Precautions when using common equipment for EMC testing

1. Antenna:

The working frequency band of EMC test is very wide. Because we do not know the number of scratches in advance, the detection must rely on various detection antennas to convert the field strength into voltage. Generally speaking, the EMC test antenna works near the site, and the test results are particularly sensitive to the test spacing. Since the ratio of electric field and magnetic field (wave impedance) is no longer a constant near the site, although these antennas have proposed correction coefficients for electric field and magnetic field, it is only reasonable when these antennas conduct far-field detection. When measuring the near-field effect, the electric and magnetic field detection results cannot be calculated according to this method.

2. Artificial power network

Pay attention to the excellent connection between the artificial power network and the grounding plate with low impedance.

3. Spectrum analyzer

According to the test requirements, attention should be paid to the selection of appropriate spectrum analyzers such as frequency resolution, sensitivity, dynamic range and total sweep width.

Frequency resolution: the minimum spectral line spacing that can be recognized, which shows that the spectrum analyzer can put the frequencies together to separate the signals.

Sensitivity: shows the level of fine signal measured by the spectrum analyzer. The noise level of the spectrum analyzer depends on the bandwidth of the high-frequency filter, generally between - 80dB and - 130dB.

Dynamic range: display the real frequency band capacity of large signal and small signal. The upper limit of dynamic range is determined by the nonlinear distortion of the spectrum analyzer. The dynamic range of spectrum analyzer is generally above 60dB, even 100dB.

Total sweep width: the working frequency band marked by the spectrum analyzer in the primary measurement and analysis link.

4. EMI measurement receiver

The spectrum analyzer only has peak detection. In addition to peak detection, EMI measurement receiver also has quasi-peak detection and average detection. Appropriate detectors shall be selected according to different specifications or regulations.

Peak detection only detects the highest value of deflection signal amplitude, and is applicable to detecting single pulse or pulse deflection signal with low repetition frequency. With the increase of repetition frequency, the objective influence of some deflection signals on the sound frequency increases. For this signal, the amplitude of the measured signal cannot objectively reflect its effect. Therefore, it is necessary to use quasi-peak detection wave. The charging time constant of quasi-peak detector is larger than that of peak detector, and the discharge time constant is smaller than that of peak detector. It can not only measure the signal amplitude of the deflection signal, but also reflect its time distribution.