EMC/EMI

Current Probe

Q: How do you convert a current probe voltage reading into the actual cable current?

You subtract the probe transfer impedance from the measured voltage in logarithmic units. The relationship is I(dBuA) = V(dBuV) minus Z_T(dB-ohm), where Z_T(dB-ohm) = 20 log10(Z_T in ohms). For a probe with a transfer impedance of 5 ohms, Z_T(dB-ohm) = 14 dB-ohm, so a 60 dBuV reading on the analyzer corresponds to 46 dBuA, or about 200 uA of cable current. Because Z_T varies with frequency, EMC software applies the probe correction factor table supplied on the calibration certificate at each frequency point rather than a single fixed value. The 50-ohm input impedance of the receiver must also be accounted for, since the published Z_T is defined into a 50-ohm load.

Q: What is the difference between an EMC current probe and a bulk current injection probe?

They are reciprocal devices but optimized for opposite directions of energy flow. A measurement current probe is designed for low insertion impedance and a flat, well-characterized transfer impedance so it samples cable current without perturbing it, typically over 10 kHz to 1 GHz. A bulk current injection (BCI) probe is built to inject RF power onto the cable harness for immunity testing per MIL-STD-461 CS114 or ISO 11452-4, so it prioritizes power handling (often 50 to 200 W) and a controlled insertion loss over 10 kHz to 400 MHz. Using a sensitive measurement probe for injection can overheat its ferrite core, while an injection probe makes a poor low-level emissions sensor because its transfer impedance is not optimized for sensitivity.

Q: Why does a current probe roll off at low and high frequencies?

The frequency response is shaped by the ferrite-core transformer that forms the probe. At low frequency the magnetizing inductance is small relative to the load, so the transfer impedance rises at about 20 dB per decade until the core reactance dominates and the response flattens. In the flat midband region the probe behaves as an ideal current transformer and Z_T is constant. At high frequency, inter-winding capacitance, core permeability roll-off, and the finite electrical size of the probe window cause Z_T to fall, again near 20 dB per decade. Probe designers select ferrite mixes (such as manganese-zinc for low frequency or nickel-zinc for high frequency) and winding geometry to place the flat band where the standard requires measurement.

/KUR-uhnt prohb/

Clamping around a wire or cable, this RF current transformer senses the magnetic field of the current flowing through it and produces a proportional output voltage at a 50 Ω receiver. Its core figure of merit is the transfer impedance Z_T = V_out/I_cable, typically 1 to 5 Ω across a flat band that spans roughly 10 kHz to 1 GHz. Current probes are the standard sensor for conducted emissions measurement and, in their high-power reciprocal form, for bulk current injection immunity testing under CISPR 25, MIL-STD-461 CS114, and ISO 11452-4. Because they require no galvanic connection to the conductor, they measure harness current without breaking the circuit or loading the line under test.

Category: EMC/EMI

Transfer Impedance: 1 to 5 Ω

Typical Band: 10 kHz to 1 GHz

How a Current Probe Senses Cable Current

A current probe is essentially a wideband current transformer wound on a split ferrite toroid. The cable under test passes through the probe window and acts as a single-turn primary; the secondary winding inside the probe couples to the magnetic flux that the cable current produces in the core. The induced secondary voltage is delivered to a 50 Ω spectrum analyzer or EMI receiver. Because the only coupling mechanism is the magnetic field encircling the conductor, the probe is sensitive to the net (common-mode) current flowing through its aperture and reads zero for a balanced pair where the forward and return currents cancel. This makes the current probe the preferred tool for finding the common-mode current that actually drives radiated emissions from a cable harness.

The defining specification is transfer impedance Z_T, the ratio of output voltage to the current threading the probe. A higher Z_T gives better sensitivity for low-level emissions work, while injection probes trade sensitivity for power handling. The split-clamp construction lets the probe be opened and placed around an in-situ harness without disconnecting anything, which is why it dominates both bench debug and formal compliance testing. Probe insertion impedance, the series impedance the probe adds to the cable, must stay low (well under 1 Ω in the band of interest) so the measurement does not change the very current it is reporting.

Calibration is performed in a coaxial calibration jig that establishes a known current through the probe window. The resulting correction factor, expressed in dBΩ versus frequency, is supplied on the calibration certificate and applied point-by-point by the EMC software. Most current probes are calibrated and used with the conductor centered in the window; off-center placement and nearby metal can shift Z_T by a fraction of a dB, which matters when a limit margin is tight.

Transfer Impedance and Frequency Response

Transfer impedance (definition):
Z_T = V_out / I_cable (Ω), typically 1 to 5 Ω
Z_T(dBΩ) = 20 log₁₀(Z_T / 1 Ω)

Current from receiver reading:
I(dBμA) = V(dBμV) − Z_T(dBΩ)

Ferrite-core frequency response:
Low band: Z_T rises ≈ +20 dB/decade (magnetizing inductance)
Mid band: Z_T ≈ constant (ideal current transformer)
High band: Z_T falls ≈ −20 dB/decade (winding capacitance, μ roll-off)

Example: Z_T = 5 Ω → 14 dBΩ. A 60 dBμV reading → 60 − 14 = 46 dBμA ≈ 200 μA of cable current.

Current Probe and Sensor Comparison

Type	Bandwidth	Z_T / Output	Current Range	Primary Use
EMC clamp-on probe	10 kHz to 1 GHz	1 to 5 Ω	1 μA to ~100 A	Conducted & radiated emissions
BCI injection probe	10 kHz to 400 MHz	Controlled insertion loss	1 to 10 A (50 to 200 W)	CS114 / ISO 11452-4 immunity
Lab current monitor	1 MHz to 1 GHz	~1 Ω	0.1 to 1 A	Bench debug, near-field tracing
Rogowski coil	DC* to 50 MHz	~10 mV/A	1 A to 10 kA	Power & pulsed current
Hall-effect sensor	DC to 100 kHz	~1 to 100 mV/A	1 to 1000 A	DC & LF power systems

*Rogowski coils sense dI/dt, so they respond down to very low frequency but not true DC.

Common Questions

Frequently Asked Questions

How do you convert a current probe voltage reading into the actual cable current?

Subtract the transfer impedance in logarithmic units: I(dBμA) = V(dBμV) − Z_T(dBΩ), where Z_T(dBΩ) = 20 log₁₀(Z_T in Ω). A 5 Ω probe is 14 dBΩ, so a 60 dBμV reading equals 46 dBμA, about 200 μA. Because Z_T varies with frequency, EMC software applies the certificate correction table at each point rather than one fixed value, all referenced to the 50 Ω receiver input.

What is the difference between an EMC current probe and a bulk current injection probe?

They are reciprocal but optimized oppositely. A measurement probe has low insertion impedance and a flat, well-characterized Z_T so it samples current without perturbing it (10 kHz to 1 GHz). A BCI probe is built to inject 50 to 200 W onto a harness for CS114 or ISO 11452-4 immunity testing (10 kHz to 400 MHz), prioritizing power handling. Injecting through a sensitive measurement probe can overheat its ferrite core.

Why does a current probe roll off at low and high frequencies?

The ferrite-core transformer sets the shape. At low frequency the small magnetizing inductance makes Z_T rise about +20 dB/decade until the core reactance dominates and the response flattens. In the midband the probe acts as an ideal current transformer with constant Z_T. At high frequency, inter-winding capacitance and permeability roll-off pull Z_T down about −20 dB/decade. Choosing a manganese-zinc or nickel-zinc ferrite places the flat band where the standard requires.

Related Terms

← Current Mirror (RF) Current Ratio →