Testing Conducted Immunity According to IEC-61000-4-6

First Posted: Aug 21, 2020 02:14 PM EDT
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Conducted immunity testing simulates conducted radio frequency (RF) disturbances to which a device could potentially be exposed during normal usage within its intended operating environment. When bundled together, noise can be inducted onto cables by both capacitive and inductive coupling. The object of IEC-61000-4-6 is to establish a common reference for evaluating the functional immunity of electrical and electronic equipment when subjected to conducted disturbances by RF noise. Some typical sources of RF emissions are transmitting radio systems (i.e.: radios, television, mobile phones, etc.) and other power electronics (i.e.: power converters and motor drivers). 

The effects of conducted disturbing signals, induced by electromagnetic energy can be tested by injecting the signal via Coupling/Decoupling Networks (CDNs) to the cabling. All cables connected to the Equipment Under Test (EUT) shall be supported at a height of at least 30mm above the reference ground.

The IEC 61000-4-6 EMC standard is about immunity testing against conducted disturbances induced by radio-frequency energy in the frequency range of 150kHz to 80 or 230 MHz, depending on the EUT (9kHz to 150 kHz test range is covered by IEC 61000-4-16). Also, equipment that does not have an external cable (i.e.: mains supply, signal line or earth connection) which can couple the disturbing RF fields to the equipment is excluded from the IEC-61000-4-6 testing.

The IEC 61000-4-6 test methods subject the equipment under test (EUT) to a source of disturbance comprising electric and magnetic fields. The disturbing signals are typically applied to one cable at a time, while keeping all the other cables undisturbed. While this method is effective, it can only approximate a real-world situation as, in a real-world setting, the disturbing sources would affect all cables simultaneously, with a broader range of amplitudes and phases. 

Test levels must be selected according to product-specific, product family standards or generic standards that define levels based on the electromagnetic environment in which the equipment is intended to be used

The Three Testing Methods

The standard allows for three methods for injecting the stress: (1) EM-Clamp, (2) Current Injection Probe and (3) CDN (including direct injection). Be sure to weigh the advantages and disadvantages when deciding which test to use on your equipment. While it is assumed that the results from each test will be equivalent, the CDN method is preferred for reasons that will be discussed later in this article.

(1) Electromagnetic Clamp (EM Clamp)

The EM-clamp is a clamping device that subjects the cable under test to both capacitive and inductive coupling of the RF stress. It was invented specifically for this test.

Advantages of the EM-Clamp Method:

  • Its principle advantage is that it is entirely non-invasive. No connection needs to be made to the cable under test.

  • It allows for adequate decoupling of the AE at high frequencies. The design is arranged so that the capacitive and inductive coupling paths reinforce one another at the EUT end and cancel at the AE end.

  • It is relatively power efficient, although not quite as much as a CDN.

Disadvantages of the EM-Clamp Method:

  • It is quite long, and to provide good capacitive coupling it has a relatively narrow inside diameter. This makes it bulky to use and restricts its application for short or large diameter cables.

  • Below 10MHz, its directivity is negligible, so the AE low frequency common mode impedance is not decoupled. 

  • It does not provide an accurate source impedance of 150 ohms across the frequency range.

Equipment Needed:

  • Broadband RF Amplifier

  • RF Signal Generator

  • Spectrum Analyzer(s) or other measuring instrument(s)

  • EM Clamp

  • Directional Couplers (optional)

  • Attenuators

  • Cables

(2) Bulk Current Injection Probe (BCI)

The bulk current injection probe (BCI probe) was not originally going to be included as part of the standard but was added later as there were already labs using it for military testing. The BCI probe acts as a current transformer who's secondary is the cable under test. The BCI probe provides inductive injection only.

Advantages of the Current Injection Probe:

  • It is non-invasive and somewhat convenient to use. It is relatively compact and can be made with quite a wide aperture, so it can be used with virtually any cable, no matter the length or thickness. 

Disadvantages of the Current Injection Probe:

  • There is no decoupling of the AE, therefore the current induced must flow into both the EUT and AE, meaning that the AE is being tested just as much as the EUT.

  • The applied stress is dependent on the cable layout and AE impedance. The current flowing into the EUT is determined by the impedance of the cable, which acts as a transmission line at high frequencies, so it may have standing waves due to mismatches in the impedance to the reference plane. This all means that this method has the highest uncertainty, and the least repeatability of all of the methods.

  • It has a high-power requirement and can be lossy. It needs to have a higher internal turn ratio to be effective, meaning that it will need more power.

Equipment Needed:

  • Broadband RF Amplifier

  • RF Signal Generator

  • Spectrum Analyzer(s) or other measuring instrument(s)

  • Current Injection Probes

  • Directional Couplers (optional)

  • Attenuators

  • Cables

(3) CDN

The Coupling/Decoupling Network is designed to couple the disturbance signal directly to the conductors or shield of the EUT cable under test, while at the same time impeding the path towards the auxiliary equipment (AE). It also provides a fixed common mode source impedance at its EUT port. 

Advantages of the CDN Method:

  • The two main advantages are the decoupling of the AE and the low uncertainty of the applied stress.

  • Minimal power is required and there is minimal radiation or environmental influence.

  • The common-mode impedance damps cable resonance, which increases the repeatability of the test and approximates very closely to real-life situations.

Disadvantages of the CDN Method:

  • It requires an electrical connection to the cable shield if there is one, or to each conductor of an unshielded cable. As a result, different networks are required for different cable types, increasing the capital investment required for general test house use.

  • Serious errors may result if an ordinary CDN is used with fewer than its intended number of wires (such as using an M3 CDN for a mains port with only live and neutral)

Equipment Needed:

  • Broadband RF Amplifier

  • RF Signal Generator

  • Spectrum Analyzer(s) or other measuring instrument(s) 

  • CDN

  • Directional Couplers (optional)

  • Attenuators

  • Cables

  • Computer Based System

There are different circumstances where each test would be the most convenient method, depending on the equipment and environment of each test. While the results from these three tests should all be equivalent, the CDN method is preferred by the IEC, as it protects the AE the most and also allows for better repeatability. There are numerous CDN types, as most CDNs are not versatile, and there are many cable types that need to be tested. The following are the different series of CDNs, and within each series there is a broad product range of models:

  • M Series: Use for unscreened AC or DC power supply lines. 

  • AF Series: Used for unscreened, unbalanced lines that are carrying low current.

  • S Series: Used for shielded cables with single or multiple conductors.

  • T Series: Used for cables with unshielded balanced conductor pairs. 

Solutions

Coupling/Decoupling Networks (CDNs): The preferred method of testing according to IEC 61000-4-6, coupling/decoupling networks are the most reliable and repeatable of the testing methods reviewed, the most power-efficient, while also providing the most protection for the AE.

Bulk Current Injection Probe (BCI): Versatility and size make this method of testing appealing, while its high usage of power and inability to decouple the AE are its drawbacks.

Electromagnetic Clamp (EM Clamp): This non-invasive method of testing is somewhat power-efficient, but its size can be hard to manage, and its inability to decouple the AE at lower frequencies can be problematic.

Immunity Systems: Some EMC companies, such as Com-Power Corporation, offer test equipment packages, making sure that all the needed equipment is included. Often, these packages can be customized for each of the three methods of testing.

Software: Certain EMC retail companies offer software that, when used with compatible hardware, can automate calibration and perform testing.

Conclusion

While there are three methods to test conducted immunity according to the IEC 61000-4-6 standard, and there are instances where each method is useful, the CDN method is preferred. Because of its accuracy and efficiency, the CDN method should be used whenever possible, given the circumstances of the test that needs to be conducted.

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