Calibration of RF Current Probes

The calibration of RF current probes can be done in various ways. Here, we will use S-parameters provided by a Vector Network Analyzer (VNA).

For the calibration, ideally, the current probe is placed inside a coaxial structure:

The so-called transfer impedance is the parameter that needs to be "calibrated" or characterized. It describes the relation between the current through the probe and the voltage measured at its output (when terminated into 50Ω).

S-parameters are widely used when working with VNAs. S21 is the relation between the "wave" from port 1 of the VNA to the "wave" into port 2.

\[S_{21}=\frac{V_2^-}{V_1+}\]

Since the termination of the calibration fixture is known (50Ω), this equation can be rewritten to

\[V_2=S_{21}\cdot V_1=S_{21}\cdot \frac{I_1}{50\Omega}\]

This finally leads us to the transfer impedance \(Z_t\):

\[Z_t=\frac{V_2}{I_1}=\frac{S_{21}}{50\Omega}\]

In logarithmic terms this gives the well-known relation:

\[Z_t=S_{21}-20\cdot log10(50\Omega)=S_{21}-34dB\]

Remarks

This procedure only holds if the calibration fixture with the current probe is sufficiently well matched. Ie. the transmission line formed by the fixture has a characteristic impedance close to 50Ω.

Older editions of CISPR 16-1-2 suggest connecting the probe fixture to a single generator and measuring the voltages at the termination resistor and the probe's output. In logarithmic units, \(Z_t\) can be determined with

\[Z_t=V_1-V_2-34\]

where \(V_1\) is the voltage at the termination resistor and \(V_2\) is the probe's output.

Last updated 2024-11-17