Column: EMC
the probe is configured to measure the common-mode current, with a distance of about 10 centimetres between the current probe and the device under test. Because of the standing waves on the cable, it may be necessary to move the current probe along the cable to capture the maximum RF current. The RF current probe is connected to
a spectrum analyser with a frequency range between 10MHz and 300MHz. This setup measures the RF current in the radiated emission frequency range, giving an indication of the unit’s far-field performance. As shown in Figure 1, emissions peak above 40dBμV in certain frequency ranges, enough to potentially cause a class B (residential) emissions failure.
Note that the reading on the spectrum
analyser is always in dBμV. With knowledge of the probe’s transfer impedance (in this case, 20dBΩ), the RF current level in dBμA can be obtained by subtracting dBΩ from dBμV. Terefore, an RF current level above 20dBμA on a cable generally raises warning signs.
Case 2 Figure 2 shows our second case study, where an RF current probe is connected to the 50Ω input of an oscilloscope. Channel 2 of the oscilloscope (also configured with 50Ω impedance) is connected to a near- field magnetic field loop. In this setup, the oscilloscope is triggered by Channel 1 as the RF current probe on the mains cable picks up the common mode current. Te near-field magnetic field probe hovers above
the PCB to pick up the “ringing” during the converter switching stage. Te ringing of the inductor shows a profile similar to the RF current measured on the mains cable, which helps identify the noise source on the PCB. In this case, the converter has conducted emission failures between 10MHz and 30MHz. By using this method, we can quickly locate the noise source and come up with solutions to fix the problem.
Useful insights With a characterised RF current probe, engineers can gain valuable insight into a product’s EMC performance. Te RF current probe is useful in troubleshooting EMI problems and can be of great help to engineers in the product development stage.
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