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.
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 ohm input
of a radio frequency interference measuring receiver or spectrum
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.
factor is the inverse of the transfer impedance, Zt. Each probe is
shipped with a graph of the correction factor versus frequency,
to the serial number on the probe. Under certain conditions, a
current probe can be used to inject low level RF signals
into individual wires or cable bundles.
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.
High power RF amplifiers with 50 ohm 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
Bulk Current Injection Probes are available in two
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.