EMC Question of the Week: September 6, 2021

Electrically small loop probe held over a circuit board

The hand-held probe shown in the figure (an electrically small loop) measures 

  1. voltage
  2. current
  3. electric fields
  4. magnetic fields


The best answer is “d.” The voltage produced by the probe is the result of time-varying magnetic flux passing through the loop. At low frequencies, the voltage is proportional to both frequency and the strength of the field passing through the loop. At higher frequencies (when |ωL| is greater than the 50-Ω receiver impedance), the frequency dependence disappears and the voltage is directly proportional to the magnetic field passing through the loop.

By definition, the circumference of the electrically small loop is less than a quarter wavelength at the measurement frequencies. These loops are generally shielded to further reduce their sensitivity to electric fields.

Although they do not measure current directly, small loop probes connected to a spectrum analyzer can be useful for locating high-frequency currents on a circuit board. High-frequency magnetic fields are created by high-frequency currents and they are generally strongest very near the circuit board traces or components that carry those currents.

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