A Capacitively Coupled Pin Injection Method for Troubleshooting Immunity Issues

Bulk current injection (BCI) tests are widely used for automotive, military, and aerospace EMC immunity tests. The test setup requires a high-power amplifier (often at least 80-watt unsaturated output power) and a BCI injection probe to achieve a reasonably high interference level on the device under test (DUT). 

In one recent example, an automotive remote controller unit experienced immunity issues during the BCI test in an accredited EMC testing laboratory. The module’s local interconnect network (LIN) lost communication in the frequency range between 5 and 15 MHz.

The same failure mode must be reproduced in a pre-compliance EMC test setup to fix the issue. For pre-compliance EMC tests, producing the same failure mode often requires a different setup unless the specific BCI test equipment is available. 

Some of the test setups often used in pre-compliance EMC immunity tests include:

  1. A workbench BCI test using an RF monitor current probe as an injection probe is described in [1];
  2. A homemade BCI probe method is described in [2];
  3. A coupling and decoupling network (CDN) method;
  4. A transverse electro-magnetic cell (TEM Cell) method.

A Fischer current monitor probe F-33-1 is often used as an injection probe for pre-compliance BCI testing [1]. The test setup was documented in detail in [3] and it was mentioned that, in order to achieve a higher level of RF interference, one would need to put some ferrite chokes on the other side of the probe to reflect more energy to the DUT.

While this method might work to some extent, it is generally not a good practice to use an RF monitor probe to inject noise unless you know the specified maximum RF power that you can feed into the probe. Besides, most of the RF monitor probes are designed to receive rather than emit RF signals. BCI injection probes typically have very large cross-section toroids to increase the saturation levels.

Today, using CDNs is the recommended choice for the immunity test, compared with the BCI test, CDN testing requires a much smaller power level to achieve a higher coupling factor. Using a TEM cell for an immunity test is also gaining popularity [4] since studies have shown that there’s a strong correlation between the TEM cell and BCI test results [5].

In this column, we’ll present a capacitively coupled pin injection method as an alternative testing method.

Introducing the Capacitively Coupled Pin Injection Method 

Test Setup

A diagram of the test setup is shown in Figure 1, and the test setup is shown in Figure 2. An RF current monitor probe is clamped to the cable to monitor the injected RF current level during the immunity test. Note that the current level depends on the output of the RF amplifier, the impedance of the capacitance value of the injection probe, and the circuit impedance of the DUT. 

Figure 1: Diagram of the test setup


Figure 2: Picture of the test setup

Making a “Flying” Probe

The injection probe used in this test is also referred to as a “flying” probe because the capacitive probe often has a small ground plane to increase the coupling between the probe and the DUT’s power/ground plane. The small ground plane looks like a wing, hence the name flying probe (or “wing” probe). 

It is easy to make a homemade flying probe, as shown in Figure 3. Steps to make the probe include:

  1. Cut a semi-rigid coaxial cable in half;
  2. Drill a hole with the same diameter as the coaxial cable in a small piece of copper and solder the piece of copper to the shield of the coaxial cable. Using a small PCB with a solid continuous ground plane is also a good idea;
  3. The tip of the coaxial cable signal line is then soldered to a 250V capacitor. The value of the capacitor depends on the level of interference current one would like to inject (see the next section).

Figure 3: An example of a “flying” probe

Selecting the Right Size of a Coupling Capacitor 

The injected RF current level depends on the amplifier’s source impedance, the capacitance value, and the load impedance. Often the load impedance is unknown and frequency dependent. But the general rule is that, at the frequency range of interest, the capacitor’s impedance should be more or less the same as 50 Ω (to match with the RF amplifier output impedance). For instance, if the DUT has an immunity issue at 68 MHz, then a 47 pF would be a good choice because the impedance of a 47 pF capacitor at 68 MHz is about 50 Ω. If the DUT has an immunity issue below 30 MHz, then a 100 pF capacitor would be a better choice. 

Although most modern RF amplifiers have a high voltage device rating against impedance mismatch, special care is needed to prevent impedance mismatch. To avoid impedance mismatch of the power amplifier, often an attenuator (such as a 3 dB one) is also recommended to be connected between the output of the power amplifier and the flying probe. 

Because the failure mode in this particular case occurred at the sub 20 MHz range, a 100 pF, 250V Y Class capacitor was selected. It is also important to note that a capacitor’s equivalent series inductance (ESL) has little impact at this frequency range. However, as the frequency increases, the long lead of a capacitor begins dominating the impedance as parasitic inductance increases with frequency. 

Therefore, if the injected noise level is in the hundreds of MHz range, the impedance vs. frequency curve of the selected capacitor needs to be checked to ensure the capacitor’s impedance is not too high. The long leads of the capacitor will undoubtedly need to be shortened in the MHz frequency range.

Test Results

This test was simple to perform. The signal generator was configured to perform a fixed amplitude, variable frequency sweep between 5 and 15 MHz. It was noticed that the LED lights of the DUT started flashing during the sweep, and the PC monitor also recorded multiple LIN communication errors. This was the same behavior the DUT experienced in the BCI test. The RF current level which was monitored through the RF monitor probe served as another useful tool to identify the potential issue on the circuit.

With the failure mode visible in the pre-compliance test set-up, fixing the issue and validating the results are more easily achieved. 


K. Wyatt, “Using higher-powered RF immunity testing,” EDN, 25 May 2021.

A. Nielsen, Application of Thrifty Test Equipment for EMC Testing,” In Compliance Magazine, December 2021.

P. André and K. Wyatt, EMI Troubleshooting Cookbook for Product Designers, Edison: SciTech Publishing, 2014.

A. Eadie, “TEM Cell and GTEM Guide For Radiated Emissions & Radiated Immunity Pre‑Compliance Testing,” EMC FastPass.

D. Trout, “Investigation of the Bulk Current Injection Technique by Comparison to Induced Current from Radiated Electromagnetic Fields,” NASA, George C. Marshall Space Flight Center.

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