Bulk Current Injection Overview
What is bulk current injection (BCI)?
The BCI test is a continuous RF immunity test designed to ensure both compliance and product reliability when exposed to conducted EMI disturbances coupled with an injection probe. This immunity test is commonly done in commercial, military, and automotive applications with varying test levels, frequencies, and requirements.
The above image illustrates the concepts of the test using a current monitoring probe common in both the closed loop and substitution methods. This conducted RF immunity test is commonly conducted from as low as 4 kHz to 400 MHz with some requirements going higher in frequency.
Regardless of if the substitution or closed loop method is 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.
The video shows the current clamp calibration setup according to IEC 61000-4-6 using the Teseq NSG 4070 conducted RF test system. While setups may very, it does provide a introduction to:
- Setup of RF Injection Equipment
- Calibration Process to Test Levels
- Using Front Panel Software
- Modifying Testing Parameters
How BCI is measured
Bulk current injection is measured a few different ways depending upon the underlying standard and method. For commercial applications, volts are used which are associated with a set energy level during the calibration. For some automotive and military applications current measurements on associated cabling during feedback loop are used typically in mA or dBuA.
Many test level requirements are based upon a series of data over a given frequency, often called a curve. The test levels on the left for ISO 11452-4:2011 provide an excellent reference guide for the associated levels over the required frequency range.
Typically these levels are preprogrammed into the software or system allowing for testing to be conducted efficiently. The RF power amplifier, as well as other associated equipment, will determine what levels can be met and over what frequencies.
BCI Test Equipment
Bulk current injection (BCI) equipment can vary by testing requirements, however typically include:
- Conducted RF System
- Associated Attenuators & Loads
- BCI Injection Probe & Fixture
- Current Monitoring Probe & Fixture
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 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:
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.
The use of a dual directional coupler with an external amplifier provides additional setup connections and considerations. The video to the left provides a guide for adding an external amplifier to an RF system and includes:
- Connecting External RF Power Amplifier
- Using Dual Directional Coupler & Importing Data
- Running System Calibration to Verify
Bulk Current Injection Probes
What is a Bulk Current Injection (BCI) Probe?
BCI probes are generally categorized by transfer impedance, frequency range, power handling, as well as compliance to a particular standard. The image on the right shows three of the most common BCI probes including power rating and frequency range:
The calibration fixture provides a stable impedance during the system calibration. Using the clamp on hinge the probe is opened and secured around the fixture then connected to the RF system. The calibration fixtures are designed to be used with a particular model of probes and are not interchangeable.
The image on the left from MIL-STD-461 CS114 provides the limitation on the insertion loss for injection (BCI) probes. The associated standards limitations should be referenced and compared to the probes' capabilities prior to testing.
RF Current Monitoring Probes
What are RF Current Monitoring Probes?
Current probes vary in their sensitivity, power handling, and frequency range. For bulk current injection testing we are mainly concerned with RF measurements from 10 kHz to 400 Mhz. Current monitoring probes are commonly used in RF immunity applications for measurements of RF levels injected on the associated cabling after the injection probe.
Additional in depth information on current probes and how they are used in EMC testing, can be found here.
BCI testing standards and requirements, which are discussed later, will determine which probes are best suited to meet testing needs. The monitoring probe(s) should cover the entire test frequency range. The image to the left shows the Com-Power CLCE-400 in the associated calibration fixture.
The most common RF current monitoring probes include:
Given the wide frequency range of conducted RF testing and the calibration process, the EMC/EMI software is a crucial component. This can be done by using the front panel of a conducted RF system, or controlling the entire system via a laptop with a computer application. Should stand alone components (signal generator, spectrum analyzer, etc.) be used, the associated drivers must be available for the software to allow for communication between components.
Conducting BCI Testing
Regardless of if the substitution or closed loop method is being used, the first step in conducting the BCI test is the calibration. Following the calibration, testing will focus on the associated method along with any standard or specific test requirements.
Prior to running any test, including the calibration, proper attenuation and safety considerations must be followed! While many systems are designed with over testing protection, if proper connections and procedures aren't followed damage to equipment can occur.
RF Signal Modulation
The two methods of modulation used for signals during the BCI test are amplitude modulation (AM) and amplitude modulation peak conservation (AM PC). The AM PC signal method is common in automotive applications where the peak of the modulation is at the same level as the peak of the CW signal.
What is the system calibration for BCI?
The calibration of the setup or system including the current injection probe is done prior to running the test. During this calibration process if there are any issues with the setup or equipment it will impact the results, often times not completing the test.Typical BCI Calibration Setup
- 1) 20dB attenuator 10 W (for >100 mA)
- 2) 150Ω to 50Ω BCI Impedance Adapters
- 3) Fischer FCC-BCICF-6-150 Cal Fixture
- 4) Fischer F-120-9A Injection Probe
- 5) 50 Ω Termination Load
During the calibration process the associated test information will be entered including test level, frequency, coupling device, as well as other equipment considerations. Typically the calibration is not done modulated, however all other criteria will be entered for testing as if testing will be conducted.
For information on common issues with the setup and equipment for BCI see the troubleshooting section below.
What is the substitution method for BCI testing?
You will notice on the image on the left, a current monitoring probe is included. This can be used to verify levels or limit the current, depending on the source impedance.
Common Substitution Requirements Include: ISO 11452-4, Ford FMC1278, & MIL-STD-461 CS114
Closed Loop Method
What is the closed loop method for BCI testing?
The image on the left shows a common test setup where the current monitoring probe is used to monitor levels going towards DUT/EUT and adjustments made using the RF test system.
Common Closed Loop Requirements Include: ISO 11452-4, Ford FMC1278, & RTCA/DO-160 CS Test
Pre-compliance/Troubleshooting Immunity Failures
Radiated immunity testing is expensive and often long periods for product modifications at a test facility are not an option. For some testing, particularly at lower frequency ranges, a BCI test can replicate similar impacts and results on an EUT/DUT
The comparable test levels presented in commercial radiated immunity tests can also be seen in conducted immunity testing (as with 10V/m and 10V). This lends itself well for conducted RF testing to be used for troubleshooting radiated EUT/DUT failures at significantly lower costs. The two largest considerations for this type of testing will be the frequency of concern and source impedance.
This method is best used at lower frequencies during radiated testing where coupling is likely occurring onto cables going towards the EUT/DUT. At higher frequencies coupling onto cables is less likely and this method becomes a less viable option.
The three most common requirements for conducted RF immunity are IEC 61000-4-6, ISO 11452-4, as well as MIL-STD-461G CS114. While each standard has unique methodologies and requirements all have equipment crossover and replicate similar events. The below image shows the most common standards, the associated frequencies, as well as a bit about the criteria for each.
The setup of the equipment is crucial for BCI testing, below you will find application notes providing a guide for test levels and setups using the Teseq NSG 4070 system.
IEC 610000-4-6 while not bulk current injection, does include much of the same equipment and employs the substitution method with monitoring,,. The preferred method of this standard for conducted RF testing is coupling decoupling networks, with the current clamps (and EM clamps) being used only when CDNs aren't applicable and testing is being conducted on screened cables. Test levels for IEC 61000-4-6:
- 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)
The volts r.m.s. requirements for this standard are measured open circuit with an unmodulated signal. This same criteria is used in the standard for other test methods as well, including CDNs and EM Clamps.
IEC 61000-4-6 Test Requirements
- Frequency Range 150 KHz to 230 MHz with 3 Test Levels
- 80% AM Modulation & Test Levels in V r.m.s.
- Substitution Method with Monitoring
- 150 Ohm Impedance for Calibration
- Limited situations where injection applicable
A unique requirement of this standard is 150 Ohm impedance as opposed to the more common 50 Ohms. This requirement can be met by adding an additional 100 Ohm adapter in the setup, or adding additional attention to compensate through the software.
The associated image shows the calibration of injection probe using the Teseq CIP 9136A injection probe, PCJ 801B calibration fixture, as well as two 100 Ohm adapters with a 50 Ohm termination load.
The test generator connected to the other side of the setup would provide the additional 50 Ohm impedance required by the standard.
ISO 11452-4 applies to vehicles, regardless of propulsion system (IE gas, electric, diesel). The standard includes both the BCI test method as well as the Tubular Wave Coupler (TWC) method. This standard can be purchased from the ISO Webstore, the most recent version has been published in 2020.
ISO 11452-4 BCI Requirements Overview:
- Frequency Range 1 MHz to 200 MHz with 4 Test Levels
- Modulation, e.g. CW, 1 kHz AM 80% cycle
- Substitution with Probe and Closed Loop Method
- Monitoring Probe only for Substitution Method
- Artificial Networks/LISNs Required for Testing
A unique equipment requirement for this standard as opposed to other common BCI tests are Artificial Networks (AN). The purpose of utilizing these in ISO 11452-4 is to act as a substitute in place of the impedance of the vehicle power lines. LISNs/ANs have three main functions:
- Provide Stable Impedance (50Ω) on mains
- Block RF Signals on Mains from Entering EUT
- Provides Measurement Port
Military standard 461 covers a wide range of EMI tests for electronic equipment used in US Department of Defense applications. Unlike other standards, this standard is free to download through the Defense Standardization Program website. Section CS114 focuses on conducted susceptibility bulk cable injection which is the main conducted RF immunity requirement in MIL-STD-461.
MIL-STD-461 CS114 Requirements Overview:
- Frequency Range 4 kHz to 200 MHz with 5 Test Levels
- 1 kHz Pulse Modulation, 50 % duty cycle
- Substitution Method with 6 dB current limitation
- Monitoring Probe Required for Limiting the Induced Current
The calibration setup for MIL-STD-461 CS114 using a conducted RF test system can be seen below. Given that the substitution method is used, the power levels are utilized from the calibration using a 50 Ohm impedance as shown below.
Troubleshooting BCI Test Setup
1. Check Connections
The first step in determining why the system setup isn't operating properly is to check the connections between the different components of the setup. It is especially important if using an external amplifier with a dual directional coupler where there are substantially more connections which can be switched.
While reviewing the connections, also check to ensure that the the RF connectors are threaded properly. Depending upon the age and wear of the connectors it can be difficult to tell if the connector is threaded correctly. This can be done by hand tightening each connection ensuring good connections are made.
2. Verify Attenuators
RF Attenuators tend to be pretty rugged, however overpowering and shipping damage can occur. Should an attenuator no longer work, it will likely be unable to run a calibration at any level. When the connections are being checked it is best practice to replace each attenuator individually and attempt to run the calibration to determine which is no longer functioning.
Attenuators can also be verified prior to putting them in the setup eliminating them as a possible culprit. This can be done using a power meter and sensor to verify the correct attenuation.
3, Check Software
EMC/EMI software and the user interface for test systems are often flexible enough to provide solutions to a variety of BCI requirements. Many issues can arise from unfamiliarity with software, this is especially true with complex test methods and setups.
A simple error when entering data or selecting criteria can cause the entire setup to fail. It is best practice to verify criteria before performing a test method for the first time. Referencing the user manual or resources to verify criteria can help resolve software issues.
4. RF Amplifier Evaluation
The RF power amplifier can be one of the most fragile components of a conducted RF test setup. The durability of the amplifiers will vary by manufacturer, but amplifiers should be evaluated on a regular basis to ensure they're functioning correctly.
Should the issue arise from the amplifier it's possible spikes in power measurements could be noticed. An example of this is shown on the left image where the amplifier failed above 150mA. Depending upon the damage, the amplifier may fail to meet certain test levels or stop at a set frequency.
Bulk Current Injection FAQ