Determining the source of radiation using near-field testing methods

In addition to the above-mentioned method of determining the interference source based on signal characteristics, the interference source can be directly found by searching for the radiation source in the near field area. The tools for searching the radiation source in the near field area include near-field probes and current calipers. Use current calipers to check the emission source on the cable, and use near-field probes to check the leakage of the chassis gap.

1 Current calipers and near-field probes

　　The current probe is a sensor that can detect the current on the wire, which is made based on the transformer principle. When the current probe is stuck on the measured wire, the wire is equivalent to the primary of the transformer, and the coil in the probe is equivalent to the secondary of the transformer. The signal current on the wire induces current in the coil of the current probe, and generates voltage at the input end of the instrument. Then the spectrum of the interference signal can be seen on the screen of the spectrum analyzer. The voltage value read on the instrument and the current value in the wire are converted through the transmission impedance. The transmission impedance is defined as: the ratio of the voltage induced on the 50? input impedance of the instrument to the current in the wire. For a specific probe, its transfer impedance ZT can be found in the probe manual provided by the manufacturer. Therefore, the current in the wire is equal to:

I = V / ZT

　　If all physical quantities in the formula are expressed in dB, they are directly subtracted.
　　For the leakage of the chassis, a near-field probe should be used for detection. The near-field probe can be regarded as a very small loop antenna. Because it is very small, its sensitivity is very low and it can only detect the radiation source in the near field. This is conducive to the precise positioning of the radiation source. Since the sensitivity of the near-field probe is low, it should be used in conjunction with a preamplifier when used.

2 Use current clamps to detect common-mode current

　　One of the main reasons for the radiation generated by the device is the common-mode current on the cable. Therefore, when the device or system has excessive emissions, the first thing to suspect is the various cables dragged outside the device. These cables include power cables and interconnect cables between devices.
　　Taishi clamps the current probe on the cable. At this time, because the probe clamps the signal line and the return line at the same time, the differential mode current will not induce voltage, and the voltage read on the instrument only represents the common mode current.
　　When measuring the common mode current, it is best to do it in a shielded room. If it is not in a shielded room, the electromagnetic field in the surrounding environment will induce current on the cable, causing misjudgment. Therefore, the power supply of the device should be disconnected first, and the background current on the cable should be measured when the device is not powered on, and recorded so as to compare with the measurement results after the device is powered on to eliminate the influence of the background.
　　If the scanning frequency of the spectrum analyzer is limited to a small range around the frequency of interest when measuring with an antenna, the interference in the environment can be eliminated.

3 Use a near-field probe to detect leakage in the chassis

　　If there is no strong common-mode current on the external cable of the device, it is necessary to check whether there is electromagnetic leakage on the device chassis. The tool for checking chassis leakage is a near-field probe. Place the near-field probe close to the seams and openings on the chassis and observe whether there is a signal of interest on the spectrum analyzer. Generally, due to the low sensitivity of the probe, even if an amplifier is used, the voltage induced in the probe by a very weak signal is also very low, so the sensitivity of the spectrum analyzer should be adjusted as high as possible during measurement. According to the previous discussion, reducing the resolution bandwidth of the spectrum analyzer can improve the sensitivity of the instrument. However, it should be noted that when the resolution bandwidth is very narrow, the scanning time will become very long. In order to shorten the scanning time and improve the detection efficiency, the scanning frequency range of the spectrum analyzer should be as small as possible. Therefore, when using a near-field probe to detect chassis leakage, the antenna is first used to measure the precise frequency of the leakage signal, and then the instrument is made to cover the interference frequency with the smallest possible scanning frequency range. Another advantage of doing this is that background interference will not be misjudged as a leakage signal.
　　For the chassis, the gap near the filter installation position is the most likely to cause electromagnetic leakage. Because the filter bypasses the interference signal on the signal line to the chassis, forming a strong interference current on the chassis, when these currents flow through the gap, electromagnetic leakage will occur at the gap.