Calibrate current clamps
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 8/1/2005
Current clamps let you perform emissions and immunity measurements in signal and power cables. Unfortunately, clamps absorb some energy from the circuit under test in order to make a measurement. Thus, they produce measurement errors. By applying a known current to a test fixture, though, you can calibrate your equipment and characterize the current clamp. You can also analyze the effect of placing the clamp on the cable.
A calibration method uses a known resistance between two transmission lines to measure current. The figure shows two transmission lines separated by a gap. A known resistance (between 1 Ù and 10 Ù) lets you measure the current passing through the gap. The resistance value comes from a combination of about 30 SMT resistors.
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| A series resistor across a gap between transmission lines provides a known resistance from which you can measure current and calibrate a current clamp. Courtesy of IEEE. |
A feed port connects a network analyzer to a metallic rod, which provides a low-impedance path to the gap resistors. The resistors then connect to a coax cable, which carries the current to a 50-Ù termination. The network analyzer measures the voltage at the fixture's exit port. Using S-parameter analysis, you can calculate the current that passes through the gap resistors.
By placing a current clamp over the gap resistors, you can measure the clamp's output voltage. Current-clamp manufacturers provide the clamp's transfer impedance, so you can calculate the current measured by the current clamp. Then, you can compare it to the current in the gap resistors, measured by the network analyzer, and calibrate the current clamp.
To learn about the theory, construction details, and test results of this method, download "A New Test Setup and Method for the Calibration of Current Clamps," by David Pommerenke of the University of Missouri-Rolla, Ramachandran Chundru of Texas Instruments, and Sunitha Chandra of Nvidia, from web.umr.edu/~davidjp/publications.html.
