At low frequencies (e.g. below 80MHz - although this will depend on the dimensions of the EUT), the EUT becomes a less efficient receiving antenna, and the connecting cables become the principal coupling route. To test for immunity at lower frequencies, therefore, conducted immunity testing is appropriate (whereas radiated immunity testing is more suitable at higher frequencies). Current injection testing also offers better repeatability, but radiated field testing is closer to the requirements of international Standards. At the Test Club, you can do both.
Testing for Conducted Immunity at the EMC Test Club is done using the Schaffner NSG 420 current injection tester, which is a portable battery-operated instrument (to avoid uncertainty that might arise from the position of a mains supply cable).
The NSG 420 generates a Ďcombí spectrum which generates energy at 39KHz intervals from a few hundred KHz to hundreds of MHz. Frequency modulation can be applied, which blurs out the 39KHz comb spectrum above approx 2MHz, helping to ensure energy is present at every frequency. Because the instrument generates a wide spectrum of HF interference, it is used within a screened chamber at the Club.
The NSG 420 drives the current through a ferrite coupling clamp, which comprises a two-part, hinged element. A section of the cable connected to the EUT (Equipment Under Test) is placed in the yoke of this current transformer, which is then closed and locked. The tester is switched on, and its power level increased until the EUT fails, or the desired level of immunity is demonstrated. The test is repeated on all the cables of the EUT.
The wide spectrum test signal shows up weak spots over itís whole frequency range, helping to show up vulnerable EUT cables and frequencies.