TEKBOX Application Note: Using RF current probes with EMCview

RF current probes are useful for measuring common-mode current on cables without disconnecting the conductor. When the probe’s frequency-dependent transfer impedance is entered into EMCview as a correction, a spectrum analyser voltage reading can be displayed as current in dBµA.

Transfer impedance and transducer factor

A current probe produces an output voltage in response to current through the conductor. Its transfer impedance is defined as Z_T = V / I and is normally specified in ohms or dBΩ versus frequency.

When voltage is expressed in dBµV and transfer impedance in dBΩ, the current is calculated as:

Current in dBµA = measured voltage in dBµV – transfer impedance in dBΩ

Some manufacturers publish a transducer factor in dB(1/Ω) instead. This is the negative of transfer impedance expressed in dBΩ, so it is added to the measured voltage:

Current in dBµA = measured voltage in dBµV + transducer factor in dB(1/Ω)

Worked example

Assume the analyser measures 40 dBµV at a frequency where the probe transfer impedance is 10 dBΩ. The corresponding current is 40 – 10 = 30 dBµA. If the probe data instead gives a transducer factor of -10 dB(1/Ω), the same result is obtained by calculating 40 + (-10) = 30 dBµA.

Create the EMCview correction data

• Obtain the probe’s calibrated transfer-impedance or transducer-factor data across the frequency range.
• Use enough frequency points to represent changes in the probe response; do not assume one value across the full band.
• If the source is transfer impedance in dBΩ, invert its sign to create an additive transducer-factor correction.
• Enter frequency and correction pairs into the EMCview correction file using the format expected by the software.
• Check the result at several frequencies with a known or independently calculated value before relying on the full sweep.

Measurement precautions

• Keep the conductor in a repeatable position within the probe aperture.
• Observe the probe’s current, voltage, frequency and safety limits.
• Characterise or account for any preamplifier and cable used between the probe and analyser.
• Check the analyser for overload when large low-frequency or switching components are present.
• Record whether displayed values are voltage, current, transfer impedance or transducer factor; the units are not interchangeable.

This article is based on the Tekbox procedure for incorporating RF current-probe correction data into EMCview. Use the calibration data supplied with the specific probe wherever available.