bottom noise testing in radiation emission testing.
The purpose of radiation emission testing to avoid environmental noise interference is to measure the electromagnetic disturbance field strength propagated through space by the device under test (EUT). If there is high background noise in the testing environment (such as radiation from other devices, radio signals, etc.), it may overlap with the radiation signal of the EUT, causing the measured values to be falsely high or even misjudged as unqualified.
The CISPR standard requires that the environmental noise at the testing site be at least 6dB below the limit to ensure accurate identification of EUT radiation signals. For example, the background noise in open areas or anechoic chambers needs to be verified to ensure that it does not affect the test results.
According to electromagnetic compatibility theory, the requirement for signal-to-noise ratio is that only when the measured signal strength is 6dB higher than the background noise, can it be confirmed that the signal comes from EUT rather than environmental interference. This is because 6dB corresponds to twice the difference in power, which can effectively distinguish between real signals and noise. For example, if the limit is 40dB μ V/m, the environmental noise should be ≤ 34dB μ V/m.
The requirement of the testing method is that in radiation emission testing, the polarization direction, height, and turntable angle of the antenna need to be continuously adjusted to capture the maximum radiation value. If the background noise is too high, it may mask the true radiation characteristics of EUT, resulting in the inability to effectively locate the interference source.
The necessity of bottom noise testing in conducted emission testing.
Conducted emission testing mainly measures the interference signals transmitted through wires such as power lines and signal lines. The propagation path of this type of noise is relatively fixed, but it is still necessary to ensure that the background noise of the testing system (such as artificial power networks, measurement receivers) does not affect the results during testing.
The CISPR standard requires that the conduction test be conducted inside a shielded room, and the background noise of the system must be verified before measurement (such as when disconnecting the EUT) to ensure that it is 6dB below the limit.
Conducted disturbances may come from the EUT itself or testing equipment (such as parasitic interference from power networks). By conducting bottom noise testing, it can be confirmed whether the measurement results are caused by the true conduction emission of EUT. For example, in the disturbance voltage test at the power supply end, the noise measured after disconnecting the EUT is the system bottom noise and must meet the requirements.
The impact of test configuration, cable layout, grounding methods, and other factors in conducting tests may introduce additional noise. For example, the length and folding method of the power cord need to comply with standards to reduce the impact of background noise on the measurement.
Radiation emission test: Bottom noise test is a mandatory requirement that directly affects the accuracy of the results. It is necessary to ensure that the environmental noise is below the limit value by 6dB
Conducted emission testing: It is also necessary to control the background noise, but the focus is on testing the background noise of the system itself, rather than spatial environmental noise.
Technical basis: The signal-to-noise ratio principle (6dB difference) and standardized site verification process are the core basis for bottom noise testing.
Through strict noise control, EMC testing can effectively distinguish between the true emission of equipment and external interference, ensuring the reliability and compliance of test results.
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