Testing Laboratories
Why It Is Important To Control and Measure CDN Impedance Per IEC 61000-4-6
By Don MacArthur, MacArthur Compliance Services, LLC
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EC 61000-4-6 is titled “Electromagnetic compatibility (EMC) – Part 4-6: Testing and measurement techniques – Immunity to conducted disturbances induced by radio-frequency fields.” This “conducted RF immunity” standard covers the conducted immunity requirements of electrical and electronic equipment to electromagnetic disturbances from intended radio-frequency (RF) transmitters in the frequency range 150 kHz up to 80 MHz.

Note: In Europe, IEC standards that become equivalent European Norm (EN) standards are designated with an ‘EN’ instead of an ‘IEC’ prefix. In most cases, there is no difference between IEC and EN standards.
Coupling/Decoupling Networks
A major component of conducted RF immunity testing per 61000-4-6 is the coupling/decoupling networks (CDNs). There are various types of CDNs used, the configuration of which depends on the circuit under test (signal, communications, input/outputs, Ethernet, etc.).

As one can imagine, the coupling (C) part of the CDN is used to couple energy into the equipment under test (EUT). In contrast, the decoupling (D) part of the CDN is used to prevent unwanted injected test signals from disrupting any auxiliary equipment used to provide functional signals to the EUT.

CDN Impedance
The impedance (Z) that the CDN contributes to the overall path that RF current flows as part of the leveling process is tightly controlled in the standard.

From 150 kHz – 26 MHz, the standard requires Z equal 150 Ω ± 20 Ω.

From 26 MHz – 80 MHz, the standard requires Z equal 150 Ω + 60 Ω, -45 Ω.

Why does the impedance of a CDN change value?
Test laboratories are busy environments where it is possible to easily damage a CDN in such a manner that its Z changes without anyone ever knowing. Failures could result from dropping, applying too much current, over-voltage conditions, spikes and transients, surges, or other similar events.
If CDN impedance changes, why is it not recognized?
The reason why changes to CDN-Z are not recognized by laboratory personnel is that the level-setting procedure results in the correct voltage regardless of CDN-Z. Laboratory personnel might be looking for 10 V, and no further analysis is conducted when the level-setting procedure obtains this result. If the level-setting procedure is only performed once per year, any changes to CDN-Z occurring after the procedure will only be recognized the next time the level-setting procedure is attempted.
metal box with wires
Why does CDN impedance manner?
CDN-Z matters because tight control of CDN-Z means fairness in testing. If CDN-Z is wrong, the test facility could either be over-testing (inadvertently applying too much signal) or under-testing (inadvertently not applying enough signal). Over-testing could result in failing a product that should have passed, and under-testing passing a product that should have failed. In addition, CDN-Z matters in situations where an EUT will be tested at several different laboratories. It is important the results are repeatable amongst these various laboratories.

Reference 1 contains two sets of example Spice simulations that show how a change in CDN-Z from 150 Ω to 300 Ω, using an identical EUT load of 1000 Ω while applying the same laboratory calibrated 10V test level, resulted in an increase in current applied to the load of 7.8 mA to 8.4 mA. This change resulted in 7.8 V and 8.4 V developed across the EUT load, respectively. This is a 7 % increase in current applied from what should have been the standard level and could make the difference between a pass or fail of the EUT.

What can be done to ensure CDN impedance is correct?
The first step in maintaining correct CDN Impedance is the training of staff.
Ensure they know the importance of maintaining the correct CDN impedance and reporting any misuse or abuse they witness. Have any suspected damage to CDNs investigated immediately and before using again.

The second step in maintaining correct CDN impedance is to have the Z of all your CDNs periodically calibrated at an ISO17025 accreditation test facility (or alternatively, learn how to do it yourself).

You may end up saving you and your customer many hours in lost revenue, re-work time, and delays.
Conclusion
This article briefly described CDN impedance requirements of IEC 61000-4-6 for conducting RF immunity testing and why maintaining the correct CDN impedance is important. Additionally, it provided methods for ensuring the correct CDN impedance is maintained.
References and Further Reading
  1. “Why is it important to control and measure CDN impedance for EN61000-4-6 testing?” The Equipment Calibration Business.
  2. IEC 61000-4-6:2013. Electromagnetic compatibility (EMC) – Part 4-6: Testing and measurement techniques – Immunity to conducted disturbances, induced by radio-frequency fields.
    Available from https://webstore.iec.ch/publication/4224
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