RFI/EMI Current Probes and Injection Probes

CURRENT PROBES

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

APPLICATION
A current probe is used as a “pick-up” device for measuring RF current in single conductors or cable bundles when connected to the 50 Ω input
of a radio frequency interference measuring receiver or spectrum analyzer.

DESCRIPTION
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 µV to EMI µAs. When the EMI current is measured in dB above 1 µV as indicated on a conventional EMI meter, the correction factor will convert the measurement to dB above 1 µA. 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.

INJECTION PROBES
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.

APPLICATION
High power RF amplifiers with 50 Ω output impedance are used to deliver voltage to the injection probe. The wire or cable through the window of the probe acts as a secondary of the toroidal transformers. This test method is intended to be used instead of earlier methods, such as CS-01, CS-02, and RS-02 of MIL-STD-461.

DESCRIPTION
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 (VSWR). 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.

Injection probes currently available are described later in this section. Development of new styles is ongoing. If a probe meeting your requirements is not found on the list, send us details and we will provide a solution.

TECHNICAL INFORMATION
Current probes used as receptors are known as “inserted primary toroidal transformers” for connection to EMI receivers. Injection probes which deliver high RF currents into wires through the window are called “inserted secondary toroidal transformers”.

The maximum voltage carried on wires through the window is limited only by the insulation of the wires. Maximum primary current in the wires through the window of current probes is listed in the table in the Specifications tab (symbol lp).

The signal input to injection probes is rated in W from the signal source as indicated in the table in the Specifications tab.

Development is continuing on new and useful probes, both current measuring sensor probes and high wattage injection probes. The following is a partial list. As time goes on, the list will grow. If you do not see what you need, just ask.