Introduction:

When measuring the common mode current conduction emission of EUT cables, it is important to ensure the repeatability of EMC-CE measurements. If the test environments and conditions in Laboratory A and Laboratory B are completely different, the two laboratories will get different common mode current measurements. Here's how to ensure the repeatability of common mode current measurements on cables.

Other impacts to be prioritized:

We've already learned that in another article:

How to improve signal-to-noise ratio when measuring with FCC current probes

How to improve the signal-to-noise ratio of a spectrum analyzer

Both of the above were written around the common mode current conduction emission test, theFCC Current Probe+ Spectrum analyzer + EUT is the component of the whole common mode current test system, because the current probe and spectrum analyzer are test instruments, while the EUT is the sample under test, so we should prioritize the first two to solve the problem of excessive signal-to-noise ratio, and when the test instruments in the two laboratories can maintain the same conditions, we will finally consider the impact of the EUT on the repeatability of the whole test.

Basic common mode current measurement procedure:

Before explaining the effects of EUT, it is also important to understand the test procedure for common-mode current conduction emission, for exampleCISPR25The current method of testing,CISPR32of the current probe test method, they can all be tested using the following test method:

  1. Connect the spectrum analyzer to the 50 ohm load, do not connect theFCC Current Probe F-52Bcable with the sample under test. Then perform a null test and finally look at the displayed data from the spectrum analyzer, here you will get the background noise in the test environment. If you see spikes in the minimum background noise, they should be added to the spectrum analyzer's correction data.
Measuring background noise by connecting a spectrum analyzer to a 50-ohm load

Measuring background noise by connecting a spectrum analyzer to a 50-ohm load

  1. If the background noise during null measurements is significant, such as exceeding the CISPR25 limit line, you should return to the process of prioritizing the signal-to-noise ratio of the FCC current probe and spectrum analyzer.
  2. When the above background noise complies with CISPR25 Current Method Level 5 and has a dynamic range of 20dB, connect the FCC current probe to the spectrum analyzer with a 50Ω coaxial cable, do not connect the DUT, and repeat Method 1 to check the background noise, and once again, let the noise at the bottom of the spectrum analyzer be more than 20dB below the CISPR25 Current Method Level 5. Observe if the background noise is the same for both tests? If it does not match, you need to look for the problem again.
Connecting a Spectrum Analyzer to an FCC Current Probe to Measure Background Noise

Connecting a Spectrum Analyzer to an FCC Current Probe to Measure Background Noise

  1. When the above background noise complies with the CISPR25 current method Level 5 and has a dynamic range of 20 dB, a 50Ω coaxial cable is used to connect the FCC current probe and thespectrum analyzerConnect, this time to the DUT, but neither the DUT EUT nor the Auxiliary Equipment AE is energized and working! Then repeat method 1, looking at the background noise and again getting the bottom noise of the spectrum analyzer to be more than 20dB below CISPR25 Current Method Level 5. Notice if the background noise for this test is the same as the previous two?
EUT and AE unpowered to measure background noise

EUT and AE unpowered to measure background noise

  1. Finally, after completing the above process and all tests see the same background noise, we can connect the spectrum analyzer, the FCC current probe, and then power up and operate both the EUT and the AE properly to perform the conduction emission common-mode current test process.
Measurement of interconnect cable common-mode currents between the EUT and AE in the energized state using an FCC current probe

Measurement of interconnect cable common-mode currents between the EUT and AE in the energized state using an FCC current probe

In short, to ensure that the background noise is lower than the EMC test standards in the conducted current method test limit of 20dB or more, so that we can make the DUT's own conducted interference signal in the spectrum analyzer correctly displayed.