How does a BCI probe work?

The BCI probe operates as a transformer. The ferrite core is split (hinged) to allow clamping around existing cable harnesses without disconnecting them. The primary winding is connected to an RF power amplifier. When current flows through the primary, it generates a magnetic field in the ferrite core, which induces a voltage on the cable (secondary). This voltage drives common-mode current through the cable's common-mode impedance (typically 50-150 ohms depending on cable type, length, and termination). The induced current I = V_induced / Z_cm, where V_induced ≈ V_input × Z_T / Z_in. The split core design introduces a small air gap at the closure, which slightly reduces coupling efficiency and must be accounted for in calibration.

What is transfer impedance and why does it matter?

Transfer impedance Z_T is the ratio of induced voltage on the secondary (cable) to the current flowing in the primary winding. Z_T = V_secondary / I_primary. For BCI testing, the critical parameter is the current delivered to the cable: I_cable = P_forward × Z_T / (Z_source × Z_cm). A probe with higher Z_T requires less amplifier power to achieve the same injection level. Typical Z_T values: 0.5-2 ohms at 10 MHz, rising to 2-5 ohms at 100 MHz due to increasing ferrite permeability effects. Z_T is frequency-dependent and must be characterized during annual calibration (per ISO 11452-4 Annex A). Calibration uncertainty of ±1 dB in Z_T directly translates to ±1 dB uncertainty in the injected current.

How often must BCI probes be calibrated?

ISO 17025 accredited laboratories typically calibrate BCI probes annually or after any physical damage (dropped, cracked core). Calibration measures: 1) Transfer impedance Z_T vs. frequency using a calibration fixture (50-ohm through-line with the probe clamped around it). 2) Insertion loss to verify the probe does not excessively load the cable circuit. 3) Core closure force to ensure the split core halves mate properly (air gap affects Z_T). 4) Input VSWR to verify the probe's input match to 50 ohms. Results are documented in a calibration certificate with measurement uncertainty (typically ±0.5-1.0 dB for Z_T). Out-of-tolerance probes must be repaired or replaced, as using an uncalibrated probe invalidates the entire EMC test.

EMC Equipment

BCI Probe (Current Injection Clamp)

/bee-see-eye prohb/

A calibrated toroidal transformer for injecting controlled RF current onto cable harnesses during EMC conducted susceptibility testing. Split ferrite core clamps around cables without disconnection. Key specifications: transfer impedance Z_T (0.5 to 5Ω), frequency range (10 kHz to 400 MHz or 1 GHz), maximum input power (50 to 100W), and aperture diameter (30 to 100 mm). Annual calibration per ISO 17025 with ±0.5 to 1.0 dB uncertainty.

Z_T: 0.5–5Ω

Power: 50–100W max

Calibration: Annual (ISO 17025)

Understanding BCI Probes

The BCI probe is the critical interface between the RF power amplifier and the cable under test. Its performance directly determines the accuracy and repeatability of conducted susceptibility measurements. A well-designed probe provides consistent coupling across its rated frequency range with minimal insertion loss and good input impedance match.

The split-core design is essential for practical testing: cables in production equipment are already routed and terminated. The probe clamps around the harness without requiring any disconnection or rerouting. The core halves must mate precisely; any air gap reduces the magnetic coupling and lowers the effective transfer impedance. High-quality probes use precision-ground core surfaces and spring-loaded closures to ensure consistent gap pressure.

BCI Probe Specifications

Transfer Impedance:
Z_T = V_secondary / I_primary
Typical: 0.5–2Ω at 10 MHz
Rising: 2–5Ω at 100 MHz

Induced Current:
I_cable = V_induced / Z_cm
V_induced ≅ V_input × Z_T / Z_in
Z_cm = 50–150Ω (typical cable)

Power Budget:
For 200 mA at Z_cm = 100Ω, Z_T = 1Ω:
V_induced = 0.2 × 100 = 20V
P_input = (20/1)² × 50 = 8W needed

BCI Probe Types

Type	Aperture	Frequency	Power	Application
Standard	32 mm	10 kHz–400 MHz	50W	Automotive (ISO 11452-4)
Large aperture	80–100 mm	100 kHz–200 MHz	100W	Aerospace cable bundles
Extended range	32 mm	10 kHz–1 GHz	50W	5G/automotive radar
Monitoring	32 mm	10 kHz–1 GHz	N/A (passive)	Current measurement

Common Questions

Frequently Asked Questions

How does the probe work?

Split ferrite transformer. Primary driven by amplifier. Cable = secondary. Magnetic coupling induces common-mode voltage. I = V_induced/Z_cm. Split core clamps around existing cables. Air gap affects Z_T.

What is transfer impedance?

Z_T = V_secondary/I_primary. Higher Z_T = less amplifier power needed. Frequency-dependent (rises with frequency). Calibrated annually (±0.5 to 1.0 dB). ±1 dB Z_T uncertainty = ±1 dB current uncertainty.

Calibration requirements?

Annual or after damage. Measures: Z_T vs. frequency, insertion loss, core closure force, input VSWR. ISO 17025 accredited lab. Certificate with uncertainty. Out-of-tolerance = invalid EMC test results.

Related Terms

← BCI Method BCJR Algorithm →

EMC Equipment

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