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EMC Engineering: A Current Probe used to Measure a Cable's Common-Mode Currents



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Above: A current probe in use. Current measurements for EMC can be in amps, milli- and micro-amp units.

Current Probe Transfer Impedance


Instead of doing a precise calculation, it’s okay to measure the voltage with a known current and frequency passing through the probe. This measurement is commonly referred to as as “transfer impedance” ( ZT ).



       

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Current Probe – An Example of Application

Let’s say one is responsible for finding the level of probe voltage which corresponds to a CM (common-mode) current on a cable -- one that just meets the radiated-emission regulatory limit.

Clamping the probe around a cable and lumping the net CM (common-mode) current into one wire of length L gives the net radiated emission a distance d away. Notice one must divide by 2, since that result was for two wires carrying a current of Ic . Assumption: FCC-B limit at 30 MHz (100 uV/m) and 1m cable.

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Updated: Wednesday, 2007-10-03 1:44 PST