Conducted RF Immunity Testing - Overview, Methods, & Equipment

Conducted RF & Immunity Testing

Unlike radiated RF, conducted RF is focused on coupling and exposure via connected lines or cables. Both capacitive and inductive coupling methods have the potential to provide transmissions of noise onto associated cables providing a path towards the electronic or electrical devices._[1]

The levels of interference traveling down these conducted cabling paths will vary in strength in relation to the field that they are exposed to. The greater the electric or magnetic (E and H) fields _[2], the higher the level of RF noise that can be expected traveling along the associated cabling. 

Conducted RF Immunity Testing

Immunity testing to conducted RF noise or interface attempts to replicate the same environmental stress by using different injection and testing methods. The below image illustrates the concepts of the test using the substitution method with a coupling decoupling network (CDN) commonly done to IEC 61000-4-6.

Conducted RF immunity testing is a continuous EMC test commonly required for commercial, automotive, and military equipment or products. Testing requirements, frequency range, setup, as well as the required test equipment will vary by standard and test level.

The image to the left provides a reference for some of the most common requirements including their associated frequency ranges. While some automotive and military testing can span a much larger frequency range, 150 kHz to 230 MHz is the most common.

Our focus will be on IEC 61000-4-6 methods focusing heavily on the requirements and calibration process.

IEC 61000-4-6 Testing

The test methods and procedures discussed will be based around IEC 61000-4-6, which has been harmonized with the European EN 61000-4-6. The below image provided by the Teseq CDN selection guide, provides the rules for selecting the injection method. The injection method will determine what equipment is required as well as EUT/DUT considerations.

Test Levels - Substitution Method

The test level requirements for this standard, regardless of injection method are :

• Test Level 1 - 1V r.m.s.
• Test Level 2 - 3V r.m.s.
• Test Level 3 - 10V r.m.s.
• Test Level X - Special (unspecified)_[1]

IEC 61000-4-6 uses the substitution method for testing (with all injection devices) where the voltage level of the calibration data over the given frequency is applied to the EUT. The calibration setup for this standard specifies an 150Ohm source impedance and uses the associated levels to conduct testing. Given the large impact that the calibration has on testing, we will spend a considerable amount of time on it.

RF Signal Modulation

The two signal modulations used for conducted RF testing are amplitude modulation (AM) and amplitude modulation peak conservation  (AM PC). IEC 61000-4-6 uses the more common AM of 80% which can be seen below.

Calibration

Regardless of the injection method used, the calibration of setup is the first step. During this process the associated test levels and corresponding power levels are saved to be used later while conducting the test.

Application Notes/Guides

The below applications notes provide excellent information on how to conducted RF immunity testing to a variety of standards using different injection methods.

CDN Injection Method

Coupling Decoupling Networks (CDNs) are the preferred injection method of IEC 61000-4-6 as seen above from the selection rules. They provide repeatable, accurate results while ensuring auxiliary equipment is protected. CDNs also have the benefit of being the most efficient method for injection allowing lower power RF amplifiers to be used.

Types of CDNs

These networks are designed specifically for a particular port or cable type (power, communication, etc.). This has the potential to lead to multiple CDNs required for testing depending upon the test plan and ports tested. The most common CDN types include:

• CDN-S1 - Screened Cables
• CDN-M1/-M2/-M3 - Unscreened Supply (Mains) Lines
• CDN AF2 & AF8 - Unscreened Unbalanced Lines
• CDN T2, T4, & T8 - Unscreened Balanced Pairs

The vast number of coupling decoupling networks available relate to the variety of different cables and ports tested. Teseq, a manufacturer of EMC equipment, provides an excellent guide that goes into great detail on the variety of CDNs available. The guide can be accessed by clicking here.

CDN Setup & Calibration

The calibration setup for IEC 61000-4-6 using the CDN injection method can be seen to the left.

• 1) Conducted RF Test System (NSG 4070)
• 2) 100Ω Adapters
• 3) 6 dB Attenuator
• 4) Coupling Decoupling Network (CDN)
• 5) 50 Ω Termination Load

Direct Injection

This method requires a physical connection from the RF generator to the cabling of the device or equipment under test. This can involve some work as it is required to make manual connections including splitting the EUT cabling. This method when used for IEC 61000-4-6 also requires capacitors and resistors put in place between the connection.

Electromagnetic (EM) Clamp Injection Method

EM Clamps are typically used when CDNs aren't applicable as a more suitable option. The clamps themselves are made of ferrite as well as conductor and insulating plates providing a combination of inductive and capacitive coupling. The ferrite used in construction of the clamp must be in good condition, both on the top and bottom, for the equipment to be functional.

The EM Clamps provide an excellent option when longer EUT cable lengths are available as they're more efficient allowing for less power to be used.

It is important to consider the cable length available and thickness of the cables or bundles when using EM clamps. Most clamps have a length between 58 inchs and 70 inchs and can handle a max diameter cable or bundle of 20mm.

Typical EM Clamp Method Calibration Setup

• 1) Conducted RF Test System (NSG 4070)
• 2) CAL 801A 100Ω Adapters
• 3) 6 dB Attenuator
• 4) EM Test EM101 Clamp
• 5) 50 Ω Termination Load
• 6) Calibration Cable

Current Clamp Injection Method

The bulk current injection method is commonly required for many military and automotive applications and can use the substitution method for injection. The component that induces the associated RF onto the cable(s) is the current injection probe (CIP). These probes use split ferrite pieces with a clamping mechanism as way of injecting required current levels onto cables and associated connected devices.

A major benefit of using this method is having a sizable window or opening on the injection probe, typically 43 mm and a clamping feature. This is ideal for situations where RF testing is required on bundles of cables, where otherwise testing would be substantially more difficult. This is also an easier solution when cabling cannot be cut, because the clamp can be opened and closed around the wires/cables.

This method is measured in either mA or dBµA with a current monitoring probe during calibration processes. Typical frequency ranges start at around 4 kHz to 400 MHz with some automotive requirements going into the Gigahertz range. This test typically requires the most power to drive enough RF though the injection probe, often requiring external power amplifiers mentioned above.  

Conducted RF Test Systems

Conducted RF Equipment Rental Categories

Conducted RF Test Systems

Conducted RF systems provide many of the commonly required equipment in a single, easy to use system that allows for effective testing. These systems can be configured with different capabilities, it is important to ensure that system is capable of meeting associated testing requirements. The block diagram of the Teseq NSG 4070 shows the different components inside of the RF test equipment.  These systems typically include:

• RF Signal Generator
• Power Meters or Spectrum Analyzer
• RF Power Amplifier
• Interface & Software

RF Power Amplifier

The RF power amplifier, along with the other associated equipment, determines the test level and frequency limitations. The amplifiers used for BCI testing can go as high as 260 watts, with 110 Watts often needed for higher test levels in automotive and commercial requirements. The most common RF amplifiers include:

• Teseq CBA 400M-110
• Teseq CBA 400M-260
• AR 100A400AM20

At lower power levels amplifiers are often built into the test systems and at higher levels tend to be external. When using an external amplifier a dual directional coupler is needed for forward and reverse power measurements typically run through the RF system. When testing at higher levels it is crucial that proper attenuation is used to help protect the power meters or analyzer.

Conducted RF Immunity FAQ

What is conducted RF test?

The conducted RF test is an EMC immunity test where equipment is subjected to radio frequency disturbances through injection on power or data lines. 

What is conducted RF immunity?

Conducted RF immunity is when EUT/DUT are able to withstand when radio frequency EMI is injected at set levels on associated cabling going to the equipment or device. 

References/Additional Information

[1]https://www.emcstandards.co.uk/files/61000-4-6_immunity_to_conducted_rfi_2_1.pdf

[2]https://atlasce.com/services/immunity-testing/conducted-rf-testing/

[3]https://www.fuseco.com.au/articles/what-is-rfi-emi-emc

http://www.unitest.com/pdf/RF_immunity_testing_e_3.pdf

https://www.rfvenue.com/blog/2014/12/14/common-sources-of-interference

https://www.teseq.com/applications/induced-rf-i.php