Current Probes and Test Fixtures

Current Probes Clamp on Current probes are used in EMC Testing for Emission and Immunity. Conducted Noise emissions measurements using a clamp on current probe: Conducted noise can be measured using a LISN (for power lines) or ISN (for signal lines), however, these techniques require that we first open the corresponding power or signal cable and insert the LISN or ISN in the path. The alternate method using a Clamp On Current Probe does not require us to open the cable to insert the LISN or ISN in the cable connection path. Instead, we simply clamp on the current probe on to the cable.

For obvious reasons, such a probe is constructed using two halves of a toroidal magnetic material hinged together on one side to allow clamping on to a cable under test. Conducted noise in the cable is then measured by the cable under test forming the primary winding of a current transformer. The secondary winding of this current transformer is wound around the magnetic material. The current measured in the secondary winding reflects the current flow in the primary winding (the cable under test). The current clamp can also be used to inject noise on to a cable in the system under test for immunity (or susceptibility). In this case, the noise to be injected is on the primary of the current transformer and is wound around the split ferrite core of the clamp on probe. The secondary winding is the cable under test. This method is also a convenient alternative to using a CDN (coupling decoupling network) which requires that you open the cable circuit and insert the appropriate CDN.

It is important to note that the design of the injection current probe would be different than the emissions probe because the immunity test currents are generally much higher test levels. Each type of probe would also require that it be designed to perform the specific current level, frequency of operation, etc.

Current probes are calibrated through the use of a calibration fixture with a coaxial-type arrangement. The fixture allows the probe to be clamped around the center conductor, while the outer conductor encapsulates the probe on four sides so that the transmission line characteristics are not compromised. This fixture is available separately.