Description

Current probes required by various EMI specifications (such as MIL-STD-461/2) are toroidal transformers designed to measure r.f. currents on active power lines or other conductors.

Direct connection to the conductor carrying EMI current is not necessary, since the probe may be opened for insertion of the conductor into the window of the toroid and then closed again to form a toroidal transformer with the conductor acting as a one-turn primary. A correction factor graph is provided to convert measured microvolts to EMI microamperes. When the EMI current is measured in dB above one microvolt as indicated on a conventional EMI meter, the correction factor will convert the measurement to dB above one microampere. The correction factor is the inverse of the transfer impedance, Zt. Each probe is shipped with a graph of the correction factor versus frequency, keyed to the serial number on the probe. Under certain conditions, a current probe can be used to inject low level r.f. signals into individual wires or cable bundles. Ask our engineers for advice in the use of current probes for this purpose. Some current probes can be easily damaged or are otherwise unsuitable for this application.

Specifications require the injection of large high frequency currents into cable bundles and individual wires, using inserted secondary toroidal transformers placed around the conductors being tested.

Bulk Current Injection Probes are available in two styles:

1. Fixed window style where the wire(s) under test must be passed through the window.
2. A split toroidal design where the probe can be opened up and clamped over the wire(s) under test.

Each probe is calibrated for insertion loss and transfer impedance in a test fixture designed for the particular window size. This fixture provides a signal path with a low Voltage Standing Wave Ratio. A typical fixture is Solar Type 9125-1, used for probes with 32 to 44 mm diameter windows. Ask for details on this and other test fixtures.