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# Use Rogowski probe to measure small impedances

Posted: 05 Mar 2014     Print Version

Keywords:current probes  Rogowski  oscilloscopes  Vector Network Analysers  VNA

You are probably familiar with clamp on current probes, but chances are you know little about the Rogowski current probe. A Rogowski coil produces a voltage that is proportional to the rate of change (derivative) of current enclosed by the coil-loop. The coil voltage must be integrated for the probe to provide an output voltage that is proportional to the current signal. This also means that the Rogowski can't perform at DC, but can operate to a frequency lower than most AC clamp-on current probes.

The table highlights the key performance attributes of three different clamp on current probes. The comparison results are based on the PEM CWT015, Tektronix P6022 and Teledyne Lecroy CP031 current probes. The three probe heads are shown in figure 1.

 Table: A comparison of current probes.

 Figure 1: CP031 (top), P6022 (middle), and Rogowski (bottom).

Both the AC current probe and the Rogowski current probe can be used with test equipment from any manufacturer. This includes oscilloscopes, Vector Network Analysers (VNAs), and Spectrum analysers. Hall-effect probes, on the other hand, are generally keyed to a particular equipment manufacturer. Comparing the AC and Rogowski probes, the Rogowski current probe excels in all performance characteristics except noise and the ability to measure DC. You can, though, manage the noise and make the Rogowski probe one of your go-to probes.

Consider the noise
The low frequency noise from the Rogowski current probe is a result of the high gain integrating amplifier needed by the probe. The CWT015 noise is predominantly below 1.5kHz as shown in the noise density measurement in figure 2.

 Figure 2: The noise density is 200µV/√Hz at 100Hz, 20µV/√Hz at 1kHz, and 2.4µV/√Hz at 10kHz. When used with a spectrum analyser or VNA the resolution bandwidth can be set to 30Hz or even lower. At 1kHz with a 10Hz resolution bandwidth the noise is approximately 100µV and at 10kHz and above only 10µV.

The frequency domain
When using the Rogowski current probe with a VNA or spectrum analyser, the resolution bandwidth can be set to 30Hz or even lower. This narrow bandwidth minimises the measurement noise. The sensitivity of the CWT-015 probe is 100mV/A. At 1kHz, the noise density indicates 20µV/√Hz noise. Converting through the sensitivity this equates to a noise current density of 200µA/√Hz.

The peak-to-peak noise can be calculated from the noise density and the resolution bandwidth as:

1 • 2 • 3 • 4

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